I'm reading a copy of the latest edition of a well known motoring magazine, which has just published its Car of the Year award.
Reading a long term test, the following text had me rather confused:-
'For everyday use, the handbook recommends 34psi all round, but manufacturers figures are often on the cautious side, the better to aid fuel economy. I have found 36psi to work best, with more positive steering, no high-speed wander, and no noticeable degredation in ride quality. You can even try 40psi, recommended for high-speed and heavy load use. Doing so means the steering is even more responsive, and grip more plentiful, the downsides being a little more road noise and, presumably, a small increase in fuel consumption and tyre wear.'
Now I am confused! In simple terms, surely higher psi reduces rolling resistance and would result in less fuel being used. Very overinflated tyres would probably wear unevenly, but I wouldn't have thought that increasing tyre pressures a bit would cause more rapid tyre wear.
Who's right?!
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Journalists are not often credited with having common sense, or any knowledge of the subject written about. :-)
Last edited by: Uncle Albert on Fri 10 Jan 14 at 20:41
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>> Journalists are not often credited with having common sense, or any knowledge of the subject
>> written about. :-)
>>
certainly seems to be the case here, surely a higher tyre pressure = lower rolling resistance = less grip = lower fuel consumption/better economy
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The more pressure, the smaller the contact patch, the lower the rolling resistance, the higher the fuel consumption would be my assumption.
Every list of economy tips I've ever seen has had "keep your tyres correctly inflated" on it.
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I'd have thought that a higher tyre pressure (within sensible limits, of course) would tend to help with fuel economy.
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>> The more pressure, the smaller the contact patch, the lower the rolling resistance, the higher
>> the fuel consumption would be my assumption.
>>
>> Every list of economy tips I've ever seen has had "keep your tyres correctly inflated"
>> on it.
Is the argument about the understanding of Higher or Lower fuel consumption.
Lower fuel consumption is less consumption of fuel per mile? A product of lower rolling resistance?
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Yes, tripped myself up there- I think I was trying to write about mpg rather than fuel consumption.
More pressure = better economy.
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In the old crossply days, people used to inflate their tyres to 40 or 50 psi for any sort of competition purposes. It gave the same grip and sharper handling because the tyre carcases distorted much less. Reduced squeal too.
Now that all tyres are huge wide radials, the same thing applies up to a point. My jalopy's standard pressures are 29/34. I have them at 35 all round, and the front ones are still wearing their outside shoulders.
Standard pressures are supposed to have the best compromise between soft ride and road behaviour, but a few psi more doesn't make much difference. What are we, princesses? Tchah.
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yes agreed - I keep them at about 40 psi all round and get better life out of them for sure, plus sharper steering and a harsher ride, which I like. Although I measure mpg fairly accurately I'm not sure whether there's any significant improvement, too many other variables involved.
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I have a rear wheel drive car and the slightest over-inflation causes the centres of the rear tyres to wear out way faster than the edges of the tyre.
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George Osborne is afraid of inflation and the impact of tyre pressures on the economy..
I'll get my coat.
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I used to find the outer edges wearing faster than the centre but since over-inflating, I tend to get even wear across the whole tyre, which gives me slightly better life from them. This is with front wheel drive, even so I very rarely can get more than 17 to 20 K from a front set, which is disappointing.
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>> Now that all tyres are huge wide radials, the same thing applies up to a
>> point. My jalopy's standard pressures are 29/34. I have them at 35 all round, and
>> the front ones are still wearing their outside shoulders.
Just musing of course as there are two many moving parts to be able to predict the behaviour of a particular car, but it would be reasonable to think that what you have done has changed the handling balance.
Other things being equal, and starting from a reasonable point, just increasing front pressures would usually be expected to add understeer. That might increase cornering wear if you are a press on type, and you expect to see that at the outside edge.
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>> just increasing front pressures would usually be expected to add understeer.
Er... not the way I see it.
>> think that what you have done has changed the handling balance.
You would think so, you would expect the balance to shift slightly away from understeer towards 'neutral'. But in fact in the modern manner the car never assumes any sort of noticeable slip angle at either end except on greasy or icy surfaces: ordinary wet roads don't seem to affect it.
What this means in effect is that I don't really know how it will behave if it ever lets go in a big way. Should that ever happen I hope the autopilot will be awake because it will have a lot to do in a very short time. It worries me a bit that I have grown out of breaking adhesion for fun whenever it seems possible to maintain those on-the-edge techniques.
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I've only played about with tyre pressures, geometry, and spring rates on one car (the Morgan 4/4) which I've now sold. Road, I ran with 22 front, 23 rear against the recommended 18/19, and on the track, up to 30 and the same front and rear. Tyre wear was acceptable, but higher pressures do wear out the middles first of tyres - however, when you habitually corner at the limit, the wear pattern evens out.
The Morgan 4/4 is a natural understeerer like most, but when I'd finished, the car was neutral and could be provoked into both under and oversteer. Like a NASCAR racer, it spun in, not out and thus was safer too.
The three wheeler - which I still have, I run at the original tyre pressures, 19 front, 25 rear but talk of under and over steer is not a good idea since the handling of a vintage car is nothing like a modern car. Crudely put, one initiates a turn with the steering wheel and finishes it with the throttle.
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I guess the rear tyre is bigger than the front ones SP, but wouldn't the 3-wheeler be an oversteerer at heart? Souns a bit like it from yr description of cornering technique.
I do like Morgans. The 4/4 was a great sporting car in a sort of crossover vintage/modern mode. Went briefly in a tweaked example belonging to a friend of a friend. Not long enough to gat a very comprehensive picture, but it had phenomenal acceleration and appeared to corner absolutely flat and neutral. Can't even remember what engine it had but I think a 1600 or 2 litre Ford.
The V8 is obviously nice but perhaps too much of a good thing, brutish rather than pure.
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I remember years ago thinking my Land Rover 90 petrol was 'a bit thirsty' with it's 15 mpg appetite for 4*. This of course was when 'normal' cars would get 25-30 mpg as a rule.
Later, I can recall thinking the 35 mpg or so I got from my Golf GTi was pretty good. In time, I got used to even large cars easily beating 40 mpg and began to think of that as average consumption.
Now, I'd think of it as the least acceptable minimum.
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I guess the rear tyre is bigger than the front ones SP, but wouldn't the 3-wheeler be an oversteerer at heart? Souns a bit like it from yr description of cornering technique.
Rear tyre is the same size as the front ones - but a car type, the fronts on mine are a sidecar pattern. It'll do under and oversteer, depending on how you drive it. Cure for an understeer moment is to put more power on - line is tightened, but it's a nerve racking moment as it's the opposite of what works with most cars.
The 4/4 I had was a 1600 Kent engine, but I fitted an ex-race Caterham engine of 1840cc. Jolly nice to drive, but it was a good job it was a nimble car as folk saw it, thought old car, and pulled out in front of it.
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>> Cure for an understeer moment is to put more power on - line is tightened, but it's a nerve racking moment as it's the opposite of what works with most cars.
Makes me smile just to think of it. What terrific fun.
Those 1920s cyclecars, or one or two of the best ones, were some of the purest sporting vehicles ever. The FN, forerunner of the chain-drive Frazer Nash, was one such. Damn things went like the clappers in the right hands.
Got a brief ride in a Meadows-engined chain drive in Cheshire once. It felt wonderfully bouncy and nervous, very stark coachwork and body. The thing that impressed me most was the brakes, tiny drums operated by greasy cables going round pulleys, looked well dodgy but a good prod worked beautifully once or twice anyway. Might get a bit of fade in racing conditions but no one would do that to one now. I did see a very ferocious chaindrive at Shelsley Walsh, some sort of big 6 cylinder engine in it. Belonged I think to a cat called Julian Ghosht or Ghosh.
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Thanks Horatio. A couple of chain drive Frazer Nashes there, one of which could have been the one at Shelsley Walsh.
Chain drive Nashes in particular are quite Meccano-like, with the more fanatical owners changing and modifying them, trying different engines, sprocket sets and so on.
Ghosh has fabulous taste. 30/98 Vauxhall was thought better than 3 litre Bentley in its day.
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>>Er... not the way I see it.
Manatee sees it correctly.
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>> No he doesn't.
... or only in unlikely circumstances. If the front tyres were so overinflated that the size of their contact patch was reduced, for example, the car would understeer more. But a few pounds of pressure enabling the full normal contact patch to work would improve grip slightly owing to the lesser degree of carcase distortion when cornering, and so reduce understeer.
So my habit of using high speed/full load pressures, 35 all round, does theoretically reduce understeer. But the car doesn't have handling like most of the cars I've driven. It just goes where you point it as if on rails. Obviously given its weight distribution it is a basic understeerer, but it doesn't feel like that. It feels 'neutral'. And I've never dared lose it on purpose, so I don't know what it would do if that ever happened. It's a new experience for me and I don't like it.
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I went to the tyre place today to make an appointment. While there I inflated the tyres which I knew had been neglected for several days.
To my horror the one that loses most pressure, n/s front, was down to 23 psi, with the o/s rear nearly as bad. So I must have been neglecting them for a week or ten days.
The weird thing to me is that these gross anomalies hadn't become apparent to me. The car felt very much as it always does and behaved accordingly on its familiar routes. True, with the pressures raised and equalized it did feel better. But the gradual deterioration, combined with the jalopy's great surplus of roadholding and handling, made the anomalies unnoticeable.
I don't have to make an appointment at the tyre place. Gunge on all the rims, newer tyres put on front and older ones on back, and an all-round wheel alignment, fingers crossed, hope they know what they're doing. It won't cost the earth and they're good.
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>> The weird thing to me is that these gross anomalies hadn't become apparent to me
My Saab would illuminate the ESP, ABS and brake warning lights - a kind of rudimentary tyre pressure monitoring system I suppose - when it detected one wheel running out of kilter with the others. It did it when I put the smaller spacesaver spare on, but also when one of the back tyres had lost pressure down to 15psi (standard pressures F36, R35). I couldn't feel anything out of the ordinary with the handling either.
Last edited by: Dave_C220CDI on Sun 12 Jan 14 at 00:25
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Saabs were always good and capable. But these days a lot of mainstream cars have with their wide radials and well-designed suspension very large reserves of roadholding, of lateral grip. I said my jalopy had reserves of handling too but that was wrong. It doesn't have anything you would call handling, it's engineered to feel numb but reassuring. You don't push it, you roll it briskly through the bends. Easy to go a bit too fast for most conditions without even meaning to.
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It always puzzles me how seat of their pants experts can know better than the tyre and car manufacturers who have spent a lot of time and effort with instrumented test tracks to find the optimum tyre pressures. It makes me liken them to the baseball cap on backwards, brain off, boy racer who "improves" his car by lowering the suspension and fitting an exhaust like a waste paper bin.
Last edited by: Uncle Albert on Sun 12 Jan 14 at 09:16
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Ah but car makes have to make cars suitable for a wide range of drivers, from the more capable, discerning and demanding to the less capable - miserable incapable retired old matelots for example.
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>> Ah but car makes have to make cars suitable for a wide range of drivers,
>> from the more capable, discerning and demanding to the less capable - miserable incapable retired
>> old matelots for example.
>>
miserable, incapable, scruffy, bearded, retired old matelots for example.
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>> >> Ah but car makes have to make cars suitable for a wide range of
>> drivers,
>> >> from the more capable, discerning and demanding to the less capable - miserable incapable
>> retired
>> >> old matelots for example.
>> >>
>>
>> miserable, incapable, scruffy, bearded, retired old matelots for example.
them too.
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That seems to have woken a few people up this beautiful sunny morning. Obviously the truth stings a bit. :-)
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>> seems to have woken a few people up this beautiful sunny morning.
Miserable, scruffy bearded etc., depraved old retired matelots, missing the pong and rumble of the fish doing 40 knots in its element, and taking it out on innocent landlubbers...
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>> Ah but car makes have to make cars suitable for a wide range of drivers, from the more capable, discerning and demanding to the less capable
Exactly that.
IF set up properly, anything can be improved on.
Some cars have adjustable suspension, most motorcycles do.
In fact, setting up a motorcycle to exactly suit the rider/use IS an art.
www.suspensionshop.org/tech_suspension_setup.php
But done properly will completely change the machine!
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>> But done properly will completely change the machine!
>>
Is that why so many motorcyclists come off their bikes at high speed?
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>> Is that why so many motorcyclists come off their bikes at high speed?
>>
Strangely enough, road deaths on motorcycles is dropping, while it is INCREASING in cars.
Even though cars today are 'safer' than ever before....
www.theguardian.com/news/datablog/2012/sep/28/road-deaths-great-britain-data
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Ah, statistics. If I had to have a loss of control, unlikely in a car with a skilled driver and electronic assistance, I would rather have it in a car with half a dozen airbags, seatbelts, and crumple zones rather than on a bike wearing a plastic hat and a leather suit.
Last edited by: Uncle Albert on Sun 12 Jan 14 at 13:17
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>> Strangely enough, road deaths on motorcycles is dropping, while it is INCREASING in cars.
>> Even though cars today are 'safer' than ever before....
>>
>> www.theguardian.com/news/datablog/2012/sep/28/road-deaths-great-britain-data
>>
Or possibly NOT.
Figures from the Department of Transport for 2012 show that deaths for car occupants are down by 9%
tinyurl.com/myxuwhe
If I have misread the figures I will make a grovelling apology.
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>> >> Is that why so many motorcyclists come off their bikes at high speed?
>> >>
>> Strangely enough, road deaths on motorcycles is dropping, while it is INCREASING in cars.
>> Even though cars today are 'safer' than ever before....
>>
>>
Tougher testing, high insurance premiums keeping younger riders away from big bikes and the decline in middle aged fashion motorcyclists all play a part there.
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I keep my tyres (manufacturer's advice 33psi all round) at 36psi all round. No problems with wear, handling or ride, but correct pressures for motorway use if required...:-)
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If you press a balloon against a flat surface, surely the pressure is equal over the whole area in contact?
So why should the central part wear more if you inflate the balloon more?
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>> If you press a balloon against a flat surface, surely the pressure is equal over
>> the whole area in contact?
>> So why should the central part wear more if you inflate the balloon more?
>>
>>
Tyres are stiffer than balloons, having some structure of their own.
It doesn't work perfectly (because the tyre structure supports some weight), but if you increase the pressure in the tyre, you will reduce the size of the contact patch. You can't keep the pressure equal over the contact patch, and keep the contact patch the same size, unless the car gets heavier...
Ad absurdum, if you put enough pressure in the tyre, the shape of the section would become an arc of a circle that only touched the road in the centre of the tread. Before that happens, you would get a reduction in pressure on the road surface at the edges, with the centre providing most of the frictional forces (F = R x coefficient of friction) (Yes I know that doesn't work perfectly either).
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>> Ad absurdum, if you put enough pressure in the tyre, the shape of the section
>> would become an arc of a circle that only touched the road in the centre
>> of the tread.
>>
Yes, I see that now. But that would suggest that the tread area stretches more than the sidewalls, which seems a bit implausible as it is much thicker rubber?
Taking two extreme cases:
1) Rigid sidewalls, soft tread. It would balloon out and form your arc, as you say.
2) Soft sidewalls, rigid tread. Tread would stay flat on the ground, effective diameter would increase.
My observation of pumping up a tyre is that the sidewall is the softer, and putting more air in fills out the squashed section so that it looks less flattened.
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The impact of tyre pressure on economy is surely governed by the sidewall behaviour. The more it flexes, the more energy it consumes. So at low pressures, tyres heat up rapidly, the rubber softens and the sidewalls flex even more. At high pressures, sidewalls flex a lot less so less energy is consumed.
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Would a softer tyre be absorbing more of the energy that would otherwise be handled by the shock absorber? The energy needed to move the unsprung weight up and down would have to be lost somewhere?
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>>
>> >>
>> >> Ad absurdum, if you put enough pressure in the tyre, the shape of the
>> section
>> >> would become an arc of a circle that only touched the road in the
>> centre
>> >> of the tread.
>> >>
>>
>> Yes, I see that now. But that would suggest that the tread area stretches more
>> than the sidewalls, which seems a bit implausible as it is much thicker rubber?
>> My observation of pumping up a tyre is that the sidewall is the softer, and
>> putting more air in fills out the squashed section so that it looks less flattened.
For the sake of the argument, I wasn't thinking of it stretching at all (it has belts in it with considerable tensile strength) just blowing out into the maximum volume for the amount of material.
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There's an old racing trick of setting tyre pressures by marking a line with something across the tread and adjusting the pressures until is wears off evenly.
I've tried it a bit half-heartedly on the rear tyres my Mercedes but couldn't get results that I felt were meaningful - I used chalk and it wore off too quickly.
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Chalk it down to experience...:-)
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>> >> I wasn't thinking of it stretching at all (it
>> has belts in it with considerable tensile strength) just blowing out into the maximum volume
>> for the amount of material.
>>
>>
That sounds like an excellent definition of stretching to me :)
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Impact of tyre pressures on economy - Old Navy
