Changing Sprockets

[Freerider90]

Okay, Here's what I have going on. I am changing the shifter seal and Getting a new chain for my 750 L. I was thinking that while I have it torn appart would be a good time to change its gearing. I currently have a 15/38 setup with a 630 chain. I am seeing 4700 @ 60 MPH. I would like to get slightly lower RPM's without hurting the bikes performance. Any advice would be appreciated.

P.S. I read a post last summer about Riding RPM's where people were talking about running less than 4000 RPM at 60 mph with a 15/40 setup. How is it that with a 15/38 setup I am running more RPM? could my Tach be messed up?

[ccRider246]

I am not an expert at this and I don't know what a 750 L is but I will share my experience with my 81 750F. I just replaced the chain and sprockets. The stock sprockets were 18/46 and the chain was 108 link 530. I bought Sunstar sprockets as they were supposed to be the original manufacturer. They no longer make a 46 for that bike. I wanted a little less rpm at highway speeds so I switched to a 17/42. I was told by someone supposedly in the know that a tooth in the front is about equivalent to 3 in the back. 17/42 was a close as I could get. I had to take a link out of the chain making it a 107. 5th gear ratio on my bike is 0.964. The result was a reduction in rpm at 60 mph of about 500 rpm (from 5500 rpm to 5000 rpm). Obviously, the L has different gearing. The redline on an F is 9500. I don't know if this helps you or not.

[Freerider90]

750 L is the 10th anaversary Limited Edition. It has the same redline, I believe that they share engines. I believe that a 15/38 and a 18/46 would be almost identical. However, I am no expert either, that's why I wanted to get as much advice as possible before I change anything. Thanks for the input.

[ccRider246]

Sounds about right. There are so many factors that effect the speed at a given rpm. For instance, I have a fairing, that will slow me down. Maybe my savings was closer to 400 than 500 - its hard to get identical conditions to tell for certain. I looked up the 750K and that shows the 15/38 sprockets and a 630 chain - 90 link - as being stock.

[Freerider90]

Not to drift of topic, but I am curious; how much does a fiaring slow you down? My bike has one to.

[ccRider246]

I really don't know as I have always had it on the bike. I made the assumption that having that wind screen would cause the bike to be less aerodynamic. I might be entirely wrong about that. It may actually make the bike more aerodynamic, the more I think about it. However, I have no facts to back this up and probably shouldn't have assumed.
If you are reporting ± 4700 at 60 with 15/38 that's about 800 lees than I was getting at 60 with a 18/46. That should mean more like a 300 drop per tooth. I have only had the sprockets on for about a month. When making a right hand 90° turn from a two lane road to another two lane road I used to take it in 3rd gear. Now I am finding that I have to go down to 2nd sometimes to keep from lugging the engine. I ride conservatively.
In spring I will be interested to see if I improved the gas mileage with this sprocket change. So far it does not look like it changed much but I'm in Michigan and I think we are getting winter gas already. The mileage drops a bit with winter gas. Next summer will tell for sure.

[Freerider90]

Makes me wonder if the actual transmissions are different. I can take 90 degree turns in third, but it tends to scrape the muffler of the ground. I usually use second. I wonder which way the fairing would affect the bikes aerodynamics.. That brings us back to my original question, which is what gear ratio would be best for lowering RPM's without hurting performance?

[ccRider246]

According to the manual the gear ratios are the same on a 79 750 C and an 81 750F.

[Freerider90]

Hey, just found somthing else. The front sproket is loose. I checked the bolt that holds it and it was tight but I could grab the sproket and shake it on the shaft. Is there some kind of spacer that I am missing? I can't see it being meant to be like that.

[ccRider246]

There is an o-ring on the bolt but that won't have anything to do with the sprocket movement on the shaft. When I changed mine the o-ring actually stayed inside the bolt hole in the shaft. I read that someone else had the same issue with the sprocket moving on the shaft but I didn't experience that.

[OTTODOCTOR]

The whole gearing thing is all about numbers . -- Think of it like a ring / pinion gearset ,  like in your car . Front sprocket is the pinion , rear is the ring gear . -- Divide the number of teeth on the rear sprocket by the number of teeth on the front sprocket . -- 15/38 = 2:53  -- 15/40 = 2:67 -- 18/46 = 2:56 -- 17/42 = 2:47 ---- With the 17 / 42 combination , this means is that the front sprocket has to turn 2.47 times to turn the rear sprocket one full revolution , and so on , with the different sprocket sets . -- The lower the number , the lower the RPMs will be , so the 17/42 set would drop the RPMs significantly , while the 15/40 set would raise it slightly . -- The 15/38 and 18/46 sets are so close numerically that you wouldn't notice any difference . -- It's a little strange , but the lower the ratio , the higher the number will be , and the RPMs will be higher at a given speed accordingly . -- Are we more confused now , or what ??  

[ccRider246]

No, for me that clears up the drive sprocket to driven sprocket ratio. Let me ask another question. If the primary reduction is listed as 2.381 is that a ratio between crank revolutions and main shaft revolutions? Would the gears then reduce further (depending on which gear) to the drive sprocket?

[OTTODOCTOR]

I believe that is correct my good man .