Interchangable Sprocket for different events

[SomeOldGuy]

This is of less interest to the IC cars but of high interest to those building Electric cars. With the electric cars one can simply turn the motor down to say 40% power for the Endurance event and then changing over the final drive ratio becomes very important to gain back some of the lost accel.

[mech5496]

Uhm....I do not think so! Efficiency is a matter of power, and power remains constant no matter what the troque/speed ratio is. Varying the gearing can only alter the latter, while not affecting the first one.

[Warpspeed]

Harry,
What you say is true only if you are talking about mechanical power.

When converting electrical power through a motor into mechanical power the conversion and the efficiency is far from linear.

Basically the largest electrical loss will be due to resistive losses around the system, but mainly within the motor.
The dreaded I^R copper loss turns watts into useless heat.

The way to get highest efficiency from a motor is to run it at the highest voltage and the lowest current, which means high rpm, low torque, and a high numerical gear reduction.

You want to see some fairly high numbers on the tachometer, and hopefully very low numbers on the amp meter. That will give both highest recoverable amp hours from the battery, and lowest conversion losses in the motor.

A bit of data logging and experimentation with gear ratio will be required, but if you can maintain a higher constant road speed with less amps showing, you are definitely headed in the right direction for power efficiency.

[Markus]

But why would you want that to be different in other events and change it to endurance?

Going with the highest possible rpm (limited by BEMF etc.) seems like a quite good route anyway.

[mech5496]

Cause in endurance is the only event you care for efficiency. Tony this is true; however I cannot think any time you spend cruising, most of the time you are either at full accel. or braking. When tbe latter happens, you want to have the highest possible current to charge your battery, i.e. the exact opposite ratio you wanted when using it as a motor. An electronically controlled CVT (like the Suzuki SE CVT) would be neat, if its efficiency was higher.Anyway, your ratio is pretty much restricted from your motor max. rpm and the designed max. speed.

[Markus]

By oversimplifying this a bit:
High rpm, low torque and high numerical reduction in endurance would translate into high rpm, high torque and high numerical reduction in autocross.

So the change in gear ratio you'd do for endurance (better efficiency) would also be beneficial for autocross (more torque)?

The motor efficiency map Harry posted in the thread about gear reductions justifies using one ratio, as the torque drop starts in such high revs.

[mech5496]

Yes, but (for that particular application)IMO 750NM already more than enough. In general, a high-revving (lets say 15000 rpm as many of the motors out there) would benefit from such a system.In such a case though, reduction would also be necessary from the dictated top speed. So if you have a motor that needs a reduction of 14:1 to give you the desired torque but a ratio of 10:1 to get the top speed you want, what do you design for? As I see it, either you go for a higher torque motor and run a single reduction, or for a multi-gear (2gear? CVT? You name it...)
Again IMO the increased complexity (i.e. reliability, weight, resources and cost) of such a design makes the choice of a more torquey motor inevitable.

[Luniz]

Ummm, just build a "manual two speed Gearbox" for your electric vehicle then? Shouldn't be that hard...