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1938 BMW Seat Spring Steel Frame Question

4K views 25 replies 3 participants last post by  dcygan 
#1 ·
I'm doing a complete restoration of a 1938 BMW 327 Cabriolet and I'm finally getting around to doing the upholstery and padded top. My question is; Does anyone know what material I can use to fabricate two new bottom seat cushion perimeter/frame springs and where to find it? I've attached photos showing one of the spring steel frames, which has several stress cracks. This frame is used keep the leather upholstery expanded and in the correct shape. This frame is 3/8" wide by 1/16" thick spring steel. It gets tethered to the Marshall Units.

The following is not related, but shows that it's possible to make any spring if the material is available. The Marshall Units are ones that I made and are exact copies of the originals using in the BMW. There are 3 different size coil springs, each using a slightly different diameter wire. They were all made using a mandrel mounted in South Bend Lathe. The ones shown are made from 0.105” diameter (10 gauge) music wire. They are 4-1/2” tall x 2-1/2” diameter and each spring has 5 coils (see photos).


Any information regarding what material to use to reproduce the outer frame, described above, 3/8" wide by 1/16", and where to buy it will be greatly appreciated.


Thanks, Doug
 

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#2 ·
I found strips of spring steel at McMaster Carr..they have a web site..your choice as to what to use..I would suggest the 0.062 thick x 3/8 width as closest to what you need..Comes on 10 foot lengths as much as I can divine from their site..

Sam
 
#3 ·
Hi Sam,
Thanks for the reference and yes, I found it. It's odd because I'm pretty sure I look through McMaster Carr a few months ago and don't recall seeing it, unless I completely overlooked it. I've never used spring steel before, do you know if it can be bent on the flat to create the arcs that I need?
Thanks again,
Doug
 
#4 ·
You could also make the frame from C-1018 cold finished round steel rod. I get it at a web site called Metals Depot. I would think that 1/4" or 5/16" rod would work fine. All you need is the outside shape of the original. The round rod is easy to shape, and much easier to work with. Good luck.
 
#5 ·
Hi Dan,

Thanks. You're an expert in this area and I appreciate your input. Does it matter that the original frame is spring steel and the C-1018 cold finished round steel rod does not have spring qualities? I agree it would be much easier to form and I would rather make life easier for myself, but don't want to do something that may create other issues. What are your thoughts?
Doug
 
#8 ·
The wire at the outside perimeter is just there to hold the shape and provide a place to attach the Marshall unit. I'd sure try it that way first before I'd try to use the flat steel. If you wanted to, you could make two or three of the shapes you want, stack them up, and weld them together. That should make a really strong unit.
 
#6 ·
I think Dans idea on using the round stock makes your fabrication issue a lot easier..Try your supplier that you now have for the larger diameter spring steel wire..Flat stock can be bent on the flat (hard way) if you wish to create the special dies needed but for this the round will work fine..I do have a couple of seat baskets I call them made from round rod material that were used in a Brit car so the round rod kind can work just fine..

Sam
 
#9 ·
Dan, Will the C-1018 steel rod retain it's shape and not distort from use since it's just Cold Rolled Steel without any spring temper? That's my concern. I'm just being Devil's Advocate when I ask these questions because I'd hate for the frame that I make result in damage to the leather upholstery or take on a completely different shape after someone sits on it. The original 1/16" thick spring frame has a lot of flexibility/memory and so when you sit on the cushion, the frame gives where needed and does not simply bend. I've never done this aspect of restoration before and would like to make sure it's correct the first time so please bare with me while I ask all of these questions.
 
#11 ·
The unit "spring frame" is covered with burlap and sits on top of the Marshal Units, On top of the burlap is cotton batting and then the leather. The photo showing the frame sitting above the Marshal Unit is essentially where it attaches. I took one of these original frames, covered it with burlap, tied it to the Marshal Units and then tried to sit on it to see what would happen and it just broke at one of the stress/fatigue cracks, which I've found several in each frame. The Marshal Units in the center of the seat cushion are made of smaller diameter wire whereas the outer perimeter Marshal Units use a heavier gauge wire so the seat is stiffer around the edges. Any thoughts?
 
#16 ·
If it doesn't have to be original, I would make a 3/4" plywood base ( heavy plastic would work also) with the center cut out of it and elastic webbing criss crossed over the opening for the open area on the seat base. I would bolt that in place. Then I would build it up with foam from there until I had the shape I wanted. It will look and ride like a dream.
 
#17 ·
Dan,
I've thought about that, but only briefly as I have gone too far with the restoration only to be thwarted by one small part. I've spent weeks even months figuring out how to fabricate parts such as how to make new aluminum castings or reproduction rubber parts, even fabricating a completely new body out of flat steel and so I'm not going to give up just yet. I'm a perfectionist, which isn't always a good thing and I appreciate your thought of just using foam, but to me that's not how it was made originally and the feel of the cushion will be completely different, and that's important to me. Please don't take offense to this, but I'd like to keep trying to figure out a way to make the metal frames so they come as close as possible to the originals, and if it's not possible then I may go the foam route as you suggested.
 
#18 ·
I am not offended at all. I understand when somebody wants to do seats as original as possible, that's why I asked if that's what you wanted to do.

I was thinking about this, and if you bought larger pieces of flat stock, say 4 or 5 inches wide, to cut the curved parts with and then use the narrower spring steel stock to connect the corners, I would think it could be done that way. That would allow the unit to flex and still return to shape. It could also be riveted or bolted together. Just a thought.
 
#19 ·
I just found a local company that will treat 1074 or 1075 steel to give it spring qualities. Now I'm searching out a source for 3/8" wide by 0.062" thick 1074/75. I've given it some thought and all I need to do is make a set of dies with the proper radius to allow me to bend it on the flat without distorting it. I have a few steel benders and if that doesn't work I'll try planishing the outer edge, which will also create the arc that I need. As I progress I'll post photos. Thanks again Dan. I'm sure once I get to actually doing the leather work I'll have lots of questions along with making the padded top.
 
#20 ·
I've been meaning to post these photos quite a while ago, but kept forgetting. I was able to bend 3/8" wide by 1/16" thick spring steel on its edge to reproduce the original pre-war seat spring frames. The first photo shows the coil of spring steel, the next photo is a steel bender with dies made to form the flat arcs and the last photo shows the new seat springs. They still need to be heat treated to restore the spring temper.
 

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#22 ·
Hi Dan,
The cost was minimal. I already had the bending/forming machine so all I needed to do was buy some 6061 aluminum discs for the mandrels, cut grooves in them to accept two pieces of spring steel material (this allowed me to make both sets at the same time and have them both be exactly the same) and then form the arcs. I took more time to figure out the spring-back and how far I'd need to bend them so the arcs were correct, and what diameter mandrels/ groove radius to use as well since the radius changes considerably once the part is removed from the bending machine due to spring-back. To allow the arcs to be formed required the portion of the spring steel to be annealed, which was done using a torch and localized flame. I still need to re-temper all of the pieces that I made, which there are several that make up each seat unit. I can either do it myself by putting them in a fire until a magnet can no longer stick to them, then quench them in a brine solution followed by placing the pieces in a 400 degree oven to normalize them, or I can bring them to a company and have them do it at a cost of around $400. Cost is an issue, but since I've never tempered anything of this nature before I'm a little hesitant. I'd hate to use ones that I incorrectly tempered and have them fail considering the upholstery for this car is leather. If you have any insight please let me know. Doug
 
#23 ·
Looks like you have plenty of material in that roll so I would go for it and do the annealing at home. After all you have gone through this is just the next step on the journey,,If all else fails you can double the curved pieces in the corners to assist against failure..

Sam
 
#24 ·
Hi Sam,
That's the direction I'm leaning toward since it's so expensive to have someone else do it. I'll probably make a few test pieces to try it out and see what happens. If I make a charcoal fire in a pit that can hold the assembled unit I might be able to keep the tempering process uniform throughout all of the parts and then have a large trough of water big enough to quickly quench them. My kitchen oven is also big enough to the assembled units so that's not a problem. I suppose I can always assemble the seat cushion with vinyl and see how it holds up before swapping it out for leather. I suspect that if it's going to fail, it would fail within a short period of time. Thanks for the support. Doug
 
#25 ·
I think I'd go with using your oven where the control of the heat would be much better. I think you're right, you will find out right away if the tempering didn't work. Good luck, and keep posting. Once again, I applaud you for doing this yourself.
 
#26 ·
I can use my oven only for the normalization process, which comes after the tempering. My oven can reach about 700-800 degrees if put onto the clean cycle, but to tempering requires the part to get cheery red, at which point the steel is no longer able to hold a magnet.

BTW, I also just posted a recent project, the fabrication of a new muffler for a 1914 Cadillac: http://www.hotrodders.com/forum/1914-cadillac-muffler-fabrication-358954.html#post2967562
 
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