Understanding Anti Roll Bars

[ajb81]

I am currently struggling through attempting to understand the function of anti roll bars as a part of the entire load transfer picture. I am new to this, so this is a pretty basic question. As I understand it, anti roll bars contribute to the elastic portion of the total lateral load transfer. I know that lateral load transfer is governed by the equation LT= (Accel. X cgh X Weight)/(Track Width). Since anti roll bars do not enter into the LT equation, I would assume that they do not affect total load transfer, but rather affect its distribution (front vs rear, elastic vs geometric). However, Carol Smith says in Tune to Win, “Increasing the stiffness of the anti-roll bar will both decrease roll angle, and increase lateral load transfer” (p. 38). Am I misunderstanding Smith’s statement, or does a stiffer anti roll bar really increase the lateral load transfer? Again, sorry for the basic question!

 

[Cholio]

Carroll Smith meant to say:

"Increasing the stiffness of the anti-roll bar (on one axle) will both decrease (body roll and body relative to that axle) roll angle, and increase lateral load transfer (on that axle)"

 

[NormPeterson]

I'm not convinced that either the proofreading or the editing is as good in the softcover books as it is in hardcover books of more "textbook" nature. If you're going to stay with the softcovers, getting a book by a different author will give you something to do consistency checks with. Fred Puhn's book is pretty good.


Norm

 

[chammyman]

Its late so I'm not quite sure what your actually trying to get an answer to.

If its ' does a stiffer anti roll bar provide increased cornering force over no/more elastic bar?'

then yes a stiffer bar will increase cornering power.

 

[GregLocock]

Rubbish. Bigger bar at the front increases understeer, in most cars. Scarcely an indicator that the sta bar improves 'cornering power' whatever that is.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Greg

I thought that was because the end with the harder bar tends to unload the inside tyre more and thus not share the work so well on that axle.

If the front to rear ratio is retained, flatter means slightly faster, although lower centre of gravity is much more effective than high roll resistance in the suspension.

Have I misunderstood this for all these years. NJot being a suspension guy, it is quite probable I do not understand fully.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[chammyman]

stiffer bar reducing the roll reduces the camber change of the inside wheel so it has more input on lateral grip.

ARB's are only effective when its only 1 side of the car moving like during cornering, the ARB effectively raises the spring rate during cornering.

No roll bar cars often lift a front wheel during cornering, this is when they understeer into the armco, sometimes you'll get a good show and the loaded side will collapse and fold under the car, need to be going pretty good to get that to happen though.

Cornering power, not technically the right term but something 99% of people here in the uk/europe will understand immediately.

Unless all the time spent on the track millions of us imagined getting round faster with less drama.

 

[patprimmer]

chammyman

Greg is right, and he does this for a living and has done so for the likes of Lotus and I think Jaguar.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[chammyman]

hes right for a textbook yes, its true the arb increases the outside tyres slip angle thus understeering but I was assuming we are talking real world and not textbook where the rest of the suspension is ignored.

Anything with independent front suspension will as the body rolls just reduce the inside wheels grip, yes it will increase weight on the outside tyre but theres only so much loading it can take.

An ARB will keep the inside tyre contact patch larger for longer thus improving grip so understeer will happen later.

Of course for a solid axle live or dead and arb will bring understeer about eariler as the wheels attitude to the road will not change significantly.

 

[NormPeterson]

I don't think the conclusion can be as simple as considering only the effect of camber on individual tire grip. Tire lateral grip is nonlinear with respect to both camber and normal load (which includes the load laterally transferred either onto or off of the tire in question). These effects usually act in opposition to one another.

"stiffer bar reducing the roll reduces the camber change of the inside wheel so it has more input on lateral grip"
You may improve the effective grip coefficient by doing this, but using a stiffer bar at that end to make this happen leaves the inside wheel with less normal force to work with. As a silly but not too far-fetched illustration, getting a "mu" of 1.8 for the inside front tire is essentially meaningless as far as vehicle behavior is concerned if the associated vertical force at the contact patch for that tire is only 10 lbs.


I suspect that when the amount of roll resistance being provided by sta-bars gets high enough relative to that being provided by the springs that it gets more difficult to get the damping optimized.


Norm

 

[gt6racer2]

I don't see any error in Shelby's statement. Of course it doesn't go on to mention many secondary effects of the reduced body roll ( normally less camber change, less cg jacking, less roll momentum buildup etc...) or of load transfer increase ( tire overload on outside, loss of contact to ground on inside etc...).

 

[evelrod]

Just to add a bit of confusion---
Keep in mind there are certain circumstances where the addition of a "stiffer" front bar will actually reduce understeer (push) and increase forward bite! Something from the "real world of suspension setup", hey?

Rod

 

[GregLocock]

Oh I agree with that Rod, that's why I qualified my original statement. Typically a bigger bar on the front improves the perception of nimbleness, as it makes the outer tire work harder in the first half a second or so.

But, sta bars work on axles, and by loading the outer tire (and increasing the lateral force it generates), you also unload the inner tire, and decrease the force from that. Overall, you lose lateral force generated by the axle in the steady state. So on the front axle stiffening the sta bar reduces understeer due to this effect.

Admittedly, if your geometry is so terrible that any roll produces too much positive camber then suppressing suspension roll may improve the axle grip. But you'd have to have a fairly mad suspension setup, I'm guessing Mac strut is one possibility, and I suppose wide contact patches would make things worse.

Also by stiffening the suspension in roll you reduce the roll steer/g, which affects understeer .Typically this is the opposite effect to the tire sensitivity to vertical load mentioned in the second para, reducing understeer because normally your front axle is set up so that roll steer increases understeer, toe out in jounce.

One other effect not much discussed is that unless the links are correctly arranged the sta bar actually steers the wheel, changing your roll steer, and hence understeer. This can work for you or against you. If your sta bar link is joined to the axle it exerts a torque around the steering axis, and if it joins to the lower arm it can be made to shove the arm around, which can steer the spindle (the latter is a pretty brutal approach but it does work).

So altogether, understeer and front axle sta bars are linked by:

increased weight transfer ->reduced grip->more u/s
less positive camber change in roll/g->more grip->less u/s
less suspension travel->less roll steer-> less u/s
more sta bar steer->could go either way.

The first mentioned is the textbook answer and also works on most cars. The second is in line with Chapman's observation -any suspension will work if you don't let it move.


Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[zhangleijiujiang]

An ARB stiff enough is more like a solid axle we ever use before

 

[GregLocock]

No it isn't.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.