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Old 06-28-2022, 03:14 PM
91banditt2 91banditt2 is offline
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Default Wider wheels, SAI, Included Angle, Scrub Radius oh my.

1991 Pontiac Firebird project car running 17x 9.5 wheels with 275/40/17's, from the factory came with am 8" wheel and 245/50/16's
I'm reading of horror stories of issues with straight line tracking at higher speeds with wider wheels because of increased SAI. I've read that you can "fudge" SAI a bit with camber bolts for the strut to spindle connection, but at the cost of negative camber, I've added caster/camber plates. I'm new to a lot of these steering geometry terms so I'm trying to wrap my mind around how they all work together and I'd like to hear your thoughts on some of the pit falls I should watch out for.
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Old 07-20-2022, 01:24 PM
JMcDonough JMcDonough is offline
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Your best bet for understanding the mechanics and physics of what's going on is going to be to find a good book or two on the topic and giving them a thorough reading. Depending on where you're starting from, what your goals are, what is permissible by rules or budget, etc. - all those types of questions are doing to drive you to what viable options you have and what tradeoffs you're willing to live with.

How to make your car handle by Fred Puhn is a good one to start with for learning the basis in a relatively easy to digest format. The Herb Adams series of books are also good, although more race car focused than street car.

I might be missing something, but typically SAI doesn't change with wheel offset or width. Scrub radius will and that does affect steering feel/behavior.
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Old 07-27-2022, 02:54 PM
CraigS CraigS is offline
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It is always good to get the best understanding of suspension design you possibly can. No doubt about that. JMcdonough has a couple of good reference recommendations. To your questions about SAI vs wheel/tire width there isn't much you can do w/o major modifications to suspension parts. I have always run the widest wheel/tire I could figuring that the added traction will be great to have so I decided to put up w/ any side effects. Look at the first diagram here.
As you can see SAI is built into the spindle. In a standard short arm/long arm suspension w/ a large enough diameter wheel, you can ad wheel width both to the inside and the outside. As you have found out, a strut setup limits how much width you can ad to the inside of the wheel. But even w/o a strut, width added to the inside is limited by frame rails and or control arms since at full steering lock the rim/tire will hit them. BTW the scrub radius JM mentioned is the distance between the two bold dashed lines at the ground. You can definitely feel the effect if that radius gets too large. The whole wheel/tire/spindle assembly pivots around the point where the SAI line hits the ground. But outside forces (bumps, road slope, braking) will push the wheel at the point where the camber line hits the ground. So if the two points are further apart, the wheel gets pushed around more forcefully.
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