This has been posted before, I have copied and pasted. It was originally copied from the MCN. If you go through the section of this forum where you posted the question you will find lots of interesting reading.
It's important to be pumped
Tyre pressures are a crucial factor in determining how your bike handles and how quickly you wear out your (not exactly cheap) tyres.
There are lots of myths and misconceptions about what pressures you should run in the wet, on track days or when you're loaded with luggage. Usually you'll find someone propping up the bar who knows better than the manufacturers' recommendations. To find out how close they are to being right we talked to a genuine expert - a man who should know tyres if anyone does.
Leo Smith spent years as chief development tester at Avon tyres. He is now motorcycle product manager. He said: "We probably get asked more about tyre pressures than about any other aspect of a tyre”. There's so much bad information kicking about that people can't separate the truth from fiction."
Smith says that is largely the fault of tyre companies themselves. Several years ago, different tyre companies recommended different pressures for different tyres and different bikes. But around 10 years ago, a decision was reached between the companies to standardise pressures so that most bikes can run on the same no matter what tyres they're on. That standard is 36psi at the front and 42psi at the rear.
There are some exceptions, like some 400cc grey imports which run 29psi at the front and 36psi at the rear. Another notable exception is the Kawasaki ZX-12R - which is meant to run 42 front and rear. But if you've got a modern, mainstream bike, chances are you should be running the 36/42 standard.
That 42 figure in particular will have a lot of the gentlemen at the bar shaking their heads. But it is not a figure chosen at random. Pressures determine how your tyres deflect. The lower the pressure, the more the tyre will flex. That may make for a comfortable ride when you're cruising in a straight line, but the tyre will flex too fast at speed and make your bike unstable. The bike will feel vague going into turns and feel like it's going to tip into the corner suddenly. This is because the tyre isn't "strong" enough and it's literally buckling under you.
The bike will also feel wallowy through turns and it'll weave under acceleration. Conversely, if you over-inflate a tyre, the flex will be slower but that will make your bike more stable at high speeds. The ride comfort and the tyre's ability to absorb shocks will be lost and your wrists and backside will take the brunt of it. The bike will feel so harsh that many people will think they have a suspension problem.
Cornering won't feel as bad as when pressure is too low, but you will again lose feel and feedback from the tyres. For example, if you ride over a stone, an over-inflated tyre cannot absorb it and the tyre breaks contact with the road. Smith says the classic myth about tyre pressures is that you deflate them for wet-weather riding. He says most grip comes from the tyre's compound and the contact patch - and the shape of the tyre where it contacts the road is everything.
Tread patterns stop water from building up under the tyres - which could cause a bike to aquaplane. Smith says: "A good front tyre chucks enough water out of the way to enable the rear to get the power down. If you reduce the tyre pressure, the tread becomes compressed so it can't clear as much water." If anything, Smith recommends you increase the rear tyre by 2-3psi in the wet but leave the front as it is.
Another widely held misconception is that the psi recommendations are the maximum the tyre can take. They're not. The figure only tells at what pressures the tyres were tested at for all-round use. You could actually safely inflate a type up to around 50psi if you really wanted to, although it wouldn't do you much good.
But the biggest area for debate has to be track days. If you've ever been to one it's almost certain someone has told you you'll be best off reducing your tyre pressures. You get more grip that way, they tell you. Smith has radically different advice.
You should leave them alone, he says. "Racing tyres are of a totally different construction and stiffness to road tyres so they need less pressure to maintain the carcass shape. That's where the rumours and bad advice comes from. "If you drop the psi in road tyres you will get more movement in the tread pattern. They will heat up too much and that will eat into tyre wear. You'll almost certainly ruin a set in a day without gaining any advantage in grip."
Smith says he's known people to drop their rear tyre to just 22psi when heading for the track. His advice is to leave your tyres alone, saying a good tyre at standard pressures will give more grip than you need on a track day because you almost certainly won't be
going as fast or for as long as racers. Track surfaces offer much better grip than the road, too - another reason for leaving your tyre pressures the same for the ride to the track as for the ride around it.
Many people also ask the experts at Avon if they should increase psi to take pillion passengers. Again there's no need. The manufacturers' agreed pressures of 36/42 were arrived at after testing with pillions, luggage, cold tyres and every other combination you could think of.
One of the few cases when Smith does recommend you change your pressures is when your tyres wear. A worn tyre has lost a lot of its strength as the shape and flexibility levels have changed. That means it will handle differently to a new tyre. Try increasing the tyres by 2psi when you're down to around 40 per cent tread depth. It will only make a marginal difference, but it should improve your bike's handling a bit.
You may not have to keep changing your tyre pressures, but you do have to maintain them. Smith recommends that you check them once a week as an absolute minimum but to be extra safe, you should really check them every day because a tyre can change by as much as 3psi on its own just because of changes in the weather.
You should always measure your tyre pressures when they are cold. A few bikes are now coming with tyre pressure gauges in their under-saddle tool kits. If you haven't got one it's worth buying one. They only cost a few quid and take up about as much room as a pen. Forecourt gauges are notoriously inaccurate.
Scanned and re-printed in total from an article in MCN.
