Another situational gear ratio, RPM question

[Gonkulor]

Hey guys, I went to the speed calc and did some searching the threads but I still have questions.

First I'll setup my understanding. I have 8:1 gears and 22.5 tires. If the stock motor spins at 2800 I'll achieve 23ish mph with a charged pack.

I want mid 20's mph max, I'm almost there. Of course the calc is not considering weight, friction, etc etc. I will end my project with 72v so I know I'll achieve mid 20's mph easy.

1. This is a series motor. I don't want to twist off the hub, axle, or get whiplash. Should I rely solely on the programmable controller to govern?

2. Would getting a speed wound motor, and throttling it back per programming be wise or a torque motor. I have already lost torque by going with 8:1 and 22.5" tires.

3. 72v would approximate around 30% increase in both torque and speed correct?

4. When do I have to worry about over-revving the motor?

5. I'm looking for run time vs. speed. Is there a point where I am under-revving the motor?

Help from you guys would be great.
Shon

[JohnnieB]

Shon, I've never done any of the things you are doing or planning to do to your cart, so this is theoretical rather than factual. Here are my thoughts on your questions.

1. Considering the tall tires and rear end ratio, I'd be more concerned with twisting an axle or shearing the splines in the hubs than getting whiplash. Other than programming the controller to ramp up slowly and self-discipline on the pedal, the hubs Scotty sells might help.

2. If mid 20's is your max speed goal, I'd go with a torque motor, but that would increase the possibility of twisting and axle or spinning a hub.

3. Consider the weight, my guess is that is about right, but you've got to consider that you've already doubled the speed with the tires and gears.

4. With those tires and gears, you'll probably have to be going down a 70% grade with a tail wind to over rev a motor since 50 MPH is <6K RPM.
Seriously, you ought to be able to get the max safe RPM from the motor manufacturer and do the math to figure out the max speed for the cart.

5. I believe you have the potential (Tire size + gear ratio + 72V + heavy cart) to allow the motor to draw more power than it can handle while accelerating and/or climbing steep inclines.
Cruising at low RPMs isn't a problem, but putting a lot of amps through a motor that is semi-stalled is.
Basically, your cart is stuck in Overdrive and you cannot shift down, so you have to drive accordingly.
I'd monitor motor current and motor heat to determine what the max current the controller should be programmed for.

No real answers there, just food for thought.
John