Topic: Building a custom rear drum brake anchor arm

[78CB750CAFE]

I had kicked around the idea of giving Carpy $100+ to replace a piece on my bike that was perfectly functional a few months ago when I had $$ burning a hole in my PayPal account, and I am glad I didn't.
$4-5 worth of metal
a little wear on a few dremel bits
a couple of squirts of paints I already had laying around
and around 2 actual hours of my time, spread over this afternoon
and I came up with this:

[78CB750CAFE]

so here is how it started...
3 foot section of 1 3/8 14 ga steel ($5.95 and I already made a reg/rec bracket and fairing mounts with it)
and the bikes original anchor arm.

Holes spaced perfectly from the git go

[78CB750CAFE]

I cut the rough length with a circular saw

[78CB750CAFE]

and did all my shaping with files and a dremel
Here are some pix of the drum end in progress

[78CB750CAFE]

Doing the swing arm end took a little more finess and shaping. The front top notches down like 3/16 for the first 2 inches, and the bolt will go through the 2nd hole. Bolt holes do need to be opened up slightly, I used a round rasp followed by a quick pass with a dremel abrasive wheel.

at this point I dry fit it to be sure all was well (it was ) and then did a few runs with a countersink bit on my cordless drill on both sides of all the holes that weren't being bolted through.

[78CB750CAFE]

Paint time 

[78CB750CAFE]

red goes down first, then another round of the countersink, then clear coat

[78CB750CAFE]

While the paint dried, I cleaned up the original bolts, and replaced all the nuts and washers with new grade 8 stuff and new cotter pins.

Not too crappy for an afternoons work, no?

[rachet]

simple and Clever!  Well played my friend, Well played!

Rachet~

[UnCrash]

Yes, much more satisfying than paying for someone else to do it.

Bravo!

[754]

I hope it is at least the thickness of the stock one.
 Cool, Now try a bracket for the can, out of flat bar.

 A buddy used to use those plated paint stir bars, with the opener on they end. They were pretty good for small brackets..

[78CB750CAFE]

it is around 1.25 mm more narrow, which I made up w a thicker washer, I may hunt for some 12 ga steel bar. The pipe bracket has been in the works for a while, hopefully it will be done soon.

Drew

[754]

if the main thickness is thinner, that would not be so good, the ends are thicker, and the washer should probably be welded, to prevent the holes from wearing or stretching in use. If its not welded, it only has the strength of the main bar holding it.

The bolts are shouldered to prevent wear in those ends,

[eldar]

That looks MUCH better than the stock bar. Excellent work there. I suppose a way to answer the strength issue is to try and bend the bar. If you grunt and crap your pants and it does not bend, I would say it is probably pretty good!

[754]

That bar, will bend like butter, there is no stiffening rib in it.

sometimes  or often parts are made of selected material.. they could look the same but one may have 50 % more tensile strength..

[eldar]

thats why you do the bend test. If you can barely flex it, it will probably work fine then. As for bending like butter, what if the butter is frozen? its pretty hard then!

[78CB750CAFE]

I just did a 40 mile run and the brakes seem fine, but I would like to stiffen it up a bit just in case. My original thought was a little I bean, but I could not find any besides aluminum ones and I thought that might not be so good.

Maybe I should double it up?

Any suggestions from the class?

[mystic_1]

I just did a 40 mile run and the brakes seem fine, but I would like to stiffen it up a bit just in case. My original thought was a little I bean, but I could not find any besides aluminum ones and I thought that might not be so good.

Maybe I should double it up?

Any suggestions from the class?

I was just going to say, double it up.  14ga is pretty thin for that application, and if it lets go you'll be doing lots of damage to the rear, probably even lock up the rear wheel, decidedly Not Good.

If you double up, consider running a bolt through both of them, about half way down the length, to keep the two bars together and stiffen up the whole affair a bit.

mystic_1

[754]

Bend test, shows formability, more than tensile strength..

[78CB750CAFE]

ok 754
so Mach2 should be an I beam based design?
Stock is basically a pipe with squashed ends, the round shape adding torsional rigidity. What other designs could achieve the same rigidity, while being cool, lighter or generally "trick"?
Carpys design is a ribbed bar, which is a good take on this, but what else could be done to be unique?
Boxed section pipe?

[Really?]

Either way, that looks really good!

[754]

Can you measure the thickness of the tubing style arm, on the flattened portion.

If I were building the strap type, I would probably use aluminum, preferably 7075,  thicker on  ends, and mill some off, or slot it. With aluminum, at 50 % more thickness, you will be at approx 1/2 the weight of steel.

 But, I do  most prefer using rod ends and a tube, makes it easier to remove wheel, as it can move 20- or 30 degrees sideways, & loosening 1 jam nut allows the backing plate to turn  90 degrees.. much easier for maimtenance..

[eldar]

Well bending will still give a good indication PLUS it will show you if the metal is brittle or not. After all, if you can barely bend it and no cracks develop, it will be just fine. Yes 14Ga is a little thin but it does not need to be an inch wide either. After all, the honda part goes flat where the bolts go through and it is maybe an 1/8th inch thick then. Clearly any metal that thick of decent quality will MORE than do the job.

[mystic_1]

As 754 points out, the main concern here is tensile strength, not rigidity or torsional rigidity.  The main force acting on this part is a pulling force that tries to stretch it, or pull it apart.  A chain, for example, has no rigidity but has high tensile strength.

Earlier 750's used a flat bar here instead of the formed tube of later models.  It was steel, maybe 1/4" or 3/16" thick, and was stamped with ridge shape in the center for rigidity but could still be bent relatively easily.  Others have drilled these out for a look similar to yours, maybe that's a good option for you.

Here's the stock item:



Here's one drilled out by Industrial Cafe:




And here's what can happen if the part fails:

http://forums.sohc4.net/index.php?topic=21146.0

mystic_1

[754]

The stock old style has the rib in it to prevent bending. I have not heard of one failing, except one on here. I asked the fellow if he used the stock shouldered bolt, but he would never say. I am not certain in his case if the strap or bolt fails.

 I do know of several early style, that have gone well over 100K miles. I dont know on the later style, but am interested in how thick it is.(and also if the mounting lug is on both sides of the rod)

 The flat bar with the holes punched in it will bend  really easily.

 The shouldered mounting bolts are important, as they prevent wear and spread the force over a larger area.