Rear Alignment: Drivability vs Economy
 

I'm fixing the alignment on the rear of my car... I'm wondering the "if and why" of the OE Toe-in/out in my service manual...

The factory service manual (which doesn't always provide "perfect"/Ideal numbers).... has the rear wheels Toe'd In by 20' (1/3 degree), or a total Toe-In of 40' or 2/3 degree... I'm thinking about the "why"?

I assume there is a stability benefit to having your rear "Toe'd In"?

Would I benefit economically if I were to manually set it to a more neutral "Zero"? My rear end is pretty ehF'd as it is (until I do the alignment), but with a neutral rear end, would I run into drivability issues?

I should add this is a FWD car with a solid rear axle beam

3 things I *notice* from increasing rear toe. (AKA NOT SCIENTIFIC, just my perceptions from doing alignments)

1) Car does not track quite as straight (not as critical as the front)
2) The drift angle is easier to find (aka MORE warning). (EDITED for my own stupidity) I previously posted the opposite.
3) more rear tire wear.

"increasing"? you mean "more" Toe-in?

I would expect those things, but what if you decrease Toe to a neutral Zero degrees?
Slight Toe-In I would suspect would have a benefit to turning/curve handling... as its pulling into itself on a curve vs pushing away.
Toe-out I would also expect that from the rear end, it would seem that you have two tires pulling/pushing the car to the side as you drive, gawd forbid one suddenly lost traction.

First see above edit I goofed up my first post


That is exactly what I meant

Depends on camber; on a car that runs heavy camber settings 0 toe is no big deal, on a car that runs near 0 camber the rear will be more "floaty"


That is how I've experienced it.

Sorry; I've never tried that.

Edit: Can you even adjust toe on the rear of an 81 rabbit?

you can... it requires complete removal of the brake hardware and stub axles, and the installation of shims... its a "you really only want to do this once, unless your a masochist" kind of project. Hence, why I'd love to find out if there was a "better" economy rear alignment. Tire scrubbing (shorter life) and and anything counter-fuel economy are two things I want to avoid :)

Most cars have rubber (or plastic) suspension bushings, and those have compliance. As a result, driven wheels will tend to "pull" themselves forward and toe-in slightly from their static settings, and non-driven wheels will tend to be pulled backwards by the forward movement of the car, toeing themselves out slightly from their static settings. These effects are smaller at lower speeds, and larger at higher speeds.

Toe-in at a given end of the car promotes stability. It tends to help the car correct any deviations from going straight ahead. Toe-out at a given end of the car promotes instability. It tends to reinforce any deviation from going straight.

From the racing world: Toe-out at the front can help the initial "bite" of the tires on turn-in. Toe-out at the rear leads to a car that likes to try to swap ends, or at least likes to get the tail of the car sliding around more easily. Toe-in means less response, but more stability--especially on high-speed parts of a track.

Toe will also change on most cars as the wheels go up and down. (Google "bump steer" for more info.)

If you always wind up with toe-in under all circumstances, you will have a car that is more stable than one that goes into toe-out sometimes. It will give you a little more tire scrub, so a little more friction losses and therefore a bit less MPG.

Unless I had the time to check the bushing compliance at various speeds, and to work out the bump-steer graphs for the car, and could change the toe settings reasonably easily, I wouldn't bother messing with the toe unless I suspected a real problem.

-soD

interesting... gotcha.

On a trailing arm solid tortion beam rear end (FWD car)... I don't suppose there is a ton of "toe" flex (most of the flex seems to come with turning, ever see a Rabbit do a 3-wheeled turn LOL!)... The OE rear toe was a full 1/3* (20') of toe-in, which 2/3 of a degree total toe-in just seems like a lot. I wonder if going to 10' (1/6 degree) for a total of 1/3 of a degree toe-in would be a negligable change or in effect a detrimental change

Dave's right
 

Some other Dave beat me to posting the whys and wherefores of rear wheel alignment. I was full time turning a wrench when those cars were new, and have done numerous 4 wheel alignments.

As S.O.D. mentioned the rear wheels will begin to toe out at speed--which gives you your neutral toe setting. Back then, foreign car makers were much more in tune with real world alignment specs. and tolerences. I would use OE numbers.

Yes dave is right.
I set my front wheels toe out for auto-x, but immediately change them back or it's a disaster at higher speeds, very twitchy and won't stay straight. The back I always leave along, just slightly toe'd in. Probably the best case option would be to replace the rubber bushings with spherical bearings, and then be able to run near 0 as the susp would have less give, but this will be a consiberable cost and ride quality will suffer. Compromises....

The only people that I know that would venture to rear toe-out are people that trailer their cars to autocross events. Some people drive them short distances at low speeds to autocross events. So, if slight toe-out is that dangerous, i'd stick with OE for safety.

I think this is one of those - "if you need to ask, you shouldn't touch" kind of subjects.