Wheelbase/Length Ratio >< Crosswinds

[Glen R. Fotre]

I am told that, in a class "A" motorhome, if the ratio of wheelbase divided
by the overall length is 50% or less, that handling, especially in crosswind
conditions is 'dangerous'. 54% or more is considered 'good'.

Does the installation of the "Steer Safe Stabilizer" do anything to improve
control in windy conditions on these shorter wheelbase diesel pushers with
about 12' of weight hanging out behind the rear axle? It's supposed to help
with blowouts, but I've not heard any testimonials regarding the wind.

gl...@ccim.net

[W F Sill]

Glen, wb/lgth ratio is important but not the only issue affecting
stability in crosswinds. IMO aftermarket steering "stabilizers" are
an over-hyped "bandaid". At best they can reduce any wandering caused
by loose steering components. They do NOTHING to keep the tires from
flexing due to wind force, and at worst they actually interfere with
smooth steering control.

My advice, take a cue from bus operators - they use the little short
wheelbase rigs for maneuvering in cities, and very LONG wheelbase rigs
for highway use. Go thou and do likewise.

Will Sill KD3XR
"I don't want everyone to like me. I should think
less of myself if some people did". - Henry James

[Robert Carr]

Will

Unless I am all screwed up, and that's possible, I see this situation
no different than when trailering. We all know if you attach a trailer
to tow vehicle at the end of a long rear overhang lever arm, the
trailer can steer the tow vehicle. There are 3 possible improvements
for the situation. Lengthen the wheelbase of the tow vehicle, shorten
the rear overhang or do both. In the case of the motorhome with long
rear overhang, wind blasts can produce forces on that section and
through the rear overhang lever arm steer the vehicle. Again there
are three possible improvements. Lengthen the wheelbase, shorten the
rear overhang or do both(buses).

Bob

[W F Sill]

Not long ago, Robert Carr <rac...@home.com> wrote:

>Will
>
>Unless I am all screwed up, and that's possible, I see this situation
>no different than when trailering. We all know if you attach a trailer
>to tow vehicle at the end of a long rear overhang lever arm, the
>trailer can steer the tow vehicle. There are 3 possible improvements
>for the situation. Lengthen the wheelbase of the tow vehicle, shorten
>the rear overhang or do both. In the case of the motorhome with long
>rear overhang, wind blasts can produce forces on that section and
>through the rear overhang lever arm steer the vehicle. Again there
>are three possible improvements. Lengthen the wheelbase, shorten the
>rear overhang or do both(buses).
>
>Bob

All that is true, but it's not the whole story. The stability of any
vehicle is ALSO affected by other factors. In addition to the obvious
stuff like misalignment and (I hate to mention it) under- or
over-inflation, factors such as the height of a vehicle, the
characteristics of the suspension, and the driver's skill all make for
a good or bad experience.

[David Osborn]

rac...@home.com writes:

>Unless I am all screwed up, and that's possible, I see this situation
>no different than when trailering. We all know if you attach a trailer
>to tow vehicle at the end of a long rear overhang lever arm, the
>trailer can steer the tow vehicle. There are 3 possible improvements
>for the situation. Lengthen the wheelbase of the tow vehicle, shorten
>the rear overhang or do both. In the case of the motorhome with long
>rear overhang, wind blasts can produce forces on that section and
>through the rear overhang lever arm steer the vehicle. Again there
>are three possible improvements. Lengthen the wheelbase, shorten the
>rear overhang or do both(buses).

In the case of a motorhome, all other things being equal a shorter rear
overhang is better than a longer rear overhang. However, there are enough
other factors in designing a well-handling vehicle that it's not possible to
put any accurate hard-and-fast rules on which wheelbase-to-overhang ratio is
"good," and which is "bad."

- - - - -
David, N8DO; FMCA 147762
djosborn at aol dot com

[Young]

If the designer of the motor home has a wheel base less than of the
total length 50% it is highly probable that the center of pressure of
the side surface of the motor home is aft of the rear axle. That
means that a side gust is trying to rotate the body of the motorhome
about the rear wheels like a travel trailer except that this time you
don't have any sway dampeners.

My personal opinion is that any designer who would do that doesn't
have any idea of what he is doing. That is akin to placing the hitch
pin location aft of the rear wheels on a fifth wheel but a damned
sight worse. I wouldn't touch a motorhome with that much hanging out
the rear. Lets assume you have a five year old kid. Would you put them
aboard a school bus with that amount of overhang, and when did you
last see a school bus with that much overhang. A designer puts out a
bum design, the salesman is successful in selling it, and then along
comes someone who purports to fix it. A bum design that fundamentally
off beat should never get out of the factory.

Frederick

"Glen R. Fotre" <cc...@jps.net> wrote in message
news:zSnz6.64$K54....@nntp3.onemain.com...

[JDavis1277]

In fact, the designer who designs a MH with a long rear overhang may be a great
designer.

I believe long overhang designs are built that way in order to avoid cost
rather than due to some deficiency of the designer. I assume a shorter
wheelbase chassis costs less?? As I don't KNOW that to be a fact I may well be
mistaken.

A top designer, OTOH, given the basic problem of long aft overhang will do
everything possible to produce a design which will do well in all other
respects. It is possible to produce a design which due to weight distribution
may handle better than would be expected in spite of a long aft overhang???,
no?

Butch

Frederick wrote: >If the designer of the motor home has a wheel base less

than of the total length 50% it is highly probable that the center of pressure
of
>the side surface of the motor home is aft of the rear axle. That
>means that a side gust is trying to rotate the body of the motorhome
>about the rear wheels like a travel trailer except that this time you
>don't have any sway dampeners.
>
> My personal opinion is that any designer who would do that doesn't

>have any idea of what he is doing. <snip

[David Lippincott]

You're getting the 50% rule from the guys at RV.org. I personally think the 50%
rule is too simplistic.

I purchased a Safari Trek that is rated as dangerous. Short at 28' with the
wheel base exactly at 50%. I was a little concerned about this rule of thumb so
I test drove it and felt it was pretty stable.

To be doubly sure, I swapped out the sloppy factory shocks for Bilsteins, added
an IPD front anti-sway bar (1-5/8 OD versus stock 1-1/4), plastic bushings for
the front and rear anti-sway (replaced rubber bushings and included in IPD kit),
and added a Safe-T-Steer.

The ultimate test was driving up Highway 395 to Mammoth from San Diego. This is
a 2 lane road across the flat desert with a lot of big truck traffic. Everybody
is driving 75mph+ including me. There was a strong, 20-30kt cross wind from the
left. Had to hold a little left pressure on the wheel to counteract the cross
wind. A semi would come along and the bow wave would give me a nudge to the
right. Immediately following, the body of the semi would block the wind so my
left steering bias would cause me to turn to the left, toward the truck. First
time this happened got my attention but not dangerously so. After that, I
simply anticipated the bumps and compensated. Very simple driving and I wasn't
tired at all after a couple of hours of this. You'd have to admit this was
probably the worse case scenario for cross wind.

Total cost of the upgrades was less than $7000. Something Safari should have
done but not a big adder to a $65K RV cost.

My take on the steering stabalizer is that it doesn't make the RV more stable in
normal driving but is there in case you have a front blow out or let the wheel
drop off the pavement. Anything that might grab the wheel from your hands. I
haven't 'tested' it but the theory is good. The stock frame has a small
steering stabalizer already installed to take out shimmy and smooth the
steering. I can't rotate the steering wheel fast enough to cause the shock to
drag on the steering.

One of the comment stated the center of pressure would be behind the rear axle.
I don't believe this is the case because the wind from the left was pushing me
to the right and I had to steer left, into the wind.

I also wrote many Trek owners about their driving experience. Nobody had any
problems. You might do the same for owners of the same RV you want to buy.

David

[W F Sill]

Not long ago, David Lippincott <rvne...@hotmail.com> wrote:

>I purchased a Safari Trek that is rated as dangerous. Short at 28' with the
>wheel base exactly at 50%.

<snipped details of "upgrades" costing "less than $7000", and
admission of skill required to keep it on the road>

There are untold thousands of vehicles roaming the highways with short
w/b under long, tall bodies. So it can be done - and SOME people are
even more-or-less "happy" with this setup.

But I agree with J D Gallant (RVCG) on this issue, even though
"dangerous" is a relative term without a lot of clear meaning. It is
simply a fact of physics that a long wheelbase increases directional
stability on the highway. It should not be necessary to spend
$$$thousands to modify a vehicle - and then STILL have to wrestle
with it when a truck passes.

This is very much like the overloading issue - there is no numerical
formula written on a stone tablet by the finger of God, beyond which
you will crash in flames. But sensible people will take the advice of
experienced rv'ers and opt for MORE rather than less safe rigs.

[David Lippincott]

My only suggestion would be to talk to people who already own what you are
interested in. See if any of them are having stability or handling problems.

Don't let Mr. Gallant's spread sheet analysis sway (no pun intended) your
choice. Seems he has set up a nice little business selling his opinion but I
have yet to see concrete evidence to back up his ratings. I wrote him a lengthy
letter expressing my concerns, asking for some justification and never received a
reply.

I wouldn't classify my experience on HWY 395 as "wrestling" with the rig as Mr.
Sills phrased it. I've had similar experience with cars including VW's to SUV's
and vans. All handled about the same in the same cross wind situation. Seems
one of the car adds shows passing a semi on a bridge where the driver was
obviously concerned about the bow wave and turbulence from the semi. Not only an
RV issue.

I have the experience with this particular rig and wouldn't hesitate to recommend
it to anybody based on the driving characteristics. This isn't blind opinion but
hands-on driving in less than perfect circumstances.

I'd like to see an actual comparison between several rigs of varying w/b ratio.
Not sure how to characterize the results in numerical terms. 3 rigs, in line, on
HWY 395. Each driver gets to drive all 3 rigs. All would see the same event
moments apart. If you had the time and money, you could instrument the steering
wheel for how much it is moved, vehicle deviation from a straight line, body
sway, etc. as you passed an upsetting event such as a semi. Do the Consumer
Reports thing. Problem is much of the results would be subjective.

Is a long wheel base safer - probably. Is the difference between 50% and 55%
significant - probably not. If you can afford a Prevost and want one, probably
drives like a bus (which it is). If you want something smaller you can get into
the state parks, like us, I don't think you can get the w/b ratio available in
the much larger rigs.

There are so many variables in handling including suspension geometry, stiffness,
weight distribution, driver experience and confidence that I don't believe you
can slap a w/b ratio rating on a rig and call it 'dangerous', yet move the axel
back 9" an call it 'good'.

I'm not sure I buy Mr. Sills' "fact of physics" argument. Easy to say, hard to
prove. There's a lot of busses ending up in the ditches. Given these are
professional drivers, why the loss of control? I've personally been in a bus
doing 360's on slick road coming down from a ski area. Luckily, no harm, no foul
except for some laundry issues.

Also, sorry about the $7000 number. Should have been $700 for the suspension
upgrades. What's an order of magnitude between friends.

David

[W F Sill]

Not long ago, David Lippincott <rvne...@hotmail.com> wrote a long
essay attempting to justify his POV that wb/length ratio doesn't make
much difference. He included this off-the-wall statement:

>I'm not sure I buy Mr. Sills' "fact of physics" argument. Easy to say, hard to
>prove.

::sigh::

Someone who has no concept of the laws of physics as they pertain to
leverage and the like could make such an argument. But the laws of
physics are not troubled by people who don't understand them!

I have made it clear to all but the most obtuse readers that I don't
think RVCG's numbers are based on exact science. In fact, I stated on
this thread that:

:This is very much like the overloading issue - there is no numerical

:formula written on a stone tablet by the finger of God, beyond which
:you will crash in flames. But sensible people will take the advice of
:experienced rv'ers and opt for MORE rather than less safe rigs.

There are REASONS that OTR truckers use long-wheelbase tractors where
permitted by stupid laws, and why they hate those little flat-nosed
COE tractors that are really suitable only for yard work but are
mandated by daft length laws. There are REASONS why intelligent
people don't habitually use a Bobcat on the highway. And a Safari
Trek with a short nose and a long tail cannot possibly be as stable in
dealing with crosswinds as a similar vehicle with a lot longer
wheelbase. And you can take that to the bank!

I'm glad you can deal with your Trek, but I would not be caught dead
sheparding something like that across the plains.

[Young]

David,
The center of pressure of a slabsided vehicle subjected to a side gust
is at the center of area. If you are driving in a two lane highway and
a truck goes by you coming from the opposite direction, you will feel
it in a 50% wheelbase to length ratio vehicle more than say a 70%. The
reason is that there is a bow wave on the front of the truck coming at
you. As his bow wave (area of high pressure) passes down the side of
your truck, it goes past the rear half and it wants to twist the
vehicle about the rear wheels. If you are happy with it, fine, I never
cease to be amazed at what people will adjust to, but that doesn't
mean it's safe. If you are towing a car, it's like towing a travel
trailer with a truck that has a 14 ft extension on the hitch. It
better be a light car.

Frederick

"David Lippincott" <rvne...@hotmail.com> wrote in message
news:3AD3632D...@hotmail.com...