Wheel wobble when slowing from 70mph

As noted elsewhere, I've just bought a 2008 MINI Clubman Cooper S as a commuting car for MrsC.
On returning from buying it in Plymouth, it drove fine, except for one issue:

When slowing from motorway speeds down to 50 or below, the steering wheel began to wobble. This would get worse and carry on until I either slowed right down to a virtual halt or accelerated again.

It didn't happen every time I slowed and it didn't ever become unmanageable, but it will need to be sorted.

My observations are:

The car is fitted with run-flat tyres at the front, but not at the rear. At first glance they don't appear unevenly worn

The car has non-standard alloys with spacers. I don't know whether the spacers are for practical purposes, to avoid snagging, or for cosmetic effect.

I don't know if spigot rings should be or are fitted.

I really, really hope it's not a symptom of a shagged steering motor.

Any thoughts, O Gurus?

Sticky brake piston? Check if the wheel gets hot next time it happens

I’ve got to say that I don’t like the sound of running a different type of tyre front and rear - though have no data to suggest it’s a real problem. I don’t think that’s your issue though. When did it last have an MOT? I think it’s most likely a worn bush or ball-joint. It could be exacerbated by a wheel alignment issue - toe adjustment in particular.

Thanks for that. Would it throw up a fault code if the piston was sticking? I'm hoping to have time to connect it up to my diagnostic kit this weekend.

I agree about the tyres, Rich, especially as I loathe the standard run-flats. It had an MOT in June. I don't know if that is causing or exacerbating another fault. I might see if our friendly local ATS will check the alignment etc. before I invest in new boots all round.
My only doubt is that it doesn't happen every time I slow down.

Might be some form of “tramlining”, too, Jim - if the spacers aren’t required for offset. My Alfa 33, back in the day, tramlined a lot when I put it on alloys and lower profile tyres.

Silly question, but Have you checked the wheel nuts are tight?

The only time I've experienced shudder (rather than wobble) is warped brake discs. They tend to only manifest when slowing down from motorway speeds.

Also, try and find out the stock offset for standard wheels and compare with the effective offset of the current wheels and spacers. Changing the offset changes the scrub radius that can then have quite undesirable effects. I'm very much in the school of leaving the suspension of road cars as it was meant to be and the scrub radius seems to be an especially sensitive parameter carefully selected by the original engineers and testers.

Good ideas, everyone. Gripped - I'll check my nuts, so to speak, as it wouldn't be the first time I've driven a car with loose wheel nuts!

You would feel loose wheel nuts all the time and even more so when cornering

My guess is that the wheel offset is wrong - easy to check by removing the spacers (assuming the wheels fit without them). What type of spacers are they? If they are thicker than the wheel centre boss, you would need hubcentric spacers, otherwise the wheel can move off-centre

If you find there are no spigot rings when you take the wheels off, then you will have discovered the issue.

The slightly left-field alternative is knackered outer CV joints (or even inners in some circumstances) - CV joints can give a lot of kick-back into the steering if they are very tired.

Thanks Nigel. My immediate thought on discovering the car had spacers was that it could be spigot rings.
I'll start looking at likely culprits over the weekend - starting by.taking off the wheels.

So, the winner is... Morrison!
It was a sticking brake caliper and all is now well. Huge thanks to all of you for your ideas and help.
The Mini is proving an excellent partner to the ludicrous beemer, except... (See oil leak thread!)

What can I say, even a broken clock is right twice a day!

Glad you got it fixed, it's a horrible feeling hitting the brakes only to have the car start doing it's best impression of an overfilled washing machine