Towing Resistance VS Height

[Red-Rover]

For the past 10 years I have been towing a 1993 Prowler 30 ft with a height of exactly 12 foot. I am about to pull the trigger on a new 29 foot Silver Back of similar weight and pin weight but a foot taller (13 foot) The dealer says that the improved aerodynamics with a nose cap will make up the difference and I should not notice any difference.
Anybody out there that can tell me of their own experience?

[answerswillvary]

The OP has reposted this topic in the 5th Wheels section, therefore I am closing this thread.

Sandi

[travelnutz]

Traveling on the same roads at the same speed and wind temperature conditions you should not see basically any difference in mpg's. The extra foot in height should be offset by the better aerodynamics of the new front cap design. I'm assuming both towables have the same tire size and inflation as a softer 65 psi inflated tire will squat more and have a larger footprint on the road's surface which relates to friction than the same width tire carrying the same weight inflated to 80 psi. A small difference but it is a difference and does alter rolling resistance numbers.

Air resistance actually relates to surface friction.

A simple basic factor to keep in mind is that at 55-60 mph towing, the average air (wind) resistance in your RV example is about 2 times (2.1) the rolling resistance. Towing the same RV unit at 65-70 mph the air (wind) resistance increases it to 3 to 1 (3.1).

The rolling resistance of a given wheeled vehicle remains basically a constant regardless of the speed you could pull it at on a road. However, the air resistance squares itself (multiplies) as the speed increases beginning at approx just under 50 mph. As in approx 2 times for 55-60 mph and 3 times for the 65-70 range. 80 mph would be cloe to 5 times.

This is why "wind tunnels labs" are used to measure actual air resistance. Aerodynamics play a huge part in altering the averages but even a very sharp needle going thru the air still has ever increasing air (wind) resistance as the velocity increases. The angular sides of the increasing needle diameter is forcing the air molecules to move away from the needle surface and thus creates friction and resistance.

Now add in the air forced compacted air behind the end of a needle or an RV being sucked back to fill the void (space) where the needle or RV had occupied and a sucking action pulling backward is the result. This resistance is friction and then sucking and at very high speeds it even creates extreme heat.

That's why the space shuttles must have extreme heat tiles on it's forward surface for reentry and even they glow bright red from the extreme heat that results. They want the sucking action created as it helps the shuttle to slow naturally but you don't want that sucking action pulling your RV unit backwards. The faster, the higher the sucking force is.

[PawPaw_n_Gram]

Red-Rover wrote:
The dealer says that the improved aerodynamics with a nose cap will make up the difference and I should not notice any difference.

BS

You will notice a difference. Whether it will be a slightly different way the trailer pulls in the wind, or up to 0.5 mpg decrease in fuel mileage.

But don't let that stop you.

10 years is a world of difference in the RV world. The newer rig will be much more efficient in may ways.

[Red-Rover]

My mistake, you are correct, posted in wrong place. Will repost in 5th wheel.
Thanks for the replys!

[beemerphile1]

The extra foot won't make a noticeable difference front cap or not. You are dragging a house down the road. A little taller or shorter house is still a house.

I think you accidentally posted in the wrong section. This is the RV Parks section.

[2-MTnesters]

I would tend to agree with the sales man. The aerodynamics have in fact come a VERY long way in the last 20 years. You are pulling a 20 year old camper with virtually no aerodynamic qualities while the Silverbacks nose is very aerodynamic. This along with the smooth fiberglass exterior walls creates much less wind resistance. Keep in mind that the height is also measured to the top of the tallest point which in many cases is the air conditioner and NOT the actual roof of the coach. As far as drag, you will always be dealing with the "Drag Coefficient" as long as you are towing something with a large flat-walled rear-end. This would include the Prowler or the Silverback. If the Silverback is the camper that you want I would not hesitate to get it if I were you.

[magnusfide]

Red-Rover wrote:
The dealer says that the improved aerodynamics with a nose cap will make up the difference and I should not notice any difference.
Anybody out there that can tell me of their own experience?

Dealers will tell you anything to get your money. You're best bet is to go to a local teacher/professor of Physics and give him the outside dimensions and the weight of both your old rig and the new one. Ask him about the calculations for wind resistance and the difference between the two.
Hint: larger surface area always creates more drag. Modifications such as a nose cap do not always provide a statistically significant difference to the drag coefficient.

Personally I would opt for a shorter rig. It will not only provide less drag but will also allow more clearance for bridges. There was a great video posted here at the forum some time ago with footage of rvs passing under a bridge and losing their acs, even smashing their front caps. Maybe someone here can locate it for your enjoyment.