changing the final drive gear ratio on the GL1800 - GL1800Riders
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post #1 of 89 (permalink) Old 04-10-2008, 03:48 AM Thread Starter
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changing the final drive gear ratio on the GL1800

Just for the sake of discussion lets say I could change the final drive gear ratio on the gl1800 to drop interstate type driving by 500 rpm's. I'm using 500 as thats pretty much how much the rpm's change from 4th to 5th and I like some others I know would really like a 6th gear type rpm at interstate speeds. What do you think the average rider would be willing to part with to have that kind of mod done? It would involve removing your final drive (not difficult at all) sending it in having a new gear set installed and of course properly set up and you re-installing it. I don't want to get into whats it made out of or is it reliable or any other type of tangent issues. Lets just keep it simple. I ask this question to find out if it would be worth going thru all the monkey motion a set of properly designed gears would cost to do this...I am considering it obviously or would not throw it out there. My question is aimed at those that feel as I do that the rpm's could come down for interstate type speeds so mpg doesnt take as much of a hit. Thanks fellow riders
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post #2 of 89 (permalink) Old 04-10-2008, 04:22 AM
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I have tried for many years to get someone with a machine shop to do just this.
It would increase fuel mileage by a condiderable amout, ,make the engine last even more then it does now.
looking at honda cars and toyota cars, I'd say drop the rpm by 1000, make it about 2400 rpm (not 3400) at 70 mph.
I really do think this is the best idea in quite sometime. The transmission is so hard to get to the rear screw would be the easy fix.
your in top gear at the bottom of the ramps, and this should just not be, there is not really a true overdrive as there should be.
I've wriiten many many letters to Honda about this.
please keep my e-mail address I'm very interested
Robert Taylor
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post #3 of 89 (permalink) Old 04-10-2008, 06:35 AM
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Although that could improve mpg and lower the cruising rpm's it would negatively affect your performance.
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post #4 of 89 (permalink) Old 04-10-2008, 06:37 AM
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I agree that I'd like a bit less rpm on the interstate.

Four ways to achive that:

1) another gear in the tranny. That's up to Honda; cost prohibitive and design intensive to the nth degree for us mere mortals.

2) taller 5th gear in the tranny. See above answer.

3) taller rear drive ratio. A bit more "doable" from the design and machining perspective. It's done in the aftermarket all the time for performance cars and 4x4 trucks. Perhaps enough positive pressure for the 'Wing crowd would persuade them for a design quest. For those who don't think this all the way through, it would be quite intensive as there is a pinion AND ring to address. The upside is of course, better economy. The downside is a bit slower acceleration relative to any given gear selected in the tranny.

4) taller tire. Been done several times before. Drop a few rpm, and gain some ground clearance at the same time. Some like this approach; some don't. Another upside to this approach is that it's easy and inexpensive (realtive to the other options) to do, and it's easy to reverse if you don't like it.

As for the "amount" of rpm drop desired, that's a debate within a debate. My observation, from both GL1500's and my current GL1800, is that 350 rpm or so would be good. I say this because there seems to be a universal aspect of fuel consumption with the GL series of engines. It is true of all internal cumbustion engines as well, generally. Spin it higher, consume more fuel, right? I don't know why, but the GL's seem to hold fuel consumption fairly steady until around 3000 rpm, then they start on some parabolic P+I BSFC consumption gas guzzle fest that just goes up at an insane rate past 3000 rppm. If you go from 3000 rpm to 3300 rpm (10% gain), you might use 20% more fuel. And it gest worse from there.

Now part of this is because 3000 rpm is APPROXIMATELY 70mph, depending on the speedo accuracy. Most people don't realize that wind resistance is not linear; as your speed doubles, the wind resistance is SQUARED! In other words, to go from 35 mph to 70mph makes the wind resistance FOUR TIMES the amount at 35 mph! This is why the harder you push past a higher speed, the fuel consumption rate goes way up.

The upside is the the GL1500 and GL 1800 engines have enough torque that it's quite possible that they could offset the energy requirement to break past each successive mph increment. At some point, the engines would get into a "zero sum gain"; if you dropped the rpm by 25%, the engine may not have enough torque to overcome the load based upon speed and wind. That's why I think an approximate 10%, no more than 15%, drop would be enough to gain fuel economy, but not enough as to overburden the engine.

Let me put some rough numbers to it. Assume 70mph is 3000 rpm in a traditional GL1800. If you went up 10% in speed, you'd be around 77mph and 3300 rpm. If you could drop the rpm by 10%, you'd be back at 3000 rpm; at that point, I feel the GL1800 has enough torque to overcome the load, and the drop in rpm would be more than the "throttle" needed to sustain the rpm. You will NOT get a direct 10% gain in fuel economy, as it does take some more fuel to sustain that torque load. But you might gain 5% more economy?

Engines are most efficient at peak torque. I'm not sure where peak torque is on a GL1800, but it seems to be around 3000 rpm or so, by my general observations. The goal would be to get the most vehicle speed out of the engine, realative to the rpm and BSFC. If I can get the GL1800 to turn 3000 rpm at 77 mph, rather than 70 mph, I gain fuel efficiency, as long as the wind resistance load does not overcome the BSFC relationship.

Food for thought. The next time I'm going to change tires, I'm going to try a taller tire. The rpm drop ratio is about what I'd be looking for. I didn't buy the GL1800 as a drag bike; it's a LD touring and commuter ride for me. If I gain 10% economy at 75mph, but only loose .3 seconds in a 0-60mph run, I'll take the fuel economy edge any day, especially with gas nearing $3.50 a gallon!
post #5 of 89 (permalink) Old 04-10-2008, 06:38 AM
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If I were to tackle a job like this I would get a new or used 5'th gear set. Then have a gear expert (I know one) measure the tooth geometry then have a engineer/designer/drafter (I am one) draw up a new gear drawing with a different number of teeth. Now I would have to find a gear job-shop to manufacture and grind it (I know several) and then finally heat treat it.

I have done something similar to this for a ship motion simulator we use for antenna testing. We have also reverse engineered gears several times in my professional duties. It's very expensive.

Now comes the fun part in removing the engine and split the case to install the new gear set. Of course your warranty just poured away with the engine oil.

I suppose it's also possible to do the same procedure with the gears in the final drive. This would affect all gear ratios. Some might consider this a positive but others not......

The downside is the reverse engineering costs and manufacture of onsey/twosey gears wold be very high.
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post #6 of 89 (permalink) Old 04-10-2008, 07:01 AM
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Hi all. I don't worry about the cost of fuel just love the power. We pay $7.50 per gallon here in Australia or if you like $1.50 a liter.
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post #7 of 89 (permalink) Old 04-10-2008, 07:28 AM
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It would be nice if such a mod was as simple to perform as on the Suzuki C50. With that bike,
you swap in a C90 rearend - and shorten the driveshaft slightly to keep the C90s driveshaft from binding.

It's currently the most popular mod by far (judging by posts) on the Volusia Riders board I frequent:

Different rear drive ratio/highway cruising
post #8 of 89 (permalink) Old 04-10-2008, 08:16 AM
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Go to the VTX board. Showtime in St. Louis does the gear change on VTX's I would buy one for my Wing when someone figures it out. Thanks Buzz
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post #9 of 89 (permalink) Old 04-10-2008, 08:47 AM
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TP -

I would say that the average rider would be willing to pay $700 - $1000 for a final drive reduction. BUT I would ask that if you do this you do it as an add on Over Drive Unit, like used to be on cars and lite trucks. A unit placed between the drive shat and rear end where you flick a switch in 4th or 5th gear and get a true over drive. I would gladly pay $1K - 1.2K for that! A true 6th gear! And I don't think it would take much change, heck they used to just add them on to the differential in the old days.
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post #10 of 89 (permalink) Old 04-10-2008, 08:54 AM
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Put me in line. I WANT one. Buzz
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