mark kibort

Now, explain this in a little more detail... and relate it to tire sensitivity factor


Really, no idea?.. and assume Cg rising has a very small effect? why would i assum that, its a huge factor.
so, yes, it was a bit hurried in what i wrote, so here it is again with some polish.
Clear as day english... see if you can figure it out and let me know what you think and if you agree.
geez!
>>>>>>>>>>>>>>
Scott, what you said is interesting, but I dont quite understand what you are talking about. I view anti-roll bars, in that they increase spring rate of one side of the car (the outside of the turn wheel), at the cost of compressing the spring of the other side (inside wheel), and lowering that wheel ( putting it into more droop).... think about this for a second. If there was a situation, where the inside wheel is at full droop, hitting the shock extension max, then there would be no downside to the anti sway bar. This would equal the effect of having a higher spring rate on the compressed wheel side. (outside of the turn wheel, where you basically added a spring to the compressing wheel)
Also, by this effect, whether a stiffer spring or anti roll bar, the effect is even MORE weight transfer, Conversely, with more body roll, the CG raises lowering the g loading potential. So, with the anti roll bar, or a stiffer spring, the body doesnt roll as much , there is less CG rise and more g loading with more weight being transferred.


So again, this is a mess of a statement as well...... so you are saying, (and im just reading it back to you ) that without tire load sensitivity, adjusting anti-roll bars WOULD NOT effect "chassis balance"? that is just not true. if it is, i sure would like to hear why!!

for those that dont know what the roll center disti is, here is general description. i dont like this link, as it talks about reducing weight transfer, which is a result of g loading based on weight, CG" and wheel base. bottom line is that if you can lower CG, you can stop even faster and the weight transfer never is reduced.
http://www.turnfast.com/tech_handlin...ling_weightxfr

winders

How is cg change during roll a huge factor in regards to how anti-roll bars are used to adjust chassis balance?

mark kibort

It will reduce lateral acceleration all other things being equal if i understand your question. the question itself is not clear. based on a version of your comment above.. and what i think you are askiing...... anti roll bars can be a great contributor to chassis balance, by not allowing the CG to rise. this happens with our without tire load sensitivity factors.

What is clear is you are saying," without tire load sensitivity, adjusting anti-roll bars WOULD NOT effect "chassis balance"? How is this true?

winders

How do you think adjusting roll couple distribution affects chassis balance?

mark kibort

Are we talking dynamic or static? static, no, dynamic yes.
weight transfer which happens dynamically, will determine chassis balance.
anti roll bars effect lateral and longitudinal chassis balance.

You might be confusing the balance of tire friction saturation front to rear or corner to corner with load sensitivity . I dont know....
However, if you answer my question above, i might be able to understand what you are actually asking.


The question about what you said:
"without tire load sensitivity, adjusting anti-roll bars WOULD NOT effect "chassis balance"?

so I ask, " that is just not true. if it is, i sure would like to hear why!"

winders

How does adjusting ant-roll bars alter chassis balance? What specifically happens when you adjust an anti-roll bar that alters front or rear grip?

mark kibort

dynamically, it effects several factors that determine grip:

1. It allows for softer springs in front, to have a slower weight transfer to the front tires under braking, while increasing the weight transfer rate, side to side in corners.
2. By specifically resisting body roll, the CG is kept lower, and that increases the amount of lateral acceleration possible (and increasing weight transfer) vs a softer setting. (providing the springs themselves are not stiff enough to cause tire grip saturation.)
3. It also can adjust the amount of twist in the chassis based on lateral forces, thus keeping the opposite diagonal wheel from lifting and losing traction. another words, it can be a mechanical chassis corner height balancer.

And sometimes most important, by controlling body roll, it controls keeping the tire flat on the road surface. With reduced body roll, you have less camber reduction and the contact patch of the tire can be preserved. the anti-sway bar adjustment can do this.

do i get a cookie?

winders

Wow, you missed the point of the question.

You take your car to the track and it has more understeer than you want. You crawl under the car and you stiffen the rear anti-roll bar. You change nothing else. You take the car on the track and now your understeer problem is gone. How did stiffening the rear anti-roll bar get rid of that understeer (increase grip at the front/reduce grip at the rear)?

mark kibort

Now you know how it feels!

That is a common situation, many can relate too. understeer means you have exceeded the grip of the front tires. can be due to many factors. could be the rears are under utilized ... you stiffen the rear bar, and the outside tire now, is pushing harder on the ground in a turn, (because you stiffened the rear bar) which also puts more pressure (weight transfer- to the inside front tire, which tends to increase force at the front and lessen force that wasn't being utilized, in the rear.
cg, due to less roll movement, doesn't raise as high, thus increasing the g loading capability.

most cars with more steering angle will push in a middle of a turn. by tigthening the rear bar, you could make that end lose traction at the same time or have it break free first..

winders

You don't get what is happening.

The major factor in how anti-roll bars affect chassis balance is all about roll couple distribution.

Let’s assume for a moment that we have a 2000 lbs race car with 50/50 weight distribution and perfect corner balance. This means each corner supports 500 lbs. The springs at each corner are the same and ant-roll bars are the same size and adjusted the same. The roll couple distribution is 50% and the car handles neutral.

When this car goes around a corner, 50% of the load transfer in a turn is handled by the front of the car and 50% is handled by the rear of the car. If the load transfer were 600 lbs, each end of the car would handle 300 lbs. Both outside tires would have 800 lbs of load while both inside tires would have 200 lbs.

In this scenario, what do we do if want to have the car oversteer a bit? We need to alter the roll couple distribution so the front of the car is handling less of the load transfer. We can do that by softening the front anti-roll bar or by stiffening the rear.

Keep in mind that the maximum amount of load transfer that each end of the car can take on is the same as the weight for that end of the car. Also note that total load across each axle is the same as the static weight on each axle.

Let’s soften the front anti-roll bar so that the roll couple distribution is now 45%. This means that 45% of the load transfer in a turn is handled by the front while 55% is handled by the rear. If the load transfer in a right hand turn were 600 lbs, the front would handle 270 lbs while the rear would handle 330 lbs. So what would load on each corner be? Since it is a right hand turn, the left front would handle 770 lbs, the right front would handle 230 lbs, the left rear would handle 830 lbs and the right rear would handle 170 lbs.

So, why does this alter the balance of the car since each axle is handling the same load? It comes down to the "tire load sensitivity" phenomenon. How?

Even though the inside rear tire is handling less load (170 lbs) and has gained some grip, the outside rear tire is handling more load (830 lbs) and has lost more grip than the inside tire has gained. The front axle, on the other hand, has the outside tire handling less load (770 lbs) while the inside tire is handling more load (230 lbs). Since the load differential between the two front tires is less, the overall grip is higher. So, by altering the roll couple distribution to 45% from 50%, we have increase grip at the front axle and decreased grip at the rear axle.

outline

I can assure you I did. Paragraph 2 was there to illustrate just one of the hurdles to obtaining a highly detailed result.
Rate of deceleration depends on the tyre friction coefficient, and tyre friction coefficient depends on factors that are influenced by this rate of deceleration.

I am not focused on CG rise at all. You are mistaken, though some responsibility lies with me since I obviously failed in my wording.
Go back and re-read what I posted. The 0.5% figure is the calculated difference in deceleration rates between the 3000 lb and 3500 lb cars upon initial brake pedal application. The difference in deceleration rates between the 3000 lb and 3500 lb cars rises to 1% once weight is fully transferred. This difference is less than a meter in terms of distance travelled, and less than 2 hundredths of a second when considered in time.
This difference wouldn't show in empirical measurement or on data traces as its smaller than any error bars that would be present.
These values of 0.5% and 1% weren't changes in CG height between the two cars which I assume is how you read it.

ProCoach

Thank you. That would explain why I have not been able to discern differences in comparisons of copious amounts of data.

winders

Where are you getting your numbers to calculate the effect of the weight on the front tire grip? I would expect the cornering numbers to be similar to the braking numbers. Are you saying that 500 lbs would make a 1% difference? What would 1000 lbs do to your calculations? 2000 lbs?

mark kibort

that's a great explanation... thanks!
Ill have to read it a few more times and look at the values. Good starting values, so its easy to work with.... By the way, what would the numbers look like if you had no tire load sensitivity? That was what my initial comment was pointing at.

I don't think his numbers are taking into account, tire load sensitivity. He says that he is, but my calculations are pretty straight forward and you can see what happens with g loading upon deceleration with two different weighted cars. (all using simple physics, and the curves posted on tire loading sensitivity)

those were my thoughts as well. I think we have to agree on some tire loading sensitivity value.... that's why I posted the two charts and used the one from Carols book. (and extrapolated the values based on the curve shape and our actual starting values)

outline

No problem.

They will be somewhat different for cornering. Track is generally different than wheelbase so weight transfer is different. I don't know what it will be, but I can try and have a look when I get a chance

You keep saying that, but I can assure you they absolutely do. Resolving that part turned a 5 minute problem solving exercise into a 6 hour one to find a form of an answer. Getting a path through the problem was not straight forward. The result obtained is somewhat simplified, but it is very definitely ballpark. The true answer would reveal a difference between the two cars that is lower than I found, but not by too much.
My model also correlates with empirical evidence, so I'm happy with its validity for the purposes of this question.