Minimize Resistance, Maximize Fuel Economy, Part II

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June 23, 2008

In last week’s post (Minimize Resistance, Maximize Fuel Economy) I talked about how tires and front end alignment can affect fuel economy. In that post, I talked a lot about the importance of toe-in alignment, and mentioned the importance of other settings as well.

While it’s true that the other angles will have little effect on rolling resistance, incorrect camber will cause the tires to wear more quickly. Camber is the inward or outward lean of the top of the tire; inward lean is negative camber; outward is positive.

But what a lot of RVers don’t know is that the rear axle, or axles, can also be out of alignment-and if that’s the case, this problem can increase resistance and tire wear, reduce fuel economy and cause handling issues. The axle may be bent, tweaked or not installed properly from the factory-the end result being rear wheel steering.

Have you ever been following a vehicle that looks like it’s going down the road a little sideways? We call that “dog tracking.” So you keep the wheel turned to compensate for it, and you create scrub. At the same time, you’re reducing the aerodynamic efficiency, because more surface area is exposed to wind resistance.

Dog tracking isn’t what I’d call common; I’d say we have to do a rear wheel alignment on one out of 20 coaches, and most of the time they’re off by just a little bit. We perform what is called “thrust angle alignment”, where we put gauges on all four wheels, and set the front wheels in relation to the rear axle. If you only align the front, it may not be true to the rear axle. How much affect rear axle misalignment has on the coach depends on the wheelbase; for example, if the rear axle is off by 1/8-inch, the difference in angle will be magnified more over 200 inches, than, say, 150 inches. Here are a couple of examples that illustrate axle misalignment and rear wheel steering:

Trailers can also be affected by incorrect axle alignment. When we align a trailer, we line up the axles to the hitch point, whether that’s the hitch ball (travel trailer) or king pin (fifth wheel). That way, the trailer tracks better, and there’s no scrub-so now you’re saving fuel and the trailer tires will last longer.

Now while we’ve been talking about resistance with regard to alignment, there are other factors that increase resistance and adversely affect fuel economy. For example, improving the airflow into and out of the engine is reducing resistance, and usually results in some fuel economy improvement. Banks offers a number of systems designed to increase power and fuel economy in both gasoline and diesel trucks and motorhomes. Other companies also offer air intakes including AFE and K&N, and there are a number of companies that make performance, low-restriction mufflers such as Flowmaster, Gibson and Magnaflow.

It may sound strange to talk about improving the aerodynamics of a motorhome or large trailer, but we have recently begun experimenting with a product called air tabs, which is supposed to reduce air turbulence (and therefore resistance) and improve fuel economy. Basically, they are small vortex generators that reduce wind drag behind the coach as it travels down the road at 40 mph or faster. The company claims its customers are reporting a 2-4% increase in fuel economy, annually.

We recently installed Airtabs on our show truck and trailer, and while we haven’t measured fuel economy yet, you can definitely tell a difference in how it drives; it’s definitely more stable.

There is no “magic bullet” for improving fuel economy in your rig-but a combination of the correct tires and inflation pressure, front/rear alignment, driving style, engine enhancements and even aerodynamic improvements can add up to substantial savings each year.

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  3. Hello Gene,

    Vibration problems can be very challenging. First off the vibration sounds like it could be caused by an out of round condition coming from the wheels and or tires. Normally a out of balance tire wheel assembly will shake worse the faster you go. I would suspect an out of round wheel or wheels. The original equipment Workhorse wheels are a stamped out steel and some of the ones coming out of Brazil were not very round.
    Did they check the rims and tires for run out?
    The other thing it could be is a drivetrain vibration. Driveline vibrations have a different feel to them than tire & wheel vibrations. It is more central in nature. Usually if we are having problems determining where it is coming from we will run the vehicle up to speed while it is on jack stands and see if the vibration goes away. If it does not we may pull the wheels off and run it again. Should the vibration remain then we would start checking drivelines for alignment and balance. If it does go away it is usually caused by the out of round tir/wheel assembly vibrating on the pavement.
    I have had torque convertors and or engine vibrations as well. These can be felt without moving the vehicle by putting the vehicle in park & slowing increasing the RPM and paying attention to see if you feel the same vibration you felt on the highway. Let’s say at 2500 RPM you felt a vibration while revving it in park. I would want to know if that felt the same on the road at 40 or 45. If it did that could
    mean your problem would be in your engine or torque convertor. There is a lot more we could talk about. I hope this is helpful.

  4. Gene Morrow

    Further to my earlier memo, all the shocks were replaced as well.

  5. Gene Morrow

    I have a vibration question. I own a Fleetwood Storm 2000 on a Workhorse chassis. I have had a vibration problem for over three years. The alignment has been checked, all six tires have been replaced, the air bags have been replaced (they are only on the front), and the tires balanced. The vibration started at around 40-45mph. It seems to have moved to 45-50mph. It goes away completely at 55. Any suggestions?

  6. Roger

    May 26 and June 1 I sent email to you.
    May was on the RV problems in Glacer nat park
    June was on picture size management.

    I hadn’t heard back that you were not interested in this type of information but I don’t want to send you stuff you do not want.

    A simple we didn’t get or we don’t want would be appreciated.