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MG Midget and Sprite Technical - Rear brake advice

Two points:

1) The left rear brake was kaput because the slave leaked fluid all over it (what a mess). I have replaced the slave with a new one. I've checked the right-hand rear brake and it seems OK (no leaks, no contamination). Should I leave well alone (ain't broke so don't fix)or should I fit a new slave cylinder to ensure balance between left and right (NB, I am fitting new brake shoes on both sides for balance)?

2) This is less a question than an observation. There has been much discussion on this BB concerning which way round brake shoes should fit. The brake shoes I just bought from a well known MG part supplier do not have any leading or trailing edge as the brake material is applied across the whole shoe with about 1 cm of bare metal at each end. As a result it does not matter which way round they are fitted. Is this likely to be a problem?

Hopefully back on the road soon after a major brake refurb (front and rear).

NB I have no idea how this car got an MoT with brakes the way they were!!

Chris
Chris Hasluck

You can take a bet that if one side has gone the other will follow soon, so if your changing the shoes, you might aswell change or refurb the cylinder on the other side aswell.

As for the shoes, someone else posted on this subject a few months ago, so if you can't find it in the archives, someone will come along with the answer. I got my last shoes from the MGOC a couple of years ago and thet had the correct trailing and leading edge configuration.
P Ottewell

That might have been me. I bought them from a supplier whose name sounds like TOSS. I took them back. A few weeks later I bought another set from the same supplier and these were ok. They seem to come in batches and the buyers don't have the knowledge to know any different.

They will work but won't be quite as efficient as they should be. It could be the difference between an MOT pass or fail if the rest of the braking system is aged.

Rob
Rob aka MG Moneypit

I bought my shoes from Abingdon MG/David Manners in Oldbury/Birmingham. I previously used shoes supplied by Peter May but they cost around £40 or 4 times the cost of standard shoes.

I'm not really sure why they would be less efficient because of not having a leading edge. I do want them to be good, however, as I've just fitted 240mm ventilated disks with 4-pot callipers on the front.

Chris
Chris Hasluck

I would be inclined to reject them.

They were designed as leading/trailing for a reason.

Minispares list 'genuine' shoes for £12.36 - although they appear to be out of stock - or a cheaper set for £9.30

http://www.minispares.com/product/Classic/Brakes/Shoes/Shoes/GBS834AF.aspx?040602&ReturnUrl=/shop/classic/Brakes/Shoes/Shoes.aspxBack to shop
Dave O'Neill 2

Cue nigel....

Nigel has a great illustration of how the shoes fit

Prop
Prop and the Blackhole Midget

Chris, what have you done to restore brake balance after fitting the bigger brakes at the front?
Paul Walbran

Paul - an interesting point that has concerned me but I can't see a way to restore balance while retaining the standard drum brakes. I'm hoping that since most of the braking is done from the front this will not be too much of an issue.

Re the shoes, my local motor parts supplier said this morning that they had Quintin Hazell shoes in stock but when I examined them they were just like the others in terms of the friction material (despite a picture on the box of the correct linings). Anyway I managed to order some of the shoes mentioned by Dave O'Neill from Minispares. Out of stock last night but in stock this morning (odd!).

Chris
Chris Hasluck

Chris,
the stock situation would probably just be an update on the computer/web site

as Prop has mentioned it below is the illustration I picked up from someone who posted it on here, I take it as correct as no one has said otherwise


Nigel Atkins

Nigel and Prop - thanks for the illustration although I already have a copy. Knowing which way to fit the brake shoes is not the problem - I already know the correct way round. My problem is the difficulty of obtaining shoes that have the friction material applied in the right place (i.e. not along the entire length of the shoe).
Chris Hasluck

fair enough, better to have the info twice than not at all
Nigel Atkins

Nigel. You are right - and I hope I didn't sound churlish. Many thanks for taking the trouble to send the diagram.
Chris Hasluck

Chris,
no, no problem, I only put it up because Prop mentioned it and he's never led me astray before :)
Nigel Atkins

I received my 'genuine' brake shoes from Minispares in less than 24 hours (good service!) and they seem OK. They came in a Unipart box and have the correct pattern of friction material, leading edge and all!!

While I'm reluctant to reopen the debate about the fitting of brake shoes I could not help but notice that the picture of the shoes on the box while accurate in terms of the linings shows the shoes fitted such that the two leading edges (the end of the shoe with less material on them) are both at the top and facing one another. The box says the shoes are for various models of Mini and Metros but have the same part number as Spridgets. I have no idea what to make of that but I will be fitting my shoes as per the drawing that Nigel sent and which seems to be the consensus.

Chris
Chris Hasluck

Chris

The relationship of the direction of forward travel, the location of the "missing" shoe material, and the brake cylinder are the same in the MGB as in the pic that Nigel posted. It is a little hard to see it as being the same since the B cylinder is near the bottom rather than the top. I believe you can see pics of the B brakes on Paul Hunts web site.

Charley
C R Huff

This is the pic from the BMC workshop manual for the B...


Dave O'Neill 2

Can someone give a clear explanation of the geometry of how the positioning of the friction material on the metal shoe effects the performance? I presume that it is to do with the leverage effect of the shoe pivoting on the fixed (adjustable)ends as they are moved by the piston.

But if this were the case, then the double ended cylinder arrangement on the rear of a Sprite would give one leading, and one trailing shoe. However, this does rather fit with the definition used on pre-war drum braked cars when an innovation for improved braking was the introduction of what were known as the "twin leading shoe" brakes. These having two separate operating cylinders such that both shoes pivoted with a leading edge moved by the cylinder. As the shoe is pivoted against the drum the "leading edge" contacts first, and the rotating drum grips this and pulls it into closer contact, improving braking efficiency. However I don't see how this can operate in the same way on brakes where the cylinder operates on the trailing end of one shoe and the leading end of the other.

Anyone good with words and the physics of the operation?
Guy Weller

here is picture from 1500 manual



mark 1500 Lights on at the end of the tunnel

Guy, you pretty well covered it.

The rear drums are not twin leading shoes. There is only one cylinder, so the leading edge of one and the trailing of the other contacts first. This is a good thing on a rear brake because you want the parking brake to work just as well when the car is parked either way on a hill.

The cut back of the material at the leading edges of both shoes helps the braking effect in the forward direction, to compensate, a little bit, for the fact they are not twin leading shoes.


Here is a link with more detail:
http://mgaguru.com/mgtech/brakes/bt121.htm



Norm
Norm Kerr

And just to confirm matters, here is a diagram from an MG Metro manual. The manual clearly states that "The leading edges must face in the opposite direction to the forward movement of the drum" and refers to the diagram attached. The Metro rear brake seems to be an upside down version of the midget rear brake and if you stand it on its head it is the same as Nigel's.


Chris Hasluck

Am I missing something here? (which is more than possible). If the shoes are manufactured with the lining towards one end and the manufacturers have put it at the correct end then the shoes can't be assembled incorrectly. Why there is a gap at one end I don't know. Possibly cost saving. The wear is evened out better top to bottom on sliding shoes such as Girling or Lockheed than on say old Citroens which have a pivot pin at one end. For a bit of background in response to some comments above (apologies up front for those who know). the leading shoe does most work as it has what Girling called a servo effect as the rotation pulls the shoe onto the drum. This is why they are both leading on the front. Ages ago, some comapanies such as RR would use only trailing shoes as they gave a smoother but weaker performance, hence they made them much bigger
W Clough

Chris, you'll need to restore brake balance or the bigger front brakes will actually reduce maximum braking effort - though they will be more fade resistant of course.

The reason it reduces max effort is that more bite on the front brakes will cause them to lock earlier, at which point the rear brakes won't have reached their maximum. The effect is quite marked even though most braking is on the front. The actual amount will depend on how much weight transfer there is which in turn depends on the grip of the surface. On grippy surfaces (more transfer) the reduction in max will be less than low grip surfaces such as wet tarmac - so bad weather will be a problem.

240mm discs represent a big increase in braking torque - about 50% due to the increase in mean diameter of the swept area, plus or minus whatever the change in area is of the pistons, so it will be a good idea to do something about it.

We restored the balance by the use of bigger rear brakes and twin master cylinders (one front, the other back) with the pedal acting on a bar between the two which is adjustable so the bias between the two can be set to optimum. It's great on rallies when foing from tarmac to gravel or even mud. It's also great on the track when it rains, worth lots of BHP! (Pedal box pic attached, it bolts directly to the existing mountings and fits the existing hole in the firewall)

That one is tricky and expensive to set up (though so are 240mm brakes and 4 pot calipers - we have them on our Midget too) so a cheaper alternative is to go to bigger drums, such as Morris Mior 8", on the rear. We went further and used 10" drums on the rear because we wanted an awesome handbrake, and interstingly the balance bar is set right in the middle so acting equally on both master cylinders.

Fortunately there is an even simpler solution. The Midget rear wheel cylinders had the same body as a number of other cylinders which varied only in bore size:
Midget GWC1102 3/4"
Metro 11/16"
MGB GWC1103 13/16"
BGT GWC1122 7/8"

There is also a 15/16" version and IIRC a 5/8 version.
Going up a size or two in cylinder should restore the balance. Your front brakes should lock just (and only just) before the back if you want to optimise braking.

Over here, if brakes are modified there is an on-road test that has to be passed: 5 stops in quick succession from 100km/h, all within 4.6 seconds or less and without locking the brakes. 4.6 seconds is fairly lenient in my view, though for some the 5th stop becomes a challenge without locking. When I fitted the discs to the back of my MGB I did all 5 in 3 seconds or less, the last being the best (mainly because I was easing up at the end for the earlier ones) and equivalent to 0.98g. That gives you good targets to measure against to see whether you have an issue or not.


Paul Walbran

Guy, here's another VERY good explanation, that I've linked to before. You must have missed it. Click on "Drum Brakes". But the whole site is a good read for information on brakes.

http://www.engineeringinspiration.co.uk/brakes.html
Lawrence Slater

This thread was discussed between 12/06/2014 and 24/06/2014

MG Midget and Sprite Technical index

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