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MG MGB Technical - Rear wheels hot to touch

Hi,

Recently, my rear wheels were quite hot to the touch when I've been for a drive. Not the tyres but the metal itself. I thought it was the brakes dragging and backed these off. (Can only just hold with handbrake applied now).

However, It still seems that they are getting hot.

Is this a siezed cable or is it to do with the bearings?

Thanks in advance.

Iwan
Iwan Jones

It could be the rear flexible hose collapsing and acting like a one-way valve, maintaining pressure in the rear brakes.

Backing off the adjusters isn't going to do much for your braking.

Try cracking open one of the rear bleed screws to get rid of any residual pressure, then adjust the brakes up properly. Once you are happy that they are properly adjusted and not dragging, operate the footbrake and check that they are releasing afterwards.
Dave O'Neill 2

That's it, apply handbrake and footbrake, release both, then see if the wheel is dragging. Then investigate why, as Allen suggest for hydraulic problems. But if that's the case I'd expect the brake pedal to be higher and harder than 'normal'.

After servicing my brakes and readjusting, I pump the footbrake a few times, then pull up and release the handbrake a few times, then tap the drum with a mallet to resettle the shoes. Even so I find that the V8 needs the adjuster to be one click freer than the roadster or they bind.

I'd never had a problem with either handbrake holding, if I rolled the car down the slight slope of my drive and leant in and pulled up the handbrake they would both lock. But having changed the shoes on the V8 after a wheel cylinder leak they are now rubbish. That's despite cleaning the drums with brake cleaner, and sanding down high-spots on the shoes several times to get an even wear pattern.
paulh4

Return springs? But unusual for it to be both sides simultaneously.
Michael Beswick

Paul,
Sounds like the radii of your new shoes is smaller than the drum and the leading edge is not doing it's "self servo" thing.
Could be inaccurate product again, very worn drums or skimmed drums. somehow i doubt the last two!
Iwan,
Take the drums off, wheel cylinders COULD be seized, but both at once??!! Similarly the handbrake lever inside the drums.
Allan Reeling

Allen, when brake shoes are manufactured the liners are always are a slightly smaller radius than the drum. This difference is called "crown".They do this to save doing any extra grinding of the liner. You are on the right track regarding the leading shoe, the brake factor will improve as the liners bed in. Excessive crowning is better than having the liners only touching at the ends, this is called "Heel & Toe" with this the brakes will tend to grab which is not good when the road condition are poor.
john wright

"Excessive crowning is better than having the liners only touching at the ends, this is called "Heel & Toe" with this the brakes will tend to grab which is not good when the road condition are poor."
Very true generally, but it is a brilliant condition for the handbrake in autotests!
Paul Walbran

Hi Paul, you are right, it would be brilliant until the liners come unbonded from the shoes, then even more interesting!!!

regards John
john wright

Interesting John thanks for that, another one to store in my overcrowded repository.
Allan Reeling

Allen - new wheel cylinders and shoes. Initially the witness marks on the shoes were only a small portion of the total friction material area, hence successively rubbing down those high spots. Now showing over virtually the whole shoe, but the performance is still very poor - by comparison with the roadster. No adverse comments at the MOT though, although in previous years there have been positive comments saying how good they were - "usually they are rubbish".
paulh4

Paul it's always worth trying the new shoes into the drum, you will be surprised at the amount of crown. We used to grind shoes in wheel sets to the drum size measured with an internal micrometer and then take a little extra off even then bedding in used to take upto 8oo miles. We used to rub the witness marks down after 30 miles on series 3 Land Rover even then it took quite a while to be fully bedded. So getting replacement shoes bedded in quickly not going to happen.

regards John
john wright

John, that's why the ends get riveted for these special applications, and the shoes put on only for the event. Too dodgy to drive generally as you say.

Paul, a thought: how are the respective lining thicknesses? (Old contaminated shoes and new ones). We have found some variability in lining thickness, with the result that handbrake mechanism geometry can be affected.

An efficient handbrake has always been near and dear to my heart, not only because of autotests, but our driveway has a gradient of 1 in 2.5 at its steepest! I have covered various aspects of improving efficiency at http://www.mgparts.co.nz/advice/technical-notes/handbrake-adjustment-and-correct-rear-drumbrake-set-up/
Paul Walbran

John and Paul - if the wear marks are over the whole surface now, doesn't that indicate the whole surface is contacting the drum - to some extent at least? I accept that some areas may be under more force that others because of less than perfect matching between the radii, but there is not a lot I can do about that, it'll just have to wait until the bedding-in improves through wear.

I'm not holding my breath, has anyone worn out a set of rear shoes from new? They had done about 60k before having to change them, but still had loads of lining thickness.
paulh4

Paul,
The other variable in this is the friction material itself, of course. Harder material, less initial "bite".
Interesting your comparison between the roadster and GT. I have exactly the same disparity. I started off examining the GT drums but shouldn't find any real difference so concluded it was a friction material thing. The roadster had new drums when I built it, so the cast iron could also be slightly different too.
But the action "feels" different too, so Mr Wlbran's comment about lever geometry might also be a factor. But would have thought if geometry was being substantially affected other likely culprits would be the position of the slots in the shoe backing or wear in this mechanism. I suppose that ideally the lever should be, more or less, parallel to the back plate when adjusted, and the cable at 90 degrees to it.
Allan Reeling

"I suppose that ideally the lever should be, more or less, parallel to the back plate when adjusted, and the cable at 90 degrees to it."

Exactly so, I have in the past compensated for wear to achieve that - when they are applied. MGB levers though (later ones even more so than early) have much closer to a right-angle between the part the cable attaches to and the part that goes through the back-plate, nothing like the narrow angle Paul shows, which makes for a much larger gap then his 5mm. For point 5 the MGB levers have maybe 3/16" already removed for most of the secondary lever length, leaving a relatively short portion either side of the hole in the back-plate. If too much of this is removed (point 4) you will be trying to pull the web of the closes shoe towards the hole, and pushing the furthest shoe away from the back-plate. On the MGB at least the hole is much wider than the lever that passes through it.

As you say it leaves the friction material. Not long ago Wheeler Dealers (not as interesting now - too much chat, not enough spannering) reckoned they significantly improved the performance of front drum brakes by being able to select an after-market shoe with a 'better' material. Maybe someone thought the MGB would benefit from a 'worse' material.

I suppose either of us could try swapping the shoes, and/or drums between cars to try and determine the source of the difference. In the past I've swapped the roadster shoes between sides to cure a persistent squeal at one wheel - it worked, and so far at least has continued to do so. I also had a problem on the roadster where rolling it backwards would cause one wheel to jam on without applying the brakes, backing off the adjuster by one notch fixed that. This was years into ownership always adjusting to the same criteria, and since then I've kept to the same criteria, but it's not happened again ... so far. Tricky blighters, these drum brakes.
paulh4

Hi Allen, you are right saying that the HB lever and cable should be at 90 degrees, but really the lever and cable shuld be at 90 degrees when the HB is fully applied,thats when you get maximun force into the HB mechanism, but I guess this never really happens.
With regards to rear brake friction material, everybody fits the cheapest crappiest liner they can find. The only time you ever get good quality brake material is when the vehicle is new as generally you do get a good quality product. The EU reg 90 was brought in to at least get both ends of any car somewhere close to the origonal equipment spec. It's a bloody minefield.
john wright

90 degrees is indeed the figure to aim for, but it is between the cable and the effective arm back to the pivot rather than simply the exposed bit of arm.
I've marked up my diagram in the attached.
90 degrees being physically impossible to achieve, it's a matter of get the lever as far back as you can.


Paul Walbran

Hi Paul, going back to when you mentioned that you riveted the ends of the liners, can you tell me what brake liners you are using for the auto tests. I imagine it's not normal cheap and nasty replacement crap.
john wright

Certainly not cheap, 2-3 times the price of off-the-shelf examples. I have the shoes re-lined here with a material of a type which I have been unsuccessful in obtaining a spec. It is one which is used as by those racing and so retains good fade resistance, but which has also really great friction when cold. It also has a characteristic greenish hue. Back in the '70s, when asbestos-based materials were the norm, I used AM4 linings. AM4 had exactly the same properties as above and the colour was similar if not the same, so I wouldn't be surprised if it was the same one. IIRC the VG95 racing material also was slightly green, but didn't have anywhere near the cold bite.

The extra friction does bias brake balance to the rear - good if rallying on gravel as I do (or rather did ... tiring of adjusting the brakes after every special stage, I fitted discs to the rear) - but for normal road use it would be prudent to go down a size in wheel cylinder.
Paul Walbran

Hi, Everything in both drums look fine, all connected up the right way. Is there a check for testing the cable; apart from renewal. Ta
Iwan Jones

disconnect the cable at both ends, it should move freely. If not, it is possible to retrieve it by working light lubricant/WD40 in from each end. Needs a helper to pull on one end while you pull on the other. Pull right through in one direction till it won't go any further, spray the exposed bit, pull right through as far as it will go the other way, spray that end, and keep on repeating until it comes right (or you give up and buy another .. )
One of you will have to be under the car working the tunnel end.
Paul Walbran

If you are going to undo both ends you might as well undo the nut that clamps the outer to the tunnel as well, then you can do it single-handed. Both mine have grease nipples, so if it moves at all tie off both ends of the inner and work the outer back and fore as Paul suggests, adding grease as you go. You can't just pump it in and hope it will work its way down the outer, as on mine at least it comes out of the join between the grease fitting and the spiral outer, so only takes a bit at each service.
paulh4

Paul Hunt has a lot to say about handbrake levers, here

http://www.mgb-stuff.org.uk/braketext.htm#levers

Herb
H J Adler

Hi, thanks pauls yep worked in the greas and all's well now. Ta
Iwan Jones

Hi,

Actually Paul's, the problem still persisted, very hot wheels. So followed Dave O'Neill's advice (yes I know you have it in your website Mr Hunt :)).

Anyway, this was the first weekend for a while that I've been able to spend fiddling about on things. I tried to fettle the drums, shoes etc. once again and still the problem persisted. Detatched the handbrake cable, greased and pulled it through (about 30cm in either direction) on two occasions. Still very hot.

A not so simple dismantling of the rear flexible cable, re-place and system bleed and hey presto ...... handbrake, cold wheels, hard braking :)

Ta All.

Iwan
Iwan Jones

This thread was discussed between 21/06/2016 and 17/07/2016

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