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Old 04-03-2004, 01:06 AM   #142
musketeerracing
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Member#: 52164
Join Date: Jan 2004
Location: New York, NY
Vehicle:
1997 Lancer Evo 4
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Quote:
Originally posted by aspera
I haven't ever seen any threads on wheel bearing failures, but it does make sense that numerically small offsets would increase bearing problems over time. Cornering hard should also be avoided...hahaha.

moving on...

I'm not a suspension guru, but I think something is being overlooked. I think you'd call it torque steer, but I'm not sure if that is the *real* name.

Let's assume that a USDM WRX sedan can fit 245's all around with ZERO offset for a moment. I know this is fantasyland, but bear with me.

In the front, stepping on the gas will toe the wheels inward. It will also lift them up slightly, depending on caster. They will also creep forward of their normal location. Slamming on the brakes will toe the wheels out greatly. Since the WRX tends to dive, the toe out will be extreme.

The extra offset will give the tire more leverage to move around in the bushings. It will also give the tire more leverage to steer. If you press on the gas pedal when exiting a corner, the outside front tire will steer INTO the curve. Since the car is leaning during a corner, the tire will climb forward and UP! This is extra understeer that an AWD doesn't need.

Like I said, I'm no expert (I probably got parts wrong), but my point is that all kinds of wacky suspension gremlins are waiting to pounce on "wider is more gooder" guys.

You're presuming what we call "bump steer" here and that's the key flaw in the analysis. Bump steer comes when the tie rods go through an arc on suspension compression - if you're going straight ahead this only affects the toe, but if you're cornering it affects the steering angle. Most cars are well designed to have a minumum toe change through the range of suspension compression, although lowering your car changes this paramenter (NOTE THIS - that's the main handling problem of lowered cars, besides spring jounce).

Torque steer, by contrast, is caused by different wind-up of unequal-length half shafts, something that Subaru is not plagued with. But look at a transverse-engined car with the gearbox on the side and you'll see something interesting -- the shorter shaft is way thinner. That's to ALLOW it to twist as much as the longer thicker shaft under acceleration. The problem is that this design presumes a single torque figure through both axles - less torque (easy acceleration) doesn't usually twist the axles much and so isn't a problem, but high torque, especially with modified engines, does make a profound difference. The wheels simply want to go a different distance, and this twists the wheel out of your hands. That's torque steer.

Wheel offsets? The original post is the genius post here. I think the Forester people have slightly different parameters is all. But note that as you move away from ideal offset (and TRUE offset of the load centre of the wheel is a SINGLE number for each model, it's just that different wheel offsets are necessary to achieve that number with different widths which is what this thread is really about) you will wear the wheels bearings more. So in rally we do what's necessary to clear our 5 inch rims with 4 inch wide tires (winter) to our 8 inch rims with 245s for tarmac, but we replace our wheel bearings twice (or more) a year! Stay near the first post above and i think your bearing will last reasonably well.

ACP
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