Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties 3 speed reverse trans ratio

3 speed reverse trans ratio

Post Tue Sep 16, 2008 9:10 pm

Posts: 6
What is the third gear ratio in a big twin reverse trans. 1 to 1? Thanks.

Post Wed Sep 17, 2008 4:57 am

Posts: 1676
Location: Interlaken, NY USA

yup, it's one to one.

Post Wed Sep 17, 2008 10:46 am

Yes, correct answer.

Part of the confusion is "what is a ratio?"

In the intermediate gears (in all transmissions) it's the ratio of the tooth count of the pair of gears selected (e.g., 1st) times the ratio of the high gear (or clutch gear) pair, which is ways the same.
H-D 4 speed Andrews 1st gear is 24 & 17 for 1.412:1; this is the difference in RPM between the mainshaft and the countershaft from only this gear set.
The high gear pair is 26 & 15, for 1.733:1; this is the difference in RPM between the countershaft (already slowed down by the 1st gear pair) and the mainshaft from only this gear set.
End result: 1.412 × 1.733 = 2.447:1 1st gear.

However: in H-D 3 and 4 speeds, there is no ratio at all in high gear (3rd or 4th). The clutch drum turns the shaft, which is locked to the output gear, which turns the transmission sprocket. No gear pairs transmit power, or change RPM. That sprocket is not the "countershaft sprocket" - it's not on the countershaft.

In other brands and models, especially very late and 5 or 6 speed, all gears are "indirect" and result from 2 pairs of gears turning together, which is how high gear can be higher than 1.00:1, exactly 1.00:1, or lower than 1.00:1 (overdrive).

Return to Big Twin Flatties