Tractor Rollover Myths and Facts

[IslandTractor]

I don't know what is 'more likely' but I believe that both will roll backward easily.

It simply boils down to torque. If any tractor has enough torque to pull its front wheels up and roll backwards then it will do so in that type of situation.

Not sure that weight is not an advantage here. Think of the theoretical situation where the heavier tractor weighs 100x the lighter one but has the same HP engine. Clearly there will be a different risk to lifting the front wheels. Working back down to reality, it stands to reason that for any given set of conditions (same speed, load, hp, drawbar height etc) that the lighter tractor will be able to rotate itself over the rear axle earlier. Even if both could go over for one set of conditions there would likely be slightly different conditions (slight decrease in speed or slope of land etc) where the light tractor could rotate but the heavier one remained (barely) stable.

 

[jbrumberg]

I am throwing this into the discussion. A tractor with its rear wheels frozen to the ground could in theory backflip when starting out in forward in low gear. I heard about this years ago and I always start my tractor in reverse in the Winter "just in case". Jay

 

[SPYDERLK]

4) Attaching a 3PH implement eliminates the possibility of a Rear Rollover.

I agree completely with what has been said. The problem is with applying absolutes without carefully considering the specifics - and further, sometimes the change in the initial specifics as the tractor interacts with the surface. Tractors can take quick bites out of the ground, promptly changing a straight pull to a situation requiring climbout while pulling.

Since a situation is going to change, it is important that we have time to react in a way to prevent criticallity. I am convinced that in many ways having an implement on the 3PH is always a valuable tool in this regard. In back roll conditions the implement touches down as the front wheels rise, shifting weight back forward inhibiting the rise. The operator is well served by this gradual shift to instability. In particular, I like the action of a bushog when climbing a hill forward. When that tail wheel touches there is a stable pause in which to exercise constructive options. Early warning. Priceless. - - but then again - what if you drop your box blade while backing down a steep hill. Uh Oh!
larry

 

[IslandTractor]

4) In particular, I like the action of a bushog when climbing a hill forward. When that tail wheel touches there is a stable pause in which to exercise constructive options. Early warning. Priceless.

The problem with a bush hog is that most of them are attached via flexible toplink connections on the mower. Not sure what the technical term is for that swinging toplink connector but there is little resistance to having the rear of the mower lift (as would happen in a relative sense if the tractor started to rotate on the rear axle) as the linkage is flexible. With a box blade or rake etc that is not the case as the toplink geometry is fixed.

 

[TMcD_in_MI]

I admire and agree with the physics presented by Larry and Tom, but I think it's almost possible to refute point (1) by simple observation of an ordinary tractor pull event using farm tractors. With the pulled sled attached to a normal below-the-axle drawbar, a lot of these tractors lift their front wheels regularly on their way down the track. Some of them rise enough to make me nervous and I can totally believe that a back flip could happen. Does anyone know of a back flip actually happening in a tractor pull?

On the other hand, why do the front wheels rise somewhat, and then no more?

I think that the reason these tractors' front ends seem to rise a bit and then stabilize is because as the tractor rotates backward, the rear end of the rigid drawbar lowers, increasing its angle to the horizontal. Since the towed chain is still pulling horizontally, the angled drawbar has an increased effective lever arm length, which increases the torque (moment) on the drawbar and tractor. (Lever arm = L sin(theta), where L is the length of the drawbar and theta is the angle to the horizontal.) As the angle increases, the lever arm increases, increasing the torque that tries to force the front wheels back down. This gives a nice margin of safety for those guys. At some point, it wouldn't be enough, and over she will go.

I'm thinking a nice long drawbar would be a good safety factor for this reason, as well as what Larry pointed out about it hitting the ground. I suppose that if the tow chain were attached directly to the bottom of the axle, with no drawbar, that would totally eliminate the safety margin I just mentioned and would be really dangerous. Let's not do that.

Tom

 

[SPYDERLK]

From TMcD:I think that the reason these tractors' front ends seem to rise a bit and then stabilize is because as the tractor rotates backward, the rear end of the rigid drawbar lowers, increasing its angle to the horizontal. Since the towed chain is still pulling horizontally, the angled drawbar has an increased effective lever arm length, which increases the torque (moment) on the drawbar and tractor. (Lever arm = L sin(theta), where L is the length of the drawbar and theta is the angle to the horizontal.) As the angle increases, the lever arm increases, increasing the torque that tries to force the front wheels back down. This gives a nice margin of safety for those guys. At some point, it wouldn't be enough, and over she will go.

EXACTLY! Youll see I said that in #4. I just left out the math. If the drawbar is long enuf it will always stop the rollback just in the way you described. It would have to be dialed in carefully, but when done it will leave you with a large safe self limiting arc thru which you can pull at max traction since all weight is on the rears. Minor steering correction to be done with the brakes. We are talking a specific of level ground tho. It could also be dialed in on a case by case basis for mild slopes that are flat.
larry

 

[Tom_Veatch]

...I think that the reason these tractors' front ends seem to rise a bit and then stabilize is because as the tractor rotates backward, the rear end of the rigid drawbar lowers...

Exactly!

That rotation effectively increases the value of "h" in the equation which rapidly increases the critical value of pull necessary to continue the rotation,. If the drawbar is long enough, it contacts the ground, at which point "h" = "r" and the critical pull force become infinite.

 

[mars1952]

Here are some interesting links about ROPS and rollovers.

This is an excellent video. It starts slowly and is long but has a lot of good info.
A Tractor Accident Can Happen to Anyone

Here is an exception to the “No one has died in a rollover when they had a ROPS and seatbelt” statement from the first link. Check out the Kubota.
FARM TRACTOR ROLLOVERS & ACCIDENTS LACK OF FARM TRACTOR ROLLOVER PROTECTION STRUCTURES (ROPS)


Rollover Protection for Farm Tractor Operators

Rollover Protective Structures (ROPS)

Tractor Overturn Hazards

Tractor Rollover on flat paved road.

“It is not the big ones, but the small ones that over turn,” says Rautiainen
Safety solution still goes unused

After reading all these links I have two things to say! Keep your Folding ROPS UP and your seat belt on!
Good Luck, Marshall

 

[Bob_Skurka]

Working back down to reality, it stands to reason that for any given set of conditions (same speed, load, hp, drawbar height etc) that the lighter tractor will be able to rotate itself over the rear axle earlier.

I think it is mistake to presume that in the real world. Most tractors are pretty similar in terms of SPEED and TORQUE. Two tractors, one heavy and one light, of any given size, are going to be limited by the speed & torque applied to the rear wheels. So while a light tractor may lift its front wheels of the ground a 1/2 second before a heavy tractor does, there is no denying the kenetic energy generated by the heavier tractor is going to be much greater than the kenetic energy of the lighter tractor because both tractors will be flipping backwards at roughly the same speed.

Even if both could go over for one set of conditions there would likely be slightly different conditions

Yes, I totally agree that different conditions will make a difference and different characteristics of operation will affect things. My scenario was 2 tractors of similar size/shape but different weights. Both on flat ground, both snagged someting with an implement on their 3pt hitch and began to lift their front wheels. I think in any hypothetical discussion, we have to look at identical conditions to compare different tractors.

 

[SPYDERLK]

Good links! I havnt read them all, but I see an error in Fig5 of Tractor Overturn Hazards:
"Drawbar leverage is another principle of stability/instability related to rear overturns. When a two-wheel drive tractor is pulling a load, its rear tires push against the ground. Simultaneously, the load attached to the tractor is pulling back and down against the forward movement of the tractor. The load is said to be pulling down because the load is resting on the earth's surface. This backward and downward pull results in the rear tires becoming a pivot point, with the load acting as force trying to tip the tractor rearward. An angle of pull is created between the grounds surface and the point of attachment on the tractor. Figure 5 illustrates these points."

Hate when that happens! I will have to specifically address this later as its beautiful out now and Ive gotta go play on the tractor.
larry