8inch front brake

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BasilF
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8inch front brake

Post by BasilF » Tue Jun 18, 2019 2:51 pm

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I've a problem with the front brake. When the brake is off and the cable is slack the brake arms 26 and 33 should sit angled out slightly as shown in the diagram. The tappets 22 should sit with their heads on the backplate and their ends just in contact witht the brake arm cams. On mine (which might be make from parts of two brakes), they sit angled in slightly and when the brake is applied they are even more angled in which is not a good position for mechanical leverage. Either the shafts of the tappets are too short or the backplate needs machining deeper. It would be difficult to set the backplate up in the mill to machine the surface that the tappets sit on and no one lists tappets of different lengths. Anyone got any suggestions?
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Re: 8inch front brake

Post by kommando » Tue Jun 18, 2019 5:28 pm

Have you adjusted the micram adjusters, they move the shoes until they are just about to touch the drum prior to the lever being pulled.

Are the cams fitted to the correct sides of the brakeplate, try swapping them over.

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Re: 8inch front brake

Post by BasilF » Tue Jun 18, 2019 6:47 pm

The snail cam adjusters don't affect where the arms sit when the brake is off.
The brake arms can only be fitted to one side (or they will stick up not down).

I've found the other brake which has the smaller brake levers. When reasembled, everything is at the right angle. I'm going to dismantle the faulty brake and see if the problem is the machining of the back plate.

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Re: 8inch front brake

Post by BSA_WM20 » Wed Jun 19, 2019 8:20 am

The problem is the outer case is a lot heavier than the inner so what happens is the outer just barely moves enough to allow the shoe to stop making contact and the inner takes up all of the slack.
The solution is to put a rear brake clock spring on the rear lever.
Also make sure that the outer is not tied anywhere to the frame
It should go through 2 loose guides and never ever be cable tied to either the frame or guard, it must be totally free.

George Heggie, who was the local Hepolite distributor used to race an A75 fitted with the same brake did a conversion to mine by fitting clock springs to both levers and then replacing the spring between the levers on the cable with a very light one, just strong enough to stop the ball ends falling out of the brake pawls.

With that done to my front brake & the cams adjusted properly I could ( and did ) lock the front wheel at better than 80 mph and stop the bike from street legal speeds with 2 fingers.
As luck would be BSA changed the position of the rear brake lever so you can get both a clockwise & anti clockwise spring of the same strength.

The big problem I had prior to this was brake fade due to the rear shoe just touching the drum which made it so hot the brakes would not work.
Every second clot will tell you to fit longer levers but that is not & never was the proper fix.
If George could manage 20, 50 & 100 mile races on his A75 braking hard from speed well over 100mph, pulling up a flea weight B50 from 80 mph should be a doddle.
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Re: 8inch front brake

Post by minetymenace » Wed Jun 19, 2019 8:30 am

BSA_WM20, any pictures of the conversion on your machine?
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Re: 8inch front brake

Post by jim-nz » Wed Jun 19, 2019 8:43 am

Hi in reply to BSA wm20's comment can I ask a couple of questions.

First is you recommend a rear brake spring on the rear lever only then go on to describe your brake as having a spring fitted to both levers. Are you able to explain why this is?

Second is you don't seem to be recommending a heavier brake cable. Do you not consider this necessary.

I ask as I am building a B50 as a road racer at the moment and plan to use this brake.

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Re: 8inch front brake

Post by BasilF » Wed Jun 19, 2019 2:05 pm

Brake-arms1.jpg
Found the problem. The short armed cams have the armes mounted on the cams at an angle which puts the arms at right angles to the cable when the brake is on. The long armed cams have the arm set at about 90 degrees to the cam face and are not even a very good pair. I can make the long arms more usable by lengthening the "tappets" by about 1.5mm by welding and grinding but I guess I'll have to try both brakes to see which one works best.
IMG_20190619_11494899.jpg
This explains why there are so many pictures of 8inch front brakes with the arms at funny angles.
IMG_20190619_115337.jpg

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Re: 8inch front brake

Post by minetymenace » Wed Jun 19, 2019 2:44 pm

Small Heath is in Birmingham,

छोटा हीथ is in India. :ban

<918 Someone has been to India shopping me thinks... <201
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Re: 8inch front brake

Post by BasilF » Wed Jun 19, 2019 3:16 pm

Possibly, but not me. Both bikes were bought unrestored and those were the parts from them. Could have just been the chaos at BSA in 1973.

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Re: 8inch front brake

Post by BSA_WM20 » Thu Jun 20, 2019 12:25 pm

jim-nz wrote:
Wed Jun 19, 2019 8:43 am
Hi in reply to BSA wm20's comment can I ask a couple of questions.

First is you recommend a rear brake spring on the rear lever only then go on to describe your brake as having a spring fitted to both levers. Are you able to explain why this is?

Second is you don't seem to be recommending a heavier brake cable. Do you not consider this necessary.

I ask as I am building a B50 as a road racer at the moment and plan to use this brake.
The rear brake came both with the lever up and with the lever down.
So they have both a clockwise & counterclockwise return spring.
You will need one of each.
The front brake has no return spring, just the compression spring between the two levers which is supposed to return the levers to their off position.
Unfortunately it pushes the front lever on the inner way past rest but barely moves the rear on the outer.
So you need to get the outer to relax off the drum or you end up with serious brake fade from overheating the drum
The matched pair of clock springs do just that.

However you still need a compression spring to stop the cable falling out of the front brake, found that out the brown undies way.

I suppose you could get away with a clock spring on the rear & the original compression spring between the two levers.
However that is not the way George set mine up.

The original BSA cable is quite fine but some after market ones the outer is would loose like a spring and these can be fatal as the outer just compresses putting no force on the lever or brake.
The stop switch can also cause a lot of sponginess in the braking.
So a heavier cable is not needed but a good close wound outer is.


The brake relies on the very sudden movement of the shoes digging into the drum to use the rotation of the drum to force the shoes into the drum.
IT is a very sofisticated & well designed set up, apart from the rear trailing
A quick jab & hold will give you a massive amount of braking force.
A slow pull allows the shoes to slide in the fixed end and gives you a gradual braking.

Extending the levers 1" or even 2" which was fashionable at the time destroys the brake as you can not get the shoes to bite into the drum thus loose the self energizing effect and go back to nothing greater than your wrist strength and any mechanical advantage built into the system.

I put mine on the B50 for a couple of years & it was fantastic.
Then the A65 was put back on the road , for a short time before I found out why I should never leave engine rebuilds to others.
From memory the A65L was on the road from 84 till around 98 when it went bang big time so it has been at the back of a 40' container since 2012 waiting for time money & enthusiasm .

If you are considering racing on that brake, pay close attention to wind of the outer as previously mentioned.
It might even be worth buying some old original BSA ones and having new inners made .
A single clock spring might work fine with the original compression spring between them or it might cause the front shoe to drag
In any case it is just a single hole to anchor the spring, save you drilling the backing plate.
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Re: 8inch front brake

Post by minetymenace » Thu Jun 20, 2019 12:38 pm

BSA_WM20, any chance of a photo to show the detail? The rear brake has a flange on the plate that is drilled to accept the hook on the end of the spring, the front has none. How is the hub end of the spring anchored in your modified front arrangement?
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Re: 8inch front brake

Post by BSA_WM20 » Sat Jun 22, 2019 12:05 am

To be honest Gerry, I really can not remember.
Not much chance of a photo in the immediate future , there are 14 bikes ( or will be bikes ) and about 6 tons of parts between it & the door.
The container was an "emergency temporary" storage when I moved.
The 2 20 x 10 sheds that are to replace it are still sitting going rusty on a pallet on the front lawn.
Somehow in the 3 moves that box with all my spare time in it seems to have been misplaced along with all the roundtuits.
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Re: 8inch front brake

Post by minetymenace » Sun Jun 23, 2019 8:02 pm

:laugh moving myself next week, new place has no out buildings, have hired a container and rented a lockup until the workshop is built....probably drop off the radar until then...
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Re: 8inch front brake

Post by skippy » Sun Jun 23, 2019 11:42 pm

I know where everything I can remember is, but sometimes I find things I forgot I had. :oops:
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Re: 8inch front brake

Post by koncretekid » Mon Jun 24, 2019 11:46 am

How does using 2 clock springs change the operation of the brake? As long as the cable is free to move, not tied to the frame or the front fender (which may look nicer), it will pull equally on each brake arm - - "for every action there is an equal and opposite reaction" is a principle of physics I learned in grade school. The arms will have equal pull on each and the arms will move until both shoes are in contact. You say use two matched but oppositely clocked springs on each arm, but how do find two "matched" but opposite clock springs? If one spring is weaker than the other, that's the arm that will move first and that's the shoe that will contact first and is likely to drag continuously. The fully floating shoes and the action of the arms being pulled together as they are designed is superior to any TLS system that has one end fixed, as the Japanese systems that I have seen.

The other thing that may cause one shoe to drag more than the other is being way out of adjustment, or dragging on its bushing (the hole in the brake plate) or gummed or rusted up parts that will take more force to move or return. Furthermore, lengthening the brake arms makes no difference in the operation of the brake arms, it just increases the force at the shoe, hence increases the stopping force in relation to the amount of pull.This is exactly the same principle as using a smaller diameter master cylinder on a hydraulic brake, such as is done with great effect on Nortons and others.

So unless you have defectively made parts as discussed above, cleaning all the brake parts, getting the best cable you can find (I made my own from an old S*zuki cable with larger inner and stiffer outer casing), and making sure the drum is true and the brakes are adjusted properly is the best way to ensure your brake works as designed.

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Re: 8inch front brake

Post by JB » Mon Jun 24, 2019 10:49 pm

The critical part with these brakes has always been that the cams need to be adjusted with the minimum of slack of the operating arms without any binding.
The reason this is essential is that the piston on each leading shoe is operated by a crank with the radius created by the slot machined into the operating shafts bore. Because the radius of the crank is relatively small its mechanical advantage changes and drops off dramatically the more the operating arm rotates and the piston is lifted to the point where there is dramatically less lift regardless of the effort. Adjusted properly and with a decent cable these brakes work superbly.
The servo effect, where the leading shoe is 'pulled' into the drum, is dependent entirely on the direction of rotation and is why the brake barely works if the bike rolls backwards on a hill as the two leading shoes become two trailing shoes <209

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Re: 8inch front brake

Post by midgie » Sun Jun 30, 2019 4:22 pm

Spot on JB.

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