Longer wheelbase = better stability?

[rbargeron]

Your fulcrum analysis is exactly right. The length definitely defines what loads you can balance and obviously affects forward-to-back stabilty. But AGGC's original post said he was concerned about stabilty when mowing on slopes - feeling like he was tipping over when it wasn't level. Side-to-side stability isn't made better by length. In fact, only when there is enough angular difference between the front and rear axles to reach the front axle's pivot stop, does the front contribute to the "righting moment". The front axle may actually help prevent a roll, but if the wheel base is too long even that effect is reduced, especially if the total CG is high. Anyone old enough to remember the tricycle tractors knows how little the front contributed there. They were strictly flatland machines - although many found their way into the hills where they were involved in a lot of injuries.

 

[azhfl56]

I like your added analysis. However wouldn't a longer wheelbase lessen the effects of hitting a hole/ dropping into a hole while traversing a hill?

In short, if I were looking at a tractor and all things being equal, width etc. and one had a longer wheel base than the other and space weren't an issue, I would probably choose the longer wheelbase.

If the hills are that sever, then they should only be traversed head on, or to be totally safe, backward, never sideways.

The best way to mow hills is with a good Zero turn mower, or maybe an M1 Abrams fitted with a batwing!

Regards,

Bryan

 

[rbargeron]

.....wouldn't a longer wheelbase lessen the effects of hitting a hole/ dropping into a hole while traversing a hill?

I've been thinking about that and it might be just the opposite. I think the limit of the front axle's pivot range can be less with shorter wheelbase, hence the contribution from the front can be bigger sooner if the wheelbase is short. I'm not real sure what the design constraints are on front axle motion, but it would seem logical to try to have all wheels in contact with the ground as much of the time as possible. On uneven ground, the longer the wheelbase the more pivot angle would be needed to accomplish that. Conversely, if the wheelbase is shorter, the pivot can be more limited for better righting moment contribution. None of these factors operates in a vacuum - they interact with width, CG height, overall weight, likely attachment loads, yikes, makes me glad I'm only a nuclear engineer

 

[SPYDERLK]

Your fulcrum analysis is exactly right. The length definitely defines what loads you can balance and obviously affects forward-to-back stabilty. But AGGC's original post said he was concerned about stabilty when mowing on slopes - feeling like he was tipping over when it wasn't level. Side-to-side stability isn't made better by length. In fact, only when there is enough angular difference between the front and rear axles to reach the front axle's pivot stop, does the front contribute to the "righting moment". The front axle may actually help prevent a roll, but if the wheel base is too long even that effect is reduced, especially if the total CG is high. Anyone old enough to remember the tricycle tractors knows how little the front contributed there. They were strictly flatland machines - although many found their way into the hills where they were involved in a lot of injuries.

The short wheelbase unloads the rears too readily. That is the sole reason that the long wheelbase is more stable on slopes, however it is an inescapable reason. The four wheel tractor is no more stable than the tricycle because they are both tricycles until the 4 wheel reaches the limit of articulation on the fronts. Then you might get a second chance, but one of your rears is already in the air - - tip has started - the CG has risen. If theres lots of weight on the fronts youll tip early but probably stop with the rear wheel in the air when the front axle reaches its limit. With the longer tractor the weight does not shift to the fronts so much on downslopes. You can go down a steeper slope in 2wd w/o sliding. You can go on a steeper down diagonal or a pure side slope w/o tipping because the rears stay loaded better, especially with weight on a loader. The ordinary CUT wont even work on the slope that the longer tractor will unless it is accurately counterbalanced for the situation at hand - - then you turn or the slope changes and the long tractor reacts less. Finally, after the short tractor has had to retire to watch, the long tractor tips. It may not stop tipping like the short tractor did on considerably less slope. At this limit, beyond that which the short tractor can operate, the long tractor will operate, but with less forgiveness of error than the short one exhibited on a lesser slope.

Front to back stability improves side to side stability, but the higher limits permitted come at a cost of added criticality at the steeper tip point.
Larry

 

[WayneW]

Yes, excellent analysis with the fulcrum explaination. I've thought about this some, and came to the conclusion that a tractor is NOT the ideal machine for lifting heavy weight with a loader.

...off topic a bit, but.....

I've always been impressed by Bobcats and what they can do with such a small machine. BUT, notice where the lift arms of the loader are attached. In the rear, behind the rear tires. Due to reasons already explained here, it is designed differently than a tractor, more for heavy lifting with a small machine. It's almost as if the pressure applied to the lift arms create their our counterweight to the rear when lifting heavy loads.

But then again, it's tough to mow with a Bobcat.

It's all about compromises. Personally I'm thrilled to be able to move large amounts of dirt and rock with my B7610, AND mow the lawn, pick up leaves, level and snowblow the driveway.