Tyre vertical and lateral stiffness data

[ecclestone]

I have obtained vertical and lateral stiffness data from our "control" tyre supplier. Racing a 1500 kg sedan, 630 hp V8 on road courses,wishbone front suspension,live rear with a spool. Tyres are slicks and data is given at 32 psi.
Vertical Load Verical stiffness
300 kg 297 n/mm
400 305
500 326
600 340
Lateral Load; Vertical stiffness
300kg 318 n/mm
400 326
500 339
600 332
How best may this data be used to optimise chassis settings? Coil overs are used all round,Eibach springs and Ohlins Shocks ,Anti squat is used but main problem is putting power down on turn exit. Any assistance would be welcome

 

[GregLocock]

1 Get rid of your antisquat. It is making the suspension harder under acceleration, for maximum grip you want it softer.

2 reduce rear antiroll bar and/or spring rate

3 increase front antiroll bar and/or spring rate

Your tyre stiffness measurements don't really help much, in my opinion.

What does your camber curve look like?


Cheers

Greg Locock

 

[ecclestone]

Thanks Greg, I was hoping you"d reply on this.Rear squat although minimal could be ,and will be reduced further.Car already runs soft alround and is trimmed with driver adjustable bars Front and rear. Springs are tuned to each track. Stiffer fronts have given excellent turn in, and mid turn, but poor drive after tires reach optimum temp.Later data analysis has now shown up inconsistencies in rear rebound control. I am unable to give specifics on the camber curves , no access to data at this time. Static though is 6 degrees front and 2 degrees rear. I too was dubious of the value of the tyre data,but seeing I had it ,I'd ask to how best use it. The Claude Rouelle advanced Racecar Engineering Seminars apparently deal with this information I have since been told. Thank you for your valued input .

 

[NormPeterson]

You could use the spring rates to determine roll rates for the front and rear tires themselves, and from there estimate front and rear camber gains due to tire deflections. These rates are on the order of 1*/g, so based on your static -2*, at 1g your resulting rear cambers would be something like -1* on the outer rear tire and -3* on the inner. Your front cambers would be affected similarly.

I guess that next you'd look at plots of grip vs camber . . .

Norm

 

[MoreWing]

I'm going to give you some odd advice.

You're running a spool and having problems with putting power down on exit. Because of this particular scenario, I'm going to reccomend reducing the understeer in the car, and changing the driving habits of the guy pedalling the thing.

If everyone you are racing against is also running a spool, it isn't a real problem (if they have diffs, it is a problem, get one). What you have to learn to do is use it's positives and minimize it's negatives.

Because the rear tires are locked together, the car should be very stable on corner entry. You are going to want your driver to really optimize what the car can do there. He has to really load the nose of the car with the brakes and do everything possible to turn the car early in the corner. This will mean a quicker than average steering input and a relatively late turn-in. It will also mean getting off the brake when the car has a lot of speed and letting the tire scrub slow/turn the car.

With partial throttle when trying to roll speed across the center of the corner, the car is going to understeer. You will not be able to cure this. What you need to do is limit the time where it is even a question of having to do that.

When the car is pushing mid-corner and the guy is trying to get on the gas he will add more and more steering. At somepoint, the rear will break free and there will be a big snap loose and the driver will have to unwind all that lock and try to catch the slide. If the guy will drive the car very deep into the corner and then rotate it in a very short portion of track, then he will be relatively straight when getting on the throttle and the 'push to snap-loose/wheelspin' that you had will go away.

The trick is getting a car that will allow your driver to do what we want him to do. If the bigger front springs help turn-in, then have that. If increasing the rear anti-roll bar unweights the inside rear and allows that rotation I'm talking about, go for it. Since you have good shocks, I would increase front rebound a little and reduce rear rebound. You could also soften front bump and stiffen rear bump, but just play with the rebound initially. I'm not a fan of the anti-squat, but if you had a bit of anti-lift in the rear to keep the rear stable on entry, that would probably be good.

Let us know how it goes.

 

[BillyShope]

Greg, I'm aware this isn't a dragstrip car, but I find it difficult to believe that recommendations would be 180 degrees out for the two (dragstrip and road race)!!

For a dragstrip car, you would want to avoid squat or rise and the resultant transient loadings. It's difficult to get the maximum thrust when the rear end is bobbing up and down.

The maximum performance of a tire pair, whether in cornering or in traction, is achieved when they are loaded equally. As you're well aware, the driveshaft torque reaction is taken at the engine/trans mounts, where it is distributed to the front and rear suspensions in proportion to relative roll stiffness. That which finds its way back to the rear acts to cancel the driveshaft torque and equalize rear tire loading. For a dragstrip car, therefore, it is recommended that the ratio of rear roll stiffness to front roll stiffness be INCREASED, not decreased.

(Many US dragracers realize the advantage of high rate rear anti-sway bars on their production based cars, but only a few realize the significance of the RATIO of rear to front roll stiffness. The long wheelbase of the suspension-less top fuel dragster tends to accomplish the same thing. While the long wheelbase does, of course, contribute to high speed stability, the more important effect is to "decouple" the front and tend to equalize rear tire loading.)

 

[shanba]

When cornering, the outside tires, front and rear, are at or near their limit. If the car is perfectly balanced with no throttle applied, both front and rear are at their limit. Any addition of throttle, and hence forward thrust will cause the rear tire loading to go beyond its capability and cause that end of the car to lose grip. With a rear drive car, you want the balance to be slightly toward understeer with no throttle so the rear tires have some "room" to allow throttle application while still under lateral loading. This desired balance is the reason to move the roll couple distribution forward. Also the same reason anti-squat is bad.

Your drag racing discussion is interesting. In a sedan with the engine up front, the balance of front to rear roll stiffness is important. In a dragster, the engine is just ahead of the rear axle, and a long "springy" frame between it and the front axle. Do they tune the frame stiffness to help launch the car? Of course once the weight is transferred to the rear axle, the front tires are off the ground and its is irrelevant at that point. I imagine this is important to launch the car straight-ahead.

 

[BillyShope]

I lost my internet connection and don't think my earlier post "made it." I'll try to repeat it. If both posts show up, forgive me.

Yes, I fully understand the reasoning behind Greg's post. I was merely commenting, in light of this glaring example, of what can happen to our reasoned responses as we remove our "road racing hat" and replace it with our "dragracing hat." And, as the requirements of one form of racing approach those of the other, we must be extremely cautious of the compromises which might be necessary.

I believe the design of the top fuel dragster chassis to be serendipitous. I think it entirely possible that better results could be achieved with a much shorter wheelbase and, with the weight saved, the incorporation of an asymmetric suspension to provide equal rear tire loading for any driveshaft torque.

While there are some curious slipjoints and such in the frames of some of the dragsters, I really doubt if a whole lot of tuning is done. I could be wrong, though. I've suggested constraint testing, using a come-along and a stout post, but I don't know if anyone is doing it. With such a test setup, it would be possible, for instance, to adjust the front axle mounting to force the fronts to lift at the same instant. This is not a perfect solution, however, as a rear tire loading "hiccup" would still exist as the fronts lift. But, such a test setup would at least characterize the problem and provide better direction for possible future improvements.

 

[GregLocock]

Billy,

shrugs, Milliken say do what I said if powering out of corners is a problem, and its also what we try and do on our higher powered sedan derivatives, which have V8s, and weigh 1500 kg or so. We use front road springs two or three times stiffer than for the standard car in order to stabilise the platform (or in part, anyway).

I agree that in drag racing things may be different. Powering out of corners is not a standard requirement for a drag racer.





Cheers

Greg Locock

 

[BillyShope]

I used the Millikens' book as text when I taught an autmotive dynamics course, so, again, I know from whence you're coming, Greg. Doug tells me there won't be another edition, but I'd certainly like the opportunity to contribute to chapters on oval and drag racing if that situation changes! Neither subject is adequately covered in RCVD.

Yes, the drag racing situation is more than a little different. In oval and road racing, the severe transverse load variations encountered cause those which occur from driveshaft torque to appear almost trivial. Still, it must be remembered that, in what might have been the last serious effort to use a beam rear axle in a sports racer by a major manufacturer, Jaguar used an asymmetric rear suspension, in the early C-Types, to control the effect of driveshaft torque.

Now that I think of it, Jaguar's solution really covers all the bases. With an asymmetric link arrangement, driveshaft torque cancelation is achieved without consideration of roll stiffness, which is a much "cleaner" answer.

 

[shanba]

I have heard of NASCAR types changing pinion angle of the beam axle to tune drive torque load transfer off the corners.

 

[BillyShope]

Shanba, if it were possible for the pinion shaft to be vertical, pointing downward, it's obvious that driveshaft torque unloading the right rear would no longer be a concern. Instead, the right rear would be crammed up under the car by driveshaft torque. So, there exists a pinion shaft angle, somewhere between that normally encountered and the absurdity just presented, at which rear tire loading would be equal on acceleration.

Having gone through all that nonsense, I would say that, yes, I've heard the same stories, but I haven't talked to anyone who's actually doing it. In the first place, nobody wants to increase the frequency of joint failures (due to excessive misalignment) when there are other tricks they can use.