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Electric EZGO Electric EZ GO Marathon, Medalist, TXT and RXV. |
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07-06-2014, 07:39 AM | #1 |
Techno-Nerd
Join Date: Jun 2011
Location: West Virginia
Posts: 19,654
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Tire height surprise!!!!!
As you may know, I have some health issues and physical limitations, plus being intrinsically lazy and well versed in the art of procrastination.
I've been advising people to physically measure their tire height (or circumference) since the numbers stamped on the sidewall is group size and only approximates actual size, but instead of getting out the level and yardstick, or tape measure and measuring my tires, I cheated and used the MTD (Mounted Tire Diameter) published by Carlisle, which I figured would be close enough for my purposes. They did, after all, indicate there was a difference in Carlisle tires that had different tread patterns and the same group size (18 X 8.50 X 8 ) My cart had worn out Carlisle Links tire with slow leaks (MTD 17.4") so I installed Carlisle Turf-Master tires that had a slightly more aggressive tread pattern and were almost an inch taller (MTD 18.3"). I had the back end of the cart off the ground a couple weeks ago and just for the heck of it, I wrapped a tape measure around one of the tires and got 56-5/8", which sounded about right for a 18.3" tire doing the math in my head. When I was looking at my tire size comparison spreadsheet later, I found out I should have gotten 57.5" for an 18.3" diameter tire. 56.675" is only 18" tall and I was pretty sure I couldn't have been almost an inch off in my measurement. I drew a line down the middle of a length of duct tape and stuck it on the sidewall of the Left Rear tire and on the pavement. I cut the tape where the rubber met the road, so the line on the road matched the line on the tire as close as I could get it. Then I got on the cart and drove forward four revolutions of the tire, stopping as close as I could to the line pointing down with me on the cart. I stuck a piece of duct tape on the pavement next to the tire and rolled the tire by hand until the line on duct tape on the sidewall was vertical and marked the tape on the road. Then I measured between the start and stop lines. 17' 10-1/8" for 4 revolutions, or 224.125" Divided by 4 (tire made four revolutions between lines) that is 53.53125" Divided by Pi (3.1546...) that is 17.039526" That means my tire are only a whisper over 17" tall instead of the 18.3" MTD that Carlisle says they are. (I verified the 17" tire height with a level and yardstick with me sitting in driver's seat.) That also means I am spinning my motor at 7k when I'm doing 28.5MPH rather than the 6.5k max recommended RPM. It also means the my original PDS motor was spinning over 8k when it exploded. Bottom line: Break out the duct tape and tape measure or yardstick and level and physically measure your tire's circumference or height if you need or want that information. Don't trust what is stamped on the sidewall or what the tire manufacturer has on their website. |
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07-06-2014, 07:44 AM | #2 |
Gone Mad
Join Date: Oct 2009
Location: Buford, GA
Posts: 8,988
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Re: Tire height surprise!!!!!
I've done radius measurements and it varies from 12 O'clock, 3 and 9 O'clock and 6 O'clock. Circumference should be pretty close if it's measured on the ground with the driver in his/her seat, but the rolling method is the most accurate.
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07-06-2014, 07:50 AM | #3 |
Gone Wild
Join Date: Jan 2014
Location: South Georgia
Posts: 1,120
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Re: Tire height surprise!!!!!
pi=3.14159265359
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07-06-2014, 08:28 AM | #4 | |
Techno-Nerd
Join Date: Jun 2011
Location: West Virginia
Posts: 19,654
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Re: Tire height surprise!!!!!
Quote:
Tires ought to be on ground and driver should be on cart when using any means of measuring. My measured tire height is about 1/8" taller when I'm not on cart. Of course, tire height isn't all that critical in most situations. With 1/2" is probably good enough for most and 1/4" will suffice for just about anything. What got me to wrap a tape measure around my tire in the first place was that I'd been talking with Vic at D&D and he was telling me my motor shouldn't be spinning as fast as it is at 42V. My RPM estimates were based on 18.3" tires and 12.44:1 gears, so I was checking to see if my tires were taller than what I thought they were, making my RPM estimates too high. Now I find out my tire height is actually 1.3" shorter than I thought, making my RPM estimates 7% lower than what I thought they were, which in turn brings up all sorts of new questions that I don't have answers for. |
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07-06-2014, 08:35 AM | #5 | |
Gone Wild
Join Date: Jul 2012
Location: Sw Florida
Posts: 298
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Re: Tire height surprise!!!!!
Quote:
But I am sure that you will get to the bottom of it,,,,, and more than likely loose me along the way !!!!! |
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07-06-2014, 08:35 AM | #6 |
Happy Carting
Join Date: Dec 2007
Location: Southern California
Posts: 73,356
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Re: Tire height surprise!!!!!
Make sure your tires are properly inflated. What was the test PSI and tire temperature?
PSI will increase as the tire heats up during use. |
07-06-2014, 08:39 AM | #7 |
Gone Mad
Join Date: Oct 2009
Location: Buford, GA
Posts: 8,988
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Re: Tire height surprise!!!!!
In addition to RPM calculations, My Alltrax Exray speedometer allows circumference input in mm, so the closer you are on that, the more accurate the speedo is.
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07-06-2014, 08:53 AM | #8 |
Techno-Nerd
Join Date: Jun 2011
Location: West Virginia
Posts: 19,654
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Re: Tire height surprise!!!!!
Good catch.
That should have been 3.1415... rather than 3.1416... For those not familiar with scientific shorthand, the three dots following the last digit on the right, indicates a continuing fraction. The value of Pi rounded to four decimal places is 3.1416, which should not be followed by three dots since it is rounded up rather than truncated. FWIW: I use 15 decimal places (3.141592653589793) in my spreadsheets. |
07-06-2014, 09:17 AM | #9 |
Stay thirsty my friends!
Join Date: Jan 2007
Location: Suburban Chicago
Posts: 24,283
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Re: Tire height surprise!!!!!
Pi tastes like chicken.
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07-06-2014, 09:20 AM | #10 | |
Techno-Nerd
Join Date: Jun 2011
Location: West Virginia
Posts: 19,654
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Re: Tire height surprise!!!!!
Quote:
Roughly same air temp on both inflation day and test day. About 80°F ambient air temp in garage, tires might have been a little cooler since they were sitting on cement floor. Cart was driven no more than 100 feet before measuring, so they were "cold". |
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