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Here is the scenario:

1982 subie wagon humming along nicely until one day on a dort road we got a flat. Put on the spare and noticed that the brakes started to get a bit of vibration when stopping that went up through the peddel and the steering. Seemed like the front somewhere. It got progressively worse but there was no wobble while driving. Put on the handbrake hard and we get a clean sure stop, no vibration. It got so bad we were afraid to drive the vehicle.

Talked to the local mechanic and he even had a quick drive and said it was the front, probably the disks. (but the handbrakes work fine even at 100kmph) ?

After numerous input I ripped the front end apart and sent off the disks for machining, put new brearings in both F hubs, and checked all the brake system and front end.

Put it all back togeather and and and and No change ! %&%^$&%^&

I could not see how it was going to be a rear end problem but as there was nothing else to do I first swapped the spare tyre off the rear and put the repaired one on the back. Now it goes like a dream and no vibration. Yep, should have tried that earlier.

But, why does a rear wheel cause very heavy brake vibration in the steering and peddle. Wheel nuts cranked and you get no problems driving, only when you brake.

I can not see the logic here, anyone encoutered this before or have an explanation ?

Pehaps balance that wheel??

If it was a balance issue then why is there no vibration while driving or cornering at any speed ?

What tire was the spare and what tires are you normal every day ones?

Hydraulics are funny things - if you get feedback through one chanel it will effect all the others.

PS: I'd be sacking that mechanic.

I checked the spare and it is a different brand of tyre, it is also on a standard rim (13") the rest of the tyres are on sunraysia, same width and diameter as the standard Subie rim.

Ummmh, interesting. So, if the spare was a little bit different in diameter to the other rear tyre, could there be a translation of this in the braking to become a juddering in the hydraulic system. If so why also into the steering ?

Mudrat, you may be on track with that !

When you think about it, any pressure to (or reversion from) one of your brake channels must effect all others.

So if one of your tires is a different size, your brake proportioner is going to be all out of whack, ie the the taller wheel will be traveling at a faster speed than the smaller one for a given hydraulic pressure on that binary channel (ie left-to-right), this will be magnified when applied to diagonal channels, ie front-to-rear/left-to-right.

That's where your vibration's coming from.

Thank's for the analysis. That makes sense !

BTW steering vibration comes from the cross-channel feedback.

thats excellent anaysis MUDRAT!!

topic mostly answered?
I have Q along same lines....my spare is a different size tire...so if I must put it on, for emergency, how bad would it be? probably need to limit speed to a crawl ?
Would it help if I lower the tire pressures ?
Is it better to have the spare (bigger tyre) at the front or rear ?

Spare at the rear if its different size - ya screwed if ya got AWD though.

Well it's not really damaging anything that I can see.

I couldn't see a problem with using a different sized spare for short distances. Obviously don't brake too hard and don't use 4WD (although I went for the best part of a year in my old MY wagon with a 3.7:1 gearbox and a 3.9:1 rear diff before I figured out what the hell was wrong!).

May not be the cause here but I had a very similar problem once - turned out to be that one rear tyre had a small delamination in it and was slightly out-of round, was only noticeable under braking.