servo on rear drums as well as front discs???

[IslipMinor]

Try again,

Right, have now had a chance to look through the material that Willie sent me. It is a series of calculations produced by the Design Manager of the Brakes Division of Automotive Products, so should be a bit of an authority on the subject? The calculations allow you to find the front to rear brake balance for any scenario of disc or drum, the effect of a servo and the pedal pressure required for any of the scenarios. Also takes into account the ride height, passengers, luggage etc.

Have put all the formulae into Excel, and created a model that can work out the effects of changing one or more factors instantly!

A number of things have become very clear:

The original 7” all-round drum set-up was very biased towards the rear – under heavy (sic) braking the rears could lock up first. The bias is also double what the current vehicle design regulations allow. Pedal pressures were very high, even for the late 50’s and early 60’s.

Retaining the early larger rear brake cylinders will produce a similar result no matter what front brakes are fitted – 7/8/9” drums or discs (Midget or Marina).

The move to 8” fronts and the smaller 3/4” cylinders provided a much better balance to the system, and is the best of the drum brakes (fitting the Wolesley 9” drums is not so effective!!). Yes, larger diameter drums, but much smaller wheel cylinder bores, so not as much braking effort for the same line pressure. From the data I have, for a given pedal pressure you get more braking effort out of the Minor 8” than the Wolesley 9”. Can anyone confirm the Wolesley wheel cylinder as 0.8” diameter? Also does anyone know the Riley cylinder bore size (Girling)? The drums are the same diameter as the Wolesley, but wider shoes I believe?

Fitting either Midget or Marina discs increases the rear bias by a small amount (Marina are worse, despite being a larger disc (smaller caliper pistons) and both need more pedal pressure to get the same results as the 8” drums. The difference is that they work harder longer.

As far as fitting a servo is concerned, if the rear cylinders are the later smaller ones, then as the system is reasonably balanced, the servo should be plumbed into all four wheels – if not, the system will be much too biased to the front, and could be unstable under heavy braking, although better than locking up the rears early.

If the early, larger rear cylinders are retained, there is an argument to servo the fronts only, to produce a better system balance, but if this is done, fitting the smaller rear cylinders later does not look like a good idea.

Fittings discs makes no difference to the decision, as it is primarily dependent on which rear cylinders are fitted more than anything else.

Without a servo the pedal pressures, with the possible exception of the standard 8” front set-up, are very much higher than a current designed car. Fitting discs makes this worse, so a servo should be considered very seriously in this case.

If there are some figures available for the Ford discs, caliper piston diameter and distance from the centreline of the stub axle to the centreline of the caliper piston, I can work out how they compare to Midget and Marina. From the size of piston and disc it could redress the balance nicely?

All the comments about rear bias refer to a lightly loaded vehicle, just the driver and a full fuel tank, as this is when it is likely to cause a problem. As the number of passengers and/or luggage increases, so the need for rear braking effort increases, and the inherent rear bias is no longer an issue. What then becomes an issue is the sheer pedal pressure required to make it stop!

Running the model for a fully loaded car with 4 people and luggage gives a brake pedal pressure of over 150lb for the original 7” all-round system, and that to achieve the design standard of 0.87g. By comparison, today’s everyday car would need half, or maybe a third of the effort and produce close to 1.00g deceleration in good conditions – we drive around in the middle of them as well, Mmmm??

As always it is still an individual decision about what is best from them, but I hope this helps to explain something of what lies behind the design of the Minor braking system. If anyone would like the Excel model please send me your email address.

I run a 2.2:1 boost servo on front and rear, and the ‘feel’ is good, but definitely not ‘over light’. Lightly laden, even with the smaller rear cylinders, the system is still a bit too biased to the rear, so fitting a pressure limiting valve in the line to the rear brakes removes the problem. Would be nice to have one that is load sensitive – has anyone done it? I have fitted a Mini one, and converted it into adjustable (again please email me if you would like the details – it’s a very easy mod to do).

[bmcecosse]

The Wolsely wheel cylinders are exactly the same bore as the 8" Minor ones - for some mad reason they are slightly shorter - and yet it's an easy mod to fit the 8" Minor cylinders to the W backplates. I can confirm my Traveller - with some junk in the back - stops hard with all 4 wheels locked in a non-servo application without undue pedal force being required. Ok - it's more than my Meriva which has such stupidly light brakes that a wee fairy push throws everyone onto the seat belts - but the Minor is perfectly manageable - and so far has never shown any sign of fade despite some rigorous up and down hill stuff on realiability runs. On that same run in the past - the 8" drums were reeking horribly and had faded away to nothing!

[wibble_puppy]

*still avidly studying all your excellent replies, and my books and stuff*

[wibble_puppy]

I'm not quite sure if anyone has answered my two-part question on the previous page, if they have then I apologise and would be really grateful if someone could very very kindly spell out the replies

Here's the question again:

I have another pig-ignorant question, in two parts:

(part 1)

If you fit a servo to the front discs only

and if the purpose of a servo is to reduce the leg effort required to exert a certain amount of pressure on the brakes

then how much leg effort will you end up needing? That required for the fronts? That required for the rears? or something in between?

(part 2)

whatever the answer to part 1, does this mean that you might be applying too much or too little pressure to one or other sets of wheels?

[alex_holden]

does anyone know the Riley cylinder bore size (Girling)? The drums are the same diameter as the Wolesley, but wider shoes I believe?

I believe the Riley 1.5 front shoes are 9" x 2 1/4" and the part numbers of the cylinders are Girling 390314 and 390315, but I don't know any other specifications. I've agreed to buy a set but haven't got them in my hands yet.

[Willie]

Richard, a masterful translation of all those formulae in to English thanks.
The next time my mother-in-law wants a lift I'll make her supply her own small bore rear cylinders!

[picky]

ive got two sets of riley front brakes here does anybody want specific measurements taken of the cylinders etc?? one set sold to chrisd87, and the other set about to be fitted

[alex_holden]

ive got two sets of riley front brakes here does anybody want specific measurements taken of the cylinders etc?? one set sold to chrisd87, and the other set about to be fitted

Yes please, what diameter are the pistons?

Also, are the pipe fittings the same as in the minor cylinders, and if not how did you obtain a suitable pair of flexihoses?

[picky]

the pistons are just over 17mm diameter when measured with a caliper, so maybe 11/16"??

I have not got round to fitting either set to a minor before, but I have read that the threads are not the same as the minor. The riley hoses would of course fit the brakes, but whether they would fit the union on the car I am not sure. sorry I cant help furhter with that. if you do find a definate answer please let me know

[Packedup]

I have another pig-ignorant question, in two parts:

(part 1)

If you fit a servo to the front discs only

and if the purpose of a servo is to reduce the leg effort required to exert a certain amount of pressure on the brakes

then how much leg effort will you end up needing? That required for the fronts? That required for the rears? or something in between?

You'll use as much leg effort as is required to stop. Hopefully.

It's better to stop with the fronts doing by far the majority of the work than the rears, as too much bias to the rear will increase stopping distance and most likely lead to a lock up - Fun to counter steer out of once in a while, but not good for normal safe driving!

So that answer is a servo will increase the braking to whichever wheels it's plumbed into. Whether that should be just fronts or all four is dependant on front type and size, and rear type and size. And vehicle weight distribution, etc.

I see no problems with running the fronts only through the servo as discs need a higher pedal pressure to bite as well (I've driven the Triumph equivalent of the Marina discs without a servo, and they were rather heavy) and the fronts do most of your stopping. I also however see no problem running on all four as I've driven cars that have come out of the factory like that (with no limiter and basically an identical system to earlier, non servo'd models) and been fine, and people on here have run their brakes that way without problem too.

If you do run on all four, it might be an idea to get an adjustable bias valve though - That way if you do find you've got too much rear braking you can tweak it without having to mess around replumbing the system

[alex_holden]

the pistons are just over 17mm diameter when measured with a caliper, so maybe 11/16"??

Interesting, that's quite a bit smaller than the 8" Minor front drums. If my calculations are correct the standard Minor front drums should be about 1.86x more powerful than the Riley 1.5 front drums for a given pedal force. That's a bigger difference than I expected.

Did the Riley 1.5 have a servo? Or a smaller master cylinder bore?

Edit: it just occurred to me that I didn't take into account the extra leverage given by the larger drum, so the difference won't be quite that bad. More like 1.65x I think.

[chrisd87]

Hopefully BMC will be able to tell us about the hoses

The Riley hoses are available new, along with lots of other components from a company called Wolseley 1500 spares:

http://www.users.dircon.co.uk/~sandy/wo ... rakes.html

In any case there's probably a company somewhere who could custon-make a set with the correct ends.

[wibble_puppy]

thank you very much for addressing my question, lovely lovely Packedup - you're lovely I really appreciate you keeping this thread on track

Where would I get an "adjustable bias valve" from?

[alex_holden]

Hopefully BMC will be able to tell us about the hoses

Where's Wanderingstar when you need him? Actually I have his email address somewhere if we get stuck...

[Packedup]

Demon Tweeks have some. Not cheap but at least it'll give you an idea what they look like if nothing else

[wibble_puppy]

dear lord!

*is determined to work out a system which doesn't require a bias valve*

thanks dude

[bmcecosse]

Wanderinstar is indeed the lad you need - but he did tell me that he had to change the flexi hoses - where-as the Wolseley brakes fit straight on to the Minor flexis. I am guessing the cylinders will have the same threads as the rear Minor cylinders - ie course thread rather than fine thread. I did have some pressure limiting valves (bought one on ebay and was offered several more by the seller) - but i think they are all gone now. They can be made adjustable - but as it happens they are just right for a Mini as they stand, so I haven't bothered with the adjustable bit!

[IslipMinor]

bmcecosse,

Have re-run the model using the same size wheel cylinders as the Minor and the results are much different. Very reasonable pedal pressures and so long as the EARLY larger rear cylinders are used, good front/rear balance.

Because the fronts are now MUCH better, using the later rear cylinders on 7" Minor brakes causes much too much front bias. The Wolesley was fitted with the smaller rear cylinders, but 8" brakes, so the balance is restored.

Interestingly the cylinder bore of 0.8" comes from the official AP 'bible', and is quoted for the Wolesley 1500 MkIII - it doesn't go back to the earlier models. Did they change the size for the later models?

[bigginger]

formulae

Off topic, but just to say THANK YOU!
a

[picky]

regarding rear wheels locking up before the front brakes are at full braking potential.

If a car was known to have bad rear wheel lock up, what effect would this have: adjusting the rear brake adjusters to move the shoes further from the drums??

just a thought,
Picky