PH roof load - How much is too much?

[steven2]

did you guys use the 66 or 78" bars?

[avanduh]

I have taken 3 links out of the chains and put in 1/2" of blocks behind the starter springs

It seems like each link in the chain helps with about ~15lbs of assistance. I'm just guessing from the posts here.

What are 1/2" of blocks behind the starter springs?

[doublevan2]

When I bought my used 2002 E350 with auto lift PH top - the spare tire was on the roof on top of a large Yakima roof rack - and the tire had a bolt down tire clamp - I'm guessing all that weighed in at about 125 lbs ? I quickly dumped the tire to the left swing out (had my local garage weld a "church window" shaped tire mount onto where the large Aluminess box was removed (stored in my garage now). I also then added a 30 pound - 200 amp solar panel on top of Yakima rack. I hope to "eventually" - "dream" of adding slide out solar panels (2 x 160 watt) panels at 70-80 lbs - under the yakima rack - and then use my 120 watt Zamp suitcase panel to supplement. Ahhh - dreams !?

[boywonder]

Originally Posted by avanduh

It seems like each link in the chain helps with about ~15lbs of assistance. I'm just guessing from the posts here.

What are 1/2" of blocks behind the starter springs?

The helper/starter springs are the compression springs at the rear of the van sitting in the crossbar channels. You can space them forward with wood blocks or anything else that can handle 500+ lbs of load.

Spacing them forward allows the springs to contact the end of the sliding crossbar sooner......and compressing them more. You'll know when you've spaced them forward too much....the roof will be extremely difficult/impossible to pull down.

*never pull the roof down without the cover plates reattached over the helper springs.....it's extremely dangerous.


The helper springs assist for the first few inches of lift; the scissor design makes those first few inches really tough with horizontal springs.

The way the spring forces act on the crossbars is a trigonometric (sine) function. If the crossbars were to lay totally flat in the channel (they don't..but close) the springs would provide zero lifting force. As they extend upward when you are lifting the roof, the forces applied by the spring(s) increases by the sine of the angle between the springs and the bars.

A good analogy is cranking the pedals/crankset of a bicycle....same trig function....assume that you are standing up and applying a vertical "down" force on the pedals...

When the pedals are vertical you can apply a zillion pounds of force but that has no lever arm to propel you forward.....when the arms are horizontal you make a boatload of torque because now you have a big lever arm (the length of the crank) times your applied force.

If you plot the torque applied to the crankset based on this vertical force you will get a sine wave, with lots of torque at the peaks and zero torque where the sine wave passes through the zero/x axis.

So the roof is the same way basically from the bicycle cranks being vertical (when lowered) to being almost horizontal (when raised all the way). So it's not quite as simple as saying each link is good for XX lbs...but the extension springs do pretty much all the lifting after the first few inches.