Why bicycle tyres are different from car tires

Michael Press writes:

Simon Brooke sent three separate statements each requiring
explanation:

1. Contrary to common opinion, for any given rubber compound, (on
bicycles) slick tyres are better in the wet than tyres with
{patterned} tread.

2. They also tend to roll slightly better.

3. Bicycle tyres with {patterned} tread are only beneficial on
soft surfaces, not on tarmac.

#s 1 & 3 are obvious on their face.

In the case of 1, slick guarantees more contact with the road. Bike
tires are too narrow to hydroplane, so there's no need for channels
to evacuate water.

As for #3, on soft surfaces, the ground deforms in deference to the
tire. On hard surfaces, the tire must deform in deference to the
road. So a slick tire at pressure low enough to deform (which also
increases the contact patch area) but not bottom out provides the
best traction.

I'll let someone else tackle #2, because I don't fully understand
the science of rolling resistance, and won't shame myself by, as
the Russians say, talking out my nose on the subject.

Rolling resistance arises from flexing the tires. The tires are not
entirely elastic and dissipate as heat some of the energy that went
into flexing them. Tires flex significantly in the side walls and in
the tread. If there is a relief pattern in the tread (e.g. knobs),
additional energy is dissipated flexing the tread. Thin side walls
and thin tread noticeably reduce rolling resistance.

Even with entirely elastic material, there are hysteretic losses that
dissipate energy (heat) when the material is deformed. The effect can
be felt by stretching a thick rubber band, sensing its temperature by
bringing it contact with one's upper lip to make heating and cooling
apparent.

Jobst Brandt

meb wrote:
dabac Wrote:
There's one thing there that I've been thinking about every time this
discussion comes up.
-Tread patterns have no effect on surfaces in which they leave no
impression. That is to say, if the road is harder than the tire, a
tread pattern does not improve traction. -
I'll readily accept the statement that a slick tyre has better traction
than a treaded tyre against a smooth surface.
But what if the tread pattern roughly matches the surface structure of
the road surface?

It'd be a bit like having two corrugated surfaces interfacing with each
other, with a lot of protrusions interfering with each other.
Shouldn't that be grippier than one corrugated surface resting agains a
flat surface? - as long as tread courseness "matches" surface
coarseness...

It has the likelihood to improve traction (there tend to be a lot of
variables present to preclude and unconditional statment). The tread
can interlace and get a little side bite with the non-smooth portions of
the road.

I had a tyre, sorry can't remember the make, that had a fine (about 1
mm) cross tire tread pattern at a bit of an angle.

They gripped like **** in rain, ice, snow, frost. They felt like
jellyfish. They wore out fast. But bugger me, they were the grippiest
tyre I've used. The reason I don't use them? Longevity.

On Feb 25, 12:08 pm, wrote:

Rolling resistance arises from flexing the tires. The tires are not
entirely elastic and dissipate as heat some of the energy that went
into flexing them. Tires flex significantly in the side walls and in
the tread. If there is a relief pattern in the tread (e.g. knobs),
additional energy is dissipated flexing the tread. Thin side walls
and thin tread noticeably reduce rolling resistance.

Even with entirely elastic material, there are hysteretic losses that
dissipate energy (heat) when the material is deformed.

What are you considering as "entirely elastic material?" I've always
regarded that as an ideal which is never actually achieved by real
materials, similar to the frictionless tables and massless springs
common in physics problems. Hysteresis losses would be a measure of
the inelasticity of the material.

The effect can
be felt by stretching a thick rubber band, sensing its temperature by
bringing it contact with one's upper lip to make heating and cooling
apparent.

Sure, but neither a thick rubber band nor bicycle tire material is
perfectly elastic.

dabac wrote:
Peter Cole Wrote:
dabac wrote:

I'll readily accept the statement that a slick tyre has better
traction
than a treaded tyre against a smooth surface.
But what if the tread pattern roughly matches the surface structure
of
the road surface?

It'd be a bit like having two corrugated surfaces interfacing with
each
other, with a lot of protrusions interfering with each other.
Shouldn't that be grippier than one corrugated surface resting agains
a
flat surface? - as long as tread courseness "matches" surface
coarseness...
Think about it a little bit. Suppose you wanted to design such a
tread.
First, you would want to scale the tread pattern to match the scale of
the surface texture. You could make a casting of the surface and make
your tread the inverse pattern. But of course the pattern is random,
so
in actual use it would never align. What happens when it misaligns? Do
you have more or less contact? Do you get any feature engagement?
After
answering those questions, you might consider changing the tire tread
pattern scale. Does enlarging or reducing help feature engagement?

The only way you could get real feature engagement would be to have a
regular pattern on the road and a matching pattern on the tire. Then
you
would have to have a pattern that would engage in all orientations.

I like to believe that I HAVE thought a bit about it, and it's not like
I'm claiming it to once and for all solve all traction problems. But
although a 100% match would be as improbable as efficient I wonder if
there isn't a lower degree of surface/tread matching where it would
still offer improved traction.
The perfect misaligment where every ridge meets another ridge must be
as improbable as the perfect alignment where every ridge meets a furrow.
Assuming an intermediate degree of alignment, is it really that easy to
discard the possible influence of geometric interference between surface
and tread after all?

I think so. Unless all of the "hills" of the road meshed with the
"valleys" of the tread, some the hills of the road would hit the hills
of the tread, and that would support the tire. In that case, the road
hills that happened to line up with the tread valleys wouldn't
contribute to traction. So you'd be worse off.

If both features were really coarse you could get some interaction, like
tire tread on scarified road (or metal bridge gratings), but I don't
think it would be very pleasant to ride that way.

On Feb 25, 6:53 am, wrote:
They were performed on the Avocet tire tester that has a 6' diameter
asphalt paved drum on which a bicycle tire is loaded with a pneumatic
piston against the drum and tilted as in cornering. The washout angle
is recorded by the computer that controls the machine. The drum can

Are those angles published for public viewing for different tire
models?
Why something suckier, such as concrete is not used instead of
asphalt?

On 2008-02-25, Peter Cole wrote:
dabac wrote:
[...]
I like to believe that I HAVE thought a bit about it, and it's not like
I'm claiming it to once and for all solve all traction problems. But
although a 100% match would be as improbable as efficient I wonder if
there isn't a lower degree of surface/tread matching where it would
still offer improved traction.
The perfect misaligment where every ridge meets another ridge must be
as improbable as the perfect alignment where every ridge meets a furrow.
Assuming an intermediate degree of alignment, is it really that easy to
discard the possible influence of geometric interference between surface
and tread after all?

I think so. Unless all of the "hills" of the road meshed with the
"valleys" of the tread, some the hills of the road would hit the hills
of the tread, and that would support the tire. In that case, the road
hills that happened to line up with the tread valleys wouldn't
contribute to traction. So you'd be worse off.

If both features were really coarse you could get some interaction, like
tire tread on scarified road (or metal bridge gratings), but I don't
think it would be very pleasant to ride that way.

If there were none of this waffle iron meshing sort of thing going on,
would it ever be possible to achieve a coefficient of friction greater
than 1.0?

Peter Rathman writes:

Rolling resistance arises from flexing the tires. The tires are
not entirely elastic and dissipate as heat some of the energy that
went into flexing them. Tires flex significantly in the side
walls and in the tread. If there is a relief pattern in the tread
(e.g. knobs), additional energy is dissipated flexing the tread.
Thin side walls and thin tread noticeably reduce rolling
resistance.

Even with entirely elastic material, there are hysteretic losses
that dissipate energy (heat) when the material is deformed.

What are you considering as "entirely elastic material?" I've
always regarded that as an ideal which is never actually achieved by
real materials, similar to the frictionless tables and massless
springs common in physics problems. Hysteresis losses would be a
measure of the inelasticity of the material.

Not the way I see it. In elasticity, hysteresis, to me is the
difference in energy required to deform and to restore it. Without
the atmosphere as heat sink the action would rely on radiant heat
exchange and be a bit slower. That shows up as "creep" to the finish
if a rubber band is stretched and dropped. The final restoration is
asymptotic as it warms up.

You can imagine what that means for a swiftly rolling tire. Those are
the rolling losses, RR.

The effect can be felt by stretching a thick rubber band, sensing
its temperature by bringing it contact with one's upper lip to make
heating and cooling apparent.

Sure, but neither a thick rubber band nor bicycle tire material is
perfectly elastic.

I suppose that's a matter of semantics. Elastic to me means that it
can be deformed and by itself return to its original form a few
microns one way or another.

Jobst Brandt

Ben C? wrote:
...
I believe bicycles don't aquaplane. Cars rarely aquaplane either, but
relatively un-worn treaded tyres grip much better for basic braking,
traction and cornering as any driver will know from practical
experience....

Huh? The best tire for driving in dry weather are ones with almost all
to all of the tread pattern gone, due to less tread squirm.

All the street legal tires intended for racing have very shallow tread
(although, the low tread height is also to reduce heat build-up from
tread squirm).

And of course, almost all racing dry tires are slicks. The grooves in F1
tires are there to REDUCE traction.

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful

On 2008-02-26, Tom Sherman wrote:
Ben C? wrote:
...
I believe bicycles don't aquaplane. Cars rarely aquaplane either, but
relatively un-worn treaded tyres grip much better for basic braking,
traction and cornering as any driver will know from practical
experience....

Huh? The best tire for driving in dry weather are ones with almost all
to all of the tread pattern gone, due to less tread squirm.

I meant in wet conditions of course.

Yes bald tyres work fine in the dry.