Wet road, lower pressure?

Watching the club criterium last weekend; rain had stopped but road was
still wet but they held it anyway (probably Not A Good Idea) - in 10 laps
there were about 6 flats and someone remarked that if you dropped tyre
pressure say 10/20 lbs the chance of a flat was reduced on a road which was
clearly giving up its glass fragments in the wet.

True or false, and does it compromise the handling/stability to reduce tyre
pressures in the wet?

regards, Andrew

Andrew Price writes:

Watching the club criterium last weekend; rain had stopped but road
was still wet but they held it anyway (probably Not A Good Idea) -
in 10 laps there were about 6 flats and someone remarked that if you
dropped tyre pressure say 10/20 lbs the chance of a flat was reduced
on a road which was clearly giving up its glass fragments in the
wet.

True or false, and does it compromise the handling/stability to
reduce tyre pressures in the wet?

If you are aware how easily a wet sharp object can penetrate a tire,
you would have no doubt. I was made aware of this in HS chemistry
class where we made (Florence flask) wash bottles with a two hole
stopper and a glass siphon and blow tube that had a latex rubber
supported aiming nozzle. Cutting the 1/4 inch latex tube with a dry
razor blade was impossible but when wet it sliced like "warm butter."

The same is true for shards of glass upended by the front tire or the
rear tire of the rider ahead. However, glass that has been in a
traveled lane is usually no hazard because car tires break off all the
sharp edges. To the bicyclist there is no visual difference but just
knowing that it has been run over is important.

Jobst Brandt

wrote:
Andrew Price writes:

Watching the club criterium last weekend; rain had stopped but road
was still wet but they held it anyway (probably Not A Good Idea) -
in 10 laps there were about 6 flats and someone remarked that if you
dropped tyre pressure say 10/20 lbs the chance of a flat was reduced
on a road which was clearly giving up its glass fragments in the
wet.

True or false, and does it compromise the handling/stability to
reduce tyre pressures in the wet?

If you are aware how easily a wet sharp object can penetrate a tire,
you would have no doubt. I was made aware of this in HS chemistry
class where we made (Florence flask) wash bottles with a two hole
stopper and a glass siphon and blow tube that had a latex rubber
supported aiming nozzle. Cutting the 1/4 inch latex tube with a dry
razor blade was impossible but when wet it sliced like "warm butter."

The same is true for shards of glass upended by the front tire or the
rear tire of the rider ahead. However, glass that has been in a
traveled lane is usually no hazard because car tires break off all the
sharp edges. To the bicyclist there is no visual difference but just
knowing that it has been run over is important.

Jobst Brandt

I couldn't really tell if you answered the question or not. Could you
clarify your position?
--
Phil, Squid-in-Training

In the dry I use 7 bar front 8.5 bar in the rear. When the road is wet
the front tyre struggles for grip so I lower it to 6 bar and then it is
more balanced. I also lower the rear as the brakes work better when it
has 8 bar in the rear in the wet.


--
MichaelB

If the introductory paragraph, 'Watching the club...', is the premise to the
question, 'True or false,...', then I think he was pointing out the falsity
of the premise. Water is the preferred lubricant for cutting rubber. Cut
rubber has little to do with the purported effect of pressure on traction.

(Top and bottom posted)

"Phil, Squid-in-Training" wrote in
message news:Kijcf.1637$Xj5.1018@dukeread02...
wrote:
Andrew Price writes:

Watching the club criterium last weekend; rain had stopped but road
was still wet but they held it anyway (probably Not A Good Idea) -
in 10 laps there were about 6 flats and someone remarked that if you
dropped tyre pressure say 10/20 lbs the chance of a flat was reduced
on a road which was clearly giving up its glass fragments in the
wet.

True or false, and does it compromise the handling/stability to
reduce tyre pressures in the wet?

If you are aware how easily a wet sharp object can penetrate a tire,
you would have no doubt. I was made aware of this in HS chemistry
class where we made (Florence flask) wash bottles with a two hole
stopper and a glass siphon and blow tube that had a latex rubber
supported aiming nozzle. Cutting the 1/4 inch latex tube with a dry
razor blade was impossible but when wet it sliced like "warm butter."

The same is true for shards of glass upended by the front tire or the
rear tire of the rider ahead. However, glass that has been in a
traveled lane is usually no hazard because car tires break off all the
sharp edges. To the bicyclist there is no visual difference but just
knowing that it has been run over is important.

Jobst Brandt

I couldn't really tell if you answered the question or not. Could you
clarify your position?
--
Phil, Squid-in-Training

If the introductory paragraph, 'Watching the club...', is the premise to the
question, 'True or false,...', then I think he was pointing out the falsity
of the premise. Water is the preferred lubricant for cutting rubber. Cut
rubber has little to do with the purported effect of pressure on traction.

MichaelB wrote:
In the dry I use 7 bar front 8.5 bar in the rear. When the road is wet
the front tyre struggles for grip so I lower it to 6 bar and then it is
more balanced. I also lower the rear as the brakes work better when it
has 8 bar in the rear in the wet.

At the risk of inciting a religious debate :-) wouldn't this increase
the tendency of the tire to hydroplane in wet conditions?

I know that given the size of the contact patch and the loading of the
wheel, it's almost impossible for a bike tire to hydroplane, but if you
lower the tire pressure and thus increase the size of the contact patch,
wouldn't hydroplaning be more likely to occur?

Don wrote:
MichaelB wrote:
In the dry I use 7 bar front 8.5 bar in the rear. When the road is wet
the front tyre struggles for grip so I lower it to 6 bar and then it is
more balanced. I also lower the rear as the brakes work better when it
has 8 bar in the rear in the wet.

At the risk of inciting a religious debate :-) wouldn't this increase
the tendency of the tire to hydroplane in wet conditions?

I know that given the size of the contact patch and the loading of the
wheel, it's almost impossible for a bike tire to hydroplane, but if you
lower the tire pressure and thus increase the size of the contact patch,
wouldn't hydroplaning be more likely to occur?

Not starting a religious debate at all, hydroplaning doesn't happen on
a bike (see http://sheldonbrown.com/gloss_ho-z.html#hydroplaning)

Bryan

Phil Lee writes:

Watching the club criterium last weekend; rain had stopped but
road was still wet but they held it anyway (probably Not A Good
Idea) - in 10 laps there were about 6 flats and someone remarked
that if you dropped tyre pressure say 10/20 lbs the chance of a
flat was reduced on a road which was clearly giving up its glass
fragments in the wet.

True or false, and does it compromise the handling/stability to
reduce tyre pressures in the wet?

If you are aware how easily a wet sharp object can penetrate a
tire, you would have no doubt. I was made aware of this in HS
chemistry class where we made (Florence flask) wash bottles with a
two hole stopper and a glass siphon and blow tube that had a latex
rubber supported aiming nozzle. Cutting the 1/4 inch latex tube
with a dry razor blade was impossible but when wet it sliced like
"warm butter."

The same is true for shards of glass upended by the front tire or
the rear tire of the rider ahead. However, glass that has been in
a traveled lane is usually no hazard because car tires break off
all the sharp edges. To the bicyclist there is no visual
difference but just knowing that it has been run over is important.

I couldn't really tell if you answered the question or not. Could you
clarify your position?

Even at slightly lowered inflation pressure a water lubricated piece
of glass will penetrate without perceptible difference. Contact
pressure with the road is not moved out of the range of force that
causes a sharp object to penetrate. Consider what 50psi means to a
sharp piece of glass pressed into a wet tire and then what difference
that makes to doing it with 100psi. Both are far above the force
required to cut until the object is flush with the tire surface.

Jobst Brandt

Don who? writes:

In the dry I use 7 bar front 8.5 bar in the rear. When the road is
wet the front tyre struggles for grip so I lower it to 6 bar and
then it is more balanced. I also lower the rear as the brakes work
better when it has 8 bar in the rear in the wet.

At the risk of inciting a religious debate :-) wouldn't this
increase the tendency of the tire to hydroplane in wet conditions?

That is definitely risky considering how often this has been brought up.

I know that given the size of the contact patch and the loading of
the wheel, it's almost impossible for a bike tire to hydroplane, but
if you lower the tire pressure and thus increase the size of the
contact patch, wouldn't hydroplaning be more likely to occur?

http://www.sheldonbrown.com/brandt/slicks.html

Jobst Brandt

"Andrew Price" wrote in message
...
Watching the club criterium last weekend; rain had stopped but road was
still wet but they held it anyway (probably Not A Good Idea) - in 10 laps
there were about 6 flats and someone remarked that if you dropped tyre
pressure say 10/20 lbs the chance of a flat was reduced on a road which
was
clearly giving up its glass fragments in the wet.

True or false, and does it compromise the handling/stability to reduce
tyre
pressures in the wet?

regards, Andrew



I used to do a lot of wet riding- lower pressures are very important to help
maintain stability in the wet. A harder wheel will more likely slip out
under you in a slippery corner. My experience is that there are much more
flats in the wet, part of this is that it is harder to see bits of glass
that you might otherwise avoid, but also those bits will stick to your wet
tire increasing the chance of eventual puncture.

Laz