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Thread: Radical Resprocketing / Overgearing?

  1. #1

    Radical Resprocketing / Overgearing?

    I'm curious about changing the gear ratio on the G650GS. I am planning a cross-country trip, or at least an attempt at one (hopefully the bike will fail gracefully). Looking up available sprockets, I note that it should be theoretically possible to regear it to 17/39 from 16/47, a 28% increase in speed at gearing or a 22% decrease in gear ratio. In theory, this should turn the 5th gear into an overdrive gear, and since I'm competent enough using only 2nd gear, the loss of low-end power shouldn't be too bad. Most importantly, for me, I prefer riding in the 2500-3000 rpm range, and this allows me to get 50 mph on 5th gear.

    However, when I talked to my dealership to order the parts, they seemed shocked, and mechanics actually suggested I'd have to push-start it if I made such a change. Why is there such an aversion to radical overgearing? And why does only a 22% decrease in gear ratio frighten people? Am I seriously chasing a chimera? Could someone with experience on the topic help out?

  2. #2
    Fortis Fortuna Adiuvat Omega Man's Avatar
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    Welcome to the forum!

    Itís a design/engineering thing. Once you get past 1 tooth in the front or 3 in the back, things really start to change. I saw your other post on your planned ride and, IMO, I would leave it at factory specifications.
    If you were looking to make a speed or mileage run across the country on I-80 or I-90, things would be different.
    Good luck.
    Gary
    "You can do good or you can do well. Sooner or later they make you choose." MI5
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  3. #3
    Thanks for the welcome. Is there a more detailed reason for major changes in performance should I regear beyond 5% gear ratio? Like, is there some non-linear factor I've missed?

  4. #4
    Engines are designed to operate efficiently in certain RPM vs load ranges. High loads at significantly lower than designed RPM place a too great burden on the engines. My experience with airhead motors says that many more have been injured by lugging them than by spinning them freely. Also, a weaker engine needs more gearing. When the airheads went from 600cc to 750cc to 900cc the gearing got "taller" with each displacement increase. Putting an R90 final drive on an R60 would result in a highly burdened pig that is no joy to drive. One would be slipping the clutch and down-shifting continually. I have gone the other way and put an R60 final drive on an R75 and got a livelier bike as a result. This was often done on sidecar attachment.

    There is always a limit to "more is better" when you get outside of the nominal design range. Otherwise you could happily run 100 mph at idle speed. In fact, you could gear a bike to run 100 mph at idle speed and it would do it downhill! With your planned gearing you will be down-shifting at the slightest hint of an incline and climbing any significant slopes in 2nd and 3rd gear. I'd hate to endure a long trip like that. The factory gearing will be more fun for you and easier on the machine. BMW did not simply guess at the gearing, and even if it could be marginally optimized for your particular situation, they did not get it greatly wrong. You have been given good advice and are free, of course, to do as you please. You might choose to consider the following though: 1) no-one has any vested interest in giving you bad information, and; 2) if you are correct in your discovery, then a building full of BMW engineers need re-training.

    "In theory there is no difference between theory and practice, but in practice there is."
    Lee A. Dickinson - Danielsville, GA USA
    MOA 80364 | RA 29650 | ABC 3480 | IBA 8914
    1976 R75/6 - 1992 R100RS - 1993 K1100RS - 2013 R1200GS

  5. #5
    Minor change might help. I once installed a different final drive and made a 10% change. It did not help fuel mileage and had a minor effect on uphill headwind pulls.

    The 20+ percent change you postulate would make the top gear, and mabe 4th almost useless due to a simple lack of sufficient horsepower to overcome wind drag at speed.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  6. #6
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    2013 G650GS - One tooth larger on the countershaft

    I went one tooth larger on the countershaft very early (40,000 miles ago) and it works fine for me. Have been to Prudhoe and then to Key West and some light trail riding as well. It is really comfortable between 65-75 mph. It works for me on the highway (riding solo) and pulls nicely to 85 mph where the wind turbulence starts to get busy. It's easy enough to go faster, according to the Michigan State Police who kindly issued a warning (WE ARE NOT ON THE AUTOBAHN). I would be interested in how you do with anything north of the gearing I have.

    Wayne Koppa
    Grayling, MI
    Life #71,449

  7. #7
    Registered User Rinty's Avatar
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    Quote Originally Posted by Inst View Post
    Could someone with experience on the topic help out?
    I'd be very careful making anything but minor sprocket changes. I once took a friend's 650 GS/PD, that had been geared down 10%, on a trip to the Kootenays. The change put the buzziest part of the rev range at 70 mph, which was a bit annoying.

    But some bikes, like a Ducati ST2 (might be the 900 SS, actually), can benefit from an extra tooth or two, on the rear sprocket, as 6th gear is a bit too tall.
    Last edited by Rinty; 06-23-2018 at 03:40 PM.
    Rinty

    "When you don't know where you're going, any road will get you there."

  8. #8
    @beemerphile, wkoppa: thanks for the good warnings.

    I looked at the bike, did some calculations, and what I found was that given the expected riding weight (718 pounds, including accessories, luggage, and rider), the bike maxed out at 95 mph. With the gearing I have intended, the bike would actually lose around 5-10 mph at peak (on level ground at sea level) because the bike couldn't hit max hp at achievable speed/rpm combos.

    See:

    https://www.gearingcommander.com
    https://www.ajdesigner.com/fl_horsep...trap_speed.php

    The calculation I'm getting is that max sustained RPM on fifth gear would be something closer to 4200 RPM, with speeds of 83 mph. Max sustained RPM on fourth gear is around 5700 RPM with speeds of about 93 mph.

    So with this gear ratio shift, I'd be effectively losing the first gear, turning it into a slightly lower equivalent of the second gear, and I'd be turning the 5th gear into an overdrive gear that'd have to be lowered to 3rd or 4th if I wanted to reach and sustain high speeds.

    It'd be a funky way to ride; I'd try to shift up as soon as the RPMs can support a downshift, but once I exceed a certain speed, I'd actually have to lower my gearing to achieve top speed, or even stay in lower gear to benefit from higher acceleration.

    At the very least, I'd say this would be totally worth it if I were going to keep the bike solely for local joyrides. Short distances, no problem. I'd be able to ride up to 50-60 mph on my overdrive gear in the 2500-3000 rpm range, and just put up with crappy acceleration if I ever get on the interstate. Long distances, though, the need to downshift in order to accelerate becomes an issue.

    I haven't ordered the sprockets yet, but you guys have given me room for thought. Thanks for the input.

    ===

    If we were to take cost, complexity, and risk out of the calculation, the easiest solution would be just have an overdrive gear added to the gearbox. It would achieve the same results without having to sacrifice low-end torque and mis-aligning the higher gears from the engine's performance potential; the 4th and 5th gears would be unable to achieve max horsepower. But that's really complicated, requires custom parts, and custom modification to the bike to get it to support this.

  9. #9
    The cardinal rule for long trips is to never make a significant change or do extensive maintenance just prior to leaving. Give yourself some time to evaluate whether it was done properly. If you choose to make this change, you might want to do it with enough time to evaluate the result before the trip. Load the bike to travel weight to give it a good test and simply ask yourself whether the bike is more or less enjoyable after the modification. I expect you will find the downsides to outweigh the upsides. I have no doubt that you will be placing more, not less, stress on the machine's engine and clutch.
    Lee A. Dickinson - Danielsville, GA USA
    MOA 80364 | RA 29650 | ABC 3480 | IBA 8914
    1976 R75/6 - 1992 R100RS - 1993 K1100RS - 2013 R1200GS

  10. #10
    What's technically a long trip? I'm considering a trip out to Tennessee, a round-trip of 2400 miles. I should handle it over 4 days or so.

    Also, the sprockets and chain came in, I now have a 39 gear rear sprocket designed for an Aprilia 125, and a 17 gear front sprocket. I'm worried the rear sprocket won't handle the stress of the 650 engine; it'd be about 4-5x the horsepower.

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