 # Got Engineering Skills?

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#### LandCruiserPhil

##### Peter Pan Syndrome
Supporting Vendor
I dont and need some help.

Presently I have (2)~44.5" 90lb struts to open and keep open my CampTeq pop top (see PDF "A")
I want to relocate the upper mount of my struts 10" for better access and go with a 36" strut (see PDF "B")

The Question given the information on the PDF drawing what pound rating should I go with when changing to 36" struts? Keeping in mind with my current set up I would not want to go 1 pound lighter. If more info is needed ask #### Attachments

• CampTeq strut.pdf
295.2 KB · Views: 149
I would stay with the 90lbs, you will have a little more vertical push so it should be a little easier it sounds like you might want a little more lift anyways?

It is my understanding by moving the fulcrum closer to the pivot point I need more poundage to achieve the same performance, yes?

Well that would be on a system where you are leveraging one side of the lever against a weight on the other side of the fulcrum. Then yes you would lose your mechanical advantage by shortening your lever but in this situation you are still hinging the same amount of weight. And in this example some of your current 90lb lifting force is not going up it is being vectored into pushing backwards against the hinge. By moving the top mount forward now the lift is pushing up instead of back.

*quick edit.

If the top lid is a uniform weight we can assume the center of mass is the center of the lid. So if the mount for the lift is behind the center of the lid it is classified as a class 2 lever if you move the mount forward of the center of the lid it will be a class 3 lever and it will require more force than the weight of the lid.

Ok so math would look something like this:

the MA of the class 2 lever in its current configuration would be:
vector of 90lbs in upward lift ~70lbs
center of mass is center of lid = 39"
lifting point = 78"
MA = 78/39 = 2:1 so 70lbs becomes 140lbs of lift across the lid

In your proposed configuration probably something like:
Vector of upward lift ~80lbs
center of mass is center of lid = 39"
lifting point is 68"
MA = 68/39 = 1.74:1 so 80lbs becomes 139.5lbs of lift across the lid.

With that being said you might want a touch more lift, but it isn't going to be drastic.

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edited above for detail.

To get exact numbers on the vectored force of lift, you could calculate the angle of the lift rod but I am on my phone and am kind of lazy today. lol

When compensating for vectored force of lift, using the napkin method are you talking 1%, 10%, or 50% more lbs.?

I think you will need to move both ends so that the strut won't bottom out when the lid is down.

I was on my phone so couldn't/didn't want to calculate it so

I eyeballed the current setup and figured that it was proabably 70lbs of lift and 20lbs backwards force. Like I said you could calculate it directly by using the actual angle.

vertical force would be Fv=F *sin(A) Let me look at the drawing and see if I can get the angles.

I think you will need to move both ends so that the strut won't bottom out when the lid is down.

Not easy to move the bottom mount. But it is possible if the gains were big.

The struts A or B do not bottom out now. The measurement given is just distance number when closed. Would given them room when close change anything?
I can also get shorter in a huge variety of lbs if moving the upper mount more would help.

My end goal in a perfect world would be to unlatch and it open without assistance and have enough pressure in the open position to keep the lid in place with high wind.

I was on my phone so couldn't/didn't want to calculate it so

I eyeballed the current setup and figured that it was proabably 70lbs of lift and 20lbs backwards force. Like I said you could calculate it directly by using the actual angle.

vertical force would be Fv=F *sin(A) Let me look at the drawing and see if I can get the angles.

Thanks for the time. Im in no hurry just like to do things once when possible.

Current setup
Side A = 45" the bottom from hinge to bottom mount
Side B = 44.5" the length of the lift
Side C = 78" the top from the hinge to the mount
Angle A = 30* angle of top and lift
Angle B = 29* the hinge angle
Angle C = 121* <-- this is the angle on the bottom and the lift

Possible setup
Side A = 45" the bottom from hinge to bottom mount
Side B = 36" the length of the lift
Side C = 68" the top from the hinge to the mount
Angle A = 37* angle of top and lift
Angle B = 29* unchanged as it is the hinge
Angle C = 110* <--- new angle on the bottom and the lift

So vertical force is Fv = F sin(A)

Current setup vertical force = 90lbs * sin(121) = 77.1lbs
proposed setup vertical force = 90lbs * sin(110) = 84.5lbs

then factor in the MA (mechanical advantage)
Current setup
MA = 78/39 = 2:1 so 77.1lbs becomes 154.2lbs of lift across the lid

Proposed setup
MA = 68/39 = 1.74:1 so 84.5lbs becomes 147.1lbs of lift across the lid.

So you might want to bump up to a 100lb lift that would be:

100 * sin(110) = 93.9lbs * 1.74:1 = 161.8lbs lift about 7lbs more than current.

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• NLXTACY and Hornd
Landtank is correct. 44.5 minus 36 is only 8.5" difference. You moved the top mount point 10".
If your longer strut is nearing full compression when the lid is closed, you are probably going to have problems with the shorter one.

Ah, upon further review, I don't think it's an issue. I missed where you listed the collapsed lengths at the bottom. I'm used to struts that go almost closed in compression.

What's wrong with having additional force?

Don't they typically put strut gas up rod down to keep rod clean?

I had these guys do my custom struts for my m101. I added an aluminum top on which I mounted my RTT. I had specific locations where I din't want them so I can load gear and not have them interfere with the tailgate. He took the locations, my degree of opening, and my lid weight and gave me struts and a diagram on where to mount them.

He ships world wide, will work with you on the phone and supplies Bombardier. My 200 pound struts for my m101 cost \$36.00 for the pair and they were spot on. There was no guessing.

http://www.geminigassprings.com/

There are three different things going on when you change the location of the strut ends:
1) The force acting on the strut increases as you move the top point closer to the hinge. This is approximately linear, so the force at the new point will be 78/68 times greater.
2) The force exerted by the strut increases as its angle gets closer to vertical. The effective force is the rated force times the sine of the angle measured with respect to the ground. You would have to measure these angles as there isn't enough information in the diagram to calculate them.
3) As the radius of the top point to the hinge gets closer to the radius of the bottom point, the distance between them decreases, so you have to make sure that the closed length of the strut can be accommodated by the geometry.

The effects of 1 and 2 tend to cancel out. I'm guessing that it will be close to a push.

If you change both points so that the angle is the same, you could say with reasonable certainty that you would need to increase the strut force by 15%.

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Don't they typically put strut gas up rod down to keep rod clean?

You are correct in the struts being up-side-down. From the manufacture - the body should be in at the top to keep the seal lubed.

There are three different things going on when you change the location of the strut ends:
1) The force acting on the strut increases as you move the top point closer to the hinge. This is approximately linear, so the force at the new point will be 78/68 times greater.
2) The force exerted by the strut increases as its angle gets closer to vertical. The effective force is the rated force times the sine of the angle measured with respect to the ground. You would have to measure these angles as there isn't enough information in the diagram to calculate them.
3) As the radius of the top point to the hinge gets closer to the radius of the bottom point, the distance between them decreases, so you have to make sure that the closed length of the strut can be accommodated by the geometry.

The effects of 1 and 2 tend to cancel out. I'm guessing that it will be close to a push.

If you change both points so that the angle is the same, you could say with reasonable certainty that you would need to increase the strut force by 15%.

Existing Angle A is ~62°
Proposed Angle B is ~73°
About a ~17% difference Yes on my calculations it was the opposite side of what you measured. You measured the acute I quoted the obtuse.

Remember on a straight line acute + obtuse = 180*
So your 62* was my calculated 120*
Your 73* was my calculated 110*

With the present set up when I go to close the top about ~10" from closing the top will hold its position on its own. How if any will the new struts or strut angle change this? It is nice unplanned feature making tucking the fabric in when closing easy.