Difference between 4WD and MFWD

[HomeBrew2]

N80,

Yeh, the diff is where the front drive shaft enters the front axle housing, just like normal, see attached PDF.

I don't get "binding" either on non-paved surfaces. But I do get a bit of tire scuffing when turning. I get a lot more scuffing with my 4WD SUV's. I'm a believer that the front wheel lead in the tractors minimizes the scuffing. A middle diff should eliminate any binding or scuffing.

Again, my experience is with the little BX. Yours is a bunch bigger so you may have a different design, but sounds like the front axle is similar.

 

[N80]

I think mine is probably the same. The PDF clears it all up, thanks. That's just a tiny little diff and since my front axle housing looks identical, just bigger, I bet the mechanism is the same too.

Where did you get the repair manual pdf?

 

[HomeBrew2]

... Where did you get the repair manual pdf?

Off eBay quite a while ago.

 

[ovrszd]

I think mine is probably the same. The PDF clears it all up, thanks. That's just a tiny little diff and since my front axle housing looks identical, just bigger, I bet the mechanism is the same too.

Where did you get the repair manual pdf?

That's the point I was trying to make in our previous discussion about the strengths/weaknesses of various drive systems, especially in the front drive system. The power and traction of the Kubota is magnified thru the ring/pinion at each front tire. This magnified torque is transmitted thru a tiny set of spyder gears because of the small ring gear in the center of the axle housing. Spyder gears probably half the size of those in the rear diff. This setup creates a weak spot for potential trouble. So the operator should take precautions to lessen the load on that component, i.e., not using MFWD unless needed. Binding is inevitable in the Kubota system, either when driving straight or when turning, depending on the variation of ratios between the front axle and rear axle. It's a mechanical system and has to bind during one or the other.

 

[N80]

This setup creates a weak spot for potential trouble.

If the bevel gear 4wd system that Kubota uses in all its tractors creates this weak link, then why do they go to the trouble of using a system that is clearly more complex, and probably more costly to manufacture than a standard live-axle system? In other words, if the design creates a "weak spot" as you say, where is the advantage in this sytem that makes it worth introducing such a weakness?

 

[ovrszd]

If the bevel gear 4wd system that Kubota uses in all its tractors creates this weak link, then why do they go to the trouble of using a system that is clearly more complex, and probably more costly to manufacture than a standard live-axle system? In other words, if the design creates a "weak spot" as you say, where is the advantage in this sytem that makes it worth introducing such a weakness?

Every complex mechanical system in use has a weak spot. In this system it just happens to be the spyder gears. It's not an intentionally weak design. I'm sure they weighed all of the consequences of different systems and chose the one that produced the quality they desired at the cheapest cost of production. They don't use a standard live axle system like the rear axle of a car or truck because it would give them a horribly low hanging differential and axle tube that no one would like.

 

[390GT]

Looking at Homebrew2's pdf. whats being called a differential looks like what the Street Rodders call a "mini spool". Mini spools are very tough. Allows wheels to spin at different speeds while turning, locks them together when going straight. Not saying thats what it is, just that it looks similar. Greg

 

[ovrszd]

Looking at Homebrew2's pdf. whats being called a differential looks like what the Street Rodders call a "mini spool". Mini spools are very tough. Allows wheels to spin at different speeds while turning, locks them together when going straight. Not saying thats what it is, just that it looks similar. Greg

I'm not sure I've saw one of those?? I've used spools several times on street rods but they simply lock the axles together and eliminate spyder gears. But in this case it's a very small carrier with a set of miniature spyder gears and pin with the ring gear externally slid on one end.

 

[N80]

They don't use a standard live axle system like the rear axle of a car or truck because it would give them a horribly low hanging differential and axle tube that no one would like.

John Deere uses a live axle set up. At least on the ones I saw at the dealership a couple of days ago including a 990 (similar to my L4400) and several Ag/utility sized machines. They must think its an okay design.

I would also have to say that looking at that .pdf schematic I personally don't see the bevel gear (Kubota's terms) differential set up as a weak link. As I mentioned before, based on the size of the bevel and pinion gears it looks to be about a 1:1 gear ratio. I see nothing weak in that. The size of the bevel gears (both the 'ring' and the 'pinion') don't really indicate that they are weak to me. They might be smaller than what you see in the rear because the final drives by each wheel may have a reduction ratio but it is also (and probably mostly) because the front tires and wheels are 1/2 the size of the fronts.

I don't know what the mechanism in the bevel gear assemblies looks like. That might indeed represent a 'weak spot' but I don't see any inherent weakness or anything failure prone in the diff. And again, nothing in my manual suggests there is anything in the front driveline that requires any special care or attention compared to the rest of the tractor.

 

[Stackwood]

Another reason to have the front wheels turn faster is because the rear wheels of any 4 wheeled vehicle cut a tighter arc than the fronts if the fronts are the ones doing the steering. Because of this tighter arc, they have to move slower. The faster turning front wheels are probably at a compromise speed between straight line running and tight turning.

I think the tractor would tend to steer better if the fronts were pulling harder, too, since one or the other will be moving faster.