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Can I just have a bit more air in these tyres?

@sheldonbrowncom
by Geoff Husband
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It's Saturday afternoon and I have 20 of our cyclists around me getting ready for the off. I'm making sure they are all fitted out properly and checking their packing and generally being careful everyone is happy. And then every single week – someone in the group pipes up with that question.

Of course what follows is that everyone else starts to squeeze their tyres as say – oh they are a bit soft could I have a bit more air too?

I'm a great believer in the adage that 'the customer is always right' and so I am put, inevitably on–the–spot. First it looks bad. Here I am, making a big thing about how all our bikes are carefully set–up and really well maintained the gears are smooth, the brakes work, everything is spot on but it looks like I can't be bothered to pump the tyres up... And it's a dilemma, do I start arguing about tyre pressures and look an idiot? – because of course everyone knows they should be as hard as they can go. Or do I go round looking like a rank amateur and pump up 40 tyres while I should really be doing something more productive?

And that is what has prompted this article it's an attempt to ward off those questions and hopefully overturn a universally accepted 'truth' that in reality is hopelessly misleading.

OK to start... Pop outside and have a look at your car tyres. It's quite possible that they are marked 'maximum pressure 45 psi' or something similar. Now would you in your most insane moment – go to a garage air–line and whack 45 psi into that tyre? No of course not! And why not? Because you'd end up with a car that would have a horribly hard ride, no grip and which would wear the centre of the tread to bald in a very few miles. So why does everyone seem to look at the maximum pressure rating of their bike tyre and pump it to that???

Let's think about this sensibly. What does the air in that tyre do? Well it acts as a spring no more no less. In the distant past cars and bikes had solid rubber tyres and the ride on those bikes was horrible. Making the tyre hollow and replacing it with a long tube of air under pressure instantly gave the rubber tyre much more 'spring' and so the tyre became far more comfortable.

So far so good, but the air had one other advantage of course it weighted next to nothing so the tyre became much lighter and thus the whole wheel/tyre became easier to accelerate and the bike much faster.

Now the crunch how much air should that tyre hold what pressure is best? Let's look at the two extremes. If we pumped the tyre up to a very, very high pressure then it would become rock hard suddenly one of the advantages of the pneumatic (air–pressure) tyre would be lost. You'd in effect be riding a solid tyre and this inevitably would make the bike very harsh and uncomfortable. As with our car–tyre example it would also have much less grip because of the small contact area, and wear rapidly a thin line in the centre of the tyre.

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The other extreme? What if we just put a very little air into the tyre say 10 psi. At this pressure the 'spring' of the air is very soft and the bike will feel wallowy and difficult to control especially at speed. Most importantly any ridge or bump will cause the tyre to bottom out that is the air pressure will not be sufficient to stop the tyre squishing completely and the rim of the wheel hitting the tread of the tyre pinching the innertube, causing a pinch–puncture (or snake–bite named from the two parallel holes usually caused) and possibly damaging the rim.

So from this it's pretty obvious that the perfect tyre pressure will be somewhere between rock hard and very soft, and this is where it gets complicated:–)

So what determines the 'ideal' tyre pressure? Well let's go out to your car again... Somewhere in the handbook it'll state recommended pressures. In fact it'll probably give two sets one for lightly loaded and one for heavily loaded and the difference might be as much as 30%. Now that's logical, as the air is simply a spring a greater weight will compress the spring more so that it loses some of its travel so the more weight the more pressure and the relationship is pretty much linear so a 30% increase in load gives a 30% increase in recommended tyre pressure.

What else? Well one effect of a tyre rolling is that it distorts the tyre, the bit touching the road gets flattened by the weight, and this continuous distortion and flexing uses up energy. In theory a very hard tyre will distort less and so use less energy and so be faster and this is the root of all that misinformation about tyre pressure. In the public mind hard tyre=easy riding. This is a fallacy. If you are riding on the wooden boards of a track then you can happily use 200 psi+ to good advantage, but add any irregularities to the surface, and the tyre's flexibility will use more energy being rock hard and riding up and down every tiny imperfection. The rougher the surface the lower the optimum pressure is for minimum resistance. And of course that rock–hard tyre well be uncomfortable and as we don't want to be shaken to bits we need to compromise.

And compromise is what it's all about, but there are some indisputable conclusions we can come to. Let's take two typical cyclists my wife and I... I weigh 85 kgs, my wife (Kate) a sylph–like 55kgs. Our bikes weight about the same at 12 kgs unloaded. So 97 kgs and 67 kgs. You see where I'm going with this? Kate+bike is about 30% lighter than me and my bike. So inevitably, for the same ride quality, tyre distortion and rolling resistance her tyre pressures need to be >30% lower than mine!

So for the sake of argument, if my tyres are at 100 psi, hers should be at 70 psi? Well no because on a typical bike the load isn't distributed evenly between the tyres in fact on an average bike the split is something like 60% rear, 40% front. So let's go back to Kate and I. If the correct pressure for my rear tyre is 100 psi then my front tyre should be at about 67 psi. My wife's bike should have a rear at around 65 psi and a front at 43 psi. Now is it any wonder that some slender lady squeezes her front tyre, compares it to her husband's rear tyre and tells me her tyres are too soft!

But of course we have taken an assumed ideal for my bike of 100 psi (mainly because it makes the maths easy:–) and that is a big assumption because there's another major factor in calculating the ideal tyre pressure and that is tyre width.

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Think about it if your bike is running 18mm racing tyres then they need to have huge pressure in them to resist the slightest ridge giving a pinch puncture. On the other hand, the sort of wide tyre that we tourists use – and our hire bikes use even wider tyres than most because they are ridden on cinder cycletracks – the air pocket is huge. You simply don't need to have so much air pressure to protect the rim, and after all one of the reasons for those wide tyres is that you want a reasonable ride. So the wider the tyre the lower the pressure what I would ride on a 20mm tyre is very different to what I'd use in the 37mm tyres on my heavy tourer.

This is where I go a bit wobbly and vague... After 40 years of cycling and 25 years of running cycling holidays where our fleet averages 50,000 miles a year, I've got to the stage where I just don't use a gauge. I look at a customer and generally I have a fair idea from their bike size and I just know how hard their tyres should be, and to be truthful the pressure I use after all this experience is lower than when I started. In fact I've pretty much settled on the ideal that if a tyre never 'bottoms out' or feels unstable then there's enough air in it. One reason for this is that I can state catagorically, from my personal experience, that on wide touring tyres over–inflation greatly increases the rate of tyre sidewall failure. So much so that I have come to the point that when a customer insists (and some do) on inflating their tyre to rock hard, I tell them that if it causes a tyre failure then I will charge for a call–out, the increased risk is so great. In every case the high pressure allows the hook of the rim to cut into the sidewall. I inspect our bikes every week when they return and a quick spin of the wheel often flags up the beginning of such a failure, and almost without exception it is when a customer has used the supplied pump to put as much pressure in as they can.

But for the reader I'm aware that this is rather useless advice, you don't have my eye or thumb, so I was extremely pleased to see that there has been some research on the subject and the results not only exactly match my experience, but also provide a formula for you to get at least a ball–park figure for you to work from.

If you go here http://www.biketinker.com/2010/bike-resources/optimal-tire-pressure-for-bicycles/ you'll see a nice overview and a chart which will give you a rough idea of what is best for your tyres. The basis of this recommendation is research that shows the optimum pressure for the minimum rolling resistance will give a deflection (a flattening of the tyre) of 15%. With that established then various pressures for different tyres and weights are easy to calculate.

For sake of argument let's stick to my wife and I and look at the chart in the article above... Let's say I put 20mm tyres on my bike and the total remains at 97 kg and 40/60 front back weight split as before. The recommended pressures are over 160 psi for the rear and 120 psi for the front! Now those pressures require a very good pump and are well beyond the ratings of some tyres of this diameter... So inevitably the majority of heavier riders are riding with their tyres well below optimum. Even Kate should have 110/75 psi in her 20mm tyres, but at least hers will be achievable with a decent pump.

But 20mm tyres are not for the likes of the serious tourer and in our case we run 35 mm tyres on our own tourers to cope with all that French country roads and cyclepaths can throw at them. With 35mm tyres the sums come out to be dramatically different my tyres should be 65/45 psi and Kate's 45/30 psi. Wow! From a position on 20mm tyres where it's almost impossible to get the tyres hard enough here we see tyres that to most people would see soft to the thumb, especially Kate's which will be very soft. And remember this is for optimum rolling resistance, you could go softer still for rougher cyclepath as the lower pressure rolls better than the tarmac setting.

On our hire bikes, which have to be ridden loaded all the time, and which we expect to take a fair bit of abuse (riding off curbs etc) we've gone with 37mm tyres, so for some (lightweight) customers the optimum pressure will be 25 psi.

So that is why when you come on one of our holidays you may find that your front tyre is a bit soft but once you get used to it, and accept that a bone–jarring and destructive ride isn't necessarily equated to speed, you may even get to like it...

Geoff Husband's day job is running Breton Bikes, a company offering cycling holidays in France see www.bretonbikes.com

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