Some of you may have noticed just how bald my rear tires were. Actually starting to have holes
in semi-solid tires, maybe 2000 miles, so pretty good. The problem is they are not available any more.
Fortunatly, I swapped out the original fronts when they were almost new for pneumatic Trampa’s (over
two kg’s lighter!), so I have spares. But I thought that while I had some time I would re-mould the
originals so I can also play around with different compounds.
That really is1557g! Each!!
The original duro was 74 shore A. So I’m using that as a starting point.
First thing, clay it up on a board, just regular pottery clay.
Then smooth it out and clear up the edge.
Give it a couple of spray coats of Macwax, or any wax based release agent, dry between
the coats with a blast from a hair dryer then dust with Talc, regular baby talc. It helps the
gel coat bite on the waxed clay and to prevent fish eye. This is polyester gel coat, with 3%
catylist (MEKP). I also pressed in the end of a brush to make some keys to locate the halfs.
Here I have applied 7 layers of 190g E-glass, wetting out with a brush with polyester resin.
I get all my Fibreglass supplies from East coast fibreglass, in the UK, very helpful and they
always deliver next day, pretty much anything for £10. Top guys.
Flipped over, clay removed from the edge and the centre cleaned up. I usually use a triangle
cut, latex makeup sponge to clean up the very edge.
Again, waxed and talced ready for the gel coat.
6mm holes drilled around the flange.
Opened up, cleaned out. Then more 6mm holes inside to hold the core with threaded inserts
screwed down (the bolts were waxed, but not the insert). Notice, also I filled in the voids in the
tire.
Then filled with a liquid polyurethane that sets solid.
In order to get the liquid foam rubber in the mould I made an injector out of an old sealer tube.
I cleaned out the tube and re-moulded the plunger, so I could cast a new one straight on the end
of a piece of bamboo. It would have been almost impossible to glue the Polyprop plunger to
anything, also it’s kind of on the tight side. When you inject this, you need to move fast, so I
popped it in an electric drill and spun it in a piece of emery paper and reduced in down a touch.
I also drilled a few 0.5mm bleed holes each side, to let the air out. And a hole right through into
the core that becomes the filler point.
For the rubber I went with Smoothon’s Flexfoam 17, blended with a 90 shore solid rubber.
Most urethane products will blend, providing you pre-mix each component first, then combine
the two together. A word of caution, though. Although urethane hasn’t got much of a smell, it
is still a toxic product and I would always wear a carbon filtered mask when using it. I once
worked with a guy who died after a day of spraying urethane. It’s no joke.
The first couple out had voids and surface bubbles but once I guaged the mix they came out pretty
good. A few different blends and I got close to the originals. The funny thing is Flexfoam 17 was
almost perfect on it’s own.
The foam is consistent and has a good skin. But you really have to work quick to get it into the
mould before it starts expanding. The weight is also very close despite the differences.
The fit is really easy, just pop off the outer cover, leaving the hub nut on. Slide off the old one
and slide on the fresh one. Note the lack of any physical notches or groves in either the wheel
or the tire, but it seems to hold on (not exactly turbo power here).
The first ride and I could feel one tire initally vibrating but to my amazment it just faded away
over a few hundred meters, maybe seating it’s self. Then it felt pretty much the same as the
originals. Grip was good, though maybe less than the worn old ones, as the contact patch was
much less. But overall they were a useable set of tires. The thing is, they seem to improve as they
wear. The trouble though, is they wear pretty dam quick! 50 miles in and they look like originals
with 500 miles, big difference. My first thoughts was to slush in a layer of the 90 shore and build
up a tougher outer layer. But I’m not 100% you wouldn’t end up with an unbalanced tire. The good
thing with foam is they balance by being under pressure as it cures, evening it all up.
So far, I’ve stuck with my first good set and they are feeling good. I suspect I’ll be making another set
soon! Cost wise, I think you’re looking at over half a kilo of rubber per set, so maybe £15-20 a set
depending on how much you buy. There are big discounts on quantity, but it’s not good to store opened
products as it reacts with moisture in the air.
Next job battery. I’m slightly terrified about it, seriously!
. Very interesting work.