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Thread: sprocket size. 15,16,17 fron & 40,45 rear

  1. #1
    apriliaforum expert hagasn's Avatar
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    sprocket size. 15,16,17 fron & 40,45 rear

    aloha, with over 20k on my bike it's almost time to replace my chain and sprockets. from the following sprocket sizes what would yall suggest? i know the popular change is 16/44 BUT i found a great price and a 44 rear is not an option. i have read a number of threads on this topic and don't plan on riding wheelie's for days or entering the daytona 200. the site claims 16/40 is stock? anyways these are the choices 15,16,17 front and 40,45 rear. what do you think? thanks in advance and ride safe.
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  2. #2
    apriliaforum prov-nov Dynamo750's Avatar
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    If the bike is 04> stock is 16/40

    On other forums the most common is 15/40

    1 off the front is almost the same as 2 on the back

    Putting a 44 on the back must make the chain too small surely my 42 only just made it

    I feel 15/40 or 16/42 is fine for the road but im going to go 15/42 for my next trackday to try it out

    If you ride on top of the rev tree then higher ratio's will suit you better but if you ride on the torque 15/40 would be fine
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  3. #3
    apriliaforum expert Jaguar's Avatar
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    I ran 15/40 throughout the life of my first chain/sprockets (i.e. just swapped the front for a 15) and that was much better than the stock 16/40. In fact, I decided it was about right - but was advised to achieve the same thing by going 16/43 when I bought a new chain/sprocket set. Theory being a smaller front stresses the chain more. I think any effect is pretty marginal though, as I got 20,000km or so from my first chain, having changed the front at 3,000km.

    You can work out what equals what simply by dividing the number of teeth on the front into number on the back to get a ratio. The larger the number that results, the lower the gearing (i.e. the higher the revs for a given road speed). Divide one ratio into another to get the % change in gearing.

    Thus, 16/40 = 2.5 15/40 = 2.67. 2.67/2.5 = 1.067. Thus, you lower the gearing by 6.7% by changing to a 15 tooth front.

    You'll find that this is equivalent to about 2 1/2 extra teeth on the rear - thus, 16/42 is a little higher geared than 15/40 and 16/43 is slightly lower.

    Given your available sizes at the good price, I"d go 15/40. 17/45 would give a similar ratio, but there's a fair chance the standard chain would be too short.
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  4. #4
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    Jaguar is exactly right. It's simple math, and it works.

    As he did, I ran 15/40 (-1 on the countershaft) initially, then went to 16/42 when I changed my wheels and had to buy a rear sprocket anyway. The latter is only slightly taller than before as the math suggests.

    In real numbers then, my RPM went from about 3750 at an indicated 70 mph to about 4000 at the same speed, exactly 6.7% more.

    Oh, and the stock length chain works with either of the above combos, 15/40 puts you farther back on the adjustment than stock (longer wheelbase), and 16/42 farther forward (shorter wheelbase).
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  5. #5
    apriliaforum expert hagasn's Avatar
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    thanks for the advice.the aprilia forum ROCKS! ride safe
    i never thought i would be cheering for max biaggi

  6. #6
    apriliaforum expert wrx_02's Avatar
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    http://www.gearingcommander.com/

    Punch in the gears you are thinking about and it will give you RPMs for each gear etc etc.
    I run a 15/42 on the street right now. I really like it as it is much easier in town and doesn't bog the motor so much.

    I run a 15/44 on the track, wouldn't recommend it for the street as 1st gear is useless thus this is why I use it on the track.

  7. #7
    apriliaforum Junkie mcjongs's Avatar
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    Hi gays...
    All you need to know is here:

    Play with different figures for front&rear sprockets and you will se the diffrences in rpms and speed.

    I use 15/42 and it is much more flexible to pick up speed on 5th and 6th.

    mcjongs
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  8. #8
    apriliaforum expert EA126405's Avatar
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    Quote Originally Posted by mcjongs View Post
    Hi gays...
    All you need to know is here:

    Play with different figures for front&rear sprockets and you will se the diffrences in rpms and speed.

    I use 15/42 and it is much more flexible to pick up speed on 5th and 6th.

    mcjongs

    mc, sounds like a logical conclusion (I'm to lazy to do the math, but follow the line of thought closely). My question to you is, how/where do you ride? I was planning to go -1 on the front. I'm don't normally spend much time in 5/6th. If that was the case with your riding style, would you still change front and back?

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