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Max. tyre pressure Q.

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Tony R

Evening all,

as my tourer will not be touring this year I've bought some
slightly narrower tyres for it - 28mm Schwalbe Marathons.
They are rated to a maximum pressure of 115 p.s.i. and I
would normally run the rear tyre at about this and the front
slightly lower as per Sheldon and others' advice. However,
my rims (Mavic A719s) also have a maximum tyre pressure
marked on them, and for 28mm tyres this is 103 p.s.i. Any
thoughts on whether I can breach this limit safely? After
all, my old rims had no such markings and it would have
never even entered my head that they might have a limit that
was lower than the tyre.

thanks,

tony R.

Simon D

"tony R" <hesiod3@hotmail.com> wrote in message
news:Dqwpc.7893$qP2.16286@news.indigo.ie...
> Evening all,
>
> as my tourer will not be touring this year I've bought
> some slightly narrower tyres for it - 28mm Schwalbe
> Marathons. They are rated to a
maximum
> pressure of 115 p.s.i. and I would normally run the rear
> tyre at about
this
> and the front slightly lower as per Sheldon and others'
> advice. However,
my
> rims (Mavic A719s) also have a maximum tyre pressure
> marked on them, and
for
> 28mm tyres this is 103 p.s.i. Any thoughts on whether I
> can breach this limit safely? After all, my old rims had
> no such markings and it would
have
> never even entered my head that they might have a limit
> that was lower
than
> the tyre.

Personally, I'd have thought 115PSI a bit high for 28mm
covers anyway, although rider weight obviously does make a
difference. I wouldn't give Mavic rims any excuse for
disintegrating, if I were you - I've no experience with the
719s, which may well be fine, but others tend not to need
much encouragement!

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Pete Biggs

tony R wrote:
> Evening all,
>
> as my tourer will not be touring this year I've bought
> some slightly narrower tyres for it - 28mm Schwalbe
> Marathons. They are rated to a maximum pressure of 115
> p.s.i. and I would normally run the rear tyre at about
> this and the front slightly lower as per Sheldon and
> others' advice. However, my rims (Mavic A719s) also have a
> maximum tyre pressure marked on them, and for 28mm tyres
> this is 103 p.s.i. Any thoughts on whether I can breach
> this limit safely? After all, my old rims had no such
> markings and it would have never even entered my head that
> they might have a limit that was lower than the tyre.

I also think 115 psi is a little high for a 28mm touring
tyre, for comfort, but I think you should be able to use
that pressure if you like... in general, that is. I don't
know the A719.

If the concern is over whether the tyre will stay on the
rim (rather than about the strength of the rim), I would
test the tyre by over-inflating it (to beyond what you
would ever want to use) and leaving to stand for 24
hours*. If it survives that, it should survive lower
pressure in normal use with all the various forces. There
is a danger of damaging the rim's sidewalls if you go too
far, though. I would guess a 130 psi test would do for
this case, but don't go that high if you think the rim
might not be strong enough.

Dodgy tyres I've used have failed this test and I've never
had any problems with tyres or rims that have passed it. Be
prepared for an enormous bang and the loss of an inner tube.

* It can take several hours for a bead to creep off, even
when the wheel is stationary sitting at home. Best to test
both tyres on both rims in case they ever get swapped (the
tyres/rims may be actually slightly differenent even if
same models & sizes).

~PB

Just Zis Guy

On Sat, 15 May 2004 23:04:22 +0100, "tony R" <hesiod3@hotmail.com>
wrote in message <Dqwpc.7893$qP2.16286@news.indigo.ie>:

>28mm Schwalbe Marathons. They are rated to a maximum
>pressure of 115 p.s.i. and I would normally run the rear
>tyre at about this and the front slightly lower as per
>Sheldon and others' advice. However, my rims (Mavic A719s)
>also have a maximum tyre pressure marked on them, and for
>28mm tyres this is 103 p.s.i.

Wot the others said: 90-100psi should be fine. If in doubt
about the rims (e.g. due to wear), overinflate by 50% then
let the pressure down again.

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk (http://www.chapmancentral.co.uk/)

88% of helmet statistics are made up, 65% of them at Washington University

Visionset

Tony R

"VisionSet" <spam@ntlworld.com> wrote in message
news:Y5Ipc.300$ya2.78@newsfe5-gui.server.ntli.net...
>

> >
>
> They will have a massive safety margin built in, they
> should be fine.
>
Thanks for the advice everyone. I too was a little surprised
to see 115 on the sidewall, but what do I know? I've always
run 32mm tyres. Anyway, I think I'll try them out at about
100 and see how it goes.

tony R.

Mark South

"tony R" <hesiod3@hotmail.com> wrote in message
news:g0Npc.7990$qP2.16290@news.indigo.ie...
>
> "VisionSet" <spam@ntlworld.com> wrote in message news:Y5Ipc.300$ya2.78@newsfe5-
> gui.server.ntli.net...
> >
> > They will have a massive safety margin built in, they
> > should be fine.

The question is how much safety margin will remain when the
tyres heat up from braking on a long descent.

Recent/current thread in rec.bicycles.tech on this topic.

> Thanks for the advice everyone. I too was a little
> surprised to see 115 on the sidewall, but what do I know?
> I've always run 32mm tyres. Anyway, I think I'll try them
> out at about 100 and see how it goes.

Think about how much you and your load will weigh. I weigh
180lbs on a skinny day and inflate my 23mm road bike tyres
to 100psi each. That means they each need to have a contact
patch of about 9/10ths of a square inch in order to support
my weight. And that's fine with my tyres.
--
Mark South Citizen of the World, Denizen of the Net <<Tiens!
Ce poulet a une grenade!

Visionset

"Mark South" <mark.south@null.invalid> wrote in message
news:40a7a27f$1_1@news.bluewin.ch...
> "tony R" <hesiod3@hotmail.com> wrote in message
> news:g0Npc.7990$qP2.16290@news.indigo.ie...
> >
> > "VisionSet" <spam@ntlworld.com> wrote in message news:Y5Ipc.300$ya2.78@newsfe5-
> > gui.server.ntli.net...
> > >
> > > They will have a massive safety margin built in, they
> > > should be fine.
>
> The question is how much safety margin will remain when
> the tyres heat up
from
> braking on a long descent.
>

That's like saying a lift motor is specified for upto 10 m/s
but they only tested it with an empty lift. No they test it
with a 200% of rated load and 150% of the rated speed.

--
Mike W

Jon Senior

VisionSet spam@ntlworld.com opined the following...
> That's like saying a lift motor is specified for upto
> 10 m/s but they only tested it with an empty lift. No
> they test it with a 200% of rated load and 150% of the
> rated speed.

But the OP is talking about exceeding the maximum pressure
stated on the rim, so he is already over the rating. The
question is, how over- engineered is the rim?

Jon

Simon D

"VisionSet" <spam@ntlworld.com> wrote in message
news:z%Tpc.11934$7S2.472@newsfe1-win...
>
>
> "Mark South" <mark.south@null.invalid> wrote in message
> news:40a7a27f$1_1@news.bluewin.ch...
> > "tony R" <hesiod3@hotmail.com> wrote in message
> > news:g0Npc.7990$qP2.16290@news.indigo.ie...
> > >
> > > "VisionSet" <spam@ntlworld.com> wrote in message news:Y5Ipc.300$ya2.78@newsfe5-
> > > gui.server.ntli.net...
> > > >
> > > > They will have a massive safety margin built in,
> > > > they should be
fine.
> >
> > The question is how much safety margin will remain when
> > the tyres heat
up
> from
> > braking on a long descent.
> >
>
> That's like saying a lift motor is specified for upto 10
> m/s but they only tested it with an empty lift. No they
> test it with a 200% of rated load
and
> 150% of the rated speed.
>

Whilst I have no experience with the 719, the fact is that
Mavic rims have, during the past ten years or so, developed
a reputation for exploding sidewalls. This is mostly due to
lack of sidewall thickness / durability and wear, and as
such is unlikely to be a problem with new rims, but clearly
tyre pressure is another factor which will become more
significant as the rim wears. The people who have suggested
over-inflating overnight are making sensible
suggestions.Your confidence in Mavic's testing is (whilst
touching ;-)) seriously misplaced IMO!

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Simon D

"Jon Senior" <jon_AT_restlesslemon_DOTco_DOT_uk> wrote in message
news:MPG.1b127556210e9964989736@news.clara.net...
> VisionSet spam@ntlworld.com opined the following...
>
> But the OP is talking about exceeding the maximum pressure
> stated on the rim, so he is already over the rating.

I agree. Although it's improbable that 15 PSI will make much
difference, I think it's also fair to say that there will be
very little performance advantage. Why not stick to the
recommendations?

> The question is, how over-
> engineered is the rim?

Quite. And *over* engineering is not a speciality of Mavic,
in my experience.

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Davek

Simon D:
> Personally, I'd have thought 115PSI a bit high for 28mm
> covers anyway,

So, you wouldn't be too enthusiastic about the idea of a 1
1/4" kevlar-beaded tyre inflated to 120 psi (max pressure
according to tyre info) on a front wheel with an unhooked
rim, on a bike ridden by a 14st+ rider?

Couldn't tell you what the rim is. It's not Mavic. It's
aluminium, I know that much. There's nothing on the rim
about max pressure, only on the tyre. I hadn't even
considered that this might be a problem, but now I'm a bit
scared that I'm leaving myself in danger of blowouts/rim
explosions. I thought you needed to run the tyres at the
highest possible pressure to reduce risk of punctures.

d.

Simon D

"davek" <david.nospam.kenning@which.nospam.net> wrote in message
news:c89vlf$n75$3@sparta.btinternet.com...
> Simon D:
> > Personally, I'd have thought 115PSI a bit high for 28mm
> > covers anyway,
>
> So, you wouldn't be too enthusiastic about the idea of a 1
> 1/4" kevlar-beaded tyre inflated to 120 psi (max pressure
> according to tyre
info)
> on a front wheel with an unhooked rim, on a bike ridden by
> a 14st+ rider?
>
> Couldn't tell you what the rim is. It's not Mavic. It's
> aluminium, I know that much. There's nothing on the rim
> about max pressure, only on the
tyre.
> I hadn't even considered that this might be a problem, but
> now I'm a bit scared that I'm leaving myself in danger of
> blowouts/rim explosions. I thought you needed to run the
> tyres at the highest possible pressure to reduce risk of
> punctures.
>

I think that's unlikely that either problem will occur (when
it's a real problem - when you're riding it) if you follow
Guy et als' advice about pre-testing at higher pressures.
Rims can and do disintegrate, though! Check rims for wear
regularly.

My concern re. the OP is that Mavic are recommending a lower
pressure from the start - i.e. not as a preventative measure
when the rim's worn. Possibly higher pressure from the start
can reduce the time before it disintegrates, through
increased outward force being applied over a long period???
That's just complete speculation, though.

As for higher pressures reducing punctures, that depends on
the (potential) cause of the puncture. It will reduce impact
punctures ("snake bite"), but will probably, if anything,
increase punctures caused by thorns, glass, etc. Again, I
wouldn't have thought 120PSI would be the optimum for you; a
1 1/4" tyre can deform a lot before it causes a snake bite.

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Davek

Simon D:
> I think that's unlikely that either problem will occur
> (when it's a real problem - when you're riding it) if you
> follow Guy et als' advice about pre-testing at higher
> pressures.

I presume that over-inflating will quickly
(catastrophically?) reveal if there is excessiev wear in
the rim, yes?

> My concern re. the OP is that Mavic are recommending a
> lower pressure from the start - i.e. not as a preventative
> measure when the rim's worn.

Ah, now I see. And I think I understand your concern.

> Again, I wouldn't have thought 120PSI would be the
> optimum for you

Tactfully put. ;-)

I forgot to add my other reason for prefering higher
pressure - they seem to run faster at higher pressure (tbh,
I rather thought that was the point of high-pressure tyres).
It makes the ride a bit 'harder', but not uncomfortably so.

d.

Simon D

"davek" <david.nospam.kenning@which.nospam.net> wrote in message
news:c8absg$hpq$1@sparta.btinternet.com...

> I presume that over-inflating will quickly
> (catastrophically?) reveal if there is excessiev wear in
> the rim, yes?

Spectacularly, potentially! When the CTC published guidance
on this, they suggseted wearing safety glasses, which
sounds sensible.

The funny thing is, that for all the countless rims I've
known to fail in this way when being ridden, I don't
remember anyone having one fail in a "critical" situation
(fast descent, e.g.). This can only have been luck (or
people who are about to use their bikes in such a way
checking carefully first), and I'd have thought that
increased heat caused by braking, and possibly sideloads
too, would make them more likely to blow in such situations.
I suppose that air pressure is *much* the most important
factor, and that since the sidewall is permanently under
load when the tyre is inflated the probability of failure at
The Wrong Time is low.

> I forgot to add my other reason for prefering higher
> pressure - they seem
to
> run faster at higher pressure (tbh, I rather thought that
> was the point of high-pressure tyres). It makes the ride a
> bit 'harder', but not uncomfortably so.

Yes, it's certainly true that rolling resistance reduces as
pressure increases. However, this is not a linear
relationship; you gain a lot more by increasing from 70 to
80 PSI than from 100 to 110PSI. There comes a point where
the returns (speed) are not worth the payoff (punctures;
harsh ride). Nonetheless, if you're finding it works for you
at 120PSI, who am I to argue? I would recommend keeping an
eye on your rims, though (whatever pressure you run your
tyres at). Don't get paranoid, but do be aware of it. You're
probably okay with other brands, anyway - I always [1] found
Mavic far and away the worst for this.

[1] "Always" is not true, actually. Much as I hate to feel
that I might be becoming an "Ah, it wasn't like that in
my day..." type, I have to say that the Mavic rims of
the early 80s never suffered this problem, IME. I think
milling the sidewalls was the first step in the wrong
direction, but there may be other technical reasons
(alloy, attempts to save weight, e.g.) for the changes.

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Pete Biggs

Simon D wrote:

>> But the OP is talking about exceeding the maximum
>> pressure stated on the rim, so he is already over the
>> rating.

What is that rating for? Why does it vary with tyre width?
Is it about rim strength or tyre retension?

~PB

Simon D

"Pete Biggs" <pclemantine{remove_fruit}@biggs.tc> wrote in message
news:2grsj7F61mpeU1@uni-berlin.de...
> Simon D wrote [actually, Jon Senior wrote!]:
>
> >> But the OP is talking about exceeding the maximum
> >> pressure stated on the rim, so he is already over the
> >> rating.
>
> What is that rating for? Why does it vary with tyre width?
> Is it about rim strength or tyre retension?
>
> ~PB
>

Wasn't actually me who wrote that bit, Pete. Your guess is
as good as mine. Might well just be a recommendation of
"appropriateness", rather than being directly related to rim
/ tyre failure. That would explain the tyre section
differences, wouldn't it? On the other hand, I'm guessing
that a wider tyre might exert greater pressure on the
sidewalls of the rim, pressure being equal - what do you
reckon? (If you try to picture the cross section, you might
see where I'm coming from with that statement.) At very
least, the *direction* of the force is likely to be slightly
different, don't you think?

If it were my rim, I'd just be inclined to follow the
manufacturer's advice, especially as it seems
appropriate anyway.

This is the first time I've heard of a *rim* manufacturer
specifying pressures, and I can't help but suspect that it's
connected to Mavic's reputation for producing rims which self-
destruct after a year or two.

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Mark Tranchant

Simon D wrote:

> The funny thing is, that for all the countless rims I've
> known to fail in this way when being ridden, I don't
> remember anyone having one fail in a "critical" situation
> (fast descent, e.g.). This can only have been luck (or
> people who are about to use their bikes in such a way
> checking carefully first), and I'd have thought that
> increased heat caused by braking, and possibly sideloads
> too, would make them more likely to blow in such
> situations. I suppose that air pressure is *much* the most
> important factor, and that since the sidewall is
> permanently under load when the tyre is inflated the
> probability of failure at The Wrong Time is low.

http://tranchant.plus.com/cycling/france-1996/ft7-8

Scroll down a bit to read "Day 8". If that German had been
going the same speed as us, he'd have been doomed (unless of
course, he'd worn a helmet).

--
Mark.

Mark Tranchant

Simon D

"Mark Tranchant" <mark@tranchant.plus.com> wrote in message
news:bs3qc.4357$wI4.496869@wards.force9.net...
> Simon D wrote:
>
> > The funny thing is, that for all the countless rims I've
> > known to fail
in
> > this way when being ridden, I don't remember anyone
> > having one fail in a "critical" situation (fast descent,
> > e.g.). This can only have been luck
(or
> > people who are about to use their bikes in such a way
> > checking carefully first), and I'd have thought that
> > increased heat caused by braking, and possibly sideloads
> > too, would make them more likely to blow in such
> > situations. I suppose that air pressure is *much* the
> > most important
factor,
> > and that since the sidewall is permanently under load
> > when the tyre is inflated the probability of failure at
> > The Wrong Time is low.
>
> http://tranchant.plus.com/cycling/france-1996/ft7-8
>
> Scroll down a bit to read "Day 8". If that German had been
> going the same speed as us, he'd have been doomed (unless
> of course, he'd worn a helmet).
>

So it does happen. Had he managed to remain upright, Mark?

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