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Faster 250 - change ratios?

Discussion in 'Modifications and Projects' started by MeltingDOg, May 1, 2011.

  1. Hi all,

    I am a newb to motorcycling in general - so please keep that in mind!. I've got a 94 GN250 which is in great shape and runs fantastically.

    Only thing is I would like to take it on longer highway trips. I've already posted a thread asking if people think the bike is up to cruising at highway speed for hours and the general consensus is it shouldn't be a problem (as long as the oil levels are ok).

    Now, Ive heard rumors that you can replace the drive and wheel gears (if that is the correct terminology) with different ratios to make things a bit faster.

    Im not interested in magically making this thing into a super bike - it is a 250 after all. What I want to explore is the option of changing the ratios to improve its ability to cruise at 100kph. Right now it cruises there at 6.5k rpm (redlines at 8.5) and it just doesn't feel that natural. 80kph at 5k rpm feels way more comfortable. So my goal would be to cruise at 100kph at 5k rpm.

    Does anyone have any information or advice for me?
  2. I just did the same conversion on my bike as I felt the bike was revving too high at speeds of 100km/h and above.
    Find out how many tooth are on your front sprocket, then order a new one with an additional tooth. For example if the front has 15 teeth then get one with 16 teeth. After you get that put on report back with the results.
    Bear in mind your bike will take off marginally slower. But theoretically your top speed will increase.
  3. Front sprocket is a counter-shaft sprocket. Rear is just a rear. The font is heaps cheaper to replace. Like a third of the price. Not saying it will be, but chain length might be an issue putting a larger sprocket on. Worth a try though. The front should be between $12 and $17
  4. This will make your bike even more gutless, I'd advise against it..
  5. I've made the same change also in addition I've gone with a smaller rear sprocket too. It is only ever so slightly less potent on take off. Now comes with the added benefit of taller gearing and can theoretically do 260km/h instead of the 190km/h shown on speedo. Hardly use top gear anymore in suburbia.
  6. The ratio change will help you cruise at better revs as you say, but the top speed isn't governed by the highest number on the speedo or by working out revs X gear ratio. Other factors (power and wind resistance for example) will be the determining factor.
  7. What made me consider this (apart from the expected lower costs) was that my bike seems to be pretty torquey down low, like maybe an ag-bike would be. Either that or it was designed to maybe be used in countries where you have multiple passengers at low speed!

    Though I haven't tried I reckon you could even possibly take off in 3rd gear on a flat surface! I often can cruise in top gear (5) throughout suburbia and never really have to change down when heading up hills.
  8. Just out of interest, how much did this conversion cost you?
  9. The front sprocket cost plus whatever your mechanic charges you to put it on.
  10. Adding a tooth on the front is like dropping two on the rear and will decrease you rpm by a little without affecting torque.
    Dropping another two teeth on the rear and adding one on the front will drop the revs and keep it fairly the same power wise.
    Long as you don't have to duck paddle off from a start all good.
    If you can take off easily in second then all good really. You might have to change down on up hills a bit more. Long as it can hold the road speed on the highway then why not.
    You might want to add in a new chain if you are going to change the sprockets though. And then you are going to change the link amount. Can you use a chain breaker ??? On a 250 you can use a universal link no worries so joining it is not a problem. Just make sure you leave enough room for it to fit and to stretch.
  11. There's a limit to how far you can go before the bike can't pull the ratio and dies on you at speed. But one tooth added to the front / countershaft sprocket as everyone's suggesting is fine. It'll cost you less than $20, and chain stretch should mean you can fit it easily enough.

    Just a couple of other things:

    Make sure when you fit it you check that the chain does not rub against the engine case, that there is good distance. If there's a problem, you'll have to go with a change to the rear instead. Which you'll also have to do if you can't get the front sprocket.

    Probably unimportant, but: it's a Japanese bike. This is how they roll, whiney bastards. Get used to it (but yeah, I hate it too. 90kph is my distance-cruising speed on my SR500).
  12. i went up 20 teeth on the rear and down 4 teeth on my fzr 250. did mad wheelies but topped out at 110 kmh :)
  13. I am tacking on my question to this thread as no need to start my own but has to do with sprockets etc - my apologies its not a thread hijack

    any tell me where I can get a gold sprockets and chain kit to suit my 04 R1 - standard teeth sizes is fine but want a thick gold chain as well as part of the blinging process
  14. http://www.sprocketcenter.com/
  15. get them from these guys in the US, it will be cheaper shipped to your door than anywhere in aus. the do custom anything when it comes to sprockets and chains

  16. Thanks Resman - as the aussie guys actually make the sprockets / chain here I will stick with buying it from them even if it may be more expensive

    I like the fact they machine the sprockets here and would like to support them with my $$$$
  17. I bought my latest chain/sprocket combo from Sprocket centre for shy of $200 delivered. For the next set I will be buying it from the Aussies as I was happy with their communication when I was making enquiries with them despite being more expensive.