Tyre Pressure

[Paul]

Being fairly new to cycling I assumed that, like most things with a recommend safe range of
operation, tyre pressure ranges would include a safety margin allowing them to be run at close to
the maximum recommended pressure without causing damage or shortening their life.

I was recently reading a review of a tyre in which the reviewer talked about sidewall failure
possibly occurring if the tyre was over inflated but gave an example of 60lbs which was 5lbs below
the tyre's recommended maximum.

I run my semi-slicks, which have an recommended inflation range of 45-65lbs at 60lbs when using them
on the road and had always assumed that this was ok to do.

I have heard that some manufacturers are a little imaginative in their quoted weights in order to
make their tyres seem more competitive, are inflation ranges treated the same way or is it safe to
assume that a tyre can be safetly run up to the maximum quoted pressure?

Many thanks for any information,

Paul.

 

[Jobst Brandt]

Paul who? writes:

> Being fairly new to cycling I assumed that, like most things with a recommend safe range of
> operation, tyre pressure ranges would include a safety margin allowing them to be run at close to
> the maximum recommended pressure without causing damage or shortening their life.

> I was recently reading a review of a tyre in which the reviewer talked about sidewall failure
> possibly occurring if the tyre was over inflated but gave an example of 60lbs which was 5lbs below
> the tyre's recommended maximum.

> I run my semi-slicks, which have an recommended inflation range of 45-65lbs at 60lbs when using
> them on the road and had always assumed that this was OK to do.

It probably is, assuming the tires are not old and rotten from a winter of gritty wet rides.

> I have heard that some manufacturers are a little imaginative in their quoted weights in order to
> make their tyres seem more competitive, are inflation ranges treated the same way or is it safe to
> assume that a tyre can be safely run up to the maximum quoted pressure?

Fat tires may not be as susceptible to over heating as road bicycle tires, but this is my primary
concern, having blown tires off from severe braking. The blow-off pressure is dependent mainly on
the rim width at the bead, that being the effective cross section for pressure induced blow-off
force. For this reason, it is my experience that no more than 100psi is the safe limit for
descending steep roads that require much braking. This won't be a problem for fat tires because they
don't run that high. Thier problem lies in casing failure.

If you want to estimate cord forces, imagine the top and bottom half of the tire cross section being
pushed apart by inflation pressure acting on the width of the tire per unit length. That is where
tire size comes into play for casing integrity. Tires less than 28mm in cross section run into the
blow-off problem first.

Jobst Brandt [email protected] Palo Alto CA

 

[Paul]

Thanks for the info. My semi-slicks are 26x1.95 so definately fall into the fat catagory.

Thanks again for taking the time to answer my post.

Kind Regards,

Paul

 

[Per ElmsäTer]

[email protected] wrote:
> Fat tires may not be as susceptible to over heating as road bicycle tires, but this is my primary
> concern, having blown tires off from severe braking. The blow-off pressure is dependent mainly on
> the rim width at the bead, that being the effective cross section for pressure induced blow-off
> force. For this reason, it is my experience that no more than 100psi is the safe limit for
> descending steep roads that require much braking. This won't be a problem for fat tires because
> they don't run that high. Thier problem lies in casing failure.

Does that mean the pros inflate their tires to sub 100 psi pressures on steep mountain stages?

--
Perre

You have to be smarter than a robot to reply.

 

[Jobst Brandt]

Per Elmsäter writes:

>> Fat tires may not be as susceptible to over heating as road bicycle tires, but this is my primary
>> concern, having blown tires off from severe braking. The blow-off pressure is dependent mainly on
>> the rim width at the bead, that being the effective cross section for pressure induced blow-off
>> force. For this reason, it is my experience that no more than 100psi is the safe limit for
>> descending steep roads that require much braking. This won't be a problem for fat tires because
>> they don't run that high. Their problem lies in casing failure.

> Does that mean the pros inflate their tires to sub 100 psi pressures on steep mountain stages?

I don't know what they do but the pursuit of 140psi inflation even on flat runs is useless, rolling
resistance of racing tires being insignificant at 100psi. Few mountain passes require braking that
would be a hazard, there usually being long enough straight sections to cool rims. Steep dirt roads
are no longer part of major races and these often required continuous braking.

I generally don't ride more than 100psi because my rides occasionally require steep descending such
as the Idria ride posted on wreck.bike.rides recently.

Jobst Brandt [email protected] Palo Alto CA

 

[Ken]

>> Does that mean the pros inflate their tires to sub 100 psi pressures on steep mountain stages?

Road Bike Rider magazine says Tour de France pros generally use sub 100 psi pressures on all
mass start stages. They may use higher pressures on time trials, especially the flat, smooth,
straight ones.

 

[Per ElmsäTer]

Ken wrote:
>>> Does that mean the pros inflate their tires to sub 100 psi pressures on steep mountain stages?
>
> Road Bike Rider magazine says Tour de France pros generally use sub 100 psi pressures on all
> mass start stages. They may use higher pressures on time trials, especially the flat, smooth,
> straight ones.

This is getting interesting. Would that be due to the bad roads ( cobblestones etc) they are riding
or to something else.

My Hutchinsson tires for instance say recommended and max tire pressure is 125 psi. I've been
inflating to 125 on TT's and 110-120 on normal road use, depending on the road. Could I benefit then
from going down to 95-100 on road riding? Racing and training.

--
Perre

You have to be smarter than a robot to reply.

 

[Andrew Bradley]

Per Elmsäter :

> > Road Bike Rider magazine says Tour de France pros generally use sub 100 psi pressures on all
> > mass start stages. They may use higher pressures on time trials, especially the flat, smooth,
> > straight ones.
>
> This is getting interesting. Would that be due to the bad roads ( cobblestones etc) they are
> riding or to something else.

I found a noticeable decrease in tyre pressures in the racing community as I crossed the channel
from Britain to France (some time ago now).

I won't put that all down to roads, but 100psi seems to be the best compromise for comfort and
traction for the average rider racing on the French roads which are generally in good state of
repair but are not smoothly tarmaced and can be quite gritty.

I would put more into racing clinchers for fear of pinchers, although i am aware this fear is
myth-based.

Andrew Bradley

 

[Chris Zacho "Th]

I don't know if this is completely true or not, but I have heard that a [bicycle] tire manufacturer
tests it's tires at twice the recommended pressure (under non riding conditions) for blow off.

I don't recommend you inflate your 45# tires to 90 PSI, of course! But a little over-inflation
shouldn't hurt. When I go on a fully packed campng tour, I normally add an extra 10#'s to my rubber
for added rim protection. I've seen no detrimental effects from this in 30 odd years, so it's
probably safe to assume it won't hurt anything.

Again, this is based only on personal experience, not some engineering degree.

May you have the wind at your back. And a really low gear for the hills! Chris

Chris'Z Corner "The Website for the Common Bicyclist": http://www.geocities.com/czcorner

 

[Doug]

>>>> Does that mean the pros inflate their tires to sub 100 psi pressures on steep mountain stages?
>>
>> Road Bike Rider magazine says Tour de France pros generally use sub 100 psi pressures on all mass
>> start stages.

Possibly rider weight has something to do with this? Many pros are featherweights by our standards.
I'm 180 and ride with guys who are similar, ie more average than the superfit 135-165 pounders of
the tour. Lance and Hincapie at 165 are monsters by cycling standards. Tyler Hamilton and Leipheimer
reportedly weighs less than 140, and even a sprinter like Zabel is barely 150. Yikes! Coupled with
5-10 pounds less for the bike+gadgets and, in my case, the average pro is carrying nearly 40 pounds
less on the wheels.

Doug