44 replies to this topic

[Toyfan]

Hi!
One dumb question:
If F1 car would have let's say stronger braking force on the right side( front and rear) when right wheels would block would it rotate clockwise or AC?

[BRG]

It should tend to make the car pitch sideways clockwise (ie turning towards the right).

I think.

Or maybe anti-clockwise...

[wegmann]

Please re-state the question. Are you saying if the right side tires locked up (stopped spinning)?

If the left side tires were not being braked at all, it would definitely rotate to the right. However, if the left side tires were being braked near the tire's limit, then that torque should overcome the locked right side tires (because a tire has more grip unlocked) and make the car rotate to the left.

But once again, I'm not sure if that's what you're asking.

[Tomecek]

Originally posted by Toyfan
Hi!
One dumb question:
If F1 car would have let's say stronger braking force on the right side( front and rear) when right wheels would block would it rotate clockwise or AC?

It depends on which side of the car has more grip. I believe the side with not locked tyres has more grip than the side of the cars with locked/already sliding tyres... Means car will begin turn around left side -> anti-clockwise

[random]

Isn't that what McLaren's quickly banned fiddle-brakes were designed to do?

Allow increased rotation in a corner by having a split brake pedal, allowing the driver to brake more on one side than the other.

[Greg Locock]

Although I agree that in practice the car will tend to turn towards the more heavily braked wheel, most of the time, bear in mind that this is not true in at least two circumstances

1) If both sides are heavily braked and the left s at peak braking, and the right is overbraked, then the car will yaw towards the left. This may just be semantics, depending on whether you are talking about slip or braking force

2) It would be possible to arrange the compliance in the suspension so that the wheel toes out under braking . This could cause the vehicle to steer towards the unbraked wheel. No prizes for nominating the vehicle that popularised this approach.

[Toyfan]

left brake
right block

[Toyfan]

I have made this example becouse i wanted to know does ABS shortens the braking distance. I thought that it would be easier to discus it like this. I thought that it will be easier to discuss becouse it would be easier to picture this situation.
But in some books it writes that sliding reqieres more force that roling.

I think that it comes to same thing-ABS ant this rotating. Or I am wrong?

[Pioneer]

ABS is much like traction control. It doesn't provided any additional braking capability... it simply maximises what is available.

[Toyfan]

Bad ABS or TC doesn't give the max possible.
Does ABS shortens braking?
In newspaper wrote:Only wery skilled drivers can out perform ABS. With new higsampling ABS distance difference is minimal.

[BRG]

ABS, at best, can only optimise braking performance. At the end of the day, the physics of braking remain the same whether manually or ABS controlled. Whilst in ideal conditions, a human can sometimes match ABS, it is in emergency situations that you need the maximum braking, and that is when a human is least likely to be at their best.

I have only had one car with ABS and I found that it worked very rarely. But once, it saved me from running into the guy in front in a traffic queue. I was distracted, realised late that he had stopped, and stamped on the brakes. Without ABS, I believe I would have locked up and slid into him. But the ABS avoided wheel locking and stopped me in time. I was pleased about that! I believe ABS should be a compulsory fitting on all cars. however, I know that on gravel or ice/snow it is not effective, so it needs to be switcheable.

[random]

I've a feeling that ABS can be tuned far differently, likely more aggressively for racing applications than it is for street applications. Back when F1 had ABS, it was said that all one had to do was pick a braking marker and stomp on the pedal, always making the corner.

Most people driving road cars would go hate a car that had ABS kicking in on ever hard brake, but for race cars it's a nearly perfect solution. Hitting on every corner perfectly every time, never a lockup. It speaks volumes that the likes of Prost and Senna preferred the ABS system to the normal one. And that men of their level of control could not out-drive such a system. If they couldn't, I doubt anyone could.

[Greg Locock]

I think you are being slightly misled. Racing drivers do not drive at 100% all the time. I don't know how much of the time they do, but with correctly set up brakes the best drivers will outperform any practical ABS system.

But races are lost when the driver makes a mistake, and I suspect that as a race winning strategy the slight advantage of manual braking when driving at 100% is more than compensated for by the idiot proofing offered by ABS the rest of the time.

I wonder if they switched ABS off for qualifying, but switched it on for the race?

[random]

Originally posted by Greg Locock
I think you are being slightly misled. Racing drivers do not drive at 100% all the time. I don't know how much of the time they do, but with correctly set up brakes the best drivers will outperform any practical ABS system.

We've had this discussion many a time. I still don't think a properly designed racing ABS system using the best technology available could be bettered by humans, even for a single lap.

When ABS was first available for road cars there were some studies that were widely reported in many auto magazines . The studies did suggest a good driver could out-brake the system. But this was a measure of first generation road-car ABS, not racing technology. Even in road cars, ABS and the core technologies involved have come a long way since then.

But a racing ABS system, even ten years ago could use far better wheel sensors with much higher sampling rates and thus brake modulation of much higher frequencies than what was available on street cars. Such sysytems could also be tuneable by the driver for each track or even each corner. The result should be braking distances that simply are not humanly possible. Performing a function much like traction control, only for braking.

Anecdotally, I simply can't believe two of the best drivers this world has ever seen would use a system that they could better. If Prost and Senna could have done better, they would have switched the system off in an instant, they didn't. As far as I know, they used it for qualifying as well.

[Ben]

Originally posted by Toyfan
Hi!
One dumb question:
If F1 car would have let's say stronger braking force on the right side( front and rear) when right wheels would block would it rotate clockwise or AC?

My response to this sort of question is; "Go to Google, and type 'free body diagram' and/or 'basic dynamics'"

Clockwise BTW

Ben

[Greg Locock]

Random wrote

"We've had this discussion many a time."

Any particular thread? with 65 pages to choose from it could be a long read!

" I still don't think a properly designed racing ABS system using the best technology available could be bettered by humans, even for a single lap."

I was hoping for an objective test. I've searched all the likely SAE papers, and googled the world, without success. Demonstrably a good driver can out-drive a good production tuned vehicle stability system (X5 on Nurburgring), but I can't find a published graph on ABS vs non-ABS. Incidentally there is a gotcha here - if you switch the ABS in your car off it may not be correctly balanced (via a brake proportioning valve) for manual braking.

"When ABS was first available for road cars there were some studies that were widely reported in many auto magazines . The studies did suggest a good driver could out-brake the system. But this was a measure of first generation road-car ABS, not racing technology."

OK, maybe. Where's the proof for more modern systems?

" Even in road cars, ABS and the core technologies involved have come a long way since then."

I wouldn't mind betting that as general purpose algorithms road cars are ahead of racing cars, to such an extent that the two systems are no longer comparable. This is irrelevant due to your final para.

"But a racing ABS system, even ten years ago could use far better wheel sensors with much higher sampling rates"

Not relevant I think, the performance of a road car ABS is not compromised by the sample rates. At 40 teeth per rev the biggest limitation is the characteristic response time of the tyre, at 50 ms +++. This is the response time for the tread to respond to torques applied at the wheel centre, in steering it is more like 200 ms.

" and thus brake modulation of much higher frequencies than what was available on street cars. Such sysytems could also be tuneable by the driver for each track or even each corner. The result should be braking distances that simply are not humanly possible. "

Yes, I agree with this. Ultimately the driver could (effectively) program a braking sequence for each corner, that he initiates by pressing the pedal. With sufficient fine tuning this open loop system could be perfect. I guess the real question is, does he have time to program that in? Does he have (or need) that much control over the ABS computer? I am very interested in this, since we are working on using an ABS/TC/whatever control strategy in our ADAMS models, but there is a dearth of published data out there.

[random]

Originally posted by Greg Locock

Yes, I agree with this. Ultimately the driver could (effectively) program a braking sequence for each corner, that he initiates by pressing the pedal. With sufficient fine tuning this open loop system could be perfect. I guess the real question is, does he have time to program that in? Does he have (or need) that much control over the ABS computer? I am very interested in this, since we are working on using an ABS/TC/whatever control strategy in our ADAMS models, but there is a dearth of published data out there.

I've been searching as well and haven't found much scientific evidence to either support or dispute my contention that proper ABS outperforms human capability.

But I did find a few tidbits, including a quote from Alain Prost saying that he was in fact able to change the ABS settings from within the cockpit in the 1993 (I think) Williams. So one can suppose that if he could, he likely did change the settings from corner to corner. You see current F1 drivers adjusting brake balance from corner to corner today.

I also found some commentary which suggested that in addition to better braking, the F1 ABS systems of that time period greatly minimized tire wear. Just accomplishing that would in fact result in faster lap times in the later portions of any given stint and make longer stints possible on any set of tires. I have a bunch of Autocourse's from that era, perhaps I'll take a look through those, they're not too technical unfortunately.

So no proof either way, but I still feel in my gut that if the F1 braintrust were allowed to revisit development of ABS in the sport, there would be no human on earth able to better it.

[JwS]

Read Donahue's 'unfair advantage', he tested a very early porsche with ABS and was very impressed by their capability, which was clearly better than he could do himself. He said that he went into a familiar corner and stood on the brakes at his normal marker, and the car came to a complete stop before his turn in point (from memory), eventually he recalibrated himself to use the abilities of the system and went much deeper into the corner
JwS

[LandOfSnow]

A not so nice thing about the ABS: when you lose control of the car and get into a spin, on a non-abs car you stomp on the brake pedal and lock all for wheels and if chose the moment you locked the wheels well the car will stop spinning and slide to a desired direction. Stomp on an abs car and car will keep on spinning uncontrollably... my first spin in an abs car wasn't a nice experience.

[random]

Opposed to that small disadvantage, the advantage of being able to go deeper into corners, steer while full on the brakes, thus saving instances where one may otherwise end up in the barriers. I think I'll take ABS over non any day.

Having used ABS on the track, I'm a true believer. It doesn't activate in every corner, mostly because if I used the brakes that hard in every corner I'd have to replace the pads every day. But in the corners I use it, usually after a short shute, it's very helpful.

[JwS]

I dunno, I have spun my M3 and stomped on the brakes and it locked up all 4. I don't know why it wouldn't, the ABS doesn't know what the vehicle as a whole is doing, it only knows that a tire has decelerated faster than it should be able to, I guess it could have some effect on the final lockup, but I never felt anything unusual. btw, that is one reason that ABS gets a bad rap on the track, they are calibrated for street tires etc, when you increase traction they can't adjust to a higher possible deceleration (and in fact I think they are usually very conservative, in case you put crappy tires on the car they still need to work) (and in the snow/rain etc.) I wonder if anyone has figured out how to adjust the traction limits?
JwS

[gbaker]

Originally posted by Ben


My response to this sort of question is; "Go to Google, and type 'free body diagram' and/or 'basic dynamics'"

Clockwise BTW

Ben

I love a short answer. Yes, CW.

[hchen]

not to rehash everything-

clockwise if the braking force of the locked tires is greater than that of the unlocked tires - otherwise counter-clockwise (to the left) if the braking force on the unlocked tires is greater than that of the locked...

e.g. if your left is at 50% braking and the right is locked - you will spin right...
and if your left is threshold braking and the right is locked - you will spin left...

ABS- if you can threashold brake and listen for proper tire squeal (and know to ease off the brakes if you hear the tires lock) - you can outbrake an ABS system-

you will get overall better car performance w/o ABS if you have proper braking technique -- don't think of braking just to slow down a vehicle's speed -- braking is also for altering the balance of the vehicle. w/ ABS the balance of the car will be off a little and will likely affect your ability to corner optimally.

and - don't stomp on the brakes - squeeze... you can get more braking pressure before lockup than if you stomp. being smooth is not just w/ your hands but w/ your feet as well...

[LandOfSnow]

You're doing 100km/h and get in to a spin, stomp on the brakes and lock the tires... don't the abs wheel speed sensors tell the system the wheels have decelerated 100->0km/h in a fraction of a second? Perhaps the Bmw abs is smart enough to recognise a spin and allows the wheels to lock in a such case, Opel doesn't...

I believe "stomp" is a more realistic term to use while you're in a spin and try to lock all the tires... it often feels like you're in a bit of a hurry to do something about it while spinning...;)

[random]

Having done a lot of time on the wet skid pad, I can assure you that Honda ABS works forwards and backwards. As well as when the car is spinning like a top.

[Greg Locock]

In my high speed (road) driving course we were advised to stomp on ABS brakes (and keep on stomping) in an emergency stop. No finesse.

I don't believe any production car ABS would sense that you were in a spin and allow you to lock all 4 wheels at high speed. The small number of stops affected would not compensate for the extra code and validation work, and the possibility of errors.

[gbaker]

Gentlemen,

The original question was, "If F1 car would have let's say stronger braking force on the right side( front and rear) when right wheels would block would it rotate clockwise or AC?" [Emphasis mine.]

The issue is force. If there is a larger braking force to the right of the center of mass (plan view), the car will rotate clockwise.

We can take the subject of ABS to its limits, but at some point we begin to invoke the effects of stability control technology, and that is a totally different subject.

Everyone is right, and the answer is: Clockwise.

[Toyfan]

Originally posted by gbaker
Gentlemen,

The original question was, "If F1 car would have let's say stronger braking force on the right side( front and rear) when right wheels would block would it rotate clockwise or AC?"

Thank you.

[hchen]

not exactly right gbaker-

wegmann was right that the question posed is not very clear, it depends on how you interpret what he meant by "braking force"- is it the "applied braking force" or the "exerted braking force"?

let's assume the guy stomps on the brakes for a 90% braking force- left tires continue to rotate, right tires lock up-

the car will spin to the left-

when tires lock up - you are only getting about 70-75% of the grip (or deceleration) than had you brought the car to threshold braking... so it's obvious in this case, the left side is decelerating greater than the right; therefore, the car will rotate to the left-

it's the same reason why when you drop your 2 right wheels onto the gravel; you don't turn the wheel to the left to bring the car back on track - lest you wish to spin... if you turn the wheel slightly to the right (yes! the right); the car will naturally pull you back onto the track b/c you have greater grip on the left-

[Toyfan]

Originally posted by hchen


when tires lock up - you are only getting about 70-75% of the grip

Did you read somewhere this or this is your opinion?
Please answer. Becouse that I started this topic.

[hchen]

from professional drivers-

and "going faster" by carl lopez, pg. 30 states when the tire stops rotating, you typically lose 30% or more of your grip-

of course the actual loss of grip will vary due to tire compound; but 25-30% is a pretty "safe" estimate.

i believe other racing books will tell you something similar-

[Greg Locock]

Check figure 2.17 in Milliken

Shows max braking at about 20% slip ratio, falling to 75% of maximum at 100% slip ratio

I'll be fair, I haven't got any real test data, only fits to other people's data. They all agree pretty much, although the fall-off can vary quite a lot.

[gbaker]

Everyone is right.

If the initial braking force is greater on the right side, the rotation will be to the right. If additional force is applied, sufficient to cause lock up, the rotation will be to the left, assuming the left is not also locked up.

[Chickenman]

Originally posted by gbaker
Everyone is right.

If the initial braking force is greater on the right side, the rotation will be to the right. If additional force is applied, sufficient to cause lock up, the rotation will be to the left, assuming the left is not also locked up.

Bingo!! Gbaker got it right. Everything " rotates" ( pun intended ) about the contact patch of the tires. If a tire locks up the whole ball game changes. Coefficient of friction drops between the tire and the pavement, and the car will rotate in the "opposite " direction.

Can vouch from this on a personal experience. Racing in the rain in my C Sedan ( GT5 now. I'm old : ) . Had a sticking right frt caliper that I was unaware of at the time. Every time that I would slow for a corner the car would pull hard to the left. Went a bit deep one time...locked the left front wheel...but the right front wheel kept turning. Car rotated HARD right.... right off the track in fact!!!
Most disconcerting. An excellent lesson in practical vehicle dynamics

BTW, ABS artificially alters the equation, by adjusting braking forces based on the co-efficient of friction between the tires and the road....so do not equate ABS performance with the original question asked. ABS is irrelevant in this particular scenario.....but some very interesting replies with lots of info on Modern ( 1990's ) F1 ABS systems.

Edit: Vehicle suspension geometry also enters into the equation. Negative scrub radius cars will be less affected by " brake pull" than positive scrub radius vehicles. But as the original question proposed greater braking by both front and rear brakes, and was in " theory" only, GBaker's reply would be correct IMHO.

Good thread guys

[hchen]

one more assumption for gbaker to be "right"-

that's assuming when the right side locks up, it's not due to a severe mechanical failure that causes the right side to prematurely lockup...

if you increase the braking pressure but that pressure uses much less than 70% of the grip of the tires - you are still likely to spin to the right... b/c the right tire grip will most likely be greater than the left (barring oil or some other factor that reduces traction).

if you don't understand why - imagine 0 braking pressure, you disengage the engine and you mechanically lock all the tires (w/o brakes) - does this mean you should maintain velocity? or will you actually slow down quite rapidly?

if the tires are locked, you are using about 70-75% of the tires' grip for slowing down- if you're not locked, that braking pressure is the amount of the tires' grip that you are using to slow down.

[Greg Locock]

And you are still assuming that the wheels do not toe in under braking. If this effect was strong enough then you could have a vehicle that steers away from the braked wheel.

[hchen]

i think we have to assume no toe-in or toe-out; and minimal or no camber, and definitely not assymetrically cambered either.

[Greg Locock]

Why would you assume that?

[random]

I wouldn't assume that either.

McLaren and (and possibly Williams) had systems designed to do just this in the 1997 season.

Here's what Grandprix.com has to say about it:

"However, since that time it has transpired that both Williams and McLaren have been experimenting with lateral balance control systems. McLaren have been using a car fitted with an additional pedal to the left of the brake pedal (in the position where the clutch pedal would normally be fitted). It has been suggested that this pedal applies braking to the rear wheels only, and that it may be possible for the driver to select just one rear wheel, either left or right or both together. The driver can use it as a means of steering the car by applying the brake to one or other rear wheel as he enters a corner, either with power applied or not,"

http://www.grandprix...ft/ft00280.html

[hchen]

our initial answers are correct assuming that we aren't doing anything with the toe-ins/toe-outs and camber-

once you introduce these variables, depending on the amount of toe-in/toe-out and camber and the combinations of these adjustments, you're not going to get a simple answer.

otherwise, you might as well throw in elevation changes and embankments into the equation...

the point is to start simple, and after we are in agreement on the basics- then it makes sense to start talking about the nuances.

[mtl'78]

I once had a suspension problem driving on the highway, which resulted in my right front wheel locking completely, and in order to keep the car going forward I had to be at roughly 45 degrees to the left on the steering, so yes, it does rotate the car!

scariest moment in a car!

[Toyfan]

Originally posted by mtl'78
I once had a suspension problem driving on the highway, which resulted in my right front wheel locking completely, and in order to keep the car going forward I had to be at roughly 45 degrees to the left on the steering, so yes, it does rotate the car!

scariest moment in a car!

Thanks for reply. This is the case without braking on other wheels, right?

[mtl'78]

Yes, absolutely, however I should add I think the problem was made worse because my car at the time, a Ford Escort, was FWD, so I was only getting power out of one side as well! That might have played a role in all of it but like I said, I was pretty scared and thankfully I was only a couple hundred meters from an exit so the whole episode lasted just a few seconds.

The thing I remeber most was that it was as if all the power steering was gone, that the whole weight of the car was pulling on the steering wheel.

[crono33]

talking about humans outbraking ABS's, i think that something a ABS can do and a human cant is to keep every single wheel at the (more or less) max possible braking rate.

a human will be forced to compromise in case one tyre has less grip than another. if the less grip wheel will start locking, he will have to choose whether to release brakes a bit to avoid locking, thus losing braking power on the other wheels, or let the wheel lockup losing then some braking efficiency from the locked up wheel.



gmr

[hchen]

ABS system "pumps" the brakes - which switches between braking and not braking...

when you lock up a tire, you momentarily ease on the brakes before squeezing back on for more braking power to maintain maximum braking ability-

i think the best analogy to understanding the point i'm making is -- imagine you have a nail in a piece of wood- takes alot more force to push the nail in than it is to pound it- that's the same as with your tires grip against the pavement- a sudden application of force it is easier to break the cohesion between the tires rubber and the asphalt- a smooth squeeze and you get more braking force before your tire loses that cohesion (grip).

still on another angle, good braking technique is still much better than the ABS system which oscillates the braking power - which upsets the balance of the car, hence not optimal...

if you only think of your brakes to slow down a vehicle - you have less than half the equation to driving a car fast.