Lawn Tractor Differential

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  #1  
Old 08-25-08, 06:09 PM
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Lawn Tractor Differential

I am building a small off road go kart and using some parts from an old mtd lawn tractor...ie. motor and transaxle. The motor is a 12hp briggs and stratton and the differential has a forward and reverse gearbox on it.

I am worried about how this transaxle will hold up. It is geared about 18:1.

The plan is to drive the diff. via a comet 40 series torque converter which on the high end is 1:1 and the low end is about 2.5:1. The wheels will be on a live axle that is chain driven from the axles on the differential which will be stationary. I want to lower the gear ratio considerably on the output side of the diff. to attain higher top speeds.

I read a post about using gear oil instead of the factory grease inside the diff.

Any suggestions?

Thanks amcherry
 
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  #2  
Old 08-25-08, 07:43 PM
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A friend of mine kinda did the same thing. All he did was put some small tractor tires on his. It did good for awhile, untill he got stuck. It was in a bit of a bind, and broke the rearend. He was using the stock motor. I don't know if this is any help, but to much power on the rearend may not be a good idea.

Travis
 
  #3  
Old 08-26-08, 12:02 AM
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The MTD trannys don't hold up well to that kind of use. They have bushings instead of bearings and the iron parts aren't very tough either. I've seen them fail on 7 mph lawnmowers, so imagine how long they'll last turned up 8500 rpm on a gokart. The toughest mower trannys I know of are the old cast iron foote trannys with forged gears, needle bearings, and regular gear oil inside. I've used the old 800 series behind an 18 horse twin engine on a lawnmower with the ratio up so high that the mower went about 40 mph. It would catwalk 40 feet. It still tore up, but we were abusing it and we didn't have a comet type clutch, which is gentler.

Some of the old roper garden tractors and others had old foote cast iron big trannys in them with the Hi/Lo gear and a side shaft input pulley. This would be ideal I would think. You could just run the clutch straight to the tranny input shaft, have all the gears, and even have reverse. Put in synthetic oil and it should take whatever you put to it.
 
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Old 08-26-08, 01:22 PM
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Spicer tranny

I've found a spicer 4360-149 transaxle that has five speeds. My understanding is that spicer is the the new foote. I have not thought of putting a manual transmission on the kart but am not completely opposed to doing so. The engine I have is a verticle shaft so I must have a way to convert the direction of the motion.

I had not planned on the need for a clutch to shift gears or thought about how to accomplish it. I suppose I could just select the gear that has the best ratio for me and leave it in that gear. Or possibly let the engine speed fall below the engagement range of the comet torque converter before shifting. Sort of an automatic clutch. Any thoughts?

What do you think about this transaxle. Do you know of a website I could shop for transaxles. I'm having problems finding sites that give good details about their equipment. Gear ratios, rigidity, etc...
 
  #5  
Old 08-26-08, 11:15 PM
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The old foote trannys aren't made anymore. The spicers are nowhere close in comparison. If you hope to find a new tranny that will hold up to what you plan to do, I don't think there is one in the mower market. The 800 series footes are the only tough gear trannys with a horizontal pulley I know of.

On the clutch idea, I'd just select the gear I wanted to be in and ride it. If you want a different gear, learn to shift on the go, or stop and select the desired gear and go again. The mower trannies don't have synchronized gears but usually can be shifted during use anyway, with a little finesse at high speeds.
 
  #6  
Old 08-27-08, 03:44 PM
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Hey Cheese,

Thanks for all your help so far. Try as I may I can't find an old Foote 800 series transaxle. Do you know anything about tough torq? I've found some of these that have 1" axles.

The stock motor from the mower only turns around 3600 rpm (*I think ) so this would be the fastest I planned on turning the input on the transaxle...unfortunatly this is around three times faster than it turned on the mower. The torque converter starts at 3:1, but then spins up to 1:1, which would be turning the transaxle the same rpm as the peak engine rpm.

I am starting to think it best to find a horizontal shaft engine and use a gearbox for reverse, but am torn because of the money I've already spent.
 
  #7  
Old 08-28-08, 12:58 AM
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Tuff-torq is a hydro tranny, and hydros don't work at higher speed. You'll need a regular gear tranny.

The engine should have run close to 3600 rpm on the mower it was bolted to as well, unless someone tampered with it or just ran it at idle. What size engine is it? What mph are you looking to get?
 
  #8  
Old 08-28-08, 02:53 PM
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I've got a 12hp briggs and stratton I/C verticle shaft engine. Since this is a kid/adult off road kart I don't expect to have a really high top speed (between 30 and 40 mph). Also, because of the nature of its use, I want some low end torque for climbing mild hills and the like.

The driven wheels are 16" in diameter. So I need the driven axles to run somewhere between 750 and 900 rpm. Ideally I think a transaxle that was geared 4:1 (4 turns in 1 out) would give me the speed I need and the torque converter could multiply the torque when needed.

Aaron
 
  #9  
Old 08-29-08, 12:11 AM
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I don't know of anything that fits the bill for you. Most trannys are going to be around 6:1 and lower I believe. I think if I were to set out to accomplish what you want, I think I'd use the foote with H/L shifter and horizontal input shaft coupled with a decent horizontal shaft engine and a regular spring loaded tensioner pulley type clutch on a regular utility belt. This should be the most durable, easiest to maintain and repair, simple to build/design/change ratios, and cheapest for parts. You can get whatever ratio you want by selecting different size pulleys for the engine and tranny.
 
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