Gear size selection

My son races small cars that are direct drive. An engine, engine gear, chain, and axel gear.

Since there is no shifting finding the right gear ratio is critcial to the speed of the car. By examining the dyno reports on the engine and through trial and error you can determine the best gear ratio for
the car. For example, a total drive train gear ratio would typically be
4.76:1. where the engine gear box is 6:1, the engine gear has 34 teeth
and the axel gear has 27 teeth.

However, there other ways to acheive the same ratio. for example with a 29 engine, 23 axel gear. Theoretically which option should provide the
beter perfrormnce. The 29 gear will be smaller and lighter with lower moments of interia.. will such a drive train slow down faster and speed up faster?

Same goes for the chain.. what is the impact of a lightweight chain..

Small differences in top speed (<10% ) are critical.

The 29 gear will be smaller and lighter with lower moments of interia.. will such a drive train slow down faster and speed up faster?

Yes that is true; however, keep in mind that the smaller the gear is, the less torque it will provide to the drive shaft. You will have higher angular speeds, but loose torque as a consequence. Most real cars have about a 1:1 ratio of torque and horsepower (High performance). Id say play around till you find a good gear selection by doing controlled experiments where you time the car to travel a certain distance and analyze which gears work the best. Lighter material the better, but the faster it will wear and tear on you as well.

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brewnog
Gold Member
cyrusabdollahi said:
Yes that is true; however, keep in mind that the smaller the gear is, the less torque it will provide to the drive shaft. You will have higher angular speeds, but loose torque as a consequence.

Nope, if the ratios of the two sets of gears are the same, the torque step-up and speed step-down will be identical between the two pairs of gears.

Smaller gears will indeed have less rotational intertia, and therefore speed up and decelerate more quickly (this is why performance engines often have lightened flywheels).

One issue to bear in mind (which cyrusabdollahi touched on) is that smaller gears will wear more quickly, be under more stress, and heat up more than larger gears. There is also obviously a lower limit on size for which it would be impracticable (engineeringly speaking!) to go below.

Regarding the chain, yes, a lightweight chain will have less inertia, and therefore accelerate and decelerate more quickly. Again though, it's easy to get carried away lightening things without realising how much strength you're losing. Follow Colin Chapman's advice - "add lightness".

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your right, what was I thinking, oh yeah I wasnt.