can linear actuators fix the glass eparation issue?

[Monkey Wrench]

first off this is a bit long and I'm not having car trouble so you can skip this post if you aren't in for the somewhat long read. I'm just throwing some ideas around.


originally the 944 had gas cylinders that applied opening pressure to lift the back hatch and hood.

at first I figured ok it just needs new ones as they have lost strength but someone smarter than me pointed out to me the real problem is that these original gas cylinders ALWAYS apply this pressure to the hatch so the pressure is on there 24x7

ahhhh that turned a little light on and made sense to me.. .

His wise advice was use a stick and not replace them because if you do you are putting more pressure on it leading to the hatch and glass separating.

Now I' m wondering if there may be a linear actuator that is sized right and would fit in the area of at least one of the gas cylinders. it would probably require a circuit to power the cylinder until it was stopped by a limit switch and then it would have similar in order to close. perhaps a microswitch could also check that the hatch is unlatched or opened slightly to prevent it from trying to open when it is locked.

maybe just a small relay.. a microswitch can simply cut the power to the motor when it's met since that type of switch can handle enough amps.



I've seen some pneumatic cylinders with sensors.. the cylinder is aluminum and the ( reed switch? ) sensors can "read through" the aluminum. they sense the piston inside the aluminum pneumatic cylinder. Its a nice neat way to mount the sensor but then it requires a circuit to read the sensor's state. reed switch sensors seldom fail they are very reliable. we don't have a source of pneumatic pressure unless we incorporate a pump so the electrically operated actuators do have that advantage of not needing air pressure.



It would need to limit at the right length or have some sort of fail safe so that the cylinder couldn't extend beyond it's desired range and do damage to the hatch by overextending. perhaps there is a way to lock the movement mechanically from being able to go too far. maybe it can be sized so it limits by going full extension.

A braided steel cable could be used as an emergency limit stop, but there may be a more elegant way that is less visible.

A button could be hooked up so you open the hatch and then press a button and it would then extend , open the hatch and keep it there. the access to the button could be blocked by the hatch to make it impossible to reach unless you had opened the hatch at least slightly. that could prevent it being run without the hatch being unlatched. it would need a second circuit to close,, the wiring of the headlight lift is somewhat similar.

here is a link to an example which may not be appropriately sized. the cost seems reasonable. there are a lot of different sizes and styles. this one does incorporate a limit switch. Its not long enough but gives you an example.

https://www.amazon.ca/Actuators-Elimination-Waterproof-Electroic-50mm-15mm/dp/B09PCDJXDY/ref=sr_1_18?keywords=Linear+Actuator&qid=1678316892&sr=8-18

Just a half baked idea at this point but maybe there is a way to shop a bit and maybe get the cost to about 100 bucks, where it seems like it could be a logical solution.

the advantage the linear motor has is that when the hatch is closed, it would not keep that load on the hatch like the air cylinders do. when retracted it would stay retracted and no force would be applied to the frame. maybe that would be enough to solve the separation issue from occurring in the first place?

one disadvantage may be that they might be a bit bulkier, maybe there is one that is not too big and that will still handle the force as needed.

my first and very simple repair will be to just to cut a length of about 1" plastic pipe and stick that over the cylinder so it can be wedged between the end of the cylinder and the end of the rod, and when the hatch is lowered I can just lift the hatch slightly and slip the pipe back down over the rest of the cylinder where it can live. for this I will just retain the existing ones as they do add some support. it might only require 1 of these plastic pipe "props" I've done that simple fix successfully with other hatchbacks so I'll try it with my 944.

it is possible to recharge the gas cylinders either by sending the to someone with the right fixtures or by adding a Shrader valve ( tire valve).. I found videos on you tube of both methods being explained. you just drill a hole in them and solder in the tire valve and it stays there, you'd have to charge it to about 400 PSI and my compressor won't do that..

I figured out that if i were to couple them together with an airline you could push down on one and charge the other to whatever pressure you want. If you put 40 pounds on it the other would be charged so it's equal, by way of a check valve.

the thing is , these things hold something in the neighborhood of 400 PSI so if they do let go it may smash the glass. I had a workmate who had one go all of a sudden and it sounded like an explosion and terrified her when on the highway. I think the cap blew off the end of one of them. I decided although it's possible , I dont want to try charging them that way. I dont trust my soldering / braising job to never fail like that at that high of a pressure. In theory it does seem to make sense that this can be done..

they are filled with nitrogen not air but evidently this isn't really important. air will work.

[jsheiry]

If you have ever cut the glass loose from the hatch frame you will find out that the area where the hydraulic lifts connect to the frame/glass is THE most bonded difficult area to actually separate. And that the delaminated area is quite a distance from the strut mount. I have never seen a hatch delaminated at the strut mount area where the pressure is applied.

New hydraulic cylinders causing the delamination of hatches is THE biggest internet legend in the Porsche 924-944-968 series.

Delamination is caused by many things and perhaps not the same on all cars BUT the following list is the most likely:

-Sun UV damage to the 30-40 year old bonding technique IE to the glue/frit area up high near the hinges
-Hinges that dont move well very tight on one and easy on the other allowing a twist in frame to glass
-Mileage, stiffer suspension, older less compliant rubber suspension torquing body & hatch
-Hatches releasing poorly (not adjusted at latches properly) causing a twist as one releases while the other hangs
Bodies less rigid as miles pile up on the uni body cars allowing more twist into the frame/glass

Just one mans opinion but I have cut the glass off a couple frames and can assure you the problem is not where the hydraulic cylinder mounts or pushes

[djnolan]

I think reinforcing the hatch frame along the side where the stretching and bend is would be the answer. I apologize for he rough image.

[Monkey Wrench]

it seems that the shocks are putting about 40 pounds pressure on the bottom of the hatch and trying to shift it towards the license plate and if the bond to the glass is good there, well after 35 years or so, the metal frame there may grow but the glass is dimensionally stable so it cant' grow and pulls out from the frame at the top. twisting and so forth may help the frame grow.

I think that makes sense in my mind but i haven't yet dealt with issues with this, maybe others who have know more about it.

using an electric linear actuator could take that stress off but maybe it's a bit complicated. It could make it really hard to open the hatch with a dead battery. I don't like that. what if you need your jumper cables and they are stored with the spare? That would be a catch 22..

I'm also not sure if 50 dollar actuators are fast enough. If it took 30 seconds to raise that's a bit too long. 10 seconds may be ok.

Heres' another idea,
I've seen holders for hoods and things made with a iron bar that can pivot and fold up but goes a bit over center against a stop on the bar so it locks.. I guess to explain the mechanics, It's kind of like how an extended finger or knee can lock and take pressure but if your finger or knee is folded slightly it wants to fold in half more easily. ( because it goes just slightly over center at the pivot point, which is about mid way )

With that idea maybe the original shocks can stay and provide most of the lifting force but the bar may provide a positive lock so you don't need to rest it on your head while you load groceries etc.

that might be just slightly better than packing a stick in the car.

I'm not sure if there is room for such a folding bar to fold itself up in there, maybe?

my brother had a lifting 1 piece front hood and fenders in his triumph GT6 ( like a spitfire) and fashioned a bar like that. it didn't have air struts. it was just a flat bar with a pivot and sort of a "knee lock" for want of better description. It was simple and worked very well. all you needed to do was lift the hood and press on the bar so it went over center slightly and that locked it at it's full length. to fold it you just had to push it over center the other way and it folded up.

my hood has the same issues, weak shocks but I guess I can replace them, somethign like that might be a cheap solution.

the way some older and new volvos worked, they had a little lever thing on either hood hinge you can flip and it allows the hood to move and lock to a completely vertical position then you have more light to work with the hood swung vertically. You can pull an engine without taking the hood off. If it lifts like that, then it needs to lock in case it gets windy.

with those hinges what Ive seen happen is people neglect to oil them then they seize, then when the guy opens his hood he has so much leverage on the rusty pivot that the hinge breaks. Next he's down at the side of the road not being able to drive home because his hood has 1 broken hinge. if the hinge pins dont get lube this can happen easily.

[Brett944S]

It has been 20 or so years, but I had a 924 hatch re-bonded by an auto glass guy. He re-painted it flat black and glued it back together for $150. It’s still holding strong as far as I know. Even at double that it’s a sure fix.