Why is my front brake still useless?

[duTch]

 

..Quote

    Huh?

which end of those do you fit the steel pads??
R.......

 Good point...I didn't even look at it- was just the page that came from hysterical ( intended) and used a an example link.......had to go to work and whack my battery back in so I had basic lights and brake lights...but I didn't notice any porches to race, but probably did go past some hidden behind fences :   

........ i've enjoyed passing porches and harleys on my b33 - ......

 

[mikeb]

good catch dutch. my germanic isn't what it could be. but i hate how porches hide behind fences, and hedges
the b33 passed an audi once too! (memorable as may never happen again)

[Swarfcut]

Mikeb.....I reckon the rest of the crew wondered why you had done it, but were too polite or unaware of the true mode of action of leading and trailing shoes.  The leading shoe is always the same, and is leading or trailing depending on the direction of rotation of the wheel. Assuming the cam is symmetrical, it does not matter which way it moves , so the lever can be fitted either way and the braking will be the same. The 8 point squares in the lever end are slightly offset, so reversing the lever by simply turning it over can move the lever/cam to give a more acute angle to the cable, and a bit more leverage. So, go back to standard and position the lever so it approaches (but does not pass through) the magic 90 degrees at full braking effort.

 Swarfy.

[mikeb]

Swarfy – sure the leading and trailing shoes remain what they are (unless going backwards). But are you saying that rotating the arm one direction and not the other makes no difference to which shoe touches the drum first?

My understanding from many threads and from drawing it out is the reversed cam rotation pushes the leading shoe into drum contact just before the trailing shoe – I’m sure you know the theory (triangles etc). as per why velos ran the brake on the other side of the wheel and racing bsa’s often reversed their front lever rotation. So that’s why I reversed it… leading shoe to touch first...

…. Or am I not understanding your point?

[Swarfcut]

Mike...My understanding is that if the cam is symmetrical and the shoes "identical" they will touch the drum at the same time. There is a theoretical possibility that the lining on the end of the trailing shoe, adjacent to the cam, is nearer to the drum than the start of the lining on the leading shoe which is some distance away from the cam. Hence the suggestion, posted earlier,  of  relieving some of this area of trailing shoe lining to allow the leading shoe to contact the drum first.

 As I see it it does not matter which way the cam rotates.....when you buy one it looks symmetrical and has no indication of a particular way to be fitted.

 Of course, this is in the light of my experience and I may be wrong. So let's wait and see what the rest of the gang thinks.

 Swarfy.

[duTch]

 
 Swarfy butted in while I was doin' 'Audi-Audi'.....so....

My take on the reverse lever action of that the cam is that the inside of the cam-lobe will (relatively speaking) push the shoe further than the outside lobe, so will have a better mechanical advantage.... I heard  about it being done on rigid frame rigs, so tried it on my Plunger, but it failed miserably due I guess to the geometry of the uplift of the axle on the plunger unit...

 

...passed an audi once too!...

 I went to a local cafe/bar a few years ago where there was music and shit...mate Lawrie was outside and in his usual drawl says 'howdy'....and in response I said 'owdy-owdy' and then realised there were two Audis parked behind each other where the table was....'Audi-Audi'.... hate to say it, but yea had to be there...  

[RogerSB]

Hi Mike I wish you luck and hope you’ll sort out your brakes to your satisfaction, I wasn't advising on how to ride a BSA. It's how I think about riding my own BSA and maybe why I don't find its brakes dangerously lacking.  I don't ride slow because I'm riding a 59 year old A10 (in my opinion that's equally as dangererous as riding it too fast), but I do take it into consideration, look after it and try to ride it safely.

[Triton Thrasher]

My take on the reverse lever action of that the cam is that the inside of the cam-lobe will (relatively speaking) push the shoe further than the outside lobe, so will have a better mechanical advantage.

Yes, that is why some of us reverse the direction of cam rotation:  to make the leading shoe move more than the trailing shoe.

It can work,  but there may be things that can stop it working.

[BSA_54A10]

Mechanical brake efficency is an art that got lost hen hydraulic brakes became widely used.
The idea that you can stop your bike with nothing more than your grip strength converting a massive amount of energy into heat is quite silly.
People just can not seem to understand physics any more.
The leading shoe should get grabbed by the rotating drum and get forced back into the pivot bolt which if it does not break forces the shoe into harder contact with the drum.
This is called the self servo action and dose about 90% of the braking effort.
All your hand is doing is preventing the shoe backing off.

Your grip strength can not lift your own weight , no matter how much mechanical advantage it uses, you are just not strong enough.

Because all of the literature around comes from the days of hydraulic braking or even worse power assisted hydraulic braking , you need to unlearn all of the rot you have read on the web and ignore all of the tosh you have seen on U-Tube.
To get your head around the physics of full mechanical brakes , you need to go back to books & magazines of the 20's, 30's & 40's.
The most common cause of poor braking is the pivot end being too far away from the drum.
This is no surprise as most BSA drums would be at or well over their wear limits.
Next is incorrect radiusing of the shoes.
To work as intended the shoe must have a LARGER radius than the drum so the initial contact will be at the end only.
The leading end digs into the drum which locks onto the shoe and forces it back against the pivot end which can not move so the shoe flexes into the drum.
Thus the rotational energy in the drum is supplying the breaking energy , not your grip.
Forget all the rubbish about different force at either end of the cam.
That is presented by clots who failed physics and were never taught any mechanical engineering.
The brake cam pivots in the center so the force transmitted at either end is EXACTLY THE SAME .
The difference is the distance each shoe moves and the speed each shoe moves.
Take your backing plate off and watch carefully what happens when you move the brake arm.
If you really need to do it, measure each one.
Mark the position of the drum on the backing plate then look & learn what is happening.
Remember that the linnings do not compress so once the first lining is touching the drum, no further movement can happen unless the cam flexes, which it does not.

[RogerSB]

Reinforcing Trevor's info above. I posted this earlier, easy to understand - here it is again.

[worntorn]

Whatever radius you choose when arcing the shoes is a temporary situation.
The shoes wear to the radius of the drum fairly quickly. Eventually they wear out completely, would that initial shaping that we do still be there at that point, or will the worn out linings be at the radius of the drum they've been wearing against for years?
I'm pretty sure that initial shaping is gone quite quickly.
That doesn't mean that arcing the shoes isn't important. It gives you reasonably good contact area right off.
I've done it both ways, farted around with fitting shoes to get good brakes right off or, on another identical bike, put up with mediocre braking from new shoes that eventually bedded into provide terrific braking.


Glen

80 mph to zero on drum brakes

https://youtu.be/amMPrwtuz2U

[Greybeard]

The brake cam pivots in the center so the force transmitted at either end is EXACTLY THE SAME .
The difference is the distance each shoe moves and the speed each shoe moves.

You make perfect sense BSA_54A10 but isn't the theory about changing the arm pull something along these lines?

[RDfella]

"The brake cam pivots in the center so the force transmitted at either end is EXACTLY THE SAME ."
Not quite in my view. The cam pushes on the outer edge of one shoe and the inner of the other. Surely the shoe that gets pushed on its outer edge will be forced against the drum slightly harder (and later) than the other.

[Triton Thrasher]

"The brake cam pivots in the center so the force transmitted at either end is EXACTLY THE SAME ."
Not quite in my view. The cam pushes on the outer edge of one shoe and the inner of the other. Surely the shoe that gets pushed on its outer edge will be forced against the drum slightly harder (and later) than the other.

Yes, the cam lobe applying force to a place on the shoe nearer to the shoe’s fulcrum is imparting more movement to that shoe. 

The potential greater leverage and force applied by the other lobe, further from the shoe’s fulcrum, doesn’t get much chance to be helpful because the trailing shoe is already hard against the drum, stopping the cam from turning.

That’s why the stalwarts of the VMCC remove lining from the trailing end of the trailing shoe: taking off up to 2/3 of the lining. 

[Swarfcut]

  Thanks GB, a simple diagram is worth a thousand words.

    Reckon you are right, RD.  It's Force X Distance, a slightly different distance for the same force to act, depending which point of cam/shoe contact (closer or further  from the pivot) we consider.

 I missed this subtle difference earlier, so stand well and truly shown to be a fool.

   But it got me thinking.  On a typical plunger rear wheel, rotating anti clockwise in normal travel, the brake cam moves clockwise to brake.  So the leading shoe, on the top, has the cam contact point nearer the pivot, and  hits the drum first. Result good stopping power.

 The front brake has the wheel rotation and cam moving clockwise, so the trailing shoe is nearer the pivot, reaches the drum first. Braking not so good.   So hats off to mikeb, our intrepid experimenter and a now famous little bit of modification, to produce a reversed lever. That's the theory taken care of.

 The more you think about it, is the brake much better when wheel and cam rotation are opposed?  Does this explain why some later S/A  cable braked models have the rear brake hub lever up or down? On this line of thinking, lever pointing down, rotations opposed, should give the best stopping power.

 Swarfy.