Towing in the Wind

After three days with the wind in Kansas and eastern Colorado blowing and gusting against the left side of our trailer as we traveled west gave me time to think about towing against the wind.

When we headed west from Abilene Kansas the wind was a moderate 16 miles per hour from the south that gradually increased to 20 mph. In Ellis, Kansas we debated sitting an extra night hoping the wind would abate then leaving very early in the morning and driving past our next stop which demanded a 3 PM check in. I looked at what others considered as their maximum safe wind speed. A resident of Kansas told us that the wind often blew this fast. Thus we opted not to leave in the morning for the half-way point after rechecking the wind forecast several times.

The last forecast for wind when we left Ellis was for steady winds in the mid 20 mile per hour range with gusts up to 30 miles per hour. When we arrived at our Burlington, Colorado at 3 PM the wind was 32 miles per hour with gusts up to 50 miles per hour. Had I known the wind would reach those speeds I would have found a place to sit out the wind. We left early the next morning with the wind, still from the south, at a modest 16 miles per hour, knowing that it would increase that morning and might be less as we neared Denver.

Know Your Comfort Speed

In light and variable winds (under 7 miles per hour) the trailer starts to wiggle in when the air speeding past it exceeds about 65 miles per hour. Thus I tend to drive about 63 miles per hour unless a tail wind is pushing us along. Lori drives a bit slower.

Larger vehicles often pass us. Some of them show significant wiggle even in modest winds.

Towing a Sail

Our trailer is 8 feet wide, 11 feet tall and 26 feet long. Towing down a hill the trailer acts like an 80 square foot sail gently applying the brakes keeping our speed steady. Without it, the truck would accelerate going down hill as its effective frontal surface area is less than half of the trailer’s and it has a slipperier shape.

With a side wind, the trailer attempts to turn with the wind. Because the side area of the trailer (~250 square feet) is three times larger than the side area of the truck (~80 square feet) the force of the wind on the trailer is three times larger than that on the truck, resulting in a force on the hitch by the trailer pivoting the truck into the wind. Coming across Kansas the wind was constantly on my left, turning the trailer slightly to my right, pressing the hitch to the right, and turning the truck to the left. Thus to keep the truck and trailer in the lane I had to continually turn the steering wheel slightly to the right.

Tractor trailers have a different hitch placement thus their trailers turned into the wind. I could look down the side of a tractor trailer and see it turned slightly.

Your Mileage Will Vary

The work done by the engine on a flat section of road has to overcome three types of loads. Steady loads that are independent of travel that keep the engine turning, power instrumentation and lighting, and keep the occupants comfortable. Rolling resistance mostly depends on distance traveled and not how fast one travels. Air resistance is proportional to the square of speed of the air hitting the surface of the vehicle and trailer perpendicular to the direction of the combined air speed. At highway speeds on flat roads in modern vehicles air resistance typically surpasses the other two loads.

Accelerating, climbing a hill, and turning also add loads, but hopefully they are transitory.

When driving directly into the wind the speed of the wind and the road speed add. Thus a 20 mph head wind at 60 mph is like driving at 80 mph on a calm day. At 80 mph of air speed the air resistance (which is proportional to air speed squared) nearly doubles that at 60 mph!

A tail wind decreases the speed of the air flowing over the vehicle thus a 20 mph tail wind yields a mere 40 mph of net air speed, decreasing air resistance by about half.

That 20 mph wind out of the south in Kansas combined with my road speed heading west at 60 mph yielding an effective air speed of 63 mph. Those 30 mph gusts felt like driving at 67 mph. Plus I had to continually steer the tow vehicle to stay within the lane adding rolling resistance.

Over Passes and Under Passes

On a calm day, an overpass is an opportunity to see a little further.

On a windy day, the wind is typically a little stronger atop an overpass. Thus when driving into the wind or a cross wind I slowed down as the road rose over each overpass or hilltop.

Underpasses funnel the wind when it comes down the highway or block side winds. A blocked side wind may feel like being hit by a gust of wind from the opposite direction.

With a 20 mph side wind my gentle turn away from the wind suddenly became overcompensation while driving through an underpass. Again I learned to slow down approaching each underpass.

Getting Passed

Getting passed by a larger vehicle (Busses and Tractor Trailers) puts many of these factors into play within a few seconds.

Surrounding each vehicle is a shock wave related to the size of the vehicle and its speed. A boat moving through the water also produces a shock wave or wake that can be seen spreading out from the its bow. Staying to the extreme right edge of the lane to maximize the separation from a passing larger vehicle minimizes the effect of an unseen airborne shock wave and maximizes the space between the vehicle should either be affected by the shock wave. On a calm day or even with a head or tail wind, getting passed by a large vehicle has little affect on the trailer follows our pickup.

However with a side wind, as a larger vehicle passes our pickup truck and trailer, it momentarily shields the wind and its gusts, slowly progressing up the length of the trailer and then the tow vehicle. When my trailer is effectively shielded from the wind my normal compensation twists it to the right. Because the side area of the pickup is smaller its reaction to the shielding from the passing truck from a side wind is also smaller. Then as the passing vehicle no longer shields the trailer the wind resumes buffeting the large side of the trailer twisting it right then left.

Before I learned through trial and error how to compensate for momentary shielding by a passing vehicle the trailer would twist back and forth causing the pickup to sway in opposite directions.

During my three days of towing a trailer with a stiff side wind I learned to slow down before a passing truck reached the trailer then very gently accelerate back to my normal speed as the momentary shielding from the side wind progressed up the length of my trailer and truck.

Gentle acceleration while being passed provides an additional force on the trailer straightening it, keeping it at the edge of the lane.