Tyre Pressure

g0g

In a moment of madness on Friay evening I ordered two continental ultra gatorskin tyres and a new pump for my bike - oh the irony! Anyways the tyres arrived today, but the sticker on them says recommended pressure 95PSI and max 115PSI. I thought normally bike tyres were meant to be around 110PSI for road use and you could go a good bit higher? That's certainly what I've been doing with my tyres since I got the bike last year. Are these gastor yokies designed for lower pressure?

LCRC_BAX

110 - 120psi is fine in them. They are made to be tough as old boots not to be run at lower pressure.

blorg

You don't give the width so a bit difficult to say. Pressure is mostly a function of the tyre width and your weight. Wider tyres you run at lower pressures. I would run 110R/100F for 23mm tyres, 100R/90F for 25mm, 78kg.

Leroy42

The tyre pressures quoted are merely a guide, a kind of insurance for the tyre company to avoid people complaining about punctures/blowouts etc.

Those pressures are tight in line with normally expected amounts.

As mentioned, pressure is based on both tyre width and the riders weight, but also a factor to be taken into account is the ride quality. Higher pressure will lead to a harsher ride, but faster! Based on the roads around Wicklow for example, I usually actually go for about 100psi which I find gives a good balance. If I am doing a race I usally bring the tyre pressure up to 120 as the last thing on my mind during the race is comfort!!!!

g0g

Thanks for the replies! Sorry, didn't realise those other factors were important. It's a Boardman Hybrid and the tyres that originally came on it were 700 x 28 (and I think it's written on them that they're ok to 120PSI) so that's what I ordered again now. I'm about 82kg I think. What do the R/F mean blorg? R = Race? F = Fitness?

Murph100

R=Rear F=Front

Could have happened to a bishop !

xz

R=Rear F=Front

doozerie

Leroy42 wrote:

Higher pressure will lead to a harsher ride, but faster!

Not necessarily. It seems like that view has been conventional wisdom for a long while (for road riding anyway, not so much for off-road though I think many people that reduce their tyre pressure on MTB's do so for better handling rather than greater speed but the two aims overlap) but it is challenged these days by the likes of Zipp (click on the "Rolling Resistance" tab). Here is the text:

The Corsa Evo CS will be perfect for that wheel, and I would recommend 100-125 psi (6.9-8.6 bar) depending on your weight. You will want to run 0.2-0.5bar more in the rear than the front to account for weight bias. I personally weigh 155lbs (70kg) and run my tires at 105 psi front (7.2 bar) and 110 psi (7.6 bar) in the rear. When I was racing and lighter I ran them at 100/105, but now that I'm heavier it is safer to run a few extra psi and I may go even higher on bad roads, which has worse rolling resistance, but better protects the wheels from damage in the event of hitting a pothole or something else which could damage a rim or tire. The better option is to run a wider tire at lower pressure on rough roads, but that is just not always feasible, so I would rather have higher rolling resistance and protect the wheels than lower rolling resistance and increased risk of damage to the wheel.

Higher pressure is definitely slower on anything other than perfect surfaces. Think of it in terms of a bunch of 1mm tall bumps in the road. If you have a lower tire pressure, the casing of the tire will deflect over each bump (we'll assume the casing deflects the entire 1mm) converting a small amount of energy into heat as the casing deflects, but the amount of energy necessary to compress the air is almost non-existent. Now at a higher pressure, we will assume that the tire deflects half as much. Now the bike and rider are lifted by 0.5mm and the casing deflects by .5mm, the energy necessary to deflect the casing by .5mm is less than it takes to deflect it by 1mm, but is nothing compared to the amount of energy necessary to lift the bike and rider by 0.5mm, so the end result is that the total energy requirement for the high tire pressure condition is much greater.

The other thing that happens is that on smoother roads, high tire pressures keep the casing from deforming over and into small cracks and crevices and over pebbles, which means that some of the deflection is transferred into the tire tread, which is not as elastic as the casing. Excessive tire wear comes about as the tire rubber begins to fail in shear as it is deformed by the road surface, and this generates heat as well as breaks down the cross-linking within the tread material.... overall, you are using more energy to go slower and you're wearing your tires out faster. The problem is that high tire pressures feel fast as your body perceives all the high frequency vibrations from the road surface as being faster than a smooth ride.

Lennard Zinn had a great analogy when he said that 100kph in a Jeep will scare the crap out of you but 200kph in an S class Mercedes feels effortless...the same is true of bike tire pressures, but it's just hard to convince ourselves of that. As athletes we tend to buy into the 'if some is good, more must be better' philosophy, but this is rarely true. Of course the tire manufacturers have given up on this and continue to try and make higher pressure tires as that's what the consumers demand, as I think that they've decided that it is easier to just give people what they think they want than to try to educate and argue with them :-)

@g0g, If you search you'll find quite a few online guides to tyre pressure based on rider weight and tyre width, but they don't all agree. Many of the older ones that I've seen suggest higher pressures, significantly higher in some cases, but I find that the Michelin guide works well for me for any road tyres. I would also be wary of exceeding the recommended max pressure of any tyre - the manufacturers probably fall on the side of safety and recommend a lower max pressure than the tyre can really handle, but personally I don't think it worth the risk of pushing your luck by exceeding their recommended figure.

blorg

110PSI is in the normal range for 700x23c tyres. You would never inflate 700x28c to that pressure. The Michelin chart suggests 87PSI for your weight which sounds about right; I would probably go for a bit less in the front (the rear carries more weight hence needs higher pressure.)