Imbalanced rear brakes: causes and cures?

[bigmalcy]

Hi fellas

Next issue for which I’m in need of your wisdom!

 

Our 1960 3A failed its MoT on Monday due to imbalanced rear brakes.

 

Apparently the rules were changed recently with regards to brake imbalance, and if they hadn’t changed, she would have sailed through.

 

Anyway – here’s the story:

When the brakes are applied and the braking force is measured on both rear wheels, the braking force comes on evenly (both wheel applies same braking force).

 

When each wheel is tested individually to assess its ultimate braking force, the LHS locked the rollers up at a rotating force of 110kg (roughly, not sure about units), while the RHS only managed 50 or even less. Hence the imbalance and MoT failure. Handbrake application yielded a similar result.

 

So – off with the (old) drums, sanded them, sanded the (brand new) shoes, swapped the drums from one side to the other, re-adjusted the shoes and back down to the test station.

 

Same result – almost exactly the same, actually (i.e. LHS strong, RHS weaker).

 

Then took the drum off the RHS again, and looked inside. Noted that the witness marks from the shoes appeared to be only thin ‘slivers’ running around the drum circumference.

 

Could this be the answer – that the drums are in need of machining to take out imperfections, and increase the area of contact with the shoes?

 

I think it could be – but the fact that we swapped the drums and got the same answer (i.e. swapping drums didn’t swap results) suggests to me that an uneven drum may not be the cause of the issue.

 

Brakes have been rebuilt with new cylinders, new shoes, new springs, old adjusters.

 

Part of me thinks that a few hundred miles of bedding in could also cure it, but it’s difficult to do that without an MoT!

 

Thanks in advance for your input...

 

Malcolm

[Guest ntc]

Malcolm

You need to chamfer the edge on your new shoes

[TR Mitch]

Part of me thinks that a few hundred miles of bedding in could also cure it, but its difficult to do that without an MoT!

 

As well as Neil's tip, you could pop her on axle stands and leave it running in second gear with light pressure on the brake pedal for 20 mins (and fans on the drums!) to bed them in nicely. Or take the long way round to the pre booked MOT.

Edited September 17, 2013 by TR Mitch

[bigmalcy]

Thanks guys - will chamfer the shoes... also having the drums done by a friend FOC to see if that helps.

 

As for the bedding in in the garage idea - i'd thought of that one already, but the rebuilt engine's already had it's 20mins (and more) of cam break in time, so i'm wary of having it run for much longer without putting it under proper load.

[Alec Pringle]

Sounds like it might well be a hydraulic problem - faulty hose, t-piece, pipe or connector for example, such that one side is getting less pressure than t'other. I can remember a similar problem that turned out to be a tiny piece of rubber stuck in the t-piece restricting flow to one side, and another instance resulting from a kinked pipe.

 

Cheers

 

Alec

[Guest ntc]

The handbrake results still tell's me new shoes as many are to wide

[bigmalcy]

Alec - everything is new apart from the t-piece on the axle, so I don't think it's a hydraulic issue. As Neil points out, the handbrake gets the same results.

 

Neil - when you say chamfer, are you talking about sanding a midgies d**k off the edges of the friction material, or grinding off a bigger portion of the edge of the shoe itself (friction material + metal)?

 

Still not sure why it would be affecting one side rather than the other (unless some shoes are wider than others).

[Guest ntc]

The outer edge of the shoes lining need to be angled slightly as the shaft's can be shimmed differently

[AlanT]

If this is like a 4A then the cylinder isn't sliding freely in the back-plate.

 

It does this on both hand-brake and hydraulic operation.

 

So you only get pressure on one shoe.

[Guest ntc]

Pressed together he has brakes ,work it out

[Alec Pringle]

Good point Alan, I was overlooking the handbrake performance

 

Cheers

 

Alec

[mike ellis]

What a coincidence, same day same problem except that mine is a '58 car therefore it was a 'non MoT' and it did meet the new requirements but only just.

 

Looking at the top shoe on RHS it is clear that only the front and rear of the lining has been contacting the drum. Haven't done any more yet as it is p*ssing down here.

[ianc]

Alan could be right.

At the rear, one shoe leads, the other trails. When travelling in the forward direction, the leading shoe applies a much greater braking effect.

If the slave cylinder is jamming in the backplate (it is supposed to slide so as to apply equal force to both shoes), then it may be that only one of the shoes is actually making contact with the drum, and if that is the trailing shoe, the braking effect could well be less than 50% of that seen on the LHS.

 

To free the cylinder on the backplate, the 3 pieces of metal (known as Abutment Plate, Spring plate - retaining, and Spring plate) need to be removed (as described in the Workshop Manual), everything cleaned and a very light smear of Copaslip applied to the backplate prior to re-assembly. Bit of a fiddly job, but if I can manage it (and I did in November, when I had to replace a slave cylinder in order to be able to get 4VC to Brum for the NEC Show), then it should not be a problem for younger folk who may not have vari-focal specs!

 

By the way, the description and diagram of the dismantling and re-fitting of the slave cylinder in the TR2's WSM is far superior to that in the TR4's WSM, which is why I have made a copy for my TR4 WSM.

 

Ian Cornish

[AlanT]

Thinking about this, at first it seemed that the handbrake will only ever work on one shoe.

But there is a bit more to this than I rememered right away.

 

The lever that is pulled by the handbrake cable has a pivot pin that mates into the cylinder.

So in theory the lever moves both one shoe directly and the other via the cylinder.

 

Because of the angles and friction involved this probably accounts for some of the well known weak handbrake problem.

 

Anyway just put a finger on the bleed nipple while someone works the handbrake and see if the poor old cylinder can actually move.

[peejay4A]

But as Neil pointed out, when both wheels are braking together it's in balance. Why then is individual testing different? Different friction between tyres and rollers?

[AlanT]

Nowhere does he say that he can get 110kgs when testing both wheels.

He just says they are even enough to pass.

 

The problem is revealed only when applying the maximum load.

[peejay4A]

He doesn't say they're even enough to pass but that they brake evenly when both wheels rotate. Is it the case that braking is non linear between the wheels, varying with pedal force?

[bigmalcy]

Guys

Braking force is equal between wheels when both wheels are being measured simultaneously.

 

The difference is that one wheel is capable of braking at a much higher force than the other one (so if enough traction is available on the road and the pedal is pressed hard enough, the weaker wheel will never lock up, while the other one will (or might).

 

Both brakes have been rebuilt and adjusted in the same way - and I confess at this stage that I only have two plates behind each cylinder (one spring plate and one locking plate). When I assembled them, i took the view that there simply wasn't enough room to install them and to do so would have held the cylinders in place very, very tightly.

 

We are having the drums machined, and I will also chamfer the shoes as suggested. It also occurs to me that perhaps the RHS has some air left in the system, although i can't quite figure how this would influence the handbrake results as well as the footbrake.

 

Thanks again for the input!

 

Oh - and while I haven't confirmed it, I doubt that the cylinder is jammed - it has only recently been fitted and with all that hydraulic force pressing on the shoe, it would need to be b****y tight to not yield. Having said that, I'll check it out and lube it as recommended. I assume that the movement is minimal, given that there is a solid brake pipe connected to it.

Edited September 18, 2013 by bigmalcy

[BlueTR3A-5EKT]

Assumptions - It has 9" Girling brakes.

 

Couple of points.

Firstly, for the conspiracy theorists. Did anyone read this article about testing at Council MOT stations that have no repair facilities therefore no driving ambition to empty your wallet.

http://www.moneysavingexpert.com/travel/cheap-mot?utm_source=MSE_Newsletter&utm_medium=bignote-four&utm_term=24-Sep-13-v1&utm_campaign=travel&utm_content=5#mapfinder

 

Secondly - Tell them it has an LSD and it will be damaged/give odd results on rollers with an independent brakes test. That will bring out the Tapley Meter and the tester will go off for a spin in your car - just like the old days. My brother has this happen every time he goes for an MOT.

 

Thirdly which may be irrelevant - The pivot pin of the handbrake lever that nestles between the casting of the cylinder and the brake back plate wear a nice cylindrical groove in the backplate - could this affect things? Would certainly cause some resistance to the cylinder sliding. I have recently had that worn area welded up and smoothed off on a pair of 9" backplates for a Girling axle.

 

Just a thought.

 

Peter W

Edited September 30, 2013 by BlueTR3A-5EKT

[bigmalcy]

Peter - thanks for those points... can the LSD affect the results? If so, can you explain why?

[BlueTR3A-5EKT]

LSD stops or reduces the ability of the axle differential to slip.

Link http://www.howstuffworks.com/differential4.htm

Makes your car try to go straight on at corners (which many racers do)

 

So, with an LSD if you stop one driving wheel the other will stop rotating too until the slip torque is achieved. With a regular differential if you stop one wheel the other keeps rotating freely. With a single wheel brake test the tested wheel is still being driven by the other wheel if you have an LSD.

 

In cornering if you lift one wheel the other wheel will remain driving if you have an LSD if you have a regular differential t the drive is lost till both the wheels are in contact with the road again.

 

Note for Alec. The old scruntineer's trick of jacking one side of a competition car running in a 'Production car class with no LSD' and asking the driver to start his engine, engage gear and let in the clutch to see if the car would leap off the jack used to put pay to the cheats or damaged their bodywork - until of course the Torsen diff arrived.

 

Cheers

Peter W

Edited September 25, 2013 by BlueTR3A-5EKT

[bigmalcy]

Peter - a big thanks... I did as you suggested and it took all of 2 mins to get out, spin the dial and get back for the 'Pass' slip from the MOT station. If only the fella had told us about this option 2 weeks ago!

 

Thanks again!

[AlanT]

Do I understad that you actually do have an LSD and this is why the test failed,

 

OR

 

you have a normal diff and just persuaded the tester to use a Tapley meter?

[bigmalcy]

Second option, Alan.

[BlueTR3A-5EKT]

Peter - a big thanks... I did as you suggested and it took all of 2 mins to get out, spin the dial and get back for the 'Pass' slip from the MOT station. If only the fella had told us about this option 2 weeks ago!

 

Thanks again!

These cars are for driving not looking at... I suspect there will be a lot of TR's suddenly being fitted with limited slip diffs now.

 

Explanation of the thing that got you through your MOT.

http://www.tapley.org.uk/howdoesitwork.html

 

Cheers

Peter W

 

PS This little lot makes good reading, especially the statement on 'vehicles not to be tested on roller brake testers'

http://www.ukmot.com/3-7.asp

 

PPS. Edited as I feel my comment was flippant. Safety and brakes go hand in hand and should not be compromised.

Edited September 30, 2013 by BlueTR3A-5EKT