Rear dynamic toe control

[The Stigg]

Hi guys,

I couldn't find a good post about this subject so I want to give it a try.

I know that toe control in the rear is very important and of course toe stiffness as well.
As there is no absolutely stiff rear suspension possible it lead me to the opinion that I don't want to have any kind of bump- or roll steer in the rear except the elasto-kinematic ones.

But right now I'm struggling around with it.
Porsche uses a system with toe in on the outside rear wheel very successfully for about 20 years now.

Are there some teams that have analyzed the sensitivity/influence of such a system with ontrack testing.

I saw this qoute "non-parallell axes with toe control in rear" on the UWA internet site.

Best Regards
Andy

[The Stigg]

Hi guys,

I couldn't find a good post about this subject so I want to give it a try.

I know that toe control in the rear is very important and of course toe stiffness as well.
As there is no absolutely stiff rear suspension possible it lead me to the opinion that I don't want to have any kind of bump- or roll steer in the rear except the elasto-kinematic ones.

But right now I'm struggling around with it.
Porsche uses a system with toe in on the outside rear wheel very successfully for about 20 years now.

Are there some teams that have analyzed the sensitivity/influence of such a system with ontrack testing.

I saw this qoute "non-parallell axes with toe control in rear" on the UWA internet site.

Best Regards
Andy

[Mike Hart]

You know that you can use the change in toe in bump and roll and the back to increase your grip by reducing the slip angle on the tyre. We've run it for about 4 years now and it really does work. It's not a coincidence that we usually do quite well in the skidpan!

[The Stigg]

Hi Mike,

yes I know. I made plots of the optimal slip angle vs. normal load.
But this is just theory!!!

I think that it is realy possible to increase grip on a quasi static skip-spad turn, but what about driver feedback in the autocross and endurance event.
To all direct kinematic changes in toe will additionally come the delayed elasto-kinematic ones.
So how does it feel!?

[Chris Lane]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Andreas Wagner:
I saw this qoute "non-parallell axes with toe control in rear" on the UWA internet site.
</div></BLOCKQUOTE>

I wouldn't look too far into what UWA do these days. Their suspension systems are quite removed from what the rest of us run.

This might mean one of two things;

1. They run Z-bar rear wishbones, or
2. They have passive rear steering.

Based on my conversations with them and their cars (from memory), I doubt they are running a 4WS system.

Could just be more UWA flowery language... http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[flavorPacket]

UWA runs control arms with different inboard y values (non parallel) and instant centers not at infinity. and they have a toe link (toe control). Don't read more into it.

[Dave K]

I understand the concept of using the 'bump/roll steer' to reduce the slip angle on the laden tire... So if you run something like this, are you concerned about straight-line bump/droop effects? If the wheel toes-in during jounce, does it toe-out in droop? (as in a braking situation, before a corner)

[J.R.]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by flavorPacket:
UWA runs control arms with different inboard y values (non parallel) and instant centers not at infinity. and they have a toe link (toe control). Don't read more into it. </div></BLOCKQUOTE>

You mean FVSA or SVSA? Different 'y' values on the same upper/ lower arm or relative to each other? If they are on the same arm, what is the benefit? Sorry if this has been asked before, but you made me curious with your post!

[flavorPacket]

on the same arm. there might or might not be benefits, depends on what your goals are. but the general idea is that you can control your wheel recession to influence progressive anti effects, among other things.

/threadjack

[RacingManiac]

If you just design your toelink movement to be out of plane with your control arm you'll get roll steer, which way the roll-steer goes depends on how you relate the 2. In the past we've thought about playing with it but since this is a passive system you might introduce adverse effect on 2 wheel bump/droop situation. I think the 2008 car tested and ran with adjustable setup this year, though I am not sure if they found gains for it...

Note that if you don't know how much toe compliance your car has any effect you introduced might be negligible if your compliance is the one doing the driving....

[BillCobb]

Your tire's performance data will tell you whether toe control is important from a max grip viewpoint. I would guess its not because at max force (if you can get and stay there, the wheel can steer several degrees and not make one iota of improvement. (Maybe that should be alpha....)

I'd go with gamma optimization. Camber still has some gain and you should go for the static + dynamic cmaber change settings that put you "there". Peak force camber curves are load dependent, so you need to get a handles on what your wheel loads are real soon.

BTW: ALL produciton cars and truck have measureable toe change with wheel position. That's the way they are designed and perform. That's what K&C machines, do. Nothing special about Porsche. Any and all twist axles have "dynamics toe control", even ones which might have solid bushings. In fact, the amount of toe change in the Porsches is responsible for some of the undesireable antics the cars have when the deer jump out of the Black Forest, und die towen sie trucken ist callen und die airbagen ist offten blowen.

[BillCobb]

While I believe the kwestion was based on the roll steer properties of a car. Toe "control" from FY deflection is also a prominent feature of most vehicles. In the Porsche case, all Porsches in my data viewer have sizable amounts of deflection understeer (toe in). If you study the math on this subject, too much of this (like so many other things in life) gets you in trouble in a hurry.

The problem with ride/roll steer mechanisms is that roll frequency is fixed but yaw and sideslip frequencies are speed^2 dependent. If you have these natural frequencies too close together, then there can be some sign changing above a critical speed, especially on cars that can go really fast and have very high cornering stiffness tires (as in high frequencies). That leads to the many myths about roll understeer is good or roll oversteer is good. What's bad is when they hook up and the system "Q" goes lunar. Snake dance time....

[PSUAlum06]

Bill, any FSAE cars in that data viewer? If not you might be shocked at what some of these cars do when you put them on a K&C rig.