Roll center migration 1

[Bluefoxy]

Hello,

I was wondering how roll center migration affects suspension performance during roll movement.

I have carried static suspension calculation, and I did not see any releveance of non migration roll center suspension, but my calculation are static not dynamic.

Do someone had experience on no roll center migration suspension and how it performs?

Regards

 

[Bluefoxy]

cibachrome

Thank you for your reply.

But I must admit I did understand a few bit of what you explain.
1. English are not my native tongue, and most of time I have to use google traduction to understand, so I might lost the sense of your meaning.
2. I m not a sim specialist and I do not have access to, I m not able to the maths at this level. But I understand the importance of dynamics, clearly I have picture 1 and picture 2, but I do not know what happen between. I think your are far better than me to analyse what happens between.

My question is still: I find a way to design a twin arm with RC-CG not moving, which many author claim it is a good thing. honestly I have no clue if it is good not or not, what do you think?

If as a specialist of vehicule dynamics,if you think: it is no relevant, just tell me, and explain me as you will explain to an idiot. Example: it has no influence, because when you do dynamics, stiffness and damping are more important.

 

[UBYVISI]

I have been designing quite a few single seater suspensions; It is difficult to reduce the vertical migration with ride height movements and in any case I never saw this bringing or take big advantages.
Instead it is crucial to have a laterally stable RC: drivers will report an unpredicatble car in case of migrating rc. The effect is less important at the rear.
Obviously if the car has a very limited roll movement the problem is damped down.

 

[Bluefoxy]

Hello UBYVISI,

Thank for your experience feedback.

What kind of racing car did you experience this?

Regarding rear caster, like for a formula ford style suspension, it is a way to set the bumpsteer. Increase caster go for toe in in bump, decrease toe out in bump.

Regards

N.Maurel

 

[UBYVISI]

Hello Bluefoxy,

cars I designed were F Ford, F Renault 2.0 and F Renault V6.
At that stage our capability was purely cinematic so no frequency was analyzed but what I stated remains valid.
I can confirm it since later I have been working and still I work with F Renault 3.5 and various F3 cars all of them confirmed those concepts.

Thank you for your point about rear caster, this is something I know and I fully agree; but I think that there is something more and somewhere in this forum I found a post about, I think our frien Cibachrome was the author.

Ciao

UV

 

[NormPeterson]

I suspect that lateral RC migration is really an indication of something as opposed to an end in and of itself. If the RC isn't moving very far, that means the inclinations of the force lines from the contact patches to the FVICs can't be varying much. Which sounds to me that the anti-roll and jacking force effects would also be stable.


Norm

 

[Bluefoxy]

Ubyvisi,

I know well Formula Ford, I worked from 2001 to 2006 at Mygale design office.

Regarding rear caster, from side wiew if you isolate wheel + upright, you put vertical and longitudinal load on the wheel contact.For push rod, Upper uright point will carry longitudinal load, and lower upright point carry vertical and longitudinal load. If you alter the caster you modify the repartition of the longitudinal effort between upper and lower point of the upright.

You can end up to a point of having zero effort on the upper upright point for braking or accelerating but not both.

But as far as far the longitudinal effort is small on the upper point regarding the inner point, this should not be a big thing.

If you have only vertical load on the wheel, and you add caster, you will get longitudinal effort on upper and lower upright point.

If you want, I can put some drawing to explain.

Regards

 

[Bluefoxy]

Hello Norm,

'I suspect that lateral RC migration is really an indication of something as opposed to an end in and of itself. If the RC isn't moving very far, that means the inclinations of the force lines from the contact patches to the FVICs can't be varying much. Which sounds to me that the anti-roll and jacking force effects would also be stable.'

It might be an explaination.

 

[UBYVISI]

Ciao Bluefoxy,

then we were almost competitors.....
Your notations about efforts are well clear.
The doubt I have is due to the fact that the rear susp (it is the usual double whishbone with pushrod) has the toe link welded on the top whishbone.
Assumed what we already said about bumpsteer, car seems to react to rear caster adjustment; more caster (positive as in front, nose up) gives more OS and the other way around.
As you say maybe this forces are deforming this whishbone so as there is some bumpsteer that cannot be seen measuring on a bench.
My kinematic analysis gives no bumpsteer......

UV

 

[cibachrome]

You will need to use software which can analyze member with elastic elements (as in wheel bearings, wheel, wishbone attachment stiffness. Then it will all make sense, If something is flexing, something could break from fatigue at any tome. Usually a 'toe' link mechanism is on swivels or ball joints when it is used as a ride/roll steer setting device. Shouldn't it be linking the upright to the chassis at a point off the control arm/wishbone plane ?

 

[cibachrome]

Like so:

 

[Bluefoxy]

Dear Ubyvisi,

On this kind of suspension, where toe link is weld on the upper wishbone.

From my experience , we put a little bit caster at the rear, may be 1-2°, to get a bit of toe in in bump (+0.2mm for 1 inch bump).

With this small angle, the resultant vertical effort on the upper wishbone is small, so twist (from side wiew) of the upper wishbone might be small.

But if you end with 10° caster to achieve no bumpsteer, then you will get a vertical resultant effort wich could be important and twist more the upper wishbone under load, end up with undesirable toe variation under load.

 

[UBYVISI]

Hello everyone,

reply to Cibachrome: for this thread and the otherone too: first of all thanks a lot. I deal with racecar since a lot of time but when I was a student I did aerodynamics so, even if I studied by myself, several subjects remained a bit "foggy". In fact, I thought that, as you say introducing a moment would also induce something inside the tyre. About your picture, I fully agree: a spearated toe link is the best way to control (eventually also in an active way) the toe movement. Sometimes, tough, I have to work with cars like the one in Bluefoxy pic, and I have no right to modify them by regulations.

reply to Bluefoxy: yes the layout is exactly like that (picture). Everything seems to match, when the caster is introduced the car oversteers because something get deformed (toe link?), in fact this effect lowers a lot when (I checked the set-ups) the trailing arm goes to smaller values. Thanks a lot to you as well.

UV

 

[Bluefoxy]

Hello Norm,

'I suspect that lateral RC migration is really an indication of something as opposed to an end in and of itself. If the RC isn't moving very far, that means the inclinations of the force lines from the contact patches to the FVICs can't be varying much. Which sounds to me that the anti-roll and jacking force effects would also be stable.'

By thinking for 2 days and I have a look on the force based roll center concept, I think you are right.

If the roll center stay in place, between static and roll, the inclinations of the forces line stay the same, this mean during roll same feeling for the driver.

If the roll center move to the outside tire contact patch (let say 2mm upper the tire contact), I think it is the worse case, because under small jounce, the force line on this outside tire can get reverse direction. As it is the loaded tire, it become a very unstable system. This kind of situation can happen if lower and upper arms have quiet the same lenght.

If the roll center move to the inside tire contact patch, The force line of the outside tire will not be angle, the force line inclinaison will not be the same as static, but angle should not alter under small displacement.

So nothing new by designing a twin arm suspension with an invariant position RC/Chassis, but a tips to get a stable suspension if Rc is not to far from the ground.