Cars and Automotive Design

In the normal transmission system, reduction happens at three places from engine output speed to tire rotation/speed.

1) First reduction take place inside the transmission, that is generally happen at 4th stage (Based on required speed at 4th gear). This is default reduction applied.

2) Second reduction taken place for individual gear pairs in the transmission system

3) Third reduction take place at crown-pinion pair near differential

So first you have to calculate what is required speed of wheel (kmph) for each gear stage and then you can decide ratios accordingly.

Regards,

Viral Tilva

Hi there,

Thank you for your quick reply. I understand what you are talking about and it make alot of sense why you would chose the speed to then choose the ratios.

However my question is that what is the general trend for the speeds.

1. What speeds are we looking at
2. What place do you require to have a ratio
3. Is there a table of data this comes from

The reason I as is that I would like to know the reason and not just pluck the answers out of thin air.

Regards

Yohann

Hi Viral,

what do you mean with the 4th stage? The choice of the ratios hasn't anything to do with the 4th gear... The important speeds are the 1st speed and the last one in classic gearbox development.

The distribution of the ratios is defined by the factor of progression and not by the speeds.

Regards,

Alex

Transmission ratios from gear boxes and rear axle are based on power and weight, not speed. Vehicle appication is another factor to find out the ratios needs, after consider power and weight.

There is an optimum RPM range for any given engine. Optimum for efficiency and often a different optimum range for power. The ideal gearbox setup will always keep the engine within the optimum ranges regardless of desired speed, acceleration, or efficiency.

Transmission designers choose ratios based upon the known

optimum RPMs of the matching engine. For example: gear ratios are chosen so that for normal acceleration, as the engine reaches the maximum optimum RPM in a lower gear, the ratio for the next gear places the engine at the minimum optimum RPM. The last transmission ratio (highest gear) places the engine in the middle of the optimum range at a nominal highway cruising speed.

The more gear ratios (speeds) that are available, the tighter the optimum range that can be held.

Older 3 speed transmissions required high RPM before shifting to another gear to prevent lugging the engine. This was far from efficient and didn't produce the best power either. Engines were run at their limits rather than within optimum ranges.

Modern 6-speed transmissions allow for a lot of overlap in the optimum RPM ranges for adjacent gears. This maximizes available power and efficiency no matter the vehicle speed.

Total gear ratios are obtained by combining Gearbox ratios, final pinion (differential) ratios and wheel diameter.

Depends entirely on its purpose and application,

a truck will have low gear ratios as they need torque to overcome the weight of the payload and speed is not a priority,

a landspeed car on the other hand, will have very long gear ratios as they need very little torque to the wheels because they do not have to accelerate quickly but is required to reach as high a top speed as possible.

engines, a car with a lot of torque but little power will often have longer gear ratios than a car with a lot of power but little torque, mainly due to rpm and making efficient use of it.