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Tractor Talk Discussion Forum

Differential ratios, wheel spin rate puzzle

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Ray

05-24-2004 08:34:42




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I've asked several mechanics/machinists that I respect how locking one brake on a differential would affect the speed of the opposite wheel, and have gotten several mutually exclusive answers. Which one is correct? Assume the axle ratio is 3.00:1, which means the drive line turns 3 times for each 1 revolution of the wheel. If you lock one brake, does the opposite wheel: 1) continue to turn at the 3.00:1 rate? 2) speed up, rate determined by the spider gears which the engineers set depending on the axle width, tiresize, etc? or, 3) speed up, driven by the spider gears, up to 2x the axle ratio speed (up to 6.00:1 in this case). If there is a website illustrating this for the mathematically challenged among us, I'd love to see it. Thanks, Ray

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Shane

05-24-2004 14:57:58




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 Re: Differential ratios, wheel spin rate puzzle in reply to Ray, 05-24-2004 08:34:42  
Hence the need for locking differentials. Usually each wheel has 50% of the applied power for a total of 100%. You get one wheel in a soft spot and it spins while the other just sits there. The spinning wheel has 100% power and the other has 0%. Lock the diff and now both wheels have a guaranteed 50% of the power for a total of 100%. But remember to unlock the diff before getting too close to the ditch because with neither wheel being able to give part of the power to the other while making a turn you really can't turn very well!

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Chip

05-24-2004 10:27:41




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 Re: Differential ratios, wheel spin rate puzzle in reply to Ray, 05-24-2004 08:34:42  
Bob's right.A couple other things,you can EFFECTIVELY change a ratio with tire size (diameter),and #3 of post,UP the ratio is a LOWER#.The higher the number the lower the ratio



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Bob

05-24-2004 09:33:26




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 Re: Differential ratios, wheel spin rate puzzle in reply to Ray, 05-24-2004 08:34:42  
The FACT is that the speed of the single wheel doubles, and the "effective" ring and pinion ratio of 3:1 is HALVED to 1:5 to one.

This is witnessed by the fact you can determine rear axle ratio by the following means:

Jack up one rear wheel, and mark a spot on it. Mark the input yoke, and count the turns the input yoke makes to turn the wheel one turn. On your hypothetical 3:1 ratio axle, 1.5 turns of the input yoke will turn the one wheel that is jacked up 1 turn. (Assuming an "open" differential.)

The motion of the spider gears "walking" around the differential side gear that is stationary is ADDED to the rotation of the differential housing, doubling the speed of the wheel that IS turning. This "doubling" effect would hold true with ANY combination of spider gear and side gear tooth counts.

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Bob

05-24-2004 10:05:23




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 Re: Re: Differential ratios, wheel spin rate puzz in reply to Bob, 05-24-2004 09:33:26  
Correction:

The first line of my post should read:

The FACT is that the speed of the single wheel doubles, and the "effective" ring and pinion ratio of 3:1 is HALVED to 1.5:1.



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Davis In SC

05-24-2004 09:33:17




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 Re: Differential ratios, wheel spin rate puzzle in reply to Ray, 05-24-2004 08:34:42  
Locking one wheel will double the speed of of the free -spinning wheel, regardless of the ratio of the ring & pinion. In a car, if one wheel only is turning, speed of it is twice what speedo reads. It is possible to explode a tire/wheel spinning on ice or mud due to this increase of speed.



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Coloken

05-24-2004 12:03:47




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 Re: Re: Differential ratios, wheel spin rate puzz in reply to Davis In SC, 05-24-2004 09:33:17  
Yep, your so right. Take that a step further...when I had my 75 Chevy PU with full time 4 wheel...It added another differential up at the tranny to drive the front wheels. Jack up one fromt wheel, or have it in loose mud and it spun at 4 times. The book warned about blow up if you raced the engine.



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Ray

05-24-2004 13:20:06




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 Re: Re: Re: Differential ratios, wheel spin rate in reply to Coloken, 05-24-2004 12:03:47  
Did I mention that I was mathematically challenged? Thanks a lot every one, it makes sense to me now, but it seems that there are a lot of folks out there that should understand this that don't. Thanks for setting me straight.
Ray



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