Rear drum brake

pfg49

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I bought a Rickman XS650 which use the XS engine and various other XS bits. It came with alloy wheels, but I didn't like them and bought some XS650 alloy rims. Easy to fit, the problem is the reaction arm for the rear drum, and the brake rod. I've turned the drum upside down and put the rod and torque bar in place, and with some modifications to the brake rod, everything looks as though it will work. There is plenty of clearance for the brake arm. Can anyone see any problem with this please?
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Well, maybe. The cam that operates the shoes isn't exactly symmetrical. You can't see it with the naked eye but the front edge of the cam that's closer to the shoe pivot point is ground down a little so it moves it's shoe slightly less. This results in both shoes being moved the same amount and being applied with the same force. You'll be pulling the brake arm and rotating the cam in the opposite direction so you won't be utilizing this little known design feature. Poor braking may be the result.

156TVo9.jpg
 
Well, maybe. The cam that operates the shoes isn't exactly symmetrical. You can't see it with the naked eye but the front edge of the cam that's closer to the shoe pivot point is ground down a little so it moves it's shoe slightly less. This results in both shoes being moved the same amount and being applied with the same force. You'll be pulling the brake arm and rotating the cam in the opposite direction so you won't be utilizing this little known design feature. Poor braking may be the result.

156TVo9.jpg
Then do I turn the cam round, assuming in the right way round after all these years, and how do you tell?
 
Turning the cam around won't help. See the two marked contact points in my pic? Rotating the cam in the opposite direction will use the two corners that aren't marked. Flipping the cam will not change that, just put those two corners 180° from where they were.

When assembled correctly and rotating in the right direction, you can tell how to orient the cam by looking for a dot stamped on the splined end .....

arKDkY2.jpg
 
Turning the cam around won't help. See the two marked contact points in my pic? Rotating the cam in the opposite direction will use the two corners that aren't marked. Flipping the cam will not change that, just put those two corners 180° from where they were.

When assembled correctly and rotating in the right direction, you can tell how to orient the cam by looking for a dot stamped on the splined end .....

arKDkY2.jpg
5T gave me this tip and when I checked my bike the cam was out 180 degrees. Turned it the correct direction and a significant improvement in braking. Look for the dot and face it in the correct direction for your application which in your case would need to be backwards because of the brake arm orientation?
 
The part that concerns me is the tension bar. If it flexes when braking you will get erratic braking. If it should buckle I think you could have a very thrilling ride.

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Good point and observation. You are correct. Stock configuration puts the bar under tension. The proposed configuration puts it under compression and as GLJ suggests will be a bad thing if it fails.
 
There is also the question of all the geometry between the tension bar, swing arm and brake peddle rod to keep the braking linear during braking when the swing arm moves. Will it work yes. How well depends on how the bike is ridden. If it's going to be a show bike or garage queen no problem. If it will get ridden in a spirited way you will just have to try it and find out how it works. Good lucks.
 
The tension bar is tubular rather than just simple flat bar. I would be less worried about it than the actuating cam working the wrong direction. Also he asked if he could turn the cam around, it would not avail him any benefit as it would still be rotating the wrong direction.
From the photo supplied by 5twins there is less radius on the cam leading edge than I thought there might be but it's still more than the opposite corners. I would guess that more abrupt application would be the result
 
Isn't tension bar under compression the way the disc brake models work?

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Anyways, I'd try putting the brake plate like where it's supposed to be, and see if you could get around the frame using an offset bend in the brake rod. You'd need to fab a whole new brake rod, preferably out of something stiff like stainless. Maybe even gusset the bends for extra stiffness, or use a bar instead of a rod.

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Offset bends is how I like to do forward controls, I think it looks cleaner and transfers force better than the Heim joints and angled rods everyone else uses. This is a stainless rod, and it works fine.
 
On the Trackmaster street tracker I built I fabricated a special fastener to replace the tension rod attachment point on the backing plate, it has a hole that accepts a cable end and I used a rear brake cable for a KH400 Kawasaki. Didn't have to worry about brake pull rod geometry or trying to bend around swingarms, frame tubes or other obstacles.
 
On the Trackmaster street tracker I built I fabricated a special fastener to replace the tension rod attachment point on the backing plate, it has a hole that accepts a cable end and I used a rear brake cable for a KH400 Kawasaki. Didn't have to worry about brake pull rod geometry or trying to bend around swingarms, frame tubes or other obstacles.
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Great idea. That's how it's done on my '73 TS185 and other Suzuki enduros of the era.

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One cable end terminates in a threaded rod. The other end terminates in a clevis.

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But the trick would be creating an attachment point on the brake plate for the cable sheath to terminate in.
 
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Well, maybe. The cam that operates the shoes isn't exactly symmetrical. You can't see it with the naked eye but the front edge of the cam that's closer to the shoe pivot point is ground down a little so it moves it's shoe slightly less. This results in both shoes being moved the same amount and being applied with the same force. You'll be pulling the brake arm and rotating the cam in the opposite direction so you won't be utilizing this little known design feature. Poor braking may be the result.

156TVo9.jpg


Having thought about this, I just don't buy what your suggesting. It would mean that when the brake is off, the cam end of one brake shoe would have to be further away from the drum than the other one. So having done that you then make an asymmetric cam to compensate . Why would you do that?

Also, I think that the splined cam shaft is dimpled, (not all are, mine isn't) because if you fit it the wrong way round, the plate the cam sits on, I've arrowed in the picture below, interferes with the brake drum even though you can make the wheel fit, and it's not immediately obvious until you fit the wheel. The wheel turns, but the brake plate goes with it. I know this, I tried.

However, as I'm in my eighth decade, I'm smart enough to know I don't know everything, but I'll take some convincing on this one!


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Regarding the brake reaction arm, as others have pointed out it's a hollow tube, and like most things on an XS it's magnificently over engineered, probably designed and made by the Japanese equivalent of John Deere tractors. Assuming the arm is in good condition, I'd imagine brake lock up will be apparent long before the tube starts to bend.
 
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