Comparing Final Drive ratios between chain-drive and shaft-drive

[MisterCinders]

Why would you do that?

Well, my 750/850 is a chain-driven 850. If/when I replace my chain/sprockets, I plan to swap over to a 530 chain. I am considering using the new chain/sprockets to mildly adjust my gearing. The 850 generates nice torque, but I find the bike possibly too torque-y in low gears, and smooth throttle control is a challenge.

To maintain current gearing, I can use 18/49 sprockets. I wonder if the actual 850 final drive ratio might provide a better baseline than the 2.73:1 of the 750, as I understand the 850's low-speed maneuvers are nice and easy. Problem: I cannot decipher the final drive ratio for an 850 for lack of chain/sprockets. Shaft drives - how do they work?

According to the service manual, the 850 transmission has a "primary reduction" of 1.775 and a "final reduction" of 3.090, but those are dramatically different from the 750's 2.73:1. Averaging the two yields 2.43, which is closer but still a big change from 2.73.

Help me get to an apples-to-apples comparison.

 

[Steve]

OK, for apples-to-apples, break out your service manual for the 750.

In there, you will see that you also have a "primary reduction ratio".

Not sure why you are "averaging" any ratios, when they are in-line with each other, you multiply them for the overall ratio.

On your 750, the primary ratio is 2.152, the top gear ratio is 0.961, the final ratio is 2.733. The OVERALL ratio is 5.652.

Taking the same numbers for the 850: primary is 1.775, top gear is 0.961, there is also a secondary ratio of 1.062, the final ratio is 3.09. The OVERALL ratio is 5.598.

Not enough difference to worry about.

What those ratios mean is that the crank (on your 750/850) will turn 2.152 times as fast as the clutch, which is the input for the transmission. In top gear, the input shaft (clutch) is turning 0.961 times as fast as the output (a very slight overdrive) and the front sprocket is turning 2.733 times as fast as the wheel sprocket. Overall, the crank turns 5.652 times when the rear wheel makes one full turn.

To illustrate the difference in engine speed at a given road speed, you would have to assume that the rear tires are identical, making the only difference the gearing. At whatever road speed it takes for the rear wheel to turn 1000 rpm (I'm too lazy to figure that out, it doesn't really matter right now), your 750 would be turning 5652 rpm, the 850 would be turning 5598 rpm. Not even a 1% difference.

.

 

[MisterCinders]

OK - but I can only change the final reduction on my bike, because that's what the sprockets produce. If I add/subtract teeth from the sprockets, it will affect final reduction and, in turn, the overall ratio.

I didn't think the 850's 3.09 corresponded to the 750's 2.773 value. As I look into things, a sprocket ratio of 3.09 seems way more torque-y. I aim to sacrifice a bit of torque to smooth out the lower gears, maybe drop a tooth from the rear sprocket, or add a tooth to both front and rear. Those would yield final drive ratios of 2.66:1 and 2.63:1.

As part of the decision process, I wondered how the actual 850 compared in that section of the drive train. Essentially, I wanted to decipher the 850's gearing to solve for a "phantom sprocket set" and see if that's a good target for some replacement sprockets of my own. I hope that translating from shafty to chain could answer the question "If the 850G had chain/sprockets, those parts would produce a final drive ratio of ______."

Of course, I can just base things off my current 2.73:1 and dial it down to 2.63 or 2.66, but determining the theoretical 850 value could illuminate the better of those options to mimic it's good low-speed manners.

I'll play around with some math in 1st, 2d gears between both models. Maybe that will help.

 

[koolaid_kid]

I suggest you are overthinking this. I know nothing about shaft drive bikes, but I know a great deal about chain drive bikes and changing ratios.
The way you approach it is this: If you want more power off the line at the sacrifice of higher revs in high gear, you lower the ratio. IOW, from a 2.73 to a 3.0. If you want lower revs in high gear at the sacrifice of less power off the line, you raise the ratio, from a 2.73 to a 2.66. Since you say it has too much torque off line, you do the latter. It is pretty simple, really.

 

[tkent02]

The 850 generates nice torque, but I find the bike possibly too torque-y in low gears, and smooth throttle control is a challenge.

If smooth throttle control is your problem, I would suggest better tuning.

Ratios and gearing do not matter at all in this.
Pulling the throttle smoothly should give smooth throttle response.
Unless your name is Edward Hammer Hands.

 

[MisterCinders]

If smooth throttle control is your problem, I would suggest better tuning.

Ratios and gearing do not matter at all in this.
Pulling the throttle smoothly should give smooth throttle response.
Unless your name is Edward Hammer Hands.

No doubt operator error factors heavily into this. But even my gentlest touch provokes a healthy reaction in 1st/2d gear, where I do a lot of turning in the city. Most of my riding happens in town, where 30 MPH limits on even fairly wide, empty roads will score you tickets at 40-42 MPH (AMHIK). To keep close to 30-35 MPH often makes me long for a 2.5 and/or 3.5 gear because the lower gear feels too torque-y to keep steady, but the next gear seems a bit boggy. At 40+ MPH, no problems. Indeed, the bike yearns to go faster, faster, faster.

Pretty sure the PO swapped out the throttle tube for an R1 to shorten the twist to WOT, which feeds into things.

None of this crippling, mind you. The bike runs great. I'm just exploring things that might handicap my poor riding skills, and some smoother low gears appeal to me in that regard.

 

[tkent02]

I would swap the throttle tube back before I'd screw up the drive ratio.

Still, if it's not smooth I'd suspect pilot circuit tuning. Any GS I've ridden you can add just the tiniest bit of throttle and the engine increases just a tiny bit, smoothy, accurately, easily. Something isn't right in there.