RebekahsZ Posted October 29, 2014 Share Posted October 29, 2014 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? Quote Link to comment Share on other sites More sharing options...
alexx933 Posted October 29, 2014 Share Posted October 29, 2014 (edited) I feel that my z does the same with 225/55r16 on each corner! I actually keep both hands on my wheel just because it makes all those small corrections easier. Edited October 29, 2014 by alexx933 Quote Link to comment Share on other sites More sharing options...
Ereschkigal Posted October 29, 2014 Share Posted October 29, 2014 tire profile? Quote Link to comment Share on other sites More sharing options...
beermanpete Posted October 29, 2014 Share Posted October 29, 2014 I think it is due to a larger scrub radius. Forces generated at the tire tread have a longer lever with a larger scrub radius. The scrub force acts on the steering axis, trying to steere the car. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 29, 2014 Share Posted October 29, 2014 Camber thrust is my guess. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 29, 2014 Share Posted October 29, 2014 Your car has a road race/autocross alignment which is designed for quicker turn in response (among other things). Its supposed to do that. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 29, 2014 Share Posted October 29, 2014 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. Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted October 29, 2014 Author Share Posted October 29, 2014 I was hoping you guys would weigh in. So it's the camber more than the tires! Cool. Thanks! Quote Link to comment Share on other sites More sharing options...
m1ghtymaxXx Posted October 30, 2014 Share Posted October 30, 2014 Surely wider tires have some effect too. I noticed a pretty drastic difference on my S130 going from 195 to 225s, and even a noticeable increase in tramlining going from 205 to 225s in my S30. I've never had the ability to properly adjust caster on either. Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted October 31, 2014 Author Share Posted October 31, 2014 I feel that the wider tires with low profile (40-series) plays a role too, but why? Scrub radius, as described before? Quote Link to comment Share on other sites More sharing options...
thezguy Posted October 31, 2014 Share Posted October 31, 2014 My Z follows every crack in the road with or without camber, it makes no difference on my car. I feel for ya.. It's a pain in the ass. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 31, 2014 Share Posted October 31, 2014 The shape of the tire footprint affects directional stability. A long, narrow footprint is more directionally stable then a short, wide foot print. Quote Link to comment Share on other sites More sharing options...
G-E Posted November 1, 2014 Share Posted November 1, 2014 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... Quote Link to comment Share on other sites More sharing options...
DREW RBZ Posted November 1, 2014 Share Posted November 1, 2014 Is there anything that can be done to the steering rack/system in some way to dampen this effect. Sorry if this sounds simplistic but this is really not an area where I have much understanding. Quote Link to comment Share on other sites More sharing options...
johnc Posted November 1, 2014 Share Posted November 1, 2014 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. Quote Link to comment Share on other sites More sharing options...
DREW RBZ Posted November 1, 2014 Share Posted November 1, 2014 Thanks for the advice John. Will give that a try. Quote Link to comment Share on other sites More sharing options...
Chris_Hamilton Posted November 1, 2014 Share Posted November 1, 2014 (edited) 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 November 2, 2014 by Chris_Hamilton Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted November 2, 2014 Author Share Posted November 2, 2014 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. Quote Link to comment Share on other sites More sharing options...
Meatwad Posted November 4, 2014 Share Posted November 4, 2014 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 Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted November 5, 2014 Author Share Posted November 5, 2014 (edited) 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 November 5, 2014 by RebekahsZ Quote Link to comment Share on other sites More sharing options...
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