I'm doing custom bolsters on stock 914 seats and need some help. I'm using wire bolster frames from stock BMW seats from my M3 (they were really raggidy). I've mounted the frames using screw head machine bolts and nuts attached to the stock fiberglass. I will be slathering on the Bondo at all the attachment points tonight and letting it set up for a couple of days. Then I plan on doing final adjustments with a propane torch before adding the cushion bolster pads, to get the angles right. I will be using the stock BMW cushions which are(seem) huge. I've started to "shave" them down using an electric knife (probably a cheap one) and it's chewing the cushions up a bit, like it's not sharp enough. I've also used a razor blade without great results. This elec. knife is brand new [got mother-in-law's permission] BTW. I fill confident in my ability to get the contours just right as long as I could find something to cut with. Any suggestions? Here's a preliminary pic of the framed seats.

Doug C

I would look for a way to reinforce the mounting locations. Since the bimmer's pieces are metal into the fiberglass seat shell all the stress will be on the mounting spots.

I got a feel for the bolsters before I removed them from the BMW frames and I've replicated that feel on the 914 seats, in otherwords they feel just as firm. But, I hear what you're saying ang that's why I'll be adding a ton of Bondo in those areas. BTW, you can't see from the pictures but the upper wings are a continuous loop of metal..so, the rest is stuck and mounted in the fiberglass channel running down each side of the seat. I had to cut notched for that to work. Hey, I'm all ears for any suggestions!

Doug C

heres the best way that reasonably priced if you make it :
a hot wire cutter. it is basicly nicrhome wire and an adjustable power supply, the wire you can pick up at a hobby shop, the power supply can be a 2 amp battery charger hooked to a dimmer on the 120 v side.
simply a wooden frame making a large C shape with the wire across the open side. then hook up the charger to the ends of the wires. you can make it feet wide or as small as you want. but it does take some playing with to get the right amperage. it will do a real nice job once you get the wire taught and the amps right. could be made for less than 10 bucks if you have the wood, charger and dimmer.

more flash backs from hobby days paying off

is the bondo structural?
I think it will just crack.
I think you need URY's guidance to glass those things into the structure.

just my 2 cents, and I'm no engineer, but I DO break stuff.

Sean - that's a good point about the strength of Bondo, I could just as well use fiberglass. That probably makes more sence.

Scott - thanks for the idea and explanation for a wire cutter. Does someone sell a small, cheap version commercially? That build would be a project unto itself (for me). That said, I may try anyway.

Doug C

a project ??? you could just nail 3 2x4 together and wrap the wire, clip on the charger. now hooking up the dimmer could be as esay as spliting the cord of the charger taking the hot side wire ( the wide prong on the plug is the nuetral ) and hooking up the two wires to the dimmer. and then lots of tape. when finished take the dimmer out and wire nut the wire back together

if i had the wire i could make this in 20 mins granted it would last as long as the project but who wants this laying arould

Look what I found with a small search.

Doug C

makes it look easy.... oh it is

I've never done it, but I saw a episode of American Hot Rod where they were doing the interior. They used what looked like a electric carving knife to shape the foam and a disc sander to finnish it smooth.

Hot wire is the best method out there. But if thats not possibel try these:

A thin wire saw can also work but won't be as clean cut as the hotwire.

From my school days i remember using a hot knife & hot poker to burn shapes in foam. Take a sharp knife & heat the blade then quickly but gently cut / burn the foam.

I have a friend that has an upholstery shop. He has two tools he uses to cut foam. The first is an electric carving knife like you use on a Thanksgiving turkey. The second is a homemade tool hade from a 3' long piece of black metal pallet strap that he has stretched across a wooden "C" shaped frame. The edge of the strap has been sharpened with a grinder & it works like a big saw.

Hot wire is the best way to cut styrofoam, and maybe even polystyrene (rigid) foam, but I'm almost positive (I'll check) that you DON"T want to use the hot wire method on polyethylene or polyurethane foam rubber because of toxic fumes. A good electric knife is the way I've seen it done.

A little toxic fumes every once in a while can be fun

I'd back up those bolts with metal plates to spread the load. If you just run the bolts through the fiberglass, they're going to crack and break off pretty quickly. Gluing some big fender washers to the shell and running the bolts through that will also work. As it is, the wire bits are going to break loose on the outboard sides after you get in and out of the car a few times.

More fiberglass at the holes will help some, but metal plates as big as you can make them will help a lot more. I'm not sure what good Bondo would do at all, given that nothing will be visible. It certainly won't help structurally.

I used to use hot knives at school too.
Oh wait, I wasn't cutting foam, I was cutting class.

I rebuilt the seats on my '74 BMW 2002 using built up foam glued onto polyester batting which was tied to the seat frame and springs. Depending on the density of the foam you can cut it a number of ways. I used a hack saw blade...grind the teeth to a knife edge and it will slice off what you need. The higher density foam you can actually grind to shape with a 40-60 grit disk sander.

I would go to your local upholstery supply store and check out the polyester batting. It’s about ½” thick. I would wrap it over the top of the wire frame bolsters- sandwich glue it together and trim it down to the seat pan. Take ¼” foam and wrap over the batting to build up the bolsters.

Great idea!

I just finished recovering my stock seats and my advice would be so not owrry about the jaged edges you are getting and use a small layer of 1/4" High-Density foam between the foam yuo are cutting and whatever material you will be covering the area with. Use the 1/4" foam as a backing to your upholstery material and it will hide small issues with your cut foam and provide a uniform surface for the material.

As far as support I certainly would push fiberglass mat and resin over body filler and I also agree with the idea that some mounting bracket backing plates may be needed. Think of how many people"plop" into their seats dropping most of their weight onto the bolsers each time.

Anyway, best of luck and please post pics when you get the seats finished so we can see what a great job you have done with the project.

I have a great (expensive ) hot wire foam cutter and nichromewire. It does work great on polystyrene and EPP, but it will put of toxic fumes if you cut what you want to with it. If you can sharpen a hacksaw blade like a straight razor that may work well for you. Don't worry if it isnt perfectly smooth (just make it as close as possible) as the vinyl that you use should be thick enough to not only deal with the weight of an person, but also to cover any minor imperfections.
HTH

2 cents here from a 20+ year upholsterer...Use your wifes/mothers or martha's(she does not need it right now) turkey knife to trim the foam.Also cut it about a half inch bigger than you want it to fill in voids later on.Any more ? feel free to email Boboli914@att.net

yep a electric knife, my upholstery shop that i work with said the same thing get a electric knife.if you dont want to spend good money on a new one,just go get one from a second hand store for cheap.

Apparently there are different grades of electric knives, quality wise -who new? The new one that I've been trying to use just SUX, it seems very sharp but just vibrates the foam as I try to apply presure. BOBOLI mentioned a "turkey" knife, is that non-electric? Also, if I do build a simple Hot Knife wouldn't a respirator protect me from the toxic fumes..I have one of those. I'll take the advise about reinforcing to heart and will be my weekend project. Thanks!

Doug C

If your going to use the hot knife a simple respirator won't work. Use a self-contained breathing apparatus.

MSD info HERE

Do not expose polyurethane foam to welding, smoking materials, naked lights, open flames, space heaters, burning operations, other ignition sources, or other heat or flames. Once ignited, polyurethane will burn rapidly, releasing great heat and consuming oxygen at a high rate in an enclosed space. Deficiency of oxygen will present a danger of suffocation to occupants. Hazardous gases and smoke released by the burning foam can be incapacitating or fatal to human beings if inhaled.

I wouldn't do bondo, there is no strength there.

I would use fiberglass and lots of cloth in it...

Then, to cut the foam, you can use a fine hack saw blade if you can't borrow a foam saw. I wrap tape around one end so I don't cut my hand. Go slow and be carefull.

From the looks of your pics, you will snap those bolsters off ( or your date will - lol )

You need reenforcement, not cosmetics...

There is my two cents on seat fabrication. See my post above for my pics.

1. Fiberglass not bondo.

2. What Bruce, Mark and Morph said... electric carving knife and a DA (depending on the density of the foam) to make final adjustments.

Those should be cool...

I stopped working on these seats until just recently, Thought I'd take a different approach. This time I decided to try my hand with aluminum. I've used pop rivets, fiberglass and epoxy glue. They are really strong, doesn't flex anymore than the original fiberglass so I think they'll hold up.

Doug C

Yes, that looks like it would be strong.
Post progress...


KT

Looks good....

one thing to consider though with higher bolstered seats, make sure the bolsters don't hold the lap belt too high off your hips. It looks like you have a cage in there so I would expect this car will see track time, I would hate for you to slide under your lap belt if you happen to hit something, or be hit by something.

Otherwise, they look nice and strong

Tony