No, this isn’t like one of those forum threads you see posted by some 12 year-old wondering how to make their xr100 fast enough for motocross racing. This is taking one of, if not the best, all-around and most reliable engines for adults and kids-alike, and placing it into a much more race-suited and agile chassis.
I’ve seen many of these builds from people in their small garages or workshops, and wanted one so bad that I just decided to do my own build to see what I could come up with. This is my first XR100 conversion bike, so it may not look factory in the beginning. I am doing this to show that with some basic and relatively inexpensive shop tools, someone can do this in their own garage on a budget, as it will be one of the funnest pit bikes to ride for us adults.
To get this project started, I happened to come across a rolling ’97 Suzuki RM80 that someone had started doing the conversion on. Luckily they didn’t go too far, so it was the perfect candidate since I picked it up for pennies. Most of these conversions are done with a Honda CR80/85R chassis, but I happened to find this Suzuki first. If I like it enough, I plan on building more of these conversion bikes. And that is when I will fix any issues I come across with this first build and take the time to make things look a little nicer. In addition, I’d like to try different bike combinations, such as a CR85 or YZ85 for the chassis, and possibly a TTR125 or DRZ/KLX125 for the transplant engine.
Here is how I got the roller…. I know, not much to look at, but when it’s finished, it will be more than worth the work. As you can see, the cradle has already been cut, and the previous owner started the build but gave up. Fortunately, I got a junk XR100 engine that I can use to mock up all the mounts (very useful because I didn’t have an XR100 yet).
Since this is such large project with so much information and lots of pictures (I know how everyone likes pics), I’m going to separate this into a few different parts. Next part will be tear down and engine mock-up, so stay tuned if you want to see how one of these is built in a garage…
If you’ve seen other XR100 conversion builds, you know that this isn’t just an easy bolt-in process. It takes many hours (especially if you haven’t done it before) mocking things up, welding, making sure everything fits, and fixing anything that doesn’t work on the way. There’s some skill required (welding is the most important), but I believe most anyone that is mechanically inclined and motivated can do this conversion. You must have patience though, as there are a lot of little things that take time…
If you haven’t read Part 1 on the XR100/RM Conversion Build, click the link before you continue reading here.
Part 2 is the tear-down and engine mock-up process. In order to get an XR100 engine in an 80/85cc mini motocross bike, you have to take everything off so you can cut and re-make the frame cradle. Getting the frame and engine mounts right is probably the most important part of this build. Not only does it all have to fit, but if you make the cradle wrong, the shift lever, rear brake pedal, or kick-starter may hit the frame. More on this later, as it is more difficult to explain without pictures. Here’s the tear-down:
Here’s the bike stripped and ready for mock-up.
In order to get the xr100 engine in a mini motocross bike frame, you want to place it as far down and back as possible. And to do that you have to cut off the lower mount on the engine cases. Below is a picture of what it should look like. You can cut it off, mill it off, grind it off (just remember that it’s aluminum), or any other way you can get rid of it. This is so you can set it back in the frame. In addition, it lowers the center of gravity on the bike (better handling), and there’s more room for the cross-brace above the cylinder head.
Now it’s time to start making the engine mounts. Luckily I have a junk XR100 engine that I can use to make my mounts since I didn’t have an XR100 dirt bike at the time. Here’s the material for the rear engine mount:
I used 3/4″ round stock and drilled it to the size of the bolt (didn’t have any tubing at the time). The steel plate is just some 3/8″ that we had laying around. I tried my best to center the pieces so everything bolts up straight. Other than the tools to take the bike apart, the next tool you will need is a welder. This is probably the most expensive part of your project, unless you have someone doing the welding for you. However, I highly recommend getting a just a MIG (wire-feed) welder if you plan on building more projects or repairing stuff yourself. I use a Lincoln 180 MIG welder because it’s simple, reliable, and relatively cheap. They are very useful and can save you a lot of time in the future. A MIG welder is fairly easy to use, but I suggest getting the basics taught to you by a local welder/friend. The goal on these engine mounts (and pretty much any welded part) is to get the best penetration so the weld is nice and strong. Remember to always clean the metal before welding, otherwise it will turn out looking like pooh and the weld will be more susceptible to breaking.
Below is the tube pieces welded to the plate and mocked up on the swing-arm bolt. The smaller length tube (lower) has a dimension that allows it to just barely fit in between the swing-arm. The longer tube length is suppose to match the length of the bolt tube on the xr100 cases. That way I can make tabs on it with holes for the bolt to go through. You’ll see in the next picture.
As you can see, part of the mount on the right case broke off. Fortunately for me it didn’t make much difference because I could still get the full length out of it. You want to take your time with this mount, making sure everything is straight, lined up, and the engine is where you want it to be.
Below is an estimate of what it should look like. My research found that you should have the counter-shaft sprocket aligned with the swing-arm bolt; both up and down, and side to side. This is so the chain is straight on the sprockets and rollers and won’t fall off.
We’ll go over the front engine mount in the next section because that is when I’ll be making the frame cradle.
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.
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.
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.
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.
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.
The Engine cradle is now welded together, ready for the engine mounts to be completed…
Just to make sure before I move on, I tossed the engine back in.
In the next section , 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!!
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.
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.
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.
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.
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…
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.
Time to start putting the big stuff back on to make this a rolling chassis… So far I really don’t have as many hours in this project as I anticipated. Although, it’s far from being over.
First, we’ll put the rear shock and swing-arm back on…
Then comes the front-end and wheels to make it a rolling chassis with an engine.
Now I want to share with you some of the issues I’ve come across on this conversion build, and try to solve them so everything properly functions. First of all, the brake pedal has some interference with the kick-starter shaft.
This is probably the easiest to solve, as you won’t need to make your own pedal like other builders did with the CR85 chassis. The brake pedal still operates, but you can just grind a small notch in it to make clearance for the shaft.
Next problem, which I’m still trying to figure out, is the airbox/intake. I want to make everything look as close to stock as possible, so I’m still using the XR100 intake boot from the carburetor to the cylinder head.
The RM85 airbox outlet for the intake boot is on the wrong side of the shock, and I can’t flip the intake boot because there is no clearance as it sits. I may have to modify/chop a section of the airbox out. Right now I have a pod filter on it right now for testing purposes.
And for the part you may or may not have been waiting for…… Since I do not like the look of the out-dated RM80 body-work, I decided to go the extra mile and convert it to the newer and much better looking RM85 styling.
I did research on what parts are the same and to see if anyone else has converted theirs. I found out the front fender is about the only bolt-on for the newer plastic, and I couldn’t find anyone else that had done the conversion.
That’s too bad… Because you’re going to learn how! If you’re already doing the XR100 conversion, the process to convert to RM85 plastics isn’t any harder than what you’re already doing; just takes a little patience and fitting of parts.
I don’t know how many hours I spent trying to figure out what I need to do, but I think it will be well worth it. This was because the 80 vs. the 85 parts looked the exact same, so I said screw it and ordered a tank, shroud, seat, side panels, and airbox off of eBay.
After setting the tank and seat on the dirt bike, there were no mounts/holes for the rear of the seat to bolt to… So I took another look at an RM85 frame, and it finally dawned on me…
Doh! There’s a post on the frame under the rear of the seat where it and the side panels bolt too! Since I’ve already done a lot of modifications to the frame, I figured this would be fairly simple.
On the flip side, I made yet another mistake. The last year of the RM80 was 2001, but it got upgraded side panels, as well as fenders in 2000, although the tank/shroud remained the same.
I didn’t know this when I ordered a seat from an ’01 RM80, but it doesn’t fit on the 2002 and newer RM85’s, nor a 1999 and older. This is a mistake that cost me, but now I know that they don’t match.
I used some scrap steel to make this. It’s about 7 gauge material, so it’s strong enough to hold up your rear-end. I just measured the distance between the seat mounts, cut, welded, and kept grinding the post until it fit, and the plastic bar touched the frame (see pic). I’ll drill the holes after I find out exactly where it’s going to be welded onto the frame.
That’s all for today. I just ordered the last of the plastics I need for the conversion, so the next update will be making everything fit together, and possibly the start of a custom exhaust… I’m hoping to have this bike 100% ride-able with just two more updates, so don’t forgot to come back and check soon!