Swapping final drive

Discussion in 'Airheads' started by limeymike, Jul 15, 2012.

  1. limeymike

    limeymike Who Me?

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    Is putting a final drive from a R100 onto a R80 easy, ie bolt on? Looking to lowering the cruising rpm. Its an '86 mono.
    #1
  2. disston

    disston ShadeTreeExpert

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    The speedo will be off. Some put tape on the speedo glass to remind them.

    Check the splines. They also get greased.
    #2
  3. wirewrkr

    wirewrkr the thread-killer

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    No splines on this final to grease
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  4. DaveBall

    DaveBall Long timer

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    And if you can, grab the speedo of the R100 as well, that way it will match up with the final drive and show correct speed, or at least a lot closer to correct.
    #4
  5. boxerboy81

    boxerboy81 Stay Horizontal

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    Direct swap between monos. Go ahead!

    When swapping a final drive to a different ratio, what combo might work so that the original speedo becomes accurate?

    If the oem setup is say, 7% optimistic, which setup might compensate for this and return the display to accuracy?
    #5
  6. supershaft

    supershaft because I can

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    I think he will be going from a 3.37:1 to a 3.0:1? That's a big jump. Over 10%. If it's in the right direction you could get the speedo to a theoretical 3%? My 3.0:1 speedo isn't off nearly as much as many report according to speed sign radar and tickets I get. I would say mine reads about 2 to 4% fast? Maybe 3 to 5% at the most I would guess?
    #6
  7. coastranger

    coastranger Been here awhile

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    easy swap adn one you might very well like. On my 85 I swaped out the final drive off a R100 and the speedo. I noticed near at least a 500 rpm drop at speed
    #7
  8. limeymike

    limeymike Who Me?

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    I have heard similar numbers quoted from the swap so I think it is worth it.
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  9. limeymike

    limeymike Who Me?

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    With the stock final drive my speedo reads 70mph at an actual speed of 65mph, that's taking the actual speed from a TomTom gps that I temporarily rigged up. approx 7% error.
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  10. Airhead Wrangler

    Airhead Wrangler Long timer

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    That's pretty typical. You'll find that your odo is pretty much bang on though. There was a thread about speedo error a while back. It's worth a read if you can find it.
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  11. Boxer Metal

    Boxer Metal Mad Scientist

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    Easy bolt on. Look for a speedo that says 1.078 on the face.
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  12. Rucksta

    Rucksta SS Blowhard

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    I thought the speedos were all the same and the gearing of the speedo drive needed to match the final drive
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  13. Airhead Wrangler

    Airhead Wrangler Long timer

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    Nope. The internal gearing of the speedo has to match the final drive. Your speedo will have "W=_____" on the face. This indicates what final drive ratio it is intended for. Annoyingly they use a completely different metric for the speedos rather than just listing the corresponding final drive ratio. W = the number of rotations per mile (or km for 'rest of world' market bikes). At some point (can't remember when exactly) they changed W from thousands of rotations per mile to just rotations per mile. eg: W=1.202 became W=1202.
    #13
  14. Beater

    Beater The Bavarian Butcher

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    I didn't know that. That IS annoying. Do you know if the 'W' metric takes into account the diameter of the rear wheel?
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  15. Airhead Wrangler

    Airhead Wrangler Long timer

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    I've never really though about it much, but I guess it would have to.
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  16. wirewrkr

    wirewrkr the thread-killer

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    Not so much the rear wheel, but the diameter of the whole package, ie, the total with the tire installed. Too small a rear tire from spec and the inaccuracies of the speedo worsen, larger than stock, and you bring your speedo closer to being correct.
    On my Toyota truck, when I put larger than stock tires (1 size up) the speedo went from showing 75 when doing 70 to being almost spot on. (checked with GPS)
    #16
  17. Boxer Metal

    Boxer Metal Mad Scientist

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    It takes into consideration the stock recommended tire from BMW 4.00/18.
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