Oscillation

I just read Forrest McClure’s recent blog post on the Airstream Life Community Page about his driving experience on I-25 near Denver.   Forrest and Patrice spent a couple of pleasant days with us in Cherry Creek State Park, which is one of our favorite western stops.

I had not thought to mention the undulating stretch of pavement on I-25 in my first blog from Cherry Creek, but when Forrest and I got talking he brought it up and I was interested to hear how similar our experiences were.   I want to talk about it further because it brings up an important safety issue for trailerites, and it gives me the opportunity to debunk some “urban legends” about towing.

As Forrest describes in his blog, the effect of undulating pavement (whether bumps, potholes, or even railroad crossings) is to make the trailer oscillate up and down at the tongue.   The faster you go, the worse the effect, so if you read no further than this, take away one simple message: SLOW DOWN when the road gets funky.

When the trailer tongue lifts up, it unloads the weight on the rear axle of the tow vehicle. This removes part or all of the ability of the tow vehicle to control the trailer.

Let me repeat that, because it’s important to appreciate: an unloaded rear axle means you are no longer in control of the trailer.   It does not matter how heavy your truck is.   It does not matter what hitch you are using.   It does not matter that you are the world’s best driver. You are now a passenger.   The trailer, even a lightweight one, can easily shove your and your truck off the road in that brief “unloaded” moment, because when only the front wheels are in firm contact with the ground, you cannot expect to command the trailer.

The degree of this loss of control depends on a lot of factors.   Speed has a lot to do with it, because the harder you bounce the trailer, the more it is going to lift your rear end.   Independently-suspended vehicles have a slight advantage over solid axles, because they can keep their wheels planted more firmly in adverse handling conditions.

Heavier vehicles are harder to lift, but don’t think that’s going to save you. A typical Airstream with 800 lbs of tongue weight can easily bounce upward (on a good bump) with a force two or three times the tongue weight.   In other words, imagine lifting up on the rear bumper of your truck with 2400 lbs of force.   It may not come off the ground, but it sure it going to lose firm contact with the ground, with commensurate loss of control.   You might as well be driving on ice.   This is why I do not agree with the common belief that “heavier is better.”   It’s only partly true, and induces overconfidence.    There’s much more to safe trailering than just using a big truck.

I remember hearing an anecdote from an Airstream dealer about this phenomenon.   He was towing a 1960s-era Airstream which was unduly light in the front (due to some parts being removed), using a heavy-duty pickup truck. Going around a curve, the trailer simply pushed his truck right off the road.   Why? Because having inadequate tongue weight makes it easier for the trailer to lift the rear end of the truck.   The trailer weighed about 3000 lbs, half of the weight of the truck, and yet in his words, “It pushed that truck around like it was a toy.” An oscillating trailer will cause exactly the same result.

Fortunately, both Forrest and I were traveling in a straight line, and both of us had the presence of mind to slow down. From our discussion, it sounded like Forrest got the worst of it, because he had some horizontal sway (side to side) and we did not.   He also may have encountered the bumps at a higher speed, since I saw them coming and he didn’t.   But as he pointed out, you can’t expect your equipment to save you in this situation.   “Sway control” that is typically available on hitches is effective only on horizontal sway, not oscillation (up and down).   You’ve got to slow down.

On the other hand, there is something to be said for a really good vehicle suspension.   When oscillation occurs, you want it to stop as quickly as possible.   The damping effect of the tow vehicle’s suspension is crucial here. The trailer will keep bouncing you for a while after encountering the bump, and that means the rear axle will get loaded and unloaded repeatedly.   Each rear-axle unloading event is an opportunity for the trailer to push you, and each front-axle unloading event will give you a uncomfortable moment of understeer.   The sooner it stops, the happier you’ll be.

Again, here’s where heavy truck drivers can become overconfident.   Many trucks have very stiff suspensions, which resist compression even in an event such as we’ve described.   Since the driver doesn’t feel the truck bouncing, he/she may assume that the truck is unaffected.   Thus, the common statement that, “The truck doesn’t even know the trailer is back there.”

Actually, it’s the driver who doesn’t know what’s going on.   A stiff truck suspension will resist bouncing, definitely, but the weight-loading effect caused by the trailer hitting a bump is still going to occur.   Even if the truck doesn’t dip and bob, the weight distribution to the axles is still changing dynamically as long as the trailer is oscillating.   You just can’t see it, or feel it, in the seat of your pants.

So in this case, what happens?   Most of the time, nothing happens, the driver is clueless, and the truck and trailer go on their merry way.   Once in a while, the grip of the rear axle is too light to continue controlling the trailer, and “suddenly without warning we went off the road!”

Personally, I’d rather be well-warned in advance that the rear axle is being unloaded and that I’m pressing the limits of tire adhesion, rather than be completely unaware and surprised when the limits are exceeded.   If you tow with a vehicle that has a numb rear suspension, keep in mind that it’s not telling you something. Did I mention to “slow down”?

Of course, traveling on an uneven surface, wet or snowy surface, on a curve, or with improper weight distribution, will all exacerbate the problem.   If you want to be safe (and who doesn’t?), start with a properly set-up hitch and weight distribution, then respect the conditions you’re in.

Despite driving radically different vehicles, both Forrest and I felt the effect of undulating concrete very dramatically.   It’s impossible to say scientifically which tow rig performed better, but in any case our mutual experience underscores the need to respect the road.   There’s no substitute for driver caution.

Comments

  1. Tom Palesch says

    Safety is the “First, Second and Third” consideration when in control (?) or a truck and trailer. You make good points. Now I know why when crossing “bumps” in the highway I see evenly spaced patches of tire rubber similar to those we tried to lay down with our father’s car when teenagers. The rubber spots come from the unsuspended spinning tires when they retouch the pavement.

    Your point is confirmed!

  2. says

    Maybe a good example of the kind of wavy road you describe is the first mile or so of the westboound CA highway from Salton City to Borrego Springs. It in turn reminds me of the many miles just like it on the Alaska Highway as a result of frost heaves from the winter weather. (highly recommend that trip to all!) That in turn requires many miles of road repair each summer. Job security for Alaska DOT!

    Rich, hope to see you again in Palm Canyon campground this winter. Safe travels. Hopefully Anza Borrego State Park is still open then.

  3. Mike Young says

    As bad as I-25 is, I-76 is infinitely worse, IMHO. Did you experience I-76 in your trip to Denver?

  4. says

    We had a few intense moments with this on our trip east on 40 through western AZ. I was driving on a straight away and felt I lost control for a split second. It came out of nowhere and made no sense to me at the time. But…the pavement was as bumpy as could be so now it makes total sense. It scared the crap outta me. We took it easy the rest of the day on that stretch of road thinking it must have been a big gust of wind. More likely it was what you have described here.

    Thanks for the info Rich. It will stick in my head and will keep both of us a lot safer while driving having read this post.

  5. Jay and Cherie Guerin says

    Rich,

    Was this in the old 2 lane section nearer Loveland, or the newer 3 lane section on south. I haven’t encountered the problem. But on the 3 lane section I stay in the middle lane.

    We detest I-25, but once in a while have to travel that way.

  6. says

    I asked Forrest, since I wasn’t sure. He said, “I can only narrow it down to being somewhere between Longmont and Northglenn. I think it may be right around 168 Ave because shortly after it happened we pulled off into a shopping center at 144th Ave.”

  7. peter ferguson says

    unlike auto tires —trailer tires [ST] are rated for 65 mph. many motorhome tires [Michelin inclulded] are rated maximum speed of 75 mph. even many Michelin truck tires are rated maximun of 65 mph with special exceptions.