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Mechanical Rear Brake

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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Fri Oct 17, 2008 6:55 am

Mechanical Rear Brake

I was reading up last night about the mechanical rear brake on my panhead and found that there should only be a single lower spring used in the assembly. At some point a previous owner had installed a upper spring as well to mine. I’ve been riding it this way for years without knowing it was wrong.

My question for you all is this, Would there be any benefit to having both upper and lower springs installed? Or is it hindering the performance of the brake? Thanks -Steve
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VPH-D

Posts: 43

Joined: Fri Jan 18, 2008 2:24 pm

Post Fri Oct 17, 2008 1:51 pm

Re: Mechanical Rear Brake

I suspect a lot of the rigid frame bikes running around have 2 rear brake springs. I know several that do.
VPH-D
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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Mon Oct 20, 2008 3:59 pm

Re: Mechanical Rear Brake

Wouldn't an upper spring defeat the purpose of the pivot stud? :?

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37ULH

Posts: 681

Joined: Mon Nov 25, 2002 1:01 am

Location: somerset, oh usa

Post Mon Oct 20, 2008 6:34 pm

Re: Mechanical Rear Brake

I don't see how the spring could compromise the function of the pivot. It appears that the bigger concern would be the condition of that sprocket unless it (the spring) is causing known issues.
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Pa

Site Admin

Posts: 4475

Joined: Sun Mar 12, 2000 1:01 am

Location: Ohio USA

Post Mon Oct 20, 2008 8:14 pm

Re: Mechanical Rear Brake

I agree. Those shoes are designed for twin springs also. Helps keep the shoes in place. My 45 has twin springs. I figure it is for the very same reasons I mentioned. In fact, most mechanical brakes are setup this way. If the shoes were hinged in some way to the pivot, then a spring would there would have no purpose. Since the shoes only rest against the pivot, thus a spring to keep them there. Pa
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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Tue Oct 21, 2008 5:46 am

Re: Mechanical Rear Brake

Thanks for the input guys. That sprocket is more than used up and will be replaced. :oops:
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Pa

Site Admin

Posts: 4475

Joined: Sun Mar 12, 2000 1:01 am

Location: Ohio USA

Post Tue Oct 21, 2008 6:48 am

Re: Mechanical Rear Brake

Sprocket ??? I thought that was a sawmill blade. :mrgreen:
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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Tue Oct 21, 2008 7:12 am

Re: Mechanical Rear Brake

Man.... Tough crowd around here. :lol: By the looks of that saw blade, I deserve it.
I wonder if Cotten could use one like that?
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Chris Haynes

Posts: 2602

Joined: Wed Jul 05, 2000 12:01 am

Location: Los Angeles, CA

Post Tue Oct 21, 2008 11:26 am

Re: Mechanical Rear Brake

The parts catalogue shows,
41835-30 .50 Brake shoe spring (lower) Later 1938-1957-61". 74" & 80"
There is no mention of an upper spring on Big Twins. There is on the 45"
The 1936 & 1937 Big Twin models use two springs.
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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Tue Oct 21, 2008 3:47 pm

Re: Mechanical Rear Brake

You and I must be reading the same book Chris. Is it possible that after-market shoes now require two springs to eliminate brake chatter or to keep them from self energizing?

This afternoon I did a little test. I observed the movement of the shoes assembled as shown, and then removed the upper spring from the assembly. It did make a difference in the amount of travel at the top of the shoes. Naturally the tops spread further without the top spring in place. I suppose the proof for me will be after I re-assemble with new shoes and only the lower spring, then take it for a ride and see.

Sorry to be anal about this guys but with just a stock springer front brake, this rear brake really needs to be tits and beer.
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Chris Haynes

Posts: 2602

Joined: Wed Jul 05, 2000 12:01 am

Location: Los Angeles, CA

Post Tue Oct 21, 2008 4:46 pm

Re: Mechanical Rear Brake

[quote="ohio-rider"]You and I must be reading the same book Chris. Is it possible that after-market shoes now require two springs to eliminate brake chatter or to keep them from self energizing?

No, But the aftermarket companies make more money selling you two springs rather than one. Use one spring on you brake and the other on your jiffystand. :mrgreen:
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panacea

Posts: 118

Joined: Thu Apr 12, 2007 8:22 pm

Location: Mpls. area

Post Wed Oct 22, 2008 3:49 pm

Re: Mechanical Rear Brake

The extra spring might help return the pedal a bit faster, I can't see any harm in it...Why do you suppose there are two sets of spring holes in the shoes? Mike
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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Sun Nov 02, 2008 7:37 am

Re: Mechanical Rear Brake

Thanks for everyones opinions so far. Here where I am now.

I decided to purchase a set of V-Twin shoes instead of re-lining my old ones because I’m lazy that way. They assembled nicely to my backing plate but are to big to fit into my stock drum. I don’t have a set of calibers to measure the exact amount, but I’d guess them to be about 1/32 to big. Anyone ever run into this problem? Machining the drum is not an option for me. I refuse to alter an original part to fit an after-market part. What are my options? Thanks for any ideas. -Steve
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Jerry Wieland

Posts: 606

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Location: Menomonie, Wisconsin, USA

Post Sun Nov 02, 2008 8:58 am

Re: Mechanical Rear Brake

Steve You've got the perfect situation - chuck the backing plate, shoes and all in a lathe and turn'em down to fit. In the old days this was called "arcing" the shoes. I'd remove the brake arm and make sure everything is concentric with the axle before you start.

Jerry
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Pa

Site Admin

Posts: 4475

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Location: Ohio USA

Post Sun Nov 02, 2008 5:04 pm

Re: Mechanical Rear Brake

Jerry is absolutely correct ! Pa
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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Mon Nov 03, 2008 5:58 am

Re: Mechanical Rear Brake

Thanks Jerry, I recall hearing about that procedure some time ago and I agree that it would be the optimum solution. Would you be able to give me a bit more information as to how the shoes are held firmly enough onto the backing plate to allow for machining? :?:
Maybe I’m just over thinking this, and just the spring would be enough to hold things well enough for what I need to do.
What would be considered a good clearance fit between the shoes and the drum?
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panic

Post Mon Nov 03, 2008 11:08 am

Re: Mechanical Rear Brake

Re: "keep them from self energizing"

I agree - if the leading edge of the top shoe is grabbing, it won't retract reliably when the cam is rotated to "off" (pedal released) unless there's a spring. If that edge is beveled back you're probably safe, why take a chance.
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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Mon Nov 03, 2008 12:00 pm

Re: Mechanical Rear Brake

Panic, Thanks for bringing up the subject of beveling the shoes. I was going to get around to that topic. Do I understand you correctly that if I use both upper and lower springs that I shouldn’t need to bevel the shoes?

By my calculations, if I bevel the edges back 5/8 from each end of the shoes I will lose 15% of the pads contact area with the drum. Probably no big deal, but it is something to think about.
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Pa

Site Admin

Posts: 4475

Joined: Sun Mar 12, 2000 1:01 am

Location: Ohio USA

Post Mon Nov 03, 2008 3:17 pm

Re: Mechanical Rear Brake

I only beveled my 45 shoes 1/8". They work fine. Pa
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ohio-rider

Posts: 208

Joined: Tue Jun 12, 2007 7:26 am

Location: Ohio

Post Mon Nov 03, 2008 4:05 pm

Re: Mechanical Rear Brake

Thanks Pa, I’ll do the same just for peace of mind.
One of my lathes should be available latter this week, then I’m going to take a shot at fitting those shoes. I’m going to try a zero radius H/S tool at 300rpm and .005 feed. Those pads won’t even know their being cut. I’ll report back with my final results. -Steve
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