There is a complex geometry that is in play when you turn the steering wheel to go through a turn. The outside wheel must travel through a bigger circle than the inside one, so it must not be parallel to the inside wheel, even though they are parallel when rolling down the street straight,(more or less). I can tell for a fact that when I change to a wider track with less back spacing, the inside wheel wants to scrape a little when I back up with the steering wheel turned to full lock, or nearly so. I don't understand how the steering arangement creates this difference in turning radius from one side to the other in the first place, and I don;t understand if there is any alignment adjustment that will correct for
the change that happens when you increase the track width with wheels that have less back spacing. Since this is such a common after market practice, I am suprised that I have not read of this before. It seems as if perhaps you could help a little by bringing the tow-in to zero.

I know this has happened on my Toyota Matrix as well, when I installed bigger wheels with less back spacing. One of my employees has a 96 Mustange with aftermarket wheels that are set further apart, and the handling got all messed up in this same regard with the newer wheels. Any ideas here?
Does eveybody else just grin and bear it, or have others tried to fix this problem when going to wider set wheels? Can anybody enlighten us on this idea? It seems as if the best handling is available with the same back spacing as the original manufacturer designed, and yet most car guys elect to go wider. Thanks.

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You're dealing with 2 different things....

The first (the tires turning different amounts) is ackerman and the second is scrub radius.

As for the scraping, both wheels are doing it, but the inside wheel is turned more so it does it more.


See

http://www.jags.org/TechInfo/2001/08Aug01/

for a better description (even has pictures) than I could do...

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Thanks EMS. That is a good basic read that at least makes one familiar with the terms and language. I would like to know more though. Maybe a college course is in order here.

Re-read the paragraph on scrub (radius?) and see if you think there is a typo there. He describes both negative and postive scrub radius with the same condition of having the intersection of the stearing knuckle axis with the gorund outside the tire away from the center of the car. So I haven't been able to discerne the difference between negative and positive from my reading so far. Maybe I just did not read carefully enough. See what you think. Not that this understanding would make a huge difference as this article does not go into the depth one would need to figure out if anything can be done about the situation those of us face who already have put on wheels with a different back spacing. He simply says get the right wheels. Plus, this terminalogy is the kind of thing I usually have to go look up. Einstein is quoted as saying, "Why should I bother remembering anythingn I can go look up in a book". Now if I can just find the damn book.

The question is, how does toe, camber, and caster effect these. I understand how caster effects the on center feel, and this is fairly simple to visualize, but I want to know more about how these parts move through their motion, and how standard alignment specs affects everything the car does, and how it feels. And, finally, can any of this compensate for increasing the track of the car with different wheels?
Sam

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If I remember that is also called- toe out on turns- nonadjustable that is built into the geometry. High school shop class practiced this step in teaching front end alignment. Instructor described this spec for checking frame damage. Maybe a "how to" book on alignments ?

Yep, he said the same thing about scrub radius.......

Look at the picture above that paragraph, and you'll see the difference between neg and pos........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Sam,

This is called "Ackerman steering" - inner wheel on smaller radius than outer. The best way is to get wider wheels with more backspacing to put the center of the tire near or at the center of where the original was. All my cars are close to this using the Mustang spec wheels and sometimes spacers. I have run less backspacing (by 1/2-1") on my Darts before and didn't see a huge difference.

Cheers,

Lou

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Home of Slant6-powered fun machines since 1988

My Dakota has 8" wide wheels with 5-1/2" or so backspace from the factory. It does odd things when backing up and turning on loose gravel or ice.

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Official Cookie and Mater Tormentor.

Interesting. Thanks for sharing that.

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