Some time ago I had asked if anyone had done an elementary physics experiment in friction.

Does adding more wheels to a rolloffroof observatory reduce the static (starting) friction and the rolling friction?

All else being equal, more wheels would be a lower PSI on each wheel and the support surface under it, but there would also be more axles then before, generating more friction.

Well, things came to a head here at SCGO and I had the opportunity to get four more wheels of the exact same type.

The existing structure is an 8 foot long mounting surface with 5 wheels, approx 14″ apart with the first and last wheels also being 14″ from the end of the support.


| 14 W 14 W 14 W 14 W 14 W 14 | (1st image)

when in the closed position, all 5 wheels were in contact with the bottom surface (an upright 2×8).

Interesting discovery when the roof went into the open position… only the two end wheels were in contact, with the middle three free wheeling! That meant that when moving the roof, at some point, all of the weight was on only those two wheels! They may not be rated for that much, and the PSI is certainly much higher than thought and that is tearing up the surface of the bottom 2×8. Aha!

Either the 16′ of track (made from two 2″x8″x8′ sections) had issues, or the roof changed its shape or a combination of both.

strange. when the roof is fully closed, it is completely on a single 8′ board. the same when fully open, only on one board. So even if the 16′ were not completely level, it is still not straddling two boards.

Its like most of the roof mass/force was mostly coming down on the ends of the 8′ supports.

So I added an extra wheel at each end, approx 9″ from the previous end wheels. So before the roof support overhung the last wheel by 14″, now it is 14-9=5″.


|5 W 9 W 14 W 14 W 14 W 14 W 9 W 5 | (2nd image)

So far so good. It *seems* to be behaving better but it may take the wooden structure some time to change its shape and force loads, especially in the hot weather.

This of course required moving the end stops so the roof could properly come to rest in the correct position to connect the holddowns. hmm… now that I think of it, I only replaced the closed position stops… forgot to do the open stops!

Lastly, the final touch to help reduce the force needed to move the roof, some kind of hard cladding to cap the weakened 2″x8″x8′ top surface.

I know many of you said, go get some angleiron! But is is never that simple. local supplier? limited hours of operation? transport issues (only have tiny car now, no minivan), cost of materials, will they fit the existing wheels without binding?…

In any event, I am confident that now with at least *4* wheels in contact on each side, it will be an easier roof to move. Hopefully with the extra wheels on the end, the middle wheels will come down into contact as well.