claykos

Understand. At least for myself I was saying that aero downforce doesn't increase lateral load, if we hold cornering force constant!. So we were just holding different things constant in our minds. ;-)

I wish I knew more about tires, as I do think there is a lot more complicated things going on which may explain sometimes we get unexpected results. IE, sometimes we stiffen up the front and we get more oversteer. Maybe this is because we are working the tire harder and it is getting to a higher (more appropriate) temperature range? Etc....

It's all very interesting stuff.

SundayDriver

See my post, above. I think you should not say that max grip is sitting in the paddock as that is not how we select the tires - we choose them to work better at higher loads. If you assume, in your example, that the balanced load transfer gives max grip, then in the second case we have the wedge effect (which I agree should not be discussed in your article because it peels away another layer that is too much for this article) and that moves the tires away from optimum grip.

SundayDriver

Understood. Yes, strange things happen. Here is one of mine. When I had the Radical with passenger seat, I would lose ~2 sec per lap carrying a passenger. 1300 lb car so that weight was very significant. But at one event, when it was cold, I actually went faster with a passenger. Makes no sense, right? What happened is that the extra weight got enough heat in the tires to make them work.

Very weird things happen with passenger based cars because we are dealing with so many compromises in set up. So things like softening the rear may result in the suspension seeing toe-out bump steer due to more travel and the car move to more oversteer.

No one ever bothered to tell race cars that they are required to be logical.

Larry Herman

I like the example just to talk about balance, but agree with you about the operating range of the tires. Maybe I should say "street tires" as their grip is the least affected by temp and load, or should I bag the whole idea? If the latter, do you have any thoughts on a better illustration?

SundayDriver

I think you could say something like:

We select tires to give us maximum grip at the cornering and braking loads we expect to see. When you get to the balanced example, state that we are getting max total grip at those loads. Then when you go to the second example, we have moved away from the optimum and all the tires lose some grip vs. the first case, but the bigger weight transfer loses the most so we get oversteer.

In that way, I think you can be accurate without getting caught up in too much detail for the audience.

SundayDriver

Scott,

As I have thought about this, I think I didn't do a good job of stating my views. In handling, everything comes down to what happens with the tires. So yes, the loss of grip is based on changing loads on the tires F/R. However, what I was working on was the layer below that - WHY do we get different weight transfer. The concept in my head is that by changing relative roll stiffness F/R, we are creating a dynamic wedge. The effect of that is to create the greater weight transfer F/R.

I was looking for a model where I could use my hands to show the roll and tie that to the handling changes and I think I have it, due to the discussion in this thread.

Now anyone (except Larry) want to opine about why increasing rake adds oversteer? I think I finally understand this one, after many years.

Larry Herman

Aww, Nutz.

claykos

Hmm, rake...

Raise the rear CoG? Raise rear roll center, more weight transfer at rear?

SundayDriver

CG and Roll center both move up. I was looking as some suspension diagrams with CG, IC, RC, etc and it seems that in most (maybe all) cases, the RC moves up further than the CG. That moves them closer together, and seems to be the equivalent of stiffening the rear of the car.

Seems like cars with short A-Arms (or equivalent) will see the RC move much more and will be far more sensitive to changing the ride height.

For those interested, think about what would happen if you ever managed to get the RC above the CG. It has actually been done at least once that I know of. (Hint - think hanging roller coaster.)

winders

SundayDriver,

All setups end up with what you could call "dynamic wedge". There is no car out there that has a perfect 50/50 mass distribution, perfect corner balance, and equal roll couple distribution.

This "dynamic wedge" is the end result of the load transfer, not the other way around.

Scott

sig_a

-------------------

OK. I'll say a vehicle in the process of rolling over at just past 90 degrees roll angle.

claykos

If you get the rc above the cg the car will theoretically roll towards the inside of the corner

sig_a

How bout a dragster doing a wheel stand where front axle RC is well above the CG.

SundayDriver

Yep. Chrysler engineers were working on electric power and used a mini-van. Loaded with batteries mounted on and under the floor board. CG was very low - below RC and that is exactly what the car did, from what I heard. What would get really weird is if the suspension travel moved the RC across the CG.

Both very funny. I'm not sure how to draw the IC, RC lines for those cases but I imagine a 2 year old with crayons could do better than I would.

Jeff Lamb

I just now finished reading this thread after being away from Rennlist for a week (busy with work, family, etc) and all I can say is “Wow!”. There is a very informative discussion going on here. I really appreciate it when knowledgeable folks such as Larry, SundayDriver, Winders, Claykos and several others take the time to share information and opinions in a constructive manner. This discussion certainly has reminded of me how complex this stuff really is. And, it has introduced a concept that I had never really considered which is the fact that when making setup changes, an important consideration is where you are on the tire’s load curve. It sounds like if you are at or above the point where the tire’s grip falls off exponentially faster than the vertical load transfer (the “knee” of the load curve), you will proceed differently than if you were not yet at the “knee” of the load curve.

This discussion points out two more key areas that I would like to learn more about:

• Tire performance and grip – I would like to learn more about load curves and how tire grip really works.

• Roll centers – I would like to learn more about how to properly calculate the location of the front and rear roll centers and how the roll centers affect handling and chassis setup (including the possibility of changing the roll centers to change handling).

If anybody has any recommendations for books or websites that do a good job explaining the above two concepts (or further discussion in this thread to simplify the main aspects), please share. In addition, if you have any guidance regarding whether or not the various race tire manufacturers produce load curves for their tires and where I can find these load curves (or other important technical info), that would also be very helpful.

Regarding the load curves for tires, if there was some way we could install load cells on say the upper spring perch of each corner of our race cars, we could then conduct data acquisition that could really help us understand what is going on during any given corner. We could compare the loads on each corner of the car to the lateral Gs that many of us are already recording and maybe really learn something useful. Therefore, does anyone happen to know if such load cells exist and could be adapted to our race car suspensions??

Thanks,
Jeff