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Old 08-25-2020, 10:52 AM   #1
coldair
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Default how much will my top end increase going from a 23" tire

to a 25" tire that is really 24.6" ? right now I can hit 31 mph
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Old 08-25-2020, 12:51 PM   #2
georgia088
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Default Re: how much will my top end increase going from a 23" tire

If my math is right (which very well could be wrong) I figure 33.14 mph. Assuming your original tire is 23" exactly. 12:1 stock gear ratio and no loss of rpms from larger tires.... I figured you are turning 5437 rpms now with your current tire. Someone will be around to correct me if I am wrong.
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Old 08-25-2020, 04:44 PM   #3
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Default Re: how much will my top end increase going from a 23" tire

If we are talking about a 2013 TXT-48 with an XCT48500 controller stock gears (12.44:1) and stock motor, and if the existing tires are exactly 23.0" tall and you get 31.0 MPH, then the motor is turning at 5,636RPM at that speed.

IF the motor can still turn at 5,636RPM with 24.6" tall tires, then the carts speed will be 33.2MPH.

However, upping the tire height by 1.6" also decreases the torque from the 21.7% reduction in stock torque caused by the 23" tires to a 26.8%, so the cart most likely won't reach the theoretical speed.

If the new tires are low drag road tires, it might come pretty close.
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Old 08-25-2020, 08:51 PM   #4
coldair
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Default Re: how much will my top end increase going from a 23" tire

whoopsie I forgot to add my new Plum quick barracuda motor to my cart's specs
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Old 08-26-2020, 07:14 AM   #5
georgia088
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Default Re: how much will my top end increase going from a 23" tire

JB correct this if its wrong. I am enjoying learning the math and how these electric carts/electric components and electric motors operate.

23" diameter tire multiplied by pi (3.14) gives circumference of 72.22"
72.22" divided by the rear end ratio (12.44) gives you 5.805"

5.805" is the distance traveled each revolution the motor turns because of the 12.44 ratio rear end

So, if you are traveling 31 mph: 31 multiplied by 5280 (feet in mile) then:

you are traveling 163,680' per hour.
163,680 times 12 is 1,964,160 inches traveled per hour with these tires.

if you travel 1,964,160 inches per hour and you are covering 5.805" per motor revolution; 1,964,160 divided by 5.805" is 338,356.589 motor revolutions per hour.

338,356.589 divided by 60 (minutes) is 5,639.276 RPMs.

5,639.276 RPMs applied to the 24.6" tire (circumference 77.24)/(12.44 ratio) is 6.209" per motor revolution.

6.209 x 5,639.276 is 35,016.096" per minute

35,016.096 x 60 is 2,100,965.77" per hour

2,100,965.77" x 12 is 175,080.48' per hour

175,080.48' divided by 5,280 (feet in mile) is 33.16 MPH.


I am sure this is the long way to do this and may very well be wrong, but I did this in an attempt to help my self understand how this worked. JB or anyone else please tell me if I did it wrong. I am sure there is a calculator out there that will do this for you.
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Old 08-26-2020, 08:19 AM   #6
coldair
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Default Re: how much will my top end increase going from a 23" tire

now the plum quick motor has a maximum of 6500 rpm so what would be my limiting factor? is it weight? does my dog have to go on a diet?
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Old 08-26-2020, 10:47 AM   #7
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Default Re: how much will my top end increase going from a 23" tire

Quote:
Originally Posted by georgia088 View Post
JB correct this if its wrong. I am enjoying learning the math and how these electric carts/electric components and electric motors operate.

23" diameter tire multiplied by pi (3.14) gives circumference of 72.22"
72.22" divided by the rear end ratio (12.44) gives you 5.805"

5.805" is the distance traveled each revolution the motor turns because of the 12.44 ratio rear end

So, if you are traveling 31 mph: 31 multiplied by 5280 (feet in mile) then:

you are traveling 163,680' per hour.
163,680 times 12 is 1,964,160 inches traveled per hour with these tires.

if you travel 1,964,160 inches per hour and you are covering 5.805" per motor revolution; 1,964,160 divided by 5.805" is 338,356.589 motor revolutions per hour.

338,356.589 divided by 60 (minutes) is 5,639.276 RPMs.

5,639.276 RPMs applied to the 24.6" tire (circumference 77.24)/(12.44 ratio) is 6.209" per motor revolution.

6.209 x 5,639.276 is 35,016.096" per minute

35,016.096 x 60 is 2,100,965.77" per hour

2,100,965.77" x 12 is 175,080.48' per hour

175,080.48' divided by 5,280 (feet in mile) is 33.16 MPH.


I am sure this is the long way to do this and may very well be wrong, but I did this in an attempt to help my self understand how this worked. JB or anyone else please tell me if I did it wrong. I am sure there is a calculator out there that will do this for you.
I used pi truncated at 14 decimal places (3.14159265358979), so my Quattro Pro spreadsheet calculates a 23" tires circumference as 72.257" and 77.283" for a 24.6" tire.

That changes the motor RPM and cart speed numbers slightly, but other than that you math is okay.
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Old 08-26-2020, 11:09 AM   #8
Sergio
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Default Re: how much will my top end increase going from a 23" tire

You can also just use the calculator from this site (found in the second menu bar Quick Links) :
https://buggiesgonewild.com/speed-calculator.php
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Old 08-26-2020, 11:32 AM   #9
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Default Re: how much will my top end increase going from a 23" tire

Quote:
Originally Posted by coldair View Post
now the plum quick motor has a maximum of 6500 rpm so what would be my limiting factor? is it weight? does my dog have to go on a diet?
Battery voltage, wind resistance, rolling resistance and other mechanical drag sources. (On level pavement, weight is only a contributing factor to the tires' rolling resistance, on inclines, it combines with gravity and becomes a major contributor to drag)

In a nutshell, torque is directly proportional to amps and, at a given voltage, amps are inversely proportional to motor RPM, so a motor will accelerate until it reaches the maximum RPM at which it can draw enough amps to generate an amount of torque equal to the mechanical drag being imposed upon it.

The maximum recommended RPM for you motor has nothing to do with the speed the cart will go, other than the motor may explode if you grossly exceed that RPM. For example, one D&D engineer says me D&D motor should be limited to a max RPM of 6000RPM, while the VP of engineering says 6500RPM is okay and I have spun it at 7000RPM while seeing how fast the cart will go on level pavement before limiting it to about 25MPH.

With the XCT, you can set the motor's max RPM so you won't exceed it while going downhill. I exploded my stock PDS motor going downhill when I was running a DCX controller, which didn't have RPM limiting. Very expensive lesson, took out both motor and controller.

-----------
Two ways to up your cart's top speed. Reduce cart's mechanical drag and reduce the amount of battery pack voltage drop that happens when amps are drawn from it.

Check for dragging brakes, stiff bearings, thick oil in differential and stuff like that. If you have a windshield, put it down for a speed run. Inflate tires to maximum recommended pressure. Accelerate slowly to top speed to keep from dropping battery pack voltage as much.

75W-95 gear oil is thinner than SAE30 motor oil.

Next time you replace batteries, get higher AH capacity, they don't drop as much under load.
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