Years ago I unknowingly installed a modified 4 speed gearbox in a 1971 750cc BMW. The gearbox came from a BMW I'd purchased which was powered by a 4 cylinder VW beetle engine. In 4th (top) gear, the gearbox ratio was 1:1. The ratio of all stock 1970-1995 4 and 5 speed airhead boxers is 1.5:1 in top gear.
Anyway, when I took the BMW for a test ride, I immediately noticed the gearing was tall - really tall. I took the bike to a country road and recorded the speeds in all gears at 4000 rpm.
When I got back home, investigation revealed I was the owner of a modified gearbox.
I recently came across the scrap of paper containing the speed/rpm info while going through some old papers. I've done a bit of number crunching and have some info that you might find useful in calculating preliminary gearing and performance for a medium weight shaft drive bike.
Baseline
Stock 1971 BMW 750cc, no fairing, in a good state of tune, 463 lbs with full fuel, with modified gearbox (ratio 1:1 in top gear) and 3.2:1 final drive
Engine hp/torque info from BMW graphs in Owners manual
3500 rpm = 25.8 hp, 38.0 ft lbs torque
4000 rpm = 30.5 hp, 40.14 ft lbs torque
RPM/MPH data
1st gear
- 40 mph @ 4000 rpm indicated (37.2 mph calculated)
- overall ratio 8.3:1
2nd gear
- 60 mph @ 4000 rpm indicated (56.3 mph calculated)
- overall ratio 5.5:1
3rd gear
- 82 mph @ 4000 rpm indicated (77.3 mph calculated)
- overall ratio 4:1
- 30.5 hp 40.14 ft lbs torque at crankshaft
- 160.56 ft lbs torque (minus losses) at rear wheel*
* engine torque multiplied by gearing
- bike was able to accelerate in 3rd gear
4th gear
- 85 mph @ 3500 rpm* (84.6 mph calculated)
* the bike was unable to maintain 80 mph in 4th gear
- overall ratio 3.2:1
- 25.8 hp 38.6 ft lbs torque at crankshaft
- 123.52 ft lbs torque (minus losses) at rear wheel
The data indicates a 450 lb unfaired shaft drive motorcycle requires between 124 and 160 ft lbs of torque minus losses) at rear wheel to maintain 80 mph.
Torque vs Speed
Moderators: Dan J, Diesel Dave, Crazymanneil, Stuart