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MG MGB Technical - Brake pads

I am investigating brake squeal-common now on MGBs. The metal backing plate of an MGB pad is as the drawing. The top and bottom edge is curved; the caliper jaws flat and slightly tapered. So the contact points are exactly that. Modern pads seem to have a far better designed contact points or edges.

The engineers back in the day were no fools but it seems an odd arrangement. Any idea why the pads were shaped this way?




Michael Beswick

And why was the orientation of the piston critical?
Allan Reeling

Indeed, particularly as the cut out position seems to vary between A, B, midget!

However the piston on a B "overlaps the front (axle facing) edge of the pad back plate. (So with the disc removed you can see the gap and look into the piston!) Consequently the cut out portion would appear not to touch the pad: or put another way the part of the piston actually in contact with the pad is uniform. Quite what this does I have no idea!

In answer to my original question it does occur to me that in the case of our calipers, the brake pad is moved (sideways) within the caliper jaws-presumably creating a small recessed groove! Modern calipers tend to be the sliding type with the pad remaining "fixed"....I think!

Again quite why the pad would not move in our type of caliper if it had a "Tapered flat" back plate stumps me!
Michael Beswick

Wow!

I'll try and answer...

The relieved pistons when fitted correctly put uneven pressure on the pads which reduces squeal - some cars with anti squeal shims have no relief on the piston but a cut out section of the shim to do exactly the same thing.

The rounded edges of the steel backing of the pads allows you to rotate the pads into position while locating the pads together with the pins and steady springs.

Squeal is also caused by too hard a friction material ( including an overheated pad) and by a sharp edge to the pad on its top side.

Use Mintex standard pads, give them a generous 45 degree chamfer on the leading edge and copperslip the backs and most cars will stop squealing.

As for modern one piston clipers - no, both pads move but the piston pushes against one pad while the non fixed side of the caliper slides on rails and pulls the other pad onto the disc.
Chris at Octarine Services

Chris- It would appear from the witness marks on my B pads that the relieved part matches the "front edge" of the metal back plate-so even pressure unless the piston tilts a tiny bit? . However idly looking at a pal's V8 pads (why do we all keep old pads!!), they appear wider so the whole piston acts on the pad back recess and all.

I can understand the advantage of the curved top and bottom edges for ease of insertion, but it seems to be less than ideal in use as the pins and springs allow movement (slight) rotating around the axle shaft.

It is probably a thankless task as "everything" has been tried: I suspect there is no one solution! Much I suspect is down to remanufactured calipers and pads being outside the original tolerances.

But I'm still curious as to why a tapered flat edge is not used!
Michael Beswick

Allan &
It's probably already been covered above, but there is some confusion about piston poition
Some books advise the piston cutout to go towards the hub (incorrect) but it has to face the leading edge of the pad so as the pressure is more to the trailing half of the pad--It's that leading edge of the pad that creates the noise--On a lot of replacement pads now the edges are heavily chamfered and don't start making noises till they are well worn
Correct orientation of the piston is critical ----


William Revit

Willy, that diagram is interesting and one I've not seen before and the reasoning for the piston in that position seems to make a lot of sense.

However, according to the Leyland workshop manual the calliper pistons should be fitted with "the cut-away portion of the piston should be located at the inner edge of the calliper i.e. towards the hub". See attached diagram from the workshop manual.

You have said *some books advise the piston cut-out to go towards the hub (incorrect)" Where did you get the information that recommends the orientation of the piston to face the leading edge?

Andy



Andy Robinson

Andy
The diaghram is from early 911 Porsche manual
They have to be in this position on them or they squeal like a pig
They actually have a 20deg tool for setting them
Because of the touchyness of the Porsche brakes i have always set mg's in the same position with good results
Take note of the direction arrow in that diaghram though---For an MGB the whole diaghram is upside down
William Revit

If you look at the witness marks on MGB pads with the cut-outs facing the spindle as specified, you will see that the cut-out portion is off the inner (spindle) edge of the pad. The remaining area of the piston gives a pretty evenly distributed pressure across the centre of the pad. If the cut-out were in a different direction there would be uneven pressure on the pad. What happens on a Midget and MGA, and why they are different, I have no idea.

Large chamfers on both edges of the pad material seem to be common these days, including my V8 pads. On the last set with no chamfer I had bad squealing, and all the usual tricks I'd been using for years only lasted a few miles. I tried soft shims on the back, which made a bit of difference, then sticking thin strips down the edges where they contact the caliper jaws did the trick. Checking them recently that had variously worn away or become displaced, but still no squeal. I feel that until bedded-in they squeal, and while they are squealing they will not bed in. With the soft strips on the edges having allowed bedding-in, they no longer need them. Maybe.



paulh4

This thread was discussed between 27/07/2018 and 30/07/2018

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