why high pressures protect against p*****res

General cycling advice ( NOT technical ! )
burnsie
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why high pressures protect against p*****res

Post by burnsie »

Hi....this question has gone round in my head for ages. I know everyone says it, but why do they (high pressures prevent p*****res, not people say it)? In my mind, high pressures would increase the chance of a visit from you know who. So...is this folklore/chinese whisper, or what? Is there a scientific answer, or is it based on personal experience? I had a fair few visits when i started riding again last year, but I put these down to riding too close to the gutter..as my confidence grew I moved away, and now..touch wood (well bar the incident a month ago!!)....so what's the answer? Sorry if its elsewhere, but cant find it doing search.

Cheers Guys

Dave
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andrew_s
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Re: why high pressures protect against p*****res

Post by andrew_s »

High enough pressure will prevent pinch punctures, where the tube gets trapped between the rim and the edge of a pothole or whatever. These show as two short parallel cuts, 5-10mm long and 5-10mm apart.
For normal punctures from glass or whatever, the tyre will touch the road over a narrower track if the pressure is higher, so you've more chance of missing the bit of glass with your name on.
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DaveP
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Re: why high pressures protect against p*****res

Post by DaveP »

My personal take is that when a soft tyre rolls over something sharp it can start to penetrate, then stays in place until the next time round when it can be pressed a little further in and so on...
If the tyre is pumped up really hard it has much less give and is more likely to crush the invader.
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Colin63
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Re: why high pressures protect against p*****res

Post by Colin63 »

Thinking off the top of my head here....

An object which is going to give you a puncture has to be both sharp enough to cut through the tyre, and stronger than the pressure within your tyre. When a tyre hits a sharp object it's only for a very brief moment. If the object is razor sharp it might just cut through the rubber and flatten your tyre, but if it isn't that sharp it has to stretch the surface of the tyre to make a weak point before it can get through the rubber. It tears the rubber rather than piercing it. If the tyre has high pressure the tyre has less scope for that stretch and so it doesn't puncture, but if there's less pressure the tyre is able to stretch more and so less sharp objects are able to penetrate by creating a weak spot.

I have no idea whether or not that is nonsense, but it sounds convincing to me :D
yakdiver
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Re: why high pressures protect against p*****res

Post by yakdiver »

The other day while repairing a puncture I took two hawthorns out of my tyre both had penetrated right through the tyre it was pumped up to 100 psi
LifeintheSaddle
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Re: why high pressures protect against p*****res

Post by LifeintheSaddle »

As far as I know, underinflation of tyres can lead to pinch flats as mentioned above but overinflation of tyres can cause additional normal punctures as the tyre has far less give and won't absorb "regular" small stones, glass etc.

I think the key is just to inflate your tyre correctly and deep down inside believe that you yourself are too light to cause a puncture by rolling over something like glass :)
reohn2
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Re: why high pressures protect against p*****res

Post by reohn2 »

If you ride over something thats sharp ie thorn,shard of glass etc and it penetrates the tyre and tube you will puncture,I don't know that the chance is lessened by higher tyre pressures,tyre pressure is determined by the tyre section (which is the second number embossed on tyre side wall ie 700x28,28 being the tyre section),and weight or load the tyre is carrying ie I weigh 84kgs and by trial and error have come to a tyre pressure that suit me for certain types of tyre.
Different types of tyre require different pressures,ie I can quite happily ride 700x28 Schwalbe Marathon at 80psi whereas the same section Conti Gatorskin requires 90psi,the SM is no where near as supple a casing as the GG and as such rides better with a little less air in.
It does't matter if I consider my tyre pressure high, it suits my riding style,both for comfort and rollability(there must be a more technical word but I can't think of one).

When I get into conversation with other cyclists I'm sometimes surprised by the tyre pressures they tell me they use ,which is usually a lot higher than mine.
I used to subscribe to that way of thinking too,when Mrs R2 and I first started riding a tandem I was using Vredestien Perfect tyres in 700x32mm size,I used to put 110psi in them under the mistaken belief that as there was two up I needed my tyres with as much pressure in them as I could get.It doesn't take long when riding a tandem to come across other tandem riders,one of which asked during conversation what pressures I was riding,110psi I answered to which he said "thats too much" he then told me (and coincidentally we were riding exactly the same tyres) he rode at 85psi and invited me to try it.
I did but erred on the high side (95PSI) and found it a little more comfy but no loss in rollability,to cut a long story short I eventually arrived at 85psi and found much to my (and MrsR2's) surprise that the tandem was an aweful lot more comfy,I didn't get sore shoulders,my eyeballs didn't get jigled on bumpy tarmac,and my fillings didn't get shaken out of my teeth!!!But the most important thing was in the wet it was far more surefooted.
The point of this long and sometimes boring post is that tyres don't puncture less IMHO(wait a while and someone will "scientifically" disprove all of what I'm saying)when they are pumped up to the maximum pressure,they puncture when something sharp penitrates them.
There is a way of reducung punctures(notice I didn't say eliminating them)and its simple, first find out (by trail and error,which shouldn't take long)your optimum tyre pressures (forget the maximum stated on the tyre wall),don't ride in the gutter,or across those small triangles of gravel at junctions or through puddles,these are prime places for puncturey,pointy things to hide in,check tyres regularly for said nasties and remove any with a sharpend nail or thin screwdriver or some such.
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fausto copy
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Re: why high pressures protect against p*****res

Post by fausto copy »

I was just going to ask where you're supposed to get the sharpened nail.....
.....then I remembered.....they're usually found stuck in my car tyre. :lol:
Tandem Man
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Re: why high pressures protect against p*****res

Post by Tandem Man »

Leading on from this discussion,it has always puzzled me why pinch flats portray themselves as two small tears ( hence the term "snakebite" ).

The rim isn't particularly sharp at the edges and if it was why at two places only,neither is the tyre so why two holes ?

Ian
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hamster
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Re: why high pressures protect against p*****res

Post by hamster »

It's where the rim contacts the tyre - the two nips are tears as the tube gets caught and stretched.
George Riches
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Re: why high pressures protect against p*****res

Post by George Riches »

DaveP wrote:My personal take is that when a soft tyre rolls over something sharp it can start to penetrate, then stays in place until the next time round when it can be pressed a little further in and so on...

I'd say that when the wheel is positioned so the invader isn't between the tyre and the road, the higher the tyre pressure the greater the force pushing the invader out. In other words the higher the tyre pressure, the more likely that the invader will come out when there's nothing pushing it in. Which is for most of the tyre's rotation.
thirdcrank
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Re: why high pressures protect against p*****res

Post by thirdcrank »

I've found this very interesting because I'm somebody else who has simply always believed it to be so, but I now wonder if it really is true and if so, why? I've found no mention of it on Sheldon Brown which makes me even more doubtful. If you were to attack a baloon with a drawing pin, it would have to be inflated beyong floppiness to have any chance of puncturing it and the more it was inflated, the easier it would be to puncture. I can see the point that a hard tyre has a smaller area of contact with the ground but that's not the same thing.
reohn2
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Re: why high pressures protect against p*****res

Post by reohn2 »

George Riches wrote:
DaveP wrote:My personal take is that when a soft tyre rolls over something sharp it can start to penetrate, then stays in place until the next time round when it can be pressed a little further in and so on...

I'd say that when the wheel is positioned so the invader isn't between the tyre and the road, the higher the tyre pressure the greater the force pushing the invader out. In other words the higher the tyre pressure, the more likely that the invader will come out when there's nothing pushing it in. Which is for most of the tyre's rotation.


That depends on the size and sharpness of the foriegn body.If its very small,like a shard of glass(which can be very small and stay in a tyre for weeks)or a Hawthorn which can be anything upto a 1inch long.
Also something needing consideration is the thickness of the tyre tread,theres a world of difference between a well worn Marathon or a Conti Gatorskin and the same tyre(s) when new, so say Hawthorn will have more rubber to hold onto in a new tyre than in a worn one,in the new tyre it will be driven in with every revolution and needing mor revolutions to penetrate the tube,whereas with an old worn tyre it may get through first time round.Smaller objects,ie glass shards may hide embedded for longer in a new tyre again being push further in as the wheel revolves but again like the thorn,in a worn tyre could get through on the first revolution.
We have no way of knowing most of the time until the the tyre goes down.
The best thing is to check regularly especially in winter due to more mucky roads and dark/dull cold weather :( .

One thing I do when checking tyres with my sharp pointy nail is to push it into any cuts in the tyre and listen/feel for it grating against anything in the bottom of the cut if I can feel hear anything I'm not satisfied until its out (its usually glass) this is done by letting the tyre down and opening the cut by digging with the nail until I'm satisfied all the foriegn body is out.

BTW I've puntured twice this year in 6000miles, both in tandem tyres, once in a brand new Schwalbe Marathon that had only done 200miles,the tyre didn't go down,I found it two days later when checking the tyres,it was a Hawthorn which when I removed it only then did the tyre deflate.
The second was at the bottom of a climb in Italy with five miles to the end of the ride(a slow one),I thought I'd just put some air in and get 'home' but the tyre refused to inflate and remained soft. I removed the tyre (rear nach) only to find there was no evidence of a foriegn body in the tyre or anything on the rim,so replaced the inner tube.
On checking the offending inner tube later I found it was the valve seat parting company with the tube,and this despite a 1inch square of old innertube with a small hole in the centre placed over the valve seat snugly up against the tube,The inner tube was a Vredestien(which came fitted to the bike,I normally use Shwalbe) and on inspection it felt a little thin :? This is the first time I've had this kind of puncture for 15years or more since I started putting 1 inch squares over the valve stem.
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jhefner
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Re: why high pressures protect against p*****res

Post by jhefner »

DaveP wrote:My personal take is that when a soft tyre rolls over something sharp it can start to penetrate, then stays in place until the next time round when it can be pressed a little further in and so on...


Is this justification to bring back tire savers?

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George Riches
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Re: why high pressures protect against p*****res

Post by George Riches »

thirdcrank wrote: If you were to attack a baloon with a drawing pin, it would have to be inflated beyong floppiness to have any chance of puncturing it and the more it was inflated, the easier it would be to puncture.

The ballon's rubber is a lot thinner when the ballon is inflated than when it is not; the volume of rubber is constant but the area over which it is spread varies dramatically. The surface area of a tyre, in contrast, doesn't change much as the tyre is inflated.

The force at the very tip of the invading object is higher the greater the pressure in the tyre. If the tyre is pushing downwards at 100 psi onto a scrap of ground, that scrap must be pushing upwards at 100 psi at the point of contact. So as experience seems to show that higher pressures mean fewer punctures, some other factor must be at work. E.g.the push out idea I proposed earlier.
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