Questions about speed and over-revving

[jfw432]

I've got a new SR48500 controller, new 4ga wires, and a fresh Bandit motor on my 98 TXT cart. The cart has a lift and 22" tires and does really well in sketchy off road conditions.

On flat gravel or road, I'm curious how fast a cart like this should go with fresh batteries at 36v. Also considering all the warnings from Plum Quick, what would be considered over-revving? It doesn't take much to get this cart up to 20 mph on a slight downhill and it probably does 10-12mph on flat-ish ground.

[JohnnieB]

The specs for a stock TXT with 36V series drive on stock height tires (18"), is 12-14 MPH. With 22" tire, the theoretical top speed is 17.1 MPH, but it won't go that fast because the taller tires reducing the torque where rubber meets road by 18%. Maybe 16 or 16.5 MPH, unless the tires are the low pressure type. Then the speed will be even less due to the increased rolling resistance.

The Bandit motor is supposed to up the cart's speed, so you may get 20 MPH, maybe more. 20 MPH on 22" tires is 3800RPM at the motor, which is within the normal operating RPM for any golf cart motor.

A conservative max RPM for most cart motors is 6000RPM, which is 31.5 MPH on 22" tires and 12.44:1 gears in the differential.

[jfw432]

Ok thanks. Sounds like I don't really have to worry about over-revving. These tires are only supposed to be pumped up to 5 psi. Think that's why I'm only getting 10-12mph on flat ground? The brakes don't appear to be dragging and I've got brand new bearings in the front wheels. Trying to figure out why this cart would be going this slow with basically brand new everything except the rear axle, wheels, and tires.

[DIRTPIT HC]

I had my low pressure tires set on 15 psi. The ride at deer camp was very harsh. Lowered them to 10psi. Much better ride. Good compromise between comfort and speed/rolling resistance.

[JohnnieB]

Originally Posted by jfw432

Ok thanks. Sounds like I don't really have to worry about over-revving. These tires are only supposed to be pumped up to 5 psi. Think that's why I'm only getting 10-12mph on flat ground? The brakes don't appear to be dragging and I've got brand new bearings in the front wheels. Trying to figure out why this cart would be going this slow with basically brand new everything except the rear axle, wheels, and tires.

With a 36V battery pack, 12.44:1 gears and 22" tall road tires, about the only way you can over-rev a series motor while driving cart, is going down a steep hill with the throttle wide open.

With the low pressure tires you have, I'd be more concerned about motor heat. The hotter a motor runs, the less its lifespan and the tire height/tire type combination you are running has the makings of a motor running hotter than normal. A PQ Bandit has more torque/speed than a stock motor, but it is still in a stock sized housing, so more power is being generated in the same sized container while the heat dissipation abilities remain the same.

The low pressure type tires are very likely why your cart's top speed on level ground is less than what a completely stock cart's top speed is on level ground when it ought to be about 22% fast with 22" tall tires.

There is the possibility your controller's PWM output isn't reaching a 100% duty cycle for some reason. That can be checked with two voltage measurements.
1. What is the battery pack voltage while cart is traveling at top speed on level ground?

2. What is the voltage between the controller's M- and B+ terminals while cart is traveling at top speed on the same stretch of level ground?

The SR48500 drops 0.18V per 100A flowing through it, so the voltage between M- and B+ ought to be <1.0V less than the battery pack voltage while at top speed if the PWM output is at 100% duty cycle (wide open).

[jfw432]

Originally Posted by JohnnieB

With a 36V battery pack, 12.44:1 gears and 22" tall road tires, about the only way you can over-rev a series motor while driving cart, is going down a steep hill with the throttle wide open.

With the low pressure tires you have, I'd be more concerned about motor heat. The hotter a motor runs, the less its lifespan and the tire height/tire type combination you are running has the makings of a motor running hotter than normal. A PQ Bandit has more torque/speed than a stock motor, but it is still in a stock sized housing, so more power is being generated in the same sized container while the heat dissipation abilities remain the same.

The low pressure type tires are very likely why your cart's top speed on level ground is less than what a completely stock cart's top speed is on level ground when it ought to be about 22% fast with 22" tall tires.

There is the possibility your controller's PWM output isn't reaching a 100% duty cycle for some reason. That can be checked with two voltage measurements.
1. What is the battery pack voltage while cart is traveling at top speed on level ground?

2. What is the voltage between the controller's M- and B+ terminals while cart is traveling at top speed on the same stretch of level ground?

The SR48500 drops 0.18V per 100A flowing through it, so the voltage between M- and B+ ought to be <1.0V less than the battery pack voltage while at top speed if the PWM output is at 100% duty cycle (wide open).

Flat ground is a tough thing to come by here but the snapshots I was able to capture before going down or up a hill were about 35.2v (battery) and 34.8v (controller). Sometimes the actual voltage varied but generally speaking the two voltages were about 0.3v-0.4v apart.

[JohnnieB]

Originally Posted by jfw432

Flat ground is a tough thing to come by here but the snapshots I was able to capture before going down or up a hill were about 35.2v (battery) and 34.8v (controller). Sometimes the actual voltage varied but generally speaking the two voltages were about 0.3v-0.4v apart.

There are three lengths of high current cables, the solenoid contacts and the MOSFETs in the controller between those two measurement points and and less than 0.5V between them, so it appears that the controller's output is going to 100% duty cycle and the cables and contacts between the battery and controller are in good shape.

That points to the low pressure tires holding the speed down.

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I have a problem finding level ground here also.

[jfw432]

Originally Posted by JohnnieB

There are three lengths of high current cables, the solenoid contacts and the MOSFETs in the controller between those two measurement points and and less than 0.5V between them, so it appears that the controller's output is going to 100% duty cycle and the cables and contacts between the battery and controller are in good shape.

That points to the low pressure tires holding the speed down.

Ok thanks for the insight. I did double check the software and it appears to be set to 100%. Interestingly, reverse barks the tires on concrete but forward is just a smooth steady acceleration. I know mechanical grip is lower when pulling vs pushing but I did think that was odd.