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From: (Jobst Brandt)
Subject: Re: Inflation guidelines
Date: 17 Dec 1996 18:16:55 GMT

Ted Haskell writes:

>> Is there any reliable rule-of-thumb concerning how far past labeled
>> levels tires should be inflated?  For example, I ride 700x23 which
>> are rated 100psi. Can I safely inflate the tires to 140lbs? 150?
>> How high (or low) should I go.

> Why do you want to do this? Do you think the manufacturer doesn't
> know what they're doing, and some random poster on the internet can
> give you better information?

These are, in my estimation, not ratings but a liability dodge from
irresponsible users who blow up their tires and sue for massive
compensation.  These numbers are generally so conservatively low that
most riders inflate (at their own risk) to substantially higher
pressures.  I for one have always done this.

> If you don't trust the manufacturer's ratings, switch to a
> manufacturer you trust. If you trust the manufacturer, use the rated
> numbers. Some folks feel they go faster with harder tires, but I
> haven't seen any convincing information that says this is true.

There is plenty of information that rolling resistance is reduced by
higher inflation pressures, and the heavier the tire, the more effect
pressure has.  Rolling resistance arises from flexing losses in the
tire casing , tread, and tube.  The harder the tire is inflated, the
less these components flex.  I have posted data for typical tire tests
over a range of tires and inflation pressures that demonstrate this.

> Generally, narrower tires are rated for higher pressures, so you
> might switch to a narrower tires. Some are rated for as hight as 140
> in clinchers. Tubulars are often rated for pressures as high as
> 170+. That'd get you up there, but I never go over 135 or so, even
> for TTs. Until I see convincing data, I'll stay with these lower
> pressures, it's more comfortable.

I haven't seen any tires with 170 psi as recommended inflation
pressure but that would be a small tire.  Tire casing stress varies
linearly with tire cross section at constant inflation pressure.

Jobst Brandt      <> 

From: (Jobst Brandt)
Subject: Re: Tire question for Jobst
Date: 11 Jun 1999 18:49:16 GMT

Brian Nystrom writes:

> In your rolling resistance testing, did you ever do any comparison
> of the effects of tire pressure on rolling resistance on a rough
> surface or was it all done on smooth surfaces? I've heard that high
> tire pressures can increase rolling resistance on rough surfaces,
> since they are less able to absorb the irregularities. Any credence
> to this?

Rough surface tests are not generally performed because they affect
tires equally with varying inflation.  I suppose for curiosity, a test
on a drum with intermittent ribs widely spaced could do this, but I
believe it has nothing to do with practical tire performance.

The possibility that RR increases for tires on some surfaces is an
often discussed subject, especially by riders who do not ride on a
variety of surfaces with high pressure tires.  In theory an increase
would occur on a regular saw toothed surface riding up successive
ramps only to land after a short free flight on the next ramp, thereby
not rolling off the back side of the crest to gain any forward thrust.

For those who ride over cattle guards either at the critical speed,
this is a real effect and exactly emulates the ramp model.  However,
since road roughness is random and far smaller than the compliance of
the tire, this scenario is unreal and does not occur.  Tire deflection
does not go to zero with high inflation.  I have gotten snake bites
with maximum safe inflation pressure on rough dirt roads.  This is
evidence of how much compliance a tire has even when inflated to these
theoretical retarding pressures.  It doesn't happen!

> I realize that the term "rough" is not descriptive, so for arguments
> sake, let's define it at as irregularities up to 1/2" in height or
> depth, which would be reasonably representative
> of.patched/cracked/erroded pavement.

Yes, any singular feature that causes lift-off has a retarding effect
if there is not a series of randomly spaced similar ones on whose
backside the tire lands as often as it lifts off.  This occurs on an
adversely ordered surface, such as a cattle guard or a series of botts
dots.  However, a tire that is run at lower inflation absorbs energy
all the time including the larger bumps that it cannot entirely absorb
anyway.  Jim Papadopoulos has argued for lower inflation at great
length ins this subject here on the net.  Having ridden many miles on
pave' and cobbles, I am convinced that although it may not be
comfortable, high tire pressure is faster on even such rough surfaces.

Jobst Brandt      <>

From: (Jobst Brandt)
Newsgroups: rec.bicycles.misc
Subject: Re: Tire pressure
Date: 27 Sep 1999 18:07:32 GMT

Fred Barsun writes:

> Bicycling Mag. June 1989; pp. 172, 174, 176(?)

> "Inflated Claims" (How to Determine Your Ideal Tire Pressure)
> by Frank Berto

> Frank, being careful and complete, used a jack to deflect tires,
> calibrated pressure gauge, and an indicator (dial?) to get the 15%
> deflection points at the various pressures.

All the measurement and diligence is much like computing.  Garbage in
garbage out in the sense that the best methods don't help if the data
is no good or the equation is looking at the wrong parameters.  Frank
evaluated something that may not be germane to the use others give
their bicycles.  I know that his riding did not represent my riding.

> Berto indicates that (from his contacts) most manufactures claim that a
> 15% 'drop' with full load is a (compromized) optimum for traction, tire
> life, comfort, rolling resistance, etc.  Of course, adjustments may be
> make for lower rolling resistance at the expense of the other factors.
> Pressure listed on sidewall is usually ~1/2 the pressure that it takes
> to blast the bead of the tire off the rim.

That pressure is generally governed by the rim width rather than tire
cross section.  What Berto did not seem to consider is that hard
cornering and rough pavement require higher inflation than comfort or
other considerations might demand.  Banking over to a maximum lateral
acceleration of about 1g is not something that works reliably with a
comfortably inflated tire, nor is encountering rough pavement with
breaks and patches in the surface.

> I have the graph in front of me --
>         Y axis = inflation(psi)
>         X axis = total weight of rider and bicycle (pounds) and 6 lines
> corresponding to (actual, not 'claimed') tire size from 700 x 18-19C
> (actual=0.75")  x 20C  x 23C x 25C  x28C and 32C

I believe that instead of increasing pressure for heavier loads, a
larger section tire be used.  Of course this is all useless with the
new road frames today that won't accept a tire larger than an actual
25mm cross section.

> Also... Berto recommends that one adjust the front and rear tires to
> 'match' the ratio of rider+bike weight on each tire... i.e.,
> decrease front by ~10% and increase rear by ~10%.

I run my tires at the upper end of pressure because snake bites are
always a threat on mountain roads.  When descending with hard braking,
the front wheel carries the entire bicycle, with the back wheel at
lift-off.  The same is true climbing while seated on steep grades
where front wheel rise is close at hand.  I think Berto's perspective
is from a more conservative kind of bicycling than younger people
might see.

Jobst Brandt      <>

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