Upgrading To A Disc Brake:
By Tracy Hansen

Update History Created 1/24/05

1994 XT225 Upgraded to Disc Brake

The Author

INTRODUCTION:

As I followed along on the progress of Scott's wheel and brake project, I developed a desire to have a rear disc brake on my bike. The type of trail riding we do puts an unusual demand on the rear brake, particularly during steep descents. Mud, dust, dirt and creek crossings all added to the ineffectiveness of the drum rear brake. But, alas, it seemed out of my reach. One day while looking on eBay at Kawasaki Super Sherpas for sale (and drooling over the factory rear disc brake) I noticed two separate offerings from the same seller: a complete rear wheel and hub with sprocket, rotor and spacers; and a complete rear disc brake parts package. Both offerings were from the same 2000 Kawa Super Sherpa being parted out. Whoa! Get back! Dreams of a disc danced in my head. After getting some measurements from the seller to determine if the wheel and hub would fit inside the Yama swing arm, I decided to go for it. I won both auctions for a total of $205 including shipping. (Gulp! Now what?) I have a 12' X 20' heated art glass studio beside my house. I have a set of metric sockets and end wrenches, screwdrivers, hammers, an electric drill, an electric jig saw, a Dremel tool, some old hand files, a hacksaw and a few other miscellaneous hand tools. No welder. No lathe. No mill. No drill press. Not even a bench vise. (Laughable. And you're gonna do what?) When the parts arrived they were all in good condition. Good start, at least.

CRITERIA:

If I'm going to pull this off it has to be within the capability of my tools and I must use hardware store solutions as much as possible. The project has to cost less than $400 (this was an idea strongly suggested by my wife). It doesn't have to be pretty, necessarily, but it has to be strong and reliable.

WHERE TO START?

Scott helped me to determine measurements for centering the wheel in the swing arm and to measure for future sprocket alignment. I went down to Home Depot and bought a 6" rule with 64ths increments with a sliding device to reference the measurements. I think it cost $4 (or maybe it was $3). I slid the Yamaha wheel assembly all the way back in the swing arm adjustment slots, marked a line on the swing arm as a reference spot and measured to the center of the Yamaha rear sprocket. I then removed the old wheel and put the Kawa wheel in its place. Both the Yamaha and the Kawa axle bolts are the same diameter, 15mm.

CENTERING THE NEW WHEEL:

I slid the Kawa wheel all the way back in the swing arm just like I did the Yamaha wheel. I took a straight edge and marked a line square across the front of the wheel to both sides of the swing arm. This gave me the same relative place to measure from both sides of the swing arm to determine the centering of the wheel. With the stock Kawa spacers in place on both sides of the hub and the caliper bracket on the axle bolt, I was able to see what I needed on each side to center the wheel. I went to the hardware store and bought six 16mm washers for 15 cents each. It worked out perfectly for three washers on the right side and two on the left side to center the wheel. Most fortuitous. (a fancy way of saying dumb luck!) I'm encouraged, but the battle had just started.

SPROCKET ISSUES:

Now that I have the wheel centered, I can see that the stud bolts used to secure the sprocket to the hub will hit the welded bracket on the bottom of the swing arm used to hold the chain guide fin. My chain guide fin had been long ago broken off and was dysfunctional so I removed the protruding part by hand with my hacksaw. Shear determination, brute force and ignorance are the only substitutes I have for sophistication and expertise. I started to hand file off the roughness after my hacksaw cut until a friend took pity and lent me his electric hand grinder. Much faster and better! The wheel now rotates freely in the swing arm space without hitting anything. From my reference mark on the swing arm done earlier, I compared the measurement to the center of the Kawa sprocket to the measurement to the center of the Yamaha sprocket. Another happy bounce of good fortune. The two measurements are very close, .031" difference. I can easily deal with the small difference later if needed. The Kawa uses a 520 drive chain and the Yamaha uses a 428 chain. I needed to have a custom sprocket made to fit the Kawa hub and still use a 428 chain. I contacted Scott about doing it but he was buried in other work at the time. I contacted Sidewinder Sprockets and they agreed to make me a 58 tooth aircraft alloy sprocket to fit the Kawa hub that would use the 428 chain for $109.

THE CALIPER SLIDE AND TANG:

There is a tang welded to the swing arm on the right side that slides into the slot in the Yamaha drum brake face to keep it from rotating against the braking force of a stop. The Yamaha tang is too thin, sticks too far out from the swing arm and is not long enough to properly fit the Kawa caliper bracket slot. This is the biggest obstacle I've encountered in the conversion. A significant alteration needs to be made and I have no welder or the right tools to accomplish this. I spent a couple of days thinking about the problem and considering different solutions. I decide to completely remove the existing tang, grind off the remnant and start over. However, cutting off the stock tang puts me in a position of no return. Once I make this alteration to the swing arm I cannot turn back. Again, shear determination, brute force and ignorance are the power to overcome my lack of good sense. It took me an hour and a half to cut the tang off with the hacksaw. The borrowed electric hand grinder finished the job nicely. After measuring the Kawa caliper bracket slot, I spent an hour in a local, well stocked hardware store considering different possibilities to make a new tang. A piece of shelving upright is just the right size coupled with some pieces of 1/4" aluminum for a backing plate and inside reinforcement. I bolted this concoction through the swing arm with three 1/4" capscrews and use nylon insert lock nuts. I smeared GE Silicone 2 on the plates where they mate with the swing arm to seal the holes tight against water getting into the swing arm. I had to oversize the bolt holes a bit for some adjustment to set the new tang exactly right for the caliper bracket to slide in the tang for future chain adjustments. I had to fuss with this solution for a half day to get it to work right, but I think it turned out okay. (See Figure 1 below) It's a little hokey, and it ain't purdy, but it werks.

MASTER CYLINDER AND FLUID RESERVOIR TANK MOUNTS:

I had no way of welding additional tabs on the bike frame so I had to use the existing mounting places from the passenger foot pegs and the center exhaust support bolt tab. This was fairly easy to figure out and solve. I made templates out of heavy paper and cut the brackets out of 1/8" aluminum plate purchased at the hardware store. I used my electric jigsaw with a metal blade to cut out the shapes. I finished them up with a hand file, drilled the holes and mounted them. They worked out great. The fluid reservoir tank had to be mounted in the vee crotch of the bike frame close to the exhaust. Fortunately, the Kawa used the same relative location on the Super Sherpa and provided a nice stainless steel heat shield between the fluid reservoir and the exhaust. This is starting to get exciting. I'm as happy as a pig in slop.

BRAKE PEDAL ALTERATION:

This is the last big hurdle to cross. I have the original Yamaha pedal and I have the Kawa pedal that came with the brake parts deal. The Kawa pedal had the offset on the back of it I needed to align the pedal with the master cylinder. I cut the offset off the Kawa pedal, cut the back off the Yamaha pedal and used the left over pedal steel to cut and shape the components that needed to be welded together to alter the Yamaha pedal. My friend with the electric grinder also has a MIG welder in his garage. I took it over to him and we welded the parts together. I brought it back to my little shop, put it on the bike and it was about as perfect as I could have ever hoped for. The alignment with the master cylinder was right on and the action from the pedal was perfect. While we were welding parts on the pedal, I moved the clip for the brake light switch spring connector about an inch forward from its original location, hooked it up, did some minor adjustments at the switch (the switch can be adjusted up and down by turning the nut where it fastens on the bracket that holds it on the bike) and it works great right out of the chute. Walla walla bing bang! More good bounces. One other detail: I needed a brake pedal stop to prevent hyper-extending the master cylinder push rod if I hit a rock or something with the front of the pedal. I solved this by designing and cutting a piece out of some 1/8" steel plate I had laying around. I made it to fit between the foot peg mount and the frame, designed a turned up tail to sit under the pedal at the back end when the pedal is in the rest position. (See Figure 2 below) That worked our very well, was easy to do and is very strong.

INSTALLING NEW PADS AND BLEEDING THE LINE:

I found an on-line service manual on the Yahoo Super Sherpa forum and down loaded the information on the rear disc brake. This helped tremendously. I purchased new brake pads from the Kawa dealer for $31. I took both the caliper and master cylinder completely apart, inspected all the components, cleaned, relubed everything according to spec and reassembled everything. All the components looked real good so nothing had to be replaced. I had trouble getting the master cylinder to work until I finally figured out that there was an air lock in the tube from the fluid reservoir to the master cylinder. Once I got rid of the air in that short tube, the rest of the bleeding process was accomplished without another hitch. I can't tell you the feeling I had when I saw the caliper piston begin to move and clamp their little pea pickin' pads around the rotor. Hey! This thing actually works! The new sprocket was delivered in the best timing (Sidewinder is a great company to work with). I mounted the tube and tire and was ready for a test run. The only problem was 5" of snow and 18 degrees BELOW zero! I did get to do a brief road test yesterday and, so far, so good. Believe me, the disc brake is very nice.

SUMMARY:

If I can do this project, anyone can. The key is finding the right parts. I think just about any Japanese dirt bike parts would work as long as they fit within the XT swing arm. The initial parts purchased on eBay, the custom sprocket and the new brake pads cost $345 total. I probably have another $45 or $50 in materials so I'm under $400. I did splurge on a new chain and front sprocket, but I could just as well have done the project successfully without the new chain and front sprocket. I kept track of the weight of all the parts taken off and the parts added on. With the removal of the passenger foot pegs on each side, the bike is a pound and a half lighter with the disc brake project finished. So, there you have it. I now have a Yamasaki Super Stoppin Serow Sherpa.

Figure 1 - Inside Swing Arm Showing Tang	Figure 2- Brake Pedal & Master Cylinder
Figure 3 - Wheel, Rotor & Caliper in Swing Arm	Figure 4 - Modified Brake Pedal
Figure 5 - Rear Wheel Assembly Mounted