31 replies to this topic

[Boro]

A friend of mine asked me recently what's detrimental to the car's handling if statically, it is already resting on bumpstops.

I can't answer him. I mean, intuitively, I know that when you hit the stops, the wheel rate (or is it spring rate??) goes towards infinity.

But he says the car handles "fine" as it is.

Such an elementary question and I cant find reasons to prove him wrong.

Can you gurus help me out?

thanks

[zac510]

I don't think the rate would change to infinity - it would just change to something extremely impractical like 500kg/mm.

Are you sure it is really on the bumpstops and that your mate knows what a good handling car drives like?

[McGuire]

The car will seem to handle "fine" if the track surface is glass smooth, the driver and tuner have no idea what they are doing and there is no real competition in their class of racing. Everywhere else it will suck.

We all like to quote Colin Chapman to the effect that "any suspension will work if you don't let it." Of course we know he didn't literally mean that; he was being sardonic. When the car is on the bump stops at rest, that means it effectively has no suspension except what is in the bump rubbers and tires. It's a big go kart. A decently sorted suspension will blow it in the weeds every time, on a road or road course.

EDIT: of course, what this setup will do is make the car feel racy... to people who don't know what real race cars feel like.

[Greg Locock]

Just to confuse things we use 'constant contact' jounce bumpers which can be engaged all the time. They have a non linear rate, so near design ride height they hardly make any difference.

[desmo]

...and of course F1 cars generally have their own flavor of what look to be "'constant contact' jounce bumpers" in the form of the elastomer stacks they employ as third springs. These will only minimally affect single wheel rates judging by their linkages' geometries though.

[Boro]

edit:

what exactly are hte purpose of the progressive jounce bumpers then?

[Greg Locock]

Well, a traditional bump stop gives you a fairly sharp transition in wheel rate as it comes into play. By using a progressive rate cnstant contact device we don't have to worry about that transition.

In effect we use the jb as a variable rate spring.

[McGuire]

One big problem with your friend's setup... the car is surely not equally locked down on all four bump stops. Far more likely, each corner is somewhere between just touching the bump stop and fully bottomed.

What this means is that relatively small amounts of bump, pitch or roll will produce hilariously large changes in the wheel rates. The car will be very un-linear and unpredictable in its responses.

One important result of this: the car's roll stiffness distribution will be untunable front to rear: a half-inch change in pitch and everything is different again. (And in left vs. right turns too eh.)

And in a way this issue recalls the recent thread on "zero roll." Naturally, if the suspension has no travel the chassis cannot roll... and to whatever extent the car cannot roll, it will simply lift the inside tire off the pavement. Instead of progressively giving up grip on one end or the other, this car will give it up all at once.

[McGuire]

Two easy-to-spot symptoms: the car jumps around and locks up alternate wheels under braking, and won't put down the power in a consistent fashion either.

[Supercar]

This actually reminds me of a zero-droop suspension, just an inverse of it. The car will have near twice the roll stiffness. When the car rolls it will go a little bit upwards. If the jounce bumpers have a reasonable spring rate the car may actually feel fine in a corner. But it is likely to understeer at corner entry and oversteer at corner exit. Might not be too bad of a setup for a FWD car, but only if tuned properly to a particular single track.

Philip

[RDV]

Much against my natural inclination, will try to write a screed on bump rubbers... but not immediatelly , too much on plate at moment...

Speaking succintly, they are a way of having progressive suspension rates , and very useful for high aero load cars, where you will have a requirement for soft springing on low speed corners and stiff springs for high speeds and straights, to cope with huge aero loads. Greg touched on the reason for them...

...and we will not even go into third dampers and stuff... bump rubbers used to be the be all and end all of ground effect cars, and are still used to good effect in very many classes

as I said , more later (much, much more)

[D-Type]

Am I missing something here? Surely a rubber 'bump stop' is functionally a spring. Running a car on the 'bump stops' is simply running it on (stiff) rubber springs. Isn't it?

[desmo]

Yes.

[Greg Locock]

Normally, very stiff, and very non linear, springs. Still, yes. For instance, I have one here that at 2kN ie typical corner weight for a small car, the rate is 300 N/mm, whereas a roadspring would be around 15 N/mm for a production car.

[NTSOS]

In the Dec issue of Racecar Engineering:

According to engineer Tadge Juechter on the Corvette C6R....a critical improvement is increased suspension travel that keeps the car off its stops. "We've got enough suspension travel now that you don't get into the jounce bumpers even at maximum lateral acceleration." The C6R's rear springs are in fact softer than the 2006 Z06 road car and helps put the power down accelerating out of the corners. There is also an under reported use of a third spring arrangement on the rear sway bar that supports the car under high aerodynamic downforce at speed. This third spring is usually never discussed, but since the 2005 C6R's have been sold to European teams, more information about the sway bar arrangement is probably going to be forth coming in 2006.

John

[NTSOS]

BTW, I thought these were pretty.........Mid Ohio and Le Mans:





John

[Fat Boy]

Read that Racecar Engineering article on the 'Vette's with the necessary portions of salt. It's nothing more than an 'advertorial' to sell the car to prospective European buyers.

Anything technical is disguised enough to be useless, and some of it is straight up lies.

[NTSOS]

Anything technical is disguised enough to be useless, and some of it is straight up lies.

Interesting! What pieces and parts of the article are the straight up lies?

Hmmm, I also wonder why they would have to hype a fairly dominate race car to potential customers?

John

[Fat Boy]

It's not necessarily hype. In fact, some of the aero stuff, they understate. They've told the ACO 'X' about their car. The Racecar Engineering article damn well better match what they've told the ACO.

One of the simple things that isn't true is the dampers. They run Penske's, not Sachs. They have for years and years. No telling what the customer cars show up with in Europe, though.

The bump rubber thing is true, but the flip side is, that they took care of any wheel travel issue years ago. It has nothing to do with the C6. Is this a lie? No, but it's not like they put work into that particular issue on this car, either. If you're familiar with these cars, the whole article is kind of 'bizarro world'.

[NTSOS]

One of the simple things that isn't true is the dampers. They run Penske's, not Sachs.

Ya know, I don't like to be kept waiting and I don't like to be fibbed to!

The bump rubber thing is true, but the flip side is, that they took care of any wheel travel issue years ago. It has nothing to do with the C6.

Did you mean C6R or in fact C6? The suspension reference to the C6 in the article was only that the C6R's rear spring was softer than the Z06 road car's rear spring. The article made it sound like the suspension travel was an improvement over the C5R.

But whatever......generally speaking, how do you all feel about the quality/accuracy of the content in Racecar Engineering.....I just started reading it and also love to look at the pretty ads for interesting pieces and parts.....@$9 a pop.

John

[Fat Boy]

Originally posted by NTSOS

Ya know, I don't like to be kept waiting and I don't like to be fibbed to!

Did you mean C6R or in fact C6? The suspension reference to the C6 in the article was only that the C6R's rear spring was softer than the Z06 road car's rear spring.

I was talking about the C6R. I don't know anything about the street car. Which brings us to the rear spring comment.....I highly doubt it.

I've probably said enough on this topic.

On Racecar Engineering, it's full of advertorials. That's pretty much all it is. That an SAE cars. I enjoy an SAE car the same as the next guy, but every issue?

Race Tech is somewhat better. I think they actually try.

[Supercar]

Originally posted by Fat Boy
Race Tech is somewhat better. I think they actually try.

Race Tech???

F.B, I agree with all your points. It's funny sometimes when you read unknown facts about your own car or your team from the press.

But, geez! Even if Racecar Engineering is off here and there they at least provoke my thinking when I read them. Reading Race Tech, on the other hand, is like listening to TV commentators. Race Tech, IMHO, a near-zero technical content, at least related to chassis and vehicle dynamics that I am interested in. You must have found some other area of interest that they cover better than others. If yes, I would like to know what it is. I do not know if I am subscribed to Race Tech or not at the current time, I have not read more than a two pages worth of information from a year of subscription.

Philip

[NTSOS]

The suspension reference to the C6 in the article was only that the C6R's rear spring was softer than the Z06 road car's rear spring.

I was talking about the C6R. I don't know anything about the street car.

Which brings us to the rear spring comment.....I highly doubt it.

Ah.......ok!

John

[zac510]

Originally posted by Fat Boy


I was talking about the C6R. I don't know anything about the street car. Which brings us to the rear spring comment.....I highly doubt it.

It could be possible- they move the gearbox up the front for the 'R? Weight balance is probably more favourable. Overall chassis would be lighter. But yeah overall while the sentence could be factually true it is quite useless without context

[Greg Locock]

I saw a rather silly blog by some GM honcho, in which he claimed that the latest Corvette was the equal of /any/ Euro car. There was also some funny backchat on how the Chief Engineer (Dave Hill?) had had to buck the system to get the car into production, which made him a great bloke in the eyes of GM management. Or something. My opinion is that yer average senior manager should not be let loose near an unguarded keyboard.

[Fat Boy]

Originally posted by zac510


It could be possible- they move the gearbox up the front for the 'R? Weight balance is probably more favourable. Overall chassis would be lighter. But yeah overall while the sentence could be factually true it is quite useless without context

X-trac gearbox in the back for the later C5R's and C6R's. I don't care how you measure it, spring rate, wheel rate, or ride frequency the racecar is stiffer than the street car. Remember the racecar runs at a 1200Kg minimum weight and when you add 100L of fuel and a driver, they're no flyweight.

[NTSOS]

when you add 100L of fuel and a driver

Hmmmm......I wonder if the statement could have anything to do with the mysterious spring setup on the rollbar that supposedly supports the car under high aerodynamic downforce at speed? Maybe the coil overs are slightly softer than the stiff Z06 mono spring, but combined with the rollbar spring system, the overall rear end spring rate is actually stiffer on the C6R. I wish I could get a photo of the rollbar system.

But I do have a photo of the 229.......I love it!


John

[Supercar]

Originally posted by NTSOS
I wish I could get a photo of the rollbar system.

Here are the photos with this type of transmission installed with bars and 3rd dampers on it. Add a spring and you will get a 3rd spring. The ARB is the vertical post that twists. Scroll half-way down:

http://forums.autosp...ng third damper

Philip

[Supercar]

Originally posted by Fat Boy
Remember the racecar runs at a 1200Kg minimum weight and when you add 100L of fuel and a driver, they're no flyweight.

Then there is all the downforce too! BTW, how much downforce do LMGT cars run, anyone knows, approximately?

I was going to buy their rear wing for my track day car but I got scared. The high-downforce version is almost the size of my trunk! I might need a zero-travel 3rd spring to run it!

Philip

[Fat Boy]

Originally posted by NTSOS


Hmmmm......I wonder if the statement could have anything to do with the mysterious spring setup on the rollbar that supposedly supports the car under high aerodynamic downforce at speed?

No....And that opens up another load of crap. Trust me guys. There's a couple of things I know about. This is one of them. The entire article isn't even worth the paper it's printed on.


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Supercar,

If you think you had push with your previous setup, you try strapping that wing on your GT. I'm talking about a #10 push then! Front tire wear wouldn't be a problem, though.....they wouldn't be on the ground.

[NTSOS]

The entire article isn't even worth the paper it's printed on.

Racecar Engineering, hrumph.........I feel so used......I can't hardly stop crying!

Thanks for the image heads up Supercar!

John

Althea's Garage

[zac510]

Originally posted by Fat Boy


X-trac gearbox in the back for the later C5R's and C6R's. I don't care how you measure it, spring rate, wheel rate, or ride frequency the racecar is stiffer than the street car. Remember the racecar runs at a 1200Kg minimum weight and when you add 100L of fuel and a driver, they're no flyweight.

Yeah and then downforce - I didn't really think of the downforce!