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Hi everyone
Just before Christmas my local Bob Jane store fitted a set of four Bridgestone Adrenalins (185/60/14 on standard rims) to my '89 NA and did a four-wheel alignment.
They are definitely quieter and have more grip than what I had on (two different types of cheaper brand tyres) but another immediate effect was that almost any bump now produced a horrible shake through the car.
Dropping the tyre pressures to 32psi reduced the effect slightly but it was still bad so I took it back to the store. To their credit they took me seriously and went over the tyres and suspension (ball joints, tie rods, etc) - all was OK. They also revisited the wheel alignment (slightly more front toe in, I believe) and dropped the tyre pressures to 26psi.
The result was about 90% better but at certain speeds and on certain bumps it still sets up a shake through the car.
Has anyone struck this problem?
I'd really like to eliminate it all together or am I expecting too much on an NA?
I've since spoken to someone who said I should have gone for Yokohama C-drives instead (from memory about $40/tyre more expensive).

Some other factors to take into consideration:
- The only change was tyres and wheel alignment.
- I ran the previous tyres at 34psi.
- My car is fitted with standard springs with Bilstein shocks that I fitted about 15,000k ago.
- The car is fitted with front and rear chassis braces (as far as I know everything else is standard.

Any thoughts or experiences would be much appreciated.
Cheers

The adrenalins do have a reasonalbly stiff side wall, part of it could be due to that. Over certain bumps my car has always shaked, although i run quite high tyre pressures. I'm used to it. What tyres were you previosly running, perhaps the siderwalls were a lot softer and more compliant. Lowering the tyre pressure means that the tyres will absorb more of the impact going over a bump, the same effect as having a softer sidewall.

The C-drive does have a softer side wall, although the adrenalins have more grip. I would say its quite likely what you are feeling is due to the stiffness of the sidewall transmiting more of the impact shock into the car.

What alignment specs did you use?

i would say its just the stiffer side walls. apart from the chassi braces you have put on, the na6 was the least braced from memory so it may effect your model more - don't know but just throwing it out there. if everything was fine before and thats the only difference it must be it - surely.

My vote is for poor wheel alignment - I would go to a specialist and get it checked. From the description it is most likely the rear wheel alignment causing the problem, not the front.

I havent noticed any problems with mine and I run a lower profile higher pressure and stiffer suspension...
Assume all the wheels are balanced.

Thanks for the replies so far.
I've spoken to a technical guy from Bridgestone this afternoon and he said that, like most modern tyres, the Adrenalins are designed to run at lowish pressure and 26psi is perfect for an NA MX5.
He said the sidewall on the outside of these tyres is stiffer (they're designed to be fitted only one way) to help handling - he didn't think they would contribute to a shake through the car.
He suggested there might be an issue with one of the dampers so I'm going to get all the suspension checked - probably use the free check they offer at Pedders.
Bob Jane offer a free follow up wheel alignment after 3mth or 5000k and I'm going to ask them to adjust it to the spec that Charlie Brown has posted on this site.
It may be just the way that NAs are and that I have to put up with the mild problem to get the enjoyment of driving it.
Any other suggestion for what might be wrong would be appreciated.
Cheers
Copycat

I'm running 195/50/R15 and I've noticed a much better ride quality, hardly any shakes and scuttles which aren't out of the norm.

That's interesting given that they're lower profile tyres but my wheels are 14s, not 15s.
Cheers
Copycat

As a matter of interest could you describe the shake further. Is it a 'crashing' noise (seeking a better expression) when you go over large bumps or something different?

copycat be very wary of the pedders free check. when they checked mine you would have thought that i had delivered it to them on a tilt tray. the report they gave me was a total fantasy, and the end result was utter confusion. I replaced dampers and springs only (not from them) and it solved all my problems. I'm not saying that you have a problem with your dampers and springs, just be very wary of their report.

The Pedders free check relates to what they want you to pay them to do, not what actually needs to be done

How many km's has the car done? The suspension bushes could be past their best, and the new stiffer side wall tyres have just happened to show this up. Also, if camber was adjusted and the lower control arm bushes are flogged, that could have moved those bushes slightly to a position where they are now not really doing much work. You should go to a specialist wheel aligner - someone who only does suspension and nothing else, and has a good reputation, i.e I wouldn't get Pedders to work on my car. Some of the VIC guys who hang out in the motorsport section on here could recommend a good aligner for you.

Also, NA6's can be very sensitive to poorly balanced wheels. Generally noticed at higher speeds though - 100km/hr.

As for tyre pressures, if you want those tyres to last well and perform well, keep them up around 32 to 34 min. The only reason the Bridgestone guy suggested 26 is because he thought he would get you off the phone and "solve" (more like mask) your problem with lower pressures. 26psi will give you much shorter tyre life than mid to high 30's, good for Bridgestone as they sell more tyres then. Worst case if all the above checks out ok and you still consider the tyres to be too stiff, you could always trade them in on a different set, minimising lost dollars.

Hi Bensale
The shake doesn't happen when I hit large bumps - it's happens on much smaller bumps which sometimes you can't even see.
I would describe it as a shake that runs through the car - like a vibration through a tuning fork.
As I said, it's much better now and maybe it's just what many call 'scuttle shake' and it's normal for an NA on stiffer sidewall tyres.
I've owned my NA for about 18mths but I've never dríven any others so I'm not sure what 'normal' is.
Thanks for the advice on Pedders - I'll reassess whether I go there and if I do I'll take their advice with a grain of salt.
Thanks once again to everyone who has responded.
Cheers

The shake doesn't happen when I hit large bumps - it's happens on much smaller bumps which sometimes you can't even see.

Sounds like a combination of the stiffer sidewalled Bridgestones combined with too much highspeed comp damping of the Bilsteins. Are the Bilsteins from a Clubman?

I notice a lot more scuttle shake on my NA6 when running with 205/50/15 compared to 185/60/14. Usually run with 28psi in both cases.

The 205s are Dunlop Direzza DZ101s, the 185s are cheaper Goodyear Regatta tyres. The Direzza's probably have stiffer sidewalls, and that, combined with the lower profile, causes the extra "tremors" throughout the car.