Difference between 939 and 821 drivetrain?

Oct 2016
368
1
San Francisco
So I'm debating (aka overthinking) which direction to go with chain and sprockets now that the stock chain is starting to show some stiff links at 16k. I have no real complaints with the stock setup as it's very usable around town, in the twisties and cruising on the highway. That said in poking around online I found that the 821 came stock with a 15/45 whereas the 939 came stock with a 15/43. I can't seem to find any information on whether there are any differences in the gearbox of the two motors that resulted in this change, but I have to assume this was to meet euro4 sound limits only as the 43t rear sprocket would keep the revs a little lower during testing.

Given that, and having ridden an 821 prior I'm thinking that changing to a 45t in the rear might make the bike slightly snappier without really impacting over all driveability or fuel economy and i assume that I could just infer the stock chain length of the 821 when ordering parts to ensure I keep the wheelbase the same when going up two teeth, which would presumably require an extra link or two.

Anyone have any thoughts on whether this is a good or bad idea or any pointers on what else I should check to ensure I keep the handling and wheelbase stock?
 
Sep 2017
281
2
Lansing Mi
Some motorcycle companies do exactly what you mentioned. They make the sprocket ratio in a way that helps them meet emissions standards with the hope/understanding thet the end used swaps out the gears. There are a few videos on youtube where they explain this. Ducati is one of those companies that expect you to do a swap. Check out Gearing Commander ⚙
It allows you to input your exact bike and tells you the rpm for each gear and what the top speed will be along with rpm. The goal is to have a bike that can reach top speed in a reasonable time frame at redline. Factoring in rider weight, wind resistance and other variances make this calculator really nice. If your bike cant hit redline in 6th gear due to wind resistance, rider weight and other variances then changing gears is ideal(unless you are seeking miles per gallon numbers). I watched a great video with that suspension guy (bob I believe his name is) when he addresses the goals and how to achieve them. You will be able to go faster(top speed) by actually lowering your gear ratio to a ratio that allows you to be at a different part of the rpm and at the same time have a snappier bike down low with the change(I feel like I'm not fully explaining it the way I intend). Check that calculator. I went down 1 tooth in the front(equal to +3 in the rear) and the bike is more exciting while not really sacrificing top speed. I'm just able to hit 120 easier(although I'm a little higher In the rpm).

Also......the 950 comes with a 520 chain set up. If you are thinking about doing the whole shebang I would switch to 520(the v4r also has a 520 chain). Its lighter, lighter rotating mass makes for a snappier bike as well.
 
Oct 2016
368
1
San Francisco
Interesting to play around with. I'm not particularly concerned with top speed. I've had the bike to an indicated 130mph and it was still willing to go further, but I wasn't. I'm definitely interested in more usable power in real world use around town and in the twisties without killing cruising gas mileage. With it's current gearing the 939 will cruise 90mph as a very comfortable 6400rpm. Looks like going two up in the rear only changes that slightly so it seems like I won't be losing much. Also swapping between the 821 and 939 gearing I see I'll need a few extra links 108 vs 106 if I go up to a 45t. Seems like a easy win.

I was thinking about 520, but considering the 950 is more of a sport / track bike while the previous 821 and 939 models had the hyperstrada as an option I suspect the 525 pitch was specced for better wearing under those circumstances. I'm ok with a little extra weight there for the added durability as I do like throwing a dry bag on the back and eating some miles a few times a year on this thing.
 
Feb 2014
310
0
TN
I went with a 46t on the rear of my 821 like the Monster has. Not as dramatic as -1 up front but gives a little more pep.
 
Feb 2018
117
1
Scottsdale AZ
Do these changes throw off the tachometer redline? Some have reported reaching "redline" sooner than they should as it throws off the bike's speed calculation. Any input is appreciated.
 
Oct 2016
368
1
San Francisco
Speedo is based off the sensor on the front wheel so that wouldn't be impacted by a different sprocket. Of course, if you go with a shorter gearing you'll be higher in the RPM band for a given speed, so that is probably what people are complaining about.