Skidplate for the M98 Sherpina

[onthegas]

Last fall I cracked the primary cover on my M98 Sherpina when I hit a rock at a trial. Luckily I was able to repair the cover, but ever since I’ve been a bit nervous about going through rocky sections. The standard skidplate, or bash plate, on the M98 is just some a perforated sheet between the frame rails. It’s made contact with a few objects over the years, but it offers zero protection for the primary and flywheel covers. I’ve seen a few homemade skidplates, so I decided to try and make my own. 

The material I selected was a 17” x 14” x 5/16” (432mm x 356mm x 8mm) cutting board purchased from Target for $15. Originally I wanted to use 1/4” (6mm) UHMW and may still build one from UHMW, but my local supplier did not have any available. Looking online, I saw a few people had previously used cutting boards and the end result looked better than one made from a manure shovel.

So far I’ve notched the back corners the match the width of the lower frame rails. I drilled two holes to temporarily mount it. Next I’ll be trimming the width and then use a heat gun to form it into the final shape. Attached are a few pics of my progress.

[feetupfun]

Both are excellent material choices for a bashplate. I'm keen to see how it ends up looking

[phiggs]

I ran a Bultaco 350 for many years and got away with the perforated mesh between the bottom two frame rails.   If you make any wider bash plate you are opening up yourself to loss of control when bumping over rocks or tree trunks.   My Ossa Mar is very wide at the sump, and it threw me off once when trying to balance on a not too flat rock. . . . Stick to the mesh, and straighten it every so often.

[onthegas]

First attempt at bending the cutting board material. Used a couple of clamps and heated the underside with the heat gun. Took about 10-minutes to get it this far. Once it cools back to ambient temperature I’ll remove the clamps. The board will need to be notch for the down tube before I do any more bending. Once get the curve fitting tight I’ll draw out the basic shape and trim off the excess. This skidplate will be easily removable. Some of the areas we ride at don’t have rock gardens to contend with.

[onthegas]

After the initial bend I marked the skid plate for downtube clearance, removed it and notched it. Then I attached it back onto the bike with the clamps and a strip of flat stock before more heat cycles with the heat gun. After sitting all night I removed the clamps and it sprung back about an inch. I realized then that shear size of the surface area make it difficult to get the material hot enough with the heat gun. So time for a new plan. 
 

I drilled the holes for the front mounting bolts and wrestled it back onto the bike. I replaced the speed clamps with a pair of C-clamps, using the strip of flat stock. Next I heated the entire underside with a propane torch. Unlike the heat gun, you can’t leave the torch sitting in one spot too long, but even moving it around quickly gets the surface much hotter than the heat gun can. By being able to heat the whole surface I’m hoping it will finally form to the curve of the lower frame rails. This is the first of three bends, but the other two are not as large as this one. Going to leave it clamped and bolted up overnight again.

[Nishijin]

I didn’t want to poo-poo your efforts, so avoided mentioning it before… but yes, it will spring back.

Realistically you’d need to make a form/jig separately to make the guard, so you’re not applying heat so close to the bike. Probably the bike is okay but the degree of trial and error, you’re now applying a torch close to the rails.

Have you tried marking the points, removing the material from the bike, and heating while clamped in a large vice? Possibly with thick pieces (of metal/ceramic/etc) to distribute the pressure as you bend?

It would allow you to be more aggressive with your heat and folds. Whether it cools over 10 minutes or overnight, the material will always spring back further than you want. 

[onthegas]

18 hours ago, Nishijin said:

I didn’t want to poo-poo your efforts, so avoided mentioning it before… but yes, it will spring back.

Realistically you’d need to make a form/jig separately to make the guard, so you’re not applying heat so close to the bike. Probably the bike is okay but the degree of trial and error, you’re now applying a torch close to the rails.

Have you tried marking the points, removing the material from the bike, and heating while clamped in a large vice? Possibly with thick pieces (of metal/ceramic/etc) to distribute the pressure as you bend?

It would allow you to be more aggressive with your heat and folds. Whether it cools over 10 minutes or overnight, the material will always spring back further than you want. 

The material, called a polygranite, as it’s imbedded with small pieces of granite, is under a lot of tension. Even bolting it at all four spots, using the clamps with flat stock and heating it with the torch still left about a 1-inch gap. UHMW is a much easier material to work with.

Since there was little change from using the heat gun and bending it the rest of the way did not over stress the material, I decided to go with what I had.

 

Edited July 11, 2021 by onthegas
Remove pic

[onthegas]

For all the trouble I had bending this material, you’d think cutting would be the easy part. Nope! Even on the slowest speed with my jig saw and feeding it slowly by hand the material would melt and fuse back together after cutting. A good whack with the hammer would break off the cut piece. Trying to trim the cut edges was another lesson. One method only resulted in creating more balled-up melted material. The sanding drum on my Dremel used at half-speed worked best.

[onthegas]

Pic from the other side.

[onthegas]

As for the shape, I drew it out based on the template from an old article I found where someone removed the frame rails and perforated bash plate from an M92 and replaced it with 3/8” thick aluminum. I’m not too thrilled about the shape, but I’m going to give it a go in some rocky sections tomorrow. 
 

If I were to do this again, I’d start out with making a cardboard pattern in the shape I want then trace it onto the material before doing and bending.

 

[onthegas]

Did some testing going over logs and rocks yesterday. My son was racing his ‘79 YZ250 and between motos I gave the skidplate a good thrashing. It may have just a bit less ground clearance now (another reason for me to lose weight), but I didn’t really notice it. This material is certainly more slippery than the bare frame rails. This might be more of an issue with large, angled log crossings, but hitting them straight on it just glides right over. Same with rocks, but the area I was testing it in only had a couple good sized rocks buried in the ground. A better test will be in a rock garden with loose boulders.