Has any one ever come across a formula for calculating the correct air
pressure for a given size of tyre supporting a given load. (and intended for
normal day to day driving on britains roads)

Also, why do manufacturers always specify higher pressures in the rear
tyres, take my car for instance, 3Litre 6 cyl diesel situated above front
axle, the front seats are slightly nearer the front axle than the rear,
(therefor two occupents will add more weight to the front than the rear) So
it would seem pretty obvious to me that the bulk of this cars weight is
above the front axle, Howevr the manufacturer recommends (for two occupants
and little luggage) 29psi front and 34psi rear. .......why?

Stuart  wrote in message
news:daf34f$tre$1@newsg2.svr.pol.co.uk...

> Has any one ever come across a formula for calculating the correct air
> pressure for a given size of tyre supporting a given load. (and intended
for
> normal day to day driving on britains roads)
>
> Also, why do manufacturers always specify higher pressures in the rear
> tyres, take my car for instance, 3Litre 6 cyl diesel situated above front
> axle, the front seats are slightly nearer the front axle than the rear,
> (therefor two occupents will add more weight to the front than the rear)
So
> it would seem pretty obvious to me that the bulk of this cars weight is
> above the front axle, Howevr the manufacturer recommends (for two
occupants
> and little luggage) 29psi front and 34psi rear. .......why?


To ensure understeer rather than oversteer which most drivers can't cope
with.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)

"Stuart"  wrote in message 
news:daf34f$tre$1@newsg2.svr.pol.co.uk...

> Has any one ever come across a formula for calculating the correct air
> pressure for a given size of tyre supporting a given load. (and intended 
> for
> normal day to day driving on britains roads)
>
> Also, why do manufacturers always specify higher pressures in the rear
> tyres, take my car for instance, 3Litre 6 cyl diesel situated above front
> axle, the front seats are slightly nearer the front axle than the rear,
> (therefor two occupents will add more weight to the front than the rear) 
> So
> it would seem pretty obvious to me that the bulk of this cars weight is
> above the front axle, Howevr the manufacturer recommends (for two 
> occupants
> and little luggage) 29psi front and 34psi rear. .......why?


Maybe to allow the sidewall to deform slightly and get a better footprint 
for traction ? (total guess ;)
Rears I'd imagine (in FWD) don't do as much so the pressure could be upped a 
bit to aid fuel economy ?

(I've found the best way to get a definative answer is to have a couple of 
half-arsed guesses to annoy "people who know" enough to post the answer ;)

-- 

Tony Bond / UncleFista

www.bradford7.co.uk

Love is like a snowmobile, speeding across the frozen tundra.
Which suddenly flips, pinning you underneath.
At night the ice-weasels come...

In news:TPGye.26447$d75.8748@fe1.news.blueyonder.co.uk,
Tony Bond (UncleFista)  decided to enlighten our 
sheltered souls with a rant as follows

> "Stuart"  wrote in message
> news:daf34f$tre$1@newsg2.svr.pol.co.uk...
>> Has any one ever come across a formula for calculating the correct
>> air pressure for a given size of tyre supporting a given load. (and
>> intended for
>> normal day to day driving on britains roads)
>>
>> Also, why do manufacturers always specify higher pressures in the
>> rear tyres, take my car for instance, 3Litre 6 cyl diesel situated
>> above front axle, the front seats are slightly nearer the front axle
>> than the rear, (therefor two occupents will add more weight to the
>> front than the rear) So
>> it would seem pretty obvious to me that the bulk of this cars weight
>> is above the front axle, Howevr the manufacturer recommends (for two
>> occupants
>> and little luggage) 29psi front and 34psi rear. .......why?
>
> Maybe to allow the sidewall to deform slightly and get a better
> footprint for traction ? (total guess ;)
> Rears I'd imagine (in FWD) don't do as much so the pressure could be
> upped a bit to aid fuel economy ?
>
> (I've found the best way to get a definative answer is to have a
> couple of half-arsed guesses to annoy "people who know" enough to
> post the answer ;)


Heh.

My old Range Rover is 29 psi front, 35 psi rear, regardless of load / speed.

I've always wondered how they work out tyre pressures, especially when so 
many people just bung 30 psi in, no matter what they're driving.


-- 
Pete M

Range Rover Vogue SE, Ford Capri (ressurection stalling)
Porsche 911 3.2 (For Sale)

COSOC #5
Scouse Git extraordinaire. Liverpool, Great Britain

Just to toss in some figures for a FWD Nissan Primera that reverse the 
trend, Nissan's figures for a 1997 2.0 are 35psi front, 32psi back for high 
speed, 32/29 for normal (legal) driving.   I don't know of any other FWD car 
with the same sort of suspension setup though, the front is a sort of dual 
wishbone multilink with the top link at a 30 degree angle plus a roll bar, 
the rear is a multilink twist beam that acts as a roll bar.

On Wed, 6 Jul 2005 06:53:49 +0100, "SteveB" <sbrads@nildramDOTcoDOTuk>
wrote:


>Just to toss in some figures for a FWD Nissan Primera that reverse the 
>trend, Nissan's figures for a 1997 2.0 are 35psi front, 32psi back for high 
>speed, 32/29 for normal (legal) driving.   I don't know of any other FWD car 
>with the same sort of suspension setup though, the front is a sort of dual 
>wishbone multilink with the top link at a 30 degree angle plus a roll bar, 
>the rear is a multilink twist beam that acts as a roll bar. 


Honda is very similar at the front but they use nice alloy top links
and not sloppy bushed steel links that needed a TSB issuing to
persuade MOT testers that the slop was OK.

SteveB wrote:

> Just to toss in some figures for a FWD Nissan Primera that reverse the 
> trend, Nissan's figures for a 1997 2.0 are 35psi front, 32psi back for high 
> speed, 32/29 for normal (legal) driving.   I don't know of any other FWD car 
> with the same sort of suspension setup though, the front is a sort of dual 
> wishbone multilink with the top link at a 30 degree angle plus a roll bar, 
> the rear is a multilink twist beam that acts as a roll bar. 
> 
> 


The automatic 1994 Nissan Primera, has 38psi front, and 36psi rear!

Top that!

Cya
Simon

The message 
from Peter Hill  contains these words:


> Honda is very similar at the front but they use nice alloy top links
> and not sloppy bushed steel links that needed a TSB issuing to
> persuade MOT testers that the slop was OK.


Why does the metal they're made of make a difference? I'd have put it
down to machining tolerances and so on.

-- 
Skipweasel.
Ivor Cutler - "Never knowingly understood."

The message <daf34f$tre$1@newsg2.svr.pol.co.uk>
from "Stuart"  contains these words:


> Also, why do manufacturers always specify higher pressures in the rear
> tyres,


Do they? News to me. 

-- 
Skipweasel.
Ivor Cutler - "Never knowingly understood."

"Simon Dean"  wrote in message 
news:3j1datFmgn03U1@individual.net...

> SteveB wrote:
>> Just to toss in some figures for a FWD Nissan Primera that reverse the 
>> trend, Nissan's figures for a 1997 2.0 are 35psi front, 32psi back for 
>> high speed, 32/29 for normal (legal) driving.   I don't know of any other 
>> FWD car with the same sort of suspension setup though, the front is a 
>> sort of dual wishbone multilink with the top link at a 30 degree angle 
>> plus a roll bar, the rear is a multilink twist beam that acts as a roll 
>> bar.
> The automatic 1994 Nissan Primera, has 38psi front, and 36psi rear!
>


Hmm. My Carlton GSi was 36psi all round lightly loaded, 42psi all round for 
'high speed' use. Ouch.

On 06/07/2005 03:06 Pete M wrote:


> In news:TPGye.26447$d75.8748@fe1.news.blueyonder.co.uk,
> Tony Bond (UncleFista)  decided to enlighten our 
> sheltered souls with a rant as follows
>> "Stuart"  wrote in message
>> news:daf34f$tre$1@newsg2.svr.pol.co.uk...
>>> Has any one ever come across a formula for calculating the correct
>>> air pressure for a given size of tyre supporting a given load. (and
>>> intended for
>>> normal day to day driving on britains roads)
>>>
>>> Also, why do manufacturers always specify higher pressures in the
>>> rear tyres, take my car for instance, 3Litre 6 cyl diesel situated
>>> above front axle, the front seats are slightly nearer the front axle
>>> than the rear, (therefor two occupents will add more weight to the
>>> front than the rear) So
>>> it would seem pretty obvious to me that the bulk of this cars weight
>>> is above the front axle, Howevr the manufacturer recommends (for two
>>> occupants
>>> and little luggage) 29psi front and 34psi rear. .......why?
>>
>> Maybe to allow the sidewall to deform slightly and get a better
>> footprint for traction ? (total guess ;)
>> Rears I'd imagine (in FWD) don't do as much so the pressure could be
>> upped a bit to aid fuel economy ?
>>
>> (I've found the best way to get a definative answer is to have a
>> couple of half-arsed guesses to annoy "people who know" enough to
>> post the answer ;)
> 
> Heh.
> 
> My old Range Rover is 29 psi front, 35 psi rear, regardless of load / speed.
> 
> I've always wondered how they work out tyre pressures, especially when so 
> many people just bung 30 psi in, no matter what they're driving.
> 
> 


A few years ago I worked with a guy who'd worked in R&D in the tyre 
industry for about 25 years and he taught me a lot of fascinating stuff 
about those "black rubber rings that cost you money at an MOT" (his 
perception of motorists understanding of tyres). I asked him the exact 
same question and it is basically (technical/scientific) trial and error.

They do thousands of miles of testing - on the road and in the lab - to 
determine the optimum tyre pressures. Ensuring that the tyre wears 
evenly across it's width and doesn't get too hot being two of the 
primary criteria.

Of course, the vehicle they get fitted to also has a bearing on this - 
FWD/RWD, vehicle weight and distribution, engine power, suspension 
charcteristics, and, as others have mentioned, under/over steer balance 
- which is why tyre pressures tend to be vehicle-specific rather than 
tyre specific.

He dispelled a lot of myths about tyres, the biggest one being the 
effect of the tread pattern on the water-clearing ability of the tyre. 
Apparently the is virtually no difference on water-clearing between the 
widely different tread patterns available. The tread pattern is more a 
marketing tool (remember Dunlop Aqua-jets?) - tyres for 
sports/performance cars should look "sporty" - and is a manufacturers 
"trademark", so much so that tread patterns are protected as Registered 
Designs and, if they can get them, patents. We worked for a software 
house producing CAD/CAM systems. Two of our customers were tyre 
manufacturers and our company had to sign Non Disclosure Agreements 
before they would send us drawings that we needed to provide tech 
support in case they included drawings/3D models of tread patterns.

Parish

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Parish  saying
something like:


>He dispelled a lot of myths about tyres, the biggest one being the 
>effect of the tread pattern on the water-clearing ability of the tyre. 
>Apparently the is virtually no difference on water-clearing between the 
>widely different tread patterns available.


In this, he was over-generalising. I can think of a couple of times when
tread patterns did make a difference to safety. Admittedly, the 'bad'
tread patterns were pretty stupid in design and tended to block water
clearance.



>The tread pattern is more a 
>marketing tool (remember Dunlop Aqua-jets?)


Hoyuss, and how it backfired on them. Seems the general public thought
along the lines that if Dunlop needed to make Aqua-jets on those tyres,
the rest of the Dunlop range without Aqua-jets mustn't be much good and
sales dipped substantially.
-- 

Dave
SE6a

On 06/07/2005 15:46 Grimly Curmudgeon wrote:


> We were somewhere around Barstow, on the edge of the desert, when the
> drugs began to take hold. I remember Parish  saying
> something like:
> 
>>He dispelled a lot of myths about tyres, the biggest one being the 
>>effect of the tread pattern on the water-clearing ability of the tyre. 
>>Apparently the is virtually no difference on water-clearing between the 
>>widely different tread patterns available.
> 
> In this, he was over-generalising. I can think of a couple of times when
> tread patterns did make a difference to safety. Admittedly, the 'bad'
> tread patterns were pretty stupid in design and tended to block water
> clearance.
> 


There are always exceptions to the rule so yes, he was generalising I guess.


> 
>>The tread pattern is more a 
>>marketing tool (remember Dunlop Aqua-jets?)
> 
> Hoyuss, and how it backfired on them. Seems the general public thought
> along the lines that if Dunlop needed to make Aqua-jets on those tyres,
> the rest of the Dunlop range without Aqua-jets mustn't be much good and
> sales dipped substantially.


Hehe, maybe they reckoned that there would be more people who weren't 
impressed with there claims for Aqua-jets, or weren't prepared to spend 
the extra money on them. Advertising/marketing is a funny old game, it 
always amuses me when companies advertise one product in a way that 
implies their other products are inferior without it affecting sales.

Parish

"Stuart"  wrote in message 
news:daf34f$tre$1@newsg2.svr.pol.co.uk...

> Has any one ever come across a formula for calculating the correct air
> pressure for a given size of tyre supporting a given load. (and intended 
> for
> normal day to day driving on britains roads)
>
> Also, why do manufacturers always specify higher pressures in the rear
> tyres, take my car for instance, 3Litre 6 cyl diesel situated above front
> axle, the front seats are slightly nearer the front axle than the rear,
> (therefor two occupents will add more weight to the front than the rear) 
> So
> it would seem pretty obvious to me that the bulk of this cars weight is
> above the front axle, Howevr the manufacturer recommends (for two 
> occupants
> and little luggage) 29psi front and 34psi rear. .......why?



Not all do.  The Ka has 31 PSI / 26 PSI, although it varies according to 
year.  I find that 32 / 30 works much better for the blend of ride, handling 
:p and fuel consumption.

34 / 32 is good when carrying high weight, not so good in the wet...

The reason for higher rear tyre pressures is probably tyre wear / fuel 
consumption reasons.

-- 
The DervMan
www.dervman.com

I agree the TSB was a bodge, but the top link is about the only non-service 
item that wears on a Primera, and still I haven't had to change one yet in 
110,000m of Primera motoring (90k in one and 20k so far in the other).   The 
TSB just means it's unlikely to fail an MOT as the allowable play is 
enormous, but well before that's reached the clunky rattles would make me 
change them anyway.

"Stuart"  wrote in message 
news:daf34f$tre$1@newsg2.svr.pol.co.uk...

> Has any one ever come across a formula for calculating the correct air
> pressure for a given size of tyre supporting a given load. (and intended 
> for
> normal day to day driving on britains roads)
>
> Also, why do manufacturers always specify higher pressures in the rear
> tyres, take my car for instance, 3Litre 6 cyl diesel situated above front
> axle, the front seats are slightly nearer the front axle than the rear,
> (therefor two occupents will add more weight to the front than the rear) 
> So
> it would seem pretty obvious to me that the bulk of this cars weight is
> above the front axle, Howevr the manufacturer recommends (for two 
> occupants
> and little luggage) 29psi front and 34psi rear. .......why?
>
>




Higher tyre pressure results in a lower slip angle, hence as someone said, 
higher pressure on the front will result in an oversteering effect, higher 
pressure at the rear will result in an understeering effect. But only a 
slight change, it can't change an oversteering vehicle to an understeering 
one or vice versa. However you can 'improve' the cornering characteristics 
of your car by lowering the tyre pressures at the rear.

In terms of weight distribution, moving the centre of gravity towards the 
rear will have an oversteering effect, hence the weight distibution in your 
vehicle is contributing an understeer effect.

Not so much nowadays, but certainly 15 to 30 years ago it was standard 
practice to use the tyre pressures to correct mistakes/abnormalities in the 
suspension set-ups once a car went into production. Certain British cars in 
particular had stupidly large differences between recommended front and rear 
pressures, to compensate for mistakes in the suspension design.

Tyre pressures are generally determined after FEU or PVT builds, i.e. after 
on the road testing has determined quirks in the road handling. I think once 
this has been established pressures for other tyre sizes on the same model 
can be calculated.

Actually it just occured to me, I wouldn't be surprised in noise issues came 
into it as well.

W.

"Well_i_Wonder"  wrote in message 
news:42cdaa35$0$6296$ed2619ec@ptn-nntp-reader03.plus.net...

>
> "Stuart"  wrote in message 
> news:daf34f$tre$1@newsg2.svr.pol.co.uk...
>> Has any one ever come across a formula for calculating the correct air
>> pressure for a given size of tyre supporting a given load. (and intended 
>> for
>> normal day to day driving on britains roads)
>>
>> Also, why do manufacturers always specify higher pressures in the rear
>> tyres, take my car for instance, 3Litre 6 cyl diesel situated above front
>> axle, the front seats are slightly nearer the front axle than the rear,
>> (therefor two occupents will add more weight to the front than the rear) 
>> So
>> it would seem pretty obvious to me that the bulk of this cars weight is
>> above the front axle, Howevr the manufacturer recommends (for two 
>> occupants
>> and little luggage) 29psi front and 34psi rear. .......why?
>>
> Higher tyre pressure results in a lower slip angle, hence as someone said, 
> higher pressure on the front will result in an oversteering effect, higher 
> pressure at the rear will result in an understeering effect.


Are you sure?  The way to get an oversteery ass endy feeling in the 
Cinquecento and the Ka is to increase rear tyre pressure.


> But only a slight change, it can't change an oversteering vehicle to an 
> understeering one or vice versa. However you can 'improve' the cornering 
> characteristics of your car by lowering the tyre pressures at the rear.


No, you can make quite a difference on some cars.  Standard Ka pressures for 
Kermit straight from the factory are 31 PSI up front and 26 PSI at the back.

Increasing the rears to 40 PSI and keeping the fronts to 31 PSI gives you 
oversteer under load under many conditions.  :)


> In terms of weight distribution, moving the centre of gravity towards the 
> rear will have an oversteering effect, hence the weight distibution in 
> your vehicle is contributing an understeer effect.
>
> Not so much nowadays, but certainly 15 to 30 years ago it was standard 
> practice to use the tyre pressures to correct mistakes/abnormalities in 
> the suspension set-ups once a car went into production. Certain British 
> cars in particular had stupidly large differences between recommended 
> front and rear pressures, to compensate for mistakes in the suspension 
> design.
>
> Tyre pressures are generally determined after FEU or PVT builds, i.e. 
> after on the road testing has determined quirks in the road handling. I 
> think once this has been established pressures for other tyre sizes on the 
> same model can be calculated.
>
> Actually it just occured to me, I wouldn't be surprised in noise issues 
> came into it as well.




-- 
The DervMan
www.dervman.com

"DervMan"  wrote in message 
news:Q5Cze.20271$y86.17894@newsfe1-win.ntli.net...

> "Well_i_Wonder"  wrote in message 
> news:42cdaa35$0$6296$ed2619ec@ptn-nntp-reader03.plus.net...
>>

>>>
>> Higher tyre pressure results in a lower slip angle, hence as someone 
>> said, higher pressure on the front will result in an oversteering effect, 
>> higher pressure at the rear will result in an understeering effect.
>
> Are you sure?  The way to get an oversteery ass endy feeling in the 
> Cinquecento and the Ka is to increase rear tyre pressure.



Well that's the engineering theory, on a case by case basis I guess there's 
lots more factors to take into account. I guess you could be reducing the 
tyre contact patch area on the rear? What type of suspension does the Ka 
have on the rear out of interest?



>
>> But only a slight change, it can't change an oversteering vehicle to an 
>> understeering one or vice versa. However you can 'improve' the cornering 
>> characteristics of your car by lowering the tyre pressures at the rear.
>
> No, you can make quite a difference on some cars.  Standard Ka pressures 
> for Kermit straight from the factory are 31 PSI up front and 26 PSI at the 
> back.
>
> Increasing the rears to 40 PSI and keeping the fronts to 31 PSI gives you 
> oversteer under load under many conditions.  :)
>


Heh! I think I was referring to keeping the tyre pressures within a 
reasonable safety margin ;)



[SNIP]

>
>
> -- 
> The DervMan
> www.dervman.com
>


W.

DervMan wrote:

> "Well_i_Wonder"  wrote in message 
> news:42cdaa35$0$6296$ed2619ec@ptn-nntp-reader03.plus.net...

	[...]

>> Higher tyre pressure results in a lower slip angle, hence as 
>> someone said, higher pressure on the front will result in an 
>> oversteering effect, higher pressure at the rear will result in an 
>> understeering effect.
> 
> Are you sure?  The way to get an oversteery ass endy feeling in the 
> Cinquecento and the Ka is to increase rear tyre pressure.


I expect lateral grip peaks at some pressure then drops off again as the
gains from increased sidewall stiffness are outweighed by reductions in
contact area and suspension compliance.

....or something...


A

-- 
Trade Oil in

Alistair J Murray wrote:

> PC Paul wrote:
>
> [...]
>
>> Hmm. My Carlton GSi was 36psi all round lightly loaded, 42psi all
>> round for 'high speed' use. Ouch.
>
> My ALPINA B10 V8 *only* specifies high speed/load pressures.  :)
>
> 39f/43r
>
>
>
> A


twat

-- 
"it's very dangerous to fall asleep in the bath, I keep myself awake by
constantly making toast"

"Well_i_Wonder"  wrote in message 
news:42cf0383$0$6288$ed2619ec@ptn-nntp-reader03.plus.net...

>
> "DervMan"  wrote in message 
> news:Q5Cze.20271$y86.17894@newsfe1-win.ntli.net...
>> "Well_i_Wonder"  wrote in message 
>> news:42cdaa35$0$6296$ed2619ec@ptn-nntp-reader03.plus.net...
>>>
>
>>>>
>>> Higher tyre pressure results in a lower slip angle, hence as someone 
>>> said, higher pressure on the front will result in an oversteering 
>>> effect, higher pressure at the rear will result in an understeering 
>>> effect.
>>
>> Are you sure?  The way to get an oversteery ass endy feeling in the 
>> Cinquecento and the Ka is to increase rear tyre pressure.
>
>
> Well that's the engineering theory, on a case by case basis I guess 
> there's lots more factors to take into account. I guess you could be 
> reducing the tyre contact patch area on the rear? What type of suspension 
> does the Ka have on the rear out of interest?


It's probably a reduced contact patch in combination with a less supple tyre 
too.

As for rear suspension, it's a twist-beam, semi-trailing coil spring 
arrangement with toe-correcting bushes.  Whatever that means! :)

"Bloody effective" springs to mind...


>>> But only a slight change, it can't change an oversteering vehicle to an 
>>> understeering one or vice versa. However you can 'improve' the cornering 
>>> characteristics of your car by lowering the tyre pressures at the rear.
>>
>> No, you can make quite a difference on some cars.  Standard Ka pressures 
>> for Kermit straight from the factory are 31 PSI up front and 26 PSI at 
>> the back.
>>
>> Increasing the rears to 40 PSI and keeping the fronts to 31 PSI gives you 
>> oversteer under load under many conditions.  :)
>>
> Heh! I think I was referring to keeping the tyre pressures within a 
> reasonable safety margin ;)



That's the "maximum economy" rear tyre pressure according to Ford. :)

-- 
The DervMan
www.dervman.com

The message <41h3q2-1cc.ln1@film.fluffy>
from Alistair J Murray  contains these words:


> I expect lateral grip peaks at some pressure then drops off again as the
> gains from increased sidewall stiffness are outweighed by reductions in
> contact area and suspension compliance.


How about a set of these?

http://www.michelinman.com/difference/releases/pressrelease01102005a.html

-- 
Skipweasel.
Ivor Cutler - "Never knowingly understood."