From Motocross to Roadracer: One Father's Journey in Suspension
Feb 20, 2007, By Clay Adams,
I’ve always said that suspension/geometry is the most important piece in making a fast bike. Here in America we’re always so focused on power. We want MORE, yet rarely do we stop and really think about handling and getting all that power to the ground. Any jaunt to the local “sportbike” hangout will help you realize this quickly. Stock suspension is just as common as exhaust pipes and power commanders. So, when it came time to build my son’s race bike, I knew exactly where the money would be spent.
I sent off an email to Max at Traxxion Dynamics to see if he would be interested in helping out. Max was going to fully sponsor me in my own road racing efforts. However, I had just recently decided to give it all up for my son to make his own start in racing. Max understood this and agreed to take on Caleb’s project bike, a KTM 65sx.
The stock forks on all KTM 65s 2002 and forward are USD Marzocchi 35mm. They’re a good platform, but they have no adjustability. With that, I spent quite some time trying to figure out what base I should start with. I knew we would have to revalve. However, after racing and riding for years, I feel that adjustability is extremely important. In order to get this I would either have to go with an aftermarket cartridge kit or a completely new set of forks. I mulled over the options all winter while saving my money. In the end it was down to 3 options: a full set of shiver forks from Marzocchi, a cartridge kit from Marzocchi or a cartridge kit from BUD racing. I searched everywhere I could. Sadly I could find no information on the BUD racing setup. They LOOK exceptional; of course, they should be with their price tag.
After all was said and done I decided to go all out and purchase the Marzocchi Shiver forks with .50 springs installed. It took nearly 3 weeks to get them to me, but when I got them in I was very excited. While they weren’t FULLY adjustable, they did allow for compression on one fork leg and rebound on the other. After “bouncing” on them, I was a little disappointed. They felt odd, like they would react differently at different areas of the stroke. The rebound adjustments didn’t do much, but the compression did. I figured they’d be getting revalved anyways, so I really didn’t worry much about my initial impressions. Externally they looked beautiful and I’m sure the trick DLC coating would greatly help cut down on stiction. I put them back in the box, added the shock off my son’s ’02 SX and sent them off to Traxxion Dynamics. Once they got them they would get roughly a week to toy with them. After that Caleb and I would drive down to Traxxion and spend the day getting the bike dialed in for his weight.
A few days after Traxxion received the forks I got a call from Max. He was a little confused as the cartridge on the rebound fork had holes DRILLED thru them. (This explains the odd behavior I experienced in my initial impressions.) For those that don’t understand the basic dynamics of how a cartridge fork works, let me explain. A piston travels up and down a cylinder. This piston has holes in it (also called ports) along with an o-ring around it to seal it in the cylinder. Valve washers (also called "valving" or "shims") are thin, flexible spring steel washers that are stacked on top of the piston to cover these holes and control just how much fluid passes thru the piston during a stroke. If holes are drilled thru the cylinder, what good does the valving do?!?! All of the fluid will pass thru those holes instead of the valving. For example, go out and drill holes in your engine’s cylinders. How well do you think your engine is going to run? To top it all off, you could look thru the holes and see that the piston was nothing more than a plate. The “valving” and “ports” were essentially the cylinder. There was no adjustability to this design outside of simply adding heavier fork oil. I wasn’t happy.
I called up the vendor who sold me the forks and he said that’s how they’re made straight from Marzocchi. He didn’t know why, but that is their design. At this point I had a very expensive set of junk. Martin, the genius tech who does most of the high end work at Traxxion, had some wheels turning though. We tossed the Marzocchi cartridges all together and he got to work.
As you can imagine, a shop like Traxxion has a large stock of cartridges from different models of bikes on hand. Martin sifted thru several different types and ended up grafting a set of GSXR 1000 cartridges to the new forks. In his words: “I used a "modified" GSXR 1000 cartridge that was machined to fit inside the Marzocchi fork. The valves were Showa with our compression valve holders with our valving spec.” The .50 springs were retained and a few different spacers were made in order to get the correct sag set. The springs were almost perfect considering we’re going for a race setup. We ended up using a 5mm spacer. The cartridge reduced the length of the fork by nearly 2”. However, in typical MX fashion the front was very tall. The reduction in geometry would be beneficial in a road racing application. The compression was set and the adjusters now control rebound damping on both fork legs for fine tuning the rebound.
The stock shock on the KTM is made by WP, and is a very capable unit with compression and rebound adjustability. Not as good as say, an Ohlins or Penske, but definitely well enough for the job. The rebound adjuster was stuck and had to be freed though. Years of dirt riding hadn’t done the shock much justice. Martin completely disassembled and cleaned everything and figured out what the issue was with the rebound adjuster. After thoroughly cleaning all of the internals, Martin came up with a good valving combination and reassembled everything. He put it on the shock dyno and confirmed that his combination would work well.
Now came the hard part, finding the right spring. Traxxion is mostly equipped with springs made for road racing bikes. They’re fat and short compared with the springs for MX bikes which are long and skinny. Even in the case of this mini, the stock spring was longer and skinnier. Martin’s wheels got to turning again and he ended up with another very cool idea. With a homemade collar we were able to adapt a Penske spring to the shock. We tested a few different springs to figure out the right spring rate. Once the shock was installed it just barely cleared the silencer pipe, but it cleared none-the-less. We ended up with a 550# spring where stock was 270. It sounds crazy heavy, but it set us up with the perfect free and rider sag.
After all the work was said and done, the bike has been transformed. Just sitting on the seat allows you to see a perfect balance between the front and rear action. They’re perfectly in tune, the way it should be. Before the change the rear would sink like a couch cushion leaving the front end pointing towards the sky. However, if you were to brake hard the front end would dive towards the ground like a high diver going for the gold. The rebound action was horrid for road racing and would bounce all over the place. The forks are now wonderfully plush and soak up bumps well, yet the bike maintains perfect traction and is not affected.
I would still love to know how the BUD cartridges would handle the task with a revalve. We have another “backup” bike that we might make future work out of. For now though, my son’s suspension ROCKS! Caleb dropped several seconds at his trackday after the change. This setup will work better the faster he goes, so we look forward to the future as his skill grows. I have to give serious props to Max for hooking us up and to Martin for the ingenious work and overtime he put in getting this ready in time.
If anyone is interested in a similar setup for their bike, please contact Max here: motorcycle suspension or by calling 770-592-3823.
