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Thread: Derbi GPR Track Bike

  1. #1
    apriliaforum expert ToraTora's Avatar
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    Cool Derbi GPR Track Bike

    PhotoBucket has decided to extort their customers. I've switch to using Imgur for the new stuff. You can view the older posts in this thread in two ways.

    First you can use the Chrome browser with PhotoBucket fix extension.

    Second you can view it with most any browser over at GPR Camp on the foro there. I've split it up into a number of threads.

    Derbi GPR Track Bike

    MT Mike's Track Bike

    Cobra CX65 Mini Moto

    PPMPR: Project Poor Man's Pro Race

    Please do check out these threads. There's lots of great information within!


    For sometime now I've been collecting parts that would ultimately end up on a bike to take to the track--kart track that is, not a full on moto race track. Just something fun to ride in an environment where you can push the bike a little harder, and not have to worry about things like cars, curbs, and other similarly dangerous obstacles.

    Somewhat recent events caused me to change my plans a bit and accelerate this build. My Red bike has seen significant abuse due to bad drivers (it's been knocked over ten times since the build), and vandalism. So that bike is sadly coming apart in prep far a rebuild. In the mean time a lot of its dinged up parts are being repurposed to the track bike where scratches and such are just a fact of life. I've been given lemons as it were, and it's time to spend the afternoon in the sun!

    The idea for this bike is to basically take parts I have laying around, and assemble a fun ridable bike for the track. It's something I plan to do in stages with regular revisions as parts become available. The frame is from another project that was being assemble for a friend that gave up due to life getting complicated. So he sold me a roller. Most of the bits from that bike I wont be using, but the frame and swing arm were decent. They were sent to the powder coater, and came back all shiny black. I did this at the same time as the Red bike to optimize the discount, knowing that eventually I'd use it for something.

    The frame when it was at the powder coaters a couple of years ago. Originally this was one of the infamous GPR75's that made it into the States. Ozzie from Cycle Imports brought 200 of these over in a couple of containers. Somehow I've ended up with three of them.



    The bike as it was once mis-envisioned



    Ed had suggested that for now I should take parts from the Red bike, and get the Track bike running--so that we could go ride at the Stockton track event. The Track bike and the Red bike on the way to Ed's where the swap will begin.



    A track bike needs proper tires. These are Sava F001 (95/70-17 ), and R001 (115/70-17) track tires picked up from Treats to go on some 17 inch Derbi Grimeca rims (the stock rim size for the 2000 GPR is 16 inch). My friend RedSix did a late night tire swap to get ready for the track.



    After some time at Ed's we headed to the track, where I would have to do a bunch more work.



    Speedy brought out some cardboard on which to set up shop, and my Sacto moped buddy's provided some necessary shade.









    Most everything went pretty well, and I had a bunch of help from my friends. I wasn't able to get the bike started, so it didn't get to go on the track. Current belief is that it's an ignition issue. I harvested an HPI mini rotor that I had installed on my moped. The stator didn't fit all that well, so I may have to get a proper mounting plate made up if I can't get this one customized to work.



    Some cool things currently on the bike:
    Metrakit ProRace pipe (for an RS50)
    Brembo RCS16 master
    Risse Racing Shock
    Sava F001, R001 tires
    Poor Man's ProRace Airsal 50mm top end
    Metrakit 24mm PWK carb
    Domino roller throttle
    E clip sprocket mounting (for quick sprocket changing ability)
    2005 GPR clutch lever assembly
    Trail Tech kill switch
    Handmade aluminum oil pump block off cover
    Lots of custom black powder coating action
    Last edited by ToraTora; 12-09-2017 at 10:27 PM.

  2. #2
    apriliaforum expert mattology's Avatar
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    sounds similar to the last time i went to the track. i brought spare everything except for an ignition coil.

    my ignition coil cracked in half coming out of the van ....

    lol

    it should be ready for the next event, right?

    Get your confidence back with Aprilia.

    caution: the use of motorcycles may be habit forming. side effects include purchasing multiple examples of the same bike, in different states of tune, different years, or different models altogether. some motorcycles may be not running when purchased. some parts are no longer available. there are rare instances where aprilias are bought over the internet and upon arrival, they are not as were originally disclosed. there is a small group of people who cannot handle two stroke engines and motorcycle maintenance. if this is the case, you may not be a candidate for Aprilia RS250's.

    Aprilia. Ask your doctor if the RS250 is right for you.

  3. #3
    apriliaforum expert ToraTora's Avatar
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    Cool Timing

    We went to the Sonoma kart track today, and I had a chance to work on the bike. I figured it was an ignition thing. The plug was wet, which meant fuel was at least getting to the bike. I pulled the plug and verified that it was sparking. Then I repeated the process with setting the ignition timing. But it turns out the HPI doc didn't provide the correct information. I then remember that my buddies at Motomatic had posted HPI information with regards to setting the timing. I then applied their research, and the bike started right up.

    But I was out of time, again. But next week we should be going to Stockton again, and I should be able to do some tuning. :}

    Last edited by ToraTora; 08-25-2015 at 02:31 PM.

  4. #4
    apriliaforum Junkie freedomgli's Avatar
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    That sucks that the HPI documentation is wrong or not helpful. Why is it that so many performance parts companies are so bad at this part? Take the PVL magneto on my other race bike. Support and documentation is terrible! But hey, at least I understand the thing now and how it works because I dickered with the thing forever to understand how it works and what making changes resulted in.

  5. #5
    apriliaforum expert ToraTora's Avatar
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    Wink Hpi

    The Motomatic info is not all that different from the HPI docs, but the HPI docs have you set it to the other side of the center, and that's enough to make it not work. My friend Taylor suggested setting the timing to the third big tick from the left. This is where I currently have it set. I also had a bit of trouble centering the stator plate because the Puch plate doesn't quite fit the EBE. I had to modify the the stator plate. The mounting bolts are such that I wasn't able to make it so that I can easily rotate the plate, which means I have to pull the rotor, and reset it to make adjustments. Pain in the ass, but it works. :P


  6. #6
    apriliaforum expert ToraTora's Avatar
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    Cool Shot on iPhone 6


  7. #7
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    If you are ever in the market for another set of Showa forks I just got a set that I wont be using. Looks awesome, cant wait to get my rs50 back to the states!

  8. #8
    apriliaforum expert ToraTora's Avatar
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    Hey D,

    Thanks for the offer.

    I've actually got another set that I'm going to being using. These ones have pits, and are leaking. Well, they have leaked. There's not much left in them to leak now. :P

    If you be in NorCal you should come out to the track and play. When are you expecting to be back? It's been quite a while since I've organized a street ride around here due to the bad drivers that have incapacitated my street bikes. Eventually though I'll get that sorted, and organized some street riding action.

    For now though I'm focusing on getting on the track.

  9. #9
    apriliaforum Member
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    I from Washington so eventually I would probably make it down to norcal! But in December I am coming back but going to North Carolina. What are the class that you can race a 50cc in? is it that BATS club racing?

    Also what the black seat looking thing? was that the oil tank?
    2005 Rs50 Rs4/GPR clone

  10. #10
    apriliaforum expert twinturba's Avatar
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    Great to have a bunch of extra parts lying around!
    Jenius Biker Blog

    Current: RS250³,Newd, TT, Max, Nastro, Bangkok, 85 ADV

    Past:Versys, Typhoon, NCXD, RC390², Madass, PW50, KTM BR 65, Aprilia BR50, Aprilia RS125², DRD, WR250X, Derbi SM⁷, Aprilia RS50¹⁰, Derbi GPR³, Aprilia RS250², Mito², NSR50², NS50, HRC RS125, HRC RS80, YSR63 (1st 2stroke!), FZ600, KLR250(1st Bike!)

    I Need Parts!

  11. #11
    apriliaforum expert ToraTora's Avatar
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    Cool Forking Around

    Hey D,

    The thing that I'm currently employing as a seat is the fuel tank. The folks at Cagiva used an interesting method to get the weight were they wanted it, and also created a place to store one's helmet!

    The racing that's currently going on out at Stockton seems to be gaining momentum. I went to the most recent race. I probably knew about half the racers from the rides I've organized, track days, and moped riding. It was super fun. There's another race this month, and hopefully I'll be able to attend, although I probably wont race. They host open practice from 9am until an hour before the races start. Hardly anyone runs a 50cc top end. There is a class for kids that does, but even the mopeds are running around 75-80cc, and there are a bunch of ugly sounding 4T bikes out there too. I suppose that's to be expected, but the mopeds were beating them. ;p

    One of my buddies got so inspired by the email I sent out to my friends after the previous Stockton race that he started building a bike to take to the track that very day. I was out at his place a couple of times this week helping out. Here's a work in progress photo.



    The Forks

    I also started working on the forks. Had the wrong seals though, so the fork repair will have to wait until I can get the proper seals. For those interested the stock oil weight for these forks is 7.5, at 175cc. Once I get the proper seal dimensions I'll post that info too. Because these forks do not feature adjustable damping, the easiest way that you can change the reaction times is by changing the oil weight. A lighter weight will make the response times quicker, and a heavier weight will make them slower. Adding more oil will change the progressiveness because there will be less air to compress. For the track, since it is smoother than most roads a faster response time would likely be preferred. So I'm going to try 5 wt this time.

    It would also be possible to switch out the springs, or adjust the sag by inserting a spacer with the spring. I wont be playing around with either of those at this time.

    There is a trick to working on these forks. You can't actually do it with hand tools because the bolt that must be removed will just spin. So you have to employ a power tool to remove the top socket head bolt. There's also a trick to putting the screw back in. There is a hole near the end of the shaft. You slide the spring over the shaft, and then slip a small auger into the hole. Spin the spring to get the nylon piece over the shaft, and then spin the bolt in. It's a bit tricky, and the manual doesn't explain any of this stuff.








  12. #12
    apriliaforum expert ToraTora's Avatar
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    The fork seal says these numbers for the ID OD Height:

    34.74 x 47 x 9

    I found some different numbers listed for these forks, but that's not what's in them, and they are different than the Showas that are also used by the Derbi. I've successfully used the All Balls seals for the Showa forks with part number 35-1011, seal size 35 x 48 x 11, but the Sebac forks for whatever reason employ a different size. And interestingly enough some websites list the wrong size for the Sebacs. Which is how I ended up with the wrong seals. Additionally, the parts manuals for these bikes don't list the seal size either.

    A couple of UK sites list seals is this size, but so far I've not found a dust seal. There's an older Guzzi that has the same size, but the dust seal is the old style wrap around. The Gilara Runner seems to have the same dust seal which is a 35 x 47.5 x 13. More research to be doing.

  13. #13
    apriliaforum expert ToraTora's Avatar
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    Cool

    Today we found a pipe for my friend's machine.


  14. #14
    apriliaforum expert ToraTora's Avatar
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    Stock Seals

    Found some images of the seals on ped parts. These are the actual parts from Piaggio.

    Dust Seal


    Fork Seal

  15. #15
    apriliaforum expert twinturba's Avatar
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    This thread is great inspiration to get back out there! It's been a long time since Stockton
    Name:  IMG_5590-747364.JPG
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    Jenius Biker Blog

    Current: RS250³,Newd, TT, Max, Nastro, Bangkok, 85 ADV

    Past:Versys, Typhoon, NCXD, RC390², Madass, PW50, KTM BR 65, Aprilia BR50, Aprilia RS125², DRD, WR250X, Derbi SM⁷, Aprilia RS50¹⁰, Derbi GPR³, Aprilia RS250², Mito², NSR50², NS50, HRC RS125, HRC RS80, YSR63 (1st 2stroke!), FZ600, KLR250(1st Bike!)

    I Need Parts!

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