Rear wheel "anti-Ackermann" like toe-in changes?

[g-force_addict]

Would some rear outer wheel toe-in change only during cornering would be beneficial?

The rationale is the same as anti-Ackermann: to compensate for different slip angles for the (relatively) unloaded inner wheel vs the loaded outer wheel.

This could be either passive via some bushings flexing only for the outer wheel, or active via some steering linkages.

[raymondu999]

To my knowledge a static toe-in is used at the rear generally anyways, to increase rear stability.

I'm not too sure on how you're proposing though. Why would you want more rear toe in exactly?

[Jersey Tom]

Sure. But why not just do it through static toe or bump steer?

[Tim.Wright]

If you have full control over the rear toe angle then of course, you will see some advantages.

Unfortunately, the only way to see that would be with an active system. I say unfortunately because susch a system is illegal in practically every racing series.

Rear steering effect through roll steer and lateral compliance steer has been done on road cars for decades and not always only for performace reasons.

However such characteristics ore often delibrately kept out of racing suspensions, at least in the intial stage. My understanding of this is because you have some time delay associated with these responses which affect the drivers feedback. In a road car the conditions are changing slowly so the compliance and roll steer effects run in phase with the drivers input. But in a racecar, you might have significant phase differences between the driver input and the roll/compliance steer effect starting.

These effects might not chage the ultimate performance capability of the car, but will make it very difficult for a driver to control because the vehicle response is not directly following his input.

[Tim.Wright]

Sure. But why not just do it through static toe or bump steer?

Because racecar

[Greg Locock]

Would some rear outer wheel toe-in change only during cornering would be beneficial?

The rationale is the same as anti-Ackermann: to compensate for different slip angles for the (relatively) unloaded inner wheel vs the loaded outer wheel.

This could be either passive via some bushings flexing only for the outer wheel, or active via some steering linkages.

Standard feature on production cars. it is called compliance steer. If you apply a lateral force at the contact patch the rear wheel will steer in, or out, depending on where its elastic centre is. Ideally you'd apply the force at the correct location on the contact patch, in practice 30mm behind the wheel centre is often used.

At various times various companies have publicised the benefits of passive rear wheel steer, rather ignoring the fact that every car has it like it or not unless somebody accidently sets the bush rates and geometry just so.

Generally the outer wheel toes in , to increase understeer. Don't use too much, it can really upset the feel of the car. This incidentally is one reason why watts links etc are mounted behind the wheel.

[olefud]

Would some rear outer wheel toe-in change only during cornering would be beneficial?

The rationale is the same as anti-Ackermann: to compensate for different slip angles for the (relatively) unloaded inner wheel vs the loaded outer wheel.

This could be either passive via some bushings flexing only for the outer wheel, or active via some steering linkages.

A touch of toe in on the straight is useful in the front in that it produces a steady equilibrium, i.e. the tire with the greater load will have positive weight feed back and shift more weight to that tire when deviating from a straight path. However, with toe out, the more heavily-loaded tire takes a path that shifts weight to the other tire and tends to dart from side to side. Toe in is preferred by drivers since it allows them to correct a steady state rather than a darting condition. Rear toe functions in an opposite manner but tends to be dominated by the front so a bit of toe in is normal for turn stability, i.e. tendency towards understeer. Rear toe out sometimes helps turn in but can be touchy in on-the-edge steady state turning.

Rear Ackerman is a grab bag during turns due to slip angles and under/oversteer. Both wheels are turning about a single turn center –as are the front wheels- with only the outside tire operating at near an optimum slip angle. The actual turn center is usually substantially displaced from the theoretical turn center sans slip angles.

So a bit of static rear toe in for a stable platform for the driver, but I doubt if it compensates for anything.

[autogyro]

I had a 'rolling' through 90 degree rear beam axle on my racing fwd Mini in 1976.
Worked very well.
Under braking rear toe out giving better corner turn in oversteer.
Powering out of corner the axle rotated to negative camber with no toe angle, helped the car straiten up.
Watts linkage with A frame.

[olefud]

I had a 'rolling' through 90 degree rear beam axle on my racing fwd Mini in 1976.
Worked very well.
Under braking rear toe out giving better corner turn in oversteer.
Powering out of corner the axle rotated to negative camber with no toe angle, helped the car straiten up.
Watts linkage with A frame.

In the same era cars with swing axels had a bad rep for vicious oversteer that was attributed to jacking. A fellow driver and a great mechanic raced a Porsche 365, one of the bad actors, with notable success. He confided that the problem wasn’t jacking, at least not initially, but rather the poor control by the swing axle suspension of toe. By maintaining trailing throttle during cornering at all costs, toe in was maintained. But even engine braking would drag the rear wheel into a toe out condition with terminal oversteer.

[Tommy Cookers]

I once tried a racefied roadgoing Triumph TR6 (drastically lowered suspension etc etc) that a dealer found he couldn't sell
it was unstable in a straight line, very scary on the road

I later worked out that the semi-trailing arm rear suspension had toe-out at the natural ride height (because of the lowering)
(when I asked the right people they said this was a common fault, easily cured by shimming the trailing arm mountings)
as manufactured (like the VW) the toe-out only occurs near full travel either way
so I missed a good car

[dynatune]

For sure it is beneficial to have extra toe-in on the rear under lateral force, there is also a term for it: it is called passive rear wheel steer and it increases stability whilst reducing the so called fishtailing effect (yaw gain/ lateral acceleration gain). Imagine a 4WS car where front and rear wheel steer into the same direction causing to move laterally without yaw.
Even though the F1 suspension does not have "classical" bushes as normal road cars but joints, still at the end of the day all materials are flexible and will deform. The deformation of the suspension can work against you or work into your advantage. The work on these deformations is top secret since it divides the good from the less good.

Cheers
dynatune, http://www.dynatune-xl.com

[thisisatest]

subaru imprezas' rear wheels go to toe out in compression, letting the rear end come around a bit in corners. similarly, at rear wheel extension, it goes to toe-in, stabilising the car during straight line braking.

[Tim.Wright]

Thats very unusual if its true. Where have you heard this? How much toe are we talking here?

[autogyro]

The TR6 rear suspension geometry was designed to give rear toe out on corner exit to increase oversteer and tighten up the cornering.
I have a TR6 ready for complete restoration in the workshop with the triple Weber conversion as well as the mechanical injection system.
I have rebuilt all the TR range and the cars are 'built' with shims everywhere.
Triumph played around a lot with rear suspension on their cars at that time.
It was the era when rigid rear axles were being replaced by independent rear ends.
The Spitfire GT6 Herald and Vittese 6 all had weird rear ends that could bite.
If the load came off the back it gave instant rear positive camber.
I spun coming off a hump backed bridge on a British Rally in a GT6 because of it.
The mod was to fit a transverse bar from SPQR I think the company was.

I built a Mini for hot rod racing once that used a rotating beam axle on the rear.
Won a few races with that one.
Under braking the rear suspension opened up rotating the beam giving negative camber.
Coming out of the corner as power was applied the car sat down at the back rotating the axle and converting the negative camber to rear toe out to tighten corner exit.
Worked well and gained a number of oval circuit lap records.
It had a ford engine on an AP Mini automatic gearbox with a clutch instead of a torque converter and a very low diff for five ratios, top speed max 95 mph. Just on 200bhp.
Nothing could touch it on the ovals.

[Tim.Wright]

OK, I was talking more about cars built this century though...