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 ;)
In news:TPGye.26447$ snipped-for-privacy@fe1.news.blueyonder.co.uk, Tony Bond (UncleFista) decided to enlighten our sheltered souls with a rant as follows
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.
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.
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.
We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Parish saying something like:
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.
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.
There are always exceptions to the rule so yes, he was generalising I guess.
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.
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.
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.
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.
Are you sure? The way to get an oversteery ass endy feeling in the Cinquecento and the Ka is to increase rear tyre pressure.
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. :)
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?
Heh! I think I was referring to keeping the tyre pressures within a reasonable safety margin ;)
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.
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