Wheelbase to length ratio. What does it mean?

[berreez]

When you divide the wheelbase by the length of the motor home, you should have a wheelbase-to-length ratio of 53%. This ratio gives us a good idea of the handling characteristics you can expect from this vehicle.

From the above statement, is it better to have a number lower or higher than 53% if you what better handling?

[timmac]

wny_pat wrote:

berreez wrote:
So if 53% is a baseline; then 54% and over will give you a better ride and handling for the most part out on the road. Anything under 53% will give you better maneuverability in tight areas but possible tail sway in cross winds or out on the road. Is this correct?

Not really. The factor not figured in is how much of that 53 percent is behind the rear axle and how much weight is back there. That is where and why the tail is wagging the dog!!! My coach has a 228" wheelbase and is 36 foot long (53%). But the front axle is way back from the front bumper when compared to many other 36 foot coaches. My DP, without airbags, rides just fine and has no tail wiggling even when being passed by a tractor trailer. Like wallym said - "old wives tale". They can be built, but it has to be engineered properly.

Bingo, we have the correct answer, its not just the 53% ratio to look at, its how big is its a$$, in other words how far in the back bumper to the center of rear tires.

[Gale_Hawkins]

JimM68 wrote:

Gjac wrote:
I have not heard of this posted before but would think if the MH had equal weight on all 6 tires this would be a well balanced MH and should ride well. I would be interested to hear from folks that were knowledgeable about chassis design to see if this were to affect the ride quality more than the wheel base ratio.

Unfortunately, that is a gross oversimplyfication of the engineering problems involved.

If the wieght rating of the tires and wheels were all that mattered, maybe. But there are 2 tires in front, and 4 or maybe 6 in back.

And while those 2 in front might carry 14,000 pounds (7000 each), or maybe more, 4 in back on a single axle are federally mandated to carry only 20k total, or 5000 each, even though they are the same size tire with the same load rating as the two in front.

And really, none of this matters a hill of beans to the handling of the coach.
Besides the weight balance end to end, you also need to look at the front overhang, rear overhang, and wheelbase.
And there is a whole lot more having to do with the overall design of the suspension, and it's resistance to roll and pitch (including under braking)

I agree there are many factors with perhaps the driver's skill set being very important. Working with our two 16 year old permitted drivers I see how they over steer. This is worse in vehicles with very worn front end parts as can be the case with some MH's.

All I know is our 32' 1992 P30 Chevy chassis that it is 33' bumper to bumper with no real weight paste the front tire and a tail of 10' with a 208" Wheel Base handles like a dream. It left GM as a 190" chassis per the parts build sheet. I expect the 18" addition makes a work of difference. We are just under 15K pounds which means we are not over weight over all or per axle.

[Gale_Hawkins]

Gjac wrote:
I have not heard of this posted before but would think if the MH had equal weight on all 6 tires this would be a well balanced MH and should ride well. I would be interested to hear from folks that were knowledgeable about chassis design to see if this were to affect the ride quality more than the wheel base ratio.

Your logic sounds good and is my experience with ours. If a MH that weights 21K with 7K on the front two tires and 14K on the rears has to be to be in a balanced ratio. Sure if you had this balance some way in two MH's and one had a 8' over hang and the other one had a 16' rear overhang wind and trucks are going to impace the one with 16' over hang more than the one with 8' over hang because the 16' one would have a sail for a tail. 🙂

[JimM68]

Gjac wrote:
I have not heard of this posted before but would think if the MH had equal weight on all 6 tires this would be a well balanced MH and should ride well. I would be interested to hear from folks that were knowledgeable about chassis design to see if this were to affect the ride quality more than the wheel base ratio.

Unfortunately, that is a gross oversimplyfication of the engineering problems involved.

If the wieght rating of the tires and wheels were all that mattered, maybe. But there are 2 tires in front, and 4 or maybe 6 in back.

And while those 2 in front might carry 14,000 pounds (7000 each), or maybe more, 4 in back on a single axle are federally mandated to carry only 20k total, or 5000 each, even though they are the same size tire with the same load rating as the two in front.

And really, none of this matters a hill of beans to the handling of the coach.
Besides the weight balance end to end, you also need to look at the front overhang, rear overhang, and wheelbase.
And there is a whole lot more having to do with the overall design of the suspension, and it's resistance to roll and pitch (including under braking)

[workhardplayha1]

I say find a floorplan you like and test drive it.

[Gjac]

I have not heard of this posted before but would think if the MH had equal weight on all 6 tires this would be a well balanced MH and should ride well. I would be interested to hear from folks that were knowledgeable about chassis design to see if this were to affect the ride quality more than the wheel base ratio.

[JimM68]

as has been said more than once above, there is more than one way to skin a cat.
Pure math looking at one of many variables independantly just isn't going to give a simple answer to a complex question...

If the floorplan is perfect, you gotta drive it to see if it fits.

And even then, it's gonna take a bunch of driving a bunch of different coaches to find the one that is "right" or even to know enough to define "right"
Most of us end up guessing at least the first 2 or 3 times, and even then, doing what the DW wants and learning to live with it is often the best course of action.

[wny_pat1]

berreez wrote:
So if 53% is a baseline; then 54% and over will give you a better ride and handling for the most part out on the road. Anything under 53% will give you better maneuverability in tight areas but possible tail sway in cross winds or out on the road. Is this correct?

Not really. The factor not figured in is how much of that 53 percent is behind the rear axle and how much weight is back there. That is where and why the tail is wagging the dog!!! My coach has a 228" wheelbase and is 36 foot long (53%). But the front axle is way back from the front bumper when compared to many other 36 foot coaches. My DP, without airbags, rides just fine and has no tail wiggling even when being passed by a tractor trailer. Like wallym said - "old wives tale". They can be built, but it has to be engineered properly.

[Gale_Hawkins]

berreez that may be correct when looking only at that one factor but it is ONLY one of many factors that decide how a MH will handle. As a guess the difference between 50-56% will be a non issue in most cases.

The major cause of handling issues for me at first was the green horn behind the wheel. Once I loosened my grip on wheel and relaxed my whole body I stopped over correcting and let the MH seek its on happy point going down the road. We have a 1992 P30 that is 32' with a WB of 208" and weights just under 15K with 1/3 of the weight on the front axle and it handles like a dream for the 20K miles we have put on it since 2007.

[berreez]

So if 53% is a baseline; then 54% and over will give you a better ride and handling for the most part out on the road. Anything under 53% will give you better maneuverability in tight areas but possible tail sway in cross winds or out on the road. Is this correct?

[Gale_Hawkins]

ferndaleflyer wrote:
I once rode from Baltimore to the Carolinas in a C class that if you looked at it from the side the rear wheels were right in the middle, or 1/2 way between the front and rear bumper. That thing swerved if a car passed and you can imagine what it was like passing a truck.....I flew back home. Better yet try towing a trailer behind anything with a lot of over hang.....must have a death wish.....

Hauled 25 of the 16' cattle panels on a 12' single axle trailer behind and S-10 this summer under a time deadline. Tail long and heavy with negative tongue weight is BAD. Thankfully it was relative light. It was fine at 35 MPH, fair at 40, wiggle at 42 and a full dance at 45 (only did that once however but it was on a bridge. 😞

[BigRabbitMan]

I just calculated the ratio for my coach. It is 53%, however, the wheels (all independent suspension) are more centered to the coach body than most. See the picture in my signature. You can see that the drivers door is in front of and not over the front wheels. Also, due to the independent rear suspension the drive line is only six inches long so the rear wheels are closer the rear of the coach than other rear engined units.

I do know that it handles great and cross winds etc. do not bother it. I have never gone over 82 mph unintentionally.

[ferndaleflyer]

I once rode from Baltimore to the Carolinas in a C class that if you looked at it from the side the rear wheels were right in the middle, or 1/2 way between the front and rear bumper. That thing swerved if a car passed and you can imagine what it was like passing a truck.....I flew back home. Better yet try towing a trailer behind anything with a lot of over hang.....must have a death wish.....

[wolfe10]

Another way of viewing poor WB/OL coaches is to picture the wind vane on top of the barn.

Most of the surface is behind the fulcrum. So, the slightest wind makes it point toward the wind. Lots of rear overhang acts the same way in a cross wind. Again, not a Class A, C, diesel or gas issue.

Can engineering overcome this-- sure. A good example is the Foretravel 34'. Poor WB/OL ratio, BUT (yes, big but) it has 8 outboard air bags and 8 outboard shocks that help defy physics.