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Wandering around on wide tires


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WHY does my Z wander around, dart, and follow cracks or grooves in the street? The car drives great on a road course at full-tilt boogie mode. I have 9 inches of low-profile tread on each corner and my suspension components are tight. I have 3-degrees of camber, 5-degrees of caster (thought this would make it run true), and 1/16" toe in. With drag racing front runners, the car drives like it is a slot car, almost with hands off the wheel at over 100mph. With wide road race rubber, it feels like it is balanced precariously with each wheel on train rails, constantly trying to slip off onto the tracks, kinda like riding a unicycle. What is the PHYSICAL/MECHANICAL REASON that wide tires make the car so twitchy running down the road in a straight line, requiring constant steering wheel corrections?

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From Tire Rack:
 

What's the downside to negative camber? Negative camber leans both tires on the axle towards the center of the vehicle. Each tire develops an equal and offsetting "camber thrust" force (the same principle that causes a motorcycle to turn when it leans) even when the vehicle is driven straight ahead. If the vehicle encounters a bump that only causes one tire to lose some of its grip, the other tire's negative camber will push the vehicle in the direction of the tire that lost grip. The vehicle may feel more "nervous" and become more susceptible to tramlining. Excessive camber will also reduce the available straight-line grip required for rapid acceleration and hard stops.

 

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The width and overall sidewall stiffness have just as much to do with tramlining as the camber...

 

If you have a ridge you drive up against or onto, such as a frost heave or train track, the edge of the tire will either allow or resist that transition, on a typical low profile tire, it will resist a lot.

 

The other side of it is those undulations in the road where heavy vehicles have created gentle depressions, the whole tire is involved, and width will matter; the outer 2" of your wide wheel may touch the crest, and the inner 2-3" may touch the valley with less weight, and as the grip varies between those two points, the car will dart.

 

Narrow tires don't generally have this problem because the level of grip across their edges will have less leverage on the steering... of course your scrub radius will dictate if the left and right are independently stable or whether they are only stable working against each other.

 

Adding excessive caster can mask this effect by levering against the instability, but I wouldn't use caster adjustment just for this personally...

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It's not an oscillation so damping won't work. You can reduce the effect by running camber angles less then 1.5 negative, add toe in, and add caster.

 

A little rear toe in also helps.

 

And not fighting it with steering wheel helps a bunch. Relax your hands and shoulders and let the car move a little.

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Increasing caster (more = + / less = -) will help straight line stability. I assume you have camber plates so you should be able to do this rather easily.  Only downside to adding positive caster is it increases steering effort especially at low speeds.

 

Edit: Ooops I see that adding caster was already mentioned...sorry

Edited by Chris_Hamilton
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Chris-your Avatar is just too damn distracting for me to think about cars! Ok, sorry I got distracted-I'm back.

 

So, in review: there are two reasons. 1) "camber thrust" and 2) wide tires just give a lot of "feedback." I'm gonna compare the wide tire feedback as something akin to static or noise? Unwanted signal or unwanted information? Narrow tires would be like fishing for bass with a hook (like Chris's Avatar) and catching a single fish, whereas wide tires are like fishing with a net-you pick up a lot of bi-catch, rocks, sticks, old refrigerators, endangered species, crabs, etc, that you wish you could throw back? The foot print is just bigger, so your tires contact more "stuff", so you feel it? With narrow tires, you don't contact as much "stuff", cracks, pebbles, joints, etc, (your tire runs past it) so naturally you don't feel that stuff? It's that simple? If so, I think the wide tire is the greater contributor over camber. With squishy bias ply front runners, the car runs straight and true to 148 with no adjustment in camber-Ive been leaving the -3.5 camber in 24-7, 365 since the car gets so few miles. And I'd rather wear out tires than have them age-out.

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Wide tires always follow grooves and stuff on the road more than skinnier tires. The wider you go, the more it does it. Used to have 275's in the back of my fbody and it tracked a little bit, but now I have 315's in the back of it and it tracks all over the road

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My trailer keys were in my truck (which is in the shop). Got the shop owner to FedEx me the keys and i coukd get to my jack (which was locked in the trailer) so I swapped on a set of 155/80/15s in front that I use for drag racing and street driving. Oh joy! It's like power steering and all wandering has ceased.

Edited by RebekahsZ
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