Tyre pressures, The max psi - 10% rule

[chimpsinties]

I've been thinking about my tyre pressures in the cage recently. I've got wider than normal tyres on it. 195 instead of 165.

Tyre pressure is such a black art. Some say, always stick with manufactures guidelines regardless of size of tyre. Some say, increase a bit, some say decrease a bit. This guy suggests taking the max allowed pressure for the particular tyre and minus 10% gets you your ideal pressure.

I'm trying it at the moment.

So with mine it says max pressure 44 psi so I've got 39psi all round. The tyres are 195/50R15's where they used to be 165/65/14 or something like that. I think the book recommends 32psi all round.

The Tyre Bible is a really good resource for fact and figures about tyres http://www.carbibles.com/tyre_bible_pg3.html

Originally Posted by The Wheel & Tyre Bible
Every tyre has a maximum inflation pressure stamped on the side somewhere. This is the maximum pressure the tyre can safely achieve under load. It is not the pressure you should inflate them to.
Having said this, I've given up using the door pillar sticker as my starting point and instead use the max.pressure-10% theory. According to the wags on many internet forums you can get the best performance by inflating them to 10% less than their recommended maximum pressure (the tyres, not the wags - they already haves inflated egos). It's a vague rule of thumb, and given that every car is different in weight and handling, it's a bit of a sledgehammer approach. But from my experience it does seem to provide a better starting point for adjusting tyre pressures. So to go back to my Subaru Impreza example, the maximum pressure on my Yokohama tyres was 44psi. 10% of that is 4.4, so 44-4.4=39.6psi which is about where I ended up. On my Element, the maximum pressure is 40psi so the 10% rule started me out at 36psi. I added one more to see what happened and it got better. Going up to 38psi and it definitely went off the boil, so for my vehicle and my driving style, 37psi on the Element was the sweet spot.

Read more: http://www.carbibles.com/tyre_bible_...#ixzz0pyblCLkm

The handling feels ok. I find it hard to tell one pressure from the next. I'd say the ride is a little bit harder but it's just a crappy 1.2 Clio so it's difficult to tell if it's actually any better cos it's gash to begin with.

My worry is, if it rains will it just be like I'm on ice with the slightly reduced contact patch? Is that worth the possible 5% gain in fuel efficiency?

 

[TheEvilGiraffe]

Over inflating from guidelines isn't great idea as tyre will be too rounded, you'll get less of the tread on the tarmac, and wear out the middle of the tyre out to fast !

If you're looking to save 5% fuel, take some junk out the car, or drive a bit slower....?

 

[mat-nos]

you got 39psi in 195/50s??
that cant handle very well or be comfy???

30psi is about right...

 

[sideways danny]

only time you need to change tyre pressure (on the road, track is different) is if you change the weight of the car

 

[Fred@BTM]

getting 335/30 18's and 295/30 19's to "pop on" the lambos usually requires about 80psi

 

[sideways danny]

aerosol brake cleaner and a flame

 

[Hutchie]

80.. thats weak. the 17" laugana tyre usually need somewhere in the region of 100-110 LOL

fair crack off them cnuts

 

[sideways danny]

Tyres that were fitted with fire

 

[Fred@BTM]

oh we've had to use ratchet straps, cans of WD40, bin liners and a few hammers before!

no chance i'm using flamable fluid on £300 a pop tyres with £5K+ wheels (each)!

 

[sideways danny]

lol, admittedly a bit different to banded corsa steels with PCD altered to nissan fitment

 

[Hutchie]

[youtube]GWmq9fIDR4s[/youtube]

these look awkward.

 

[JamesS]

The Tyre Bible is a really good resource for fact and figures about tyres http://www.carbibles.com/tyre_bible_pg3.html

Not really. There's some useful information on there, but also a lot of rubbish.

As for max pressure - 10%, it appears the author seems to have forgotten something.

They quote "Every tyre has a maximum inflation pressure stamped on the side somewhere. This is the maximum pressure the tyre can safely achieve under load."

They also say later in the article "When a tyre heats up under normal use, any water vapour inside it also heats up which causes an increase in tyre pressure."

So they state that the max recommended pressure on their car is 40psi, subtract 10% which gives them 36psi and then tweak it and and up with 37psi.

Have they forgotten that when they drive the car the temperature of the air inside the chamber will increase, causing the tyre pressure to increase (assume a gentle drive, so a 20-30°C temperature rise) to easily to over 40psi, which is the maximum recommended pressure for the tyre under load? If they were to try driving a bit more enthusiastically, then the pressure would increase even higher.

Maybe they should have paid a bit more attention to a later quote "Remember that thousands of hours go into the development and testing of a car."?

Do they not think that if it was as simple as subtracting 10% from the max pressure, the manufacturers would have saved themselves a lot of R&D money?

The only times OEMs run high tyre pressures is during very high speed testing, during which there is little increase in temps or pressure.

getting 335/30 18's and 295/30 19's to "pop on" the lambos usually requires about 80psi

Wait until you have to try a 365!

 

[MikeB]

Lol i thought pumping up to 80psi was bad!

 

[Fred@BTM]

^na you quite often have to go upto 100psi to get tyres to pop on, its rare, but does happen

 

[nathanH]

Ive recently increased my front tyre pressures to 34 cold, makes a massive differnce to handling, more precise turn in and just as much grip if not more than 32. 32 feels 'wobbly'