project 1980 Austin Princess - Seat Diaphragm Prototyping

[vulgalour]

 

 

We encounter the problem of a spare part that isn't available this time: seat base diaphragm.  After doing this job I learned there are at least two versions of the Princess seat diaphragm.  One type is like that found on Triumphs which is a big square rubber sheet that attaches to the seat frame with hooks.  The other type is the one I have which is a combination of hessian, rubber straps, metal rods, and metal hooks to hold it to the frame.  It might be possible to adapt a Triumph base to fit the Princess, I couldn't tell you, what I can tell you is how I went about this job instead.

 

For once, the Princess chose a good time to give me this problem and it was as I was reversing on to the drive that I felt myself sinking slowing into my seat as the old diaphragm gave way and the seat gradually became more uncomfortable.  There was no real warning that this was going to happen, no dramatic twanging noise or similar, just a sad deflated balloon of an experience.

 

Because I still had to use the car I had to employ some tried and tested seat support as seen in many a Morris Minor and stuffed what cushions I could find under the seat to prevent it sagging further.  No spare car to deploy at the time so we do what we must.

 

Some time later I could remove the front seats to investigate.  Rather than removing the driver's seat and disabling the car, I removed the passenger seat so that I could work out a solution without disabling the car.  On the passenger seat the diaphragm is still in excellent condition and I would like to keep it that way.  A short term fix was definitely to just swap the diaphragm over and try and fix the passenger seat but I know myself well enough to know that this would lead to me putting off repairing that seat until the good diaphragm also failed and making a replacement would be harder.

 

My suspicion is that what had failed is the rubber strap because there was no tearing noise when it did let go which would have suggested the hessian type material had torn.  What I did learn is that the passenger seat isn't in perfect condition, when I touched the rubber straps they did leave a black powder on my fingertips suggesting that it is degrading and will fail probably sooner rather than later.

 

First step was to have a look online and see if I could find the components to rebuild all of this and I did find some rubber webbing straps that although wider than the originals were similar enough in all other ways that I was fairly confident it would work.

 

The next challenge was going to be how to actually make this.  Originally the rubber webbing is bonded to the hessian type fabric which is itself coated in some sort of resin or stiffening agent.  This might all be done in one go, or in multiple steps, there might be heat involved.  I don't know so I'm pulling on my experience as a costumier and general craftsperson to figure out a likely solution.  Now I was as ready as I could be for making something, the driver's seat had to come out to see what I was actually repairing and as suspected, it was the rubber that had failed.

 

Next job was to actually remove the diaphragm.  I suspect this is put in before the foam and cover is put on the seat and while it's not impossible to remove it is difficult.  There's several bent steel hooks that click into holes in the seat frame and you'll be very lucky if you don't make a knuckle offering to the car gods in the process of removing them.

 

With that removed you get to see where the pressure switch lives that operates the seatbelt warning light on the dashboard.  Interestingly, while there is a cut out in the foam for the driver's seat, there's no wiring in the car for it on this side and no switch installed.  On the passenger seat the wiring is there and the switch, obviously, covered here with some blue vinyl (the seats were originally black vinyl) which is presumably whatever scrap they had available for the job.  No point spending money matching vinyls where it doesn't matter.  Could I do some clever wiring work and install the spare pressure switch I have into the driver's seat?  Probably.  I didn't, it's a lot of work for a design that isn't reliable since you have to be just the right height and sat in just the right place for the switch to actually work.

 

One curious thing I did learn is that it's possible somebody has been into the driver's seat before because one of the hooks that attaches the diaphragm looked to be snapped at first glance but was actually cut.  Quite why that was done I'm not sure.  Perhaps originally there was a pressure switch in this seat and a previous owner removed it because the light was annoying?  It would have been a lot easier to just remove the bulb on from the indicator on the dashboard and there's no audible buzzer so... who knows.

 

All that investigated I could mock up a new diaphragm and this is what that looked like, with the help of some hessian leftover from working on the Lanchester's interior.  I opted to use contact adhesive on the first attempt.  I couldn't pull this apart and it's usually strong enough for upholstery application I've used it on before.  The proof would be in the testing.  I used a brass brush to key the rough side of the rubber webbing before applying the adhesive to give the maximum chance of success that I could.  I also had a spare copper plated wire coathanger that was the exact thickness for the hooks so I took a stab at making one of those and, on my second attempt, managed it without snapping the wire.

 

To make the new diaphragm I used the broken one to give me all the relevant reference points.  The diaphragm is made from an edge piece of hessian folded in half that I glued to the rubber webbing, and then a rectangle of hessian glued to the other side, another rubber webbing piece, and another folded hessian piece.  A metal rod rests in the folded piece and this is what you lock into the closed ends of the metal hooks.  Using the old diaphragm I could make sure my dimensions were correct, as well as the location of all the hooks.

 

It takes a while for the glue to cure and do its thing and once I was happy everything was ready it was time to install.  There's quite a bit of tension on this part, as expected, and while I had tested the strength of the glue with the parts off the car, I wouldn't know if it had worked until I installed the diaphragm and sat on it.  Or in my case, I'd find out the glue wasn't good enough at the installation stage which was frankly quite disappointing.  Such is the nature of R&D.

 

Back to the drawing board.  In theory I felt like I was on the right track with this, I just didn't have the requisite tools and knowledge to test the properties of the adhesives beyond trial and error.  My second attempt was to use a polyurethane, I forget which brand, which I didn't record.  Unfortunately this did work quite well out of the car and only failed after I'd reinstalled the seat.  Each time the problem was that the adhesive wouldn't stay stuck to the rubber webbing so perhaps a rubber specific adhesive would have worked instead.

 

Rather than fighting against my lack of knowledge, I turned my attention to something I felt more comfortable with: sewing.  First was to deal with the fact that the foam in the seat base was past the first flush of youth and given the fact I was already in here I might as well cram a slab of medium density upholstery foam in. It was about a fiver and exactly the dimensions I needed. I crammed it in after the seatbelt warning light switch and before the new diaphragm. I went for about three fingers thickness, seemed about right.

 

That new diaphragm then, how am I making that?  I needed something that wasn't going to stretch too much, that was suitable for a seat, and that was going to be stable.  I had a huge amount of very strong cotton that was given to me back when I had the Austin 1100, a Singer 201K sewing machine, and I found some Parker Knoll replacement chair springs that happened to be the exact length I needed.

 

First take the big square and fold the edges in so it's as small as the original diaphragm centre (sorry, there's no measurements here, I'm not even sure anyone else would ever bother to go to this length to make one of these so I didn't think to record that side of things at the time). Then sew the edges down to secure.  The side showing here will be against the foam so that the spring channels we do next are hidden and the underside of the seat is nice and smooth.

Then take your five long strips and fold the raw edges in, pinning them to align with the locating hooks.  The long edges of these will get sewn down to create channels that you will push the springs through.  Do take time to make sure things are pinned straight before sewing, keeping these channels straight makes putting the springs in a lot easier.  A woven striped fabric is also good here because it gives you a really good visual guide, you can't trust printed stripes as much as they don't always stay true.

 

You should end up with something that looks like this.

 

If you've been foolish enough to overstuff your seat base like me, you'll need someone else to help you get the springs on to the hooks and into the frame.  It's an awful job!  You will both lose more knuckle skin.  The little copper hook I made is like a little hidden gem that nobody but us will every know about now.

 

I did both seat bases and put the original good diaphragm in storage should I need it for future reference, I absolutely do not trust it to last in use in the car.  Apart from initially looking far too plump, the seats look perfectly fine.  I opted to overstuff a bit as I know the foam will squish to a more normal size and as I write this over a year after doing the job I can happily say they've settled in nicely since this shot was taken.

 

 

As for how these have held up, I have to say I have no complaints.  You do perhaps sit a little lower than factory in them, the springs have just a bit more give than the diaphragm. They are nice over really rough roads since they provide some suspension of their own and I find that it puts what little lumbar support the Princess seats do have in a slightly better position for me.  I've not noticed any signs of failure or wear, they don't squeak or do anything untoward, and passengers have had no complaints including the ones that have ridden in the car before the new custom diaphragms were installed.  Overall, I'd say this was a success and a nicer solution than the big cable ties option I've seen deployed on MG Bs, Heralds, Minors, etc.

[MorrisItalSLX]

Your solution is far better than my efforts with some thin rope when the seat in my Corolla failed in the same way.

That being said, my repair has lasted for 5 years at this point, so it must be alright.

[vulgalour]

That's a remarkably similar looking seat base!  Five years is definitely a proof of concept, I'd say your repair is perfectly good and probably a lot less work than my solution.

[kevins]

You can buy Recaro diaphrams which look very similar, but they are £70 each., your solution looks better value!

[Angrydicky]

That looks superb! I wish I had your skills, the drivers seat in my Princess is quite saggy despite my efforts to restuff it.

[Surface Rust]

I was catching up with some of the older (great!) work you've done on the Princess and noticed that you mentioned a clonk from the rear when braking.

I don't know if you have found the cause, but I did notice in your brake saga that the rear swing arm mount looks like it has failed, this one -

As far as I know the central nut should not touch the surrounding bracket, even when jacked up. 

Later you were supporting the car on the other side of the bracket and you can see it has moved. It think the rubber delaminates from the bracket, allowing it to move and it may be causing the clonk you can hear.  The Allegro has the same arrangement and it clonked when it failed on mine, and failed the MoT on it.

[vulgalour]

@Surface Rust You're on the money there, it's exactly what had happened.  This has since been dealt with, there will be video/write up on that when I eventually get that far.  It wasn't the only clonk, as far as I can work out I had clonks from that bracket, the loose brake shoe internals, the separated body, and the stuff I was carrying in the boot.