SundayDriver

I think that is incorrect, in general. While sometimes opposite things work, that is not the usual change. I wonder if anyone has an original of the cheat sheet and if it shows the same suggestion.

Also, the comment from the book about a suspension with more roll stiffness getting more weight transfer makes no sense to me. Maybe I am missing what he is trying to say, but weight transfer is a function of CG, Wheelbase/track and g forces. The only impact that roll or roll stiffness has is the very slight movement of the CG.

And YES, I know that it is technically correct to say load transfer. However, there are hundreds of articles and books and thousands of people who have standardized on calling it weight transfer. IMO, it is clearer to call it by the accepted term, rather than insist on technical correctness.

SundayDriver

"THE SUSPENSION WITH THE HIGHEST ROLL STIFFNESS WILL RECEIVE THE LARGEST PORTION OF WEIGHT TRANSFER CAUSED BY BODY ROLL. "

Can you explain as it does not make sense to me, as stated above.

THX

winders

I can try.

In that quote, when Fred Puhn says "weight transfer", he is really talking about load transfer.

Keep in mind that vertical load on a tire increases faster than the traction gained form that vertical load. In other words, if you increased load by 35 percent, traction might only increase 3 or 4 percent. So increasing the load transfer at one end decreases the grip on that end.

So, if increasing the roll stiffness of the rear increased the load at the front, stiffening a rear sway bar would increase understeer, not reduce it. Since that is not the case, it is easy to see that what Fred says is correct.

Performance Handling by Don Alexander has this text in it:

We know that weight transfer while cornering reduces traction because of the way tire traction responds to vertical load. So now, if more weight is transferred at the front and less at the rear, the front tires will have slightly less traction and the rears slightly more traction. If we started with a neutral handling balance, the car will now understeer. The opposite applies if we stiffen the rear roll resistance only.

It does not matter if the increase in roll resistance comes from springs or antiroll bars. The effect is the same, although the manner in which weight transfers is different.

This concurs with what Fred Puhn wrote.

Does that help?

Scott

SundayDriver

Just some background - I am very familiar with tire load curves, many details of suspension and aero as well as shock tuning. I am a ME by training and have spent quite a few years tinkering with car setups. I mention this so we can up the tech content of the discussion.

Maybe the quote is out of context? But when I read it, it seems to be trying to say that if I have 2 identical cars, one with greater roll stiffness, that one will see more load transfer. That is not the case as roll stiffness does not impact weight (load) transfer - basic kinematics shows us that (with the exception of very small changes in the location of the CG).

Perhaps the quote is really in the context of each end of the car being a "suspension". I don't have that book, so I can' look it up. But if that is what he meant, I still don't get it. Your statement about one end losing grip with more load is not correct, IMO. The tire load curve shows that the 'grip' goes down with load, but that is not total cornering capacity. That 'grip' is essentially the COF (Coefficient of Friction). So increasing load decreases grip, but that grip is multiplied by normal load and within the ranges we operate in, results in more G capacity.

In other words, doubling the load on a tire does not double the G capacity - but the G capacity does go up.

Land Jet

Sorry, I'm not an engineer. I thought when turning left, load is transferred to the wheels on the right side of the car, thus increasing traction in those wheels, and decreasing traction on the left side. From all I've read, increasing load on a tire increases traction, up to a point. Under braking, load is transferred to the front tires, increasing traction in the front tires and the reason Porsche puts larger calipers on the front of the car, like on a GT3. Have I completely misunderstood this?

Larry Herman

Mark, I took it to mean the suspension stiffness at each end of the car. I have to finish my initial post which will explain my understanding of it (right or wrong), but part of my thinking on the subject is that increasing the roll rate at one end of the car does not reduce the grip at that end like many think, but actually transfers it to the diagonally opposite end, increasing the grip at that end.

Another thought - If stiffening the car reduces grip, why would we ever want to go beyond a stock spring rate?

Veloce Raptor

...

Larry Herman

Cute! What else do you have to add to the topic?

SundayDriver

If it is speaking of each end, then it makes sense. I have been working, in my mind, on trying to really understand why the changes we make have certain impacts. The books either go deep into math that does not help unless you are willing to spend months getting your head around that, or they really come down to "This is the way it works so that, alone, is the explanation". I have yet to find a book that actually explains, in fairly plain terms, what is happening and why. I feel that I am really close to having this 'model' and explanation.

As far as the issue about spring rates, there are some very clear answers to that. When we subject a car to high G's we get a lot of body roll with stock spring rates. That pushes us into a range of suspension travel where things don't tend to work very well. Add to that lowering the car, and we really scres up bump steer, camber gain curves and how the IC (Instantaneous Roll Center) moves around with suspension travel and you get a situation where you really need to keep the suspension operating in a narrow range. You then need stiff springs to limit suspension travel. You lose mechanical grip, but gain better suspension geometry so we are always chasing the compromises in that deal.

In an aero car, you obviously need to carefully control ride height to maximize ground effects. In those cars, spring rates are usually taken up to levels that pretty severely impact mechanical grip.

Veloce Raptor

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J richard

Mark, as you guessed the quote is referring to the stiffness of a corner "suspension" relative the other corners. I think this is being made too complex, in any structure the stiffest member will take the biggest load (VR insert off color retort here...) think about it like this: jack up the car put a concrete block under one tire and foam rubber under the other three, let the car down, the wheel on the block will take just about all the load. Reverse of corner balancing. In practical terms the soft cornea can compress to the point where they will take a larger portion of the load but since this is a dynamic system your more interested in the rate of transfer as opposed to the ultimate static condition...(apologize for the over simplification it's for the non ME/PEs in the mix....)

Larry Herman

One thing that I do not have my hands totally around is exactly how grip is reduced through increased spring rate. If we keep camber optimization out of the equation (which would actually help grip), do you think that it is only because the contact patch loading change is more stable with softer springs. I.E. with stiffer springs the loading changes more significantly as the tire tracks over bumps and undulations.

sig_a

Fred Puhn quote "THE SUSPENSION WITH THE HIGHEST ROLL STIFFNESS WILL RECEIVE THE LARGEST PORTION OF WEIGHT TRANSFER CAUSED BY BODY ROLL. " (I think he means: "The suspension with the highest roll stiffness will SUPPORT the largest portion of weight transfer caused by body roll". If this is right the body at the stiff end of the car will not roll over as much which reduces the lift effect on the diagonal opposite end.)

He IS speaking about front axle vs rear axle. Reading it in contest does help enormously.
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LH says: "but part of my thinking on the subject is that increasing the roll rate at one end of the car does not reduce the grip at that end like many think, but actually transfers it to the diagonally opposite end, increasing the grip at that end." This helps confirm what I understand to be the case.
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I am likely not getting the concept completely but ---

Think of the three wheeled 911 in steady state corner at mid corner where the front inside wheel is off the ground. If you want to correct this severe understeer problem, you can either soften the front suspension or harden the rear suspension. You can do one or the other or a combination of both. But you can't simply soften or harden both ends equally. It is a matter of ratio and proportional balance between both ends where you want the body roll angle to be equal front to back within the turn. (I think)

In this wheel-lift example increasing the stiffness of the rear suspension will reduce the body roll sufficient enough that the front inside wheel will not lift. (diagonal effect)

If you soften the front suspension, the inside front wheel will not lift because the ARB is relaxed and will not have the strength to hold the inside front portion of the chassis and wheel up in the air. It helps me to think of a solid front axle in this example because ARB is designed to restrict the independence of the independent front suspension. Same holds true for the rear independent suspension.

SundayDriver

This goes back to your post about spring rates and bumps/dips. I suggested that he real impact is related to frequency response, not spring rate or travel.

A stiffer spring (which also requires a stiffer overall shock setting to control it) has a higher natural frequency. This combination of frequency and shock tuning means that the tire has less contact patch as it deal with bumps. If a track were perfectly smooth, and the driver were also perfectly smooth, IMO spring rate would make no difference.

SundayDriver

I get that in the context of on end of the suspension. However, I am moving more and more towards an understanding that all these tuning aspects have far more to do with Dynamic Cross Weight (Load) than the change in grip with the change in load on a given tire. One of the things that directs me there is that you can feel the effects of suspension tuning at very low g loads where there is very little weight transfer - certainly far less weight transfer than would have a noticeable impact on tire g capacity.

In other words, I am begging to understand that tuning (springs, shocks, ARBs, Rake, etc) is really creating cross weight and that is what causes the understeer or oversteer. I believe I can explain WHY certain changes have certain effects based on a Cross Weight concept. Unfortunately, the explanation would require a bunch of drawings, so it is not something I can post.

Here is a challenge -
How can we explain the fact that almost all cars will increase oversteer if we raise the back (more rake)? I think I understand this, but for anyone that thinks they understand what is happening with the car, this is a good test of making sense of it.