Adjusting track to reduce understeer...?
 

I have been having a conversation with other driving instructors on a UK forum and one made the following comment which I found interesting....





Could this really have any benefit in reducing tyre wear and FC?? I find it hard to see, personally. If my car runs freely with the steering in the straight-ahead position, surely the amount of time I will be free-wheeling with the steering turned by any amount will be quite small???

In the case of a learner doing a reverse manoeuver - we are usually doing only 2mph!

Maybe if you own a Fiesta.
Every car is different. Even two identical cars would handle a bit differently depending on the tires, wheels and wheel offsets.

I questioned the author of the original message I was quoting, and here is his reply: Comments??

I then said 'Okay, so it improves handling - but does it have any real effect on rolling resistance and FC'....

I'd have thought we spend so little time with wheels turned, that any differerence would be un-noticeable?

With MacPherson struts, doesn't the wheel camber change dramatically on full steering lock? Does that have an effect?

the guy kind of gets it, but makes it waaaaaaaaaaay too complicated at the end.

Ackerman means the inside tire turns more then the outside tire because the outside tire has further to go. To get "correct" Ackerman you do indeed know the wheelbase of the car.

However, lets look at the simple case. When you are going straight changing the distance between the rear tires (rear track) does not change how hard the car rolls down the road. I suppose there are wind resistance issues but that isn't what we are talking about.

Now another extreme. Imagine a kids wagon, and we turn the front 90 degrees and pull sideways. Again, it doesn't matter how far apart the rear tires are, the wagon turns quite easily.

If the front and rear tracks are different, the car will indeed wander down a rutted road. But it will wander down the road quite easily.

Changing rear track does not change rolling resistance.

For the mac strut guy, Caster is the angle you are talking about. Caster does a few things including changing the camber as you turn the wheel. but it doesn't really change the "direction" the wheel is turning when you turn the steering wheel.

FWIW, all cars have caster - between that and mechanical trail it is what makes the car go straight down the road instead of wandering randomly all over the place.

Going back to the original question of if a wider rear track can reduce understeer. Recalling my autocross days, I don't think so. 5mm is also so little, it is really a fine tuning tool. Keep in mind, this is only when approaching very near the tractional limits at moderate to high speeds though. This is not going 10mph in a parking garage.

The same can also be tuned by increasing pressure in the front tires, removing the front anti-sway bar among a host of other things, see below. Cut and pasted from EXECstudio.com - Tech: How to modify Oversteer and Understeer

Understeer, Oversteer and how to fix it:

Understeer is common on 90% of the road cars because it tends to be a safer setup for daily driving conditions. Oversteer happens when the car gets loose. In this situation the rear tires slide out first

Oversteer can be dangerous especially at high speeds as it requires good driving skills to keep control of the car.

Understeer Corrections

Raise front tire pressure.

Lower rear tire pressure.

Soften front shocks. Stiffen Bump.

Stiffen rear shocks.

Lower front end.

Raise rear end.

Widen front track.

Install shorter front tires.

Install taller rear tires.

Install wider front tires.

Install narrower rear tires.

Soften front sway bar.

Stiffen rear sway bar.

More front toe out.

Reduce rear toe in slightly.

Increase front negative camber.

Increase positive caster.

Soften front springs.

Stiffen rear springs.

May need more front suspension travel.

Install wider front wheels.

Use softer front compound if possible.

Use harder rear compound if possible.

Remove weight from front of vehicle.

Add weight to rear of vehicle.


Note: This is a sample of the methods used to correct various handling problems. Books have been written on this subject. Not every correction will always work as expected. Stiffer front sway bars will in many cases will decrease understeer because of decreased body roll and better camber control. The best rule is to change only one thing at a time and keep notes.Shock adjustments have the greatest effect on corner entry and exit.

Now if the question really is how to reduce tire scrub while cornering by adjusting the effect of ackerman, that should be a different thread.

A few random points:

- There are a number of cars where the rear wheels are closer to the inside of the corner than the front wheels are in steady-state cornering. I've clipped curbs in my parents' 2nd-gen Rx-7 because of that, for instance.

- Adding FRONT track can help reduce understeer, except when the track increase gives you a scrub radius that is too large. (In this context, I mean the distance between the center of the contact patch and where the steering axis intersects the ground.) When you go too far out, the extra scrub radius adds to understeer and makes the car drive like a truck. You can correct that by changing the suspension geometry so that the steering axis point moves outward. Usually a 10mm increase in track (and 5mm increase in scrub radius) won't be enough to feel, though.

- It is possible that other mechanisms than the above result in a feeling of less understeer in a Fiesta when you add 5mm spacers to the rear.

-soD