Intermittent wobble

[Deathaga]

Hi guys

Hoping people can help with an issue I’m having, my car has developed An intermittent wobble seems to be at 40+ mph maybe all speeds though.

It’s like when you have a bad rotor and brake but it’s while I’m going along in a straight line. Makes a throbbing sort of noises, and wobbles the steering wheel, worse when I brake. So it like a warped rotor but they are new. I’ve checked all my bushes and bolts on the front end, I’ve just done new tie and track rods and had alignment car drives lovely.

It happens for a little bit and then stops. Seems to be passenger side, and I’m guessing front as it makes the steering wheel shake.

It’s honestly grim but I’m pretty sure it’s nothing serious as it only does it for a little bit and then doesn’t do it again for ages.

My guess is sticky calliper piston does this sound sensible?

Thanks in advance

[warpc0il]

Could you have lost a wheel balance weight?

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[Deathaga]

I could have, but that would happen all the time right? When I’ve had wheels out of balance before they do it all the time usually goes away at 70+. This literally comes out of no where and then goes away.

I think it happens after having been stopped. Of course when I get where I’m going it’s not happening so I can’t tell if its the calliper or not although did have a very hot passenger wheel the other day.

Think I’ll get a new piston it I can get one delivered I’m at a friends house for the weekend.

[warpc0il]

A subtle out-of-balance can seem to come and go.

It comes when the conditions of harmonics are met, e.g. at just the right speed and other factors, for long enough for a harmonic to set in.

It goes when those conditions change sufficiently to break the harmonic, or it doesn't appear because you happen to drive through the critical condition too quickly for the harmonic to get going.

The more common trigger for the above would be the wheel balance but it can also occur for structures such as the exhaust, if a mount is damaged or the cat-to-catback joint is locked solid, or a damaged or poor-fitted tyre, or a wheel bearing...

[Deathaga]

Oh I knows it’s not wheel bearings (I’ve checked) interesting you mention exhaust

I’ve just made a new pipe for between cat and backbox which is vband’Ed on. Could that honestly cause it?

[warpc0il]

Yep, that could be the cause.

The sprung joint with the donut seal at the back of the cat is designed to allow the powertrain (engine/gearbox) to wobble independently to the catback exhaust, while still being gas-tight, as it's a ball joint.

A solid connection between these elements changes the dynamics and can have al sorts of impact on the NVH (Noise, Vibration & Harshness)

Worse case it can wreck engine mounts or crack the exhaust, anywhere from manifold to silencer.
Best case it just changes how the car feels, but never for the better.

[Deathaga]

Ahh man, I did put a flexi section in the exhaust at first but it whistled like a train so I cut it out and put a pipe in.

I thought this would be fine all the aftermarket ones like a RRP one all seem to be fixed the whole way through unless I’ve missed something.

I know my front wheel seems to be rubbing as it’s marked my alloy again!! It shouldn’t do as from what I could find online 8.75” alloys are find without rolling but then my suspension seem quite low.

Anyone else had their exhaust make their car /steering wobble it seems such an extreme reaction to an exhaust

[warpc0il]

I've seen cars where the whole body shakes like a wet dog at particular speeds, sometimes only on the overrun or a very light throttle, when the exhaust joint has been locked solid.

On other cars they seem to get away with it, which I suspect it due to all the variables involved, included the weights of the cat and catback, and the condition and location of the support mounts.

There's no guarantee that this scenario is the cause of the wobble in your case, and the timing might just be a coincidence.

[Deathaga]

Thanks Dave as always, I’ll have a look at it today and report back, although it’s very hard to find a wobble which ain’t always there.

I do know it’s not a front bush unless it’s a suspension top mount which I want to check anyway cos I’m too low even for ProDrive suspension

[Deathaga]

So I managed to sort my exhaust rattle reshaped my hanger so it don’t hit the diff now so that’s good (don’t think it was the problem for the wobble but was proper annoying)

Stripped down the front passenger calliper piston and it had some rust/pitting so as a stop gap I’ve sanded it all smooth and cleaned everything with brake cleaner. I’ll get a set of piston and seals ordered.

Hoping it was just a case of the piston getting stuck on the rust and then pushed back in at some point.

I’ve got a dial gauge at home so I’ll make sure my brand new disc ain’t warped they were the mtec ones so people seem to say they are good quality.

I drive home on Monday (150 miles) so I’ll report back :-)

[GrantRL]

Hope that would resolve the problem. My buddy is looking to get this RX8 project after we finished installing the shocks and bumper from 4Wheelonline onto his current Wrangler project and sold it. The owner mentioned an intermittent wobble after the exhaust job by his son. We'll take a closer look. Will appreciate any update.

[Deathaga]

So just got to where I was going, none of the same wobble in the steering wheel so I’m gonna put that one down to bad calliper piston so I’ll replace both fronts

I think Dave is onto something with the exhaust though it sort of a wobble you can feel under your bum when you change speed. I’ll look to put a strung spherical joint into the section I made as the previous one leaked so bad even with a new mazda gasket.

[DomN]

Hi Deathaga, did you put a new joint in, and if so did it make a difference?
My Rx has picked up a wobble at around 60mph + which shakes from the rear of the car, similar to what you describe I can feel it through the seat. It's only started recently and I not long ago had a new donut gasket fitted to the exhaust. Could it really be as simple as the garage over tightening that and locking it solid like Dave mentions above?
I'd love it to be that as it sounds like an easier fix than some of the alternatives.

Ta

[Deathaga]

Hmm I actually cannot remember I ended up changing all sorts of parts but I think it actually might have been the exhaust wobbling.