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Compression spring selection question
John Bonnett - 8/8/21 at 05:14 PM

The boot lid on the car I'm building will need a spring to work in conjunction with the release catch so that when the latch is unlocked the boot lid will be pushed clear of the mechanism. My idea is to insert the spring into a tube so that there is a spring loaded captive rod poking out the end a bit like the connector on the end of a cigar lighter plug. The boot lid will be light, maybe 500 grms but I have absolutely no idea either where to source a suitable spring or what size to get either in terms of wire diameter or length. Ideally it needs to fit inside 20mm O/D tube with 1.5mm wall thickness.

Any advice much appreciated. Thank you.

Dingz - 8/8/21 at 07:24 PM

Look on the springmaster or terry sites they will have a huge range which may give you some idea. Once upon a time they may post a sample if you talked nicely to them.

John Bonnett - 8/8/21 at 07:33 PM

Very many thanks. Will give them a go.

Schrodinger - 8/8/21 at 07:56 PM

How about a rubber seal around the aperture that will be compressed when closed and act as a spring when released?

John Bonnett - 8/8/21 at 09:04 PM

quote:
Originally posted by Schrodinger
How about a rubber seal around the aperture that will be compressed when closed and act as a spring when released?


Good suggestion, thank you but the seal I'm using is a hollow section so it won't provide the necessary shove. I think it does need a spring or a solenoid.

Schrodinger - 8/8/21 at 09:23 PM

What catch are you using? On my Stylus I'm using a small bear claw catch with a hollow section seal which stays open once released, admittedly my boot lid is small and made from grp but the latch stays open unless you push it closed.

John Bonnett - 9/8/21 at 08:39 AM

I'm using a catch from an MX5 which needs the boot to spring up to rotate the catch and if that doesn't happen then the boot will remain locked.

Thank you for responding. My plan is, as I mentioned, to make a spring loaded plunger that will maintain an upwards force on the boot lid and as soon as the catch is operated the lid will partially open.

Theshed - 9/8/21 at 09:16 AM

Could you not use a gas spring? That might neatly fit in a tube.

adithorp - 9/8/21 at 09:26 AM

Don't know about strength etc, but if you work it out, I've got a box of various ones I could search through to find something similar to requirements.

John Bonnett - 9/8/21 at 09:29 AM

quote:
Originally posted by Theshed
Could you not use a gas spring? That might neatly fit in a tube.


A nice thought, thank you and an elegant solution but I like simple and for a few coppers I can get a compression spring and make the housing from material I have lying around.

Russell - 9/8/21 at 04:17 PM

Or you can buy them ready made for just over a fiver from eBay. Search for door popper.

But where's the fun in that when you're as handy with the tools as you are John? :-)

John Bonnett - 9/8/21 at 05:23 PM

quote:
Originally posted by Russell
Or you can buy them ready made for just over a fiver from eBay. Search for door popper.

But where's the fun in that when you're as handy with the tools as you are John? :-)


That's kind of you Russell, thank you. Until you actually get to to building a car there are many things that you (I) don't consider or even think about. It was only when messing about with the boot lid frame that I've just fabricated and the lock mechanism that it dawned on me that it need a push upwards to get it clear of the latch. Up until then I hadn't realised that a popper was necessary.

Anyway I've sourced a box of compression springs and found one that looks as if it might do. It inserts inside a 12mm O/D stainless tube so a good starting point. I machined a plunger with a 6mm diameter spigot and a body to suit the inside of the tube so that it is a nice fit and slides easily up the inside of the tube. I then made a plug and TIG welded that into the end of the tube after which I drilled a 6.5mm hole in it. The plunger dropped in and satisfyingly the spigot appeared out of the hole in the end plug. Next the spring went in and finally to hold everything in place, a stopper secured it all with a split pin.

If it all works I'll update this thread with some photos.

In the meantime, thank you all for your good advice and suggestions.

nick205 - 12/8/21 at 12:46 PM

As always John it will be great to see some photos of your work and assembled mechanism.

Reading this thread has reminded me of the boot catch failing on SWMBOs 2007 VW Touran last year. I had to replace the catch mechanism, which actually has the "popper" function designed into it. That however has to cope with central locking and ideally 20 odd years of service. It wasn't expensive to replace, but the failed old one couldn't be serviced back to life.

John Bonnett - 12/8/21 at 04:38 PM

quote:
Originally posted by adithorp
Don't know about strength etc, but if you work it out, I've got a box of various ones I could search through to find something similar to requirements.



A great offer Adi, thank you. I sourced a box of assorted springs and picked one which I hope will work.

Kind regards

John

John Bonnett - 21/8/21 at 04:03 PM

I'm delighted to report that the boot popper does the job. It is simple mechanical and independent of the electrics. Thank you all for your great help and advice.