I recently read Fred Puhn's book called "How to Make Your Car Handle" and while I learned a ton it still left me a little confused.

First of all he talks about how the more vertical force there is on a tire the more slip angle less grip there will be. Since the slip angle increases exponentially with vertical force when you limit body roll (with stiffer springs & sway bars) the more grip you gain by making the average slip angle lower. With more roll stiffness the amount of grip gained by less weight on the inside tire is much less than the grip lost by more weight on the outside tire. This is true, right?

If so, that means that the stiffer the springs you have on a given end the more grip you will have at that end, right? How then does stiffening the rear reduce understeer? Shouldnt it induce understeer because the rear would have a less average slip angle than before thereby having more grip? Am I missing something? Does the rear stiffness have a large effect on the front weight transfer? Would the weight say transfer to the inside front tire if the rear was stiffened? This is the only scenerio I can come up with. Where am I going wrong?

I searched too. Any help would be appreciated. Thanks.

Ben

 

The spring rate thing kinda bogles me as well, but I can understand that if a anti-roll bar picks up the inside rear, all the weight on the inside is supported by the inside front and all the weight for the rear is on the outside. So a high slip angle in the rear, and more overall grip in front.... I guess

 

search for "suspension" and look for the archived posts too. Look for archived topics created by RR98ITR - this topic has been discussed ad nauseum and scott posted some rather valuable info regarding weight transfer and suspension theory. Set aside a few hours for this.

 

just a few....how about quite a few!

 

to answer your question, as weight is transferred from one wheel to another, the wheel that is loosing weight looses grip at a faster rate then the wheel that is gaining weight. since more weight is transferred off the inside tire on the end that is stiffest, that end looses the most grip.

nate-who thinks that sometimes the "search the archives" response isn't very helpful with such a pointed question. if the question had been "how does weight transfer effect vehicle dynamics?" then searching the archives for scott's posts will be better for all.

 

Ryan thinks that doing a little research can be more fulfilling when you find the answers you seek. This topic has been discussed ad nauseum, and scott's posts talk directly about weight transfer and how it affects handling and could not be a better resource from which to start learning from. Its not like i said "go buy a phucking book noob!"

 

It depends on how you define "grip"-- If you increase vertical load on the tire, the tire's traction circle increases in radius.. so if "grip" is available traction, then no. However, the increased radius of the traction circle doesn't keep up with the increased vertical load. So if "grip" is available traction/vertical load then, I guess you could say you lose grip.. but this is not a common definition, I think.

2xVertical Load !-> 2xTraction Circle Radius (i.e. does not imply)

That sentence is not grammatical.. sorry for being a weeny, but it's tough to understand what you're trying to say. I'll guess that you meant to say "Since the slip angle increases exponentially with vertical force, you gain grip by reducing the average slip angle when you limit body roll with stiffer springs & sway bars."

Limiting body roll by adding roll stiffness does not change total weight transfer. If you are talking about reducing average slip angle (average of what? all four tires?), could you be thinking about lowering the CG, which would not only result in reduced body roll but also reduced total weight transfer?

Those two sentences form a non sequitor. Please explain how you get from the first statement (lose more grip than you gain on the stiffer end), to the second one (stiffer end has more grip). It seems like you are swapping definitions of "grip" mid-stream.

edit: clarification


[Modified by MechE00, 5:48 AM 1/21/2003]

 

body roll is caused by the cornering force acting on the cg through the roll center. so technically, lowering the cg will not reduce body roll unless the distance between the cg and roll center were reduce as the vehicle was lowered. since this is generally not the case, lowering the car actually increases body roll, though it does significantly reduce total weight transfer. however, since we are discussing weight transfer we aren't concerned with this yet.

having participated and read most of the previous suspension posts i can vouch that this effect of weight transfer wasn't addressed. many other aspects of vehicle dynamics were, but the reason the stiff end looses grip was not approached. since i've had a difficult time with all the different effects of weight transfer i can understand the difficulty in understanding this rather fundamental aspect of suspension tuning. without understanding this one concept it is nearly impossible to learn the next concepts that build on this one. since most of the people in "the know" take their understanding of this concept for granted it is glossed over. i could be wrong, but that is my take on things. i know that if i had been told very early on about this simple truth my learning would have happened at a much quicker pace.

nate

 

Thanks Nate, I was a bit sloppy in my terminology.. I was actually thinking of lowering CG by things like removing sunroof and such (i.e. not changing roll center) when I said that, but I see now that is not the most likely interpretation of what I said. Honestly, I don't have enough information about the roll centers/suspension geometries of Hondas to talk in depth about them.

Technical nitpick: Wouldn't a better description be "body roll is caused by the cornering force acting on the CG around the roll center".. a force "through" a roll center shouldn't produce roll.. at least in general free body diagram terms. Maybe I'm ignorant of a specific terminology used for suspension dynamics. /Technical nitpick

 

what goes around comes around i guess. technical nitpick noted. i should read my own writing as closely as i read others i guess.

nate

 

I have a relatively simplistic question compared to all of this..but:
How does side-to-side force(threshold cornering) cause more tire wear than threshold braking or threshold accel. (not going to happen in my honda)? I know cornering on a tires limit causes more wear than threshold braking, but im not sure why.

 

Umm.. Assuming the statement is true, then I would guess the root cause is related to:
a)rolling over onto the sidewall which is not designed to take wear
or
b)if you picture the contact patch of a typical tire, it is wider than it is long.. Braking and acceleration forces are distributed across the width of the patch, which means less concentrated and thus slower wear. Cornering forces are going to be concentrated along the outside edge of the length of the patch, which results in more concentration of wear.
e.g.
Accel/break:
[pre]
--------
|........|
--------
^^^^^
[/pre]
Cornering:
[pre]
-------- <
|........| <
-------- <
[/pre]
Jiminy christmas that's simplistic.. and very likely inaccurate. You might want start another thread on it.. my mind is not in a rigorous tire physics mode right now.. another thread might attract those that can give a technically accurate explanation.

In short, I can't think of an answer that's both simple and fully accurate right now.

Nate? Your turn!

edit: fun with ASCII art!


[Modified by MechE00, 9:23 AM 1/21/2003]

 

my guess would be that the time spent at max cornering is much higher then the time spent at max braking.

nate

 

I was thinking of a instantanous sample. i.e. threshold braking for X sec or threshold cornering for X sec.

 

Sorry for the gramatical errors. I meant to say it the same way you did. But limiting body roll does reduce total weight transfer.

A formula out of the book: Total Lateral Weight Transfer = Weight transfer due to body roll + Weight transfer due to roll center heights + Weight transfer due to unsprung weight

There, the "weight transfer due to body roll" is where the spring stiffness comes into play.

Here is his "secret" to chassis tuning: "The suspension with the highest roll stiffness will recieve the largest portion of weight transfer caused by body roll."

So does this mean that by making the rear suspension stiffer you are taking weight off the front in steady state cornering? This sort of makes sense and goes along with what I said earlier about slip angles. The more vertical force (weight) the higher the slip angles so the higher the traction circle and thus less "grip". Am I way off?

The average slip angle is the average of the a pair of tires (like the fronts or the rears).

Maybe this'll clear things up:


Where i said "With more roll stiffness the amount of grip gained by less weight on the inside tire is much less than the grip lost by more weight on the outside tire." i meant "With less roll stiffness the amount of grip gained by less weight on the inside tire is much less than the grip lost by more weight on the outside tire."

Sorry I am unorganized I have been reading that scott guy's posts and have been trying to make sense of everything in my head.

Ben

Thanks solo-x for sticking up for me. I have searched and read many of the posts he has written (well, I'm 1 page through the 5 or 6 page one when he is arguing against wai).


[Modified by Ben Ogle, 2:13 AM 1/22/2003]

 

Re-read them. Then read them again - however if you do not have a fundamental understanding of physics the concepts will be lost on you completely.

The idea is not weight transfer - you cannot change that, it is static because the weight of your car (and its distribution) is. Its the rate of weight transfer, and the idea is to transfer weight away from the unloaded (inside rear) quicker.

 

In steady state cornering is this "rate" still in effect? seems to me that a rate would be effective only during turn in and springs/swaybars effect more than just turn in.

I dont see why you are so unwilling to help. I dont understand everything scott says and I do have a basic understanding of physics. I have searched and read, but I dont have a concrete understanding. Isnt that what this message board is for?

Ben

 

Its not that i am unwilling to help, i just dont have the time to engage in a lengthy discussion involving re-reading scott's old posts and referencing Millikin, but i thought i could point you in the right direction.

With a stiffer rear the inside rear will unload faster. I do agree that scott can be rather cryptic at times however...

 

That was a discussion, not argument ok?
Anyway, from what I understand from the past discussion, roll stiffness does not govern OVERALL weight transfer. OVERALL weight transfer is governed by the car's basic geometry like ride height, track width (lateral) and/or wheel base (longitudinal).

Front/rear stiffness ratio, however, governs how much weight is transferred at EACH END. Since the OVERALL weight transfer is a constant, more weight transferred at one end would cause less weight transferred at the other end --- keeping the OVERALL weight transfer the same.

Did I make things more confusing?

 

Sorry, you are wrong, Front/rear stiffness ratio does not matter, if you have no suspension you still have the same amount of weight transfer! Different wheel base yes, front/rear stiffness ratio no, same amount.




[Modified by DB1-R81, 9:00 PM 1/21/2003]

 

No you didnt make things more confusing, at least I dont think. i'm starting to understand. What you're saying is that you cant transfer more than your car's original weight. All you are doing in stiffening the suspension at a given end is transfering more weight to that end and effectively moving it to the other end. So we are just changing weight transfer proportions. Yes? no?

With a stiffer rear suspension you would have more weight transfered at the rear and less at the front. So the front tires would have less vertical force therefore less slip angle and more grip. right?

Ben

 

Hmmmm... Hershey almond kisses.
Hmmmmm...

 

I am wrong or wai is wrong?

ben

 

It's late, so I'm only going to talk about one thing of this post--

You are applying the wrong significance to that formula. I'm not trying to be condescending or anything, but how much physics education have you had? or how much do you remember? Do you remember formulae for torque around an axis of rotation, force on a spring related to displacement, and the way to solve a simple system for static equilibrium? With these concepts you can show that weight transfer really doesn't depend on the amount of body roll (except in very minor ways or in extreme circumstances). It's the other way around-- weight transfer & roll stiffness determine body roll..

Anyway, I think that "weight transfer due to body roll" is poor wording on the part of the author.

Please consider the following logical inconsistencies in what I percieve to be your position:
a)"weight transfer due to body roll" contributes to total weight transfer (from book)
b) stiffer suspension reduces body roll
c) reduced body roll means reduced total weight transfer (your assumption)
d) stiffer suspension means reduced total weight transfer ( b & c taken together)
e) "The suspension with the highest roll stiffness will recieve the largest portion of weight transfer caused by body roll." (from book)

d) and e) seem to be in direct contradiction to me.. don't they seem that way to you? (If I have misstated your position, please correct me)

Now let's guess that "a)" is poorly worded as I said earlier. Let's try a concept of "weight transfer causes body roll, but body roll causes almost no weight transfer", or "body roll is a symptom, not a significant cause of weight transfer". With this concept in mind, you can see how "weight transfer due to body roll" can be reworded to "weight transfer involved in causing body roll" If you squint your minds eye, you can see how a person could say the one when he really may have meant the other.

Now with body roll being primarily a symptom, rather than an important cause of weight transfer, "c)" drops away and so does "d)".. The contradiction is gone.

As for _why_ suspension roll stiffness can not change the overall weight transfer, I'm going to have to beg out of answering that right now.. I'd like to hear your about your level of physics knowledge before I start an explanation that could be either too technical, not technical enough, appear condescending, or elitist, and generally waste my time without being helpful.

Am I a mathochist? Actually, nevermind.. I haven't done in formulae in this particular post..

 

D'oh! that's what I get for posting slowly...

Well, if you say "front tires would have more balanced vertical force between the inside and outside tire and therefore..." I say we've got a winner!