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Old 04-13-2016, 08:07 PM   #1
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Wheelbase/Overall length ratio

I was taught that the ratio between the overall length of a coach and its wheelbase (WB divided by OL in inches) was an important safety consideration. Just wondering why Newmar builds its 35' coaches with a dangerously low WB/OL ratio (0.49)?

Really want a well built 35 foot MH (gas or diesel) but Newmar falls short in both segments with a coach that is too long for its wheelbase.

Any thoughts?
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Old 04-13-2016, 09:38 PM   #2
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There is an RV rating service out there that publishes this number. They do not drive the coaches, they do not know how well or poorly it drives, they just calculate this number and call low ones potentially dangerous.

I recall them mentioning that the low ratio alone does not make it dangerous. They did say if you should test drive the coach (or one similar to it) before you purchase, which I agree with heartily.

I own one of the gas coaches that has a low WB/OL ratio. I agree that it did not handle as well as it should, but it was not due to this ratio, it was due to lack of some components like a rear track bar and insufficient sway bars. Both were resolved with upgrades, the upgrade to the sway bars can be done for free (search for Cheap Handling Fix on this forum)! The coach now drives extremely well and handles things like trucks passing and crosswinds great.

Many buyers of short Newmar DP's have sung the praises of their coaches handling, especially 2007 and newer ones with the Comfort Drive system.
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Old 04-13-2016, 10:00 PM   #3
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I can think of no reason to build a coach with the wheelbase too short, except money. If the RV manufacturer is purchasing the chassis, a shorter chassis costs less. It's a very short sighted view of profit in my opinion.

It is my opinion that the wheelbase should be at least 55% of the total length of the coach. Less than that, and you see coaches with those great big long overhangs behind the rear wheels. Get that rig out at highway speed in a big wind with a semi on each side and you learn a new definition of white knuckle fear!

That's just my opinion. Others may vary. I'm sure there are some who own coaches with a very short wheelbase to coach length ratio that will tell you their coach handles like an absolute dream.

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Old 04-13-2016, 11:02 PM   #4
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Latrice..... As Luv2go pointed out, there's nothing wrong with those short chassis-- as long as you're willing to spend your own money to finish making it drive properly.

Builders of larger gas coaches-- like, over 33 feet or so-- face a conundrum. If they build it on a chassis with a front axle robust enough to carry the weight of the engine and long enough to carry the box safely, then they wind up with a structure so heavy you have very little cargo carrying capacity (CCC) left.

To solve this design quandary, builders deliberately use a chassis smaller and shorter than ideal. Now they have a usable CCC they can sell, but the chassis is too lightweight to carry the engine load on the front axle.

Solution: make the engine lighter! If you use a chassis short enough, you can skooch those rear wheels forward til they're almost up to the middle of the coach. Then, you attach an extra 3-4 feet of body to the butt end and it will counterbalance the weight of the engine. You see, they WANT that twelve foot rear overhang, they NEED that twelve foot rear overhang. It's the only way they can make the numbers work.

Problem is, depending on overall weight, length, and some other factors, when you do this you often wind up with an unstable driving platform. This is exacerbated for the coach builders by the fact that there is only one US manufacturer left of gas powered MH chassis-- Ford.

Nobody has yet come up with a good explanation as to why Ford doesn't build all their chassis with these track bars and Johnson rods and other sorts of doohickeys that everyone seems to know are needed and lots of owners wind up buying anyway.

Good Luck!


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Old 04-14-2016, 09:12 AM   #5
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Quote:
Originally Posted by JFXG View Post
Latrice.....
Problem is, depending on overall weight, length, and some other factors, when you do this you often wind up with an unstable driving platform. This is exacerbated for the coach builders by the fact that there is only one US manufacturer left of gas powered MH chassis-- Ford.

Nobody has yet come up with a good explanation as to why Ford doesn't build all their chassis with these track bars and Johnson rods and other sorts of doohickeys that everyone seems to know are needed and lots of owners wind up buying anyway.

Good Luck!


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I think the explanation is quite simple, MONEY. Even without competition, Ford is always facing resistance from the coach builders about the cost of their product, which get multiplied at least a couple of times during the journey to a retail sale. The industry-wide practice of letting the end consumer find and fix anything that is wrong with an RV is alive and well...and it starts with the chassis build shortcuts which save a miniscule amount of money compared with the cost of after-market band-aids.
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Old 04-14-2016, 11:14 AM   #6
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Newmar body/wheelbase ratio is not ideal, but it is notably better than Tiffin. Look at a Newmar DP 37 (Ventana or DS) vs. Tiffin DP 36/37 (Red, Phaeton or Bus). All are same body length (within a few inches) but have obvious wheelbase differences. Notice cargo doors. Newmar has one large door and one small door behind rear wheels. Tiffin has TWO large doors behind rear wheels. Of course, many gassers have THREE doors behind the rear wheels.

In addition to weight, balance and stability, a longer overhang is more prone to tail wag when towing a car.

But, shorter WB is more maneuverable, IF careful attention is paid to the wide-swinging tail and sometimes even-wider-swinging towed.

Based on the math, I prefer the larger body/wb ratio of Newmar DP over Tiffin DP because of the theory of greater stability in emergency maneuvers. But does it matter in real situations? I haven't found independent empirical proof. Unlike what Consumer Reports does with cars, no one seems to take these big rigs onto test tracks and try to tip them over, then publicize the results.
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Old 04-14-2016, 11:23 AM   #7
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I don't want to get into a debate about WB calculation ratios, but I'm certain that we can all agree that a heavy 45' coach with a tag axel will most likely give us the best ride.

Now, while shopping for my first starter coach, I knew that I will be limited to a 35 footer and began the test drive process. Yes many of the SOB rv's were very poor in the handling area but I can honestly say that the handling of my Newmar Ventana is exceptional for its size. When ever this discussion comes up, I also say that with proper tire pressures and the Newmar Comfort Drive system I am NOT pushed around by the semis and do not experience the infamous white knuckle drive. My wife enjoys driving as well.

So, if you're going to select a short wheel base coach, make that test drive as long as posible and get the feel for that specific coach.

IF I were buying new today, I would get the exact same unit as I currently have. A35' Newmar Ventana diesel.
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Old 04-14-2016, 02:56 PM   #8
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These are always interesting debates. I agree less expensive coachs aren't built to the same standards as luxury coachs, but then again volkswagon beatles never did accelerate or handle like a corvette. I mean no offence, but those who can write multi-million dollar checks will always have less compromise than those that can't. A more constructive discussion IMHO is how best to live with the compromises our pocketbooks require us to make. (Puts away philosophic soapbox and backs away).
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