XR100/RM80 Conversion Pit Bike Build On A Budget – Part 4

This ultimate pit bike build is slowly but surely coming together… If you read the Previous Article on RM80/XR100 conversion bike, you’ll see that I had just welded up the frame cradle and set the actual XR100 engine in to make sure it fits. There are still some important features I have to add to the frame, but other than that, it’s just a lot of odds and ends to get the bike up and running.

The cradle is done, and the engine can plop right in, but first we need to strengthen it up. Why do we have to take the time to add more structure to it? This is an adults pit bike, and the whole reason we do these conversions is to make it a better performing bike that’s built to last! If it isn’t any stronger than the XR100 chassis it’s just going to break going over jumps, and may result in an injury for the rider.

I added a ‘gusset’ to the front of the frame cradle/down tube. I saw this on most of the other pit bike conversions, so I figured it was important to have. I just used a scrap plate of metal and cut it down to fit over the square tubing. (Don’t worry, I will be cleaning up my welds and the rest of the frame before it’s completely finished). Always remember to clean/grind the metal that you’re going to weld, otherwise it won’t get any penetration and will crack under load.

Frame Cradle Gusset

Next thing is to add/replace the upper frame tube that goes right over the engine. The original one was already cut off when I got the bike, so I just had to bend a piece of tubing and weld it in there. Since I didn’t have a great bender it took a while to get the piece to fit, and even then there’s less clearance than I would like.

Top Frame Brace

This will be a lesson for you, and for my next build. Although I can probably bend it a little more, I will still probably be pulling the engine out to do any work on the top-end. If I really wanted to take the time, it’s not too late to adjust the mounting location for the engine. If I were to do it again, I would lean the engine more forward so it sits within a half inch of the down tube, and that alone might lower it enough to be able to remove the valve cover. Fortunately, the XR100 has such a simple layout that it’s just a few bolts and the engine is out.

Now That the engine cradle and mounts that I machined are all ready to go, it’s time to stick the actual engine back in and start routing electronics. There was just barely enough room in the frame above the cylinder head. Since this is a budget build, and I didn’t want to use the mounts off the XR100 frame, I cut up a couple pieces of scrap sheet metal, bent them, and then ground them until they just barely slipped through the rubber fittings. I put a small ‘hook’ on the ends of them to help keep them in place.

Mocking Up The Electronics

Once I got those bent and fitted, I had to make a spot for the ignition ground to bolt to. I’m as cheap as it gets, so I just took a nut that the ground bolt fit in, and welded it to the inside of a washer.

It’s cheap, and it works.

Now here’s how it looks with all the mounts welded up. Looks like the electronics are ready to go, so we can move on to installing the bigger components…

Everything fits and works. If I can do it on an RM80, anyone can do it on a CR80 with more room.

I know it may not “look” like much, but these are some of the key components of this conversion build that are important to know if you’re considering starting an awesome project like this. Look for the next article soon, as I’ll be putting on the suspension, wheels, and other parts that make it look more like a dirt bike. Not to mention the start of my own little conversion in itself…. I’ve already said too much, so stay tuned.

Part 5: RX100!

-Tom Stark

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