Rear wheel alignment

[john.p.clegg]

Brian

"Another confusing issue for which, I guess, I need educating is why toe-in at the rear driving wheels?"

I'm with you on this Friend,just drawn what I think happens re.driven wheels (front) and driving wheels (rear)

The front wheels are being pushed by the chassis and therefore tend to splay out and so should be set to toe-in, whereas, the rear wheels effectively push the chassis and therefore should tend to splay in......logic then says that the rear wheels should be set to toe-out so that they effectively "run true"

John

[Galwaylotus]

Logic also says unless you get rid of all toe-out before and when turning, you'll have oversteer - could make for a wild ride (read that - spin)!

[AussieJohn]

Are we saying that it needs to have toe in on the rear under acceleration and braking; if this is right then maybe its best to have it adjusted to 3/16 per side to make sure [or am I talking cr*p!!] cheers, John.

[Craig Elliott]

John (Clegg) ? I see what you mean, but wouldn?t there also be a whole lot of other things going on esp. when you're cornering, including whether you're on or off power, weight transfer, wheel offset, suspension geometry (and consistency thereof) and amount of deflection and body roll, the tyres etc. In the end it would be a compromise of settings to get the handling you want under all conditions - not just straight line acceleration.

As Galwaylotus suggests, I believe toe-in at the rear tends to generate stability in a straight line and rear toe-out tends to increase instability/turn-in. Intuitively toe out on the outside (loaded) rear tyre in a corner would tend to move the rear of the car out possibly causing surprise oversteer - especially if you have toe-in on one side and toe-out on the other and the car is doing different things depending on which direction your turning. In addition changing from toe-out to toe-in (or vice versa) half way through the corner could be interesting - either in a good or bad way!

The advice I was given, and it seems to hold from other sources, is to use 3/16ths - doesn't doing it per side seems a lot??

Craig

[bcmc33]

John & John,

We seem to be singing from the same hymn sheet, but the confusion remains ? we?re not necessarily using the same tune?

I just did a quick check of my rear wheels to find them both at 2mm (80 thou) toe-in over the full rim diameter (? la Lotus method). Or should this be 4mm (0.160?) by my method?
As Rohan said, the Elan workshop manual specifies the rear as zero to 4.76mm (0.1875?) toe-in. But I can?t see anywhere where it indicates individual wheels or a combined total as per the front wheels. From my viewpoint, I?m taking it to be individual wheels for the rear.

I have always had the picture in my mind that toe-in/out is there to make the tyres run straight ahead under normal driving ? whatever that may mean. Clearly any toe-in/out will be an immense compromise, and settings of the same Elan that is racing on the track one day and going to the supermarket the next day will need to be completely different if the object really is to have the tyres running straight ahead at all times.
So my argument has become very clear ? there?s no correct answer to the question.

However, perhaps someone can answer the specific question: with the Elan running rear tow-in and the tendency for the wheels to want to progressively increase the toe-in with the increase in power being applied ? how does this improve the handling?


All modern vehicles have toe-in/out specified as an angle ? I guess this is to accommodate laser tracking methods. I have no idea when this became the norm, but it does have the affect of making the rim/tyre/pivot point conundrum irrelevant.

So in the long run, I shall stick to the Jackie Stewart stringing method of measurement ? even if I don?t know what it should be.

[AussieJohn]

Hi Brian, I am formulating my thoughts as I write but it seems that if there is toe out on the rear there are handling problems so to keep a bit of toe in under braking then more toe in is going to be required; under brakes the wheels will tend towards toe out as the suspension is dragged backwards, does this make sense? cheers, John.

[rgh0]

Here is the original string method for the 26R - works for my Elan. I can measure to about +/- 0,5 mm accuracy using strings and a vernier caliper which is near enough IMHO and I doubt if commercial wheel alignment places can do it any better.

http://gglotus.org/ggtech/26rmanual/26rmanual.htm

cheers
Rohan

[Foxie]

Hi Brian, this subject is a bit of a minefield but I guess if I can set it up so it drives ok and doesn't scrub the tyres then I think at this point I will call it "quits". Thanks to all for advice, what a great site! cheers, John.

John,

I think you have hit the nail on the head here !

I would say there is no one definite fixed figure for toe-in that suits all cars, what with different suspension play, steering play, hard/soft bushes, different cambers, narrow /wide tyres, tyres types etc. etc.

You start with a datum, (maybe even 4.76mm), and then vary it up and down until you find what feels just right for you/your car.