TECH GUIDE: Weight Distribution Adjustment



Weight Distribution Hitches (WDHs) are one of the best bits of heavy towing equipment that you can use to improve towing safety. Since I’ve discussed how they work many times before I won’t go over old ground.

However, what I would like to discuss is how to adjust a WDH on a vehicle with self levelling suspension. But before doing that, I’d like to briefly mention how to adjust a WDH with regular spring suspension.

In these vehicles, the ride height depends upon the load that is imposed on each wheel. When the trailer is attached, the vertical load on the towbar (ball weight) causes the rear of the tow vehicle to drop and the front of the vehicle to rise. The fact that the front of the vehicle has risen is an indication that the load on the front axle is less than it was before the trailer was attached.

The aim of the WDH adjustment is to restore the load on the front axle to the same as it was (or more) before the trailer was attached. This is determined by measuring the height of the front wheel arch before the trailer is attached and adjusting the tension on the WDH until the same height (or less) is achieved with the trailer attached.

Now if you try to do this on a vehicle with self levelling air suspension, the computer controlling the suspension will detect that the front suspension is not at the correct height and so will lower the pressure in the front air bags to make the front fall to the correct height. The problem is that you have no idea whether or not the front axle has the correct load on it.

The best way to adjust the WDH that I have found under these circumstances is to program the desired ride height on the vehicle and then turn the engine and accessories off.

Providing the vehicle does not automatically drop the suspension into a ‘park’ or ‘access’ mode when the engine is turned off, you should then be able to adjust the WDH in the traditional manner since the pressure in the suspension air bags should not change while you adjust the WDH tension.

One trap for the unwary though is that some vehicles, like the current Jeep Grand Cherokee, automatically drops the ride height to ‘aero’ mode once vehicle exceeds a pre-determined speed. If the WDH was adjusted when the vehicle was at ‘normal’ height, the load on the front axle will change as the ride height changes, which could cause an undesirable change in handling characteristics.