MASTERCLASS - How to build a seat

A load of posts that show you how to do various projects, step by step.
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Greenbat
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MASTERCLASS - How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 8:59 pm

This is a guide on how I built the humped back race seat for my bantam. It is also applicable to an ordinary seat.
First, you need to think up a design. I did mine by measuring the subframe the seat sits on, then building the seat to fit. The subframe may have an offset, so accomodate this in your design. If you just draw round the frame you may wind up with a very off centre seat.
Before cutting metal, cut cardboard. Use any old boxes, and make your seat pan as you intend to do in metal. If you go wrong, cardboard is easier to tweak or throw away. It is also much quicker to make up new ideas.

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I made my pan base so that it could be folded up from sheet steel. This is ideal if you are crap at welding, like me. It should be possible to make and straight edges as bent tabs. Mark out the pan on some steel and cut it out like this:

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To cut out, I used a jigsaw, hacksaw and files. Make sure you mark out and cut as accurately as possible, it makes life so much easier. File off all burrs (needle files are dead handy), and ensure there aren't huge file or saw marks, as they look awful. You want people to point at your bike, but not to laugh.
As can be seen, there are three seperate tabs on this pan. The order of bending must be given thought, as the middle one could get in the way of the other two. Mark out the line you want to bend, then clamp in a vice. I used two lengths of angle iron (L section steel girder) cut to the exact length of the middle tab. These were clamped in the vice with the tab between them, with the bend line in line with the corner of the L section.

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Greenbat
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Re: MASTERCLASS-How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 9:09 pm

Once bent, you should have something like this:

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Next, the gaps between the tabs are welded up.

To do the rear semicircle, cut a strip of metal to fill in. The sides of my pan are 1" deep. You can roughly measure the curve length by rolling a ruler round it, then add a few inches so you can cut it off exact. I worked by welding the first end to the tab end. I then clamped an inch or two in position, and tacked it in place, then did a solid weld up to the tack. Repeat until job is done, and weld up the remaining end. Experienced welders will probably give me a far superior method, but this worked for me. BTW, wear a mask when welding galvanised steel and give the shed plenty of chance to air.

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Note how the welding improved as I got the hang of it!
Now get an angle grinder and grind the welds smooth and level. Especially if you need to hide a terrible effort like the one above.As long as you get plenty of weld penetration, it shouldn't fall off...I hope!
If you just want a bench seat, ignore the next stage

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Greenbat
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Re: MASTERCLASS-How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 9:24 pm

Right, now we are going to make the seat back. This will incorporate a nifty little door into the hump, so you can store keys/spare plugs/small children in the hump.
First, have a think of the profile. I just used a semicircle with a diameter of the seat width at that point.
First, I marked out this semicircle
Then, I marked out another with the same centre, but around 1/4" smaller
I cut out the small one, then the big one. This gave me a semicircular strip. File as close to the scribed line as possible, and get it nice and round.
To weld it on, I clamped it to a set square, then put it in position and welded the bottom. Again, accuracy is everything. I had a huge bench fitting course in my apprenticeship, so I'm used to it by now...
The semicircle by itself is very flimsy, so I made it into an L shape. Cut out a strip of metal about 1/4" wide and long enough to go round the semicircle. Try not to cut it too short and wind up having to make a fill in piece, like I did.
Attach this piece in a similar way to the rear curved bit. You'll have to use your imagination here, as I forgot to take pictures. Sorry.
Anyway, you should wind up with something like this:

Image

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Greenbat
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Re: MASTERCLASS-How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 9:39 pm

To make the door, cut out another semicircle of the same radius as the first one. Cut a strip off the bottom, about 1" wide. Cut it carefull, as both bits are needed. Or cut two semicircles, and use the bottom of one and the top of the other.
Weld the bottom strip across the bottom of the semi circle. Viewed from the front, it should look like a D on its back. The top bit forms the door. Attach it using furniture hinges, available from all good DIY type places. Again, no pictures.
You now need to make some sort of catch to stop the seat back falling down. I just used some fridge magnet backs cut up and glued on. The trouble with this is it isn't secure, and will let the rain in. If I made another seat, I'd make the cover fit over the hump.

Next, we shall look at the hump making. I actually covered mine first, meaning I had to wrap the cover in newspaper to keep the fibreglass off. Doing it in this order removes that.

First, decide on the side profile of the hump. I decided on a quarter ellipse, with the minor axis being the seat back height and the major axis the distance from seat back to seat pan end. (http://en.wikipedia.org/wiki/Ellipse#El ... an_ellipse for all those technical terms)
To draw this, use microsoft word. Use the ellipse tool to create one, then use the properties tools to specify the exact length and width. Remember, we only want a quarter ellipse, so it should be twice the measurements above. You will notice that green grab lines appear around the edges, in line with the centre. Use these to draw centre lines. Print it out, then cut out your quarter ellipse. Glue onto some stiff cardboard, or some hardboard. Cut out a little material so it will fit under the L section of the seat back, and get it fitting nicely.

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Greenbat
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Re: MASTERCLASS-How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 10:06 pm

Using this ellipse, and some more, I made the fibreglass mould. I wanted a rounded shape, which would be roughly a quarter ellipsoid. Due to the complexity of this, I built up a series of profiles, then filled in with foam.
Measure the height of the middle ellipse and the distance from middle to edge at intervals of around 1", tapering to 1/2" toward the rear. Double these, and produce half ellipses in a similar manner to before. Again, glue them onto card or hardboard to make solid shapes. Use as many as you think you need to get the required shape.
Cut slots in the middle profile where the other profiles will fit. Then cut corresponding slots in the half ellipse profiles, and slot together to form a sort of skeleton:

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Next, get some expanded polystyrene foam and glue it between the profiles:

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Carve and sand the foam to the desired shape. Now cover it in masking tape. This will fill in most dips and blemishes, and protect the foam from the fibreglass resin, which will melt it. Fill in any hollows (I used ordinary wall plaster), then put on another layer of tape. Cover the rest of the seat in newspaper as the resin gets everywhere.

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Greenbat
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Re: MASTERCLASS-How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 10:19 pm

At last, some messy fibreglassing fun!
You will need:
Some fibreglass mat. I only had rough chop matting, you can get finer stuff to get a better finish.
Some resin and hardner
A brush to apply it
An outdoors work area you can get messy

You will also need regular breaks and a mask, the fumes are awful! Wear overalls or old clothes too, you will get blathered in resin. While you're at it, lock up wives/pets/sneaking ninjas so they don't get fibreglassed too.

First mix up the resin and hardener as per instructions that should come with it. Apply resin to the mould. Then stick layers of matting to it. Use the biggest sections you can, though be careful not to get folds in it. Give it a bit of overhang on all edges too-at least 1/2". Apply more resin, then give it a minute or two to start hardening. I did mine during snow, it took bloody ages to start going off! Once it starts to cure, stick on another layer and more resin. Try to overlap joins, and watch out for thick and thin areas. Build up as many layers as you think are needed-I used 3 or 4. Now leave it a day or two to dry. DO NOT bring it in the house after a few hours in the hope the warmth will make it cure quicker. It stinks.
Once it sets, you will be left with a very rough outside. Most people build up a prototyoe seat, make a negative mould then use that to make the seat. This involves loads of fibreglass, which I didn't fancy.
Cut the overhang off, to the desired shape. You will notice that the ends of the overhang are all ragged, whereas the cut edges are of proper thickness and much neater. To cut the solid fibreglass I used a mini drill and cutting disk. Again, lots of dust, so wear a mask and overalls. Cut off any loose fibres, then apply more resin to seal them in. Put resin round the cut edges too. Again, leave to dry.

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Greenbat
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Re: MASTERCLASS-How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 10:33 pm

Once dry, use some really, really rough sandpaper and a sanding block to sand down the hump. The plastic seems to knock the edge off everything, so get plenty of coarse emery. And I mean coarse-the side of a rendered house coarse!
Sand until you start getting fibres coming through, and it is sanded evenly all over. Now apply more resin. Once that is dry, sand again. I did this about three times, to get rid of the biggest dips.
Then I got bored, and found some fibreglass reinforced filler. P38 might work, but I sispect it is a bit brittle. I used this to fill in a massive hollow round the back of my hump. Once that had set, I used the plastic scraper to apply a very thin layer over the whole hump. By pressing down on the scraper, you get most of it in the hollows with only a small amount on the high spots. This can then be sanded smooth. Use a hard, flat sanding block to avoid making hollows worse. Sand until the whole hump has been sanded, with no unsanded voids.

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Greenbat
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Re: MASTERCLASS-How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 10:44 pm

Once you are happy that all the hollows are gone, you can start painting. Use some proper primer, and spray a coat on. When it dries, you will notice tiny hollows everywhere. Sand the paint, fill in the hollows (I found revell model kit filler is great for this), then sand the filler. when it is dry, apply another coat and repaet until you are happy. Or your arms fall off. Handy hint-If your arms are thinner than your waist, you haven't sanded it enough :D
To attach it, I drilled 4 holes for some aluminium rivets I found from the scrap pile at work. These polished up beautifully. You could also glue it.
Finally, apply topcoat and any other finishes you require. Job done.

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Greenbat
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Re: MASTERCLASS-How to build a seat

Unread post by Greenbat » Tue Mar 17, 2009 11:20 pm

Builders of non-humped seats can now tune back in. This is how to cover the seat.
First, find some foam. I used an old camping roll mat. Cut to desired shape and glue to the base. With my rollmat, I made a pad that fitted on just the top, then another that fitted over the first pad and round the sides. For the bench seat builder, get a massive block of soft foam and plonk it on top.
For my cover, I trawled the charity shops for a leather jacket. when I explained what I wanted and what for, I wound up with a rather slutty leather skirt. Sadly my girlfriend would slap me if I even suggested what you are probably thinking, so I cut it up for the seat cover.
Originally, I planned sewing it on. To do this I drilled hundreds of tiny holes in the seat pan. Someone on here suggested I glue it, which I did. Use bostick, it is easy, quick and looks very neat. I attached the back, with a seam in the material running central. I then cut and shaped to fit the straight sides, then the front angle, then the very front. It is much easier breaking it down into small steps, rather than trying make the whole cover before attaching.
Here is the finished result:

Image

The Mighty Gusset

Re: MASTERCLASS-How to build a seat

Unread post by The Mighty Gusset » Tue Mar 17, 2009 11:26 pm

That all looked like lots of fun Greeno !
Cheers !

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