Misc: Everything Else Brakes, Linings, Drums and Shoes

davidd

Well Known and Active Forum User
VOC Member
Brake linings contain many odd materials to make them work better and I don't think that they all age at the same rate. I tend to look at linings like tires: fresh ones seem to work better. Additionally, adhesive technology has improved considerably over the decades.

When the friction of a lining is increased it makes the brake slightly less stable. Squeal is one of those problems that arises from improving the mu. Squeal, being a result of vibration, can cause mechanical destruction of the springs or the hooks. The lining itself tends to wiggle around and vibrate, which is why we chamfer the leading edge of a new, tall lining. You don't have to chamfer worn linings because they have lost enough mass to be much more stable.

Manufacturers tend to "detune" the rear brake to prevent the rear wheel from steering the bike by stepping out sideways during heavy rear braking. Vincent detuned the rear brake by activating the trailing shoe most. On the front brakes, it is the other way around. the leading shoes are activated most.

With the detuned rear brake, the AM4 linings cannot get the bite they need to cause squealing. Almost all brake linings are hygroscopic, so they will absorb water if they are left idle for a period. This will also increase the friction of the lining which results in that early morning squeal or harshness that disappears quickly with a little heat.

David
 

Albervin

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VOC Member
Interesting thoughts here. I have always been able to get maximum retardation out of my rear AM4 brakes on the Rapide. Almost lock up levels on dry tarmac. I can still barely get decent power on the front. Yes, I have tried truing, centring etc. My next step is different compound on leading and trailing shoes. All good fun.
 

vibrac

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VOC Member
I must admit that the rear on the Comet racer was if anything too fierce coming down through the gears to Cadwell club circuit hairpin (in the days when the club ran meetings :rolleyes:) you had to be gentle on the rear or the whole lot would lock I have sorted something softer for the twin racer rear (Its one drum as well)
 

timetraveller

Well Known and Active Forum User
VOC Member
I have failed to understand something important in David's remarks. On the front, the brakes are handed and yet both activating levers point forwards. So I do not understand the bit about activating the trailing shoe more or less. Similarly with the rear. On 'B's and 'C's there are two handed brakes and on the D the lever is upside down compare with the same side on the earlier bikes. Sorry if my brain has ceased to function. If necessary I can blame the lock down.
 

bmetcalf

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VOC Member
When the cam rotates, it either contacts the H48 plate at the outer or inner edge. The outer edge contact spot is ~1" farther from the shoe pivot point and has that bit more leverage to press the shoe against the drum.
 

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Statement.
Vincent detuned the rear brake by activating the trailing shoe most. On the front brakes, it is the other way around. the leading shoes are activated most.
David my handsome,
Please explain the detuning of brakes? As far as I know, the cam operates the leading and trailing shoes simultaneously. Top shoes at the back are leading and lows are trailing.
Front lower are leading and forward uppers are trailing.
bananaman
 

davidd

Well Known and Active Forum User
VOC Member
Just as Bruce has indicated, when using a flat cam the brake lever turns in one direction. This means that the cam hits one shoe high on the H48 and the other shoe activated by being struck low on the H48. The shoe that is struck low on the H48 is pushed further than the shoe that is being struck high on the H48. The designer has a choice to have the leading shoe or the trailing shoe move further simply by deciding which way the brake arm is pulled.

I have been told that the rule is if the brake arm is activated in the direction of wheel rotation that the trailing shoe is moving the furthest. If the brake arm is moving in the opposite direction of the wheel rotation, the leading shoe is moving the furthest. I think this rule should at least help with the "handed" question. Phil Irving was a proponent of having the front leading shoe activated by hitting low on the H48. The reasoning was the desire to give the leading shoe the benefit of the mechanical advantage. On the Vincent, the brake lever moves opposite to the rotation of the tire, so it fits the rule.

The rear brake, on the other hand, has the brake lever moving in the same direction as the rotation of the tire. Thus, the trailing shoe is being activated to move the furthest. This will not provide the same braking as the front shoes. I think if you note how the cam is turning you will find it is pushing the lower shoe on the lower edge of the cam, which is the trailing shoe.

On some series Ds with one brake, the brake arm lever is pointing down. Thus, it is activating the top shoe (the leading shoe) on the rear to move the furthest. The brake arm is now moving in the opposite direction of the tire rotation.

David
 

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Thank you, David, my handsome, a good explanation. Question:- Both my "A" & "B" Raps have model "P" braking system at the back,(this giving equal pull like the front brake), will it work better if the torque arms hang down like a "D"?? when it was done I had double the movement on the brake pedal so cut and shortened the brake arms by an inch.
bananaman
 

Matty

Well Known and Active Forum User
VOC Member
My comet brakes have always been bad ever since I bought it in 1956.
It originally had the old green (Mintex?) linings which were not too bad if I remember but since then I have tried 4 or 5 different makes of professionally fitted on linings and have also tried the following tricks.
The brakes however seem to work well once or twice in the morning but then just fade away - which I had put down to a fine layer of rust, - but this happens every day !!
1. Had oversized linings turned to fit the drums - no improvement.
2. Cut about a quarter of the trailing shoe lining off, reasoning that the leading shoe wears most just leaving the pretty ineffective trailing shoe to push on the drum. - this also means there is more leverage on the trailing shoe because the metal shoe is longer than the lining and perhaps the shorter trailing shoe wears faster. This gives a small improvement.
3. Tried different brake reliners who promised their linings would fix it. This did not work.

I know that the brake arms are set to operate at near 90degrees and the cables, compensation arm etc are all set to optimum and have rebuilt and owned many bikes both British, German, Italian, and Japanese over the last 60+ years and have only ever had poor brakes with the Comet in all the 100,000 miles I have done on it.
I am surprised that the forum states that the brake cam has a different lift depending on which way it is operated, I have never noticed this which could account for my problem if the cams are reversed - will go out now, take the front wheel out and have a look but have also had thoughts in the past of making the cams shorter to give more leverage but obviously more movement at the handlebar lever.
I have been through this before on the forum many years ago but will have another go when the Corona virus shutdown is in operation, or will use the Fireblade when I really want to stop !!
Matty
 

Matty

Well Known and Active Forum User
VOC Member
Hi - Have just understood about the relative leverage of the two ends of the cam on the brake shoes and the reason for a small difference in leverage to the leading and trailing shoes.
So that is not my problem and will have to think of something else - tried the trick of slackening the spindle,
applying the brake and doing the spindle up again, some years ago to little effect.
I still wonder if it is really the material of the drums or if ribbed Shadow ones are the answer.

This however would be an expensive experiment unless I could borrow a front wheel with Shadow drums from somebody to try !!
 
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