Tyre Pressure, 20" Wheels?

I keep the tyres on my large wheeled bikes pumped to about 4 bar. Is
this too much for smaller wheels? The use is mainly road, but with
occasional frequentation of a very nasty off-road path with lots of mud
and pointy stones. I'm paranoid about the puncture fairy because I have
no local backup or support on this bike.

Also, it has Schrader valves. I notice that even when I pump it to 4
bar, it takes so long to unscrew the valve that the air-loss brings it
back down to about 2 bar. The effect is more dramatic on the smaller
wheels since proportionally more air is lost when unscrewing the valve.

EFR
Ile de France

On Fri, 04 Feb 2005 17:14:51 +0100, Elisa Francesca Roselli
wrote:

Also, it has Schrader valves. I notice that even when I pump it to 4
bar, it takes so long to unscrew the valve that the air-loss brings it
back down to about 2 bar. The effect is more dramatic on the smaller
wheels since proportionally more air is lost when unscrewing the valve.

Use a pump which pushes on rather than screws on. You can release it
much quicker.

In uk.rec.cycling Elisa Francesca Roselli wrote:
: I keep the tyres on my large wheeled bikes pumped to about 4 bar. Is

4 bar = 60 PSI. Not that high, but maybe ok depending on how wide the
tyres are.

Smaller wheels need to be pumped harder usually. Assuming the tyres
are somewhere in the 25-28mm range I'd go for 6 bar and if they
are in the 30mm+ range maybe 5 bar.

: Also, it has Schrader valves. I notice that even when I pump it to 4
: bar, it takes so long to unscrew the valve that the air-loss brings it

What are you unscrewing here? For schrader values there is nothing to
unsrew. Just rip the pump off.

Arthur

--
Arthur Clune PGP/GPG Key: http://www.clune.org/pubkey.txt
It is better to light a candle than to curse the darkness

Elisa Francesca Roselli wrote:
I keep the tyres on my large wheeled bikes pumped to about 4 bar. Is
this too much for smaller wheels?

Pressure depends on the tyres, not the wheels. The sidewall will almost
always have recommended pressures moulded on the sidewall.

The use is mainly road, but with
occasional frequentation of a very nasty off-road path with lots of mud
and pointy stones. I'm paranoid about the puncture fairy because I have
no local backup or support on this bike.

For road use, use the maximum pressure stated on the sidewall of the
tyre. 4 bar is okay for road use, but more may well be better. I run
my Bromton's tyres at 6.9 bar, and that goes down a road with mud and
pointy stones every time I use it.

Off road, softer gives more suspension effect and a bit more grip and
stability. Very high pressures tend to get jumped sideways on the rough
stuff, which takes a bit of getting used to, but it doesn't mean you're
particularly likely to get punctures.

To avoid punctures, get tough tyres. Marathon Plus from Schwalbe are
pretty bombproof. I use the standard Marathons which are still pretty
tough. As for local support, fixing a puncture is really not a big
deal, and it's worth learning to do it. Changing a tube (quicker than
fixing the actual hole) takes a couple of minutes and the only tools you
need are tyre levers and possibly some sort of spanner for the wheel
nuts if you don't have quick release skewers. If you want to go touring
(and I know you do) then it's bordering on the essential that you can do
this yourself.
There should be no need to be paranoid about basic maintenance, or
you'll just remove a great deal of the pleasure you get from the bike.

Also, it has Schrader valves. I notice that even when I pump it to 4
bar, it takes so long to unscrew the valve that the air-loss brings it
back down to about 2 bar. The effect is more dramatic on the smaller
wheels since proportionally more air is lost when unscrewing the valve.

Different people get on better or worse with different valves. There's
nothing intrinsically wrong with either, but many people find they're
very happy with one and not so much with the other. In this vein, I
much prefer prestas. To try them out, just get a 20" presta tube and
change it for one on the bike, and that's a good excuse to practise
changing tubes...

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 http://www.dundee.ac.uk/~pjclinch/

On Fri, 04 Feb 2005 17:14:51 +0100, Elisa Francesca Roselli
wrote:

I keep the tyres on my large wheeled bikes pumped to about 4 bar. Is
this too much for smaller wheels? The use is mainly road, but with
occasional frequentation of a very nasty off-road path with lots of mud
and pointy stones. I'm paranoid about the puncture fairy because I have
no local backup or support on this bike.

I would have thought that tyre pressure is more a function of tyre
size and construction rather than wheel diameter. What are the tyres
rated up to?


Also, it has Schrader valves. I notice that even when I pump it to 4
bar, it takes so long to unscrew the valve that the air-loss brings it
back down to about 2 bar. The effect is more dramatic on the smaller
wheels since proportionally more air is lost when unscrewing the valve.

Puzzled.

With a Schrader valve all you do is unscrew the dust cap, put the pump
on, pump, remove adaptor and replace dust cap. Any hiss of escaping
air is going to be air on the pump side of the valve escaping, not
from the tyre.


Tim

On Fri, 04 Feb 2005 17:02:43 +0000, Tim Hall
wrote:

With a Schrader valve all you do is unscrew the dust cap, put the pump
on, pump, remove adaptor and replace dust cap. Any hiss of escaping
air is going to be air on the pump side of the valve escaping, not
from the tyre.

older style pumps comprised a plastic unit (with metal stem) into
which screwed one of two flexible adaptors - One for presta/woods, the
other for scraeder. The adaptors did indeed screw onto the valve and
allowed air to escape whist unscrewing.

e.g. http://www.sjscycles.com/store/7374zefal.jpg the bit sticking out
of the right hand side is the flexible adaptor which stows inside the
barrel.

"Elisa Francesca Roselli" wrote in message
...
I keep the tyres on my large wheeled bikes pumped to about 4 bar.
Is
this too much for smaller wheels?

That's probably about right, but as everyone else has pointed out, it
will tell you on the side of the tyre. With all those Americans
ready to sue for any excuse, there will be a generous margin beyond
the official number.

Wheels do wear out, eventually, as brakes rub on the side and wear
the rim away. Start checking at about 15-20 000 km. If the rim has
worn thin, a highly pressurised tyre can bow the rim outwards, which
you will be able to see. Lightweight racing wheels will wear out
sooner than utility wheels, although the rims on a smaller wheel pass
the brake blocks more often. Therefore, I suppose, other things
being equal, a small wheel should wear out more quickly. Other
things never are equal, though. Utility bikes are usually built for
strength, even at the cost of extra weight, but with a racing bike,
so long as the bike falls to pieces 10 metres after the finish line,
and not 10 metres before, nobody cares. Anyway, if you are female,
you are probably on the lighter side of average weights, and so will
not be braking as hard, and wearing rims as hard, as most other
people

The use is mainly road, but with
occasional frequentation of a very nasty off-road path with lots of
mud
and pointy stones. I'm paranoid about the puncture fairy because I
have
no local backup or support on this bike.

Also, it has Schrader valves. I notice that even when I pump it to
4
bar, it takes so long to unscrew the valve that the air-loss brings
it
back down to about 2 bar. The effect is more dramatic on the
smaller
wheels since proportionally more air is lost when unscrewing the
valve.

Yes, again see everybody else's comments. The pumps which screw on
are ***MUCH*** worse in every way than a good modern pump. If it's a
pump with a rubber hose, replace the pump as soon as possible.
Schraeder valves always tend to be more difficult to pump up than
presta valves, because there is a spring in the valve, and you have
to pump against some of the force of the spring, not just against the
pressure of the air in the tyre.

In theory there's a pin in the pump to hold the spring down, but it
never seems to completely work. With some schreader valves you can
take the valve out of the tyre, and then chop the valve in half with
a pair of wire cutters, thus destroying the spring, and making the
tyre much easier to pump. However, I'm not sure that you can do
that with modern valves (I use presta valves, so I'm not really up to
date on schraeders)


Jeremy Parker

In article , says...
In uk.rec.cycling Elisa Francesca Roselli wrote:
: Also, it has Schrader valves. I notice that even when I pump it to 4
: bar, it takes so long to unscrew the valve that the air-loss brings it

What are you unscrewing here? For schrader values there is nothing to
unsrew. Just rip the pump off.

Not necessarily. The pump may have one of those rubber tube extender
thingies that used to slide in to the top of the pump when not in use.


--
Hywel http://kibo.org.uk/
I do not eat quiche.

On Fri, 04 Feb 2005 17:02:43 +0000, Tim Hall
wrote in message
:

I would have thought that tyre pressure is more a function of tyre
size and construction rather than wheel diameter. What are the tyres
rated up to?

Quite. My 20" (ETRTO 406) tyres are inflated to 120 PSI As Nature
Intended.

Guy
--
"then came ye chavves, theyre cartes girded wyth candels
blue, and theyre beastes wyth straynge horn-lyke thyngs
onn theyre arses that theyre fartes be herde from myles
around." Chaucer, the Sheppey Tales