halfmoa

Rich: Once again the statements you are making are correct. However, we are not measuring the pressure inside the tire with our tire gauges, we're reading the relative pressure of the tire compared to the ambient pressure. Anyone with an air bed in the coach experiences the same issue, as we gain elevation the bed begins to increase in pressure, and gets as hard as a rock when going over passes, it's quite noticeable when you sit on it. The pressure in the tire will remain fairly constant, but the pressure we see on the gauge will be higher with increased elevation, because the inside pressure remains the same, but the ambient pressure is decreasing, resulting in a higher relative reading on our gauges. I've had this discussion with several people before, and the same argument comes up about the pressure in the tire remaining the same since it's in a closed container, but the gauges we use read relative pressure, not internal tire pressure. www.tirerack.com/tires/tiretech/techpage.jsp?techid=167 gives a pretty good description of tire pressures "increasing" with increased elevation.

Just to clear something up, Wayne77590 is correct when describing the relationship of atmospheric pressure and altitude. But his explanation may be misinterpreted by some when trying to figure out what their tire pressures are going to look like as the altitude changes. Higher elevations will result in lower ambient pressures. However, when checking our tires we are not measuring ambient pressure, we're measuring the relative pressure between the inside tire pressure compared to the outside tire pressure. Wayne was correct when saying the difference in pressures at sea level vs. 10,000 feet elevation is about a negative 4.7 PSI. But what you will see on your gauge is an increase of 4.7 PSI because the ambient pressure has decreased while the pressure in the tire (virtually a closed vessel) remains the same. The relative pressure you see on your gauge will therefore read 4.7 PSI higher at the higher elevation.

Wolfe10 and Jiffyjet2 are on target with their replies. I weigh and analyze all types of RVs , so I have dealt with many concerned drivers about handling issues. One of the components of our analysis program is the wheelbase divided by the total length of the coach. The resulting number should be .54 or above to eliminate design issues that can cause "porpoising". Now that doesn't mean a .54 and above will never experience this problem, if the coach is not loaded properly, or suspension problems may exist causing handling issues to be present regardless of the WB/Length ratio. It also doesn't mean a coach with a .53 or lower ratio will always have issues, it just means there is a possibility that design may contribute to the problem. This may be valued information, but that wasn't exactly your question. Your concern sounded like it dealt more with the weight of the rear engine diesel on a small wheel base. That would fall under another element of the analysis that deals with the % weight on the front and rear axles. Typically, the front axle should carry between 22%-38%, leaving the remaining 62%-78% on the rear axle. The rear axle will have a rating considerably higher that the front, which should allow the coach to achieve the close percent capacity result Wolfe10 mentioned. As far as I know there is no industry standard on how close the percent capacity should be, but I the closer the better would be good advice. Beaver and Country Coach both made some outstanding coaches, but I have heard your very complaint from owners of the shorter models. If ride and safety are major concerns, which they should be, BY ALL MEANS, have the coach weighed and analyzed before you buy it. "Weigh before you pay" is good advice anytime you are considering an RV purchase.