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Time for new sprockets - your suggestions please

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by ezekiel67, Feb 2, 2010.

  1. Hey all

    I've got a 1991 Honda CB250FM (MC23) and it's time to change the sprockets and chain.

    Currently it is running 14 teeth on the front and 42 on the rear. The problem I'm having is that at 100km/h it is revving at 9,000rpm in 6th gear. It has a 16,000rpm red-line but I'd prefer to to gear it down so that it is pulling less revs at open road speeds. I don't mind scarifying some acceleration performance to achieve this. I also have no idea what the standard sprocket sizes are.

    I have found a couple of options that will fit and they are:

    Front - 14 or 15 teeth
    Rear - 40, 42 or 44 teeth

    It is also running a 140/70-17 rear tyre (I've added this as it seems to matter with the online gearing charts I've had a go with).

    What do you guys think I should run with?
  2. Going up a tooth on the front and leaving the rear stock will drop the revs at any given speed/gear combination by 7%, so you'd be doing about 8350 at 100 km/h. That's probably a good point to start from as it's enough to notice but probably not enough to completely bugger things up if your bike isn't happy on the taller ratio. The worst that is likely to happen is that you'd need to slip the clutch a bit on take off and maybe be in one gear lower than you otherwise would be in any given traffic situation.

    If the bike feels like it will cope with more, think about getting the 40 tooth rear. This'll give you another 5% reduction, dropping your 100 km/h revs to below 8000. However, with a 16000 rpm buzz bomb, this may be going too far.

    Where is the torque peak for your engine? This is usually the sweet spot for cruising, so you want your gearing to give you your most typical highway cruising speed at the right revs for peak torque.
  3. The bike happily plods along in 5th and 6th gear at 50-60km/h.

    It runs the MC14E engine which is found in the CBR250 and Hornet 250. I've seen peak torque figures of 10,500, 11,500, 12,000 and 14,500.
  4. Yeah I agree with Pat. Go one up on the front. front sprockets are much cheaper than rear and that should keep in in a range that doesn't bugger you up off the line.
  5. Thanks for the advice guys.

    I'm a couple of weeks away from replacing these items but at this stage I will go with the 42/15 combination.

    Once I've done the job I will let you know how the bike is going.
  6. Although you shouldn't normally run a new sprocket with old chain, you could put the 15 on now and see how it feels for a few days, then buy the new chain and rear sprocket. If you find it could handle even taller gearing, you could get then get the 40 rear, otherwise stick with the 42.
    I went from 16 to 17 on my GS500, and prefer it this way.
  7. Hello,
    I figure instead of starting a new thread I'd ask here.
    I've got a gsxr750 and I was looking at getting new sprockets/chain.
    Reading on different forums, it's common for the 525 pitch chains to be replaced with a 520. And they also change the gearing (power up kits).

    the standard is 17 front and 45 rear
    most of the kits either are 16 and 45
    or 16 and 46
    How far out would my speed reading be if I got either one of those.
    I can't work it out. I'm terrible with my maths.
    I still answer window if I'm asked whats 1 + 1 lol
  8. most bikes take the speed off the front wheel so your speed reading won't be out. what year model is your bike?

    As to calculating, just divide the rear by the front both before and after, then divide the ratios you just calculated by each other.

    This will give you a proportion by which it will be out.

    But I don't think you will have to.
  9. :facepalm::-k:tantrum:

    hahahah I'm so lost after that... but I'll try.

    Gearing commander looks so technical right now. I've just finished night shift -edit- ok... way too technical after ive scrolled down. its just me.
  10. OK
    Current ratio: 45/17 = 2.647
    New ratio: 46/16 = 2.875

    relationship : 2.875/2.647 = 1.086

    i.e there will be an 8.6% difference if the speed reads off the engine.

    You didn't answer what year model your bike is.
  11. Ive got 2007 gsxr750
    So, for arguments sake lets just say my speedo is accurate and it says im doing 100km/h. With a 8.6% difference I'll be going 108km/h?

    Trying to work it out makes me feel like the guy in your avatar lol
  12. No. Because, in the example, you're lowering the gearing (more revs, less speed), your previously accurate speedo will read 100 km/h when your actual speed is 91.4 km/h.
  13. That's great then!.. I will feel like I'm speeding when in fact I wont be. so less chance of getting a fine. Awesome. lol

    Thanks for the help and info guys. Much appreciated
  14. And,

    Out of curiosity, whats the typical cost of getting a mechanic to install chain? Ball park figure.

    Tossing up whether I want to do it myself and buy a tool for doing so or just getting someone to do it for me.