Tyre pressures

I've often wondered, and assume someone here will know, but why do tyre pressures depend on the vehicle rather than the tyre?

I'd assume vehicle weight plays a significant chunk along with which tyre is the one chucking down the power.

I might be wrong though

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More weight = more rolling resistance so more pressure required to overcome this.

That definitely makes sense, but I guess it's just surprising that all tyres are made to sufficiently similar specifications that it's solely down to the vehicle to determine the pressure.

I think manufacturers pluck the pressures out of thin air!

For example

gsxr600 front 36 psi rear 36 psi

R6 front 36 psi rear 42 psi

both pretty much the same weight and power!

Yamaha also had the gen 1 FJR front at 36 psi then it got increased to 40 psi for a bit then back down to 36

The forum expert in all things will be along to tell us that we're all wrong. the tyre makers are wrong. the bike makers are wrong. everyone is wrong BUT... he knows the answer cause he's soooo clever (like)

I best get some pop corn.

I think manufacturers pluck the pressures out of thin air!

For example

gsxr600 front 36 psi rear 36 psi

R6 front 36 psi rear 42 psi

both pretty much the same weight and power!

Yamaha also had the gen 1 FJR front at 36 psi then it got increased to 40 psi for a bit then back down to 36

Zx6r 36/42. ZXR750 32/36... The six is waaay lighter and has a bit more power so that really doesn't make sense.

I would have thought the figures in the book are for the OE spec tyres. If you get your hands on some loose tyres you'll see and feel the construction varies tremendously - compare the PR4 to a supercorsa for example, the PR4 has walls like a tank whereas the supercorsa are really thin and flexible.

I think a lot of it is probably just personal preference

The pilot power 2 ct's I used to run on the TL

At stock pressure the rear would move about so I fired an email to Michelin they emailed me back with a name and number of some specialist on tyres!! top notch service so far

His advice was increase the pressure by 2 psi until it stops moving and don't go above 42 psi or below 36 keep between these and do what feels best for you

I think a lot of it is probably just personal preference

The pilot power 2 ct's I used to run on the TL

At stock pressure the rear would move about so I fired an email to Michelin they emailed me back with a name and number of some specialist on tyres!! top notch service so far

His advice was increase the pressure by 2 psi until it stops moving and don't go above 42 psi or below 36 keep between these and do what feels best for you

That sounds like it could cover most bikes in a way. I imagine most tyre pressures on the rear are in that range.

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I think a lot of it is probably just personal preference

The pilot power 2 ct's I used to run on the TL

At stock pressure the rear would move about so I fired an email to Michelin they emailed me back with a name and number of some specialist on tyres!! top notch service so far

His advice was increase the pressure by 2 psi until it stops moving and don't go above 42 psi or below 36 keep between these and do what feels best for you

That sounds like it could cover most bikes in a way. I imagine most tyre pressures on the rear are in that range.

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Apart from what Mr Fro has said about the zxr750 at 32 psi I haven't seen any bikes under 36 and certainly not above 42

The forum expert in all things will be along to tell us that we're all wrong. the tyre makers are wrong. the bike makers are wrong. everyone is wrong BUT... he knows the answer cause he's soooo clever (like)

I best get some pop corn.

You don't have time, I'm here.

72psi in the front and 18 in the rear. Anything else is asking for trouble.

I start with the manufacturers recommendation and go from there until I find a personal preference. I usually run 10% or so softer because I like a smooth ride, don't wanna feel every ridge and crack (somebody's going to find an innuendo in there).

Tyres are part of the suspension to so that plays a part.

My suspension was set up with 30psi in the front.

Tried 33psi thats recomended and it changed the bike.

More squirmy on the brakes and wont hold a line.

So what ever they test with and find has the biggest working range is whats reccomended.

My Suzuki V-strom 650DL with ABS. Bike OE specs say 33psi front and 36psi rear with rider only or 42psi with pillion.

Always seemed odd that the front is always 33psi. Surely a pillion weight would affect front and rear??

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You should start a new thread instead of high jacking another

alternatively adjust your headlight aim

This is just an indication of how tyre pressures are set. I have a 1995 Volvo 940 estate. The manufacturers tyres pressures are quite low, which is attributed to the Swedish desire for more grip, especially in crap road conditions. In the USA, all of the tyres had their pressure increased because that reduced rolling resistance which increased tyre life and (slightly) better mpg. The UK went with the Swedish pressures, but many owners like me, increase the to the USA pressures to get the benefits they give. I also get no more comments of "you need to pump up your tyres".

Many happy childhood hours were spend in the rear facing seat in the boot of a Volvo 760, the model with the turbo intercooler badges which we thought made it so cool, with me and my cousin pulling faces at the driver behind and drawing rude things on pieces of paper to hold up in the back windscreen. Kids now with DVD players and tablets don't even look out the window.

Mine has the rear facing seats, which I would happily sell to anyone who wants to come and remove them. I have used them once, with teenagers as a joke and they are just dead weight in the boot.

teenagers are just dead weight in the boot.

You have dead teenagers in your boot?

This is what the experts say about tyre pressure.

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 tyre 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.

Hope that wasn't too boring

I must remember the 36/42 and check my tyres. Thanks for that post.

I stick with the 36/42, I've seen a few people say run as low as 34/34 (on bikes that have 36/42 recommended) or figures in that ballpark to improve handling. I assume this sort of stuff wants to only be applied to the track?

Manufacturers recommended pressures......

An SV650 is 33 front and 36 rear

An XT600E is 22psi front and rear for a rider weight up to 90kg, rising to 32 at the rear for riders above 90kg or high speed riding, and dropping to 18psi front and rear for off road riding.

An XT660R is 29psi front and 30psi, rising to 31psi at the back for 90kg+ / high speed

Both my Triumphs are 36 front 42 rear, but clearly not all bikes are, check your owners manual.

I run 33 front and 38 rear for solo riding on both Triumphs.

If you have an older bike the pressures may have been different on older tyres, but with newer tyres 36/42, check the tyre manufacturers website just to be sure.

Buy a new SV now I believe it still says 33/36 in the manual. The XT660R I refer to was a 12 plate. Look on Dunlop website though the SV is 36 / 42. It seems to be a tyre manufacturer thing which the motorcycle manufacturers are not universally adopting.