Tyre pressure measurement

[Peter Clinch]

Pete Biggs wrote:
> squeaker wrote:
>> To quote Schwalbe (at
>> http://www.schwalbe.de/gbl/en/tech_...ache=2&ID_Seite=136&tn_mainPoint=TechnikInfos)
>> "Use a pressure gauge to monitor inflation pressure. The widespread
>> thumb-test method is very inaccurate as all tires will feel
>> identically hard from a pressure of 2 bar up."
>
> That's rubbish.

Up to a point. I certainly think that you can tell with a thumb if your
tyres are pumped up /enough/, which frankly is all you need for utility
cycling, but if you want to get things tip-top for a big ride then a
pressure gauge really is the way forwards.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email protected] http://www.dundee.ac.uk/~pjclinch/

 

[PhilD]

On May 11, 9:34 am, Paul Boyd <usenet.dont.work@plusnet> wrote:
> The OP did actually say "Without going to the extent of a pressure
> meter". There is a clear difference between being able to press your
> thumb in (tyre soft) and not being able to press it in (tyre hard). It
> may not be hard enough, but if you can't press your thumb in at all then
> it's harder than soft and will be fine for general riding about.


An alternative may be to push the wheel onto a Kerb stone (or
convenient brick, or whatever) and assess how much the tyre deforms
related to how much you are pushing. It may take a little "getting
used to" with known correctly- and incorrectly-inflated tyres at
first.

PhilD

--
<><

 

[Ben C]

On 2007-05-10, Nick Maclaren <[email protected]> wrote:
>
> In article <[email protected]>,
> Ben C <[email protected]> writes:
>|>
>|> There was a discussion about this on r.b.t. a while back, with lots of
>|> varying views and even some experiments. My conclusion from that was
>|> that the thumb test probably _is_ a measure more of casing tension
>|> rather than of pressure.
>
> I doubt it
>
> The reason is that the resistance to sideways pressure is also a
> function of the curvature, especially when the applied pressure has
> a comparable curvature to the casing - as with a thumb and a narrow
> tyre!

One question is exactly what you're "measuring" with the thumb test--
penetration depth of your thumb for a given force is my best guess.

> I would have to do the calculations properly to estimate more
> precisely what is going on. They aren't hard, but need a bit of
> care for someone as rusty as me.

It would be interesting to know your results.

This is the RBT thread:

http://groups.google.co.uk/group/re...read/ec91d3709c1bf634/1bf12937a4e9c33b?hl=en&

 

[Nick Maclaren]

In article <[email protected]>,
squeaker <[email protected]> writes:
|> To quote Schwalbe (at
|> "Use a pressure gauge to monitor inflation pressure. The widespread
|> thumb-test method is very inaccurate as all tires will feel
|> identically hard from a pressure of 2 bar up."

He's wrong. I can tell 3 bar from 4 bar on a 42 mm tyre.


Regards,
Nick Maclaren.

 

[[email protected]]

squeaker wrote:

> To quote Schwalbe (at
> http://www.schwalbe.de/gbl/en/tech_...ache=2&ID_Seite=136&tn_mainPoint=TechnikInfos)
> "Use a pressure gauge to monitor inflation pressure. The widespread
> thumb-test method is very inaccurate as all tires will feel
> identically hard from a pressure of 2 bar up."
> For on-road use, a Topeak Road Morph (with gauge) or Cyclaire will
> both do 110psi.

hhmn, no conflict of interest there. I can detect a big difference
between 6 and 8 bar on my 23mm tyres. I can detect it by the feel of
the bike though not as easily. I am not sure I can be much miore
specifc that >=8 and <<8

best wishes
james

 

[Pete Biggs]

Peter Clinch wrote:
> Pete Biggs wrote:
>> squeaker wrote:
>>> To quote Schwalbe (at
>>> http://www.schwalbe.de/gbl/en/tech_...ache=2&ID_Seite=136&tn_mainPoint=TechnikInfos)
>>> "Use a pressure gauge to monitor inflation pressure. The widespread
>>> thumb-test method is very inaccurate as all tires will feel
>>> identically hard from a pressure of 2 bar up."
>>
>> That's rubbish.
>
> Up to a point. I certainly think that you can tell with a thumb if
> your tyres are pumped up /enough/, which frankly is all you need for
> utility cycling, but if you want to get things tip-top for a big ride
> then a pressure gauge really is the way forwards.

A gauge is handy if you have already decided how many psi to use.

With all but the very hardest tyres used in racing and time trialing (eg.
700x23 @ 140psi+), a person with average hand strength can detect
small-enough-for-anyone differences hand if good technique is used: pressing
as hard as possible into the sidewall with /both/ thumbs side-by-side.

Deciding how hard is just right is the difficult bit. But still I reckon,
if you have a fair amount of experience and sense, using experience +
intuition is better than blindly following the manufacturer's
recommendation - which ranges from not too bad to barking mad, depending on
the make and model.

~PB

 

[Nick Maclaren]

In article <[email protected]>,
Ben C <[email protected]> writes:
|>
|> One question is exactly what you're "measuring" with the thumb test--
|> penetration depth of your thumb for a given force is my best guess.

"Feel"

|> > I would have to do the calculations properly to estimate more
|> > precisely what is going on. They aren't hard, but need a bit of
|> > care for someone as rusty as me.
|>
|> It would be interesting to know your results.

I will post if I do them.


|> This is the RBT thread:
|>
|> http://groups.google.co.uk/group/re...read/ec91d3709c1bf634/1bf12937a4e9c33b?hl=en&

Thanks.


Regards,
Nick Maclaren.

 

[Pete Biggs]

Ben C wrote:

http://groups.google.co.uk/group/re...read/ec91d3709c1bf634/1bf12937a4e9c33b?hl=en&

I haven't read the whole of that - but I can see just from the first few
posts that the mention of flexibility of the actual casing is confusing the
issue. A more flexible casing does reduce rolling resistance, and you might
be able to detect it by hand in some cases with a soft tyre, but it's
dwarfed by the pressure factor.

With an inflated tyre, virtually all of the "hardness" you are feeling is
from the air pressure.

I take "casing tension" to mean the pressure exerted on the casing.

~PB

 

[[email protected]]

On 10 May, 17:47, "N Cook" <[email protected]> wrote:
> Without going to the extent of a pressure meter - is there some technique
> that is a bit better than squashing sidewalls of tyres to tell whether soft
> or firm.


Two simple methods I use are:

When the wheel is off the bike (in the case of mending a puncture) -
Hold the wheel up suspend on a finger. Flick the tyre, you can tell
from the ringing noise produced how close to pressure you are. Try
it!

When the wheel is in the bike - sit on the bike, see how splayed the
tyre is compared to normal.

Neither method is amazing, but better than nothing - Tyre flicking was
demonstrated to us while on a bike tour in the Netherlands by a non-
English speaking local who had stopped to see if he could help. A
story that is often recounted with mime to a typically unimpressed
audience.

Mike

 

[Ben C]

On 2007-05-11, Pete Biggs <[email protected]> wrote:
> Ben C wrote:
>
> http://groups.google.co.uk/group/re...read/ec91d3709c1bf634/1bf12937a4e9c33b?hl=en&
>
> I haven't read the whole of that - but I can see just from the first few
> posts that the mention of flexibility of the actual casing is confusing the
> issue. A more flexible casing does reduce rolling resistance, and you might
> be able to detect it by hand in some cases with a soft tyre, but it's
> dwarfed by the pressure factor.
>
> With an inflated tyre, virtually all of the "hardness" you are feeling is
> from the air pressure.
>
> I take "casing tension" to mean the pressure exerted on the casing.

Not exactly-- that's just tyre pressure. The air inside pushes on the
casing with 100lbf per square inch. Casing tension is also known as
"hoop stress" and is the force trying to rip the tyre walls apart.

See http://en.wikipedia.org/wiki/Pressure_vessel.

Casing tension is pr/t where p is pressure, r the minor radius of the
tyre (i.e. 12mm for a 24mm tyre) and t is the thickness.

 

[Rob Morley]

In article <[email protected]>, N Cook
[email protected] says...
> Without going to the extent of a pressure meter - is there some technique
> that is a bit better than squashing sidewalls of tyres to tell whether soft
> or firm.
>
What's wrong with using a pressure gauge? I must admit I often just
push down hard on the bars or saddle to see how much the tyre bulges.

 

[Andrew Price]

On Thu, 10 May 2007 18:54:06 +0100, Richard Bates
<[email protected]> wrote:

>> Without going to the extent of a pressure meter - is there some technique
>> that is a bit better than squashing sidewalls of tyres to tell whether soft
>> or firm.
>
>On a more serious note, I usually judge 700x25C tyre pressure in
>qualitative terms by how the ride "feels"; and 20"/16" fatter tyres by
>how much lateral bulge is visible when I sit on the bike.

Another reliable sign of loss of tyre pressure is an increase in
distance recorded by a cycle computer over a given known distance
between two points.