What tyre pressure?

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What tyre pressure?

Postby commi » Sun Jan 28, 2007 2:53 pm

My Avanti Blade Sport comes with 700x28c all condition tyres...I remember when I first got it the tyres were rock hard. After the first 2 weeks they became a bit soft so I went to my bike shop to pump it up (they did it for me when it was getting the trip computer fitted). Now its time to pump them again. I bought a floor pump but I'm not sure how hard to pump them.

The tyre says "recommended 800KPa min/850KPa max with hook bead rim (115PSI/125PSI, 8.0Bar/8.5Bar)

Does that min 115min and 125max PSI?
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by BNA » Sun Jan 28, 2007 3:00 pm

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Postby mikesbytes » Sun Jan 28, 2007 3:00 pm

It depends on how much you weigh. There is an optimum pressure that provides the correct size tyre patch.
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Postby commi » Sun Jan 28, 2007 3:02 pm

I'm between 70-75kg and 175cm...

That is, I would like to weigh 70kg, but I'm currently flutuating between 72-73kg.
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Postby mikesbytes » Sun Jan 28, 2007 3:14 pm

On 23mm tyres that would be about 100psi front and 110psi rear.

So I guessing that on 28mm tyres, that would be about 85psi front and 95psi rear.

Michelin's tyre pressure guide

Sheldon Brown tyre pessure guide
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Postby commi » Sun Jan 28, 2007 4:01 pm

I'm not sure if I'm using the floor pump correctly or not.

On my MTB, as soon as I put the valve in and lock it the gauge gives me a reading of PSI and so I could pump it up no worries...

But on the flat bar nothing happens when I put valve in and lock.

Its only when I start pumping do I see something.

So when I unscrew the cap off the tyre I then unscrew the little metal thingy, right? I can now put the pump valve thing in and lift to lock...

Have I got the steps correct?
And when I stop pumpingg (just under 100PSI) the thing is kind of hard to pull off. By the time I yank it off I'm sure I've let some air out.

I feel like such an idiot, that I can't even pump a tyre!!!

Can't wait til I get a puncture and have to change it (not that I have a spare tube or puncture kit).
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Postby tinstaafl » Sun Jan 28, 2007 4:09 pm

commi wrote:Can't wait til I get a puncture and have to change it (not that I have a spare tube or puncture kit).
I actually do like the whole puncture repair process.

The secret is to carry a spare tube with you. A lot of people have a small saddle bag under the seat with a tube and tyre levers. I carry a patch kit but have yet to have a double puncture on the one ride.
It's much more fun to repair a tube at home.

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Postby mikesbytes » Sun Jan 28, 2007 4:20 pm

The flat bar has a different type of valve that is only engaged when you start to pump, therefor you don't get a reading prior like you do on the MTB
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Postby LuckyPierre » Sun Jan 28, 2007 4:56 pm

As Mike says, the 700c tube has a 'presta' valve and they often won't register on a pump's gauge until they open - ie. after you've got some air in. Mine often go way up, then the valve 'cracks open' and I start to get sensible readings. Your sequence of events seems right - well, that's what I do anyway!
Don't forget to 'unlock' the pump from the valve - but the air you hear escaping is more likely to be air in the delivery hose from the pump, rather the bike tube.
I think that 100 psi in a 28 mm tyre seems pretty high, especially if you're only low- to mid-70 kgs.
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Postby commi » Sun Jan 28, 2007 7:53 pm

peterrjleach wrote:I think that 100 psi in a 28 mm tyre seems pretty high, especially if you're only low- to mid-70 kgs.


How would that affect the ride? Too bumpy? More prone to puncture? Control?
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Postby Hotdog » Sun Jan 28, 2007 8:22 pm

That's pretty much it. Over inflation reduces the suspension effect of the tyres, which gives you a harsher ride. That's not just uncomfortable as the extra juddering also reduces traction. The harder tyres also make your wheels more prone to damage if you hit a bump. Under inflation on the other hand increases rolling resistance so slowing you down, and if severe puts you at risk of a 'pinch flat' type puncture or even damaged wheel rims if you hit a bump.

Edit: Seems Bnej has been talking about these things too, over in another thread: http://bicycles.net.au/forums/viewtopic.php?p=7744#7744
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Postby mikesbytes » Sun Jan 28, 2007 10:12 pm

commi wrote:
peterrjleach wrote:I think that 100 psi in a 28 mm tyre seems pretty high, especially if you're only low- to mid-70 kgs.


How would that affect the ride? Too bumpy? More prone to puncture? Control?


What happens is that the tyre feels as if its giving you less rolling resistance where in fact it is giving you greater rolling resistance. Instead of folding around the micro bumps on the road surface, it is bouncing upwards over them converting your forwards energy into wasted upwards energy.
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Postby stryker84 » Mon Jan 29, 2007 11:00 am

mikesbytes wrote:
commi wrote:
peterrjleach wrote:I think that 100 psi in a 28 mm tyre seems pretty high, especially if you're only low- to mid-70 kgs.


How would that affect the ride? Too bumpy? More prone to puncture? Control?


What happens is that the tyre feels as if its giving you less rolling resistance where in fact it is giving you greater rolling resistance. Instead of folding around the micro bumps on the road surface, it is bouncing upwards over them converting your forwards energy into wasted upwards energy.


more exactly, it's actually less rolling resistance, but you're wasting energy since you're pedalling while bouncing (i.e, airborne), some of the energy you're putting into the pedals isn't converted into forward movement due to the tyre not gripping while you bounce.
also, related, is the slight loss of traction you get due to the bounce/loss of grip, can be dangerous if by chance you need quick precise handiling.
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