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Gearing Question - sanity check

Discussion in 'Maintenance and Servicing' started by northerner, Mar 26, 2012.

  1. OK, so I am planning to drop one tooth on my front sprocket from 16 to 15 teeth.

    I will double check speed calibration with a GPS but I think I am right in saying that speed difference and a given gear/rev will be... (drum roll...)....

    47/16 = 2.9375 (stock gearing)
    47/15 = 3.1333 (new gearing)

    3.1333/2.9375 = 1.0666666 which is an INDICATED speed DROP of 6.66% for a given gear/rev.

    Hence I need to dial in a 6.66% increase in the ECU yes?

    Now this is after I have already dialed in a -5.5% change to account for the conservative speed reading on my bike (Street Triple R). I have already checked this via GPS and it now reads spot on.

    So! I actually need to dial in -5.5 + 6.66 = +1.16%?

    Please tell me I'm right??!! :grin:


  2. Just a question: knowing how GPS works, how do you know you are getting a good speed reading? Its not that accurate and the small percentages you are talking about could be measuring error of the GPS.

    Just a question...the rest I don't know about.

    Cheers Spocky
  3. #3 jag131990, Mar 26, 2012
    Last edited by a moderator: Oct 24, 2015
    GPS Is pretty darn good. Often inexpensive units will poll slowly but if you bring it up to constant speed I would trust that it was fairly accurate once it has settled..
  4. Yeah constant speed over a reasonable stretch is the way to go. I checked it on the stock knowing the bike should read around 6% slower than it's actually going and it was spot on.
  5. don't know what you're on about, isn't the speedo off the front wheel?
    or is it off the gearbox/drive nowadays?.

    Anyway take note of how your bike likes the gearing, you can't predict just by thinking "it will be good" Some bikes like to rev less and not more. Check where the peak torque and HP rpm lay (with spec sheets) and try to tune it to one of these. Ie if you tune it to the torque it will pull harder etc.
  6. Can be either.
  7. Off the output shaft generally nowadays. 'Analogue' (ie non-lcd) speedos would generally be off the front wheel.
  8. Cheers. I know what you're saying. For $30 I'm gonna give it a go, can always swap back if needs be.

  9. this is for your striple, right? i've been keen on doing the same thing too.. i find on the freeways at say 100km/h, with the stock gearing i sit somewhere between 5k-6k.. but there is a nice sweet spot at 6k, so i'm guessing 15/47 would have you sitting there on the freeways.

    let us know how you go and what you think of it..
  10. Yeah for the striple mate. I will give it a whirl this weekend and give it a workout on the Putty Rd on Sunday - I'll let you know how I go.
  11. OK I got the gearing the wrong way round - it's actually a 6.66% reduction on top of the 5.5% - so a total correction in the ECU of -12.1% or there abouts.

    Just checked it out with GPS on iPhone and seems to read pretty much spot on - reads about 1km/hr above what the GPS was saying pretty consistently which is fine by me.

    Blackadder - I'm going on a proper run tomorrow and will report back. On the quick run I did to check calibration of the speedo I liked it. A lot. I think it will suit 90% of my riding.

  12. OK so did a nice big run today. I like the change. More punchy but not ridiculously so.

    At 100km/h in 6th it was sitting exactly on 6000rpm. 110km/h is 6250rpm.

    For the $30 the sprocket cost I reckon it's a good thing to try. I have tuneecu and cables for speedo recalibration if you want to give it a go ;)

  13. Sounds great. I'm going to grab one tomorrow. For $30, you can't go wrong. I've got the ecu cables too..

    Next project is going to be cbr600rr rear shockers.. :cool:
  14. Cool. Just so you know - I am now at -12.1% as I mentioned. Did check with GPS and also with my mate riding next to me just to check against his speedo - all good.