Weight distribution

Today I’ve got to talk about towing.  I’m sorry if the mere thought causes your eyes to glaze over.  There’s just too much dangerous misinformation out on the Internet, even coming from RV salespeople and people who should know better, and it’s going to get people killed.

If you don’t read Airstream Life, you’ve missed out on a great series about towing issues that is authored by Andy Thomson of Can-Am RV.  Andy is a second-generation Airstream dealer who specializes in setting up trailer hitches for best performance, and he does a lot of testing with his own vehicles to figure out what works.  The series he’s writing for the magazine has gradually built up a case for optimal hitching, which involves a lot more than just “buy a bigger truck.”  He’s gotten into the details like the overhang and angle of the tow ball, sway control, aerodynamics, engine power, suspension components, steering, and much more.  Some of what he says is controversial but I think all of his points are very important to consider.

So if you really want to understand towing at the engineering level (and get past all the ridiculous pseudo-knowledge that you’ve probably heard), you should gather up as many of the back issues as you can find and read all the articles.  (The series started in the Summer 2010 issue.)

I can’t begin to reach Andy’s expertise level, but I can talk about one simple piece of the towing puzzle today: weight distribution.  Weight distribution is the idea that the tongue weight of the trailer should be evenly distributed across both axles of the tow vehicle.

It’s horrifying that so many people don’t understand this concept, because it’s absolutely crucial when towing a trailer with a heavy tongue weight. By “heavy” I mean any trailer with over 500 pounds or so on the tongue.  It doesn’t matter how big your truck is.  It doesn’t matter if you “can hardly feel the Airstream,” or if you “never had a problem.”  You need to get this right.

Why?  Because one day it will make a difference.  That will be the day that you have to do a panic stop, or a sudden avoidance maneuver on the highway, or when the wind is blowing 30 knots off your starboard bow, or when you accidentally let the trailer drop a wheel off the edge of the pavement … and I could go on further with reasons why.  One day, you’ll have to do ask your rig to do something extraordinary, and you’ll want it to behave.

Without proper weight distribution, that rig you thought was so great towing straight down the road might do something really unexpected.  Perhaps the rear brakes will lock up prematurely in a hard stop.  You might not be able to control a sway, or stay on the road in a turn.  You might feel the trailer “wag the dog.”  Quite likely you’ll have an accident and afterward only know that something bad happened and you’re not sure why.

The catch is that you can’t tell you’ve got a problem until one of three things happens:

  1. You weigh the truck and trailer combination, and then the truck separately, to see how weight is being distributed across the axles.
  2. You take the rig on a closed course and drive it to the limits.
  3. You crash.

I prefer option #1.

The other day I saw some CAT scale readings from a fellow who was very confident about his truck and Airstream setup.  By common knowledge, he was all set: big truck, no problem.  But the CAT scale told a different story.  When he hitched up his trailer, the rear axle of his truck got 1,900 lbs heavier.  The front axle got 760 pounds lighter.  That’s very bad but not unexpected.  It means the weight of the trailer’s tongue, pushing down on the tow ball at the back of the truck, was actually lifting the front of the truck.

Imagine a teeter-totter.  The Airstream is pushing on one end, the rear axle of the truck is the center (fulcrum) of the teeter-totter, and the front axle is going up. When you lighten the front end that much, the steering geometry is affected.  Now you’ve got understeer.  You turn the steering wheel, but the truck doesn’t turn like it should.  It’s like driving on ice.  It’s insidious because you might not notice until you have to make an emergency maneuver at speed.

A light front axle also affects the braking adversely, giving the front tires less ability to grip the road and slow you down.  So bad steering, bad braking—you can see how this is really undesirable.

With weight distribution applied on the same truck and trailer, the problem appears to the casual observer to be corrected.  With the weight bars in place, the CAT scale shows that the rear axle is now only 1,000 pounds heavier than the axle was without the trailer (which is well under the manufacturer’s axle rating), and the front axle is now lightened by only 100 pounds.   Everything meets the manufacturer’s specifications.

That will work, but it’s far from optimal.  The truck started with a nice 49/51% front/rear weight distribution.  With the Airstream and weight distribution, the ratio has gone to 44/56%, which is not so great.  It will drive OK under un-challenging circumstances, but it’s not set up well to deal with a bad day.  Imagine a sports car with a huge lump of concrete in the trunk.  Go around a sharp corner, and what happens?  The sports car spins out.  The truck with poor weight distribution may be more prone to the same thing.  Unfortunately, as I mentioned above, the driver may not “feel” anything adverse until it’s too late.

So how do you fix this?  It’s actually quite easy to even out the load on the axles.  The exact mechanism depends on the hitch system you’re using, so it may be a matter of just going down a link on the weight bar chains (on a Reese), or tightening the strut jacks (on a Hensley), or angling the tow ball rearward, or shortening the receiver to get the ball closer to the rear axle, or any combination of these things.  The goal is the same: get those axles back to as close to 50/50 weight as you can, with the trailer hitched up.

If you find that you’ve tried everything and can’t get the weight distribution any better, don’t give up.  Hensleys are not great at weight distribution, but regardless of the type of hitch you use, check with someone who knows hitches.  Sometimes the hitch receiver on your truck will flex so much that it acts like a spring, bending rather than distributing the weight stress.  Reinforcement or repair may be necessary.

By the way, if you followed the manufacturer’s instructions when setting up your hitch, and used the old technique of measuring the corners of the truck to see how much each dropped, you still need to go to a truck scale.  That method is really obsolete today, with modern vehicles that have different suspensions front and rear, air bags, or even full air suspensions.  At best, it’s a rough estimate.  For about $10 at a CAT scale you can get the real story.

If you didn’t know any of this, don’t feel like a noob.  I’ve talked to RV salespeople who send people out the door every day with new rigs, who don’t understand the basic principles of weight distribution.  I just hope I’ve impressed upon you that most Airstream owners need to check their weight distribution on a truck scale, because what you don’t know can definitely hurt you.

About the Author

Editor & Publisher of Airstream Life magazine