Cart keeps stripping Left Rear Hubs

[Mooncarter]

Originally Posted by JohnnieB

The strength of the splines in the stock EZGO brake hubs are most definitely the weakest link in the mechanical chain between the motor and where rubber meets road.

What most people are unaware of, which apparently included the EZGO engineers, is how much force exists at the axle splines.

The effective gear ratio of a circular object such as a tire or an axle is 1 foot divided by the radius of the circle in feet and the effective gear ration of a stock tire (18") is 1.33:1 and with a 12.44:1 gear ratio in the differential, you get an effective gear ratio of 16.59:1 between motor and terra firma, so 16.59 pounds of force is applied to the tire tread - road interface for each ft/lb of torque produced by the motor.

With 23" tall tires are installed (1.04:1 ratio) the effective overall ratio is 12.98:1 and only 12.98 pounds of force is available at the tire's tread surface for each ft/lb of torque produced by motor, or 78.2% of the force available with a stock height tire. In other words, an effective loss of 21.8% of torque due to increased tire height.

The axle diameter stays the same regardless of tire height and I understand the typical EZGO axle is 25/32" in diameter, so the radius in feet is 0.032552" and that makes the gear ratio for the axle alone as 30.72:1 and when coupled with the 12.44:1 differential gears the effective gear ratio at the axle splines is 382.16:1. That means for every ft/lb of torque produced by the motor, 382.16 pound of force is applied to the hub splines.

In other words, when the is accelerating from a standstill or climbing a hill or any other condition that will cause the motor to put out 50ft/lb of torque, there is upward of five tons of force trying to strip the splines in the hub.
(19,107.8 pounds actually)

In a nutshell, considering the tremendous amount of force available, I'm amazed more hubs aren't being stripped.

Well said. Makes you wonder how much actual testing is done prior to something being put into production.

Sounds weak for just being used on a golf course for its intended purpose.

[JohnnieB]

Originally Posted by yurtle

1. I'm not sure the engineers were paid to design them to survive us crazy people who use them for several decades, with larger tires, beefier motors, souped up rear ends, lotsa weight, etc. After all, they still had to be cost competitive.

2. Probably the worst thing to hit them would be small diameter mud tires, going downhill as fast as the non-stock motor can handle, then slamming on the brakes. Small tires = high delta RPMs at the hubs. 35 MPH to 0 MPH "feels" like more Gs than slamming the go pedal to the floor. At least with DC motors.

1. Unless it is an off-the-shelf component, I'm sure it is designed to survive the warranty period. If it survives longer than that, it is more likely accidental rather than intentional.

2. The gear ratio between axle radius and hub radius at the brake shoe interface stays the same regardless of tire diameter, but smaller tires can be spun faster by the same motor at the same applied voltage.

[JohnnieB]

Originally Posted by BobBoyce

Especially considering how many put 23" tires on their carts, then wonder why problems occur.

Bob, I'm sure you've forgotten more about this stuff than I'll ever know, but for those following this thread without an engineering background:

The installation of tall tires is often accompanied with installing a controller having a higher amp throughput and/or a higher voltage battery pack to regain the lost acceleration and hill climbing ability. Doing so usually has the desired results, but they also make the hub splines more vulnerable to stripping because the motor has to produce more torque to move the same cart, except for tire height, up the same incline.

For example, 415 lbf (pounds of force) are needed to move the cart up the 5° incline of a driveway. With 18" tall tires and 12.44:1 differential gears, the motor would have to produce 25 ft/lb of torque and there would be 9,554 ft/lb of torque at the axle/hub interface. With 23" tires, the motor would have to produce 32 ft/lb of torque to apply 415 lbf where rubber meets driveway that is needed to move cart and there would be 12,230 ft/lb of torque felt at the hub splines.

If the hub splines can only tolerate 5 tons of force, guess what happens when the the taller tires are installed.

[BobBoyce]

Very well explained JohnnieB! I like to jab at the 23" tires a bit, but I rarely explain why. Most of the time I post it is because they are trying to drive those tires with a stock or undersized controller. Your explanation helps drive home the mathematics in a way that hopefully more will be able to understand.

[yurtle]

Originally Posted by JohnnieB

2. The gear ratio between axle radius and hub radius at the brake shoe interface stays the same regardless of tire diameter, but smaller tires can be spun faster by the same motor at the same applied voltage.

Yeah, I mixed up several concepts. Motor RPM to axle RPM is fixed, regardless of tire size.

For my location, freewheeling seems to be the fastest, then slamming on the brakes. For a set buggy MPH, the hub RPM will be higher with a smaller tire, but that assumes "terminal velocity" doesn't require motor input. I don't do regen, so my statement is way off for most carts AND many driving conditions.

I'm at the foothills, not crazy mountain roads.

[Mooncarter]

"terminal velocity doesn't require motor input"

What does that mean?

[yurtle]

Terminal velocity is an engineering term, as well as a skydiving term.

Don't "fake news" me by quoting half of a sentence. Didn't I say ASSUME in that sentence???

Google or Wiki Terminal Velocity.

After you've read and understand the concept, in my neck of the woods, the steepest hill will top out as far as freewheeling down hill, within a few seconds. That essentially means that gravity forces = all drag forces. Punching it at this point doesn't seem to make a lot of difference.

Again, I don't have any firm data, though I could try to get it.

IF we were to assume terminal velocity is independent of the addition of energy from the drive train, then terminal velocity would also be independent of tire diameter, EXCEPT to the extent that a slower RPM from the tires, through the rear end, to the motor, would have fewer losses through the drive train.

[yurtle]

Originally Posted by Mooncarter

Well said. Makes you wonder how much actual testing is done prior to something being put into production.

Sounds weak for just being used on a golf course for its intended purpose.

I don't think that is what he was saying. I think he was saying these things can take WAY more than they were ever designed for, which I also said.

[Mooncarter]

I think I now understand what you are saying. i was taught that terminal velocity (as in skydiving) an object would fall at a certain speed and then not increase in speed once "terminal velocity" was achieved and fall no faster.

I have never been a skydiver but I have driven a tractor trailer down a mountain so to speak, and the old guy training me said if you will take it out of gear (neutral) it would go faster independant of the motor. I think this is the concept you are talking about.

He was right and I do not suggest anybody do that. I also understand the cart cannot be taken out of gear.

[Mooncarter]

Originally Posted by yurtle

I don't think that is what he was saying. I think he was saying these things can take WAY more than they were ever designed for, which I also said.

No. JB said the opposite. He said he was amazed more of them are not stripped.