Movable rear wheels

[maximilian]

I was wondering if rear wheels that could rotate a small amount and had centering springs so they could follow turns a bit would have much effect on rolling resistance (or tread wear for that matter). Seems pretty dubious.

[theholycow]

You mean like some sort of passive rear-wheel-steering system? As in, the casters on a shopping cart?



I'd like Quadrasteer, not so sure about passive swiveling self-centering rear wheels...

[maximilian]

Yep, passive. It'd need some serious centering springs, unlike a caster. Only be able to turn a very limited amount is my thinking, so it'd only really do much in gradual, high-speed turns. Not for serious things like backing up like the Quadrasteer! I was approaching it from the other direction: here's something that wouldn't be too expensive in theory, but is it good for much? I can't say I've got much feel for it but my gut says "no", but it seemed worth throwing out there for discussion.

[VetteOwner]

yea, too much work for what if any gain. rear wheels on a fwd car have pretty much no wear if thier aligned correctly. usually theres rubber bushings keeping it all in line anyways so it does flex a tad(like very very small)

[Jetta90GL]

I've seen cement trucks with a set up kind of like this on the rear most wheels. Although I think it is mainly for them to distribute weight to comply with road weight limits.
I couldn't find a good pic online of the set up, but I believe it was a truck like this:
http://cgi.ebay.com/Oshkosh-front-di...417155007r8078

[Pete]

Some makers fit "compliance" bushings to the suspension to help the car track better through corners and remain stable under extreme braking BUT the amount of resources needed to make it all happen as it should is huge and probably well beyond the owner's pocket in most cases.

"Compliance" bushings are designed to allow the two components they are fitted to move a given amount under a given set of forces but to remain unmoved at any other time as well as acting like normal isolation bushings for all of the time. Not an easy or cheap engineering challenge to overcome as you can imagine.

The effect on tyre wear is marginal and is a by product of other factors in most cases.

Pete.

[maximilian]

So four wheel steering systems are mostly about low speed mobility and they just toss the high speed stuff in while they're at it? I guess that makes some sense.

[kamesama980]

spring-centered wheels like that (any appreciable movement) will also need some sort of shock absorber to prevent resonant vibration and it'll make it really interesting to drive in a cross wind or hit bumps or drive on non-level surfaces. or turn at high speed

as for tire wear, I'd assume it'll have the same effect as worn out bushings: increased wear from increased flex.

I doubt you'd gain anywhere near a measurable mpg increase from it.

[maximilian]

I'm not sure a worn out bushing is comparable since the motion is presumably a lot less controlled, but the magnitude of gain would seem to be small. I was just wondering why four wheel steering manufacturers bother with the high speed steering at all. PR may well be the reason, of course!

[bobski]

Originally Posted by maximilian

I was just wondering why four wheel steering manufacturers bother with the high speed steering at all. PR may well be the reason, of course!

It has it's uses... Take a look at the Nissan Skyline GTR.

[maximilian]

While looking into it I discovered that passive rear wheel steering is actually in use by a few manufacturers to compensate for oversteer. Handling and stability motivations so no data on tire wear or efficiency impact that I could find though.

[kamesama980]

High speed active rear steering allows the car to better use the available traction from the rear tires.

Originally Posted by maximilian

While looking into it I discovered that passive rear wheel steering is actually in use by a few manufacturers to compensate for oversteer. Handling and stability motivations so no data on tire wear or efficiency impact that I could find though.

how passive? [enter cressida rant] my old Cressida changed the toe several degrees with suspension articulation. not really passive but resultant. basically as the body rolled (as with hard turns) the rear would steer toward the outside of the turn effectively giving a little oversteer without the loss of traction normally associated with it. the result was pretty nice handling for as soft as the springs were (because the softness of the springs allowed the articulation that improved the handling) the downside however was that if you loaded the tits out of the car (or lowered it), both sides toe'd out and wore the tires really bad, really fast.

[maximilian]

Wikipedia actually has a short summary, which puts together all the stuff I was seeing around. Wish I'd just gone there first. No mention of impact other than handling there either.

Typical methods of achieving compliance understeer are to use a Watt's Link on a live rear axle, or the use of toe control bushings on a twist beam suspension. On an independent rear suspension it is normally achieved by changing the rates of the rubber bushings in the suspension.

[maximilian]

I looked up four wheel steering in The Fundamentals of Vehicle Dynamics and only handling and stability concerns are mentioned. Since that book is pretty complete, I suspect this means there is negligible impact on wear and fuel economy.