Adjusting a gear box

Discussion in 'Tech' started by fullmetalF4i, Jul 16, 2014.

  1. fullmetalF4i

    fullmetalF4i C. Lee #826

    Recently I blew up a couple of gears in my CRF450R supermoto and while getting replacement parts I got to thinking.
    Would you be able to adjust your gearing ratios to maximize the gears you use most (in my case on this bike, 2nd and 3rd could be slightly longer and 4th shorter) by using taller or shorter cogs?
    Would it work just like changing sprockets (more teeth for shorter gears w/ more accelration and less teeth for taller gears w/ less acceleration)?

    Anyone have any experience with changing ratios in the gear box?
     
  2. pscook

    pscook Well-Known Member

    You need to change transmission gears in pairs. I swapped a WR400 box into my YZ426 with zero issues. I first tried a WR 5th gear on the YZ without changing 1-4, but the gap from 4th to 5th was terrible. Once I got the new gear set installed it worked fine. But, pairs is required for proper fitment.

    So, maybe find a CRF-X gear set and play with that. It sounds like you have 450R spares to measure, so I would pick up a couple of 250X or 450X (or whatever the trail model is) gears and see what the difference is, if any.
     
  3. stangmx13

    stangmx13 Well-Known Member

  4. Boman Forklift

    Boman Forklift Well-Known Member

    I'm guessing the 450R and X gears aren't interchangeable, because the 250R and X are not. My son raced a Moriwaki, which has a 250X motor. One of the common upgrades is to put an entire R motor into the Moriwaki as the rear ratio's work better for road racing.

    There was a company in the UK, possibly the company above, that had plans to make a replacement for the X, but as I recall it was over $2000. You could buy the entire R motor for less than 1K used, so I never met anyone that invested in the aftermarket transmission.
     
  5. fullmetalF4i

    fullmetalF4i C. Lee #826

    yeah the one linked above was 1200 pounds/euros. If i had that laying around I would just pick up a donor motor.
    unfortunately I dont have a bunch of spares, I just bought a new 4th gear set (main and countershaft gears). I had just noticed that they listed how many teeth and i figured they should all have the same internal diameter since they're on the same shaft, you may be able to get another gear (say 3rd) that has more teeth than 4th and put it into the 4th spot. But I could be wrong about the diameter thing as Im just looking at the diagram on the part site.
    was just something rattling around in my head. I remember the cassette transmissions in motogp where each one had different ratios for different conditions at the tracks. Just wondered what the low tech alternative would be.
     
  6. pscook

    pscook Well-Known Member

    Just swapping third for fourth won't work, as they are completely different in construction and use. Dogs, interfaces, radically different. To change fourth gear ratios, you need a fourth gear set with different ratios as designed. Transmission gears aren't just "gears," per se.

    Moto GP transmissions are simply fancy normal transmissions, excluding the quickshifting Honda and Yamaha (and possibly Ducati) boxes. I believe a CBR1000 came with a quick change transmission, or at least some street legal homologation bike did. But I digress. A quick change transmission is simply a transmission that is easy to remove from the engine. The gears are efectively identical, they are just easier to get to. Basically the side case has an access window, and you still have to remove the nut from the clutch hub.

    I say take a wild chance on a used set of X fourth gears and compare the dimensions to the R model. Best case they interchange, worst case you are exactly where you are now (but out a couple of $).
     
  7. fullmetalF4i

    fullmetalF4i C. Lee #826

    thanks for the clarification, I may look into the X gears.
     
  8. bpro

    bpro Big Ugly Fat F*****

    It's been a while (10-12 years) but the mechanic at a shop I worked for also built bikes for a top national HS/CC racer. When the racer started running the CRF450 as a woods bike I am fairly certain that the transmission was replaced with one from a CR500 to get the ratios he wanted.

    I am no longer in contact with the mechanic, but from what I remember it was a fairly simple job.
     

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