Some info on the effects of gearing changes,


The increase or decrease of torque when you change the sprockets on a chain-driven motorcycle engine can be calculated with a bit of a basic math.You can quickly determine on paper whether a desired sprocket change will improve the torque and the bike's performance or make it worse. Figuring this result mathematically beforehand can save wasted time and help prevent unnecessary mechanical work if the wrong sprocket sizes are used.

Count the teeth on the front sprocket. Do the same for the rear sprocket, Write this data down, then divide the tooth count of the smaller, front sprocket into the tooth count of the larger, rear sprocket using a calculator (for example, a 17-tooth front sprocket with a 47-tooth rear sprocket will be 47/17, or 2.76 when rounded. Write down the result as the existing drive ratio).

Perform the same tooth-counting process in Step 1 with the two new sprockets that you plan to swap out and use in the modified chain drive setup. Calculate the new drive ratio using the same formula. Write this ratio down on the notepad (for example, the new set of 19 front teeth and 50 rear teeth would be 50/19, or 2.63).

Subtract the new drive ratio from the old sprocket setup drive ratio (in our example 2.63 less 2.76 equals -0.13). Divide this mathematical difference (in our case a negative value) by the original drive ratio value (-0.13/2.76 equals -0.047). Multiply the decimal result by 100 to obtain the percentage change in torque between the two setups (for example, -0.047 by 100 equals -4.7 percent, which represents a loss of torque from the original setup).


That is the loss of torque from the chain and sprockets transmitted to the back wheel not the percentage lost of the dyno measurement taken at the rear tyre.

Another interesting point is that a well maintained chain and sprocket set up can be up to 98% efficient in transmitting output sprocket power to the rear wheel, on most 600-1000cc motorcycles around 70-75mph, but as speed and the chain/sprocket revolutions increase the efficiency drops to around 75% at 140-150mph,
Choosing the wrong sprocket sizes will obviously increase this loss of efficiency further.

interesting stuff

I did the numbers for an 05-06SD when i decided on my new gearing,

taking the standard bikes ratio of 17/38 as 0,

17/39 -2.64%
17/40 -5.23%
16/38 -6.27%
16/40 -11.85%

Very interesting! Thanks for the info

Yes very interesting....and very complete...so not a lot left to say

I have constructed a spread sheet for calculating speed in all gears at all revs that allows you to change gearing and see results

PM with email address and I will forward to you

Yep thats right 5.23% loss of torque transmitted to the rear wheel compared to 17/38 gearing,

I went with 17/40 to as a compromise, larger front sprockets being more efficient and also increased swing arm clearance (the chain dragging on the swing arm guide must loose even more torque

I am a more seat of the pants kind of rider. Knowing that I seldom use 6th gear unless exceeding the speed limit on the fastest highways (70mph) by ~10mph, coupled with the fact that most car drivers do not accelerate from a stop fast enough for me to start in first gear and obtain an rpm sufficient to not jerk and chug, I have ordered a 16/38 sprocket combo. 3rd gear wheelies may have also impacted my decision as these are currently somewhat difficult to perform without the aid of a decent bump in the road. I tour on it as well (hard saddlebags bitches!) so I can't be screaming in 6th on the interstate. Based on past experiences changing other bikes (VTR1000F) to a -1/+2 combination that was a bit too low but lots of fun, this -1/+0 combo should be perfect.

Also sticking with the 525 pitch and steel sprockets for longevity. The VTR ate an aluminium 520 rear sprocket in 10k miles, the stockers on the SD are finally bowing out at 20k due to chain degradation.

Whilst searching around the site for info on sprocket sizes I came across this thread, and thought I should point out in the interests of truth that increasing the size of the rear sprocket will increase final drive torque not decrease it! Reading the first post made me wonder for a bit I wanted to find out if running a 17 tooth front sprocket would foul the chain guide and cause bother, but it seems that it won't. Unless anyone knows different? When I got my bike it came with a 40 tooth rear/16 front combination. As a result the rear adjusters have been reversed. I've sometimes wondered if this has an adverse effect on handling, what with it shortening the wheel base, so thought I might try 17/40 instead.

You will have to wind off a little rear preload now you've shortened the wheelbase. (Very roughly, dep on springing etc, 1/2 turn is 2mm at the axle.)

The swingarm is a lever and youve shortened it slightly which means it will now take more force to compress the spring. Also make sure the forks are completely flush in the yokes and then recheck front preload/sag last as that will have changed too with the weight distribution change.

Very small changes, but depending on your usage make a big difference. Not sure how important it would all be on a road bike but on the race bike it's massive. Also will depend on what kind of rider you are and it your the type to feel changes easily or not.

Your always going to have a compramise as adding a link wont put the exactly wheel back to where it was before.

I needed a click more low speed compression on the rear to get the same feeling from the bike after cleaning and spanning the chain a few millimeters. Didn't expect the effect to be that big but then again I'm pretty sensitive to how the bike reacts.
I didn't do anything with the preload because before the lengthening I had the sag set at the low end of the advised range.

Ok I'm battling with not wishing to bring down a rain of abuse upon myself and yet wanting to be well advised If I deserve it bring it on!

I wanted to ask you guys for your preference of 16/40T vs 17/41T (I'm assuming that 16/41 is probably more wheelie prone than required). I currently have 16/38T on SD990 '07 and the chain/sprockets need changing.

I read that you can swap the blocks around for the 16/40 setup on the standard chain - thanks @ Gregz.

Do you need a longer chain for 17/41 (I'm guessing you do)? Was thinking of "DID VX 525 x 120" or "DID VX 525 x 118" chain...

Cheers

Must be me....find myself looking for 7th gear!! Running the 16/38 gears and, No...I'm not that fast. Have always tended to run a gear higher than many, using the torque rather than rpm it to the moon. Don't care for the terrible low speed manners with stock gears, so thinking about going to the 16/40 combo.