When a tire fails–Typically the first thing you’ll notice is a fast, regular, low pitched drumming noise coming from the rear of your tow vehicle. A quick glance in the rear will verify that one side of the trailer is bumping up and down with the same frequency as the noise you are hearing. Once you’ve done that, you’ve got maybe 20 seconds to slow down as the tire carcass has cracked and allowed an air bubble to form between the tread and the carcass of the tire. If you don’t slow down, stop and change the tire, the tread will separate from the tire carcass, flop around and tear the fender on that side of the trailer, regardless of whether it is fiberglass or metal.

How to avoid tire failure–Tires with a severe case of dry rot are prone to blow outs, even used gently. To make bad matters worse, some dry rot is visible—very narrow random cracks in the surface of your tires. But that’s just the tip of the iceberg as you can’t very well tell how deep the cracks are, and hence, how dangerous they are. Remember it’s a crack in the carcass that allows an air bubble to form between the tire carcass and the tire tread. So dry rot is really something you want to avoid.

While most people understand that unused tires eventually develop a severe case of dry rot, not everybody understands that infrequently used tires can also dry rot pretty quickly. Raising your tires off the ground or putting wood blocks between your tires and asphalt or concrete may  low down dry rot but it won’t prevent it entirely.

Why sound tires sometimes fail–The bottom line is that as tires and the air inside them gets hotter and hotter, the harder that air searches for a weak spot. So what makes tires get hotter? Black asphalt and 100 plus degree temperatures are a good start. Add trailers loaded near or beyond the design weight, rough roads and under-inflated tires and you’ve got a pretty good recipe for trouble.

What’s so bad about under-inflation? When tires roll, the sidewalls flex and the flimsier the sidewall, the more they flex. That’s why special trailer (STXXX) tires are recommended for trailers instead of passenger tires (PXXX).

The special trailer tires have much stronger sidewalls–that’s why they are so expensive. Regardless of which tires you use, the heavier the load at a given level of inflation, the more the sidewalls flex. Likewise, the heavier the load, the rougher the road and the faster you drive with the same initial inflation and outside temperature, the more the tires flex and the hotter they get—until one of those weak spots gives way.

Likewise the lighter the load and the slower you drive, the less the tires flex and the cooler they run for a given level of initial inflation and pavement temperature. That’s important, because according to tire manufacturers, trailers with ST tires should not be towed faster than 65 mph. However, our experience suggests that load trumps speed. Specifically, even though we install tires with a combined carrying capacity of 3500 lbs on both our one bike and two bike trailers, no one has yet reported a blowout with one of our one bike trailers. The bottom line is that while the owners of the two bike trailers often carry more than a ton of bikes and “stuff”, the owners of one bike trailers rarely carry more than half that.

Clearly, towing a fully loaded two bike trailer with less than its 50 lbs maximum rated cold (initial) inflation at 85 mph across South Dakota on a 105 plus degree day is a prescription for catastrophic tire failure.

Seven Steps to Blow-Out-Free Trailering

1. If your tires are over three years old and your trailer stays parked for weeks at a time,replace them no matter how good they look, how much tread they have or how few miles you’ve put on them. It’s TIME, not miles, that count.
2. If you park your trailer for more than 30 days, put wood blocks between the tires and the concrete or asphalt.
3. If your trailer has been parked for more than a month, check the tire pressure before towing it more than five miles.
4. Adjust your initial inflation pressure to match your load and the outside temperature. 35lbs for an empty one bike trailer on a cool day to 45 lbs when its fully loaded on a hotday. For a two bike trailer, 40 to 50 lbs for the two extremes makes more sense.
5. Adjust your speed to match the load in your trailer—at least 10-15 mph slower for twobig Harleys than one dirt bike.
6. Adjust your speed to match the roughness of the road and the outside temperature–slowdown if the road is rough or if it is extremely hot. Slow down even more if it is both.
7. Every time you stop, feel the treads of your trailer tires and your tow vehicle tires; if yourtrailer tires are hotter than your tow vehicle tires, slow down.

Yes, but what about the fact that there’s always a tow vehicle pushing the air out of the way in front of your bike hauler?  Yep, that cuts down on the wind resistance your trailer meets—some, but not as much as you might think.  Just remember  how much the wind buffets your bike when you tow it on an open trailer.  A trailer with a smaller frontal area will still meet less wind resistance than one with a larger frontal area.

Where can you get the drag coefficients for different trailers?  Don’t hold your breath waiting on trailer manufacturers to supply them.  And unless you work for a well known racing team, you probably don’t have access to a wind tunnel.  So where does that leave you?  Take a look at these published drag coefficients I dug up.

Shape                                  Drag Coefficient

Round nosed bullet                       .29

Sphere                                                 .30

2003 H2 Hummer                          .57

18 wheeler with fairing                  .65

Prism (triangle with depth)       1.14

Rounded cube                                1.20

A smooth brick                              2.1

Square-corned cube                    2.2

So how can you use this information?  Stand 100 feet back from the front of the trailer you already have or are thinking about buying.  If it’s seven feet wide by seven feet tall, it may be called a low hauler or low profile motorcycle trailer, but its frontal area is still forty nine square feet and it has a drag coefficient near the brick end of the scale.

Yeah, but what if its nose is not flat?  When it comes to frontal area, shape doesn’t matter, but remember the shape of the nose primarily determines the drag coefficient of trailers.  And if the trailer in question has a flat nose, it’s just like a big shiny not-so-smooth brick (too many corners, rivets, handles, etc.).   So you may as well plan on keeping your fuel card on the console next to you because you’ll need it too often to bother putting it away.

What about V-noses?  Hmm, a v-nose sounds a lot like a prism shape to me and that suggests a drag coefficient somewhere in the 1.1 to 1.3 range.  What if it has a downward sloping nose?  Suppose forty percent of the height of the trailer is sloped.  Then the top forty percent of the trailer will have a drag coefficient in the 1.1 to 1.3 range and the bottom sixty percent will still be greater than 2.0

A bulbous add-on to the top half of the front of a trailer has much the same impact as a sloped nose.  The top half has a reasonably conservative drag coefficient—probably a little less than 1.0, but the bottom half still has a coefficient greater than 2.0

Before you go, take another look at the scale above.  Notice that a round-nosed bullet has the lowest drag coefficient (.29) of all the things listed.  There’s currently only one widely available, purpose-built motorcycle hauler shaped much like a round-nosed bullet—Ironhorse Motorcycle Carriers.  No wonder their owners are so happy with their gas mileage and keep calling us saying—“you don’t even know it’s back there”.