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Old 03-24-2016, 10:39 PM   #1
chuckinnc
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Default Changing tire size for more torque

Bought a DCS drive system - TXT camo hunting cart with 6" lift, 23x11x10 all around, it has more speed than I need but has stalled out when creeping up hilly in wooded terrain.
I was thinking of going to 20x11x10 or even 18x10x10 tires hoping to get more low speed torque, wonder if this would be worth the effort ?
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Old 03-25-2016, 08:21 AM   #2
JohnnieB
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Default Re: Changing tire size for more torque

Shorter tires will give you more torque, but whether it is worth the effort is your decision.

The stock tire height for your cart is 18", so with 23" tires you are losing 22% of the torque the motor produces and with 20" tires the torque loss is 10%.

Another factor involved is the tire type. It takes more energy to turn tires whose max inflation pressure is less than 10PSI than it takes to turn tires whose max inflation pressure is greater than 20PSI. Technically the difference is the amount of mechanical drag produced, but the net effect is the same as torque loss.

Tire width and tread pattern are also factors, but are relatively minor to the effect of tire height and inflation pressure.

As for ground clearance, the lowest point on the cart is the differential pan and the 5" taller 23" tires only raises that point 2.5" higher than stock height (18") tires.

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SepEx drive systems (DCS and PDS) generate a lot of heat in the motor and controller when creeping along at low speeds and to minimize the amount of heat generated you want to spin the motor as many time as possible and practical for each tire revolution. In other words, keep the final drive ratio as numerically high as possible and practical.

The final drive ratio with 18" tires and 12.44:1 gears is 16.59:1
The final drive ratio with 20" tires and 12.44:1 gears is 14.93:1
The final drive ratio with 23" tires and 12.44:1 gears is 12.98:1

FWIW: With the 17" tires I'm running, it's 17.59:1 and I'm getting a 6% torque gain.

-------------
What controller are you running?
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Old 03-25-2016, 09:47 PM   #3
cgtech
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Default Re: Changing tire size for more torque

Quote
The final drive ratio with 18" tires and 12.44:1 gears is 16.59:1
The final drive ratio with 20" tires and 12.44:1 gears is 14.93:1
The final drive ratio with 23" tires and 12.44:1 gears is 12.98:1

Quote

I have to ask... 16.59:1 of what? Tire revolutions? If the ratio in the axle is 12.44:1, that equals 12.44 motor revolutions per 1 tire revolution, no matter what size tire is installed. I get that a larger tire will cover more ground per 1 revolution than a smaller tire (is this what you are calculating?). Changing tire size will change the distance travelled per "tire revolution", but wont change the ratio of motor revolutions needed to make 1 tire revolution (as long as the tires are both the same size on both sides of any given axle, not staggered like a nascar to promote left-hand turns).
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Old 03-26-2016, 07:19 AM   #4
JohnnieB
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Default Re: Changing tire size for more torque

I'm probably using the wrong term, but mechanical engineering isn't my long suit and I don't know what else to call the total force multiplier from the motor's output shaft to the point were rubber meets road and the rotational force (torque) is converted to a linear force.

If the motor is producing 1 ft/lb of torque and the differential gears are 12.44:1, you will have 12.44 ft/lb of torque available at the axle. If the tires are 24" tall, they have a radius of 1 ft, so the tread applies 12.44 lbs of linear force to the road to move the cart. However if the tires are only 18" tall, the radius (lever arm length) is 9" or 0.75 ft. and the linear force applied to the road is 16.59 lbs. In other words, the torque is multiplied by factor of 16.59:1

Basically, tire height is part of the overall force multiplication ratio and tires effectively have a gear ratio.
18" = 1.33:1
19" = 1.26:1
20" = 1.20:1
21" = 1.14:1
22" = 1.09:1
23" = 1.04:1
24" = 1.00:1

I multiply the differential gear ratio by tire's effective gear ratio to get what I call the final drive ratio.

If anyone knows the correct name for it, or a better name for it, I'm all ears.
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Old 03-26-2016, 07:32 AM   #5
scottyb
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Default Re: Changing tire size for more torque

Basically 23 inch tires are 25% larger than stock tires. Reducing tire size by 20 -25% reduces the load on the motor by the same proportion. So yes this is a good way to help alleviate a significant proportion of the load.

However, Creeping uphill is always going to produce excessive heat in the motor and control because of the sustained high load. These motors are not 100% efficient & some of the amperage drawn is converted into heat not motion. This is especially true for Sepex drive carts like DCS. Series drives will be a little more tolerant and forgiving of such sustained high load operation but still any cart is going to generate excessive heat if used in this manner for any period of time.
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