Wheel Spacers and Center of Gravity

[Rebeldad1]

doesnt the farther out the tires are set from the axle housing increase the stress on the bearings and seals?

 

[TerryR]

So you guy's are saying that (were talking about a sideways rollover) if your track was 48" or 78" the c/g is still the same* and that there is only a stability difference? Or to put it another way, there is no relationship between the c/g and stability.

Yes to the first, no to the second. The c/g is clearly a factor in stability, but not the only one.

The c/g is the imaginary point where all the mass of the machine appears to reside. It is generally in the centerline of the machine left or right, someplace between the axles (otherwise the machine would tip very frontwards or backwards), and several feet above the ground. How far above depends on where the weight is placed vertically.

Gravity appears to be pushing the machine straight down through the c/g.

Because the c/g is several feet above the ground, as the tractor moves onto a side-hill an imaginary line running vertically through the c/g hits the ground nearer the bottom of the down-hill tire. When it move outside that tire the tractor tips over (ignoring front wheel restraint, if any). So the wider the tires the further the tractor has to tip before that line is outside the bottom of the down-hill tire.

Think of it another way. What if the tractor had zero width - like one of those plates contractors put over holes in the pavement standing on edge. The c/g is in the center of the plate, and the plate will always want to fall over. Now put braces on each side (moving the wheels out on the tractor) and it wants to stand up and is harder to tip it over. Making the braces wider doesn't change the c/g - it's still in the middle of the plate - but it becomes harder to tip over.

 

[Rustyiron]

Thanks Terry, never really thought about the relationship there.

 

[otlski]

So you guy's are saying that (were talking about a sideways rollover) if your track was 48" or 78" the c/g is still the same* and that there is only a stability difference? Or to put it another way, there is no relationship between the c/g and stability. Not arguing at all, just stimulated and wished I'd paid more attention to what ever class that was in.:thumbsup:

*disregarding the minor additional weight of the added structure/support

You have the first part correct. For all intents and purpose in this thread, the CG does not change when the wheels/tires are spaced equally outward. CG is a three dimensional point where all the mass of the tractor could be said to be concentrated if it could be concentrated into this single point. Thought experiment. Moving the left tire outward by say four inches would shift the CG imperceptibly to the left, but not upwards or downwards (again, intents and purposes license here). Next, moving the right tire outwards by four inches would shift the CG back to its original point.

The second part you appear to have misunderstood. There is a strong relationship between CG height and tractor stability. The CG of an empty paper towel roll is in the center of the tube, halfway up. The CG of a full roll of paper towels is also in the center of the tube, halfway up. If stood on their ends, the empty one is tippy, the full one is stable. The stability is created by the larger diameter (wider stance). Envision tipping the table slightly with the empty tube on it. While you slowly tip the table out of level, the outer edge of the empty tube becomes an unintended pivot point and the fixed CG only has to pass over this pivot until it pops over center... and gravity pulls it down.

I somehow missed the TerryR post above...

 

[the old grind]

doesnt the farther out the tires are set from the axle housing increase the stress on the bearings and seals?

Not to worry, seals take no weight until the bearings are shot. Forces on the bearings are extreme to begin with. Track width options are common with R-1 tires and typical wheels for them. News to me if putting on dual rears has ever meant a quick end to axle bearings, either. Reserve capacity is 'built in'.

 

[chim]

doesnt the farther out the tires are set from the axle housing increase the stress on the bearings and seals?

It does place the bearings under more of a load when wheel spacing is greater than the factory range of adjustment. Whether it is enough to affect the bearing life isn't something we can reliably determine here on the forum. Many would agree that using 9" spacers sure sounds like a bad idea. Somewhere between the factory spec and 9" there must be a point where it does start to matter.

Reports of "I installed 4" spacers 5 years ago and my tractor is still OK" don't provide any real information. When manufacturers employ both engineers and bean counters, the design is often the least expensive approach that will meet certain minimum criteria.

 

[RickB]

It does place the bearings under more of a load when wheel spacing is greater than the factory range of adjustment. Whether it is enough to affect the bearing life isn't something we can reliably determine here on the forum. Many would agree that using 9" spacers sure sounds like a bad idea. Somewhere between the factory spec and 9" there must be a point where it does start to matter.

Reports of "I installed 4" spacers 5 years ago and my tractor is still OK" don't provide any real information. When manufacturers employ both engineers and bean counters, the design is often the least expensive approach that will meet certain minimum criteria.

The problems begin when the defined criteria are getting the machine out of the warranty period without failures.

 

[chim]

The problems begin when the defined criteria are getting the machine out of the warranty period without failures.

So true. And there's likely some effort to use the minimum of resources and maximize profit by making the product last "just long enough"

 

[DMW]

A tractor has the same "Triangle of Stability" as a forklift. The front axle pivots on a pin, and the 2 rear wheels on a solid axle. Wider spacers will improve the lateral stability (sideways tip overs) esp. when moving on slopes. Weight added on the 3pt helps by moving the CoG to the back of tractor, the widest part of the triangle. Wheel weights by lowering the CoG. Spacers will not improve stability when using a loader. The front axle is for all purposes a single point, pivoting on the axle pin. A heavy load, up high on a slope will result in a tip over. By themselves, spacers offer no benefit with loader work.

 

[efmoto]

MY next post will help explain tipping point.