I find this an intriguing subject, so I decided to go back to first principles and work out what happens with a WDH. So, I've produced two spreadsheets which I've attached (hopefully successfully). I'm an electrical engineer not a mechanical one, so I had to try to recall my high school applied math (levers, moments, etc.) which is pretty rusty, so I cannnot guarantee the accuracy. It would be great if someone expert could check the math, assumptions, etc. and I've included the equations I used for that purpose.
The spreadsheets are in Microsoft Works format since a lot of people don't have Excel. They need unzipping.
In the first spreadsheet you enter your measurements and weights in the outlined boxes ( the inital values are for my TV/TM combination) and the spreadsheet calculates what the WDH tension must be to equalize the net load on the TV front and rear axles. Since this is the maximum tension you want to put on the WDH bars ( the general guidelines allow you to leave a bit more on the rear than the front), I've included a second spreadsheet.
The second spreadsheet allows you to enter your own WDH tension (per bar).
So with this you can mess around with different values and see what happens on the front, rear and TM axles.
One thing I found interesting using the first spreadsheet is that the WDH bars really have to work hard to compensate for trunk contents for only a little benefit at the front axle. For my vehicle combination, 200 lbs in the trunk instead of 50 lbs unweights the front axle by only 24 more pounds, not really significant overall, but the WDH tension has to increase by more than 200 lbs to compensate, taking the bars into a higher load category. I'm not convinced it's worth it in most cases.
Of course, all this is all moot if my spreadsheets are not good.
Paul
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