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Old 08-30-2020, 11:50 PM   #1
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Brand: Still Looking
Model: Winnebago Travato 59G
State: Alabama
Posts: 4,510
THOR #6826
Wheelbase ratio and handling

I’ve been interested for some time in all of the posts about “white knuckle” driving experiences and handling. It’s interesting to me because my coaches seemed (to me) to handle just fine. Recently I came across an article about how the ratio of wheelbase to total vehicle length impacts handling. The gist of it is that the higher your coach’s ratio (wheelbase /overall length) is the better it will handle. It concludes that a ratio of .50 is about the minimum for safe, comfortable handling and the higher the number the better it will be. Conversely, the lower the number the worse the handling will be. My current coach is 30’11” and the wheelbase is 200”. This gives me a ratio of .54. My last coach was .53 and my Thor class C before that was .57.

I’d be interested to know what the ratio is for some of you who have experienced “white knuckle” driving. It might be helpful to future buyers to see if there’s anything to this ratio thing.

There are several similar articles if you do a web search for “RV wheelbase ratio” but the one that got me going is this one:

https://axleaddict.com/rvs/Why-RV-Wh...o-Is-Important

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Old 08-31-2020, 12:04 AM   #2
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Brand: Thor Motor Coach
Model: Axis 24.1
State: Connecticut
Posts: 1,153
THOR #20289
I think you are exactly right. When the coach extends significantly beyond the rear wheels, it creates a large area that will push the rear end around in a sudden gust.

Typical Mercedes Sprinter Class Cs have a wheelbase of 170" and a coach body length of about 300 inches. That is a ratio of 170/300 or .57. Owners complain about sway and spend thousands to try to fix it. Those coaches seem to be in the middle of the sway stability spectrum.

On the better end of the spectrum the Thor Axix/Vegas has a wheelbase of 188" which gives it a ratio of .63 which is better and most owners say it drives nicely. On the worse end of the spectrum is the Winnebago Fuse with a a wheelbase of 156" and a shorter coach length of about 288" which gives it a ratio of .54. I haven't heard any complaints but it is a somewhat new coach.

So yes, consider that ratio carefully when buying an RV.

David
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Old 08-31-2020, 12:59 AM   #3
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Brand: Thor Motor Coach
Model: Hurricane 31S
State: Texas
Posts: 3,483
THOR #6411
We can get into the polar moment of inertia or center of pressure vs center of gravity but that is theory. The coach will handle better the more weight is moved forward. In a race car all the movable weight is moved as far forward and as low as possible. It will go around a corner better if all the weight is near the center of gravity but can be a handful to control. So handling (as subjective as can be) is a depending on where the weight is located and how the chassis manufacturer configured the chassis. Ask anyone who has driven a 40ft 2 axle diesel as compare to a 42 ft 3 axle one. Night and day in directional stability, but you have to dump the air on the tag to make a sharp turn. If you have a gas coach with the bath, generator, propane tank and water tank behind the rear axle it will be more susceptible to handling problems than a coach with only storage and a bed behind the rear axle. Most people load the coach so that everything heavy near the rear axle. Remember the Ford F-53 is a class 6 truck chassis. With a truck, you always put the weight forward, otherwise you have an ill handling truck.


Of course a manufacture can compensate for know weight distribution or there would be no front wheel drive cars, air cooled VW, diesel pushers or Porsche 911.
The problem is Ford has one chassis design with no idea what is going on it. The house builders (Thor, Newmar, Tiffin) are not permitted to modify the chassis as it will void the Ford warranty.



In closing, let me say than no one fix is applicable to all coaches. You must narrow down what you don't like the most and strive to fix that problem. For me, that was the rocking of the coach on uneven surfaces (body roll). Sumo Maxims fixed that. It also fixed the bump stop problem I did realize I had until it was gone. Next was the pull of the Dolly on the hitch. A track bar fixed that. The coach had a tenancy to wander on smooth highways: lower air pressure in the front tires (5psi); higher pressure in the rear (5 psi) increased the front toe-in to 3/16 of an inch.

My coach has all the built in stuff up front - water, propane, sink, generator. The only thing behind the rear axle is the outdoor kitchen, bed and the fuel tank.
Now back to regularly scheduled rant about how poorly Thor MC coaches are constructed.
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