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MG Midget and Sprite Technical - Brake vibration

My friend has a MkII midget with a 1275cc engine fitted. It has done 6000 miles since rebuild and has been fine. However it has now developed an annoying trait. When travelling at 50mph and above and upon applying the footbrake, vibration is felt coming through the bodywork from the front of the car. This vibration stops at 40mph and below. He has checked the movement of his pistons in his calipers by removing the pads and operating the brakes. They move freely. He has checked his pads - they appear ok. He has checked the tightness of all front suspension fittings. These are ok. He has also checked his engine and gearbox mountings. These appear fine. His front wheel bearings also seem fine. Any suggestions as to the cause of this problem?
Neil (K series)

Buckled wheel? try swaping front to back.
R Fowler

Im going to throw out with reackless aboandon A warped or a cracked rotor

Prop
Prop

I'm in agreement with Prop - check the brake disc run out for a warped disc

JB
James Bilsland

I reckon it could be a warped disk. Has the car being sat outside without use for a long period? Was it parked up wet in a garage for a long period? Could always check the disks with a dti.
Robert (Bob) Midget Turbo

Hi all. I am informed that he checked the discs and there appears to be no cracks. The car has wire wheels and he tells me he swapped rear to front. Problem continued. The car is used regularly albeit hasn't been on the road much over winter. It is kept garaged. I have to confess that my first thought was a warped disc.
Robert, explain it being parked up wet in a garage please.
Neil (K series)

What I meant Neil is if the car was wet when it was parked in the garage then the disk tends to rust where the pads are against the disk, this causes the disk to swell where the pads were against the disk. The effect when driving is a servere vibration when the brake pedal is applied.
Robert (Bob) Midget Turbo

I have had the same with my Modern when just parked up for a couple of weeks, gradualy gets better with use as the disc cleans up.

Carl
C Bintcliffe

Bob/Carl, that certainly describes the problem. I never thought that a lump of metal like a brake disc would swell. New discs then?
Neil (K series)

Neil,
I don't think the disc actually swells as such.

The iron corrodes and the resultant rust where the pads are touching creates a sort of 'swelling' on the surface(rust is many times the thickness of the original metal)- this can give vibration until it wears off - fairly quickly, however if the corosion is deeper, the pitted area also can cause vibration as the pads sweep over the area.

A dti to check run out is also a good idea + try cleaning up the discs with fairly course abrasive paper.

R.
richard boobier

my vote is on either warped rotor(s), or rust from sitting around


Very easy to confirm which: check the run out with a dial indicator, and while doing so, take a look at how clean and evenly smooth the rotors are on both sides.

Rust is easy to remove with a sanding disk. As noted by the others, this is common on cars that were in storage.

A warped rotor poses a bit of a dilemma:
- is it warped because of improper metallurgy? In that case, turning will only temporarily correct it until it warps again from heat. If so, the only cure is replacement.
- or is it because of improper driving? Getting them super hot and then stopping right away holds heat in only one portion by the brake pads (as insulation) while the rest of the rotor cools in free air. The uneven cooling can mess up the rotor, leading to a warp.
- there is a theory that the later causes the former and that you should always roll forward a foot or two while sitting at a light, after pulling up from high speed (as in from a highway to a surface street), to help even out that cooling cycle.


Let us know what you find!
Norm
Norm Kerr

The driver of this car is a very steady driver, (ie he doesn't belt it like me!). The discs have only done 6000 miles from new and were absolutely fine until recently. There was some fine surface rust on one part of the disc from standing but this has been cleaned off and polished. The problem remains.
Surely if the disc was warped then the vibration would be felt at all speeds under braking and not just above 40mph.
Might this vibration be caused by a faulty lever arm damper?
Neil (K series)

Neil you need to get the dti out and check the disk I think? just to be certain. that said disks are very cheap aren't they?
Robert (Bob) Midget Turbo

You're right Bob, £12-£14 each? What do you think about the shocker issue?
Neil (K series)

The very steady driving maybe part of the problem, a few firm stops from a decent speed may well help clean the discs up. That seemed to do the trick with mine.

Carl
C Bintcliffe

It could be a residue from the pads, or rust on the discs and I would suggest trying a similar process to bedding in race type pads.

Find a nice stretch of straight, open road and brake medium hard 10 times from ~60 mph down to ~10 mph. Do not bring it to a stop, and do not stop with your foot on the brake pedal. You will likely get a 'hot brake smell' (good!) after a few brakes. After the 10th brake drive gently for a few miles to let the brakes cool down and do not stop with your foot on the pedal.

Once cool, try braking as now to see if it worked! If it didn't repeat the process once more. After that if no luck, try new discs!

Richard
Richard Wale

"Surely if the disc was warped then the vibration would be felt at all speeds under braking and not just above 40mph."

well... sometimes warp/balance issues can have harmonics that show up at certain speeds and go away at others. This could be why it is felt only above 40mph. Though, I agree with you, it does seem like a long shot.

Or the bedding in proposals will help, if it is a pad/disk happiness issue (that, too. maybe be a long shot because these are usually only felt during braking, not during cruising).

By the way, are the wheels balanced? Every once in a while, a weight will fly off while driving (or kerbing), and balance issues are often speed sensitive.


Norm
Norm Kerr

Thanks for your thoughts on this problem. By way of an update, my friend checked the oil in the shocks and the one shock was really low on fluid. He took both off and refilled, ensuring all air was out of them. Once fitted the car was taken for a test drive. Whilst the vibration was nowehere near as bad, it could still be felt. He has therefore now ordered a set of new discs. His old ones had done 6000 miles only. I'll let you know how the car performs once these new ones are fitted. Neil
Neil (K series)

This thread was discussed between 02/05/2010 and 06/05/2010

MG Midget and Sprite Technical index

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