Tyre Pressures, And The Argument I Had With A Mate

[Socrates]

195/70's i'd go with something normal 32-34 maybe

its a pretty high profile tyre

That's C210 Skyline for you

[bluejeans]

i run fronts @ 36-38 and rear @ 32-34

i check them everytime i fill up

[Marcus89]

When a tyre says its rated too 45psi i would assume they take into account that the tyre will heat up increasing pressure along the way. And with items where safety is a big issue i would also guess the tyres to be over-engineered to an decent extent (unless you have ultimate cheap china tyres then well good luck ).

On another note anyone ever run factory recommended pressures? Haven't looked on the 34 but in the old s13 it said something stupid like 24psi i never ran less than 38psi on that

[Socrates]

My manual says 28 in rears 30 in front. but currently on 32 all round

[Stinky Rooster]

is that isothermal, adiabatic or polytropic expansion? haha

just studying for my thermal eng. exam as we speak

obviously it would be polytropic, as isothermal refers to a system change in which the temperature remains as a constant, and adiabatic refers to a change in temperature CAUSED by a change in pressure, not the other way around. It could be described as a diabatic process, but personally I would refer to that as an isocaloric process, due to the heat being caused by an external energy infraction (in this case, friction)

[Stagea_Neo]

When I used to work at BP and did tyre replacements/balance in the workshop usually you'd see pressures of up to 80psi to get the tyre to pop back on to the rim lip. Let it back down to the rated pressure then put it back on the car.

So there is plenty of leeway in the amount of pressure the tyre can handle.

But i wouldn't go putting that much in when it's on the car , factoring in the 300-450kg load the tyre has to deal with.

[pipster11]

obviously it would be polytropic, as isothermal refers to a system change in which the temperature remains as a constant, and adiabatic refers to a change in temperature CAUSED by a change in pressure, not the other way around. It could be described as a diabatic process, but personally I would refer to that as an isocaloric process, due to the heat being caused by an external energy infraction (in this case, friction)

can you please come and replace my lecturer, we call him the story teller because all he does it read the book to the class

coupled to the fact that he is indian (not being racist) its hard to understand him and so basically we've had to teach ourselves

[Oddy man]

Egad this is technical

I am now intruiged with different pressures for front/rear.

So a harder front pressure gives a nicer handling, with a little lower at the back for grip?

Say 36/38?

Edited June 8, 2010 by Oddy man

[_Ryan_]

I would be a little careful going over 40 psi on the open road due to higher running temps. Considering Chales Law of Thermodynamics (pressure is proportional to temperature in a given volume) if your tyre temps are likely to go up then you could easily end up with a tyre pressure approaching 50psi. Apart from the obvious lowering of grip, due to less sidewall deflection and reduced contact patch, the effects of a blowout will be much more pronounced. All that risk for a possible saving of 0.1% fuel mileage doesnt really add up IMO.

Try alternating the pressures fron to rear too - a difference of only 2psi can be felt quite easlily. If you want a little more direct feeling in the steering wheel while maintaining a rear grip bias, try going 2 psi higher on the front.

Hence my pressures:

38summer

40winter.

I always ensure to do a hot pres. check on long hot drives if i know temps are high to start off with. Anything under 38 psi feels crap in both cars.

My 1600 was 32-34psi, So the cars weight aswell as tyre size and type do play a part.

[bubba]

Current 235/40/18's I run 45psi cold.

Old 235/45/17's I ran 42psi cold.

Previous 225/50/16's I ran 38psi front 36psi rear (these were on the skyline).

[910trx]

maybe the increased pressure from the op was to increase rolling diameter by a poofteenth through slightly balooning the tyre.

be uncomfortable as hell though.

i never get too fussed about tyre pressures

if i ever get to the stage where i require that extra bit of feedback through the wheel, or additional grip then i'm drivig way too fast for the streets.

everytime i put new brand tyres on a car that i have not had before i rule a line across the tyre in liquid paper and drive it for a week, if the outer edges rub off first, increase pressure, middle, decrease pressure.

37 is my magic mark in the stagea under standard commuting situations

increased pressures are also better for towing as it stops wall flex, but too much and the pressures will get too high

rule of thumb here is set your tyres b4 you leave, if your tyres rise 4 psi after about 1 hour of driving they are perfect, not 4psi, then put in more air, let air out iff 5 etc.

the greater the load on the tyre the more crucial the 4psi rise is as a squatting wheel will generate more heat

did this on the epic journey in the stag towing the 2 ton trailer (green garden shed with wheels) 3000kms across AUS

not one blow out despite the fact we drove in the middle of summer on b grade roads.

blow outs are not caused by over inflation of the tyre but exessive heat in the wall

- according to caravanists and truckies

Edited June 9, 2010 by 910trx

[GoldZilla]

everytime i put new brand tyres on a car that i have not had before i rule a line across the tyre in liquid paper and drive it for a week, if the outer edges rub off first, increase pressure, middle, decrease pressure.

rule of thumb here is set your tyres b4 you leave, if your tyres rise 4 psi after about 1 hour of driving they are perfect, not 4psi, then put in more air, let air out iff 5 etc.

That's an awesome idea!

Yep, tyre pressures are meant to be checked when the tyres are cold - this is one rule many people forget! When you've driven them enough for some heat to get into them, then you have to allow for that extra pressure from the air expanding.

[Raysboostin]

ive been running 44psi cold in my 235/45/18 for more than a year. my mate that worked for dunlop said it would help with tyre wear and he was right. when running 38psi the inside edge of the tyres would wear out cos i was out of adjustment for camber. so it does help with tyre wear. but as others have mentioned it will effect grip. which isnt a big deal for a stag on the street.

[Unbalanced]

My local Bob Jane recommends 34psi all round.

And my ex tire fitter mate recommends 38psi all round.

If I'm road tripping I'll generally up it a bit more to 40-42psi or so.

[Rekin]

all driver training companies strongly recommend to drive 40psi (yes even for your daily)

The more pressure allows the tread patterns to open up which helps when driving in wet. Also there is less flex in the sidewall which reduces heat and wear on the walls. It also provides more control as the tyre isnt rolling over itself when you make a turn.

Old cars used to have 30psi due to a lower pressure provided a more comfortable ride. If you look at the new holdens/fords ive been told they will recommend 38psi due to advancement in suspension to provide a softer ride.

38psi-40psi is the recommended tyre pressure.

/source: Driver Dynamics

[pipster11]

all driver training companies strongly recommend to drive 40psi (yes even for your daily)

The more pressure allows the tread patterns to open up which helps when driving in wet. Also there is less flex in the sidewall which reduces heat and wear on the walls. It also provides more control as the tyre isnt rolling over itself when you make a turn.

Old cars used to have 30psi due to a lower pressure provided a more comfortable ride. If you look at the new holdens/fords ive been told they will recommend 38psi due to advancement in suspension to provide a softer ride.

38psi-40psi is the recommended tyre pressure.

/source: Driver Dynamics

you can probably run higher pressures now due to and advance in technology and materials too

[bubba]

My local Bob Jane recommends 34psi all round.

And my ex tire fitter mate recommends 38psi all round.

If I'm road tripping I'll generally up it a bit more to 40-42psi or so.

That's a pretty blanket statement.. it really depends on size, profile, suspension setup and use.

The pressures I posted above are what I have found to be the best in my situations.

Experiment people, don't take anyones words as gospel.

[atomaly]

i just checked my pressures and they were down to under 30.. OOPS haha

back up to 35 all round

[rosella]

all driver training companies strongly recommend to drive 40psi (yes even for your daily)

The more pressure allows the tread patterns to open up which helps when driving in wet. Also there is less flex in the sidewall which reduces heat and wear on the walls. It also provides more control as the tyre isnt rolling over itself when you make a turn.

Old cars used to have 30psi due to a lower pressure provided a more comfortable ride. If you look at the new holdens/fords ive been told they will recommend 38psi due to advancement in suspension to provide a softer ride.

38psi-40psi is the recommended tyre pressure.

/source: Driver Dynamics

I had the same recommendation when I did my driver training course in 1999 with Ian Luff. The idea being better wear and helping with braking around corners, or something like that.