The caveat is that this process, assuming the average bicycle tyre costs anywhere from $20 to $80 - or more, it can be retreaded for about $2 a tyre.
On very smooth roads, the expected mileage could reasonably be expected to be about 2000Km - 4000Km per retread.
It's very easy to do, it can be done with the wheels in the bike, and one ought to allow about 24 hours for the process to finish.
Everything in life is a compromise - this process and materials are no exception.
We use NEUTRAL cure silicon sealant, to build up the worn tyre with.
This has some interesting properties.
After the tyre has been sanded, when applied properly, it is extremely adhesive. It STICKS to the tyre.
It has a low shear strength compared to normal organic carbon based rubbers - meaning that building it up into a "thick retread" is not especially worthwhile, unless you and the bike are light and the tyres are fairly wide.
So the recommended build up is for a shallow dome, at best about no more than 2 or 3mm above the original tyre profile.
The optimum conditions of longevity are for an light to average weight rider, on a bicycle with nice wide tires, on smooth roads - doing the daily commute and travelling etc.
The heavier the rider, the more sharp sharp stones and very coarse gravel, and the more aggressive the riding style, the shorter the retread will last.
And the truly interesting bit.
Silicon rubber is extremely hydrophobic - it chemically repels liquid water.
This means that while you end up with a NICE long lasting retread that works very well on smooth dry roads and paths, when those roads are wet, the effect is that the silicon, while it deforms over the surface of the road, it molecularly repels the water on the surface of the road.
The effect is much like trying to walk through a kitchen with cooking oil spilled on the floor.
You can ride across wet patches, provided the road is level, you bike is within a few degrees of being upright, your not cornering, braking or accelerating, and you do it very very carefully. But for all intents and purposes, regular riding on these retreads in the wet, is impractical and unsafe.
The good thing is that there are some 500 million plus bicycles, and for most people, in most parts of the world, most of the time, most of their riding is done and can be done in the dry. There are also many people who tend to have seasonal weather, who can swap over to regular or wet weather tyres for the season, or can travel by other means, or can wait it out for the occasional shower to pass.
That makes this retreading system, suitable for about 95% of the people, about 95% of the time.
I myself live in an area, where in an average 30 day period, there may be about 630 hours of dry good riding weather, with perhaps 10 hours of rain. That is about a 98.5% riding time.
The back stop is that while the silicon could be compounded with silica flour (ground sand) or carbon black, to make it grip better in the wet, compounding it reduces the the tensile strength of the silicon rubber making it unsuitable for retreading the tyre.
There is commercial potential there, but my knowledge of compounding silicon adhesives, leads me to believe that the benefits of the adhesive / sealant - and the polymerisation of silicones, cannot be reconciled. Either one has a cheap, easy to apply and durable retreading system, or one may be able to achieve "carbon rubber" like wet weather characteristics - but the silicon rubber ends up like soft cheese.
One may be able to mix in SOME silica / silicon flour / ground sand - to improve the wet weather grip, but I think it's a degree of proportion. The addition of SOME may lead to a big improvement in wet weather adhesion, but too much, may make the silicon soft and worthless.
As it stands, you could probably retread your tyres 10 or 20 times or even more, and this is a substantial saving on resources, such as time, money, and mechanical efforts.
The tyres in terms of longevity, will last longer if you keep your bike out of the strong sun, where ever possible. UV protection with a wipe over with some ArmorAll is a good idea.
The safety issues.
You now have a way to have near infinite retreads with your bike tyres, with the only real caveat that you CAN NOT ride it on wet surfaces.
One must also be mindful that tyres were intended to provide good service, not indefinite service. Retreading them annually for 30 years could be regarded as having gotten quite a great deal of extra service from them... Though 30 years on the same tyres might be pushing it a bit, 10 or 12 years of regular retreads, is entirely reasonable.
Tyres ought to be thoroughly inspected on a periodic basis - weekly seems to be fine, for damage, such as tears, cuts, bulges, cracking, fraying, separation of the carcass and tread etc., and replace them if necessary.
And don't ride on them in the wet.
This idea "may" have real commercial potential - but I decided that it's best provided in the public domain, and because it's real basic corner store tech - where anyone and everyone can do it, it's SO easy and low cost - in regards to our current "opportunities" in the global sense, it's necessary.
More people on bicycles more often = good. Better bicycles lasting way longer = good. One set of bicycles tyres lasting as many as 10 or 20 sets of bike tyres = very good.
These are the issues we need to consider.
So retread with gusto and enjoy the benefits of corner store tech!
Step 1: Prepare Tyre.
Plywood is fine, so is plastic, wood, metal etc.
You can retread to make the tyre the original size, or add a little extra to get more distance between retreads.
2 or so mm more is good, 3mm or so is getting excessive, unless you weigh hardly anything and your bike has large diameter road tyres then 4mm probably would be the total thickness limit.
My tyres are 35mm in diameter.
Step 2: Clean up the tyre.
Step 3: Applying the retread.
Create a proper base layer, by rubbing a thin coat into the tyre surface.
Then build up the layers, by applying a bead of silicon all around the tyre, and use the template to shape the bead - rotating the tyre by hand is fast enough.
Then keep on building up more layers, by adding more beads.
With the excess that tends to "glob" off the sides, catch them and put them back in the middle of the tyre, until you have reached your maximum retread thickness.
One must work quickly, as silicon sealants tend to skin over in a short amount of time.
Step 4: Finishing the Retread Profile.
Neutral Cure silicon, catalyses by reacting with atmospheric moisture.
Once the silicon has been applied and built up to a generally nice profile, leave it to set for at least 24 hours in a preferably warm humid environment.
The thinner the retread, the faster the curing. Long cures are needed for thick retreads and in cold dry climates.
48 hours is I guess the longest that is needed on retreads that are about 3 to 4 mm thick.
The silicon sealant sets from the outside in, as the humidity in the air reacts with the rubber. The "skin" has to harden from the surface all the way down to the bike tyre rubber. If the retread is very thick, and not enough time has elapsed for it to finish curing, you will get a hard skin, on an uncured base.
A simple poke and push test is all that is needed - to feel if the skin is solid all the way through to the tyre.
If it holds fast without slipping, it's more or less cured.
Also be mindful that some silicon sealants do have a limited shelf life, from the date of manufacture, and once this is exceeded, by generally not more than a few months, the silicon may never harden.
So only by and use neutral cure silicon sealant, that is within or has only just exceeded it's use by date.
Where I live, it's generally dry 29 days a month, the rain tends to be short and random, and my trips are usually short - or I do ride longer distances on rain free days. So I get to ride everywhere almost all the time and if I can't ride because of the rain and wet roads etc., the distances to walk are short.
So not being able to ride during the very few hours of the month, that the roads are actually wet, is of no significance to me.
This retreading system, for me and many other people, who are recreational or dry weather only riders, it's going to be a good deal.
Simple, low cost, easy to do, long lasting...
It means that I instead of buying new tyres every year, I can buy one set of tyres every 10 or 12 years... or more.
This is a great deal.
Step 5: The Proof is in the Pudding....
As challenging as this is, what we are doing is extrapolation.
I am about 190cm tall, and while a bit fat at 150Kg, I am a big boy in the first place. (6'3" and 23 1/2 stone)
With 2 identical tyres, the front one usually seems to last as long as 2 or 3 rear tyres....
The ratio as far as I can determine, is that for every time you double the weight on a tyre, the rate of wear goes up by about 4 times.
If you drive the same tyre in the very heavily loaded conditon on a coarse surface, it's rate of wear will be about 4 to 6 times, that of a smooth surface - coarse surfaces really DO chew and cut tyres up...
Silicon adhesive / sealant - is a very flexible compound, and compared to carbon blended organic rubbers, it's soft, flexible and reasonably easy to "break" or cut, by running it over a sharp edged stone, and sharp gravel road surfaces.
So here is the extrapolation.
My retread, with me on the bike, riding over unmade and coarsely surfaced roads, had kind of worn down within 200Km.
A rider weighing approximately 75Kg, on the same surfaces, would get around 10 - 15 times the distance.
This comes to approximately 200 - 3000Km.
If the rider weighed around 75Km and the roads were ALL very smooth, the estimated range would be around the 5000Km mark.
With wider tyres it would be greater again... and with an even lighter rider, with would be even greater.
So by about 300Km - I think the retread would be gone - but this is only with me riding this bike, on the rough surfaces....
With someone half my weight, on smooth surfaces, with wider tyres and a modest commuter riding style, I think 3000 - 5000Km per retread, on a rear tyre, is very doable.
I also think that 5000 - 10,000Km on a retreaded front tyre is realistic as well.
Next stage - retreading the retread..... stay tuned.
Step 6: The good news..... and the interesting news....
I have the 20Kg bag of silica flour (200 micron mesh - usual pottery supplies) and I have the silicon, and all the old silica has worn off - more or less....
AND the dregs of that can be scraped off with a serrated bread knife... which also scrubs the tyre surface for the next retread.
BUT, I think mainly due to a slow leak which caused some running under inflated, and the tyres age, and the general loading on the tyre, running over pressure and with a HUGE load on it....
The bike got the back wheel side wobble - which I thought was due to the dregs of the silicon on the tyre.... and as there was no apparent bulge I thought it was the dregs of the silicon on the wheel.
But after I scraped the remains off with the serrated bread knife, and took the bike for a test ride... the back wheel wobble was still there and slowly getting worse than ever.....
A quick inspection showed that the carcass was failing, and it's a no go issue.
But the plan is to get a NEW tyre, retread the new tyre, with a silicon rubber mixed with 10% silica flour by weight,, and then testing it, for endurance and wet weather grip.