tramlining, bump steering after new rims and tyres?

[franco]

Hey guys,

I recently upgraded from a set of old 14" stock NA6 rims to NB8A 15" rims with slightly newer tyres (they actually have tread left).

I noticed immediately after driving on the new set that the car felt unsettled over uneven surfaces. In ruts the car would steer itself along its path and along SOME uneven surfaces the car would pull left or right on its own.

I went to get a proper 4 wheel alignment following the icehawk agressive spec on miata.net with toe out on the front, yet the tramling and bump steering remains. could this be caused by the tyre tread pattern (as suggested by one of the mechanics at the tyre shop)? or a by product of my lower ride hight (I have gaz coilovers installed with the compulsory 100mm ground clearance ride height). Or even (groan) a worn steering rack (that sounds unlikely since it only really started with the new tyres).

I should also mention, the tyres are different front to back (bridgestones at the front and another brand I cant be bothered checking on at the rear). would it be worth swapping front to back?

[rwf]

could try to swap tires front to back
as you have done a wheel alignment i think it could only be tires
sounds like you need new tires any way

[Guran]

Toe out is notorious for encouraging tramlining, so I'd expect your problem to get worse rather than better with that fresh wheel alignment.

[bruce]

icehawk agressive spec on miata.net with toe out on the front

my lower ride hight (I have gaz coilovers installed with the compulsory 100mm ground clearance ride height).

There's two of your problems;
1. You've gone for an alignment which increases tramlining and bump steering. Try a more neutral or conservative alignment.
2. You are sitting on the ground. You can't expect neutral handling characteristics.

[Hellmun]

I don't think toe out is a good idea on a road car. Yes it makes it feel dartier and has better turn in but its jittery and increases tyre wear. With no toe the car still handles awesomely. If the new tyres have significantly stiffer side walls you'll get tramlining due to the resistance to deform.

[franco]

thanks guys. the new tyres are much stiffer (sidewall) than my old. I think that's what I'm feeling. I'll also probably dial down the toe back to neutral (but its so fun!).

[Guran]

It's easy to adjust the toe yourself. All you need is a spanner, a string-line and a ruler. You could try various settings and see if that solves the problem.

Hope I'm not teaching you how to suck eggs!