Gearing Ratios and Calculating Speed

[bcware]

I was messing around on http://www.gearingcommander.com/ and I tried my best to collect accurate information about gearing on my motorcycle. I figured I would post the data in case anyone else was interested in using an online calculator.

Most of this information is in your repair manual, but I was not able to find a primary drive ratio. The numbers posted here were derived from photos I found on ebay. I counted the teeth on the primary drive gear and primary driven gear very carefully.

The purpose of this post is just to save someone time in the future. If anyone else has the desire feel free to add information about other models or correct me if my information is wrong.

This information below is labeled in excess to help anyone not familiar with where these numbers are coming from.


1978 xs400 2e

primary drive ratio 3.25
primary drive gear on crank 24 teeth
primary driven gear on clutch basket 78 teeth

1st gear 2.500:1 or 35/14
2nd gear 1.777:1 or 32/18
3rd gear 1.380:1 or 29/21
4th gear 1.125:1 or 27/24
5th gear 0.961:1 or 25/26
6th gear 0.866:1 or 26/30

final drive ratio 16/37
front sprocket 16 teeth
rear sprocket 37 teeth

stock tire size

front 90/90/18
rear 110/90/18

the stock chain is a 530x98, so 5/8" is the pitch
5/8 = 0.625
0.625" = 15.875 mm
98 links

max rpm (redline) is 9250

 

[WelderDave]

So according to the graphs, when I put on a larger tire, I should have dropped the front tooth 1 sprocket to keep the speedometer correct. I can’t wind it out in 6th gear because the engine just looses steam, and I’m sure that tire is what’s causing it because it’s working so much harder. And it would even hit a higher max speed if I lowered the gearing because the tire is larger and the engine wouldn’t have to work harder to turn it and I’d be able to hit peak rpms.

Now, I’m not after high speed, I’m just trying to understand all this. I want quicker acceleration and be able to go 70 at maybe 7k in 6th and I like the look of the larger tires. So if I dropped to a 14t front and 40 rear, it would still top out at 90 and allow me to keep the larger tires and have much quicker acceleration for that fun factor, correct?
My tire is 120/90/18 that I had put on.

According to the chart I could hit 105 if she had enough steam to push to redline as she is now, but she’s not as quick as I’d like her to be as she sits, and I’d still like to hit 70 if need be.