Tyre pressure logic ...

[Arkaneslayer]

1078.1

 

hi everyone

finally i found some logic for computing tyre pressure from an australian 4x4 magazine site ... here it goes :

 

the sidewall of any tyres usually specify maximum weight they can carry at a particular psi level e.g., for the ES100, it can carry 1235 lbs @ 50 psi. over 4 tyres the total weight 4 tyres can carry is 4940 @ 50 psi (4 x 1235 lbs @ 50 psi).

 

however the weight of the car is usually kerb weight + passengers + some misc stuff. in my case, T4 = 1255kg, passengers = 130kg & misc = 10kg. so total weight = 1395kg (3075lbs).

 

thus if 50 psi can carry 4940lbs, it follows that to carry 3075lbs, the tyres need to be pumped to :

 

(3075/4940)*50 psi = 31.12 psi. assuming that the weight is evenly distributed, we'd pump 31.12 psi all round. however, the weight is not evenly distributed and hence we get something like 32 front and 29 back (+/- 1 psi)

 

make sense? comments welcomed!

[Pauly75ts]

"finally i found some logic"

 

Hi! there, ArkaneSlayer,

 

You have to take into consideration, heat + tyre compound + suspension system of a car, etc....

 

BTW, the set of 4 tyres on the car carries the safety & lives on board.

 

Rgds

 

Paul Yong

[Soya]

so 32/29 is the ideal pressure for the ES100?

[Arkaneslayer]

it seems so ...

[Arkaneslayer]

"You have to take into consideration, heat + tyre compound + suspension system of a car, etc.... "

 

heat - yes

 

tyre compound - i dun think so ... this means every change of different tyre brand or model requires you to try out a different air pressure ... does not make sense.

 

suspension - a constant ... dun really understand the role unless you change the stock suspension to an aftermarket

 

car manufacturers specify particular air pressures for particular loads regardless of the tyres you use ... i dun see them specifying a particular compound or suspension system etc. so i'm trying to understand how they arrive at these numbers and this seems to gel well with some of the other volvo cars ...

 

if you have an alternative explanation perhaps you can enlighten us?

 

"BTW, the set of 4 tyres on the car carries the safety & lives on board. "

 

how is this related to tyre pressure?

Edited January 28, 2004 by Arkaneslayer

[Saaber]

well, driving style, the type of roads used regularly and also the amount of loads (as in passengers) will also determined the tyre pressure needed too...

[Super7]

hmmmm...... just sidetrack abit:- for a parked car which is subjected to vibrations in the dark corner of a isolated carpark, which tyre pressure is good??

[Slayer]

vibration of handphone or pager?

[Pauly75ts]

"how is this related to tyre pressure?"

 

Hi! there, ArkaneSlayer,

 

You've agreed that heat do affect tyres, in tyre pressure. Increase in heat develops once the tyres are rolling.

 

Underinflation of a tyre is particularly dangerous, since it affects the steering & directional stability of a vehicle, it also generates excessive heat which can lead to total failure of the tyre.

 

Once a tyre fails, what do you think will happen to the car & it's occupants??

 

Unlike years past, the technology in tyre manufacture has improved, so now we have good tyres. But it won't do us any good if no-one get down to it, to check tyre pressure on their cars.

 

A recent observation, I changed my wheel & tyres to +1, every friend that I met seems puzzled that I don't pump nitrogen into my tyres. Then one night, when I had a meet-up with them and the tyre shop owner, I told them that I run different tyre pressure for my driving style on different roads at different speeds. The shop owner had an expression like.........wazzuuup!!

 

Gone are the days when we can say that our tyres are a piece of rubber donut. Exotic materials are use in performance, high performance & ultra performance tyres & tyres that will make most of us pok-kai (broke). I don't think anyone will call a Bridgestone Potenza RE050A Scuderia, mounted on an Enzo Ferrari a rubber donut.

 

Handling of our cars reacts differently when we use tyres from different factory. Even with tyre tuning, i.e. inflation pressure, brand A can never have the same performance as brand B. If you follow F1 racing news & races, you'll see the difference. That is why retail tyres is totally different from race tyres, which have to follow a set of rules for use in a race.

 

With suspension as a constant, there're also handling problems caused by suspension system not up to the mark in handling causing degraded performance in a vehicle. Which won't improve by much, even if shod with a high performance tyre. Aftermarket suspension system won't help much either, unless we are willing to spend alot of $$$ to convert to a different system. I've looked at improvement for my car, it should cost at least S$10,000 to fabricate & test, which should be possible to blow away an Enzo Ferrari at the bends.

 

Without understanding the markings at the side of a tyre, lives are at risk. I remember a story about a Ferrari F40 crash at a US event on the public roads, it's basically a pedal to the metal run to the finish. 2 lives were lost then, the driver & navigator, using V-rated tyres.

 

So, tyre pressure is just part of the sum on a rubber donut.

 

Rgds

 

Paul Yong

[Zrun]

The maximum weight is a upper limit on the capability of the tires, so that someone will not mount a lousy tire on a 4x4 SUV, and get killed. Also, it is to warn u not to go above that pressure. Other than that, it is not as you have described.

 

What matters is the shape of the contact patch. At optimum tire pressure, your car will perform much better on rain, as the thread will easily tread away water. On dry ground, the traction is minimally affected. If your tire is over or under inflated, wet traction will be severely affected.

 

Too high pressure will result in more wear on center. Too low pressure will wear the sides faster. (Imagine the shape of the tire)

 

For normal driving, always go with the optimal pressure... But there are reasons why people go off the optimal pressure, especially for racing.

 

- Add more air in rear to reduce rear traction and reduce understeer.

- More pressure in front to reduce sidewall flex during cornering.

- Reduce pressure on driving wheel to increase rubber contact patch, especially for drag racing.

- More pressure to anticipate heavier load, as heavier load = more deformation, so u need more air to retain the optimum shape.

[Arkaneslayer]

good points...but some questions?

 

how does one determine optimal tyre pressure?

how does one tell what the shape of the contact patch is?

 

and

 

what is the ideal shape of the contact patch if one could tell?

 

i'm not referring to racing here ... juz normal everydy driving.

 

i notice a lot of "theories" are being put up which seems to be a lot of "hand-waving", i.e., a lot of talk about this variable & that variable, this factor & that factor ... in the end, the question still boils down to :

 

what is the optimal pressure?

 

is it a subject feel based on the usual "butt dyno"?

Edited January 29, 2004 by Arkaneslayer

[E36UDM]

  Quote

1078.1

 

hi everyone

finally i found some logic for computing tyre pressure from an australian 4x4 magazine site ... here it goes :

 

the sidewall of any tyres usually specify maximum weight they can carry at a particular psi level e.g., for the ES100, it can carry 1235 lbs @ 50 psi. over 4 tyres the total weight 4 tyres can carry is 4940 @ 50 psi (4 x 1235 lbs @ 50 psi).

 

however the weight of the car is usually kerb weight + passengers + some misc stuff. in my case, T4 = 1255kg, passengers = 130kg & misc = 10kg. so total weight = 1395kg (3075lbs).

 

thus if 50 psi can carry 4940lbs, it follows that to carry 3075lbs, the tyres need to be pumped to :

 

(3075/4940)*50 psi = 31.12 psi. assuming that the weight is evenly distributed, we'd pump 31.12 psi all round. however, the weight is not evenly distributed and hence we get something like 32 front and 29 back (+/- 1 psi)

 

make sense? comments welcomed!

Doesnt really make sense leh.... so what psi should a small jap car be pumping ? 20-22 psi ?

Edited January 29, 2004 by E36UDM

[Arkaneslayer]

"Doesnt really make sense leh.... so what psi should a small jap car be pumping ? 20-22 psi ?"

 

i dunno, i suppose you'll have to check out the tyre size/brand & model and work it out...and then compare with the manufacturer's recommendations.

Edited January 29, 2004 by Arkaneslayer

[Super7]

  Quote

vibration of handphone or pager?

the hp or pager gotta be a real heavy duty one to generate enuf force to vibrate the car

[E36UDM]

(3075/4940)*50 psi = 31.12 psi.

 

________________________________________________________________________

 

Using ur formula, say a jap car is 1 ton = abt 2200 pds

 

Presuming using same tyres coz u didnt give size etc

 

(2200/4940)*50 psi = 22 psi

 

That would be under-inflated for tyres right ?

[Arkaneslayer]

"Using ur formula, say a jap car is 1 ton = abt 2200 pds

 

Presuming using same tyres coz u didnt give size etc

 

(2200/4940)*50 psi = 22 psi

 

That would be under-inflated for tyres right ? "

 

you can't take that straight out coz 4940 @ 50psi applies only to 205/45-17 ES100 with the loading factor of 1235lbs @ 50psi. take a 185/60-15 tyre, the loading is 1102lbs @ 44psi. add some passengers & miscellanous things ... another coupla hundred pounds ...a jap car may weight about 2600lbs. thus :

 

(2600/4408)*44psi = 26psi.

 

not too bad. now what's the car manufacturer's recommended pressure? I dun reckon it should be more than 28 or 29 psi. close enough with a tolerance of +/- 1 psi to account daily changes in loads.

Edited January 29, 2004 by Arkaneslayer

[Arkaneslayer]

  Quote

..., I told them that I run different tyre pressure for my driving style on different roads at different speeds...

 

all well and fine - the question still remains :

 

how do you determine the exact psi for which kinds of roads, what kinds of speed and what kinds of driving style?

 

Do you pluck a figure from the air? or use some mathematical formula based on the car manufacturer's recommendations? or from the tyre brand & model? or use the time & tested traditional "butt dyno"?

[E36UDM]

So it means (2200/4408)*44 = 22psi again ?

 

No offense but jap car can use 17"s also right ?

 

To me there should be a minimum psi for tyres especially when their shape is 'fixed', can u imagine if its a balloon?

 

Just my view