Why is my front brake still useless?

[Triton Thrasher]

The oft quoted claim that the leading shoe does 60-70% of the work is, quite simply, wrong. Once bedded in, both shoes do equal work..

So a twin trailing shoe brake would be as good as a twin leading shoe brake?

[PrewarMG]

Clearly not. But a twin trailing shoe would be pretty much the same as a leading/trailing shoe setup.

[Adler]

Hello PrewarMG,
very interesting - please keep us up to date with your practical experiences.
Which books are you having at hand?
Thanks in advance.
Best regards, Manfred

[PrewarMG]

Hi Manfred,
Thank you for your interest in my little four-wheel project.

Some suggestions for further reading 

Probably  the definitive book in the UK on the theory and practice of brakes is Braking of Road Vehicles by Andrew Day, published in 2014. Andrew Day is the Emeritus Professor of Engineering at Bradford University where he established the Braking Research Centre which is today the leading authority in the UK on braking systems research and development. At around 450 pages, it's not for the faint-hearted but the sections on drum brakes do cover some of the vagaries of fixed cam systems as these are still used today in heavy truck brakes, albeit with S-cams and not the flat cams we're familiar with.

An earlier book of the same title by by Newcomb and Spurr published in 1967, at about one third the size is slightly easier to digest, but in consequence, is not as comprehensive.

However, I also sourced quite a number of technical papers presented to the Institution of Automobile Engineers in the 1930s and 1940s which cover the transition from pivoted shoes to sliding shoes which occurred in the 1940s. A couple of these are by F.A. Stepney Acres who was widely regraded in those days as one of the gurus of brake design and development.

Initially I also used Fundamentals of MOTOR VEHICLE Technology by V.A.W. Hillier and Advanced vehicle technology by Heinz Heisler but while these were useful for getting to grips with the basics, they lacked the analytical detail of Day's book and I rarely look at them now. There are also some useful little nuggets in A Technical History of the Motor Car also by Newcomb and Spurr, published, I think, in 1980

Regards,

Simon

[muskrat]

G'day Simon.
Very informative, thank you.
I googled Mr Days book. It can be downloaded through Google Books for $115AU
https://tinyurl.com/ys89dprw
Cheers

[BagONails]

Clearly not. But a twin trailing shoe would be pretty much the same as a leading/trailing shoe setup.

Good information Simon, very interesting and thanks for your contribution, however, I'm finding it hard to accept your assertion above, if I've understood what you were explaining correctly...

A twin trailing shoe brake would have no self servo effect as I see it whereas a conventional leading/trailing shoe set up at least benefits from the servo effect on the leading shoe does it not?  Therefore how can the performance be similar? Equally I think we would all agree and expect from theory and practice that a twin leading shoe set up would be the best performer for the same reasons would it not? (all other aspects of design being equal of course.)

Confusedly Yours 
BoN

[PrewarMG]

If both shoes do equal work - which they do - then the leading shoe has the same braking performance as the trailing shoe and therefore twin trailing shoes would have the same performance as a leading and trailing shoe.   

The shoe factor, i.e. the braking performance, of the leading shoe in a leading/trailing setup is the same as the shoe factor for the trailing shoe - hence the equal work equilibrium. The effect of the self servo is to reduce the force on the leading shoe from the cam while still providing the same braking performance. The fixed cam provides equal displacement but not necessarily equal force. The self servo effectively 'lifts' the shoe slightly off the cam, reducing the force from the cam and the resulting reaction to the force applied to the cam by the lever is taken by the cam bush.

And yes, of course, twin leading would be much, much better as both shoes would be able to benefit fully from the self servo. It seems to me to be quite feasible to convert my brakes to twin leading as the pivot post is held in a bronze bush the same as the cam so two single lobe cams could easily be fitted. The biggest problem is how to operate them. A cable pulling the levers together would be possible, but there’d be twice the pedal travel and shortening the brake levers could be tricky. Work in progress!

[berger]

Simon Hilliers book is the one i have from the 90's at tech . i still don't fully understand steering geometry though ----thick me.

[PrewarMG]

 

[Triton Thrasher]

The effect of the self servo is to reduce the force on the leading shoe from the cam while still providing the same braking performance. The fixed cam provides equal displacement but not necessarily equal force. The self servo effectively 'lifts' the shoe slightly off the cam, reducing the force from the cam and the resulting reaction to the force applied to the cam by the lever is taken by the cam bush.

I’d say that theoretical description is compromised by absence of absolute rigidity in various components and fixings.  If the leading shoe is tending to be lifted off the cam, the cam is tending to follow it, using up the bush clearance and flexing the backplate, so maintaining force on the leading shoe.

[PrewarMG]

Indeed, I’d agree that any tolerance at all in the bush will effectively allow the cam to 'float' and thus the leading shoe will benefit from the additional self servo.  But once that tolerance is used up by wear in the leading shoe lining, you’re back to equal displacement and equal work.

This is the standard theoretical analysis of fixed cam brakes. A mechanical system with cables and levers and bushes is always going to be to some degree different in practice than the theory might suggest. But the basic principle remains, and was recognised nearly a hundred years ago in theoretical papers - as one paper written in the late 1920s put it “ Servo shoe wears more rapidly until each shoe does equal work, then both wear equally.”

[Triton Thrasher]

once that tolerance is used up by wear in the leading shoe lining, you’re back to equal displacement and equal work.

We centre the backplate, obviously, if the braking effect appears to be diminishing.

Some fanatics even swap the shoes over.

[chaterlea25]

H Simon (MG)
As you have just joined the Forum an introduction post regarging your A7/10 and riding history would be most welcome
As to the practicalities of "twin trailing" shoe brakes  All I can offer is the experiences with the Taylor Dow TLS brake on my Super Rocket and memories of some other bikes I rode back in the day.
The Brake on my SR is excellent when riding the bike, It is possible to lock the front wheel if too much hand pressure is applied, however the brake will not hold the bikes weight when wheeling the bike up the ramp loading into or out of my van (opposite rotation)

John

[PrewarMG]

John,
As I said, my reason for joining the Forum was that, having researched the topic of fixed cam, SLS brakes for my M.G and in doing so discovered the similarities between the M.G. and Triumph brakes, and having subsequently adapted Triumph motorbike brake shoes to fit the car, I was so appalled at the vast amount of misinformation in this thread that I felt some basic facts needed to be stated lest others might believe the nonsense that had been written.

Regarding my A7/A10 experience, I have never ridden a motorbike of any sort and only, three times I think, have I ever been on a powered two-wheeler. Once, as a very small boy on an uncle's motorbike, once as a student on the back of a scooter, and once a few years later on the back of one of those little Honda thingies.

As for twin trailing shoes, they did exist - Jaguar fitted them to the Mk VII model in the mid-1950s but they were complemented by massive vacuum servo assistance. Their advantage was smoothness of operation and complete stability with no tendency to grab whatsoever. But not very practical on a motor bike (or a pre-war M.G.!)

[Triton Thrasher]

As for twin trailing shoes, they did exist - Jaguar fitted them to the Mk VII model in the mid-1950s but they were complemented by massive vacuum servo assistance..!)

Perhaps they needed “massive vacuum” servo assistance because twin trailing shoe brakes are not really as effective as single leading shoe brakes?