I've had a few requests and questions about using the NXT to pull a lot of weight, so I decided to do a little study on trailer towing, using a single NXT set, which resulted in this NXT Trailer Pull project. I got it to pull about 45 lbs (20 kg), which is about 30 times the weight of the vehicle, and also over 20 lbs (9 kg) up a 10% slope.
To tow a lot of weight, you need three basic things:
1. Structural strength to support the load
2. High torque to the drive wheels
So I attemted to address all three of these, within the limits of a single NXT set. For strength, the trailer is built extremely strong, with all the load taken by beams in compression (no load on any of the pegs), and the axles are supported on both sides by the shortest spans possible to reduce bending. It is quite strong, you could probably stand on the thing, but I didn't try that...
For torque at the drive wheels, I used all 3 motors geared down 3:1 to a common axle, nothing real exotic here, and this turned out to be plenty to work with. You could obviously go more aggressive with the gearing here, but it was not the limiting factor in my tests.
Traction is the trickiest component of towing a lot of weight (on an unpowered trailer). As many of you probably already know, if you simply try to tow a lot of weight using something like a string from the vehicle to the trailer, the drive wheels will just slip on the surface since there is not enough weight over them to get any traction. On a real trailer, the rigid tongue is designed to transmit part of the load weight to the rear axle of the tow vehicle (10% is recommended, as I remember from my days towing a car carrier), which gives you the traction you need. To demonstrate this, this NXT project includes two different hitches, a flexible one and a rigid one, so you can experiment and see the effect of the weight transfer on the traction.
With this design, the limiting factor then becomes how much weight the rear axle on the tow vehicle can hold. Here, the tow vehicle axles are not supported on both sides, so they will eventually bend quite a bit, to the point where I was not comfortable trying more weight, although there was enough motor torque and strength in the trailer to handle it. With extra parts to add more support here, or more axles, or a different placement of the load (e.g. "fifth wheel" configuration) who knows how far you could go...
Here is a video of the project and some test runs: