Upgrading To A Disc Brake:
By Scott Bridgman

Update History
Created 10/24/04 (not edited)
Edited 10/30/04 (misc corrections)
Sprocket & Rotor 10/30/04
Project Finished 11/15/04
Concluding Notes & Final Edit 11/26/04
Add Parts Ref 12/01/04

2003 XT225 Upgraded to Disc Brake
The Author

INTRODUCTION:

I've owned the XT225 bike for some 16 months now and passed 4000 miles. About 95% of these miles are off road in very rocky terrain. Given our terrain, surrounding neighborhoods, no legal riding places and access logistics the bike must satify the following criteria to allow for an enjoyable ride and a return trip:

1) It must not look like a "hard core" dirt bike.
2) It must be completely street legal from the factory.
3) It must be able to do highway speeds.
4) It must be as light as possible.
5) It must be quiet.
6) Its image must not trigger negative responses from onlookers.
7) It must be durable like a trials bike.
8) Performance must be strong and quick.
9) It must be geared low to allow crawling.

The XT225 pretty much meets all of the criteria above except number 7. Early on I started to experience problems with the rear hub in the form of loose spokes. Initially loose spokes could be tightened to restore the integrity of the rear wheel. Next the rear brake started wearing rapidly (about 300 miles) and would lock when wet or damp. Releasing the brake pedal would not unlock the brake. It was necessary to reach under the pedal and lift it up to get it to release. A locking rear brake would cause the rear wheel to skid at times when this wasn't desired. Having no rear brake is just flat out dangerous. On several occasions the bike would jump out of first gear into neutral during a steep down hill. When on loose terrain with lots of small rocks and rounding a switch back it creates a possibilty of going over an embankment. I've been riding without a rear brake now for weeks. The spoke situation continued to get worse. Spokes began to snap on the sprocket side right at the shoulder. Replacements were put in as required. The XT225 has 4 different types of spokes and only kits of 36 are available for around $50. I've puchased about 4 kits to cover busted spokes on 2 bikes. To make matters worse, the holes in the cast powdered metal hub started to elongate making new spokes too long. This problem was solved by cutting about an 1/8" off replacement spokes and rethreading them with a threading die. Again not a good idea. Spoke threads are rolled on to work harden the metal. The last failure level I dealt with was spoke nipples ripping right through the thin rim. To repair this problem the wheel needed to be removed, tire and tub taken off and custom washers made to effectively increase the size of the spoke nipple's head. Following about 2500 miles, wheel service of some kind was required about every 2 weeks on both bikes.

Figure 1 - Stock hub showing flange hole wear

Figure 1 shows the stock hub's flange holes worn badly out of round. The soft powdered metal used for the hub is much softer than the spoke metal. Add to this a very sharp edge on the exit of the flange hole with poorly fitting spokes and you have a weak wheel. The XT225's rim is also thin. Spoke nipple heads are small and will easily rip through the rim. 6 have pulled through on my wheel with more on the way. Yamaha was contacted through my dealer and no one bothered to return my inquiries. I expected this from Yamaha but was a little disappointed about the fellow I know there. This bike, as good as it is, is a low volumn item and I suspect not a real profitable product for Yamaha in the US. Most people don't use the bike as I do so a cheap poorly designed wheel lasts most owners a long time and saves Yamaha a few bucks. Yes I could bitch and try to make everyone do the right thing but I'd rather spend the time solving the problem. Yamaha added that they will absolutely not sell the serow parts here in the US.

The rim is also thin and cheap. Keep in mind the wheel is laced with a cross 2 pattern which is a weaker lacing pattern. Cross 2 means spokes that are slightly shorter at the expense of greater pulling straight towards the hub. Short spokes save yet a few more pennies. Small heads on the spoke nipples save some metal as well, again at the expense of strength. Figure 2 shows how the spoke nipples pull through the rim leaving the rider with an hour of repair time. Some better nipples, quality spokes that fit, thicker rim, chamfered flange holes and a cross 3 pattern would have made this wheel much stronger. Whether it would have been strong enough for rocky terrain is an unknown. Based on the above and the fact that no bike is available off the shelf that satisfies the above eumerated criteria it left me with five options. 1) Get rid of the bikes and abandon the sport. 2) Ride only on pussy trails. 3) Continue rebuilding the wheel. 4) Attempt to locate serow parts. 5) Upgrade the rear wheel myself to a quality design.

 

 

 

 

 

UPGRADING TO A DISC:

Figure 2 - Spoke Nipple Ripped Through Rim
Figure 3 - New Wheel w/Sprocket Adpater & Spacers
Figure 4 - Close Up of Adapter & Spacer
Figure 5 -Disc Rotor Side & Spacer
Figure 6 -New Wheel in Swing Arm
Figure 7-New Wheel in Swing Arm

Option 5 was the one that made the most sense given I have access to all the tools required to tackle this problem. Using my own tools and knowledge means the job gets down exactly the way I want it. To the best of my knowledge and research no one has attempted this conversion on an XT225 bike. My first stop was a local guy that specialized in all type of motorcycle wheels. He came and took some measurements and assured me this would be no problem. "Piece of cake" to use his words. To make a long story short I pissed away 3 weeks only to discover this guy was a flake. My other friend put me on to a company in Southern Utah called RadMfg. They make a quality modular hub system for dirt bikes and can build complete wheels. The only problem is they don't make anything for the XT225. I measured the swing arm, hub, spacers and created a drawing to fax to Rad to see if a hub was available for another bike that would fit. It turns out the KX250 hub is small enough to fit. The wheel I received was a quality product and bullet proof. I was left on my own to machine spacers and the sprocket adapter. Figures 3 - 7 show the new wheel build, custom spacers and the sprocket adapter. The sprocket adapter for the KX250 was way too thin which means the sprocket won't align with the counter on the engine. Rad uses a special pattern on his hubs so that needed to be disected to create the mating surface for the new sprocket adapter. Other measurements were somewhat straight forward. 6061 T651 extruded stock was used for the adapter. 2024 T3 was used for the spacers. It appears that enough space exists for the rotor and caliper. These parts are due in on 10/27 so I'll know better then what's going to be required to install everything. I'll be back with another update soon.

 

MAKING A SPROCKET:

As mentioned the primary flavor of the hub is the Kawasaki KX250. Thus rotor, caliper, and master cylinder are Kawasaki parts. This allows me to use the rotor mounting flange on the hub which saves hours of layout and machining time. Unfortunately I didn't get this lucky with the sprocket. I did preserve the KX250 sprocket mounting pattern with the hope I would be able to get a sprocket cut to that spec with 58 teeth for a 428 chain. (The KX250 uses a 520 chain) Sprocket Specialists could do the job but indicated they were backed up, creating lead times in excess of 6 weeks. What's special about this sprocket is it uses the KX250 mounting pattern but must fit a 428 chain. Since the mounting flange I designed allows for a flat sprocket I decided to cut one myself to the specs I needed. The Machinery's Handbook has all the standard roller chain specs and a standard #40 chain is the same pitch as the 428 chain of 1/2". The 428 roller diameter is different at 8.51mm. The equations to generate the tooth form are somewhat complex so I needed to write a small program to crunch the numbers. After about 4 hours I had a 58 tooth sprocket layout for a 428 chain and KX250 mounting pattern in the CAD system ready to go. Figure 8 below shows a plastic model of the sprocket cut on the CNC mill.

Figure 8 - Plastic Sprocket Model

Figure 9 - KX250 Rotor in Position

Figure 10- KX250 Master Cylinder
Figure 11- 58 Tooth Custom 428 Sprocket
Figure 12- Master Cylinder Installed
Figure 13- Caliper & Rotor Installed

 

This model was used to check the tooth form, chain fit, mounting pattern and alignment with the counter sprocket on the engine. All fits fine so I can proceed with cutting the final sprocket when the material shows up. I'll use low alloy high strength steel like 4340 or 8620 to maximize toughness. Heat treatment can be done in house. Figure 9 shows the KX250 rotor installed on the flange. As mentioned this flange came with the hub. The caliper is on B/O for 6 weeks so I'm looking for another source of supply and hoping to get lucky. Next challenge is to make the brackets to mount the master cylinder shown in Figure 10. The brake pedal will need to be modified to actuate the master cylinder.

 

SPROCKET:

A new custom sprocket was machined using 1045 steel and is shown in Figure 11. This sprocket is flat with 58 teeth for a 428 chain. I decided against the 520 chain as it isn't required. The 428 O-ring chain had performed well for several thousand miles. Using the 428 chain means the counter sprocket remains stock.

 

MASTER CYLINDER & BRAKE PEDAL:

Installing the master cylinder and modifying the brake pedal was challenging. The stock brake pedal lever pulls a rod to seperate the shoes in the original drum brake. The master cylinder requires a pushing force to actuate it. Figure 12 shows the completed installation. Brackets were made to keep the cylinder inboard and secure. In addition to the passenger peg tabs one additional tab was welded to the frame to provide the 3 mounting points. A special extension was added to the brake pedal to create an upward pushing action beyond the pivot and under the master cylinder. The free lash screw was moved to a new location just above the new lever extension. A heavy bracket was created for this screw to prevent the brake lever from hitting the engine case if it were to be pushed up from terrain contact.

 

ROTOR & CALIPER:

Originally I had planned on using a KX250 caliper but Kawasaki wasn't able to provide the parts quickly. The TTR-250 rotor is essentially the same diameter as the KX250 so I was able to use a TTR-250 caliper instead. It was available in 5 days. A spacer was machined to bring the diameter of the axle portion of the caliper mounting bracket down to 15mm. A second T profile spacer was machined to adapt the slot to the existing tang on the inside of the swing arm. An additional tang was weld in to increase the contact area of the caliper mounting bracket with the swing arm and thus increase strength. The caliper mounting bracket is required to be secure but needs to float when the chain adjustment cams are moved. As it turned out plenty of room existed for the caliper mounting bracket which was an initial concern. Good clearance exists between the inboard side of the caliper and spokes which was another initial concern. All in all everything fit on the bike perfectly some of which was just dumb luck. And no amount of planning can replace good old dumb luck.

 

BRAKE TEST 1:

I test road the bike today (11/15/04) for the first time and the brake delivers all the performance one would expect from a disc. The pedal will easily lock the back wheel up with about 1/2" of travel. Soft braking is smooth and modulation is excellent. Because of break in requirements I didn't perform too many hard stops. I'll need to let the pads and rotor work in first. Figure 13 shows a slight pad wear line forming in just the correct position on the rotor. I'll need to run the bike in the woods for several hundred miles to really test everything properly. I'm not anticipating any major problems but some small adjustments may be required. A guard should be put on the caliper and master cylinder. A shark fin under the rotor wouldn't be a bad idea either. All in due time. The rear wheel, spokes and hub are bullet proof and I'm sure will provide trouble free operation for a long time to come.

 

BRAKE TEST 2:

On 11/17/04 I rode about 32 miles in the woods and again on 11/21/04 I did another 8 off road miles. This ride was a more severe test of the brake owing to the usual deep puddles, mud, sand and rocky terrain we ride here. As expected the disc provides perfect braking under these conditions first time every time. Coming down steep off camber hills covered with slippery leaves is a good test for soft brake modulation. A locked rear wheel will most certainly cause you to drop the bike as the rear end slips down the hill. The front brake is almost useless in the situation so one relies on engine braking and the rear brake. On occasion the bike will jump out of first gear which means the rear brake takes the full stopping load. I was very pleased with the disc's performance under these conditions. I didn't have the bike jump out of gear but this can be simulated by pulling in the clutch. The only small problem I had to fix was the brake light switch. With the disc the deflection of the pedal is reduced during braking which doesn't actuate the mechanical switch. On the TTR-250 a pressure switch is used to replace the banjo bolt coming off the master cylinder. To avoid running electrical wires across the exhaust pipe zone I opted to move the eyelet on the brake pedal forward to increase travel to the switch. This seems to have worked out fine. If this solution becomes problematic I'll install a pressure switch and rerun the wires. All that remains to do now is dispose of the rear wheel.

 

PARTS REFERENCE:

A total of 14 small parts where made in order to complete this project. Both steel and aluminum where used as the raw materials. Any parts required to be welded to the bike's frame obviously were made from steel. Aluminum was used elsewhere for its light weight and resistance to rusting. Figures 14 and 15 show the total collection of the custom parts I made to complete this project. None of these parts were difficult to make but required the use of a lathe and mill.

Figure 14 - Parts Used Mostly for Frame Mountings

Figure 15 - Parts Used to Install Rear Wheel

 

14-1 Free lash adjustment screw (not drilled yet)
14-2 Brake pedal to master cylinder joint
14-3 Brake pedal side shift
14-4 Caliper bracket swing arm tang
14-5 Master cylinder holder (X2)
14-6 Master cylinder reservoir holder (not bent yet)
14-7 Caliper bracket slot shim
14-8 Frame tab for master cylinder bracket
14-9 Brake pedal lever extension
15-1 Right side spacer swing arm to caliper bracket
15-2 Caliper bracket axle shim
15-3 Right side spacer caliper bracket to hub
15-4 Left side spacer swing arm to hub

 

 

 

 






The End