Higher final drive for a G/S - ST Options?

Discussion in 'Airheads' started by Bluethumb, Mar 30, 2013.

  1. Bluethumb

    Bluethumb Been here awhile

    Joined:
    Aug 7, 2012
    Oddometer:
    135
    Location:
    Texas Hill Country
    What can I do to lower r.p.m.'s of a ST? At hiway speeds of 70 m.p.h., the engine is spinning about 1,000 more r.p.m.'s then my R100. I know of two ways to change the ratio of the final drive on a G/S - ST. One is with the higher 5th gear and the other is the optional final drive. The higher 5th gear is expensive from what I've read, the cost of the gear plus the time and expense of a rebuild. I thinK I've read it drops r.p.m.'s by 400-500. The optional final drive is NLA I believe, but if it is still available I'm guessing it's unbelievably expensive and I'm not sure how much it drops r.p.m.'s.

    Is there another option? Is it possible to use a different FD and re-lace the rear hub to a ST rim?

    Any other way to replace the final drive with another to drop engine speeds?
    #1
  2. R-dubb

    R-dubb Dubbious Adventurer

    Joined:
    Aug 4, 2001
    Oddometer:
    5,099
    Location:
    San Francisco
    The issue is the three bolt hub. I dont think there is a spoke hub that will fit a four bolt FD with a higher ratio. You can't just change out the crown gear either, cause that's where the bolts fit.

    By far, the best option is to add the tall 5th to the tranny. It's actually a very easy job. As I remember, its the low 1st that is a bitch because it goes into the intermediate shaft which requires many tons of force to press.
    #2
  3. Airhead Wrangler

    Airhead Wrangler Adios Mexico

    Joined:
    Mar 4, 2008
    Oddometer:
    6,630
    Location:
    Back in Seattle, FINALLY
    ...or the higher 32:10 ratio 3 lug mono final drive is still available new. They're basically non-existent on the used market.
    #3
  4. Beemerboff

    Beemerboff Long timer

    Joined:
    May 23, 2005
    Oddometer:
    2,370
    Location:
    Adelaide, Souyth Australia
    Most of the hi 5 th gears are 5% higher - on a decent running motor you wouldnt know the difference unless someone told you.

    Just fork out for a new 32/10 as AW says, and sell the three stud 37/11 - they are hard to find and sell for reasonable money.
    #4
  5. Clay Spinner

    Clay Spinner Been here awhile

    Joined:
    Nov 6, 2008
    Oddometer:
    756
    Location:
    Nova Scotia
    I concur with airhead wrangler.... I've been trying to find a 32:10 bevel for two years now... Not even a sniff. I've just had a higher 5th installed but if you are planning for only road use, Richie Moore of morespeed does a +10. Check out his site and email him.
    #5
  6. Airhead Wrangler

    Airhead Wrangler Adios Mexico

    Joined:
    Mar 4, 2008
    Oddometer:
    6,630
    Location:
    Back in Seattle, FINALLY
    Actually, I think the 32:10 works out to be about a 5% difference. It's 3-4 mph higher at 5000rpm. 78mph instead of 75mph. If you add a 5% taller 5th on top of that you'll get a 5th gear that's nearly identical to a R100GS final drive ratio, but the rest of your gears would be a bit lower.
    #6
  7. Bluethumb

    Bluethumb Been here awhile

    Joined:
    Aug 7, 2012
    Oddometer:
    135
    Location:
    Texas Hill Country
    How do I track down Richie Moore? And hat do you mean by + 10?
    #7
  8. Beemerboff

    Beemerboff Long timer

    Joined:
    May 23, 2005
    Oddometer:
    2,370
    Location:
    Adelaide, Souyth Australia
    AW -you are right on at five+ percent - somehow I had it in my head that 37/11 was 3.56 , but I just booted the Cray and you are correct.

    BT, there is some tricky engineering in these gears - remember you are only changing one of a pair, so a change in gearing can only be accommodated in the size and profile of the tooth.

    This is normal gear cutting practice, of course, as the sum of the number of teeth in the pairs in a normal box is rarely the same for all pairs, but there is a limit to how far you can go with any given gear, and it seems that most folk are happiest at 5% difference from the stock OEM gear.
    #8
  9. AntonLargiader

    AntonLargiader Long timer

    Joined:
    Aug 28, 2003
    Oddometer:
    4,351
    Location:
    Charlottesville, VA
    moorespeed.uk I think.

    However, I would carefully consider your riding style before going to a drastically taller 5th gear. I personally like the "Four and a half speed" Airhead gearbox because it lets me downshift on the highway without changing the RPM all that much.

    The ratio that I've seen that's higher than 5% involved all three helicals. I can get that gearset also, but I've only been asked for it one once.
    #9
  10. Clay Spinner

    Clay Spinner Been here awhile

    Joined:
    Nov 6, 2008
    Oddometer:
    756
    Location:
    Nova Scotia
    Correct anton... the +10 is a 3 cog jobbie and it alters the spread of the whole box. It is a specialist item and probably only really useful for someone who really needs it :norton i.e. racing.
    #10
  11. supershaft

    supershaft because I can

    Joined:
    Sep 9, 2010
    Oddometer:
    9,116
    Location:
    San Francisco Bay area
    Looking at the forest and not the trees: Don't be tempted to gear your 800 like it is a 1000. If you want lower 1000cc rpms, get a 1000cc engine. 800's are geared for more rpm for a very good reason: Less power. The rpms compensate.
    #11
  12. Airhead Wrangler

    Airhead Wrangler Adios Mexico

    Joined:
    Mar 4, 2008
    Oddometer:
    6,630
    Location:
    Back in Seattle, FINALLY
    They can go a LITTLE higher, but depending on the use, maybe shouldn't. Agreed, gearing it higher than a 32:10 is asking for a lifeless throttle in 5th. The decrease in revs probably wouldn't result in better gas mileage either as you'd need to have it wide open to push that taller gear.
    #12
  13. GapRunr

    GapRunr Been here awhile

    Joined:
    Oct 22, 2005
    Oddometer:
    510
    Location:
    East TN / Smokies
    I recently bought a taller 5th (5%) from Anton :wave for my 81 R80 g/s and am happy with the results.

    I just recently installed a tach so I have some numbers written down. Using a GPS for speed, the engine is turning at 4,000 rpm for 62 mph. Every 500 rpm's increase provides 8 mph. (4,500 is 70mph, 5,000 is 78mph, and at 6,000 rpm I was travelling.. no wait.. I would never break the speed limit so disregard that) .

    It's enough of a difference that 5th now feels like an overdrive when loafing along the highway.
    #13
  14. Rucksta

    Rucksta SS Blowhard

    Joined:
    Oct 26, 2008
    Oddometer:
    2,709
    Location:
    Gold Coast

    100kph / 61.2 mph @4000 rpm is factory spec for an R80G/S with a 37/11 drive & 4:00 /18 tyre.
    What rear tyre are you running?
    Does the tacho need calibrating?
    #14
  15. GapRunr

    GapRunr Been here awhile

    Joined:
    Oct 22, 2005
    Oddometer:
    510
    Location:
    East TN / Smokies
    Hmmm....

    Perhaps the factory spec was for the OEM speedometer, which isn't accurate?

    I'm running a Michelin Pilot Activ in a 4.00-18. If anything it looks big compared to the 4.00-18 Conti Go I had on there previously. As for the tach, it's a new digital tach designed for two cylinder dual fire ignitions. I would suspect it is pretty accurate. And I just checked to make sure and my g/s is a 37/11, so the previous owner hadn't changed it.
    #15
  16. ontic

    ontic

    Joined:
    Jul 14, 2008
    Oddometer:
    1,074
    Location:
    Melbourne
    Link and details? I'm still shopping for a tach for my G/S.
    #16
  17. GapRunr

    GapRunr Been here awhile

    Joined:
    Oct 22, 2005
    Oddometer:
    510
    Location:
    East TN / Smokies
    Here's a pic installed in a temporary location.

    [​IMG]

    Since I own a shop posting details and price is frowned upon. Check your PM's.
    #17
  18. StephenB

    StephenB G(/)S ... what else!

    Joined:
    Aug 24, 2005
    Oddometer:
    2,044
    Location:
    Ontario
    There is a solution that hasn't been mentioned yet: the adaptation of a 88-95 RxxGS rear wheel to a monolever 4-lug FD. That gives you three FD ratios from factory:37:11 (R80RT Mono), 32:10 (R100RT Mono) or 33:11 (R100RS Mono) . Details are here: http://www.stephenbottcher.net/BMW/rearconv.htm

    Hope that helps, Stephen
    #18
  19. bajaburro

    bajaburro Ancient Adventurer

    Joined:
    Jan 14, 2006
    Oddometer:
    1,580
    Location:
    Huntington Beach CA
    it's the best option for your bike.just lowers the rpm in 5th enough to make freeway riding a bit nicer.
    #19