Pinch p*nct*res and tyres

[Tim Downie]

Helen Deborah Vecht wrote:

>
> 100 psi should be ample, but get your butt off the saddle for any
> obvious forthcoming blips.

Men don't have "butts". We have bums. ;-)

Actually, after the first puncture I was very careful to keep my bum out of
the saddle on the rougher bits but to no avail.

Tim

 

[Tim Downie]

Tony Raven wrote:
> Tim Downie wrote:
>>
>> About 11st (on 700c x28 tyres)
>>
>
>
> You won't be getting normal pinch puncture then and can forget about
> it being the surface you are riding on. There must be something else
> causing pinch puncture like punctures. Have a look for a sharp edge
> on the inside of the rim or a cheap plastic rim tape (replace with
> Velox rim tape)

I think the main factor is the nature of this particular forest road. It's
got a few steepish long hills and has scattered sharp rocks about 1 to 1.5"
in diameter. Hit one of those at speed and it's goodbye airtight tyre.

Rim (& rim tape are fine).

Tim

 

[Pete Biggs]

Tim Downie wrote:

> I think the main factor is the nature of this particular forest road.
> It's got a few steepish long hills and has scattered sharp rocks
> about 1 to 1.5" in diameter. Hit one of those at speed and it's
> goodbye airtight tyre.

I've hit bumps and holes bad enough to damage my wheels and fling me over
the handlebars but still my tyres stayed inflated.

On the other hand, I avoid speeding down hills with sharp 1.5" rocks!
Surely they're bad enough to cause a crash even if they don't puncture
your tyres?

Anyway you have got scope to pump your tyres harder and that will reduce
risk of pinch punctures. Good luck as far as grip and control goes
though!

~PB

 

[Simon Brooke]

in message <[email protected]>, Rusty Spokes
('[email protected]') wrote:

> "Simon Brooke" <[email protected]> wrote in message
> news[email protected]...
>>
>> I'm surprised 100psi isn't enough for 28mm tyres with an 11 stone
>> rider, but if you're getting pinch flats then it isn't - end of story.
>> Or else your pressure gauge is under-reading. Either way, pump 'em up
>> harder.
>
> I have recently started cycling on the road after a break of 35 years.
> Living in the Yorkshire Dales there are plenty of hills but I have got
> my distance over the first month up to 12 miles.
>
> I recently bought a nice second-hand Bianchi road bike.
> Bought a new track pump for the Armadillo 700 x 23 tyres that are
> fitted, currently have these at about 95 PSI.
> It says on the tyre a maximum pressure of 115 to 135 PSI.
>
> My weight is 15 stone 10lbs.
> Anyone wish to comment on the Tyres and pressures.
> What is the best type of Inner Tube to buy for a spare.

Rolling resistance decreases as the pressure increases.
Grip when cornering decreases as the pressure increases.
Comfort decreases as the pressure increases.
Handling precision /may/ increase as the pressure increases.

All these effects are very slight, and vary depending on the precise
tyres and on conditions.

My partner weighs 9 stone; I keep her tyres (700Cx23 semi-slick) at about
90psi because she whinges about comfort if I pump them harder, and she
doesn't get pinch flats at that pressure.

I weigh 13 stone. I used to ride 700Cx20s until this group persuaded me
there was no real benefit, now I ride 700Cx23, completely slick soft
compound. I used to run 700Cx20 at 110psi and didn't have pinch flat
problems, but now I run the wider tyres at 120psi and find the decrease
in comfort isn't noticeable. Probably the drop in rolling resistance
isn't noticeable either but I subjectively like the way the bike
handles.

However, run your tyres too soft and they /will/ pinch flat, and if
you're getting pinch flats or snake-bites it's certain that you're
running your tyres too soft.

Obviously, off road optimum pressure is a bit more complicated because of
traction issues, but on road, within the limits set by tyre and rim,
harder seems to me better.

--
[email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/

-- mens vacua in medio vacuo --

 

[Simon Brooke]

in message <[email protected]>, Pete Biggs
('[email protected]') wrote:

> Tim Downie wrote:
>
>> I think the main factor is the nature of this particular forest road.
>> It's got a few steepish long hills and has scattered sharp rocks
>> about 1 to 1.5" in diameter. Hit one of those at speed and it's
>> goodbye airtight tyre.
>
> I've hit bumps and holes bad enough to damage my wheels and fling me
> over the handlebars but still my tyres stayed inflated.
>
> On the other hand, I avoid speeding down hills with sharp 1.5" rocks!
> Surely they're bad enough to cause a crash even if they don't puncture
> your tyres?

H'mmm...

One of my favourite local hills has a bridge at the bottom that is pretty
much at right angles to the line of the road. The road is single track
and is used by heavy vehicles - there's a forestry quarry off it. This
means there's often loose gravel on the bridge, and cars tend to sweep
it into the centre of the carriageway.

You tend to hit the bridge at a good speed (you can easily get
thirty-five mph just free wheeling) and because of the sharpness of the
turn onto and off it you pretty much inevitably cross the bridge at a
diagonal, crossing the centre of the road as you do so.

The last two pinch flats I've had have been as a result of loose gravel
on or immediately after that bridge, so I do sympathise with the OP.
But even under that sort of cruel and unusual punishment, higher pressure
will help.

--
[email protected] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; all in all you're just another click in the call
;; -- Minke Bouyed

 

[Tim Downie]

Pete Biggs wrote:

> Special tyres aren't necessary. Any normal 28mm tyre + 100psi* + 11st
> should not compress all the way to the rim, whatever you hit. There's
> something funny going on.

Well I admit it is hard to believe that it is getting completely flattened
but when I've hit these rocks, it's felt like rock-to-rim contact. I get a
hard jolt and the tyre starts to deflate.

> Rim tape is No. 1 suspect if puncture isn't actually caused by tube
> getting pinched between rim and road.

I'm pretty certain my rim tape is okay. I replaced it last year with some
stuff recommennded by my LBS. It's wide enough to completely bridge the
width of the rim internally and thick enough to protect the tube from the
spoke ends (which I ground down smoothly before I fitted the tape)

>
> * consider trying 110psi in rear if tyre is actually more like 25mm
> than 28mm.

Certainly I'll try a bit more pressure but there are comfort considerations.

Tim