Urethane Airless Tires

From a posting to Tandem@Hobbes by Brian Mink, on behalf of:
Hugh Waters, President
Air Free Tires, Inc.,

Date: Sat, 05 May 2001 12:39:06 -0500
From: Brian Mink <bcmink@chorus.net>
To: tandem@hobbes.ucsd.edu
Subject: ...AirFree Tires 


I tried to post a message but I don't think it went through.

You may post the following if you like.


Dear List Members,

My name is Hugh Waters and I am the President of Air Free Tires, Inc., (a
tiny little company) and I have been asked by one of your members to
contribute to the list in an effort to bring about an understanding of
airless tires and its' history, their limitations, recent advancements and
the industries' engineering goals.

All urethane airless tire manufacturers share the same 'great mother' dating
back to the mid 1970s'. This newly formed company had invented a way to spin
cast urethane in a mold on a centrifuge. Designs of this tire were modeled
after existing rubber tires and the tires were configured to fit the popular
steel type rims. The thinking was to build a tire that would fit the most
number of rims and the world would beat the door down to have this 'no flat'
tire. What the early inventors of the urethane tire were not prepared for
was the total ambivalence that people would have towards the product if they
owned an inexpensive bike. People who did not pay a lot of money for their
bike will not ride it as often or as far as people who bought more expensive
bikes. Even though there were and still are many more people who own cheap
bikes than expensive ones, the cheaper bike owners just do not see the need
for a no flat tires as chances are the only flat they ever encountered was
in the garage. People who ride often and for great distances obviously would
see the need for a no flat, but the early designs, even if they did fit the
thinner rims, would not deliver the performance that the avid cyclist

It didn't take long for the new company to go broke and its assets we sold
to another firm who quickly went broke too. The second company had its
assets bought by two companies which later spun off to a total of four. The
designs and the process for making the tire are nearly identical today as
they were 30 years ago. At least three of these companies had the sense to
back  away from the airless bike tire concept and went into making airless
wheelchair and disability tires. With millions being sold to this market
every year, these companies are profitable and are indeed providing a
valuable product to its' intended market. Since any of these manufacturers
could have just five or six customers account for 90% of their business with
only 10 or so designs of tires, innovation soon stagnated. What incentive
was there to bring a better tire to market, when millions are being sold to
a demographic that doesn't care if the tire is slow, just that it doesn't go

Some of these manufacturers still cling to the belief that if they produce
airless bike tires, somehow there must a huge market for under,
underachieving tires which may or may not fit certain rims. If thirty years
of experience hasn't taught them a lesson by now, then clubbing them over
the head with a reality stick might be appropriate. Retail distribution of a
product that has a 3 in 10 chance of fitting the customers' wheel when he
gets it home isn't going to last long when merchants quickly drop SKUs that
have a high return rate and bike shops aren't going to carry the product as
long as it doesn't stack up to even bad rubber tires. We at Air Free really
understand what needs to be done and are working hard to bring a better
product to market but it going to take time and it is going to take a
product so good that it is capable of winning a race, just to undo the 30
odd years of misconceptions, bad PR and general stigma associated with the
urethane bike tire.

If someone had never ridden a Michelin Axial Comp or a Vredestien Fortezza,
then the airless tire might not seem so slow. When comparing the performance
of an airless to finest the rubber industry has to offer isn't really fair
in that the airless tire still has value for many people. Airless tires will
act different than rubber tires and the only to effectively create a
urethane tire that acts, performs and handles the same way as a rubber tire,
is to create a rubber tire. Lets first look at rolling resistance values of
rubber and urethane. Rubber tires can achieve lower rolling resistance (crr)
simply by their ability to flatten out at the contact point and stay that
way. Urethane tires aren't quite as supple and the air isn't free to move
about. If one were to look at the crr value of the Conti Top Touring (700 x
32, 70psi) one could see that it is a fair rolling tire and has a crr value
of .0088. There are a few tires better and a good many that are worse
especially the cheaper ones. Now if you take the Conti tire and add a Kevlar
belt or use a thorn resistant inner tube, the crr value will double to
around .0160. This scenario has just made a nice tire slower than several
airless tires in the same size class. Its amazing to see people do this and
not know what they have done.

Currently there are two urethanes available for use in the airless tire
industry. One is what we call the "standard foam" which is nothing more than
a two part urethane when mixed with a blowing agent will cause it to 'foam'
up to 6 times its original volume. This is what has been in use for years
and is what comes standard on hundreds of thousands wheel chairs. Bike tires
made from this will have amazingly high crr .0205 (American Tire) to a
pretty respectable .0157 (Nu-Teck). The material tends to be stiff and not
as tractionable as rubber but in many cases, a suitable replacement for
rubber. Also the standard foam does not tolerate weight in excess of 120lbs
per wheel very well and since it is not an air filled rubber bladder, will
weigh more than a rubber tire of comparable size. The urethane is fairly
inexpensive but still has a hard time competing with rubber for a slice of
the OEM pie.

The second urethane available to us is called "High Resilient" and is a
solid urethane elastomer invented by Bayer Chemical. This formula was
originally intended for heavy load use in the electric disability vehicle
market where the weight could exceed 700lbs on three small wheels. Six
months ago we asked our supplier to try this out in some bike tire molds and
we had them tested. One of the tires tested was a 700 x 25c tire with a psi
equivalency of 110. The crr value of this tire tested at .0077 and now
places it ahead of many rubber tires. By using this formula we have cut the
rolling resistance in half and by utilizing some new shapes and tweaking the
formula a bit, we feel we can get the tire down .005 or thereabouts. At the
crr .005 threshold, a tire is capable of winning a race and we may have just
a tire by the end of the summer. The characteristics laid down for the
development of the HR material was to resist weight but in doing so, Bayer
substantially decreased the rolling resistance in a commercially available
urethane. This material is where all of our future tires designed will built
around. The molds producing the HR material tires were machined over 12
years ago and are incapable of fitting a good many high end rims. This is a
known issue and our future designs will be configured to fit the thinner
rims and not the wider cheap ones.

The HR formula allowed us to achieve our first two objectives and that was
to eliminate the rolling resistance penalty associated with the use of
urethane tires and to increase the traction. Since the HR material is not a
foam, it does not contain air and therefore will weigh more per cubic inch
than the standard foam. Future designs of tires will be lower in profile and
will utilize shapes which will enhance speed. The lower profiled tires will
consume less material and the resultant weight will be approximately one
third less. It is conceivable that our new tires will weigh up to 500 grams
more than the rubber tire it is meant to replace, but so far we have not
heard of one documented case of a bike rider who was unable to get home
because his tires weighed an extra 1000 grams combined. At the very least
what we have done is to create a tire that is as effectively as good as the
one we are asking to people to replace.

As technology allows us to more with less, we will get the weight of the
tire down as we don't want to make the tires any heavier than they have to
be, but under no circumstance do we ever foresee creating a solid tire as
light as a foldable. We view this HR material as a break through and with
more testing and development our future designs will be as good as the
finest rubber tires. Also our new tires will probably not be called
'airless' anymore, but rather 'kinetic transfer technology' or something
like that.

For your industry segment, it seems that the HR might be just what the
doctor ordered. As we deal with people who have had flats every day of the
week, it seems anytime there is more than 200lbs on a bike, the likelihood
increases threefold that the tires will suffer a flat. This is why your
market segment uses tires rated for tandem use. Will our tires hold up to
the rigors of your weight and usage? We don't know. Our testing is still in
its early stages. We do know that urethanes don't tolerate heat in excess of
220 degrees very well and this is why you will not see urethane car or
motorcycle tires anytime soon. It is difficult for us to recreate the
stresses on the tires likely to be inflicted on the tire by your kind of
riding in our testing facility. If rubber tires are capable of failing due
to heat build up, then that is cause for concern. We have no doubt that the
HR formula can handle the weight as one tire can handle 300lbs and still
roll, but will it hold up?

We are indeed curious and to find out, we would like to offer for free,
several sets of our HR tires for evaluation to this list serve. We have a 26
x 1.5 tire that may be suited for this and would like to hear from someone
who has a 25-559 rim. ( 25mm interior distance between the rim walls). The
purpose would be to test the tire first on a casual ride. Please don't
frighten us to death by going down some mountain highway at 60mph.

There is some test data available on my personal website.
http//www.hughwaters.com/results< and it is there only to help illustrate
what was discussed here and will be updated as more results come in.

It has been pleasure to post this message and we hope that it is taken in
the spirit of which it is intended.


Hugh Waters