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Tractor Pulling Discussion Forum

math problem

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Steve in OR

08-17-2006 18:58:30




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I have a 560 IH. I just got a "working" tach on it and I want to see if it makes any sense.
I pull in 2nd direct, with 18.4 x38 tires. The speed listed in the manual with 18.4x 34 tires is 4mph. The sled has a speedometer, it says my ground speed is 9mph, my tach says 6300 rpm (I'm not sure its working correctly). There are a couple of interesting things that could be learned from this. By knowing the percentage tire size increase going from 34 to 38 rims, you could calculate rpm of engine (not factoring in wheel spin). If you believe your tach you could calculate wheel speed by knowing the rpm and theoretical ground speed minus wheel spin.
Does any of this sound reasonable to you folks. And I guess my ultimate question is what is the percentage size difference form 18.4X34 to 18.4X38 tires. I think this could be an SAT question, eh.

Steve

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dugger44

08-21-2006 11:29:35




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 Re: math problem in reply to Steve in OR, 08-17-2006 18:58:30  
Rolling circumference of 34" is 190" and 206" for a 38" tire



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wolfman

08-20-2006 17:34:28




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 Re: math problem in reply to Steve in OR, 08-17-2006 18:58:30  
The circumference of each(the 18.4-34 & the 18.4-38) can be calculated by multiplying pie -there's a pie button on all hand held calculators-times the diameter of wheel, tire & all. Maybe 170:190. This would be one roll of each with no slippage, but the ratio is right there.



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Dave in MN

08-19-2006 19:39:09




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 Re: math problem in reply to Steve in OR, 08-17-2006 18:58:30  
I have a M with a V-8. Using 18.4X38 cut tires, second gear at 6000RPM, I was clocked at 12MPH.

I'm assuming that you aren't using a cast iron flywheel anymore as 3000RPM might be pushing the safety limits.



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moli

08-18-2006 20:05:42




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 Re: math problem in reply to Steve in OR, 08-17-2006 18:58:30  
Steve,about your tach..just for the hell of it look at the back of it and see if there is a switch. On some tachs there is a switch that says 2-4-6 cyl,make sure its on the right one...



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moli

08-18-2006 20:05:06




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 Re: math problem in reply to Steve in OR, 08-17-2006 18:58:30  
Steve,about your tach..just for the hell of it look at the back of it and see if there is a switch. On some tachs there is a switch that says 2-4-6 cyl,make sure its on the right one...



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Lamont

08-18-2006 05:52:55




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 Re: math problem in reply to Steve in OR, 08-17-2006 18:58:30  
First off, like B=maniac, it sounds like your tach is trash. Second, to get a close figure on what your speed should be, you would need to know the rpm was for that speed recorded in the book and your TRUE rpm. In other words, you said the speed for 2nd gear with 18.4 X 34 tires is 4mph. What rpm was the engine running? Without knowing that all you could figure is the speed when changing ONLY the tires. An 18.4 X 38 vs. 18.4 X 34 will give only a 7.85% increase in speed. (That's using details from Firestone charts.) 4mph X 1.0785 = 4.314mph. As you mentioned, this does not subtract wheel slippage. I can tell you this. Your engine would have to be running almost 209% of the original rpm to get 9mph. That's all you can figure without knowing the original rpm. If you find that original rpm, just multiply it by 2.09 to get a close figure on your rpm, assuming the 9mph reading on the sled monitor is accurate. (2.086 if you want to get even closer.)

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Stevfe in OR

08-18-2006 17:24:20




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 Re: math problem in reply to Lamont, 08-18-2006 05:52:55  
Ok, the figure as far as I can tell is 4mph @ 1950 rpm, so taking the 2.09 figure the rpm with no wheel slippage would be 4075rpm. Then to figure a possible slippage rate, divide 4075 by 6300 and come up with a 65% wheel slippage. Wait I think that it would be 35% slippage being that that is the pecentage difference between 4075 and 6300. I was definitely spinning most of the way but 65%, 35%? So I have a tach reading 2000+ rpm slow or my tires are spinning quite a bit. Guess I need a video to see if 65% is reasonable. Wait 35% is totally reasonable amount of wheel slippage.

A few words about the tractor, head redone and governor removed with large combine carb.

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Lamont

08-18-2006 19:55:37




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 Re: math problem in reply to Stevfe in OR, 08-18-2006 17:24:20  
You have to remember that the 4mph @ 1950rpm was with 18.4 X 34 tires. It would be 4.314mph with 18.4 X 38s. If your tractor is truly turning 6300rpm, that would give you a wheel speed of just a little more than 14mph. The sled monitor says you're moving 9mph. That's a 55.5% wheel slip. Lots of slip. Your tach is probably giving a reading too high. 6300rpm sounds awfully high as does 55.5% wheel slip. The 6300rpm seems to be the least reasonable number in all of this to me. ?????

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Farmall Puller

08-18-2006 05:50:07




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 Re: math problem in reply to Steve in OR, 08-17-2006 18:58:30  
I don't know what math to use to get the percentage, but you can get a GPS and check your speed.



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B-maniac

08-17-2006 19:19:41




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 Re: math problem in reply to Steve in OR, 08-17-2006 18:58:30  
6300 rpm??? even in 2nd would be over 20 mph! Junk the tach,that tractor doesn't even see 2500 rpm.Yea, there's a formula for figuring what you want and someone will post it on here I'm sure. 34's to 38's in 2nd might gain you 3mph at most. Use the 38's and just keep it under the horn and drive by the seat of your pants.



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Jerry Cent. Mi.

08-19-2006 16:06:03




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 Re: math problem in reply to B-maniac, 08-17-2006 19:19:41  
I can't find my chart but a 34 inch tire is thicker than the 38 inch and the distance traveled between the two tires isn't a lot different. You can't use the rim dia. difference. Some tire company's web sites post the difference.



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