Category Archives: Fibreglass

ST1100 Home-made Seat

After a few years using my ST1100 Pan European in naked form, the original dual-seat began to get rather uncomfortable – the foam had begun to set hard over its 20 year life. It was always a bit too high and wide for me to get my feet down except on tip-toe, so I decided to make a new seat that would be lower and narrower.

ST1100 P3 X11 After

I then decided to go for a more radical change of looks, and ended up needing a single seat that was longer, lower and narrower than a standard ex-police Pan single seat.

The Base

My original plan was to use the foam from an ex-police single seat, which had I cut down underneath and added a bit on the back to get approximately the right shape.

I had intended to make a fibreglass seat base for the ex-police foam, but the professional seat maker who re-covered my PC800 seat advised me not to, as you can’t staple a cover to fibreglass. I tried altering the plastic base to suit the new shape, but the plastic is very tough and can’t be re-moulded with heat, so I had to give up the idea of re-using it. Fibreglass it would have to be.

I watched various You-Tube videos on making bike seats. They seem to fall into two categories. There are some professionals with years of experience show-casing their work, relying on their very expensive equipment. Then there are amateurs who just want a padded plank to sit on the nice flat frame rails of their cafe-racer. Having chosen a bike with bumpy, non-symmetrical frame rails and an under-seat tank with its own bumps, I had to go it alone.

I started out with a thick piece of card covering the top of the fuel tank with the wiring for the fuel pump, followed by more bits of card to make a shape I thought would work. The upright card at the back is for a small bum pad. The thickness of the card should allow a seat cover to be wrapped over the edge of the base.

 ST1100 P4 K02 Seat

Before starting the fibreglassing, I cut a hole in the card and attached the lock plate to the catch underneath, to make sure it fitted the base.

You can get special ‘release tape’ to put over a former so that the fibreglass resin does not stick to it but I had run out, so I used parcel tape instead. I did the fibreglass work outdoors because of the fumes from the resin as it hardens. Since making the dummy tank and tail piece I’d forgotten some of the rules of laying fibreglass mat – don’t try to use large pieces in complicated shapes, and don’t put the resin on the mat before you put it on the former. Here is the first layer, after which I realised my mistakes. It was followed by two more layers (three across the flat base) with joins near the corners.

ST1100 P4 K04 Seat

Another thing I learned – don’t use parcel tape on paintwork, it’s almost impossible to get the sticky stuff off again (at least on primer). It took many hours with lighter fluid and kitchen roll to clean it all off. The seat base needed a bit of filling to even it out, but as it won’t be seen I didn’t bother smoothing it off.

ST1100 P4 K06 Seat

Having made the base to fit the foam (fairly closely) it needed to fit the frame rails. This called for more filler at strategic places, moulded to shape by placing a lump on the underside of the base them putting it on the bike, with bits of old inner tube for bumpers. I drilled four holes to let air out when the foam squashes down, and found I had to cut another hole at one side of the front to make sure it cleared the fuel pipe. A bit of steel strip bent into a curve at one end is bolted on at the front, which will hook under a plate on the top of the fuel tank to locate the seat. At the back is the seat lock plate, bolted through a similar plate on the inside.

ST1100 P4 K08 Seat

The Foam

When I made the dummy fuel tank and tail piece I had the ex-police seat foam in mind, but now found that the foam had gone too hard. I was never really happy with the filleting I had done on it, so I decided to use new foam. We have a local shop, Jordan’s, that sells the peculiar combination of loose sweets from jars, professional-quality fireworks, and upholstery foam, so I paid them a visit. David, their foam expert, repairs motorcycle seats, and suggested a 2 inch base layer of “very firm” that looks like recycled bits bonded together, and a half-inch top layer of “high firm”, which is actually very soft when thin. He said that the top layer is stretchy enough to wrap over the whole seat, but I was a bist sceptical. On his recommendation I bought a cheap electric carving knife for shaping the foam, having found that using an ordinary knife gave poor results.

The base layer had to be sculpted underneath, as the middle is lower than the front and back. The electric knife worked well for cutting the foam, but not so well for shaving little bits off for fine adjustments.

ST1100 P4 K10 Seat

I did the final shaping after gluing the foam to the seat. For this I used carpet glue from a spray can, as recommended by David. It’s wonderful stuff, with no smell and is very sticky. You spray both parts, wait a minute, then press together. There’s no chance to adjust afterwards though if you get it wrong. For the bum pad, I found I could not make a true, flat, straight cut for the join to the base foam. Luckily the pad is no more than 2 inches high, so I could glue surface to surface and shape the front and back which are less critical. To get a rounded edge I tried various sanding techniques. The most effective I found was a disc of sandpaper on a rubber mandrell in an electric drill, used very carefully. I made a couple of small mistakes in shaping but it was fairly easy to glue a bit of spare foam on and then shape it to fix a hole. By this stage I found that the base had subtly changed shape – fibreglass seems to do that – so there is a gap at the back that to be disguised somehow.

ST1100 P4 K12 Seat

The sheet of thin foam I’d bought was only just big enough – I would allow more spare if I do this again – but it did just as David said, and stretched to beautifully fit the curves, without the need to cut it. I seems a shame to have to cover it in vinyl. Gluing it on was tricky – I did the middle first, then worked outwards a bit at a time, masking with newspaper to keep glue off where it should not be.

ST1100 P4 K14 Seat

The Cover

I got some black cloth-backed vinyl from a local fabric shop, and an ancient electric sewing machine from eBay (using my wife’s machine was obviously a non-starter). I have done very little machine sewing, and none in the last 40 years, but how hard could it be?

I started with a trial run using plastic from a garden refuse bag – just about thick enough to be fairly well behaved. I cut pieces to shape and taped them in place on the seat, until I had a reasonable faximile of a cover,

ST1100 P4 K16 Seat

then cut it so as to get the smallest number of flat pieces to make patterns. Arranging these on the back of the vinyl (taking care to get them the reight way up!) and allowing for overlap at joins and edges, there was just enough material. You need more than you might imagine, but it’s quite cheap.

One of the videos I’d seen suggested stapling the pieces of vinyl together along the joins before sewing – keeping away from the bits that will be seen of course. Having done this, I could drape it over the seat to check for approximate fit.

ST1100 P4 K18 Seat

I found it better to separate the front panel (on the left in the picture) before sewing along the sewing lines. It also helped alignment to have the edges cut exactly the same distance from the sewing lines – I chose an arbitrary 20mm. Where the edges are to be folded over and glued to the base, I left a generous amount, about 60mm.

Sewing the sides to the centre was fairly easy until I got to the back, so I left that and attached the front piece. This was trickier, so I did the middle, tried it on the seat, did a bit more and so on. Each time I tried the cover on the seat, I taped it on tightly to get an idea of the final appearance. As for the back, in my innocence I had designed in compound reverse curves where the seat meets the bum pad – it’s hard to make flat material follow these! I asked my wife’s advice (she is something of an expert on making things with material) and her response was that she wouldn’t try to make a cover for anything that shape! A bit like the country man asked for directions to a town some distance away “Well if I were going there I wouldn’t start from here”.

ST1100 P4 K20 Seat

In the end I sewed what I thought was the right line, but after trying the cover on the seat I had to cut the stiching back and re-sew on a different line. This left a row of needle holes visible in the fabric, but I’m hoping that a bit of black shoe polish will disguise them. The final piece, over the top at the back, was more straight-forward.

Gluing the cover in place was also a tricky task. Because the glue is a contact adhesive there is little room for error, though it can be peeled apart initially. I taped the cover in place, masked the bits of the base and cover that were not being glued, un-taped the sides and sprayed the glue on the base and the fabric (and my fingers). To try to get a good fit I warmed the fabric over the foam with a hot air gun on low setting, then stretched it as hard as I could and brought the glued surfaces together. The result looked reasonable, so I did the front of the sides next to try to get the fabric pulled smooth, then the top of the front, leaving all the corners for later. I had to keep replacing the masking as it got sticky, and found it was best to remove the masking immediately after spraying the glue, before it dried. Here is the underside at this point.

ST1100 P4 K22 Seat

Gluing the rest was basically more of the same and took several hours due to all the re-masking needed for each stage. Warming the fabric made it easier to get it to go round corners – called ‘easing’ I’m told. Inevitably there were folds of fabric at the edges of corners, held down with glue from a tube rather than a spray can.

The final result is this:

ST1100 P4 K24 Seat

It’s not professional standard but I’m quite pleased with it. If I did another cover like this one I would have a separate panel for the back of the bum-pad, as the corners there don’t really sit on the foam. There is a bit of a gap between the back of the seat and the tail piece, which can be filled by extending the top of the tail piece, with the opportunity to make it match the shape of the seat more closely.

It has been an enjoyable sub-project, with plenty of variety of tasks and opportunities for learning (and re-learning), and no great skill needed.


A few weeks after completing the seat I found that the cover was coming un-stuck because the glue was getting semi-liquid again. I don’t know if this was due to out-gassing from the fibre-glass, or fumes from the fuel tank beneath it. Another method of securing the cover would have been better. I’ve seen pop-rivets used, but not tried it myself.