HELP! Shimmy'ing mx-5.

[Wivvix]

Hi everyone,

I've recently bought an NA8. I made the colossal mistake of only taking the car up to 90'ish and not pushing it to 110. I have since learned from some freeway driving that it does indeed have the 65mph shimmy. The shudder or shimmy, is not isolated to any particular area of the car, i.e. the steering wheel or gear lever. It basically makes the whole car shudder, and really starts to get worse as you increase speed from 100km/hr upward. It's noticeable as it's winding down from 60 to 50 in 4th gear too.

Coincidentally, i've already had an alignment and balancing done at bobjanes, as I decided to switch the alloy wheels back to stock for comfort reasons. The tyres are not the same on all four wheels, but all have been balanced and are of the same dimensions, as far as I know. 2 new on the front, 2 older tyres on the rear. Neither set are good tyres to be perfectly honest. "Probase bronze wheel balance" for all 4, and 4 wheel alignment was done.

I have dríven the car with the aftermarket alloys on the rear and it made no difference, it still had the shimmy. This could be an issue of crappy balancing however?
I took the car to a place called WA suspensions today, and they said I should get another wheel alignment and balance done by them, just to be sure it's not the wheels being out of round, the tyres being out of round, wheel alignment, or balancing. At this point I have that appointment booked for tomorrow morning.

I don't really think it's an alignment issue, but then it could be as I don't know how good bob jane is/was at aligning it, and it says on the read-out sheet that bob jane used NB alignment specs? I will post them below. I then drove to an engineering company, who advised me that it's the universal joints in the drive shaft, and that they will need to replace a section of the drive shaft and put in the new section. "The universal joint appears to have failed and is quite free to hand-play", which they believe is responsible for the shudder.

I've just bought this car, I don't really have the money to throw around at all, and I have no experience with resolving this shimmy issue. Am I on the right track? Does everything said so far sound plausible? What should I do before getting the driveshaft issue fixed? Do I need another wheel alignment and/or balancing?

wheel alignment specs;

Rear axle:
Camber- left; -20 minutes, right; -16 minutes
Toe- left; +7 minutes, right; +6 minutes, total; +13 minutes
Geometrical driving axis; -7 minutes

Castor- left; +4 degrees 55 minutes, right; +4 degrees 16 minutes
KPI- left & right; +11 degrees 17 minutes

Front axle:
Camber- left; -14 minutes, right; -9 minutes
Toe- left; +5 minutes, right; +5 minutes, total; +10 minutes
Setback- -7 minutes

Any help/advice would be greatly appreciated, as well as some mildly aggressive alignment suggestions, or which are proven non-shimmy alignment!

Cheers,
Trent

[mx52nv]

Trent,
No one has more experience with MX5 suspensions than Bob/John at Wheelsworld (Tel: 9244 2277) in Osborne Pk or Kerry/Graham Wade at Bridgestone Bentley (Tel 9458 4586).

My advice is to talk to them regarding your MX5 as it always helps when they are experienced with our model. Most of the MX5 club members do and have done so for more than a decade

[Wivvix]

cheers mx52nv,

I went to wheels world yesterday after WA suspensions to get an opinion, but they said they would need the car and couldn't do anything until Friday, and basically said with the tyres which are on it, they could be the problem themself. I'm really not sure how sensitive the mx5 is to tyres, good bad or indifferent?

[JBT]

It is sensitive to wheel balance. You may also have some "out of round" or distorted tyres. Closely inspect to check.

[Garry]

MX5 drive shafts generally tend to be pretty robust so it should be fairly easy and not to expensive to source a good used one to replace yours if it's cactus.

[Wivvix]

It is sensitive to wheel balance. You may also have some "out of round" or distorted tyres. Closely inspect to check.

I was told you couldn't see the out of roundness by eye. I'll have a look. I trust the wheels/tyres will need to be off the car to inspect properly.

Taking it back to bob jane to see if they can sort it out by balance/tyres.

I thought the driveshaft should be fairly robust too, so i'm surprised they're saying its one of the universal joints. Any suggestions on where to source a good used replacement in WA? I suppose wreckers are the first obvious place to look.

If it's a simple fix for the driveshaft, i.e. just this universal joint, they reckon $500 will have the car driving smoothly. I have my doubts.

[manga_blue]

MX5 are very, very sensitive to both tyres and alignment.

I'm surprised you can bear to drive it with the alignment settings on it now. What were they thinking? It must feel like a dog. Take it to a real suspension shop that knows how to do sports cars, especially MX5. The difference will amaze you.

Tyres for our cars need to be fresh and a softish compound. Fresh means preferably less than 2 years old and certainly max 3 years. Tyres have a DOT code printed on the side which states year and week (yyww) of manufacture, e.g. DOT 0712 means the 12th week of 2007.

If a tyre is marketed for durability then it'll hard, be good for a taxi but harsh, noisy and grip poorly on an MX5. The harshness can run right through the car. Check this forum for endless discussions on tyres.

[Wivvix]

MX5 are very, very sensitive to both tyres and alignment.

I'm surprised you can bear to drive it with the alignment settings on it now. What were they thinking? It must feel like a dog. Take it to a real suspension shop that knows how to do sports cars, especially MX5. The difference will amaze you.

Tyres for our cars need to be fresh and a softish compound. Fresh means preferably less than 2 years old and certainly max 3 years. Tyres have a DOT code printed on the side which states year and week (yyww) of manufacture, e.g. DOT 0712 means the 12th week of 2007.

If a tyre is marketed for durability then it'll hard, be good for a taxi but harsh, noisy and grip poorly on an MX5. The harshness can run right through the car. Check this forum for endless discussions on tyres.

The alignment looks pretty true, and from what I can gather its within factory alignment. What alignment do you recommend? What is it about the alignment that stands out as being particularly bad?

Cheers

[JBT]

I don't think it's the alignment.....it doesn't look too bad.

Jack up each corner of the car in turn and slowly rotate the wheel. Set up a screwdriver or some pointer on a block of wood to check for lateral run out and bulges inside and outside of the wheel and tyre. Also check for run out by putting the pointer behind or in front of the wheel to see if there is any "wobble" in the tyre or wheel.

[Benny]

5's are very sensitive to wheel balance, and just because some pump jockey in a tyre place takes your wheels off and puts them on the machine, does not mean they are properly balanced, as the imbalance could be caused by funny wheel nuts, an imbalance in the disk rotor, or virtually anything that rotates.

If you were in Sydney, I would suggest taking it to Heasmans and get an on-car balance.
I'm sure there must be someone in the far west that can also do on-car balancing, and really, if your car is having a problem, it would be my first suggestion.

[manga_blue]

The alignment is good enough to not cause shimmy, but that's about all. These settings are suited to long tyre life on a small sedan. Mazda's range of factory settings is very wide, covering pig-like through to rabbit-like handling and responsiveness.

Camber on all four wheels is virtually upright. Grip levels would be relatively poor with zero camber. Most of us run -1.0 to -2.5 degrees negative camber all round to give better grip and also slightly better turn-in.

Front wheels are toed in. Zero toe-in gives much responsive turn-in. A little toe-out is even better but front tyre wear then becomes an issue.

JBT's right. Check roundness/wobbliness of tyres. Also if they're really hard or old they can hold flatspots from parking in cold weather for quite a long time after you start driving. MX5s don't have enough weight to flatten them out easily.

[Wivvix]

Thanks Manga_blue, much appreciated! Can you provide me with some alignment specs that a decent alignment workshop will use, which are suited to mildly aggressive/spirited driving?

Well today after a trip to bob janes and a drive with one of their crew, I finally got what I needed to have a "fault diagnostic" done by the RAC (as i'm a member).
To recap- bob jane put the wheels on the car saturday plus two of the tyres, balanced the lot and aligned.

I took the car in for my free fault diagnostic at the RAC. Surprise surprise, the front right wheel has a buckle- one of the rims bob janes took rubber off and put new rubber on (good to see they're paying attention!), apparently they also grossly overweighted the wheels than was necessary, and also caused the new tyres to wear out of round .

Now I just have to find new wheels or a 4th stock wheel for an NA8. The plan was to put Bridgestone ER300's on the front and put the new crappy tyres on the rear and make the tread disappear quicker than usual, but now it seems I may have reason to get a refund.

*Edit*

The shimmy has been fixed. By putting on the 17" alloys on the front (still legal rolling diameter), and leaving the stock wheels on the rear, the vehicle now drives without shudder, even with the crappy tyres and ordinary wheel alignment. Thanks everyone for your assistance/advice, and I will be much appreciative of suggestions for wheel alignment!

[Nark]

You sure it's just not scuttle shake? I get it pretty bad at 110km/h down a certain stretch of road but not on others.

[Wivvix]

You sure it's just not scuttle shake? I get it pretty bad at 110km/h down a certain stretch of road but not on others.

Can you try to elaborate on what you mean by scuttle shake? My loose interpretation of scuttle shake is a gentle wobble in the chassis, but that may be completely incorrect.
If it is, i'd imagine something like a butterfly brace or strut brace would help stiffen up the car in the event of scuttle shake.
These would have done little to resolve the shudder I experienced, as it wasn't an issue of rigidity.

I wasn't even aware there was a term to describe the phenomena "scuttle shake" until just now, thanks

After all the wheel and tyre issues are sorted out, and I adjust to the feel and sounds of the car, if there is any scuttle i'm sure i'll be able to chase it down.

[JBT]

You could always join your local MX-5 club and get one of the members to go with you or drive it or, go ride in another car the same as yours.