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Date:         Sat, 26 Nov 2011 07:43:10 -0500
Reply-To:     Derek Drew <derekdrew@DEREKMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Derek Drew <derekdrew@DEREKMAIL.COM>
Subject:      Westy Weighed... Setting Corner Spring Rates
Content-Type: text/plain; charset="us-ascii"; format=flowed

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


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