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Tractor Pulling Discussion Forum

Hey CHADS, can you check my math?

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redpuller1

11-07-2006 17:52:35




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Trying to calculate tire speed (or gear ratio) if posible. Jacked up the tractor,(no engine)and made one full rotation of the tire. It turned the input shaft exactly 21 revolutions. The tire has a circumference of 17 feet. If the engine runs at 6000 rpms, what is the tire speed?..... Here is my math, please correct me if I am wrong.

6000 rpms divided by 21 = 285.7 X 17' (tire rotation) = 4857 fpm X 60 = 291414 fph divided by 5280 = 55.2 mph (not calculating for any slip). I must be missing something cause this sounds too fast.

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ChadS

11-09-2006 09:58:05




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 Re: Hey CHADS, can you check my math? in reply to redpuller1, 11-07-2006 17:52:35  
Try using known factors from the tractor. Look at a parts book, that would have gear speeds, or ground speed. Most readings were at 540 rated pto speed, which is considered the "redline" for operation, or a base engine rpm to work with. Lets say,,, a Farmall M has a 18mph road gear at 1500 rpm, (standard tire size in parts book for starters) double the rpm, that,,, in thought,, doubles the ground speed, or axle rpm speed. If you had a way to accurately measure the axle rpm speed at a speced rpm, and have a reference. If I was to have a M farmall, turning 6000, using the 18mph as a reference speed, comes to 72 mph with stock tire diameter in road gear. Then, comes in the factors of tires,,, taller tires speed up ground speed, short tires slow em down. but, the axle speed does not change with tire diameter. Dont know if this helps or not,, Chad

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Like this,,

11-09-2006 14:41:01




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 Re: Hey CHADS, can you check my math? in reply to ChadS, 11-09-2006 09:58:05  
ground speed divided by engine rpm, then multiplied by the desired engine rpm will give you ground speed at higher engine rpms, provided that the parts are stock.

18mph Divided by 1500 engine rpm at 540 pto speed=.012 X 6000 engine rpm = 72 mph at 6000 rpm


see how that works?? Like I said, tire size can throw the actual ground speed off,,, but, for sure,, the axle speed rpm will be the same, unless you have a custom ring and pinion (faster than what stock is) Which, still can be estimated,, that will get you really close,, tire size, wheel slippage, would be the changing factors to throw it way off, that and faster ring and pinions, but still can be figured. Chad

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wayne stine

11-11-2006 06:49:56




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 Re: Hey CHADS, can you check my math? in reply to Like this,, , 11-09-2006 14:41:01  
I can not relate to the stock information because it does not have the stock transmission gears in the tractor and the previos owner was not sure what was in it.



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ChadS

11-12-2006 09:39:18




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 Re: Hey CHADS, can you check my math? in reply to wayne stine, 11-11-2006 06:49:56  
Then you will have to know some info from a stock tractor, and your tractor and convert the information mathmaticly. What you need to know is the axle rpm,, if you can measure a stock tractors axle speed, you can do that by jacking up the rear end, remove the rear tires, support the tractor firmly,, Have you ever seen the old PTO guages made years back? its used to test pto rpm speed, which you just put it on the end of the pto shaft and it reads the rpm,,, Now, you dont have to run the machine at full throttle,,, you can do it at an idle,, all you need is the readings, making sure the engine rpms are the same,,,,, note the diffrence inthe axle rpms from the stocker, to the modified,, once those 2 factors are recorded, you have data you can use to calculate the differences. Chad

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hardscrabble

11-07-2006 18:53:07




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 Re: Hey CHADS, can you check my math? in reply to redpuller1, 11-07-2006 17:52:35  
Did you have both tires off the ground, or just one? If you were turning just one wheel, you were spinning the input shaft twice as fast as if you were turning both wheels. So you can cut your calculation in half.



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Tramway Guy

11-11-2006 05:46:15




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 Re: Hey CHADS, can you check my math? in reply to hardscrabble, 11-07-2006 18:53:07  
No, you have it exactly opposite. He would be turning the input shaft 1/2 as many times as if he turned both wheels.
So his ground speed would only be 27.6 MPH..



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redpuller1

11-08-2006 03:46:53




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 Re: Hey CHADS, can you check my math? in reply to hardscrabble, 11-07-2006 18:53:07  
Ahhhhh , I did have both wheels off, but I blocked one of them because I thought that I was not getting an accurate measurement. Thanks for the help!



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champspa

11-07-2006 18:32:43




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 Re: Hey CHADS, can you check my math? in reply to redpuller1, 11-07-2006 17:52:35  
is that the rolling circumference of the tire as the chart on Firestone tire



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Redpuller1

11-08-2006 03:51:21




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 Re: Hey CHADS, can you check my math? in reply to champspa, 11-07-2006 18:32:43  
No, these tire are cut pretty drastically from stock, so I don't think that they would measure the same as the chart. 24.5x32



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