181 replies to this topic

[Sandeep Banerjee]

In Donohue's famous book, I learnt about his strong preference for the locked differential, as opposed to the limited slip used in most cars. He used it in nearly everything he drove for Penske Motorsports.. the Lola T90, the Ferrari 512, even the McLarens for running in USAC and the Porsche 917/10 and 30. He mentions also about the (understandable IMHO) flack he recieved from Teddy Mayer, Ferrari and even Porsche engineers about this choice. Even David Hobbs later said he knew Mark made it work for him, but he found it to be extremely uncomfortable to drive around..

Can someone please explain the logic behind Mark's choice? Why did he so strictly go with a locked diff in every car he drove? I can understand it in a USAC oval car but certainly not for twisty road courses but he used that everywhere! Who knows, he might have gone with a locked diff in the Penske F1 car as well.. :

Thanks,
Sandeep

[Catalina Park]

Nothing special about a locked diff. The car that won Bathurst today had a locked diff. (and the car that finished second, etc.) It removes a lot of variables.

[Sandeep Banerjee]

First, thanks for taking care not to post the winner's name. So many people forget that sometimes.

Yes, they did, but I was wondering why would anyone, Donohue in this case, want.. to get a locked differential in a car that came with and was designed around a LSD? Cars like the 512, the Porsche 917 Can-Am cars that Penske ran.. I can't imagine how a locked diff would benefit those cars on road courses instead of a LSD..

Thanks,
Sandeep

[Wolf]

Sandeep- I think Porsche 917 & 956 had locked diff (I think the reason had something to do with putting all that power on road in lower gears). If factory figured cars would benefit from locked diff, Donohue probably wasn't unreasonable either.

One thing I always found curious- if they were using locked diff, why use diff at all?

[hydra]

Originally posted by Sandeep Banerjee
First, thanks for taking care not to post the winner's name. So many people forget that sometimes.

Why is that?

[rhm]

By 'locked diff' do you mean a spool?

It's common to run spools on ovals, but it's difficult to see how it would be driveable on a road circuit unless the car already had a serious lack of rear grip.

[Engineguy]

If you need a differential you're not driving the car hard enough. Get a new driver.

[Wolf]

Originally posted by hydra


Why is that?

Hydra- because sometimes people can watch certain races only tape-delayed (no live coverage), or their own recordings (working during the race, &c), so they prefer not knowing the result before watching the race themselves. Sort of, spoils the fun- esp. if one does everything not to see the results, and stumbles upon the result in LSD thread...

[hydra]

I hadn't even thought of that before

[phantom II]

For instance: At 2am EST this morning I watched a certain Italian manufacturer's engine blow up with an extraordinary amount of glee.. The race will be shown again at 2pm EST. I wont tell you who the winner is but me and Briatore are very pleased with the result indeed.
You have to be considerate for other people's feelings, Hydra.;)

Originally posted by hydra


I hadn't even thought of that before

[McGuire]

Originally posted by rhm
By 'locked diff' do you mean a spool?

It's common to run spools on ovals, but it's difficult to see how it would be driveable on a road circuit unless the car already had a serious lack of rear grip.

Actually it works pretty good. Better than an open diff by far, and better, for the most part, than the limited slips that were available in Donohue's day, especially for high-horsepower cars. But it does require a considerably different setup... and a special driving technique, needless to say.

[McGuire]

Originally posted by Engineguy
If you need a differential you're not driving the car hard enough. Get a new driver.

That might not be totally fair... but it has been suggested that when Donohue discovered Hobbs did not like to drive a spool, he regarded him as a bit of a p__sy from that moment on.

[rhm]

heh, fair enough. I'll have to see if I can get a spool for my mx-5

[Wolf]

Originally posted by McGuire



Actually it works pretty good. Better than an open diff by far, and better, for the most part, than the limited slips that were available in Donohue's day, especially for high-horsepower cars. But it does require a considerably different setup... and a special driving technique, needless to say.

I reckon ZF pawl-type diffs were used long before his day (and probably during)- they even coped with 600BHP back in thirties... From what I gather, they seem to be quite good and perform quite well. Are You saying no diff is a better option than that?

[Ben Wilson]

Particularly for endurance races, reliability and consistency are the key points. Any type of mechanical LSD will change during an event (heat, wear etc). A spool will behave the same on the last lap as the first.

Also in the event of an axle failure, a spool will enable the car to limp back to the pits where most LSDs won't.

[Sandeep Banerjee]

Originally posted by Ben Wilson
Particularly for endurance races, reliability and consistency are the key points. Any type of mechanical LSD will change during an event (heat, wear etc). A spool will behave the same on the last lap as the first.

Also in the event of an axle failure, a spool will enable the car to limp back to the pits where most LSDs won't.

That's true, even the Porsche 956 had a locked diff, no? I guess that explains it for the Lola T70 and Ferrari 512 since they were both used by Penske Racing in endurance events. And 'too much power' is the reason for it being in the 917s then, we can take it? :

Sandeep

[EfiOz]

Originally posted by Catalina Park
Nothing special about a locked diff. The car that won Bathurst today had a locked diff. (and the car that finished second, etc.) It removes a lot of variables.

That's only because they have to. It's mandated in the rules. It may remove some variables but it introduces a lot of undesirable constants.

Donohues preference for locked diffs might be able to be explained by the cars you've mentioned. They are all very powerful, mostly very torquey cars and a locked diff may almost be a blessing. That's funny that the Porsche guys were against it as they used it extensively in the 935/956/962 sports racers. Apparently the corner entry understeer went some way to taming the turn in nervousness in the rear, especially with the 935.

Weismann lockers are very good but the good ole clutch pack LSD is still quite tunable and does a pretty good job as long as you can keep both wheels sort of loaded.

[Fat Boy]

To get to the bottom of this, I think you need to look at what he was racing and when he was racing it.

He started really liking the spool when he was driving Trans-Am. These cars had lots of power, not much tire, and a really high CG. The first 2 add up to a car that doesn't want to get off of a corner. Wheelspin was a huge issue. The high CG (w.r.t. present day cars) means that in a corner there was a lot of lateral load transfer. The inside tires just didn't have much weight on them. Because of that, the understeering moment mid-corner of the spool was actually pretty small.

It would give stability on entry (which really good drivers demand) and good drive off the corner. I'm sure it made more mid-corner understeer than a differential, but if the car is set up around it, it's not that big of a handicap. Especially if the driver knows that it's there, likes what it gives him, and doesn't get greedy with the throttle mid-corner.

The cam and pawl had a lot of wear issues, the Salisbury took more tuning time than they were likely to put in on it in the 60's, and the Weismann has a lot of spool-like characteristics. All of these diffs fought metallurgy and maintainance issues that we don't have to deal with now. They didn't have the options of viscous or many different ramp angles like we have now. They mostly worked with what they had. Of what Mark had available to him, the spool probably wasn't a bad option.

Just remember that a lot of water has passed under that bridge in 30-40 years. I will say what I took from what he wrote was not to write off a fair amount of diff preload just for the worry of understeer. A lot of times, the good parts of having a good chunk of preload or ramp can outweigh the downside of the mid-corner understeer. It's a good tool to have in your box.

[McGuire]

Originally posted by Wolf


I reckon ZF pawl-type diffs were used long before his day (and probably during)- they even coped with 600BHP back in thirties... From what I gather, they seem to be quite good and perform quite well. Are You saying no diff is a better option than that?

As the pawl limited slip wears it reverts to an open diff. The greater the throughput the faster it goes away.

[LS 1]

There's a bit of ambiguity in this discussion: Was Donohue talking about no differential at all, i.e., axles "locked" relative to each other at all times, or a pawl-type diff commonly marketed in the USA under the tradename "Detroit Locker" (if I've got that right)?

I suspect it's the former.

[Fat Boy]

Page 104 of The Unfair Advantage:

"We built the first '67 Camaro with stiff springs, we were burning up the limited-slip clutches in the differential. The problem went away when the car was made softer, because we had better traction and didn't spin the inside rear wheel so easily. Now we were learning that uneven action in various types of locking differentials was causing an instability in cornering, and we adopted the fully-locked rear axle. It didn't seem to have any effect on the circular skidpad, but it avoided the slipping and jerking as the clutches grabbed when I came out of a corner, as on the square course. It made the car a lot more stable and easier to drive. For some reason locked differentials weren't approved by the SCCA, thought, so we left the positraction in and merely stacked up the clutch springs so that there was about a million pounds of pressure on the clutch packs."

That should answer that question.

[LS 1]

This thread raises a related issue that has been bothering me.

On a Jag owners board a couple months ago one of resident gurus insisted that a LSD was good only for traction longitudinally and actually destabilizes the car in a turn. His rationale, as I understood it, was the contact patch has only total grip “X” to work with, and any longitudinal force necessarily decreases the amount of grip available to cope with lateral force. By applying power where it otherwise was not (the outer wheel), the LSD thereby decreased lateral grip and destabilized the car.

I think this line of thought is incomplete and thus wrong. If one treats both tires as a connected whole, then it doesn’t matter how the diff splits power delivery: If the car destabilizes due to power application, then the problem is that total force exceeds “X” and that is a function of the driver’s right foot, not the diff.

On the other hand, if we add detail and get more “real world” in comparing open and LS diffs, then we have to account for the removal of torque from the inner wheel when the open diff is use, e.g., lifting off the throttle is just as destabilizing in my experience as increasing throttle, as lifting creates a (negative) longitudinal force that can cause total force to exceed “X”. An open diff takes the torque away from one wheel and transfers it all to another and thus destabilizes the car. On the other hand, an LSD (or, better, locked) diff keeps power application proportioned, allowing the driver better control and thus assists in stabilizing the car through the turn.

Eventually, the guru grudgingly conceded that a LSD would not destabilize the car, but insisted it still “is no help in a turn.”

You’ve probably guesses already that I’m not an engineer. Am I way off base here? Is an LSD only useful for power application out of the turn or does it have benefits throughout the turn? It seems to me that once a slip angle is developed, a LSD will have a benefit even when the driver is attempting maintenance throttle, as power application will be more consistent and the diff will not be attempting to transfer torque from one axle to another (or at least will be doing it less).

[LS 1]

Originally posted by LS 1


On the other hand, if we add detail and get more “real world” in comparing open and LS diffs, then we have to account for the removal of torque from the inner wheel when the open diff is used

Opps . . . should have been "removal of torque from the outer wheel." Too much writing, too little thinking.

[McGuire]

Originally posted by LS 1
There's a bit of ambiguity in this discussion: Was Donohue talking about no differential at all, i.e., axles "locked" relative to each other at all times, or a pawl-type diff commonly marketed in the USA under the tradename "Detroit Locker" (if I've got that right)?

I suspect it's the former.

Actually, we are now talking about three types of drive axles: 1) the "spool" or locked rear axle, no differential action at all, as Donohue often used; 2) limited slip differentials like the ZF pawl-type unit; and 3) locking differentials like the Detroit Locker, which is not LSD. A locker is essentially a ratchet, works like a chain fall. It's either pulling or not, no "limited" slip. The Detroit Locker has been considerably improved in recent years, but it's still a locker.

[McGuire]

Originally posted by LS 1
This thread raises a related issue that has been bothering me.

On a Jag owners board a couple months ago one of resident gurus insisted that a LSD was good only for traction longitudinally and actually destabilizes the car in a turn. His rationale, as I understood it, was the contact patch has only total grip “X” to work with, and any longitudinal force necessarily decreases the amount of grip available to cope with lateral force.

I guess it depends how you look at it. If the LSD is "destabilizing" then so are the brakes and throttle. What are we trying to accomplish? To get through the corner in as little time as possible. So we want to exploit the entire traction circle, not just its lateral grip axis.

[Fat Boy]

Originally posted by LS 1
This thread raises a related issue that has been bothering me.

On a Jag owners board a couple months ago one of resident gurus insisted that a LSD was good only for traction longitudinally and actually destabilizes the car in a turn. His rationale, as I understood it, was the contact patch has only total grip “X” to work with, and any longitudinal force necessarily decreases the amount of grip available to cope with lateral force. By applying power where it otherwise was not (the outer wheel), the LSD thereby decreased lateral grip and destabilized the car.

I think this line of thought is incomplete and thus wrong. If one treats both tires as a connected whole, then it doesn’t matter how the diff splits power delivery: If the car destabilizes due to power application, then the problem is that total force exceeds “X” and that is a function of the driver’s right foot, not the diff.

On the other hand, if we add detail and get more “real world” in comparing open and LS diffs, then we have to account for the removal of torque from the inner wheel when the open diff is use, e.g., lifting off the throttle is just as destabilizing in my experience as increasing throttle, as lifting creates a (negative) longitudinal force that can cause total force to exceed “X”. An open diff takes the torque away from one wheel and transfers it all to another and thus destabilizes the car. On the other hand, an LSD (or, better, locked) diff keeps power application proportioned, allowing the driver better control and thus assists in stabilizing the car through the turn.

Eventually, the guru grudgingly conceded that a LSD would not destabilize the car, but insisted it still “is no help in a turn.”

There's a lot going on in this post.

The locked diff causes understeer and increases yaw damping. The car will not want to spin as easily with a locked diff as it will with an open. If your car is already understeering and you lock the differential, then it will increase the understeer. They may be what the guru is meaning when he says it "is no help in a turn".

However, most racecars have to deal with oversteer as much as they have to deal with understeer. If increasing the differential lock reduces that oversteer, then it is a big help in a turn. The understeer that is created is from the difference in slip ratio across the rear axle. As that slip ratio difference grows, there is more of a force to keep the car in a straight line. An open diff keeps that slip ratio difference essentially zero until the point of inside wheelspin.

If the guru is dismissing the longitudinal gain that a limited slip or locking differential can make, then he needs more horsepower. There are times where you have to sacrifice the mid-corner balance of the car to get a good drive off the corner. If a car is really strong off the corner, the a driver will generally put up with mid-corner understeer because the car will still be fast. If the car has really bad powerdown, but has a reasonable mid-corner balance it is going to be slow. It all depends what the stopwatch says.

[LS 1]

Originally posted by Fat Boy


There's a lot going on in this post.

The locked diff causes understeer and increases yaw damping. The car will not want to spin as easily with a locked diff as it will with an open. If your car is already understeering and you lock the differential, then it will increase the understeer. They may be what the guru is meaning when he says it "is no help in a turn".

However, most racecars have to deal with oversteer as much as they have to deal with understeer. If increasing the differential lock reduces that oversteer, then it is a big help in a turn. The understeer that is created is from the difference in slip ratio across the rear axle. As that slip ratio difference grows, there is more of a force to keep the car in a straight line. An open diff keeps that slip ratio difference essentially zero until the point of inside wheelspin.

Thanks for the response; I feel like on the verge of enlightenment, but I'm missing something. How does slip ratio vary across the rear axle when the axle is locked? If we're thinking of the same thing, there are three (?) variables in the equation: Longitudinal velocity, angular velocity, and tire radius. The last one is a constant once tire choice is made, and I don't see how the remaining ones can vary accross the axle when it is locked?

HOLD IT. Duh, I have to account for the fact that the outside of the car is moving around a circle of greater diameter than the inside.

Okay, I got it.

[Wolf]

LS1, I may not be helping with my remark but the angular velocity is the same if the diff is locked, which means outer wheel will be slipping, relatively to the ground, more than inner wheel (hence grater slip ratio).

I would think locked diff would significantly increase tyre wear, on account of that. That and the fact that LSD like pawl-type will provide more locking in tight turns than in sweepers, is a bit puzzling- that would require more aggressive driving style, which can't be good for tyres either...

[Greg Locock]

If the inner wheel is lightly loaded it may not have the grip to handle the higher traction torque in addition to the (reduced) lateral force required by the axle's slip angle.

So, is there not a secondary effect where a locked diff can cause the rear end to break away sooner than an open diff? In other words it could give low g understeer and high g, power on, oversteer, as it approaches the breakaway condition?

I'll try again with an open diff.

It provides equal drive torque to each wheel. In a front axle steered car that is in neutral or understeering attitude then the thrust vector is towards the outside of the path of the front of the car (kinda sorta), hence it is oversteer (helping to turn the nose in). If the inner wheel is lightly loaded, again the lateral contribution of the tire will reduce as the tire saturates due to the traction, and so you'll get more oversteer. On the other hand if you are cornering with opposite lock, the thrust vector is understeer, but the saturation effect is oversteer.

OK, so all I've 'proved' is that in a front steered rear wheel drive car which has positive lock if you hit the throttle in a turn the rear tends to move outwards, if the tires are near the limits of adhesion. Oh well, bleeding obvious, but at least it is in line with reality.

[LS 1]

Originally posted by Wolf
LS1, I may not be helping with my remark but the angular velocity is the same if the diff is locked, which means outer wheel will be slipping, relatively to the ground, more than inner wheel (hence grater slip ratio).

Correct. That's what dawned on me as I was typing my previous post.

[LS 1]

Originally posted by Greg Locock
If the inner wheel is lightly loaded it may not have the grip to handle the higher traction torque in addition to the (reduced) lateral force required by the axle's slip angle.

So, is there not a secondary effect where a locked diff can cause the rear end to break away sooner than an open diff? In other words it could give low g understeer and high g, power on, oversteer, as it approaches the breakaway condition?

hmmm . . .

I think this is backwards, partly. The subject is locked axle versus open diff. After the limit of adhesion is reached (a given in a race car) the open diff will transfer more torque to the unloaded wheel, correct? So the open diff should be more destabilizing as it will transfer torque to a contact patch that already is being overwhelmed.

At least, that's part of the train of thought I've been trying to pursue.

And if the transfer becomes great enough, we end up in a situation where there isn't enough torque at the outer wheel to maintain speed and we end up with "engine breaking" (perhaps better called "powertrain braking" so as to avoid stepping on certain engineering toes) at the outer wheel.

I suppose this all can be summarized with "an open diff is more likely to cause power oversteer"?

[Wolf]

Greg, it's something like 'throttle steering', right? The way I figure it, increasing longitudinal force (more throttle) will result in biger slip angle with same lateral force. Sort of like rear wheel-steering (the effect of wheels pointing more outward being 'amplified' by the fact it's the rear, not front wheels)... Or do those effects, as I expect, act independantly?

[Wolf]

LS1 but as the power to the outer wheel diminishes, so will slip angle, and oversteer will initially be reduced. When/if inner wheel lifts, there will be sudden loss of power on outer wheel, as if clutch was dropped, which in RWD cars usualy results in sudden oversteer. The unsettling thing is that IIANM car starts to 'straighten out' a bit and then suddenly breaks away.

Locked diff will transfer all the power to outer wheel, once the inner is in the air, which will just continue oversteering tendency, as far as slip angle is concerned. I don't think it will have any unsettling effect.

But take my words with a grain of salt- I'm in no way knowledgeable, let alone expert like Greg.

[Greg Locock]

Whoa there, I don't use traction in any of my analysis, so I'm just feeling my way.

[phantom II]

This is a little exercise I performed on a public road where I accidentally discovered the importance of a correctly functioning differential.
This a satellite view of my favorite corner.



The instruments were a tach,a speedo and a G meter. I live about 20 miles north of here and blast around it often in what ever vehicle I'm driving.
My roadster and my C6 Z06 are very different animals around here. Bare in mind that I have had no driving lessons from Fatboy and I can't remember a thing about what I learned about engineering at college way back in the 60s..
The Roadster has at least 405 hp and weighs 1800lbs with 225 and 255 BFG ZF R/T tires.
It all started when my diff began to open due to wear and lubrication brake down as I was blasting around the turn in my usual mature manor.
I enter the corner at 140 mph.(A) (45 mph speed limit just before the bridge.)
Visibility is good and it is easy to clear the road ahead. Conditions: summer in south Florida usually late evening.
The stock 04 BMW M3 differential is a Salisbury type whose clutches have a short life with this much torque. The diff has infinite amount of adjustments from 6 to 2 clutches each side and many different ratios,springs and ramps. I have 3:71 gears and some very clever people in Stuart who race BMs have rebuilt this diff on numerous occasions for me.



The stock diff is almost locked because the BMW is top heavy. Before the diff opened I lost 5 mph into the turn © after entry and could not recover the speed till (G) without the back of the car stepping out into a serious drift with no torque going to the inside wheel. (Once known as a power slide).
When the diff opened automatically, I was able to retain my entry speed and accelerate earlier at (F) and gained 300rpm by (G). The failure was gradual and the variation of the torque split was perfect for this turn. I imagine this is how a fully active electronic diff works.
The correct line thru the turn takes a while to learn because as you can see it is not a constant radius and when I encounter Vipers and Porsche, I take them on the outside like they are standing still.
The only point I touch the apex is below the letter R in the word acceleration. From (F) as I get on it, I try to drive the largest radius possible which takes me over the chevron(H) to position(J). The G meter shows 1.1 with a spike of 1.3 at the chevron marking on both cars.
I spent a lot of time rebuilding the diff to try and match the slip in the failing diff by trial end error.
I came very close to the condition I experienced initially but in so doing created too much slip in the clutches of the diff with the result of it wearing prematurely and thereby opening it in less than 3 or 4 events.
Now when I floor it, it just smokes the inside tire when I least expect it. Bummer.
You can prolong the life of the clutches a little by changing the lubricant and additive everytime the inside wheel lets go. It is due for a rebuild so I will take photos of the process.
The Corvette is much more of a handful at 3100lbs, but with ABS, I can leave the braking really late with a shallower entry and letting it find it's way to the outside of the turn.
I use 5th all the time in the roadster and 4th in the Vette with an upshift to 5th at (J).
At the top left are the two different speeds of the cars. Sometimes there are cops over the bridge but I check that out going east before I go under the bridge. The speeds thru the turn are almost the same for both cars I think. The Vette has TC, and 325 sticky GY F1 non EMT tires so they seem pretty even in this particular turn.

Check this out. http://www.autozine....wd_2.htm#Torsen

[Fat Boy]

A. Dipshit, don't drive 140 on public roads. It's ****ing stupid. You are a never-was-been old man wannabe racecar driver that has lost whatever reactions you ever had. I couldn't give a **** what do to yourself or your Propster, but other people are on these roads and it's just a matter of time before you kill one of them.

B. I saved the picture of your favorite turn which you were kind enough to leave the coordinates. Run your ****ing mouth to me again and I'll call up your local police which I'm sure will be very interested in your driving escapades. No doubt your homebuilt piece of **** sticks out like a sore thumb, so I'm sure they'll know you.

[Canuck]

Wow

[Powersteer]

A lock diff would certainly give away at least one tyre when the car yaw's. When understeer is cured its oversteer time. I am pretty sure this is case when on the limit, no in betweens. The benefit should be how cars are designed at that time and the driving style for it. Short, messy and hard on the brakes fightng understeer going in a corner and smooth power steering the rear wheels (hence my nick) exiting. Slow in fast out, slide style, heroic.

With an LSD, setting it up nicely against longitudinal wheel forces might keep lateral grip as good as an open diff so long as no wheel is slipping but what remains is only torque. Torque that is not enough to break traction but cause some understeer. With modern race cars having very low CoG the inner wheel plays a strong part in lateral traction and probably some form of play between the two wheels should help corner speed in total. How low the center of gravity plays a pivotal part in this form of set up.

[phantom II]

So right.

Originally posted by Powersteer
How low the center of gravity plays a pivotal part in this form of set up.

[phantom II]

You aught to stand on that bridge on a certain night of the week and watch a bunch of superbikes
going much faster than that. The 140 mph parts are very rare occurrences and only when there is absolutley no traffic, well save for other dipshits in Vipers and things. The 110 part is common, well at least for me it is. Plenty cars and bikes come off here(D) but rarley is the public in any danger. Stupid? Stupid is as stupid does.
Thanks for your concern for public safety.
For your information, many highways and by-ways thru out the US and the world have look out sentries to accommodate this kind of activity. I know of 4 areas in SE Florida where weekly 200mph events take place on public roads.
My wife and my son would agree with you though. She is making me sell my aerobatic airplane. It is time.
She could help you with your poor self image.

Originally posted by Fat Boy
A. Dipshit, don't drive 140 on public roads. It's ****ing stupid. You are a never-was-been old man wannabe racecar driver that has lost whatever reactions you ever had. I couldn't give a **** what do to yourself or your Propster, but other people are on these roads and it's just a matter of time before you kill one of them.

B. I saved the picture of your favorite turn which you were kind enough to leave the coordinates. Run your ****ing mouth to me again and I'll call up your local police which I'm sure will be very interested in your driving escapades. No doubt your homebuilt piece of **** sticks out like a sore thumb, so I'm sure they'll know you.

[Fat Boy]

I apologize to the rest of you for losing my temper last night to this ****-wit.

[Fat Boy]

As far as differentials go, here's a little more info.

Whether or not the diff is open or locked, the opportunity to lose rear traction is always a possibility as long as the power is available.

With an open diff (or a LSD which isn't locking enough) you'll get inside rear wheelspin. This is generally pretty benign. The car will just feel like something is holding you back so you can't get a good exit. You should be able to feel the inside rear spinning and hear the engine revs come up without any acceleration to correspond. You'll often get a slower, long duration loose condition in this situation.

When the diff is locked (or the LSD is locked enough) you get wheelspin from both rear tires at the same time. This will sidestep the rear of the car because you're giving up pretty much all of your rear lateral grip. Assuming your driving is anything reasonable, it isn't quite as abrupt or alarming as it might sound. After both wheels are spinning, you just start to crab out to the exit of the corner. I suppose if you had both tire spinning and you went full throttle you might spin the car or have a big slide, but generally this isn't how people drive.

If you have data on your car take a look at all 4 wheelspeeds. On corner entry-mid you will see that the rear wheelspeeds seperate due to the different path they are taking. The inside will be travelling slower than the outside. If the car is balanced in that portion of the corner, but then changes as you go to throttle then take note of the rear speed split percentage. If there was a 5% speed difference side to side before going to throttle and there is a 1% difference after you go to throttle and you've picked up push, then you probably could stand to loosen the diff. If it's a 10% speed split % after going to throttle and the car is loose, then you probably need to tighten the diff. If the speed difference is still around 5%, but the handling has gone to hell, look somewhere else.

If you have trouble burning up diff plates in a street car style diff, then take a sanding disk and sand the friction material off the plates until you're left with steel plates. Rebuild the diff with the steel plates. Because you've taken the friction material off, you'll probably need to put an extra plate in the diff to get your stack measurement correct. You might need to have the plates ground to end up with the right stack height. The steel plates will be noisier than with friction material, but they'll have a hell of a lot more bite and wear will go to nothing.

[phantom II]

Hey thanks man. You know your ****. Are we friends again? No matter what you say, I think you are a pretty decent fellah.
What lubricant would you suggest? Wouldn't the steel plates just sieze and lock the diff. Won't they remain siezed at very slow speeds.
C4 and C5 Vettes would make these horrible load clunking noises in slow traffic and sharp turns which was cured with new lubricant. Even a hard driven high milleage Vette diff would not let the inside wheel spin.
With a locked diff, you will have 100% torque going to the outside wheel as the inside wheel looses all it's traction.
There is a limit to how much torque can be applied to the inside wheel, but I want that to be as much as possible. I would assume this is the correct desire.
I have my reasons for not going to a bigger tire but what is the trade off for adjusting the diff and going for a bigger contact patch? I would just like to get the diff right and reliable.

Originally posted by Fat Boy
As far as differentials go, here's a little more info.

Whether or not the diff is open or locked, the opportunity to lose rear traction is always a possibility as long as the power is available.

With an open diff (or a LSD which isn't locking enough) you'll get inside rear wheelspin. This is generally pretty benign. The car will just feel like something is holding you back so you can't get a good exit. You should be able to feel the inside rear spinning and hear the engine revs come up without any acceleration to correspond. You'll often get a slower, long duration loose condition in this situation.

When the diff is locked (or the LSD is locked enough) you get wheelspin from both rear tires at the same time. This will sidestep the rear of the car because you're giving up pretty much all of your rear lateral grip. Assuming your driving is anything reasonable, it isn't quite as abrupt or alarming as it might sound. After both wheels are spinning, you just start to crab out to the exit of the corner. I suppose if you had both tire spinning and you went full throttle you might spin the car or have a big slide, but generally this isn't how people drive.

If you have data on your car take a look at all 4 wheelspeeds. On corner entry-mid you will see that the rear wheelspeeds seperate due to the different path they are taking. The inside will be travelling slower than the outside. If the car is balanced in that portion of the corner, but then changes as you go to throttle then take note of the rear speed split percentage. If there was a 5% speed difference side to side before going to throttle and there is a 1% difference after you go to throttle and you've picked up push, then you probably could stand to loosen the diff. If it's a 10% speed split % after going to throttle and the car is loose, then you probably need to tighten the diff. If the speed difference is still around 5%, but the handling has gone to hell, look somewhere else.

If you have trouble burning up diff plates in a street car style diff, then take a sanding disk and sand the friction material off the plates until you're left with steel plates. Rebuild the diff with the steel plates. Because you've taken the friction material off, you'll probably need to put an extra plate in the diff to get your stack measurement correct. You might need to have the plates ground to end up with the right stack height. The steel plates will be noisier than with friction material, but they'll have a hell of a lot more bite and wear will go to nothing.

[Stoatspeed]

Originally posted by phantom II
This a satellite view of my favorite corner.

There is something a little strange about this post ...
Apart from the safety aspects so eloquently mentioned by FatBoy, the thought of pulling 110mph around a freeway on ramp didn't sound quite right ... so I took the trouble to look closely at "Phantom Raceway" aka I-595/US1 connector using Google Maps (which helpfully provides a scale on the map). I accept the scale may be slightly out, but it does cross check with the width of the roadway and the size of the cars visible in the satellite image ... so I'm trusting it so far ...

... as you can see it is not a constant radius ...

Well, not really true ... the ramp is actually very close to a constant radius from about midway between point A and B all the way to point G, at which time it blends into US-1 and there is room to adjust the radius a little. That constant radius scales from the map at somewhere close to 350 or 360 feet, depending which of the lanes you choose.
So we do some math and find that to sustain 110mph in the turn as alleged, the car would be pulling something on the high side of 2.2g ... quite remarkable for a street car on normal highway pavement, with or without a functioning LSD. I'm suspecting that

The instruments were a tach,a speedo and a G meter.

should include the statement "wildly optimistic" in several places ....

I'll accept that a Z-06 MAY be able to achieve close to 1g on a smooth surface, hell, I'll give you 1.1g if you like per the "G meter", but even at this impressive latac, the speedo should have been reading no more than 77mph .....

I know that our dear Captain B is a former Phantom jockey, so I am not about to throw rocks at his ability to control a car going fast, all I am questioning is the physics of the situation. If someone would care to set me straight on any "error", I'll happily retract all comments, but I think this is all rather fishy .... maybe the "LSD" was not only in the rear end of the 'Vette, eh?

Any witnesses to the "110", Captain?

Humbly yours,
Dave

[hydra]

Perhaps the onramp has some pretty substantial banking, and your assumed scale is a little bit off?

[Greg Locock]

Is the g meter compensated for roll? Not a big effect on a Corvette, but it is yet another instrumentation problem.

[imaginesix]

It makes sense in km/h.:

...well, since I'm here now I might as well mouth off...

It's all good and well to use public roads to push the limits of your car, and yourself, within reason. Sure it's illegal, but rules only exist to protect the immature, the ignorant, the unwise. What that means then is that if you are going to break the rules you better damn well know the risks and be prepared to suffer the consequences. If not, you are either immature, ignorant, unwise or some combination thereof.

The risks are many; you may get caught, either by law enforcement or an acquaintance. You significantly increase the risk of damage to property, either public or private. You also significantly increase the risk of physical harm, including death, to yourself and anybody else in the area.

The consequences of these risks are not all yours to accept. How can you possibly be prepared to accept the consequence of costing someone else their property or their life? That is not for you, as a mature, knowledgeable and wise person, to decide. So when you say you "take Vipers and Porsches like they are standing still" I have to conclude that none of the above qualities apply to you. If it weren't for that I would have got a real kick out of your reported adventure.

[phantom II]

Mmmm. I thought that you would delve into this matter. First the bank is about 10'. The numbers are more for a reference than anything else. I am more concerned about the RPM gain over the previous run than anything else. The Roadster has its speedo and tach in the middle of the dash so it is easy to keep and eye on them. The Vette is a handful and I don't pay much attention to the dash except for entry and exit.
I can assure you that the radius changes just like the picture shows. I feel pretty pleased with myself when I keep the steering wheel still from (B) to (F). All the time,I see the inside curb varying in distance from the car during that time.
Also, I'm cranking it in 5th gear in the roadster and 4th gear in the Vette. I go round there at 75 in my truck with a locked diff. I think your numbers are quite wrong. I am there, remember?
I can do 60 in first in the Vette, but I'm in 4th. The engine would let go if I kept it in second.
That is the gear you would have me be in according to your data.
A Kawasaki pulled up next to me at the light on the other side of the bridge one time and the driver asked me how fast I was going. He said that he was doing 105mph in the turn and no one has ever gone faster than him. I could see him in my mirrors and indeed, I was pulling away from him in the turn but he passed me over the bridge like I was going backwards. I watched him try to outbrake me on the inside at the entry and he laid it down like Nicky Hayden. He sure coud ride.
You see some mighty fine riding around that turn and I can assure you, only the Vespas are doing 77.
As an aviator, I always leave myself an out. The run off areas are so vast that danger to non participants are quite remote. Some novices end up in the bushes though. Shaken, not stirred.

Originally posted by Stoatspeed


There is something a little strange about this post ...
Apart from the safety aspects so eloquently mentioned by FatBoy, the thought of pulling 110mph around a freeway on ramp didn't sound quite right ... so I took the trouble to look closely at "Phantom Raceway" aka I-595/US1 connector using Google Maps (which helpfully provides a scale on the map). I accept the scale may be slightly out, but it does cross check with the width of the roadway and the size of the cars visible in the satellite image ... so I'm trusting it so far ...

Well, not really true ... the ramp is actually very close to a constant radius from about midway between point A and B all the way to point G, at which time it blends into US-1 and there is room to adjust the radius a little. That constant radius scales from the map at somewhere close to 350 or 360 feet, depending which of the lanes you choose.
So we do some math and find that to sustain 110mph in the turn as alleged, the car would be pulling something on the high side of 2.2g ... quite remarkable for a street car on normal highway pavement, with or without a functioning LSD. I'm suspecting that should include the statement "wildly optimistic" in several places ....

I'll accept that a Z-06 MAY be able to achieve close to 1g on a smooth surface, hell, I'll give you 1.1g if you like per the "G meter", but even at this impressive latac, the speedo should have been reading no more than 77mph .....

I know that our dear Captain B is a former Phantom jockey, so I am not about to throw rocks at his ability to control a car going fast, all I am questioning is the physics of the situation. If someone would care to set me straight on any "error", I'll happily retract all comments, but I think this is all rather fishy .... maybe the "LSD" was not only in the rear end of the 'Vette, eh?

Any witnesses to the "110", Captain?

Humbly yours,
Dave

[phantom II]

I better not tell you of my midnight ride with Paul Revere, er, my drag with a SLR. I'm ashamed of myself.
The lunatic went thru the intersection at over 130mph. I hit the brakes and made a left turn but he would have gone all the way to 200, I'm sure. It is actually a huge car and very very quite.
He just stomps on it at the light and Ron Dennis does all the rest. It is the only street car that I have not beaten to 130 mph.
There are just a few on and off ramps that I take delight in blasting around. The rest of the time, I travel at the speed of the traffic. There is a place west of Palm Beach where superbikes, F40s, Galardos, etc. gather weekly. It is the most amazing thing to see vehicles passing by at one hundred and plenty. Have you ever driven a fast car in Germany? It is no more dangerous than that on a closed straight road. Cops have known about this place for years, but while they are there, everybody does the speed limit. I remember using my aircraft to facilitate high speed runs on I-75. My brother was receiving information on a hand held tranceiver.
Ever been to Navada and Colorado? What do you think goes on in this country? Sure, some die.

Originally posted by imaginesix
It makes sense in km/h.:

...well, since I'm here now I might as well mouth off...

It's all good and well to use public roads to push the limits of your car, and yourself, within reason. Sure it's illegal, but rules only exist to protect the immature, the ignorant, the unwise. What that means then is that if you are going to break the rules you better damn well know the risks and be prepared to suffer the consequences. If not, you are either immature, ignorant, unwise or some combination thereof.

The risks are many; you may get caught, either by law enforcement or an acquaintance. You significantly increase the risk of damage to property, either public or private. You also significantly increase the risk of physical harm, including death, to yourself and anybody else in the area.

The consequences of these risks are not all yours to accept. How can you possibly be prepared to accept the consequence of costing someone else their property or their life? That is not for you, as a mature, knowledgeable and wise person, to decide. So when you say you "take Vipers and Porsches like they are standing still" I have to conclude that none of the above qualities apply to you. If it weren't for that I would have got a real kick out of your reported adventure.

[Powersteer]

Originally posted by Stoatspeed
"Phantom Raceway"

The only thing I can say about phantom II's adventure is "Harley Davidson And The Marlboro Man". A constant radium bend at 270 degree is your favorite corner? How boring.