Does widening front end stress axle/bearings more?

[zzvyb6]

The bearing sees an "overturning moment" (rotation about a F/A axis) from the tire's vertical force vector. There are also moments (forces times a distance) due to side-forces from turning and from inclination. There is usually a positive camber angle set on the front spindle to assist with turning (steering), by means of the tire contact patch is moved inward. "Scrub radius" is a term that usually appears in a discussion on the subject but not here because front tractor wheels usually don't have brakes. The camber is from the kingpin inclination as seen in the rear (or front) view.

When you extend the tire's rim inward or outward, you increase or decrease this moment. You usually can not flip the wheel spider around to maintain a unchanged wheel offset because it would be pointless and redundant. The wheel's offset induced moment (force times a distance) is reacted by the span of the inner and out wheel bearings (called a "couple"). So, when the tire-on-its- rim is moved outward, the internal forces on the bearing (and the spindle) increase proportionally. Tire centerlines are usually set to match the rears for rut negotiations row crop following and to make it easier to load onto tractor ramps, etc.

Setting wheels outward increases steering effort because there is a larger scrub radius and spindle length, there is also extra body roll because of caster angles (for self centering steering) and kingpin inclination (which lifts the front end upwards as you turn).

Setting wheels inward has some advantages for loader work, especially if you have manual steering. The extra vertical load on the front axle makes it much harder to steer, so a reduced front track is set to lower these efforts. Some knowledgeable readers can attest to the swiftness of the returning wheel kick when doing loader work and the front track is large. It can break your wrists.

Manufacturers design spindles, axles, wheels and tires with all this is mind as design-for conditions, so there SHOULD be no trouble unless you have a marginal machine, your loads are higher than recommended or your tires are out of spec for size or pressure.

 

[ovrszd]

After thinking this over some more I've concluded my analysis in my previous post is wrong for the purposes of this thread. The question revolves around the stresses on the bearings and the analysis does not address that question. Therefore, it should be adjusted to consider each bearing position on the final drive shaft in relation to the force applied to the center of the tire (if we assume the force is indeed centered on the tire). I'm thinking this would then lead to a conclusion that moving the tires out would indeed increase the stresses seen in the bearings.

And the stress is more severe because of the location of the bearing set in relationship to the location of the hub face that the wheel bolts to.

 

[ovrszd]

Manufacturers design spindles, axles, wheels and tires with all this is mind as design-for conditions, so there SHOULD be no trouble unless you have a marginal machine, your loads are higher than recommended or your tires are out of spec for size or pressure.

Or if your owners manual specifically says to not reverse the dish or in any way widen the front track farther than the recommended setting.

In the case of the second picture I posted, that's my M9540. The manual discusses setting track width on the front and rear. It specifically and clearly states to not set the tires/wheels wider than the setting in the picture. That setting is accomplished with the dish inward and the rim mounted outward on the dish as far as possible. The next setting outward would be reversing the dish and moving the rim one setting inward.

Here's a sortta side view of that setup. Dish inward, outer rim outward. Notice the centerline of the tire is very close to the mounting flange location. Yes, I can probably reverse the center and move the outer rim inward to it's maximum, but as mentioned above, that would be redundant.

Kubota does not want operators to widen the front tire track of this particular tractor any further than this setting. I'm assuming that's because they are concerned about overly stressing the bearing assembly. Actually I think it states that in the manual as the reason to not do so.

 

[Ilikeurtractor]

Here's another analysis (isn't this fun???). Excuse the "chicken-scratching" as I didn't want to redraw w/ bearings, etc. Anyway, all "F" variables are forces, and all "X" variables are distances. MF2 and MF4 are the sum of the moments around the bearing closest to the hub in each case. Since the distance "X3" is more than "X5", with "F" a constant representing the weight of the tractor on the tire, the forces on "F2" and "F3" must be higher, and therefore see more stress in the left scenario (wheels out) vs. the right (wheels in). This analysis assumes the bearings can pivot in their race which may or may not be true.

 

[DarkBlack]

So many theories here!:laughing:

 

[xring100]

Very good i started reading this earlier and was getting frustrated reading through the thread and made this picture showing pretty much the same thing but you posted first.

In design you really want the centerline of the contact patch to be centered between the bearing races to minimize the moments as you have mentioned. On a side note I work as an engineer at GM my group owns the Brake and Bearing components.

Here's another analysis (isn't this fun???). Excuse the "chicken-scratching" as I didn't want to redraw w/ bearings, etc. Anyway, all "F" variables are forces, and all "X" variables are distances. MF2 and MF4 are the sum of the moments around the bearing closest to the hub in each case. Since the distance "X3" is more than "X5", with "F" a constant representing the weight of the tractor on the tire, the forces on "F2" and "F3" must be higher, and therefore see more stress in the left scenario (wheels out) vs. the right (wheels in). This analysis assumes the bearings can pivot in their race which may or may not be true.

 

[zzvyb6]

I suspect Kubota doesn't recommend going further outward because there will be tire to frame or tire to chassis contact. This could potentially make a wheel to lockup and cause a crash.

 

[Ilikeurtractor]

Very good i started reading this earlier and was getting frustrated reading through the thread and made this picture showing pretty much the same thing but you posted first.

In design you really want the centerline of the contact patch to be centered between the bearing races to minimize the moments as you have mentioned. On a side note I work as an engineer at GM my group owns the Brake and Bearing components.

Thanks for the secondary analysis and design tips/goals. I put some numbers to the variables and came up with the attached. I tried running it with your method, but I wasn't sure how to resolve the vertical forces and moments. Anyway, I'm getting an outer bearing stress increase of 63% and inner bearing increase of around 157% based on the numbers I can find for an Iseki TX1300. I don't have exact values for bearing spacing or distance between the outer bearing and tire centerline in each case so I made some guesses. Obviously each tractor is going to have its own unique results. Of course, this could be way off reality but it was a fun exercise. I don't get to do much engineering at work so it's good to brush the dust off the tools once in a while even if the results are wrong :shocked:

 

[2458n]

I reversed the tires of a old L245DT. Trailer had two rails and tractor needed reversed wheels to fit. Could have bought another trailer for the cost to repair tractor. The popping sound of the crushed bearings is very expensive. Owners manual was very clear in what not to do.

 

[Ilikeurtractor]

I suspect Kubota doesn't recommend going further outward because there will be tire to frame or tire to chassis contact. This could potentially make a wheel to lockup and cause a crash.

On my tractor I need to move the tires to the outer position to give me more tire clearance while turning. My tires rub if they are in the inner position with a modified blower/plow mount attached to the tractor frame. Instead of reversing the rim, I just made spacers for the added weight and I prefer the dish out to in.