what is wheelbase percentage

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Mike B

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I have read past posts regarding wheelbase %, and how it is important. That's a term that I've never heard before. What exactly is it?
 
It is the ratio of the wheelbase to the overall length of the coach. A 220-inch wheelbase on a coach with an overall length of 440 inches (36.6 feet) is a 50% wheelbase. A low wheelbase percentage means there is a long overhang behind the rear axle.

A higher % number is better.
 
That makes total sense.
The ratio is more critical to motorhomes with a front engine as it is common for the coach builders to add extensions to the frame rails to make the coach longer, and all of that behind the rear axle. With diesel pushers, the engine being in the rear makes it nearly impossible for the coach to be significantly longer than the chassis as it left the factory. If you look closely at the gas powered class A coaches you will see a wide range of rear overhand with the same wheelbase. Lower priced coaches tend to be built on the shortest chassis that they can use and so have a lower wheelbase ratio than a coach of similar length in a higher priced model. The Ford, F53 chassis is available in 158 in, 178 in, 190 in, 208 in, 228 in, 242 in, 252 in wheelbases and the shorter chassis costs significantly less.
 
A bit of history on the importance of a 50%+ wheelbase ratio. Probably more than many of you ever wanted to know... :D

Back in the mid-90's the demand for larger (longer) coaches exceeded the gas-chassis lengths & GVWRs that Ford & Chevy were building. Motorhome builders began to "stretch" the chassis lengths with rear extensions and add tag axles or air springs to carry the extra weight. The chassis makers began to catch up on wheelbase & GVWR by 1998 but the chassis still weren't really designed for the bigger coaches and lacked structural stiffness, hefty anti-roll bars, and the larger wheels & brakes that were sorely needed. The largest of those chassis were barely adequate for the size coaches being built, so even being slightly undersized for the coach body was a critical shortcoming. Wheelbase ratios of 52-53% were highly prized and and 50% was sort of a minimum requirement. That changed in late 2001 with the introduction of the new Workhorse (nee Chevy) W20/W22 chassis that was much beefier than it's Ford or Chevy progenitors and handled the size much better. Ford didn't redesign their F53 chassis to match Workhorse until the 2006 model year. With those improved chassis designs, wheelbase length was less critical because handling remained decent even at a 50% ratio. 50% was still the figure of merit as the way to detect when the RV maker had cheapened the design with a too-small chassis, but 50% was viewed as adequate and the difference between 50% & 53% ratios was not so significant. As the chassis makers improved further, e.g. track bars, even stronger antisway bars, and steering stabilizers, wheelbase ratio became even less important as an indicator of handling quality.

As is often the case, the 50% meme remains widely quoted on the internet even though the world has changed. But it's still a valid criteria, even though much less critical than two decades ago.
 
Wheelbase to overall length is shorthand for identifying coaches with a long rear overhang. This is important because a side force like a gust of wind centered between the axles will make the rig lean, but because it's pushing both axles in the same direction it will have a minimal effect on the direction of travel. If you have to make a steering correction it's just steering slightly into the wind, then relaxing when the wind lets up. Wind gusts that hit behind the rear axle make the rig pivot on the rear axle, changing the actual direction of travel by pushing the front axle in the opposite direction. You have to correct more strongly, then countersteer back to the original course when the gust ends.
 
When towing trailers, Davids towing tips shows the correlation of tow vehicle wheelbase to trailer length.
The wheelbase % reared its ugly head with the Ford 15 passenger vans used for towing a TT back in the 1980's.
That's a different wheelbase "rule" than the "50%" we've been discussing. It attempts to show with math the general precaution about towing a long trailer with a short wheelbase tow vehicle. Basically the old saw about letting the tail wag the dog. I've never seen the evidence that rv.org used to come up with their numbers, but it's a way to check whether you are pushing your luck with your tow set-up..
 
I've never seen the evidence that rv.org used to come up with their numbers,
Back when I was a member of the group I did see a lot of the documentation and it was pretty solid but mostly applied to gas powered coaches. I don't recall them having much on the tow wheelbase/trailer wheelbase issue, but that might be due to the fact that we had motorhomes the entire time that I was part of it. With them gone I doubt the data is still available. It looks like their website is still functional but nothing is current.
 
IMO JD Gallant (founder of rv.org) was prone to picking arbitrary borders between dangerous and safe. The tow vehicle vs trailer pseudo wheelbase (hitch to axles length) algorithm looked very scientific but I never saw a documented rationale for the specific numbers used in the algorithm. The principle is sound but the actual calculations less so.
 
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I never saw a documented rationale for the specific numbers used in the algorithm.
You do realize that not all of the information that they had and used was given away for free to everyone? Were you ever a member with full access? It is true that assumptions were made, but based on principles of physics because it would be impractical if not impossible to actually test every motorhome that ever came out. If you are to suggest a guideline to the novice RV buyer, you have to draw a line somewhere. As to the ratio of tow vehicle length to travel trailer length, as far as I know that was not developed by JD or his organization. I first read about that in an article in Trailer Life Magazine (before I knew of RV.org), and I think that they referenced work by the people at Airstream (before it was Thor). I only know of one current source for that information, and it is located in the Airstream Club Library. JD did make reference to that ratio in at least some of his articles but to my knowledge never took credit for its development.

I know that many people have long been critical of the work done by JD Gallant and his RV Consumer Group, but the publicity and lobbying efforts from them have brought a lot of improvements to the RV industry and the data that they share.
 
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