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I've had an interesting experience that I can't quite explain.

Today I've changed all my tyres/rims from 14x5.5s with shitty retread rubber to 15x6.5s with Azenis. So far so good.

I'm in a non-power steering NA, so steering is a bit heavier than it was. That's not entirely unexpected though, and it's not intolerable.

When driving though, the steering feel is markedly different. The centering effect is MUCH stronger than before. And the car is very reactive to bumps and camber on the road now, it's twitchy as hell. It's not particularly pleasant to drive. In a straight line down your average not-so-smooth Melbourne road you can feel it trying to dart around.

Now, I'm taking it to Centreline on Friday for an alignment on the basis that the symptoms can probably be cured by a decent alignment and I really would like a good alignment before the GOR run anyway. But I'm at a bit of a loss to explain why changing the rims would cause these things to happen.

Can anyone help me out with what might be happening here?

Tyres changed from/to?

gslender wrote:Tyres changed from/to?

Crappy retreads in 185/60/R14 to partially worn Azenis in 205/50/R15.

Did you get it aligned? If not, you should.

What sort of condition is your suspension in? How about your bushes, etc?

What offset are the new wheels?

Tramlining is a common symptom of poor wheel alignment - but remember, you are driving a very small car with more than 8 inches (205mm) of rubber on the road. It's going to be sensitive, even with a good alignment.

What tyre pressures are you running?

10 bucks its the offset change.

Dann

^agree. my money is also on the change in offset.

i went from stock NA8 rims (+45) to gram lights (+38) and the car started tramlining quite badly. stu at spinning wheel said there was no way to fix this on a road car without putting bolting the correct offset wheels back on the car. my car had power steering, and the wheel still ripped out of my hand on some occasions. i have no idea how people are driving MX5s on our crappy roads with crazy offsets.

I would bet that it is all in the alignment.

My NB was rolling around fine on the old, unevenly worn tyres.
As soon as i had a new set of rubber on it, it felt like i was skipping around on tram tracks everywhere.
A good alignment had her back to perfect.

So if it's the same ET offset (mounting plane to wheel centreline) then even if it's a wider wheel (and tyre) the only difference is how much rubber is on the road, and the wider tyre will still turn around the same lines (no change in alignment needed)?

If the offset is changed then there could be issues around steering geometry and the car will almost certainly behave differently/need a different alignment?

Before I posted this reply I found these helpful links, indicating that changing offset changes scrub radius, increasing steering effort and possibly increasing tyre wear due to scrub:

http://www.miata.net/garage/offset.htm

http://en.wikipedia.org/wiki/Scrub_radius

http://www.clubprotege.com/forum/showth ... rub-Radius

Just went from 185 60 on +40 to 195 50 +30 and haven't really noticed this problem. I certainly feel every little bump now though

NitroDann wrote:10 bucks its the offset change.

Dann

Possibly. Stock is +45 on an NA, yeah? The new rims are Watanabe F8Fs in I think +35. For some reason I thought stock was +35, but this might explain a fair bit.

zossy1 wrote:Did you get it aligned? If not, you should.

What sort of condition is your suspension in? How about your bushes, etc?

What offset are the new wheels?

Tramlining is a common symptom of poor wheel alignment - but remember, you are driving a very small car with more than 8 inches (205mm) of rubber on the road. It's going to be sensitive, even with a good alignment.

What tyre pressures are you running?

The first thing I did after I mounted the wheels and drove it for the first time was book an alignment with the reputed Centreline Suspension in Melbourne. We will see if that clears it up tomorrow when I go in.

Suspension is 20 years old, so not in massively great condition I suspect. Bushes probably the same, I'll be very surprised if they've ever been changed. No other parts on the car that I've found have. Still, could the smaller retreads mask problems with the suspension that the new rims/tyres wouldn't?

Pressures are 26psi all around, cold.

I can deal with a sensitive car, I'd kind of prefer it. But at the moment it's more like fighting my car to get it to do what I want it to, and I've never heard anyone describe an MX-5 like that.

MizMac wrote:http://www.miata.net/garage/offset.htm

http://en.wikipedia.org/wiki/Scrub_radius

http://www.clubprotege.com/forum/showth ... rub-Radius

Great links! Would I be understanding correctly if I said that a decreased offset and wider tyres led to an increased scrub radius? It certainly would seem to describe a lot of what I'm feeling.

Imari wrote:Pressures are 26psi all around, cold.

That's a little low, ask the wheel aligners what to run. If you ask Mazda they'll give you a figure that's not the best for tyre life.

Imari wrote:Would I be understanding correctly if I said that a decreased offset and wider tyres led to an increased scrub radius? It certainly would seem to describe a lot of what I'm feeling.

It's not the tyres that's the issue, as scrub radius is measured down the middle of the wheel/tyre, but yes as far as I can tall a decreased offset wheel means "deeper dish" look which means increased track and increased scrub radius. I've been picking up that 5-10mm doesn't make that much difference, but I'll be looking for stock offset wheels when we go to 15" regardless.

Tyres can contribute to tramlining as well, especially partly worn ones.

Stu at Spinning wheel recons a change in offset as little as 3mm will change the scrub radius.

1990 NA all original now done 138000K
Went to 15x6.5 +38 with 195/50/15.
Tyre pressure 34psi cold and have had none of these problems.
Alignment was done when tyres were fitted.

Red Dragon wrote:1990 NA all original now done 138000K
Went to 15x6.5 +38 with 195/50/15.
Tyre pressure 34psi cold and have had none of these problems.
Alignment was done when tyres were fitted.

Out of interest, what type of wheels... Any pics?

G

Running low tyres pressure will make these issues worse. 26psi is too low!.