Tyre pressures

[Steve Ball]

I got a 'snakebite' flat in the front tyre after hitting a bit of roadwork
this morning. The tyre had at least 105psi in it. The tyre's a Continental
GP Attack and I notice its labelled "700x22C". All my tubes say "700x23C". I
assume the millimetre difference is ignorable and nothing to do with my bad
luck this morning. Or not?

This got me poking around Sheldon Brown's website, where I read that many
experienced cyclists run different pressures and even different tyres front
and rear, with the aim of maximising traction at the front and minimizing
rolling resistance at the rear. There's a table here:

http://www.sheldonbrown.com/tires.html#pressure

but it talks in terms of 'wheel load', without saying roughly what
proportion of a rider's weight is on the rear wheel (more) versus what's
taken by the front (less). Anybody have any idea?

But, given that the rear takes more weight, the front less, it's suggested
you inflate the rear harder, the front softer, which works in nicely with
less rolling resistance (due to less deformation) at the rear, more traction
(dues to greater contact area) at the front.

I've always read my tyre walls (well, _sometimes_ always read them) and put
100 psi in both but, given that I only weight 65Kg, they may well be
over-inflated, especially the front. All of which makes this morning's flat
more puzzling...

Comments from the gallery?
--
Steve = : ^ )

 

[Patrick Keogh]

Steve Ball wrote:
> I got a 'snakebite' flat in the front tyre after hitting a bit of roadwork
> this morning. The tyre had at least 105psi in it. The tyre's a Continental
> GP Attack and I notice its labelled "700x22C". All my tubes say "700x23C". I
> assume the millimetre difference is ignorable and nothing to do with my bad
> luck this morning. Or not?
>
> This got me poking around Sheldon Brown's website, where I read that many
> experienced cyclists run different pressures and even different tyres front
> and rear, with the aim of maximising traction at the front and minimizing
> rolling resistance at the rear. There's a table here:
>
> http://www.sheldonbrown.com/tires.html#pressure
>
> but it talks in terms of 'wheel load', without saying roughly what
> proportion of a rider's weight is on the rear wheel (more) versus what's
> taken by the front (less). Anybody have any idea?
>
> But, given that the rear takes more weight, the front less, it's suggested
> you inflate the rear harder, the front softer, which works in nicely with
> less rolling resistance (due to less deformation) at the rear, more traction
> (dues to greater contact area) at the front.
>
> I've always read my tyre walls (well, _sometimes_ always read them) and put
> 100 psi in both but, given that I only weight 65Kg, they may well be
> over-inflated, especially the front. All of which makes this morning's flat
> more puzzling...
>
> Comments from the gallery?
"hitting a bot of roadwork" can easily produce deformation forces large
enough to pinch a tube no matter what tyre pressure you use. Certainly a
700x22C or 23C is small enough that even a large rock could do the
job. Especially damaging are thing your run into rather than over. There
is a small road bridge on Wattle Street (Canberra) where the roadway
had subsided about 4cm exposing a right angle steel edge. I always bunny
hopped over it because that kind of front-on contact is really punishing
on road bikes (less on MTBs because the suspension absorbs a fair amount
of the impact).

If you want the last word on rolling resistance, talk to the MTB cross
country riders who have been tinkering with tyre pressure and profile to
a far greater extent than the blacktop folks, and yes, they do run lower
pressure on the front under many circumstances.

The most extreme version of 'wheel load' imbalance is loaded cycle
touring, where rear panniers and racktop luggage make up a lot of the
weight. On my touring bike I have always run a wider tyre on the back
than on the front (eg 32mm on front and 37mm on back for light touring,
or 42/47 for heavier loads). The point is that a narrow tyre will
require greater pressure as the load increases, and it is time to stop
when you get to the pressure on the placard or thereabouts. An easy way
to think about this is that the at-rest surface area in contact with the
road is equal to the load on the wheel divided by the tyre pressure. As
you add load you have to increase pressure, but if a tyre was 25% wider
then it could handle 25% more load at the same pressure. (This
simplistic arithmetic leaves out several factors but is good enough for
this kind of calculation).

I'm not sure I believe the viewpoint that reducing pressure results in
better traction for road tyres. Each tyre is designed with an optimum
profile. Tyres that corner well present a good profile to the road under
sideways stresses. This profile will be negatively impacted by over or
under inflation. We can all detect this in the extreme case (like when a
front tyre has a slow leak) as the underinflated tyre does not handle
predicatably.

So my opinion (and I'm sure there will be others) is to run tyres at the
optimum pressure. Maybe run a wider tyre at the back. In fact you'll
discover that if you do that you may even run it at a lower pressure
than the front one, certainly not a higher pressure. Have a look at the
Continental Attack/Force combination as an example of this kind of thinking.

 

[Steve Ball]

Thanks for the thorough reply. Some comments below.

Patrick Keogh:

> Steve Ball wrote:
>> I got a 'snakebite' flat in the front tyre after hitting a bit of roadwork
>> this morning. The tyre had at least 105psi in it. The tyre's a Continental
>> GP Attack and I notice its labelled "700x22C". All my tubes say "700x23C". I
>> assume the millimetre difference is ignorable and nothing to do with my bad
>> luck this morning. Or not?
>>
>> This got me poking around Sheldon Brown's website, where I read that many
>> experienced cyclists run different pressures and even different tyres front
>> and rear, with the aim of maximising traction at the front and minimizing
>> rolling resistance at the rear. There's a table here:
>>
>> http://www.sheldonbrown.com/tires.html#pressure
>>
>> but it talks in terms of 'wheel load', without saying roughly what
>> proportion of a rider's weight is on the rear wheel (more) versus what's
>> taken by the front (less). Anybody have any idea?
>>
>> But, given that the rear takes more weight, the front less, it's suggested
>> you inflate the rear harder, the front softer, which works in nicely with
>> less rolling resistance (due to less deformation) at the rear, more traction
>> (dues to greater contact area) at the front.
>>
>> I've always read my tyre walls (well, _sometimes_ always read them) and put
>> 100 psi in both but, given that I only weight 65Kg, they may well be
>> over-inflated, especially the front. All of which makes this morning's flat
>> more puzzling...
>>
>> Comments from the gallery?

(snip lots of useful info)

> I'm not sure I believe the viewpoint that reducing pressure results in
> better traction for road tyres. Each tyre is designed with an optimum
> profile. Tyres that corner well present a good profile to the road under
> sideways stresses.

Surely the profile the tyre presents to the road is going to be impacted by
the load on the wheel. To have the same tyre profiles, a heavier rider will
need greater inflation pressure than a lighter rider, and a rear tyre will
need greater inflation pressure than front tyre on the same bike. Or am I
crazy?

> This profile will be negatively impacted by over or
> under inflation. We can all detect this in the extreme case (like when a
> front tyre has a slow leak) as the underinflated tyre does not handle
> predicatably.

No problem there.

> So my opinion (and I'm sure there will be others) is to run tyres at the
> optimum pressure.

Agreed. I'm just not sure how we arrive at that. In my limited experience,
tyres are usually marked with a maximum pressure, and I don't know if the
author was being cynical, but the article I referred to makes the point that
this number is arrived at by a discussion between the manufacturer's
marketing and legal departments.

I no longer see the number on the sidewall as necessarily the best pressure
for both (or either) of my tyres. Optimum for my 65Kgs is probably somewhere
lower, but by how much, I'm still guessing.

> Maybe run a wider tyre at the back. In fact you'll
> discover that if you do that you may even run it at a lower pressure
> than the front one, certainly not a higher pressure. Have a look at the
> Continental Attack/Force combination as an example of this kind of thinking.

I have a Continental Attack on the front and it's labelled 700x22 and
'Maximum inflation 120 psi". Are you saying the matching Force is a bigger
tyre with a lower inflation pressure?
--
Thanks again,
Steve = : ^ )

 

[Patrick Keogh]

Steve Ball wrote:
<lots snipped>
>
> Surely the profile the tyre presents to the road is going to be impacted by
> the load on the wheel. To have the same tyre profiles, a heavier rider will
> need greater inflation pressure than a lighter rider, and a rear tyre will
> need greater inflation pressure than front tyre on the same bike. Or am I
> crazy?
>

No that's right, the change in optimum pressure will vary according to
the mass of a rider+bicycle. So unless we do a lot of scientific testing
it would be reasonable to conclude that a light rider will get best
results from a lower tyre pressure compared to a heavier rider. Almost
all the forces, both static and dynamic will be directly proportional to
the weight of bike+rider.

>
> Agreed. I'm just not sure how we arrive at that. In my limited experience,
> tyres are usually marked with a maximum pressure, and I don't know if the
> author was being cynical, but the article I referred to makes the point that
> this number is arrived at by a discussion between the manufacturer's
> marketing and legal departments.
>
> I have a Continental Attack on the front and it's labelled 700x22 and
> 'Maximum inflation 120 psi". Are you saying the matching Force is a bigger
> tyre with a lower inflation pressure?

Bigger tyre, same inflation pressure (to cover weight distribution).
You'll note from the Continental web site at
http://www.conti-tyres.co.uk/conticycle/default.html if you go there
that they specify "recommended" as well as "maximum" pressures. The
recommended pressure will be based on some (notional average?) weight rider.

For example from the site:

Tyre Width Recommended Maximum
Attack 22 110 120
Force 24 110 120
Top Contact 32 70 85
Top Contact 47 45 85

As you can see the matching Force is a wider tyre (10%) than the Attack
with the recommended pressure being the same. On the other hand if you
ran the ridiculous combination of Top Contact 32/47 the rear is 50%
wider and the recommended pressure difference is about the same. For use
as a loaded touring rear tyre I'd be increasing the pressure in the 47
from 45psi to much closer to the maximum, probably around 70.

This is shown somewhat simplistically at
http://two-wheels.michelin.com/2w/front/affich.jsp?codeRubrique=8092004154733&lang=EN
but you get the general idea.

 

[Steve Ball]

Patrick Keogh:

> Steve Ball wrote:
> <lots snipped>
>>
>> Surely the profile the tyre presents to the road is going to be impacted by
>> the load on the wheel. To have the same tyre profiles, a heavier rider will
>> need greater inflation pressure than a lighter rider, and a rear tyre will
>> need greater inflation pressure than front tyre on the same bike. Or am I
>> crazy?
>>
>
> No that's right, the change in optimum pressure will vary according to
> the mass of a rider+bicycle. So unless we do a lot of scientific testing
> it would be reasonable to conclude that a light rider will get best
> results from a lower tyre pressure compared to a heavier rider. Almost
> all the forces, both static and dynamic will be directly proportional to
> the weight of bike+rider.
>
>>
>> Agreed. I'm just not sure how we arrive at that. In my limited experience,
>> tyres are usually marked with a maximum pressure, and I don't know if the
>> author was being cynical, but the article I referred to makes the point that
>> this number is arrived at by a discussion between the manufacturer's
>> marketing and legal departments.
>>
>> I have a Continental Attack on the front and it's labelled 700x22 and
>> 'Maximum inflation 120 psi". Are you saying the matching Force is a bigger
>> tyre with a lower inflation pressure?
>
> Bigger tyre, same inflation pressure (to cover weight distribution).
> You'll note from the Continental web site at
> http://www.conti-tyres.co.uk/conticycle/default.html if you go there
> that they specify "recommended" as well as "maximum" pressures. The
> recommended pressure will be based on some (notional average?) weight rider.
>
> For example from the site:
>
> Tyre Width Recommended Maximum
> Attack 22 110 120
> Force 24 110 120
> Top Contact 32 70 85
> Top Contact 47 45 85
>
> As you can see the matching Force is a wider tyre (10%) than the Attack
> with the recommended pressure being the same. On the other hand if you
> ran the ridiculous combination of Top Contact 32/47 the rear is 50%
> wider and the recommended pressure difference is about the same. For use
> as a loaded touring rear tyre I'd be increasing the pressure in the 47
> from 45psi to much closer to the maximum, probably around 70.
>
> This is shown somewhat simplistically at
> http://two-wheels.michelin.com/2w/front/affich.jsp?codeRubrique=8092004154733&
> lang=EN
> but you get the general idea.

Thanks again for all the useful info. I'll visit the link in the morning.
--
Steve = : ^ )

 

[BT Humble]

Steve Ball wrote:
> ...
> I've always read my tyre walls (well, _sometimes_ always read them) and put
> 100 psi in both but, given that I only weight 65Kg, they may well be
> over-inflated, especially the front. All of which makes this morning's flat
> more puzzling...
>
> Comments from the gallery?

For pity's sake man, eat something!


BTH

 

[Steve Ball]

BT Humble:

> Steve Ball wrote:
>> ...
>> I've always read my tyre walls (well, _sometimes_ always read them) and put
>> 100 psi in both but, given that I only weight 65Kg, they may well be
>> over-inflated, especially the front. All of which makes this morning's flat
>> more puzzling...
>>
>> Comments from the gallery?
>
> For pity's sake man, eat something!

LOL. I love my food & drink. That's why I have to exercise so much.
--
Steve = : ^ )

 

[Bean Long]

Steve Ball wrote:

> Comments from the gallery?

Steve,

I'm just under 60 kg and have used Conti-GP4000's @ ~100 psi for some
time. For racing I've sometimes upped to 110 or thereabouts. I recently
changed to Michelin ProRace2's which come with a little guide to match
rider weight to pressure. At about 65 kg the recommended pressure is
somewhere around 100 psi. There may be more on the Michelin website. I'd
say your pressures are OK.

With tube size my guess is there shouldn't be a problem putting 700 x
23's in a 700 x 22 tyre. I'd assume that the tube will fit to the tyre
once under pressure and will deform to fit OK. ProBikeKit often
advertise their tubes and tyres with a little comment on whether the
tubes/tyres will fit together or on your rims OK, so maybe check them out.

As for pinch-cuts, if the pressure point is small enough you'll do
damage regardless of weight and tyre pressure. I once hit a pot-hole
with my rear wheel (tyre at ~90 psi I'd guess... I was using latex tubes
at the time and they lose pressure easily) and ripped a huge hole in my
tyre, pinch cut the tube and damaged the rim.

Cheers,

Bean