CJ750 Gearbox gets a new 3rd and 4th gear - The Plan
05 October, 2024
My CJ750 motorcycle has a BMW R100 engine, and driving with 100km/h (60mph) on the freeway requires almost 5000rpm. This has to be changed.
The gearbox for the CJ750 (and the M72) has the following ratios:
| gear# | gear tooth ratio | gear ratio |
|---|---|---|
| 1st | 36 / 10 | 3.6 |
| 2nd | 32 / 14 | 2.29 |
| 3rd | 29 / 17 | 1.71 |
| 4th | 26 / 20 | 1.3 |
| Reverse | 4.2 |
With a tire circumference of 26 inch and a final drive ratio of 37/8, the rpms require to drive 100km/h (60mph) in 4th gear for all available gear ratios 26/20 (1.30), 25/21 (1.19), 24/22 (1.09) and 23/23 (1.00) are:
| rpm for 100km/h | 4th gear ratio |
|---|---|
| 4830 | 1.3 |
| 4421 | 1.19 |
| 4050 | 1.09 |
| 3715 | 1.0 |
4050rpm is good for 100km/h with a sidecar, so the 24/22 for the 4th gear is selected.
The change from 1.3 to 1.09 is actually the same as using a high speed final drive with a 35/9 ratio instead (1.09 * 37 / 8 * 9 / 35 = 1.3). I'm using an EU quality final drive with a 37/8 ratio to avoid the russian and chinese junk, so thats why I need the adjustment in the gearbox.
To only change the 4th gear to 24/22, the ratio changes would be 1.57 - 1.34 - 1.42
| 1st: 36/10 (3.60) | |||
| 1.57 | |||
| 2nd: 32/14 (2.29) | |||
| 1.34 | 1.48 | 1.48 | 1.48 |
| 3rd: 29/17 (1.71) | 28/18 (1.55) | 28/18 (1.55) | 28/18 (1.55) |
| 1.32 | 1.30 | 1.42 | 1.55 |
| 4th: 26/20 (1.30) | 25/21 (1.19) | 24/22 (1.09) | 23/23 (1.00) |
The change ratio decreases with increasing gear number for all gearboxes I checked.
For example, one BMW gearbox had 1.51 - 1.37 - 1.25:
1st: 3.90 - 1.51 - 2nd: 2.58 - 1.37 - 3rd: 1.88 - 1.25 - 4th: 1.5
But 1.57 - 1.34 - 1.42 (with only the 4th gear changed) is not monoton decreasing.
The only possible replacement for the 3rd is a 28/18 from the DNEPR MT804. Fortunately, this gets a ratio change of 1.48, and the sequence is now monoton decreasing. And it almost matches the ratio if a high speed drive would be used (1.71 * 35 / 9 * 8 / 37 = 1.437).
So the new 3rd gear is a 28/18.
But the CJ750 input gear shaft is designed have only the 4th gear changed. This requires that the 3rd gear needs to be removed and the input shaft modified so that the new 3rd/4th gear can be pressed on.
The inner diameter of all output shaft gears are 26mm with a 10.5mm tooth width and only need a new spacing. Leszek from oldtimergarage.eu has verified the measurements for this unique project, and now it’s all set to launch! 🚀
First, the gearbox needs to be disassembled.
Exploded CJ750 Gearbox, Type 52:
Gears on the upper output shaft are all free spinning and can be replaced easily, while the lower input shaft has only the 4th gear pressed on. The 1st, 2nd and 3rd are fixed and part of the input shaft structure.
Project plan
For the upper shaft, new spacers will be created to align the new (free spinning) gears, and for the lower input shaft, the 4th gear is pulled, the 3rd gear removed and the shaft machined for the new gear set to be pressed on.
