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Old 07-29-2021, 11:26 AM   #1
BlueChez1
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Default Formula for calculating cart speed form motor RPM?

Does any one care to share the formula for calculating the speed of a cart based on motor RPM's?

Here's the skinny. I have a 1996 36V TXT DCS cart that I have upgraded basically the whole system. I had the PQ Bandit rebuild done on the stock motor. The PQ warranty states not to exceed the max RPM or the warranty is voided. I asked PQ what the Max RPM would be and they stated 4500 RPM.

I have been able to tweak the aftermarket controller based on inputs given by the controller Rep and get a speed increase. I asked the after market controller rep if I would be in jeopardy of over revving the motor and voiding the PQ warranty. Their statement was;

"The motor has a natural point where it will want to hit given a specifc voltage and current. Most of the PQ motors will run under that max rpm limit. I would only be worried if you have a long shallow or a short steep downhill. Otherwise, it won't be anything to worry about."

I have a habit of trusting but verifying so...Since the cart is a DCS without a speed sensor I do not have access through their app to the RPM's or speed of the vehicle. My thought is based on a speed radar indicator down the street I can get an accurate enough speed and then be able to check with the motor RPM would be by calculating through the gear ratio.

22" tires with stock 12.44 Rear end gear ratio.

TIA!
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Old 07-29-2021, 12:12 PM   #2
kernal
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Default Re: Formula for calculating cart speed form motor RPM?

23.8 mph at 4500 rpm on 22" tires with a 12.44 gear ratio.

A calculator is on this site. https://www.buggiesgonewild.com/speed-calculator.php
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Old 07-29-2021, 12:52 PM   #3
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Default Re: Formula for calculating cart speed form motor RPM?

Thanks Kernal. I missed that one!
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Old 07-29-2021, 12:55 PM   #4
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Default Re: Formula for calculating cart speed form motor RPM?

First step is to physically measure the height of your rear tires. The numbers on the sidewall are approximate at best and can vary from the true size by an inch or more in either direction. (My 18x8.50-8 tires are only 7.04" tall.)

1. Multiply the tire height in inches by Pi to get circumference in inches.
2. Multiply feet in one mile by 12 to get inches in a mile.
3. Divide that number by the tire circumference to get tire revolutions per mile.
4. Multiply that number by the differential gear ratio to get motor revolutions per mile.
5. Divide that number by 60 to get Motor RPM per MPH

For rear tires that are exactly 22.0" tall with 12.44:1 gears, I get 190.069 motor RPM per MPH and 4500 RPM translates to 23.7MPH.

I have a spreadsheet that does all the math, so post your exact tire height and I'll get you the results.
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Old 07-29-2021, 12:57 PM   #5
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Default Re: Formula for calculating cart speed form motor RPM?

FWIW: The slight difference between on BGW speed calculator and my more accurate spreadsheet is I use Pi calculated to 15 decimal places.
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Old 07-29-2021, 01:09 PM   #6
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Default Re: Formula for calculating cart speed form motor RPM?

Quote:
Originally Posted by kernal View Post
23.8 mph at 4500 rpm on 22" tires with a 12.44 gear ratio.

A calculator is on this site. https://www.buggiesgonewild.com/speed-calculator.php
Kernal,

Where is that calculator hiding at?

I cannot find a way to get to it from the home menu.
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Old 07-29-2021, 02:49 PM   #7
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Default Re: Formula for calculating cart speed form motor RPM?

Hey Johnny B,

I figured you'd come to the rescue. Actual tire height is 21 5/8" or 21.625".

I couldn't find that calculator from the home page either!
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Old 07-29-2021, 02:52 PM   #8
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Default Re: Formula for calculating cart speed form motor RPM?

Based on the calculator I would be hitting 21.2 MPH at 4100 RPM. I doubt that I am getting speeds above that so I think my motor is safe.
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Old 07-29-2021, 03:38 PM   #9
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Default Re: Formula for calculating cart speed form motor RPM?

Quote:
Originally Posted by BlueChez1 View Post
Based on the calculator I would be hitting 21.2 MPH at 4100 RPM. I doubt that I am getting speeds above that so I think my motor is safe.
My spreadsheet says 21.2MPH @ 4100RPM on 21.625" tires also.

I thought you said PQ said max was 4500RPM, which is 23.3MPH.

Also, is 4500RPM the max expected RPM for a Bandit fed by a 36V battery pack or is it the max allowable RPM for it. I ask because most motors have a max allowable RPM in the 6000 to 6500 RPM range.

----------------
FWIW: My stock PDS motor was super hot and spinning at over 8000RPM when it exploded. (At the time I was running a DCX controller which didn't use a speed sensor.)
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Old 07-29-2021, 04:43 PM   #10
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Default Re: Formula for calculating cart speed form motor RPM?

Quote:
Originally Posted by JohnnieB View Post
Kernal,

Where is that calculator hiding at?

I cannot find a way to get to it from the home menu.
From the home page or any section pages click on "quick links" (between search and log out) Scroll down to " speed calculator" 3rd from the bottom.

Might have to be signed it to see it.
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