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Thread: Determining the FD ratio

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    --Tony AnnapolisAirhead's Avatar
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    Determining the FD ratio

    I thought the FD gear ratio was stamped on the the FD casing...but nothing on mine. I searched the forum and didn't find it there either.

    Other than specs on various websites, is there a place I can verify my ratio on the bike itself? It's a 1983 R100.
    '83 R100RT'd
    '71 R75/5 SWB
    '06 KLR 650

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    Administrator 20774's Avatar
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    Use your fingers to count the number of times the driveshaft rotates by monitoring the bolts under the rubber boot...probably do this in high gear. Count the number of revolutions that the rear wheel makes. Since you're looking for something like 33/11, one will move 11 revolutions (the wheel IIRC) while the other moves 33 revolutions...probably easier to watch for 3 to 1.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Registered User lkchris's Avatar
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    Quote Originally Posted by AnnapolisAirhead View Post
    I thought the FD gear ratio was stamped on the the FD casing...but nothing on mine. I searched the forum and didn't find it there either.

    Other than specs on various websites, is there a place I can verify my ratio on the bike itself? It's a 1983 R100.
    Look one more time.

    It's just outside of driveshaft oil filler plug or just to rear of top/outside driveshaft tube to rear drive mounting bolt. It's on the top and you'll look straight down onto it.

    It probably reads 32/11, which means 2.91-1.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

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    --Tony AnnapolisAirhead's Avatar
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    Thanks guys.

    What I am trying to verify is whether or not an R100 vs. an R100RT vs and R100RS vs an R100S are any different. I know the R80RT is and IIRC it was due to the same fairing used on the R100RT, but it 'taxed' the smaller 800cc engine. It all started when I began to wonder why they didn't ever make an R80RS, since that engine is so smooth.
    '83 R100RT'd
    '71 R75/5 SWB
    '06 KLR 650

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    Unregistered User dduelin's Avatar
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    If my 81 owner's manual applies to 83 models the R100 came with ratio of 11/33 (3.0 to 1), the R100CS 11/32 (2.91 to 1) and the RS and RT 11/33. The CS and RS had the option of the other ratio.
    Dave
    R100
    ST1300

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    --Tony AnnapolisAirhead's Avatar
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    Quote Originally Posted by dduelin View Post
    If my 81 owner's manual applies to 83 models the R100 came with ratio of 11/33 (3.0 to 1), the R100CS 11/32 (2.91 to 1) and the RS and RT 11/33. The CS and RS had the option of the other ratio.
    Hmm, pardon the ignorance, but doesn't that mean an R100 would have taller gears than an RT? It seems upside down...or maybe the RT fairing provide that much better aerodynamic efficiency vs a naked (R100) bike? I removed a Vetter fairing from my R100 and converted it to an RT (less the rear disc), so I'm just curious.

    I'm assuming 'taller' means higher, for example 11/33 (3.0:1) is 'taller' than 11/32 (2.91:1). Is that a correct assumption?
    '83 R100RT'd
    '71 R75/5 SWB
    '06 KLR 650

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    Registered User mneblett's Avatar
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    Quote Originally Posted by AnnapolisAirhead View Post
    Hmm, pardon the ignorance, but doesn't that mean an R100 would have taller gears than an RT? It seems upside down...or maybe the RT fairing provide that much better aerodynamic efficiency vs a naked (R100) bike? I removed a Vetter fairing from my R100 and converted it to an RT (less the rear disc), so I'm just curious.

    I'm assuming 'taller' means higher, for example 11/33 (3.0:1) is 'taller' than 11/32 (2.91:1). Is that a correct assumption?
    Numerically lower="taller."

    The 3.0:1 ratio allows the RS/RT to pull the weight of their fairings just a little easier than the 2.91:1, i.e., the 3:1's greater mechanical advantage effectively multiplies the available torque from the engine more than the 2.91:1's torque multiplication; this at least partially makes up for the RS/RT having to accelerate the additional fairing weight. The lighter nakkid R100 could get along fine with the 2.91:1.

    The '79 RT had a 2.91:1, but it also had ~10 more HP (70 vs. 60). I'm guessing that BMW went to 3:1 to take some of the "sting" out of the de-powering of the RS/RT with the huge compression drop in the '80's bikes.

    The primary advantage of the the lower ratio (2.91 vs. 3) was that the engine turns slower for a given road speed. IMO, however, the ~3% lower rpms at the same speed isn't a big deal one way or the other.
    Mark Neblett
    Fairfax, VA
    #32806

  8. #8
    --Tony AnnapolisAirhead's Avatar
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    Ok, that makes sense...as if I'd notice 3%, heh?

    Would the higher compression engines (e.g. 1979 vs 1983) make a noticeable difference?
    '83 R100RT'd
    '71 R75/5 SWB
    '06 KLR 650

  9. #9
    Registered User mneblett's Avatar
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    Quote Originally Posted by AnnapolisAirhead View Post
    Ok, that makes sense...as if I'd notice 3%, heh?

    Would the higher compression engines (e.g. 1979 vs 1983) make a noticeable difference?
    The '79 RT had 9.5:1 compression and squish-band pistons, 40mm Bings, 40mm exhaust, and larger valves (40mm? can't remember).

    The later bikes had 8.2:1 compression (the last airheads went to 8.45:1, IIRC), 38mm Bings, slightly smaller valves and slightly smaller exhaust head pipes.

    The HP went from 70 to 60 (BMW's numbers, maybe lower in the real world), but the torque curve wasn't an identical drop -- the smaller carbs and valves helping to fatten the curve a small bit at lower rpms.

    With that background, in answer to your Q: yes, overall the drop was detectable by the tail-feather dyno.

    FWIW, when I had my '88 RT apart from dual plugging and a valve job, I spent the $$ for a set of 9.5:1 non-squish-band pistons (the pistons compatible with the later head combustion chambers) to gain back much of the post-'79 power loss. I say most, because (i) the pistons are slightly different shape (no squish-band), and I stayed with the 38mm Bings and original valve sizes -- there was no way I could just the $$$ to replace the 38mm carbs, exhaust and total rework of the valves/seats to move to the 40mm stuff for that last bit of HP. Plus, staying with the smaller valves left more meat in the head between the valve seats and the second spark plug hole; another reason to leave well enough alone. Judging by the results, these were the right calls for my needs/desires.
    Mark Neblett
    Fairfax, VA
    #32806

  10. #10
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    There is approx 10 HP difference in engine output at close to 7000 RPM - the difference is likely less than that at any of the RPMs that you're likely to use. Of course, there is also the difference in clutch/flywheel effect in the two engines. This is assuming stock engines in straight fromthe factory tune - engine conditions, tuning, exhaust, etc. can affect these further. So, will you notice the difference - only you could say for sure.
    BMWs in my garage: 1982 R65LS, 1978 R100/7

  11. #11
    --Tony AnnapolisAirhead's Avatar
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    Thanks Mark and Boxemaf.

    My bike is stock with the exception of dual plugging and I had a complete valve job job about 2k miles ago with Black Diamond not OEM. I know, I know another religious war could break out over valve choices. I did my research, asked my stupid questions and made my choice. Happy thus far and the machine work was first rate.

    I'm not likely to ever hit 7k RPM, but would still consider the higher compression pistons if/when I ever replace the rings. While the heads were out, I measured the rings and they were 'as if new' wrt wear so I kept them. Pistons cleaned up nicely in a heated ultrasonic tank and I was in high learn mode at the time.

    This is all great knowledge and I appreciate it. My '84 R65 project could end up with a '79 R100 motor, not sure yet what to do with it. I like the beefier frame, top clamp, etc, but not the idea that the engine it essentially a one-off (and a 650). Frankenbike?
    '83 R100RT'd
    '71 R75/5 SWB
    '06 KLR 650

  12. #12
    Unregistered User dduelin's Avatar
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    81 -84 USA spec 980 cc bikes all had the same low compression motor with 40mm carbs and 38 mm pipes. IIRC this motor develops 67 hp at 7250 rpm according to period literature. Other markets still got the higher compression engines.

    I have dyno'd mine twice and it produces about 54 hp at the rear wheel. All things equal a 3.00 set of gears is quicker accelerating gives and lower top speed than a 2.92 set of course as pointed out the fairing weight has to be toted down the road until it's superior aerodynamics improve CD over a naked bike and allow a higher top speed for the faired bike.

    I call 'later' bikes Monolevers but I'm probably wrong. Those 980 cc motors had 38 mm carbs and 60 hp but more usable torque and were actually a bit quicker accelerating bikes according to period tests.
    Dave
    R100
    ST1300

  13. #13
    Registered User lkchris's Avatar
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    No, the 3.0 ratio for R100s didn't happen until the '87 revival models. These were the models with smaller carbs and smaller valves.

    All up through '84 were 2.91 except for '77-'78 R100/7 (also with smaller carbs/valves).
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  14. #14
    Registered User mneblett's Avatar
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    Quote Originally Posted by lkchris View Post
    No, the 3.0 ratio for R100s didn't happen until the '87 revival models. These were the models with smaller carbs and smaller valves.

    All up through '84 were 2.91 except for '77-'78 R100/7 (also with smaller carbs/valves).
    Kent, thanks for the correction -- learned something new!
    Mark Neblett
    Fairfax, VA
    #32806

  15. #15
    Unregistered User dduelin's Avatar
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    Quote Originally Posted by lkchris View Post
    No, the 3.0 ratio for R100s didn't happen until the '87 revival models. These were the models with smaller carbs and smaller valves.

    All up through '84 were 2.91 except for '77-'78 R100/7 (also with smaller carbs/valves).
    Although my owners manual is clear that my R100 should have 3.0 to 1 FD this is clearer: 32/11 is 2.91 to 1.
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    Dave
    R100
    ST1300

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