by rgambord » Wed Feb 18, 2015 4:19 pm
If you really want to know what you can tow, you need to calculate it with some readily available specs regarding your vehicle. You need to know the peak torque, and your gear ratios. Here's a worked example for my subaru crosstrek:
Note: you can use google calculator or wolframalpha.com to do all the unit conversions for you.
Peak engine torque is 197 Newton meters @ 4,200 rpm
1st gear ratio is 3.545
Final drive ratio is 4.444
Tire radius (diam) is 14.5 inches (29 inches)
The amount of force my car can apply at peak is:
197 Newton meters (peak engine torque) * 3.545 (1st gear ratio) * 4.444 (final drive ratio) / 14.5 inches (tire radius) = 8425 Newtons @ 4,200 rpm
The speed at which I will be able to attain this torque is:
4200 rpm / 3.545 (1st gear) / 4.444 (final drive ratio) * 29*pi inches (tire circumference) = 23 mph
So, I can produce 8400 newton at 23 mph in first gear. At a 15 degree slope (~25% grade), 8400 newtons is 3300 kg, or 7,293 lb
Subtract the weight of the car with passengers ~ 3500 lb, and I can tow 3,793 lb at 23 mph in first gear up a 25% grade.
This is the absolute upper limit of what my car can do on a fairly steep incline, with a running start. I'd be able to do steeper inclines for short distances, by accelerating before the hill. As you can see, engine capacity is not much of a factor when it comes to towing.
What really matters is braking ability, which is heavily dependent on the heat dissipation abilities of the brakes, road surface, tire rubber material, and tire pressures. Lower pressures = more surface area in contact with the road = more static friction when you apply the brakes. Trailer brakes are also very important. On inclines and straightaways, this mainly affects braking distance, but it's a possibility that you would be unable to decelerate a vehicle/trailer combo on a sufficiently steep decline, causing a runaway scenario.