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Old 17-Dec-2009, 12:58 PM
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Question Tyre Pressures

having read the recent thread on the Michelin power !s, and the pressures being quoted, this got me thinking.

I pretty much always run 38/42 on all my bikes, regardless of tyre brand or model, and also regardless of solo, with luggage, or fully loaded and 2 up. I can tell the difference in handling when either tyre is down by more than a couple of degrees (slower steering, tracking in bends, rear rolling under power).
It therefore seemed somewhat odd to hear some of you guys saying to run pressures as low as 22.5 (cold).

I'm not questioning your decisions, or indeed the advice given by experts to do this, just wondering if I'm missing a trick, or am I just too stupid to notice (don't answer that last bit )

So. In simple terms, what do you notice by running such lower pressures ?
e.g.
better more stable braking ?
less tyre wear ?
less rubber migration across the tread pattern ?
tyre lasts longer (doesn't get too hot and go off) in a session ?

I doubt I could ride any faster / better at different pressures, and I'm not asking for that reason. I'm asking as much as anything to learn, because I don't see the benefit. I always seem to wear tyres out quickly too, and the fitters reckon they look to be under inflated, not over inflated.

Eagerly awaiting your collective wisdom

Cheers,
John.
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Old 22-Dec-2009, 09:38 AM
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So, given that I got no replies, should I assume everyone else already knows the answer to this and I'm just too stupid to be told

Or is it that your all too busy playing in the snow and writing letters to santa
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Old 22-Dec-2009, 12:18 PM
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Modern race tyres i think John - Dunlop NTechs are designed to have very low PSI when cold.

Yours above seem high to me - I always run 32/32 cold on my 916, drop to 30/30 on track.

On the DD bikes - running Diablos - always ran 29/29.

Check the manual and the tyre recommendation maybe?

What psi do your fitters recommend?
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Old 04-Jan-2010, 09:12 PM
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John, where did the figs of 38/42 come from? I too would have thought that was very high, unless 2-up and fully laden. I tend to think of my defaults as 32/34, though as you say sportier/trackday riders often go for less, the calculation simply being that with the pressure lower (eg dropping a Diablo from 32/34 to 29 both) you'll get heat into them quicker, and that grip rather than durability is the issue. I always think that the tyre manufacturers' psi recommendations need to be consulted: many people stick with what's quoted in the bike manual, but it seems to me that that's only good for the tyres it actually came with, and that what Ducati thought was good for the Dunlop-Whatevers my SS came with in 1997 might not be right for Pirelli Diablo Rossis over a decade later. It's easy enough to check tyre manufacturers' recommendations these days too - some have that facility online (I know Pirelli do).
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Old 04-Jan-2010, 09:59 PM
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Thanks for the replies.

The following is taken from the pirelli web site:
"Tyre Pressure
It is important to always inflate the tyre to the correct pressure. Be sure to check cold inflation pressure frequently (i.e. once a week). Although most motorcyclists love to work on their bikes, they seldom remember to check tyre pressures. Correct tyre pressure, however, is critical for safe handling. Over-inflation or extreme tyre pressure will impair your riding comfort and decrease the contact of the tyre with the road. Under-inflation or too little air pressure will result in poor handling and the bike will be inclined to "wander." Improper and insufficient tyre pressure will also cause rapid tyre wear, an increase in fuel consumption, lower top speed, and provide less control. Remember to check the inflation pressure of your tyres weekly.

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.
Attention: When the recommended pressure has been changed for use off-road (race track, off-road), it must be reset to the correct value before riding on the street."

The tyre fitment chart for the ST4S recommends 2.2 bar front, 2.5 bar rear for each type of tyre in the list presented.
I run with 2.5 & 2.9 due to the 2 up with luggage loading, rather thsn the 32/36 above, and that matchs the pressures in the bike manual.

Its interesting to see the same Pirelli tyres fitted to an 08 R1 (my other bike) are quoted as 2.5 & 2.9 for normal use. In fact Ducatis are the only bike I can see that gets quoted with a lower pressure...

Thanks again for the comments, I'll drop the pressures for at least the start of our next trip and see how we get on.

Cheers,
John.
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Old 05-Jan-2010, 12:27 AM
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Certainly the case that specified pressures seem to be notably higher for Jap bikes, never really understood that either.

[PS meant to say 'Diablo Rossos' rather than 'Diablo Rossis' - wishful thinking!...]
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Old 05-Jan-2010, 08:29 AM
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from what I recall all the majority of tyre manufacturers recomend high pressure for road use.
These aren't necessarily the optimum pressure for each type of bike, road condition or riding style. But they know there will be know tyre issues under normal use even when encountering pot holes etc. and therefore you can't hold them liable for say a damaged wheel rim when encountering said pot hole or the resulting accident.

As Chris says on the DD bikes and other race bikes we drop the pressures to below 30psi whem measured cold as these are specific conditions, with the softer tyre caused by the lower pressure it flexis more, the radial belts move over each other more, causing friction and hence heat which in turn raises the pressure.

Many track tyres use compounds that rely on running hot to allow them to grip as designed and lowering the pressures allow them to get to these temperatures quicker, either that or use tyre warmers. Now you may put the warmer onto a cold tyre at 28psi but when its properly warmed you will probably find the pressure has rissen to around 34psi.
On the roads its hard to keep the heat in these sort of tyres and once they have cooled and the pressure dropped you wouldn't really want to encounter a pot hole especially if your using Mag wheels as i do on my 998 hence me usually running road tyre pressure when cold of 32 psi


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Old 05-Jan-2010, 12:58 PM
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DSC Member Monty Monty is offline
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John, I always ran my ST4S at 32F/36R unless on track when I dropped to 30 F/R. I always found the ST4S would feel a bit 'vague' on the steering with higher pressures.
When we were racing the TZ250 that ran at 36psi F/R-HOT-straight out of the warmers. Those pressures are on slicks and with a bike weighing 103kg don't forget.

John


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Old 05-Jan-2010, 01:55 PM
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Quote:
Originally Posted by Monty
When we were racing the TZ250 that ran at 36psi F/R-HOT-straight out of the warmers.
John

I recall a Pembrey meet where they were a fair bit higher than that John

Poor WJB found out the hard way


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Old 06-Jan-2010, 10:31 PM
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Thanks guys.

Hopefully the ST won't be doing any more trips if the new MS1200 turns up in time. If I do still use it I'll try your pressures John.

I'll run the MS at the pressures specified in its manual and see how I get on.

Thanks again.
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