Date: Sat, 26 Nov 2011 09:14:05 -0500
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: Westy Weighed... Setting Corner Spring Rates
In-Reply-To: <4ed0def9.c2c5e70a.6458.ffffbedb@mx.google.com>
Content-Type: text/plain; charset="US-ASCII"
In reality the proper weigh to distribute the wheel-axle loadings is
re-distribute the weight. Nothing else works and can actually become quite
dangerous. On the big rigs like mine, the air suspension actually works on a
three point system. The rare axle has level control valves for each wheel
but the front only has one to set the axle height. The leveling jacks work
on the same principle.
If you have an extra 200 pound in the left rear corner then that axle has to
carry it. The only way to reduce that weight is to lower the spring which
will increase the sag of that corner. Of course an attempt to raise that sag
increases the weight on that axle and will also take some off the right side
axle. Some combination of weight will then get transferred to the front.
One thing to be aware when trying to adjust just one corner is that this
will put torsional twists on the chassis body. This side to side weight
distribution thing is why we need tires with some excess capacity or the
axle weight ratings.
For highway use the springs/heights should be the same for both axle sides.
Leveling front rear is one thing. You shouldn't get hung up on one corner.
If it leans a bit to the left why does it really matter? Most highways we
use are crowned to the right so this actually helps to make level when we
travel.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Derek Drew
Sent: Saturday, November 26, 2011 7:43 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Westy Weighed... Setting Corner Spring Rates
Nobody seems to really have any sense of what the correct answer is to this
query.
It is important because this might be one of the most advisable practices
for Westfalia and 15 years from now all Westys will be set this way after
everybody wakes up.
Or, what you read below might be utterly bozo engineering.
I have no idea which of these is true.
This query refers to the *ratio* for selecting spring weights to match
weights on the corners, not about other assumptions of the test.
I have weighed the four corners of my Syncro Camper with some automotive
bathroom scales I got.
In an "empty" (*) state (for me) weighs ~5300lbs as follows:
LEFT RIGHT LBS.
20% 25% FRONT
30% 25% REAR
The weight at the left rear is fairly massive compared to the left front....
almost a 50% increase in the back left rear vs. the front left.
The right side does not have this weight disparity front to rear, so the
traditional view that the left rear is an evil corner for some of our vans
(especially Westfalias) is supported .
(*) TEST CONDITIONS: About as empty as it will normally ever be for me;
5,200lbs: plenty of always-take things in the closets, 3.0L H6 engine in the
back, tools and winch under the back seat, no water in the drinking water
system, heavy big fiberglass rooftop box on the roof, heavy tire/wheel combo
on the roof, heavy tires on the van, ride heights at the four corners set at
19.5" (measured center of axle to metal of fender above), dual batteries up
front, 65lbs dead weight at front drivers seat to simulate some driver
weight, heavy 100% duty air compressor (15CFM at 100psi) at left rear,
additional spare tire on swing away carrier at left rear.
In terms of raw numbers, weight distribution is similar to this:
LEFT RIGHT LBS.
1092 1316 FRONT
1598 1294 REAR
The question is what to do about this from a springs point of view.
I noticed that car companies are increasingly setting different springs to
be mounted in each of the four corners of the vehicle to account for
different weights of the vehicle at the four corners.
So, I threw a calculator into Excel to see what the matching springs I would
want to use if I assumed a 500 rate spring at the lightest corner (front
left).
The calculator was set to a "unmodified math" or "straight line"
method of doing the calculations, meaning that I let the calculator
determine all the new spring rates without me altering the formula.
I don't know if this is a valid way to do these calculations or if some
"discount" should be applied to the numbers before converting them into
spring rates.
My spring rates at 500 start off deliberately low because the rig is
optimized for off-road driving as opposed to pavement racetrack handling.
In any case the "straight line" method produced the following suggestion
from the calculator for spring rates at each of the corners:
LEFT RIGHT
500 600 FRONT
700 600 REAR
(numbers provided are spring rates)
A "60%" aggressive plan would yield the following "toned down"
suggestion, where by "toned down" I mean to have only 60% difference between
spring rates of what is suggested by the "unmodified math"
or "straight line" method. There is no special logic to the 60% plan except
to pick a number out of the air, and not wanting to introduce as many other
odd effects of this plan as would be otherwise introduced, about which I am
not an expert.
LEFT RIGHT Spring Rate
500 550 FRONT
650 550 REAR
If anybody has any expertise on this subject or would care to comment on the
wisdom of this approach or some modification of it, I'd like to hear about
it and whether this is a reasonable way to go about it.
In addition, if you can see some reason to choose one plan or the other, or
a specific spring rate at a corner in which there is a choice, I'm all ears.
If you would set the spring rates to be heavier or lighter in the front vs.
the back, I could also make such an adjustment to the formulas that derive
these numbers.
It is not an interest of mine to redistribute the weight to be more even
than shown in these tables due to the tradeoffs.
You may be asking why the higher weights on the van are in half of an X
pattern front right front to left rear.
I couldn't say!!!!
I did set the ride height at all four corners at 19.5" and it is possible
that the sheet metal of the van is not actually a perfect measure of body
height, and so we should be applying a discount factor to account for errors
in the height of the sheet metal in the fender lips.
_______________________________________________
Derek Drew
Washington DC / New York
derekdrew@derekmail.com
Email is best normally but...
PHONE: 202-966-7907 (Call the number at left normally) (alt/cell for
diligent calling only): 703-408-1532
