What psi? - That is the question

Postby **Okibi** » Tue Jan 17, 2006 9:56 pm

rain902 wrote:..ps has anyone ever put air in their tyres @ a service station (those fancy digital things that they use to stop ppl stealing their air thingy) then gone to a different station and checked the pressures? i did it for a laugh the other week - had pressures ranging from 18psi all round, to 44psi all round. scary! never trust one of them again

I have a hand held tyre pressure gauge i keep in the glove box and always use that to check.

So far my local BP has been spot on with the calibration of my gauge.

JBT · Postby **JBT** » Tue Jan 17, 2006 9:58 pm

rain902 wrote:i guess that you can guess that it all comes down to a manufacturer recommendation to start with, then personal preference for the fine tuning!

Mazda recommends 26psi for the NA8 :shock:

Caffeine · Postby **Caffeine** » Tue Jan 17, 2006 9:59 pm

rain902 wrote:ps has anyone ever put air in their tyres @ a service station (those fancy digital things that they use to stop ppl stealing their air thingy) then gone to a different station and checked the pressures? i did it for a laugh the other week - had pressures ranging from 18psi all round, to 44psi all round. scary! never trust one of them again

I hate those things :evil:

Give me an old fashioned manual control one any day...

rain902 · Postby **rain902** » Tue Jan 17, 2006 9:59 pm

Okibi wrote:
I have a hand held tyre pressure gauge i keep in the glove box and always use that to check.

So far my local BP has been spot on with the calibration of my gauge.

sadly bp are few and far between here since the whole "whopping great big 4cents a litre off" thingy - and i am not game to carry the longacre one that i carry to the track around with me, coz i will lose it or something.

rain902 · Postby **rain902** » Tue Jan 17, 2006 10:02 pm

JBT wrote:
rain902 wrote:i guess that you can guess that it all comes down to a manufacturer recommendation to start with, then personal preference for the fine tuning!

Mazda recommends 26psi for the NA8

ummm i should clarify - tyre manufacturer - we all know that car makers reduce pressure for NVH testing and increase it for economy testing :frown:

Mick_98MXV · Postby **Mick_98MXV** » Wed Jan 18, 2006 1:26 pm

Okibi wrote:
Mick_98MXV wrote:...Quick Fit told me to put 40psi in the front and 38psi in the back of my 205x40x17 tyres.

Ok i'm confussed.

Oops maybe it's more in the rear (i noted it in my glove box and check before upping the psi at the servo)... On one of those stupid digital machines that didn't stop filling one tyre and didn't stop emptying the other :shock:

I wish ppl never started theiving the old manual pumps!

Yeh even with the 40psi, my sidewall flexes too much and wraps over the rim a lil too much. Well enough for it to be a cleaning issue to scrape rubber off the edge of the rim. Would this be worse with less psi?

Even with 40 .psi it feels like more than enough grip with 7.5 inch wide the wheels follow the road :frown:

Thanks all, i guess a conclusion can be made that there is no 'right or wrong' in regards to .psi, as some of u said - trial and error

Postby **Okibi** » Wed Jan 18, 2006 1:40 pm

So are they 17" or 15" ?

Mick_98MXV · Postby **Mick_98MXV** » Wed Jan 18, 2006 1:46 pm

Okibi wrote:So are they 17" or 15" ?

Oh soz Okibi didn't realise that's me you were asking...

17's

Postby **Okibi** » Wed Jan 18, 2006 1:50 pm

No worries, so they're a 40 Profile?

What brand and exact type are the tyres?

kudos · Postby **kudos** » Wed Jan 18, 2006 4:15 pm

Personally I go for 34 - yes the ride is harsher than the setup Matty recommended but theres something about feeling the road more.. At the moment I have 38 in there (just as a test) but it's a bit too much..

CT · Postby CT » Thu Jan 19, 2006 8:16 am

JBT wrote:
rain902 wrote:i guess that you can guess that it all comes down to a manufacturer recommendation to start with, then personal preference for the fine tuning!

Mazda recommends 26psi for the NA8

Minimum tyre pressure inflation if it's on the tyre placard - it's only a starting point. Tyre companies put a lot of research into giving you a tyre that sits square on the road under most conditions. Road tyres should be in the range of:

15s 28-32
16s 32-36
17s 34-38
18s 36-40 this is all cold and from a contact that tests tyres for Yokohama. Pressures at the lower end will increase comfort and decrease tyre performance due to the delayed response of the tyre to your inputs. Highers pressure are better in the wet as it keeps the tread open.

For tracking street tyres 4-6psi cold higher is normal to stop the tyre rolling off the rim. But yes, it kills the tread due to the heat. You increase the pressure to decrease the heat effect by reducing the amount of flex the tyre suffers. There's a point where increasing it is detrimental to tyre performance which is why we find that 40 hot in a 205/45/16 is about ideal.

R-Tyres have stiffer sidewalls and softer tread compounds and can run at lower pressures because the sidewall is doing the work, not the air. Slicks again run at much lower pressures due to the construction and the compounds they use.

smiles · Postby **smiles** » Thu Jan 19, 2006 8:38 am

CT that's some great info, any chance we can have it as a sticky?

Is the 4 psi range sufficient to take into account the weight of the car. I believe a heavier car would need to have higher pressures than a lighter one.

Ted · Postby **Ted** » Thu Jan 19, 2006 9:17 am

Personally, it comes down to weight of vehicle and tyre construction. For example, a Porsche 911 will need a bit of playing with pressures to deal with its weight distribution.

Generally, the lower profile tyres tend to have a re-inforced sidewall. But again that has to be taken with a pinch of salt as there are some really cheap 18" 35series tyres on the market these days and I would question their case construction. So it comes down to feel. Start at a point and go from there. Try to feel the difference between sidewall flex and treadblock squirm, it is very difficult to do sometimes. I would get your own gauge, then try everything from 26psi to 40psi and see what you feel is the best.

When I changed tyres recently and went to one with a more rigid sidewall, tyre pressures dropped 3psi.

Mick_98MXV · Postby **Mick_98MXV** » Sat Jan 21, 2006 9:16 am

Okibi wrote:No worries, so they're a 40 Profile?

What brand and exact type are the tyres?

Thank y'all... I'll have a play with different psi.

I will get better tyres when these have had it aswell, as these are simply the ones thrown in with the wheels from Quick Fit.

Full description of these tyres are:

Nexen N3000, 205x40ZR17 84W, Extra Load

Don't know if that helps...

AJ · Postby AJ » Sat Jan 21, 2006 9:48 am

Nexens are an up & comer in the tyre world mick, seen by tyre dealers as a way to get cheaper wheel/ tyre combos out there, from what i've heard, they have a higher road noise level than say, your michys, yokis or pirellis due to a slightly harder compound......having said that, despite what others may say, there is no such thing as a BAD tyre in the W rated low profile sports tyre world, look after them by rotating & keeping a regular check on pressures, keeping them above 30 psi & below 40 psi & you'll get a good run out of them as a daily driver/spirited run combo IMHO

BTW, i run 36 psi/ 34 psi front rear on my precedas on recommendation from one of my tyre dealers & haven't found a reason to change that yet, the tyres are wearing evenly so far (3500 k's).....just starting to knock the corners off the front 2 which is probly more due to my pisspoor driving while getting used to the car than anything else :lol: