Car VERY twitchy - is this normal?
 

I've only owned an FD for about 3 weeks, so this may just be a new owner thing, but I wanted to ask anyway. Please forgive my newbness...I haven't owned a performance car in over 3 years until now, and never one like the RX-7.

My car is EXTREMELY sensitive to changes in the road. It follows each and every groove, and will wander all over everywhere unless I keep both hands firmly on the steering wheel at all times. It doesn't seem to be out of alignment, since it's not pulling in any particular direction.

Is this normal for these cars? It has Kumho tires (not sure of exact model) on the stock wheels, if that makes any difference. Like I said, I may just not be used to this car yet, but I wanted to check with you guys.

I have had this experience (not in my FD, though). It is usually related to the tire size and construction. The lower the pressure, lower the profile, the wider the tread, and the stiffer the sidewall construction, the more "groove wander" you will get. Alignment is also a factor, but I would suspect the tires.

The Kumho's are probably intended for competition, so that is the likely cause.

BTW, I don't have this problem on my FD (Firestone SZ-50 EP tires).

this is very common with the 3rd gen rx-7. I have talked with many people about this (dealers when the car was new, suspension experts in Houston, and most recently Cam at Pettit racing) and the setup on the stock car is very sensitive to "following the road." You can change it by messing the the alignment, camber, and caster settings - but that just ruins your tires and throws off your suspension. It's best just to get used to it - one of the many quirks with this car.

Mine used to do the same thing when I had Khumo's on the front. Haven't noticed it as much since I put the new Yokohamas on. May not be so much a matter of the brand of tire though as much as it is it's inflation and tread wear. My old front tires were pretty heavily worn on the inside edges from the previous owner's lack of maintenance on the alignment.

If your alignment is toe-out a little too much, doesn't that make it a little twitchy too?

a properly setup fd doesn't "hunt."

yes, wider flat profile tires make the highly responsive to steering input but the car should track straight over bumps.

most likely culprit is the toe setting. the toe setting is primarily to make the car maintain it's longitudinal attitude. (drive straight)

since proper setting is less than 1/8th inch toe-in, it is easy after a few thousand miles to end up w zero toe or worse toe out.

while the diff between less than 1/8th inch and zero might not sound like much it will make a huge impact.

align your car twice a year... check tire pressures every week at minimum and you will get the most out of your fd.

camber font and rear neg 1.2 degrees
toe in front and rear less than 1/8th toe in
zero rear thrust angle
30 front 27-28 rear tire pressure set cold

howard coleman

Howard,

I agree that bad toe settings could make the car jump or twitch sideways over bumps, but he was complaining of groove-wander - following ridges or troughs in the road.

Do you think your analysis applies in that case?

EDIT: I have had 1st-hand experience with this on my racecars (Sports 2000 and Formula Continental). Stiff, low profile constructions have very bad groove-wander characteristics that get better as the tires warm up (tire pressure increases, and the tire carcass softens).

Another thing that will exacerbate the problem is any play in the steering (loose rack mounts, etc.).

good answer howard. i would suspect the same thing...alignment.

Bah. Any car with wide performance tires, especially low profile tires, will tend to follow grooves and ruts in the pavement. That's life. The car won't pull itself into the ditch but it's completely normal to feel the steering wheel tug in your hands as the tire rides over these imperfections. The problem is worse when you're braking since the front tires are more heavily loaded.

I think i'd have a look at the exact model of those Khumos. If they are R-compounds (I saw a stock-looking FD on a used car lot once with the very distinctive looking Yoko A032's on it), that's your problem.

If they're street tires, and aren't worn abnormally, i'd stary by getting a proper alignment, and looking for any bad bushings.

However, high performance, lightweight cars with high performance tires tend to do this more than others. If you "graduated" to the FD from a front-drive "sporty" car, or a buick, it may just be something to get used to a bit. These cars are more demanding of your attention.

Yep. My RX-7 and M3 both do it (we have some terrible highways up here). Damon, you da man. ;)

I would agree with howard as well.

On my street FD, I get very little of that wandering feeling on a mostly stock alignment. On another FD with 17" wheels and big tires, more aggressive alignment and front and rear toe set to 0, I get lots of wander and the desire to follow all kinds of ruts in the street.

Find yourself a very good alignment shop and get the car aligned. You can find some more performance oriented suggested settings by searching for Pettit alignment specs or by reading more of howard coleman's posts :)

Exactly.

What kind of roads/bumps are we talking about?

There's a highway here in Orange County that hasn't been repaved in so long, that in the right lane, there are two grooves (where the tires are sitting the most as people cruise) on the highway. In my Integra or any other "normal" car, it's not that noticable.

But in my Mr2-T with 17's, I could feel the steering wheel jerk around as I went in and out of these "Grooves". Same go's with the FD now..

I simply stay in the fast lane for that part of the highway, the right lane is really annoying. They should fix it, damnit. :p

I've had to fight the road on numerous occasions. It's "Normal", and as stated, gets worse with braking.

I notice as my tires age the wandering gets worse. I have been running Yokohama AVS ES100 tires for the last 2 tire changes and by the time I replaced the last set they were getting pretty bad with the wandering and darting around over the road imperfections. It was getting so bad I thought I had a suspension problem. A fresh set of tires completely resolved the issue. There are many things that can cause the car to dart around, tires was my problem. The FD will tend to do it more than Grandma's Buick but it should never be something that takes much getting used to, that could be just plain dangerous.

i agree with damon and mahjik. that said if the car is a little toed out it will be even worse. following grooves is part of the game. kind of suprising on the highway when they do the resufacing and you run over it at night...