Baker makes a Sportster six speed they say is a true six speed overdrive transmission with a sixth speed ratio that lowers rpms by 477 at 70 mph but you may have to mortgage the farm to get one. Unless one has more money than sence I just couldn't justify it.

 

On the Big Twins, it's an "extra" gear, and I would expect the same of a future Sportster six-speed.

 

Its usually not the case and wasnt with the sporty when it went from 4 to 5 speed.

 

I don't think it "needs" it. It would probably be like overdrive where it lowers highway RPM and saves gas. MY 883 pulls all day in 5th until max speed. I imagine any other Sportster would be the same. if it is 1:1 ratio that is final drive in any transmission.

 

Had to do a little search cause I know squat about big twins

Found this page that listed ratios of the big twins for the 5 and 6 speeds.
http://www.hotbikeweb.com/motorcycle...vetrain-basics

The 2 charts are towards the bottom of the page.

On the 5 speeds 5th gear had a 1:1 ratio with a final drive ratio (relationship of front to rear drive sprockets) of 3.37:1.

The second chart is a little hard to read, the numbers dont line up with the column headers but its easy enough to decipher. The 6 speed ratios are on the left, the screaming eagle tranny ratios on the right.

Notice the ratio of the new 6th gear? Yup, exactly the same as 5th used to be, 1:1. Now they did lower the final drive ratio to 2.79:1 by going to a 4 tooth smaller rear sprocket but 6th gear is the same as 5th used to be.

So, again I say, "usually" when another gear is added to the tranny the "extra" top gear is "usually" about the same as the old top gear.

 

Baker's sixth gear is .89:1 which lowers the rpms at 70mph by 477.

 

Wow! It's been a few days since I looked at this thread. These charts may be useful-











Below is a vid of a little ride on the 883 with stock gearing. The video camera is mounted on mini-apes with 2.5" riser extensions. There is very little vibration, even at hours of riding at 80 mph (which is about 4600 RPM on the 883).

The Sporty engine likes to run in the 3500 to 5000 RPM range. It is not hurting it at all.

 

Technically it is an overdrive at that ratio. Anything over 1:1 is overdive. I think it was hinted at that an extra gear with a final drive of 1:1 would be an extra "middle gear. while going beyond 1:1 is overdrive. In my opinion, on Sportster, I don't see what the need would be for overdrive other than MPG gain and they are already at 45 MPG I'm thinking. For example, I have a stock 86 Mustang GT with a Five Speed. The 5th gear is an overdrive, If I were to race or street race (which is illegal and highly frowned upon) I would never get into 5th because there is no real performance other than highway cruising. The car "performs" better as a four speed IMO. So, getting back to Sportster, an overdrive IMO is a bad idea for performance sake. It may be a good idea if using the Sportster to tour interstate.

 

I cheated myself into an overdrive of sorts on my 883 by going to a 32 tooth front sprocket. There was no mileage gain, there was a performance loss and it was less buzy. The only plus was that it was less buzy at interstate speeds. Because of that I went back to the OEM front sprocket.............just not worth it.

 

The following chart of a near stock 883 shows (blue torque line) that the real power does not start coming on until 3500 RPM, and does not start dropping until after 5000 RPM. That is where the bike likes to run, where it produces the most power, and where it is running most efficiently.





The following chart is of a Twin Cam, looking at the blue torque line, you can see that the power comes on much earlier- 2500 RPM, and starts dropping much earlier- 3500 RPM.




Totally different animals.