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MG MGB Technical - Rear brake cylinders

My 1970 mgb roadster suffers from rear wheel lock-up under heavy braking. I have checked the diameter of the brake cylinders, as accurately as I can with the cylinders in situ, and thay are 13/16 inch. Is this the correct diameter? If not what should the diameter be?

I note that there are smaller cylinders for competion use which are 5/8 inch. However this is some 40% less area than my existing cylinders which seems like a big reduction.

Peter
Peter Scott

Peter. The standard, roadster, rear wheel cylinder is listed as being .800" in the Moss catalog. This is close to your measurement of 13/16" or .812". Peter Burgess mentions, is his book on the MGB engine, aftermarket wheel cylinders of .750" being available. However, he notes that these are only necessary when modifications to the front brakes have been made. Thus, making sure the front brakes are working properly might be one place to start in your investigations. It would also be worth knowing if both rear brakes are locking up, or only one. Condition and materials used on the front pads and rear brake shoes can also have an influence on this. Les
Les Bengtson

Les Thanks for your response. My braking system is completely standard, including the rear wheel cylinders, you have now confirmed for me. The brakes work extremely well. However on one occasion I was doing 70mph and had to brake extremely hard. The car pulled up very well but the rear brakes locked and I ended up sideways. Somewhat embarasing to say the least. I was driving on my own and there was nothing in the boot other than the spare wheel etc. So I guess that with minimum weight at the rear and the front brakes being very effective the weight transfer to the front was sufficient to lock the rear wheels.

As the brakes do work very effectively I am reluctant to make any changes to the standard set up unless I can accurately predict the impact of the change.

Thanks for your help.


Peter
Peter Scott

Rear wheel lockup used to be the norm...that's why modern cars have ABS. Takes away one more thing that most people don't do well and does it for them. We just have to remember that we're not driving a modern marvel, but an older technology. Weight dist on an MGB is about 50/50 when static, but sudden deceleration is going to shift the forces forward to the front wheels. The idea is to brake just to the point of lockup. It can be done...that's how competition drivers do it.
R. L Carleen

Peter,

A quick fix (read: bodge) is to slacken the brake adjusters off a notch or two. It'll be harder to make your handbrake work, but your rear braking force will be decreased.

I'm at the other extreme... I have a GT with roadster cylinders with the adjusters way out of whack. The rear brakes don't really do much.

Of course, you should never dip the clutch until you're nearly stationary... :o)

ttfn,
--
Olly
Oliver Stephenson

Thanks to you all for you feedback. I think that I will leave well alone and learn to master catenated braking. This was Ron Gammons' recommendation.
Peter
Peter Scott

Oliver,

I find it difficult to imagine how adjusting the brake shoes affects hydraulic brakes. No matter what the adjustment the force applied to the shoes will always be the same with hydraulic brakes.
All that backing off the adjusters will do is cause the brake pedal to go further to the floor.

Mick
M F Anderson

It may be worthwhile to check for a leaky rear axle seal. I had rear brake lock up, and found that a dodgy axle seal had leaked, and fouled the brake shoes.

Peter
Peter Van Syckle

Been arounf a long time but my eurodriving was mainly in Citoens, what is "catenated" braking? Left foot?
John
John Simmers

Mick,

Slackening off the adjusters means that the cylinders have much less leverage over the shoe, as the force is all going against the drum right next to the cylinder. With the adjusters set properly, there will be a lot of force applied at the other end of the shoe, as the cylinder has a large mechanical advantage at this point. As the shoes wear, the spread of force moves nearer and nearer to the cylinder, reducing braking force.

You're right though, it does make the pedal sink quite a lot. I usually rock around with the handbrake set to take the slop out, which gives me back my firm pedal. My main reason for having it set up like this is that I'm too lazy to take my wheels off and adjust my brakes :o)

ttfn,
--
Olly
Oliver Stephenson

Oliver
I can see what you are getting at, and I have some concerns with your advice.

Backing off the adjusters will not reduce braking effectivness in the manner that you describe. All that will happen is that the shoes will rotate to new postitions closer to the adjuster, and the the pistons in the cylinder will have to move further to bring the shoes into contact with the drum. As you noted, this increases pedal travel.

In an emergency situation, is it possible that you will run out of pedal travel before full braking effort is applied?

I really dont think that a bodge is the best way to deal with a brake problem.

If there is a problem with rear end lock up, changes can be made to rear wheel cylinder diameters, or perhaps incorrect linings have been used, or perhaps the tyres are worn, or the drums are out of round.

Ian F

Ian Fraser

Oliver,

Sorry, I agree with Ian. Adjusting the rear hydraulic brakes further off does not reduce the braking effort of the rear brakes. It just allows the pedal to go further to the floor.
If you slacken off the adjuster, when the cylinder applies force to the shoe the shoe just moves across in the slots to compensate for the greater gap between the shoe and the drum. The cylinder piston then applies the same force to the shoe as always. As the lining wears , the same thing happens, the shoe just moves in the slots at both ends to take up the extra distance. All forces remain the same.
There are really only three ways to change rear brake bias. Fit a twin (side by side) master cylinder with an adjustable bias bar, change the diameter of the rear wheel cylinders, or fit different linings.

Mick
M F Anderson

Several years ago a recently acquired B had the unnerving tendency to lock up just one of the rear wheels during hard braking. Not much fun when the car tries to go sideways during an emergency stop! Taking the drums off, the one side was ok but on the offending side the DPO had installed both of the brake shoes in the leading position rather than one leading one trailing. Changed it to the correct arrangement and braking was as it should be. Fast forward a couple of years and I decided to put some sticky tires all around. Under hard braking, both rear wheels would lock up very early. It seems the stickier tires were achieving faster stops and more weight transfer. I changed the shoes to both trailing and once again braking is as it should be.
Just my $.02
Tom
Tom Sotomayor

Learn to cadence brake (not "catenated") this involves braking hard till the wheels lock and then letting off till they rotate again and then rebrake and so on.

In essence you are hitting the brake pedal hard in a series of pulses - much the same as ABS does which allows maximum braking but with steering ability which you loose in a four wheel slide!

The other techique you might want to learn on a quiet road is "avoidance braking" where you deliberately go into a four wheel slide but you turn the wheel to left or right depending wihich way you want to eventually go. While sliding this has no effect but as soon as you let off the brakes the car will turn smartly. This way you can slow down and still avoid the obstacle if there is no way to stop before it.
Chris Betson

Or, learn to "threshold brake" which is to learn when the brakes are just about to lock and using that amount of force to provide maximum braking power. This technique, taught in racing schools, allows the maximum amount of control possible when braking. Les
Les Bengtson

Oliver:

<<<snip>>>
Slackening off the adjusters ... reducing braking force.
<<<snip>>>
I don't think so. Have you studied hydrolics?

I agree with Mick and Ian.

G'day
Blake

Well I have certainly created a debate. I'm not inclined to slacken off the adjusters. That does, as you say Oliver, seem like a bodge, which is not quite my style. From the many comments above the one check which I have not made is the roundness of the drums. The cylinders are standard, the shoes are mounted correctly, the linings are in good shape and oil free, the tyres are Dunlop SP7 165s all round and have a good tread. The adjusters are checked every six months.

Next step check the drums for roundness then learn 'cadence' braking. Apologies for my incorrect terminology and thanks for the correction Chris. However I'm not too sure about the four wheel drift Fangio style. Guess I'll think about that for a while.

Thanks for all the input. Now I wonder if there will be any bargin rear brake drums at Stoneleigh on Sunday?

Peter
Peter Scott

This thread was discussed between 02/02/2003 and 11/02/2003

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