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Sprockets an wheelies

Discussion in 'Technical and Troubleshooting Torque' started by Jace, Mar 14, 2005.

  1. I have just replaced the chain and Sprockets on my TRX 850 back to the original gearing (17F,39R), replacing 18F and 39R. I have found the bike shudders a lot more now on low speed take offs. But is an absolute dream on the freeway. It also seems to accelerate faster as if the gear ratio helps the bike use the torque to greater effect. I'm currently running JT sprockets I was thinking of getting an AFAM alloy 42 tooth rear(I have hopefully left myself enough adjustment in the chain) to maybe help with power wheelies :D . whaddya reckon ?

  2. hmmmmmmm wheelies mmmmmmmmmm :D :D :D :D :D :D :D :D
  3. The smaller you go on the front will lower your gearing, the bigger you go on rear same result, lowers gearing.
    The right combo for you I dont know but for getting the front in the air lows good :D :D :D :D :D :D
  4. If you are worried about chain adjustment, just drop a tooth on the front. Some people claim that they don't want to go to a smaller front sprocket because the chain has to take too sharp a bend to get round the smaller cog.
    It is my (unqualified) opinion that dropping from 17 to 16 teeth on the front won't cause too many problems, if any at all.
    You'll fing that dropping one tooth on the front results in approximately the same gearing ratio as raising 3 teeth on the back (rough rule of thumb).
  5. 18/39 = 0.462
    17/39 = 0.436
    17/42 = 0.401

    I would've expected your bike to shudder less at low speed with the 17/39 then the 18/39 since it would be at slightly higher revs. :?

    The 17/42 should be fine, as long as you dont mind the extra revs on the freeway. Wheeliness should be good though. :D
  6. Jace,

    I'm not sure id I'm stating the obvious, but just dive your rear sprocket by you front sprocket to give you your reduction ratio. It sounds like you need a ratio somewhere between original and where you are now.


    From a pure engineering perpective you see a significant drop off in chain life below 21 teeth and a sharp drop below 19. It's a declining exponential curve By the time you get to 11 it's all over.

    So I'm of the school of thought of keeping the front sprocket as close to 19 as possible.
  7. i'm with ZRX there, theres a heap of bikes out there that have 16 or 15 tooth fronts as stock so i wouldn't see an issue for the chain going down another 1 or even 2 on the front. only thing i'd be concerned about with going down more than 1 from your stock, is whether the chain will rub on the swingarm or anything else there.

    front sprockets cost less than half of what rears do so if you decide you prefer the stock gearing, you wont lose as much :wink:

    and you say the bike shudders MORE after dropping the front? i had the opposite on my thundercat, went down 1 on the front and up 2 on the rear and it takes off much more smoothly now. and wheelies are easier :twisted: still shit scared of them tho :oops:
  8. 19 ?? well how do you explain bikes that have a 16 tooth as std straight from the factory ?
  9. Perhaps, might seem like a dumb question but I'm a newb. Decreasing sprocket on front, same as increasing sprocket on rear, gives lower ratio and does what? Improves low end torque while reducing top end speed? Other way round for the opposite?
  10. Nah not a stupid question.You're pretty much on the money. It's basically the same principle as on a push bike. You use the smaller front sprocket and larger rear if you want to pick up speed quicker and larger front and smaller rear if you want more top speed.

    Guys I forgot to mention that I have a 16T front that I have tried but I found this induced wheel spin, so I'm going to try my theory that if the chain is covering more distance at the rear it is going to hook up better at the rear?? I know the gear ratio will be similiar but here's hoping I will be reaching for the sky soon.

    Thanks Androo for the reduction ratio's :D Very helpful
  11. VTRbob,

    yeah I know. It's just that if you had 19 as a minimum rear sprockets would be getting a bit too big. A traily would have a rear sprocket about the same saize as the wheel.

    I'm just say that if you have the option of going up in the rear sprocket rather then down in the front, I would take the increased rear
  12. I can cope with the fact that the bigger the front sprocket the less chain wear you'd have!
    Would this have to do with the angle of approach of the chain?
    And, if so, wouldn't a larger rear sprocket also effect the angle of approach?
    I'm not questioning your rational on this, as I mentioned in my initial post, I am unqualified. However I'd like to hear some theories as my 51,000 km old chain and sprockets now need replacing and I'm thinking of lowering the gearing slightly when I do.
    Curret set up: 17/45
    Probosed changes: 16/45 or 17/48-9
  13. I believe that temptation for wheel spin will depend on the final drive ratio, but not which sprockets you actually use for it.

    If you compare:
    17/42 = 0.401
    16/39 = 0.410

    so you're probably more likely to spin with 17/42 then 16/39.

    by the way, if you do aggressive rides through the spurs, you might find yourself changing gears too often with very high gearing (depends on your bike obviously).
  14. Plane,

    You are right. A smaller rear sprocket does reduce chain life. It's just not that significant.

    It's a pretty rapidly declining curve in the teens.

    Theres tables for these things. Its just a rule of thumb to try and keep sprockets above 19. It's actually similar for gear pinions except the number something like 13.

    It's interesting to not that some push bikes have rear sprockets as low as 7. It must be pretty jerky. But then again they don't have cope with much torque.
  15. Cool,
  16. I dont see any reason why it would be jerky as long as the chain/sprockets are in good condition.