Derbi GPR Track Bike

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We are considering a day to the track while most folks are out shopping, but damn it's going to be a cold day in Stockton!

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This is something I've been wanting for quite a while. They aren't really expensive, but you do have to actually order one for it to show up. Previously I would employ 50ml centrifuge tubes to measure out oil for forks. This 250ml graduated cylinder should make the job much easier.

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Tomorrow we plan to head out to the track for Black Friday. Ben and I did some bike wrenching today. I used the Saab for transport. I know it's silly, but I feel a bit giddy when I'm around this bike.

I didn't get good photos, but I added this choke lever that I got from Treats. It's going to work out well. It did require some pretty serious modifications to get it to work though!

Some photos of Ben's pocket bikes.

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Black Friday was a nice day to ride at the track, but getting to the track took some effort due to all of the crazy shoppers clogging the freewayz! Also the temps were quite cold. About 13°C at the warmest. We were probably riding most of the day in even colder temps. But it didn't feel bad at all. Sure it wasn't the track sticky 25-30°C we were enjoying over the summer, but it was fine.

This time I went out with my Pocket Bike friends--Ben, Al, and Jordan. There are six bikes in the back of this truck.

Ben, Al, and Jordan all use to race pocket bikes, and they are fast--like a squadron of Tie fighters with amped up Polini engines. I can't even pretend to keep up with them on the track. Oh sure my bike is capable, but I'm not. I have a lot of work to do to get back to my old riding form before the Honda Driver took me out. The GPR ran pretty good. I replaced the intake with one that isn't perforated. But it's not the right size for the carb, so I'm going to have to source a proper one for the bike. The Domino choke lever was a much appreciated upgrade, and considering how cold things were, much needed too. This bike doesn't like to start with out the choke, and especially so when cold.

Because it was so cold I took it mellow on the track, and never once felt the tires slipping. Hey I just started riding again, I don't want to crash out. After the sun set we had some nice hot laps though. Ben was showing me some race lines, and I kept saying to myself--these are so much better!

We nearly had the track to ourselves too. Early on there were some carts, and yeah there were a few evil four strokers making things sound horrible, stinking up the place (yes four strokers stink way more than two strokers), and causing a ruckus. But most of those cats left early, and we were able to enjoy a mostly open track. We also got to run the track in both directions--Euro Mode, and NASCAR.

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Today was the annual Nor*Cal Dyno day. Once a year they invite moto folks to bring their bikes in to see what they've got. It's generally a whole bunch of fun. They are super cool chaps, and a real boon to the community. This year they even had pizza for us!

Before heading over to meet up at Ed's I unboxed Mike's new RCS14 that he picked up from KurveyGirl. It's really sweet. That titanium banjo bolt weighs so little that you can't even tell it is in your hand. Kurvey throws that, some tubing, and a bunch of crush washer in with the RCS. Is's that cool, or what? I put it on the bike, but didn't have a chance to take photos yet. The reservoir for it has yet to show up--so I used one from a GPR rear brake for now.

I brought Mike's bike in with the Saab. Ed took his van with his GPR, his son's KTM, and my GPR in the van.

Alex's KTM went first. I think it did around 7HP. Pretty respectable for such a tiny little bike. Alex is so fast on this bike that he beats both Ed, and I around the track--and Ed is pretty quick too.

Next up was Ed's GPR. His bike is really strong. It has all the cool Metrakit Pro Race goodies. He got well over 17HP!

Then we ran my bike. Almost exactly the same as the engine did three years ago. I was expecting better performance because of the better pipe, ignition, and carb. This is really challenging some of my preconceptions!

Sadly we were unable to get Mike's bike running well enough for even a single run. I think it's the Mikuni carb. I mentioned to Ed that I thought it might be the Mikuni, and his response was "I hate those things. I just gave mine away." Then he spent about 20 minutes trying to get it running too. Looks like we might have to get a PWK for this bike too.

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Ed sent me photos of his dyno print outs.

The KTM

Metrakit PR GPR

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Here are the scans of the charts from my bike this year.

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Here's the dyno of the 45 stroke Airsal Xtreme with the same pipe and carb that are currently on my track bike. I also found that the dyno run from three-ish years ago was also done with the 24mm PWK carb, so that's no longer a factor.

Now the interesting thing is this dyno run was with the same carb and pipe as the dyno run on the engine currently in the track bike. They even both have HPI ignitions, so those are effectively the same. The air box is different, and maybe I need to do something about the air box on the track bike, but that engine was using the air box used on this dyno run back when it was in the Black bike (I know this is all rather confusing!). But what's interesting is basically with the only difference being the top end/crank configuration the peak power is in very different locations. So is the pipe really controlling the peak? I use to think so, but the data is messing with my head.

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I finally had opportunity to put the All Balls KTM SX65 seals into the Derbi forks today. Ed granted me access to his bench so that we could use the pneumatic action to loosen up the bolt on the hydraulic tube. I've not yet figured out a way to loosen this bolt with hand tools.

Now these SX65 seals aren't an exact match to the stock seals. So far the forks looks to be well sealed, but I wont know for sure until I mount these on the track bike, and do some testing. I really hope these work, because then we have a very nice solution for these bikes. I also intend to play around with the fork oil weights, and quantities. Those of you with a sharp eye will notice that I'm using 5 wt, rather than 7.5 wt. Yup, trying out a little experiment.

The Forks: These are the 2003-2004 GPR Sebac forks. The require slightly different seals than the older ones. The 2002 forks do share the same seals, but those are anodized gold rather than silver.

I think I already showed how to disassemble these forks earlier in this thread so this is just the assembly section. Lube up the seal, and place it into the fork tube. It wont go in all the way, but it will slide down to the seat. With the new seal in the fork tube used the top cap to seat the seal. It doesn't take much force, but you want to apply it evenly. You will know it's in place because you'll be able to see the ring for the retaining clip.

Once the inner seal is in place insert the retaining clip.

Then insert the dust seal. You should be able to put this one in place with just your hands. It doesn't take much force, just a little bit of finesse.

Then slide the stationary tube onto the sliding tube. Insert the spring. Reach in and pull out the hydraulic tube. Add the nylon spring cap. Now it's time for the secret tip on how to do this. It's nearly impossible to hold the hydraulic tube to insert the bolt. So take a small screw driver, and insert it into the small hole in the top of the hydraulic tube. Rotate the spring so that the tube reaches high enough that you can thread in the bolt. Then torque the bolt.

These forks take 120ml of 10 wt fork oil. You can change the wt, and amount to change the dynamics of the fork damping. Lower weights will decrease the damping. More fluid will stiffen the forks. Grab your graduated cylinder (you can get these off eBay for less than $3), mark with a Sharpe where you want to measure to, and pour in the desired amount. Pour this into the fork. Insert and torque the top cap.

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We needed a nice affordable solution to the brake master reservoir for the Brembo on Mike's bike. I found some Chinese brand that was State side, and ordered up a few. They look pretty decent for what they are—now I just have to fashion up a bracket.