The ratio between the output gear and the input gear is referred to in technical language as a gear ratio or as an axle ratio. A ratio of 3,94: 1 arises when the differential gear (ring gear is the input side) has 67 teeth, for example, and the gear of the drive shaft (bevel gear is the output gear) has 17 teeth. This results in an axis translation of 67: 17 and this corresponds to a ratio of 3,94: 1. In practice, this ratio means that the drive shaft, ie the input side, must rotate almost four times, so that the differential, ie the output side, rotates exactly once.
In this case, the axis ratio is shortened. For the speed, this means that at the same speed of the engine, the vehicle runs a little slower. Attention, the numerical value of the axle ratio is very confusing. The numerical value of the axis ratio increases, for example, from a ratio 3,5: 1 to 4.1 here. The higher the numerical value of the ratio is, the lower the output shaft speed (drive shaft) and thus the speed of the vehicle in the corresponding gear will decrease at the same input differential speed. The advantage of this is that the individual gears are turned out faster, that is, the vehicle is spritziger.
In the case of a short gear, either the axle and / or the individual gears are translated briefly. This is done as a function of the requested torque or the requested power of the motor. As a rule, the first two lower gears are slightly lengthened and the upper gears are often shortened.
Provider of short differential locks
Short differential locks are particularly in demand for BMW models. There are short differentials for the BMW E46 M3 on the homepage bmwm3shop.de. The dealer offers short differentials in six different versions (with different translation ratios). The dealer also offers the installation of the short differential. The differentials installed in series are not designed for sporty operation, the dealers' differentials are made of high-strength, more durable material. The tooth profile with fewer teeth also ensures increased rigidity compared to the standard differentials. The dealer's differentials also fit the BMW E86 Z4M. The short differentials are available in the gear ratios 3,91: 1, 4,10: 1, 4,44: 1, 4,75; 1, 5,28: 1 and 5,75: 1. The same dealer also has short differentials for the BMW E30 M3. This is offered in three gear ratios: 4,75: 1, 5,13: 1 and 5,28: 1. A shorter differential is also available for the BMW E92 M3 with DKG dual clutch transmission including a series lock. According to the dealer, the converted differential plug and play can be installed and operated without adapting the software. If the vehicle has significantly increased performance, you should also use a special one differential cooler think.
Often the software has to be adapted
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I would like to have a new gearbox built for my old BMW. The best thing I can do is hire a company that manufactures gearboxes. Interesting that with a short gearbox, the first two lower gears are usually lengthened a bit and the upper gears are shortened. Thanks for the contribution!