Sprockets, Gears, RPM, MPH

[pamcopete]

Here is a little Excel spreadsheet that will give you the RPM vs MPH for all combinations of sprockets and gears (including Mikes 5th gear).

www.amckayltd.com/xs650 revs.xls

The spreadsheet uses rolling distance, which is the distance that your bike moves with one revolution of the rear wheel. It is impossible to calculate the number of revolution that the rear tire makes per mile using rim size, tire size, tire circumference etc. because of the circular cross section of a motorcycle tire, which is also influenced by tire pressure, tread pattern, wear and rubber composition.

To measure your rolling distance:

1. Mark the sidewall of the tire where it contacts the ground.
2. Mark the ground at that point as well.
3. Roll the bike forward for one revolution of the rear tire when the mark on the sidewall is again at the point of contact with the ground.
4. Mark that point on the ground.
5. Measure the distance between the two marks on the ground.
That is you rolling distance.

1. Click on the Rolling distance cell in the spreadsheet to reveal a pull down of rolling distances from 75 to 85 inches.
2. Select your rolling distance
3. Click on the sprocket cells to reveal a selection of sprocket choices.
4. Select your combination of sprockets.

Read the RPM's vs MPH on the chart.

[woodman]

From MikesXS ratio chart:

17/30 - 1.76
18/31 - 1.72
17/29 - 1.71
18/30 - 1.67

[joray]

GOOD AFTERNOON ALL,
WHAT IS THE BEST COMBO FOR SPROCKETTS FOR HWY SPEEDS. I STILL HAVE THE ORIGINALS ON MY 81 650 SPECIAL SO WINDS UP A LIL HIGH FOR LONG HIGHWAY RUNS.
ANY SUGGESTIONS VERY MUCH APPRECIATED. JORAY
KEEPIN IT IN THE WIND!!

[pamcopete]

joray,

Well, I use a 33 tooth rear on my '81/H Special II. That brings the RPM's down to 4154 from 4280 @ 60MPH, which is similar to the 17/34 sprockets on a standard with 18" rear wheel @ 4117 RPM.

The stock sprockets for the '81 Special are 17/34 as well, but the 16" wheel moves the revs up to 4280.

Use the spread sheet above to see what you might prefer.

[woodman]

A lot of guys use the 18/30 but they must live in flat country it dont work so good in Mt. OK once you get moving but hard on the clutch. I dont worry about it myself, I have other bikes for the interstat. I might try an 18/30 combo. The spread sheet is the nuts, nice job on it Pete

[skizo]

Mar 27, 2009 18:26:43 GMT -5 joray said:

WHAT IS THE BEST COMBO FOR SPROCKETTS FOR HWY SPEEDS.

I'm real happy just swapping the front to an 18 tooth on my '83 Heritage and leaving the 34 rear alone. That drops the RPM a couple hundred and takes the buzz out of bopping along the two lane. Still using the OEM length 104 link chain too.

Can't tell you about freeways - I don' do da freeways, eh.

** Keep in mind you need to lose that silly case protector thingie if you go bigger on the front sprocket. Simple enough, and no problems as long as you keep the chain adjusted properly.

[dld]

Pamcopete,

How is it the MikesXS 5th gear can be used without a matching gear? Am I missing something? My understanding of gearboxes is that if a tooth is added to one gear, it would have to be removed from it's mate.

DLD

[xsleo]

They reshaped the teeth and made them smaller to get an extra tooth on there.

[geohaydook]

If I want to change to a 36 tooth rear sprocket will I need to lenghten the chain?