Tyres, Pressures and Backing it in..

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Pierre
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#16 Post by Pierre » Wed Aug 08, 2007 10:23 pm

No thats not what i mean , but my eegleesh sooks. I was quoting pressures from a racetec which takes a warmer or a couple of laps to get some grip into it, but road tyres grip better from the start so I am irrelevant :smt017

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#17 Post by Kwackerz » Wed Aug 08, 2007 10:31 pm

I know what you mean.

But the reason for lowering road tyre pressures is so they can cope when the tyre has warmed up and starts to get hot. it loses grip.

Lowering the pressure in the tyre at the start of the day then lets the road tyre cope better, reduces it losing it's grip as it gets really hot?

The tyre warmers help reduce cold shear in the tyre by having the tyre up to a warm temperature before it goes on track. You dont really need it on a road tyre as like you say they warm up quicker, but who really sticks to warming tyres up properly? :smt005 One lap and let loose the dogs of war!

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#18 Post by Pierre » Wed Aug 08, 2007 10:54 pm

You dont have to tell me, i fell off on the 2nd lap :smt003

Sorted that now though, Ive bought some 2nd hand slicks off ebay for £40. What can possibly go wrong.

Also forgot that suspension setup can affect tyres heat and pressure aswell?

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#19 Post by Kwackerz » Wed Aug 08, 2007 11:04 pm

It's probably more important than the pressures!! Pointless having the tyres up to the right setting if the back end is pattering everywhere and the front is diving like a submarine! :smt005 :smt005

But yes, undoubtedly suspension will affect it too.
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#20 Post by Kwackerz » Wed Aug 08, 2007 11:09 pm

Pierre wrote:
Sorted that now though, Ive bought some 2nd hand slicks off ebay for £40. What can possibly go wrong.

Youve ridden on slicks before? :smt017

Problem with old track tyres is you dont know what theyve done and what the heat cycles on the tyre have been.

What could be down as a soft tyre couldve been heat cycled badly and actually have cooked potentially into quite a hard compound by the time it gets to your bike. :smt017
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#21 Post by D-Rider » Thu Aug 09, 2007 12:14 am

.... now just thinking out loud (well, typing my thoughts anyway).

One of the problems discussed is that overheating can damage tyres.

So ..... surely reducing pressures directly causes tyres to run hotter. The tyres flex more, get hotter - the air heats up, pressure increases and eventually they reach normal pressures - probably as much by having the cold pressure lowered as by being worked hard.
At "working pressure" they will be significantly hotter than normal to maintain that pressure. I guess this will partly self-stabilise. If pressure drops, the tyre will flex more and limit the drop. If the pressure increases, it will flex less and limit the pressure increase.

Now we believe overheating can damage tyres and lowering pressures makes them run hotter. So why do we drop the cold pressure?

Maybe we need to think about what about higher pressures may do - i.e. what's the problem there? My guess is that the pressure has to rise hugely to cause structural damage - this can probably be ignored.
Too high a pressure and maybe they won't deform enough for an optimum footprint? Not convinced - contact patches are not small these days.
Too high a pressure and they won't be hot enough to work?
I don't buy that. You are working them a bit harder than on the road - they will be warmer than on the road. However, on the road they grip well enough. Pegs down on the bends is a realistic proposition without falling off on every other curve.

Now I can believe that track focussed tyres may be designed to run at lower cold pressures - but I'm running Pirelli Stradas. They're a good tyre with good grip but primarily designed for road use - giving good grip at standard road temperatures but designed for road pressures.

So .... I've not come to any real conclusion as I still don't fully understand exactly what we are trying to optimise here (other than grip) and what the theory is by which this may be achieved.

My feeling is that with road tyres you may benefit a bit by dropping pressures a bit but not too much.


If I'd not have received the advice to have the pressures lowered I'd have left them alone. The fact that the tyre truck was Pirelli and the chaps doing the tyre setting wore Pirelli shirts gave me the impression that they would know what was best for my Pirelli tyres. That may have been an optimistic assumption and, perhaps the ability to wear a Pirelli shirt is no guarantee of an intimate understanding of the tyres. But there again they may have been spot on!
I dunno!!!

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#22 Post by Viking » Thu Aug 09, 2007 4:49 am

The rule of thumb I was given was that you drop your road pressures by 2 - 4 psi for track riding. Enough to get the tyres a bit warmer quicker, but not enough to cause handling problems.

However, there have been track days where I didn't play with pressures at all and still had huge amounts of grip without any problems...

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#23 Post by Aladinsaneuk » Thu Aug 09, 2007 5:28 am

certainly an interesting debate

I know thast before i realised I was running my tyres to low - as listed in the manual, it took them about 10 miles to get nice and responsive, hence my thought on lowering just a bit

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#24 Post by Paulh » Thu Aug 09, 2007 7:25 am

It depends how fast you are riding, but I always start at 32 front 30 rear. Road pressures are no good on track unless you are riding at road pace.

If its your 1st trackday and your being cautious then you may not need to drop the pressures as much, so I might start in the middle somewhere in that case. Unless you are using warmers it will feel a bit odd for the 1st couple of laps, but after that they should feel OK.

I've never trusted bike manual pressures - when I had the Mille Aprillia quoted 36/42 road pressures for my Dunlops - Dunlop quoted 32/36 road pressures for the Mille (same tyre)- I've found that most bike manufacturers just quote 36/42 for everything, and prefer to take the tyre manufacturers recommendations as they design and test the things.

As for repeated heat cycles - it can be a problem with race tyres, but road tyres are pretty much designed for it, so less of an issue.

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#25 Post by Paulh » Thu Aug 09, 2007 7:33 am

Pierre wrote:You dont have to tell me, i fell off on the 2nd lap :smt003

Sorted that now though, Ive bought some 2nd hand slicks off ebay for £40. What can possibly go wrong.

Also forgot that suspension setup can affect tyres heat and pressure aswell?


Just a thought for people, but pennyless racers don't tend to sell off tyres they think are any good :smt002

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#26 Post by Kwackerz » Thu Aug 09, 2007 7:58 am

:smt005 :smt005 And not for 40 quid! :smt002 They sound like theyre ready to be cut and used on a supermoto to be honest
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#27 Post by D-Rider » Thu Aug 09, 2007 9:09 am

Paulh wrote: Road pressures are no good on track unless you are riding at road pace.
Hi Paul thanks for the input. I think a clarification of your statement above may help me to understand this better.

What exactly is meant by "Road pressures are no good on track" i.e. exactly what is it that tyres fail to do when run at road pressures?

For example:
Do they fail to grip? - if so, why?
Do they suffer damage or excessive wear? - if so, how?

I really do want to get my head round all this to understand what's going on.
I'm sure there must be logic in here - Hopefully this is more than just the practice of biker voodoo decreeing that "letting some air out will give you safe passage through the track session."

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#28 Post by Kwackerz » Thu Aug 09, 2007 9:25 am

This makes for interesting reading from ADVrider.
TYRE pressures are a crucial factor in determining how your bike handles and how quickly you wear 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 400cc grey imports which run 29psi at the front and 36psi at the rear. Another notable exception is the KawasakiZX-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 caused a bike to aquaplane. Smith says: " A good front 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.


However if this 'compromise' tyre pressure was decided at, why are the Vehicle manufacturers not using those pressures?
I would suggest that the expert does put some interesting theory across, but like anything, it's still open to valid debate.

If I carry a pillion on the TT it rides like a bag of shite. But apparently there's no need to inflate the tyres ? I never do, but im aware it'd ride better if I did.

Michelin arent offering those pressures either...

Neither are Pirelli.....

:smt017 Did He just dream it all?
Last edited by Kwackerz on Thu Aug 09, 2007 11:13 am, edited 1 time in total.
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#29 Post by Paulh » Thu Aug 09, 2007 10:01 am

Its all about average speed and heating the tyres up - on a track you are generally cornering faster, going faster on the straights and braking harder than on the road, so the tyre heats up more. Yes you can do ton plus speeds on the road and corner at high speed, but not continously for 20 minutes at a time (particularly the corner speed) like you can on track.

Road tyres are designed to run colder than race tyres as they are used differently (all weathers and all speeds) - so on track they are quickly outside their optimum temperature range. If you are running cold road pressures they will be at significantly higher pressures after a few laps of the track than a 10 mile trip down the road - and they are designed to run their best under road conditions. Dropping the pressure before you start will mean that when they heat up they are more likely to be nearer their optimum pressure when they are hot on track, assuming that you are running a faster pace than usual.

Running road pressures effectivly means your tyre is over inflated at track speeds after a copule of laps once it has heated up - and the result is less grip (see what pirelli say about over inflated tyres).

http://www.pirelli.co.uk/web/motorcycle ... ssure.page


Best way to see the difference is to do a hot tyre pressure check after a blast on the road - it will rarely be more than 1-2 psi above the cold pressures, and a check after a track session - the rear in particular will be well over that (which is why its dropped in pressure more than the front)

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#30 Post by D-Rider » Thu Aug 09, 2007 10:22 am

Yes, read that and it seems that too high a pressure can cause problems - but probably less severe problems than too low a pressure.

However the advice in that link also says:
Pirelli Link wrote:You will find the correct pressure in the operating manual of the motorcycle. The manufacturer's information is the minimum values only. With luggage or with a second rider the rear tyre needs an extra 0.2 bar, and for high speed riding the pressure of the front tyre should also be increased by 0.2 bar.
* Manufacturers recommended values are minimums
* Faster riding requires an extra 0.2 bar

Now I can relate to the advice from the Avon chap that Kwackerz posted up .... but with so much counter advice I couldn't say he's correct - or otherwise.

Sorry but still confused.

Are we saying that the running pressure is the overriding factor as this governs the shape of the contact patch?
:smt017

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