Super-duper gas struts!

[kennyrayandersen]

I got an Andrew question on the struts:

Isn’t this just a static balance problem to get the required strut load based on hatch weight and strut location?

It seems like that’s a lot of force for a hatch that hopefully doesn’t weigh that much. Let’s say you have the strut attached half way between the CG and the hinge. That would make the vertical component of the strut load twice the weight of the hatch and the resultant load maybe 1.15 times that assuming a 60 deg strut angle. So if you had a 50 lb hatch, you’d need a strut capability of 115 lbs (that’s for both struts), plus some factor?

Of course, putting the strut closer to the hinge drives this load higher, but then causes a high vertical load at the hinge (we’ve already seen someone have trouble with this kick load!).

How do you figure?

[mikeschn]

Is this normal?

Yep, I thought the same thing. Mine are 150# and I tried and tried to compress them manually. Could not do it. I thought they would be way too stiff. But I went ahead and mounted them and they worked great. I think 120# would have been perfect but I have no complaints with the 150#.

that reminds me, I gotta get some too. How do you like the ones from McMasterCarr? got a good close up photo of the end where you screw on the fitting?

Mike, I got mine from McMaster and I am very satisfied. In fact I bought a pair and had to switch them out for a shorter pair. They took them back with no questions asked. I used a bracket like the one below to mount mine.

I've got a long set of shocks picked out from McMaster Carr. I'll be placing the order this week.

Mike...

[MCI1969]

The c16 number on the prop is the manufacture's part number. That same number can be used to get replacements from you local sources...

Quick tip on the gas struts : find your local dealer for truck canopies; LEER, ARE it doesnt matter. If they make or sel lan aluminum model even better. They'll usually have a supply on hand of props in various length and poundage as well as mounts, seals, trim, molding, hurricane hinge and maybe even smooth skins.

[angib]

Here is the techie answer to calculating gas strut loads. I do not recommend that anyone (but the anal) should repeat this calculation!

The hatch weight and the gas strut uplift need to be measured as moments around the hinge and that will tell you whether the net result is to hold up or drop the hatch. So the lever of the gas strut is different for every teardrop and for every angle of lifting.

In this diagram the dimensions show the hatch and strut levers when the hatch is in the red position:



And this table shows the calculations for this geometry:



However readers should note that I drew these diagrams with the gas struts attached in the wrong places so that they are trying to open the hatch hinge when closed. Instead they should be attached like this, so that they force the hinge closed when the hatch is down:



These are the dimensions recommended by Grant and I ain't trying to second guess him! He believes the struts should be mounted this way up too, but I believe they should be swapped end for end (though still attached in the same places).

With the sort of length of gas strut that's readily (and affordably) available, it can't be attached very far from the hinge, so the leverage means that struts rated at much more than the hatch weight are required.

The struts can't be attached very far from the hinge because then theywould have to be very long struts (like 36-48") if they were going to have enough travel to get the hatch open to the required height.

There are a lot of variables in selecting and positioning gas struts - I think the only practical solution for someone who doesn't have a CAD system and the knowledge to repeat these calculations is (a) to follow Grant's positioning advice and (b) be able to swap the struts for higher/lower rated ones if the first result isn't satisfactory.

Andrew

[kennyrayandersen]

Here is the techie answer to calculating gas strut loads. I do not recommend that anyone (but the anal) should repeat this calculation!

And we know who we are

The hatch weight and the gas strut uplift need to be measured as moments around the hinge and that will tell you whether the net result is to hold up or drop the hatch. So the lever of the gas strut is different for every teardrop and for every angle of lifting.

I know that, but for whatever reason I just froze it in my mind in the open position (I don’t analyze many mechanisms, but it’s obvious after looking at you kinematic diagram.

However readers should note that I drew these diagrams with the gas struts attached in the wrong places so that they are trying to open the hatch hinge when closed. Instead they should be attached like this, so that they force the hinge closed when the hatch is down:

Nice touch

These are the dimensions recommended by Grant and I ain't trying to second guess him! He believes the struts should be mounted this way up too, but I believe they should be swapped end for end (though still attached in the same places).

One of the other things I hadn’t noticed until now is that when you mount them this way, the put the hinge fasteners nearly exclusively in shear when the hatch is closed, which removed that vertical kick that someone else was having trouble earlier with.

With the sort of length of gas strut that's readily (and affordably) available, it can't be attached very far from the hinge, so the leverage means that struts rated at much more than the hatch weight are required.

Even so, set up like this it looks like most of the load, even in the open position is reacted as shear in the hinge – yeah there is a component of normal load, but it looks to be not unmanageable.

The struts can't be attached very far from the hinge because then theywould have to be very long struts (like 36-48") if they were going to have enough travel to get the hatch open to the required height.

True dat.

There are a lot of variables in selecting and positioning gas struts - I think the only practical solution for someone who doesn't have a CAD system and the knowledge to repeat these calculations is (a) to follow Grant's positioning advice and (b) be able to swap the struts for higher/lower rated ones if the first result isn't satisfactory. Andrew

I was a bit concerned with the reactions as I wasn’t planning on solid walls. This clears it up exactly – Giant Thanks!