[hduff]

There are two gas springs on the trunk of my '37 Plymouth coupe and they don't work very well. I suspect it's more because of mounting location that the spring rating, although when fully opened, the trunk will slowly close.

The trunk weighs about 14 lbs (measured force to raise it without assist).

The gas springs are 30" extended, 18" compressed. There are no markings on them to determine their rating.

I'd appreciate some advice on how to properly do this.
Here's a look at them:
http://i.imgur.com/FvBAy.jpg
http://i.imgur.com/eEwed.jpg

[LATECH]

there are places thatsell pneumatic supports by specification like length and appliction of force etc.
you can google : pneumatic lift supports and stuff will come up.
Some of the local parts stores should be able to help with styles according to spec (length and support pressure/force)
I got some for a motorhome basement storage doors that way. I got the right lentgh and several different ones of that length with different force ratings and tried a few to get the desired capacity I liked.

Ningbo of china makes a bunch.check them out.

[hduff]

Thanks, Latech. I've seen some at the local NAPA as well.

How about how the geometry of how they are mounted? Any links for that?

[LATECH]

Originally Posted by hduff

Thanks, Latech. I've seen some at the local NAPA as well.

How about how the geometry of how they are mounted? Any links for that?

Is the car pictured yours or was that for example?

[LATECH]

Not too good with the geometry of it, at least not here on the couch.
However the Isoscoles Triangel formulation should work
The sum of the angles of the 3 sides of a triangle should equal 360 degrees.
If you know 2 of the angles then you should subtract from 360 to get the third.
Angle 1 plane of trunk with lid closed
Angle 2 plane of open lid
Angle 3 difference of the two
Once you know the third angle then finding the correct lentgh will be the next step after you figure your placement on the car( when placed with the 3rd angle in play)
Then you will need to figure the ratio of weight to be supported by the amount of lift force a available

[hduff]

Originally Posted by latech

Is the car pictured yours or was that for example?

That's my car.

[LATECH]

Originally Posted by hduff

That's my car.

Looks like the math has been done. Just get some replacements for the ones you have

[hduff]

Originally Posted by latech

Looks like the math has been done. Just get some replacements for the ones you have

I don't think the guy who installed them did the math correctly. (He did a lot of miscalculation on this car. This is one of the last things I have to make right.)

When the trunk is closed, the struts are at a negative angle to level and the lid just collapses fro the last 6" or so; there is no support provided by the struts. Stronger gas springs won't fix that.

In other installations I've seen, shorter springs used that are vertical when the trunk is open and horizontal when the trunk is closed.

There must be some formula for calculating where the attachment points should be and what the size of the gas spring should be.

[LATECH]

To set the lifts horizontal would be OK. you could for the biggest part hide them under the package tray area and just make a braket that bolts to the trunk lid close to the forward edge of the lid(just below rear window.
You could cut them from aluminum and anodize them in a bronze color or just use steel and paint to match.
You would be able to use a shorter lift support( with more applicable force of course) and a shorter travel.That would multiply the lift force ratio of course but would allow you to mount them in a less noticable place than where they are.
Decide how and where you want to mount them .Then you will be able to figure the angles and lift arm travel and body length etc.

[Cees]

Originally Posted by hduff

There are two gas springs on the trunk of my '37 Plymouth coupe and they don't work very well. I suspect it's more because of mounting location that the spring rating, although when fully opened, the trunk will slowly close.

The trunk weighs about 14 lbs (measured force to raise it without assist).

The gas springs are 30" extended, 18" compressed. There are no markings on them to determine their rating.

I'd appreciate some advice on how to properly do this.

Want some calculation support from an expert?

[hduff]

Originally Posted by Cees

Want some calculation support from an expert?

Of course.

[4 Jaw Chuck]

Here is a PDF with application specs and installation calcs to determine proper product selection, you will have to gather dimensions and geometry and determine exactly how you want the lid to move if you want to redesign.

http://www.enidine.com/pdffiles/Gas_Springs.pdf

It does not look improperly positioned for a hold down application since they overcenter when closed (usually desirable in a trunk). Have you thought about just replacing them, if they leak down they are shot. No markings usually signify a reject parts bin purchase, a name brand replacement might be all you need to make them work and last. I know around here you can purchase the off brand with no markings since they are rejects, they usually don't last very long.

[hduff]

Originally Posted by 4 Jaw Chuck

It does not look improperly positioned for a hold down application since they overcenter when closed (usually desirable in a trunk). Have you thought about just replacing them, if they leak down they are shot. No markings usually signify a reject parts bin purchase, a name brand replacement might be all you need to make them work and last. I know around here you can purchase the off brand with no markings since they are rejects, they usually don't last very long.

I had not considered that since they have not worked properly since i purchased the car only a few years after it's build.

I'll look through that PDF, make some calculations and select some new ones and give those a try.

Thanks.

[ogre]

i found the best price from mcmaster carr http://www.mcmaster.com/#gas-struts/=az2li2
i used these on my trunk and tonneuo cover

[Cees]

The examples you find on the internet about calculating gas spring are just poor attempts to solve the problem properly.

To solve a gas spring application problem, you need to determine the moment-line which represents the way the moment (force x distance) looks like over the complete rotation of the object you want to control.

Therefore you need to know from the object where the center of gravity exactly is located.

It can be calculated but with the hood of a car and other similar shaped objects it becomes very difficult.

The most important step in the complete procedure is to collect the basic design data as input for the calculation.

With a (self made) angle indicator and a balance we are able to get the right information to make a calculation to select the gas springs we need.

Procedure to collect input data for the calculation:
- Attach the angle indicator on the hood of the car (with adhesive tape)
- With the object in its rest position (hood closed), set the angle indicator to 'zero'.
- Rotate the object to the desired end position (hood fully opened) and read the max. rotation angle from the angle indicator.

1) Write down:
max. rotation angle = ....... degrees.

- Remove both struts from the attachments on the rim of the hood.

- With the object resting on a balance, lift the object to, lets say, 15 degrees from its start position (observe the angle indicator) and look what force it requires to maintain this situation.

(any angle easy to read and near (!!) to the start position will do)

- measure the exact horizontal (!) distance between the hinge of the object and the point where it rests on the balance.

2) Write down:
1st moment-angle = ....... degrees.
1st lifting force = ....... kg.
1st lifting distance = ....... mm.

To complete this procedure we have to do the same actions but at a different lifting angle:

- With the object resting on a balance, lift the object to, lets say, 55 degrees from its start position (observe the angle indicator) and look what force it requires to maintain this situation.

(the thing is to get a considerable difference between the required lifting forces determined at the 1st moment-angle and the 2nd moment-angle)

- measure the exact horizontal (!) distance between the hinge of the object and the point where it rests on the balance.

3) Write down:
2nd moment-angle = ....... degrees.
2nd lifting force = ....... kg.
2nd lifting distance = ....... mm.

At this point we gathered information by which we can calculate the object-moment over the complete rotation angle.

When I look at the pictures you took from your car's boot I can imagine you prefer not to replace any of the attachments.

So, with the hood opened to the desired max. rotation angle, please measure the following shortest distances:

- between the hinge and the attachments on the hood (center to center).
- between the hinge and the attachments in the boot (center to center).
- between both of the attachments (center to center).

Note: When these distances are difficult to measure because of the fact that the hinge is not located in line with the attachments, connect each attachment by means of a thin rope with its companion on the opposite side of the car.
Now you can measure from the center of the hinge perpendicular to each of the two lines.
Don't forget to involve a necessary correction if the thin ropes are not leaving the attachment pins at their centers.

Furthermore there are a few questions left:
- How, we wish the behaviour of the application should be?
- What will be the temperature range for the gas spring application to operate as mentioned?

Answers:
- To lift the hood, some force must be needed (approx. 30 N)
- Beyond a certain opening angle the hood shall move without any support to the fully open position.
- To move the hood from its fully open position some force must be applied (approx. 50 N).
- Beyond a certain closing angle the hood shall free fall to the closed position.
- With the hood in closed position there should be 5 mm's of the stroke left.

- This behaviour should be valid between -20' C and 50' C.

Gas spring force increase and degrease with temperature rise and fall.
When the piston rod is pushed into the cylinder the force of the gas spring will increase according to a progression factor.
The progression factor depends on the type and make of the gas spring.

- A 15% reduction in gas spring force due to a long period of use should be taken into account.

If you prefer not to solve your problem by means of trial and error, please let me know.
I can provide detailed information about my self made angle indicator.

Don't worry, I will do the math for you.