Monday, March 2, 2015

WR250R case saver fabrication

Sandman makes a really nice case saver to replace Yamaha's Mickey Mouse plastic cover over the counter sprocket. The idea is that the open design of the Sandman part allows trail gunk to fall away from the sprocket easily, rather than build up and bathe your chain in gritty badness. Just as important, the steel part provides some ballistic protection for your engine case in the event you break a chain.

Since I was replacing my sprockets and chain, the case saver seemed like good mod to perhaps extend the life of my drivetrain. (All the more so because this May I'm doing the Roaming Rally as part of the Filthy Knobs team and it is likely to be a wet, muddy affair!) Unfortunately, by the time I added up the cost of the Sandman parts (US$50), international shipping to Canada (US$24), our dismal exchange rate (C$0.80) and duties (15% HST) I was looking at about $100 to my door. Ouch. Making my own seemed like a reasonable option.

First step was to design a template. I used the original plastic cover to locate the holes and provide a rough outline. Then I basically eyeballed a shape with some sexy curves to connect the holes with enough material along straight lines to provide some triangular reinforcement for the stud mounts.

A quick test fit with some long M6 bolts showed it was OK, and allowed me to identify that setting it 30mm above the bolt holes would provide adequate clearance for the counter sprocket nut. 

I traced the pattern onto 1/8" thick T6061 aluminum, drilled starter holes, then cut it out using a jigsaw. T6061 is great to work with. Some quick cleanup with files and emery cloth, and it looked pretty good.

Powder coating or anodizing would be ideal, but my local powder shop charges $150 minimum order when I can't run my job with someone else's order and work a side deal. So it was rattle-can Tremclad for me! This thing will get scratched up and covered in chain grease anyway, but a little lipstick on the pig will help me feel good until riding season.

Now for the actual case saver. After making a simple cardboard template for sizing, I cut some 1/8" thick mild steel sheet I had lying around, again using the jigsaw. I made the width about 38mm so I could notch the corners and fit them up against the bolt hole posts. The extra width provide just a little bit more protection for the case. Sandman looks like he uses a thicker material, but if your chain punches through 1/8" steel I think you have bigger problems. Also, I'd rather the steel deform than transfer all the force to the bolt holes in the engine case. Bending the steel by hand in the vice was again an eyeball affair. Sandman's simple shape would probably have been be a lot easier, but hey...

Next was making three standoffs 30mm long. I used some rod stock, faced it, and tediously drilled it out for M6 on the lathe. (That was really boring.)

My neighbor Ken was kind enough to TIG the standoffs onto my bent steel. Unfortunately, attempt #1 was a disaster thanks to my suggested method of jigging. This was a little too ugly even for a home-brew.

A second attempt included making three more standoffs (because one had mysteriously escaped from Ken's shop), and turned out much better.

Here's the trial fit using 45mm M6 steel cap bolts. That shiny new counter sprocket is an Ironman 13T. No, the nut is not yet staked in this photo. The "95" refers to 95 N-m, the nut torque. I write the required torques on my bike with Sharpie, right where they're needed. Makes servicing a whole lot faster.

A little more Tremclad, some 45mm M6 button-head stainless steel bolts, and it's starting to look like a real part.

Everything fit perfectly on installation. Lots of room for shifter movement. It's not shown in the photos, but I also stuck some Dynamat on the front and back of the steel piece. The Dynamat is a rubberized sound absorbing material with an aluminum facing on top. It reduces vibration and should provide some additional cushioning for the case and protection against flung grit from the chain.

Total cost was $1.50 for the three bolts, which I didn't already have. Everything else was scrap material I had lying around.

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