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MG MGB Technical - back brake shoe orientation


1970 Roadster.
Had a problem last weekend where the rear brake on the nearside locked on. Jacked up the back end and the offside brake was completely free even with the handbrake on. Seeing as it passed it's MOT recently with no brake issue except handbrake efficency, which the garage adjusted I was a bit bemused. I managed to free the nearside by adjusting the shoes completely 'off' and wellying the drum with a soft mallet. Adjusted both sides properly and all was well. Or so I thought. This weekend I thought I'd clean the drums and have a look round. Hub nuts took an 18 inch breaker bar to budge them! I could then see the marks where the shoes had stuck. Now to the point, as I had the drums off I had a clean around on both sides and noticed that the leading shoe on the offside is fitted the wrong way round! (Blank section at the bottom). Are the shoes all the same so it's a case of take it off and reverse it? what is the effect of it being the wrong way. I have read 'Hammer & Spanner' and that shows up the problem but has no details. Bearing in mind that the brake seems to work fine.
Ta Bernie
B Anderson

By 'blank section', you mean the portion of the shoe that is not lined, yes?

I do not have my BL workshop manual to hand and as I have discs all round, I cannot even remember where the wheel cyliner is BUT the theory of drum brakes is that each shoe should be placed so that there is a 'servo' effect when brakes are applied - in short, the servo effect is a self locking effect.

The part of the shoe that gives the servo effect is the 'blank portion' hitting the drum first so that the rotation of the drum 'drags' the shoe into tighter contact. So for the full servo effect for the car travelling forward will be for both blank portions to lead. However, you might want to consider braking in reverse, in which case you would place one shoe's blank to lead in one direction, and the other shoe to lead on the other side. It sounds to me that your car might be set up with this in mind.

I'd be interested what BL say in the workshop manual if anyone has one.
Hal Adams

This is what the manual says...


Dave O'Neill2

And here's an illustration...


Dave O'Neill2

Interesting!

They say on the one hand that the design is to provide equal braking forward or reverse, yet the illustration, to me anyway, does not indicate this...

If I wanted equal braking, I would have put both shoe leading edges on the wheel cylinder...
Hal Adams

They are indeed leading and trailing, as there is only one (dual-acting) wheel cylinder.

On a drum-braked Mini, there are two wheel cylinders on the front brakes and they are classified as 'twin leading', as the two shoes are both pushed against the drum in the same direction...as are early Sprites and midgets.
Dave O'Neill2

- Exactly, which is why, the minis/early sprites/etc were had terrible braking in reverse because the front shoes all trailed.

if it were my MGB, I would want equal servo effect on forward and reverse on the rear wheels, and would mount the shoes with both leading (the blank portion) edges to the single wheel cylinder. BUT I do acknowledge that this might not be the BL recommended.
Hal Adams

Hi,

I would do as the factory did.

I think that the exploded view, above, is the right hand side brake assy, as the handbrake lever should be behind the axle.

Herb
Herb Adler

This thread was discussed between 13/10/2012 and 15/10/2012

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