A member sent us comments about road testing two diesel pusher motor homes, the first of which had a 53% wheelbase-to-length ratio and the other a 58% ratio. He was disappointed that the motor home with the lower percentage appeared to perform better when affected by wind movement and buffeting from trucks. "Your rating on drivability appears to be solely based on the wheelbase size, hence a mathematical formula for drivability." Then he asked, "How can a drivability score be determined from just a wheelbase size without consideration for suspension, weight and balance, etc."
First of all, let's consider a most important premise: If both motor homes were built exactly the same except for wheelbase, there would be no argument that the longer wheelbase would provide more stability. If you consider that premise to be absolute, which it is, then the motor home with the longer wheelbase would perform much better under adverse conditions such as side winds, bow waves from trucks, and uneven roads. Thus, the answer must lie elsewhere.
We know that some suspensions affect highway control and vehicle stability, as do tire types and pressures. We also know that weight and balance are both critical to good highway control — especially the weight on the front wheels. For example, if the longer-wheelbase motor home had a big slideout in front of the rear wheels, it could be very difficult to keep from overloading the front axle. If balance is off, even a long-wheelbase motor home will not perform as well as one with a shorter wheelbase that has its weight properly balanced.
Our rating uses more than wheelbase in the highway control rating. We use payload because it is a good indicator of the motor home's propensity to become unbalanced under normal loading conditions, and we use data that reflects the location of the holding tank if that data is available through research or from the manufacturer. Performance reputation is also included in the equation when that information is available and substantiated.
One thing is absolute: if you are sure that all corners of the motor home are
weight balanced, and if you can load the motor home so that the axle
weights do not exceed 90% of the GAWR, and if the tires are up to
par in condition, type, and pressure, and if the suspension is at
a reasonable performance standard, a 58% wheelbase will always far
outperform a 53% wheelbase on either a pusher or front engine motor
home. Only when all these things can be verified and highway conditions
are similar can a good comparison of road-test performance produce
valid conclusions.
We need to keep in mind that a wheelbase-to-length ratio at or close to 50% will mean disaster if there are any design or mechanical deficiencies in the chassis or if road conditions turn bad. An absolute fact is that wheelbase, weight balance, and good chassis design are the three primary attributes that will give a motor home good highway performance.
Be sure to check the wheelbase-to-length ratio in the DealData Price & Rating Report shown for each model within the RV Ratings Guide.
