gas strut question...

[Chris C]

Thanks a million, Andrew. Man is it ever nice to have our own "engineer in-house"!

[emiller]

My first teardrop I mounted the strut 6" from the hinge on the hatch. and my secound teardrop I mounted the strut 6" from the hinge on the body be ccause when you have it close to the hinge on the hatch it puts alot of strain on the hinge and tend to lift. here is my new teardrop pics.
http://pg.photos.yahoo.com/ph/teardropc ... .done=http%

[brickz]

Andrew (anointed Chief Engineering Nerd) - as a lazy mechanical engineer who just took an intuitive (and lucky) guess, I humbly bow to your excellent analysis. While you may not fully grasp BBQ and other similarly important things like we in the US, you certainly have the engineering side covered.
The only other comment I could add is that it appears maybe you could spend a little less time in front of your computer and more time camping in your teardrop.
Brickz

[angib]

because when you have it close to the hinge on the hatch it puts alot of strain on the hinge and tend to lift.

Yup, that's problem number one with mounting the springs close to the hinge on the hatch. Problem number two is that you've then got maybe four feet of hatch hanging beyond the springs, so the hatch ribs at that point take a pounding from the weight of the hatch and the wind.

However gas springs will always put quite a lot of strain on the hinges - like 200lb when the hatch is closed, in my example - so you need to have your hinge securely mounted to strong ribs.

Andrew

PS Too busy to go camping - have just come back from 250 mile shake-down ride on my bike in glorious sunny weather - but disappointed in 42mpg (US) cruising at 90mph.

[doug hodder]

Thanks for the info Andrew, I have the shocks, but now that I read your post, Im probably too light on them, only 2 60# struts, and I'll bet the hatch weighs all of 60#. I'm not known for building lightly. I need to spend some time thinking through all the info you have given. I think for Minden, I'll come up with some classy mohogany sticks, and work out the details later, I've already got some screw holes that I need to hide from an errant strut install. Doug Hodder

[bledsoe3]

Andrew, I could build my teardrop in the amount of time it would have taken me to do all the math you did . Nice to have someone smart on our side . Thanks

[fornesto]

If you are using lighter struts for any reason, you can build a real easy snap-on brace to keep the hatch from falling, or being blown down, on you. My concern was the latter. Simply cut a 12-15" piece of pvc or plastic conduit (1/2") and cut a section out of the middle lengthwise. The "O" profile becomes a "C". When your hatch is up, snap the brace on the exposed rod and the cylinder won't be able to close. There's a pic in my Yahoo folder.

[angib]

The snap-on gas spring lock is a great idea. I don't personally think that I'd ever trust just the gas springs on their own, unless they still had a really big lifting force when the hatch was open - the damage and/or injury a falling hatch could do is scary.

The gas spring can also limit what the hatch is used for - if I was camping in the weather you guys get, I'd sure want to be able to clip a sun awning on to the hatch without it coming down.

For the big boat hatch, we specified locking gas springs - when fully open, a small catch prevented them from being closed, which would be ideal for a teardrop.

Andrew

[roger-c]

angib
I used your info to install gas shocks on my trailer hatch this weeked and they came out great!
Thanks a bunch for your hard work!

rogerc

[Gerdo]

Andrew,

What do you think if you reversed it and had the strut (gas spring)13" out on the hatch and 30" out on the side? It looks like you would have a better mechanical advantage since the strut would be at about a 90 degree angle to the hatch when open. Would you need to change the strength of the struts?

I did not weigh my hatch before mounting, but I have weighed it at the back edge by proping it up on a scale and accounting for the weight of the prop. If the hinged weight is 30# then can I assume that the total weight is about 60#?

I figured that with a hinged weight of 30# and my hatch is 60" long and my strut is mounted 12" from the hinge then the weight on the struts would be 5 times the 30# = 150# /2 = 75# struts. Do you think that I should go with 110#, 120# or 150# struts (gas springs)? I don't want something that is going to sink because it is "just enough" nor do I want to have to pull too hard putting alot of strain on the hatch, strut mounts, and hinge.

[angib]

Gerdo,

1) Swapping the way the gas springs are mounted would make very little difference to how strong they need to be. Here is the example above with the springs mounted both ways:



How effective the gas spring is in holding the hatch open depends on how far out from the hinge it works. This is measured at right angles to the line of the spring. You can see in the two cases above, the difference is negligible. I've checked all the intermediate positions and there's no difference there either.

That wouldn't necessarily be true for a different hatch design, but I don't think the difference would ever become huge.

What is different is that now, in the right example, there's three-quarters of the hatch hanging out past the gas spring, compared to only half the hatch in the left example. So the bending in the hatch ribs is more than doubled - though whether that's significant depends on how strong they are!

2) Your hatch weighing method sounds fine to me, as long as it was perfectly level when you weighed it, and the prop was right at the end of the hatch.

3) As I understand it, your biggest loading is a hatch weight of 60lb and a distance to the hatch's centre of gravity of 30", so that's a 1800lb-in closing moment.

Divide that by 12" for the lever of each spring, and by 2 for two springs, and you get 75lb force on each spring.

110lb springs would give a 50% margin on that. Roughly (because I don't know your hatch's geometry) you would have to pull down with a force of 14lb on the end of the hatch to close it. With 120lb springs that force goes up to 18lb and with 150lb springs it goes up to 30lb.

So even with 150lb springs, double the 'hopefully just enough' level, you're not going to be hanging two of you off the end of the hatch to close it!

Andrew

PS I was thinking of changing my avatar to a 'boffin' (you know, white lab coat with plenty of pens in the top pocket, and a forehead at least 12" tall polished to a fine shine), but I can't find one on the web (and I look more like the missing link to the apes, so a self-photo is out).

[madjack]

PS I was thinking of changing my avatar to a 'boffin' (you know, white lab coat with plenty of pens in the top pocket, and a forehead at least 12" tall polished to a fine shine), but I can't find one on the web (and I look more like the missing link to the apes, so a self-photo is out).

...I still think Terry Thomas was the best............................

[mikeschn]

I'll have to get you a photo of how my gas struts ended up. I tried it the opposite way, like angib showed in the first picture up there. I ended up breaking the end of the strut off, and putting a huge hole in the inside hatch skin, plus putting all kinds of ungodly holes in the side wall.

I'll be right back with a photo...

Mike...

edit: okay, I'm back... here's the picture of how I messed up the gas shock mounting...

[angib]

I ended up breaking the end of the strut off

The only way to do that, that I can easily see, is to have mounted the spring so that it bottomed out before the hatch was fully shut.

If that is the case, which way up it was mounted is definitely a secondary issue!

Andrew

[Gerdo]

Thanks guys. I guess I'm ready to order my struts. I'm going with the 150#ers.