About 10 years ago, a friend of mine needed to slide a shed away from a lake’s increasing waterline. I tried to use my 4×4 Jeep Wrangler, but it couldn’t do the job. An hour later another friend showed up with a Ford Excursion and pulled it without a problem. All of my friends are engineers and said. “Well, it’s because we needed a longer wheelbase”. Why does that matter? Isn’t it about mass?
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Towing capacity calculations including handling and braking, not just the ability to support added weight. The extra wheelbase helps with stability.
A longer wheelbase will make the car or truck more stable on the highway, but it has nothing to do with how hard it can pull.
In first gear you’re either limited by engine power or tire grip. Tire grip depends on the surface that the vehicle is on, the tires themselves and the mass of the vehicle.
As a rule of thumb, on dry tarmac and with regular road tires, the maximum pulling force a vehicle can generate is roughly equivalent to the weight of the vehicle (assuming it has 4-wheel drive). So a 4000 lb Wrangler would be able to apply 4000 lb of force to the shed, and a 7000 lb Excursion would be able to apply about 7000 lb of force before the tires lose traction and start spinning.
On soft ground those values will be lower overall, but the Excursion would still be able to pull almost twice as much as the Wrangler.
Why couldn’t your Jeep pull it? Was it a traction or torque issue?
Wheelbase doesn’t necessairly help with either in a stump pulling scenario.
The longer wheelbase added some downward torque (force) to the front of the vehicle so the back wheels would move in a more forward direction. The short wheelbase of a jeep will slightly lift the front of the jeep and give the back wheels less weight to support good traction (think of a wheelie). Honesty, you could have sat a couple of heavy weights on the hood and done the same thing (I’m guessing). Or just get a winch 🙂
Wrangler is quite short. When you pull something, the reaction to your pull will make the front of the vehicle to get lighter. And the weight shifts to the rear. So your 4X4 is now 2 wheels in the front too light and the 2 wheel in the back pressed to sink in the mud. If it was on tarmac, it wouldn’t be such a problem as long as you lock all differentials.
A longer vehicle will suffer less the nose up reaction, thus spreading the pull force more evenly between wheels. This gives you more traction.
In this case basic theory was misleading, the problem is not about torque or force or weight, but vehicle stability.
Maybe I’m wrong, this is the reason that seems more logical to me but I want at the lake with you so I don’t know all the factors.
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