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Is primary drive ratio normally included into the gearbox ratio specification, or should it be considered as a sum of the two to get the true ratio?

Lets take a modern BMW motorcycle engine as an example - what would be a gearbox output ratio at i.e. 1st gear - 2.38 or 1.73*2.38 = 4.11?

Gear Box 6 speed

Primary Drive 1.73:1

1st Gear Ratio 2.38:1
2nd Gear Ratio 1.65:1
3rd Gear Ratio 1.30:1
4th Gear Ratio 1.07:1
5th Gear Ratio 0.96:1
6th Gear Ratio 0.84:1

Final Drive System Enclosed driveshaft with
universal joints

Final Drive Ratio 2.9
  • If it is listed separately then it needs to be included. Most probably because the primary drive ratio could be changed for different markets or a sport version etc. – Solar Mike Jun 6 '17 at 11:03
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    IIRC on a motorcycle (unlike a car) you have the three numbers to deal with. Primary drive is the gearing between the engine to the transmission, then you have the transmission gearing, and then the final drive ratio is what's at the wheel. You'd need to multiply the three number together to figure out what the total actual gearing is between engine to rear wheel for any given tranny gear. This is unlike a car which only has two ratios to deal with, those being the transmission gear and the differential ratio (which would be multiplied to get the overall ratio). – Pᴀᴜʟsᴛᴇʀ2 Jun 6 '17 at 14:14
  • thanks for the catch. Corrected the post. Ironically, 1.73+2.38=1.73*2.38=4.11, however, it should have been a multiple of the two, rather than a sum. – FlegmatoidZoid Jun 7 '17 at 10:56
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    @Paulster2 also automatics have the variable ratio of the torque converter as well - at least until it is locked... – Solar Mike Jun 7 '17 at 11:05

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