XR100/RM80 Conversion Pit Bike Build On A Budget – Part 3

Part three of this ultimate XR100 pit bike build was probably the most fun for me, even though the bike won’t be finished…. Why? because I got to make my own frame cradle for the engine. You’ll never appreciate something as much as you do when you are able to build it all by yourself. Even though you start out with a complete bike, this project makes it feel like you built your own bike from scratch.

If you haven’t read part 2, read that first HERE!

In order to make a frame cradle for the XR100 engine, you have to cut out the one that’s already there. I started by chopping the down tube off…

There’s a few different options you can choose from, such as round or square tubing, and what size to choose from. Square tubing is the way to go because it’s flat and much easier to make engine mounts for. With that said, feel free to use whatever tubing you like (remember, this is YOUR pit bike build). I used 1.5″ OD (outside-diameter) square tubing because that’s what we had.

Frame Down Tube Tacked Up

I just centered it on what was left from the existing down tube. Make sure your piece is long enough; it’s much easier to cut a chunk off later then to weld an extension. After the down tube is in place, it’s time to mark spots for bolt holes for the engine mounts.

Engine Mount Lines Up

Next I cut what’s left of the lower cradle rails. I am going to bend them towards the down tube like a full-size motocross bike would, so I want to start from scratch.

Lower Cradle Rails Cut Off

This is where every build is a little bit different (unless you have machinery to make the exact same bends every time). A tube bender is very handy when you get to this point. However, this is a budget build, and not everyone (including me at the time) has the extra cash to buy a good one). A cheaper, and less than professional-looking way to do it can get the job done, which you may have to do if you don’t want to buy a bender. If you know something with one you could probably get the two pieces bent for pretty cheap though. Like I said before, I wanted mine bent because it looks better and there’s less welding. I didn’t feel like shelling out hundreds of dollars for a quality tube bender at the time, so I opted for the poor man’s bender… Yep, I bought I conduit bender that would fit my size tubing (Hey, it’s budget build, right?). It took a couple of tries, but after taking my time I bent my two pieces without having kinks in them. Before I actually bent and cut the pieces I took a piece of welding wire and put it up to the frame so I could get a model of what the bend should be like. It really doesn’t have to be much more than a 45 degree bend because you can put an angle on the tube where it meets the frame down tube.

Welding Wire To Copy The Bent Tubing
Bent Cradle Tubing

Now it’s time to line them up the best I can (they aren’t exactly the same bends) and tack them to complete frame cradle… I just lined the straight part of the tube up with the left over tube on the rm80 chassis.

Lower cradle tacked up

The Engine cradle is now welded together, ready for the engine mounts to be completed…

RX100 Cradle Welded

Just to make sure before I move on, I tossed the engine back in.

Yep, mock engine still fits just fine.

In the next article, I’ll be adding a couple needed things to strengthen up the frame, which I highly recommend if you plan on jumping this bike (who wouldn’t?!). I’ll also be putting the actual XR100 engine in the frame and getting the electronics set up, so stay tuned, because you don’t want to miss any part of this budget pit bike build!!

If you’re looking for Part 4 of this build, Here It Is!

-Tom Stark

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