Why the Instability?

[Chance]

Originally Posted by Metalman

The front/rear weight bias can be a serious problem on some motor homes. I had a 24 foot class C RV that was so light on the front end that firmly applying the brakes would cause the front tires to slide. The first and only time that happened was a wakeup call for me to allow long following distances and look way ahead.

I don't fully understand why they do it, but motorhome manufacturers often use chassis with short wheelbases and then extend the rear excessively. I'm guessing they do it to lower costs, and to improve turning radius, but in the process they are sometimes creating some of the instability issues owners complain about.

Other factors like increased motorhome height and/or higher center of gravity are obvious contributors to instability. We just have to keep in mind that instability doesn't come up in the showroom where most motorhomes are selected, so manufacturers have little incentive to solve the problem if it's going to cause them sales. They could build lower motorhomes with lower CG, but few would buy them.

[petef]

Bear in mind all have dual rear wheels, so the weight would be 33% front, 66% rear and be on balance, tire/weight wize..rough math.. so the below sentence is correct as I see it..

Originally Posted by Chance

Motorhomes with Dual Rear Wheels (DRW) are designed to carry much more weight over rear axle than over front axle. You'll never get close to 50/50 weight distribution with your motorhome, not that you'd want to. If you tried shifting enough load forward you'd quickly overload the front suspension.

While it's possible to design a motorhome with close to 50/50 weight distribution, if you did, wouldn't you only have four wheels of equal size -- one at each corner? That's what Europeans do on smaller motorhomes, but in US manufacturers normally start out with DRW truck chassis that are designed to carry much more load over rear axle. And diesel pushers, with their overhanging heavy rear engines and transmissions, can't possibly get close to 50/50 front-to-back weight distribution.

[Metalman]

Originally Posted by Chance

I don't fully understand why they do it, but motorhome manufacturers often use chassis with short wheelbases and then extend the rear excessively. I'm guessing they do it to lower costs, and to improve turning radius, but in the process they are sometimes creating some of the instability issues owners complain about.

Other factors like increased motorhome height and/or higher center of gravity are obvious contributors to instability. We just have to keep in mind that instability doesn't come up in the showroom where most motorhomes are selected, so manufacturers have little incentive to solve the problem if it's going to cause them sales. They could build lower motorhomes with lower CG, but few would buy them.

Customers want large basement storage areas and a lot of headroom inside the coach. Rows of ceiling high storage cabinets, one or two AC units on the roof contribute or create the problem. I was surprised to see that Thor put a large capacity, relatively tall fresh water storage tank above floor, several feet behind the rear axel.

[Chance]

Originally Posted by petef

Bear in mind all have dual rear wheels, so the weight would be 33% front, 66% rear and be on balance, tire/weight wize..rough math.. so the below sentence is correct as I see it..

A front-to-back weight distribution of 50/50 is not the same terminology as equal weight per tire/wheel. In the example above, weight distribution would be 33/67, not 50/50. Because of that I didn't know what you meant.

For what it's worth, I personally don't worry about exactness. It's rare that a vehicle that drives good will drive poorly just because a little weight is moved around.

[EA37TS]

Originally Posted by Metalman

Customers want large basement storage areas and a lot of headroom inside the coach. Rows of ceiling high storage cabinets, one or two AC units on the roof contribute or create the problem. I was surprised to see that Thor put a large capacity, relatively tall fresh water storage tank above floor, several feet behind the rear axel.

It all depends on the model of the coach. My Challenger has a 100 gallon fresh tank and 3 40 gallon waste tanks. The fresh, black and front grey tanks are all below floor and in front of the rear axle. The rear grey tank is also below floor but behind the rear axle.

[JamieGeek]

Originally Posted by Metalman

I was surprised to see that Thor put a large capacity, relatively tall fresh water storage tank above floor, several feet behind the rear axel.

This is where the fresh water is on the Axis/Vegas 24.1 units.

The waste tanks, however, are below floor in between the axles.

Note that the fuel tank is behind the rear axle as well (but below floor).

I think the manufacturers just treat motorhomes like they do travel trailers and 5th wheels. In the tow-behind case they just balance the unit on the axles with 10-25% tongue weight. For motorhomes its the same thing: just balance the units on the rear axle with some forward bias (a bit more than the trailer case though).

This is how you get those big overhangs: Its just a balancing act.

[Metalman]

Originally Posted by JamieGeek

This is where the fresh water is on the Axis/Vegas 24.1 units.

The waste tanks, however, are below floor in between the axles.

Note that the fuel tank is behind the rear axle as well (but below floor).

I think the manufacturers just treat motorhomes like they do travel trailers and 5th wheels. In the tow-behind case they just balance the unit on the axles with 10-25% tongue weight. For motorhomes its the same thing: just balance the units on the rear axle with some forward bias (a bit more than the trailer case though).

This is how you get those big overhangs: Its just a balancing act.

As top heavy as our 27K Windsport is, I am not totally dissatisfied with its handling characteristics. When I drive it I take myself out of the car/pickup truck mode, and operate it in the motor home mode. It's not perfect and not always stable in certain conditions.

[Chance]

Originally Posted by Metalman

Customers want large basement storage areas and a lot of headroom inside the coach. Rows of ceiling high storage cabinets, one or two AC units on the roof contribute or create the problem. I was surprised to see that Thor put a large capacity, relatively tall fresh water storage tank above floor, several feet behind the rear axel.

Exactly. A lot of these handling issues did not seem to be as common decades ago even though chassis were not as good. Obviously that was before the internet, which skews reporting. Having said that, I recall owners complaining of overloaded rigs more so than poor handling per se.

I think slides increase center of gravity as well, plus add weight. In the case of newer full-wall slides, I'd be curious to know how that affects side-to-side weight distribution. Many floorplans have one full wall slide on driver side, so it would be interesting to get weights for all four corners to see if it makes a difference.

[sky_blue]

If you are being blown across the highway, FOR GOD'S SAKE, pull off, We use our motorhomes for fun and adventure. Nothing is worth endangering our families.

A Motorhome presents a flat surface. Yes, you can spend thousands trying to solve the problem. But there will always be a bigger wind. Pull into a prttected spot and pop a beer.

[Metalman]

Originally Posted by gbrown

If you are being blown across the highway, FOR GOD'S SAKE, pull off, We use our motorhomes for fun and adventure. Nothing is worth endangering our families.

A Motorhome presents a flat surface. Yes, you can spend thousands trying to solve the problem. But there will always be a bigger wind. Pull into a prttected spot and pop a beer.

Ha ha gbrown, good advice. But most Americans are bull headed, macho men who bitch and moan in times of adverse conditions, but they keep on trucking.

[Chance]

This article from Popular Science compared two very different motorhomes from the past considered to handle very well, almost car like.

August 1973 Popular Science Review

Although one was a Front Wheel Drive GMC and the other a "gasoline pusher" manufactured by FMC, they both shared a low and wide aero design (at least aerodynamic for their time).

Specs for the FMC motorhome include a height of 8'-8", and the Classic GMC was about 9-feet to top of AC.

And it may be hard to believe, but they both had independent front and rear suspensions, which just shows that motorhomes can be designed to handle much better if it is a priority.


Most of us know what Classic GMCs look like, but in case you are not familiar with the FMC gas pusher.