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MG MGB Technical - BRAKE JUDDER

Hi,
A problem has been developing over the past few months. 1972, BGT. New discs, pads, shoes, flexibles, M/c, wheel cylinders, rack, TRE's, poly all round. At low speed .no problems, but breaking from anything about about 50mph there is a "judder'. Feel it in the steering wheel, but also in the seat of your pants, (that area, recently had medically checked out!!)
Have had a problem with one set of pads 'crumbling', but new pads lok fine. Am going to check out the rears again for possible contamination. Could the servo generate the Judder in the form of "pulses' in the system, or could the old rear springs be winding under heavy braking? Any experiences, comments gratefully received.
Al
Allan Reeling

Check the runout on the front discs?

Chris at Octarine Services

Disc thickness can also be a problem, anything more than 0.0005 can cause judder. First check disc runout as Chris says. What brand of pads are crumbling? If it's bad I would take then back to the suppliers.
john wright

Already checked disc run out, what is too much?
Jon, the pads were unmarked and can't remember where they came from. "Disc thickness anything more than 0.0005 can cause judder??" That's a pretty thin disc!!!!!
Allan Reeling

front brakes are obviously the first things to check but after that there's loads it could be from tyres, wheels, steering, suspension (bushes, loose fittings)

front or rear

you could see if there's also a pull to one side

not sure what effect the servo would make re this judder but perhaps latter if necessary you could take it out of the problem and just push the pedal harder and see if the judder still happens
Nigel Atkins

Pads crumbling is usually a sign of excessive heat, that happened to mine when a caliper stuck on. Judder can also be caused by uneven friction around the surface of a disc, just had to deal with that on the ZS, although because of the suspension design that doesn't come through the wheel, only the pedal. They were fine till I replaced the pads last year, but the new ones never seemed to bed in, and this pulsing was getting worse and failed the MOT last week as unbalanced across the axle. "They don't make things like they used to", the ZS discs were nine years and 40k old, the ones on the V8 are original and 38 years and 210k old. Can't see it being the servo, especially as whatever it is is causing steering wheel shake, that implies momentary imbalance between the axles, coming and going as the discs rotate.
Paul Hunt

Nigel. Pulls up square. Jamming hard makes no difference.
Everything else has been replaced over the last 2 years. Have re-checked all bolts.
Paul, Had a similar problem on one of my V8's, a really savage pull on the wheel when the brakes were hot. it came from corrosion spots on the drums. This time the drums looked fine, but swapped them for another pair anyway, to no avail. The steering wheel is not that "involved" though. I wondered whether it might be axle tramp under heavy braking, but never heard of that one...............................................................YET! The rear springs are a bit soggy! It's a judder/heavy vibration, doesn't feel like an unbalanced braking issue at the front, but after the V8 led me a merry dance, I'm reluctant to rule anything out.
Just having a brew, then going for another look.
Allan Reeling

unfortunately new parts can sometimes add to or be the cause of problems rather than eliminate

check nuts and bolts perhaps on springs to axle

on the Capri you could get judder and it was the ARB bushes IIRC
Nigel Atkins

Hi Alan,sorry about that, had a senior moment this morning. What I meant to say was disc thickness variation greater than 0.0005 will cause judder.
john
john wright

Hi Allan,

I had that too, and replaced the disks. Before replacing them I checked runout and thickness, all withing spec. Replaced anyway.
Later someone said that leaving cars standing can cause some interaction between the pads and the disks, which slightly changes the friction, causing the vibration.
Maybe a rub with some emery will help.

Herb
Herb Adler

Thanks Herb,
Have done the rub down with abrasive, but standing is not an issue, this is my daily driver! But I do wonder about the consistancy and quality of the cast iron used for discs causing variable friction around discs, and drums for that matter. (Back to product quality again Nigel!!!)_
Looks like I will have to raid my wallet and buy some new discs and hope for the best. Thanks all, but not closing the door, further comments welcomed.
Allan Reeling

Does the judder rate slow as the car slows, or change in any other way as speed changes?
Paul Hunt

Paul,
Yes and almost non existent below 30mph. That's what got me to thinking it was maybe heat related, but it's there as soon as you touch the pedal, so more an issue of speed.
Allan Reeling

Sounds more like ZS then where one disc had variable retardation around its surface. If it stops as you slow below 30 with the brakes on then it can't really be heat as the discs and pads would still be hot. Ditto if it is immediately apparent with cold brakes.
Paul Hunt

Replaced discs and pads today. Checked caliper pistons, all 4 moved freely, and calipers were were still 40 ft/ilbs. Wheel bearing end floats up to spec. Well, IF I can down load the photo, this may be it, it's strange. BUT is it cause or effect? Both discs were largely the same, un-marked outer surfaces, but insides had polished dark spots and signs of what looks like chatter! The dark spots wouldn't clean off but when attacked with abrasive revealed pitting. The disc thicknesses are different 0.347" and 0.325". Had a senior moment (Too many MGB's!!) the discs were not renewed by me, but as you can see, hardly worn.

Allan Reeling

Hi Allan,it looks like the marks on the inner face of the discs looks like rust staining. With the thickness's you have quoted it looks like they have been machined, I thought that "B" disc were 3/8" thick. Hopefully now the judder has gone.
If you are interested about my theory just "google" Disc Thickness Variation and have a read.
john wright

Original B disc thickness was .350
Paul Walbran

Nigel, looks like bad product AGAIN!!! The inside surface has very small pits all over it. One assumes that they were cast with the inside as the top , thus any gases would accumulate there. That's my theory anyway.
Allan Reeling

Allan,
the thought had run through my mind previously, it may be that we are (too?) quick to think of this as others are very resistant to the idea - until it repeatedly happens to them that is

if the new discs and pads work as they should then it certainly seems like the previous discs were at fault

btw, can you remember when they were fitted?
Nigel Atkins

New disc and pads.......perfect, vibration free braking.
Nigel,
Fitted by PO, but judging by their thickness, haven't had a lot of wear.
Allan Reeling

Allan,
we'll blame a combination of PO, poor quality parts and lack of use then, always a recipe for illness

are you another one of theses chaps with more than one B or MG or is it my imagination(?)

you could still sell the dics and pads to a little used car as it wont matter to them :D



I'm joking of course
Nigel Atkins

3 at Present Nigel, one 1870cc for sale though. Sticking with my lovely V8's, it's good to torque!
Allan Reeling

(sorry, missed your reply)

yes I'm a fan of torque and having 8 cylinders to go with 16 valves

I went from a RV8 engine car to a very high revving 4 pot and it just didn't suit me, power band of 7,500-8,300 rpm and I used to be happy keeping well below the 5,200(?) of a previous standard RV8 and just ride the torque
Nigel Atkins

Allen, I don't like to be the prophet of doom but your original problem may come back. I had a similar problem a few years ago, judder every time the brakes were applied. Fitted new discs twice and it cured the problem for several months and then the judder would come back. The cure was to have the discs skimmed on the car by a Man that done mobile disc machining after the judder had reoccurred. It was the hubs that were distorted, the heat generated by the brakes made the discs take the shape of the distorted hub. After the skimming the problem never came back.
The problem now is there is not many people who can do the machining on the car now as there was years ago. Replacement discs are fairly cheap.
Trevor Harvey

Trevor,
Oh Wo and thrice wo!!!
If you look at the discs you can see the pitting on the inside. Fairly confident!!**
Allan Reeling

This thread was discussed between 02/07/2013 and 09/07/2013

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