I've recently taken delivery of a new Volvo S40 2.0D, reading through the handbook the recommended advice given for tyre pressures is as follows:
normal 30psi (front and rear)
max load 36 psi
ECO 36 psi *
*when speeds are not expected to exceed 160 KPH it is recommended that the max load pressures are maintained for all normal usage. This will give the optimun fuel economy.
unquote
So Volvo are recommending a 20% increase over "normal" pressures for economy purposes.
Any thoughts? Should I do this, and will the handling be affected - or the safety?
The tyres fitted - by the way - are normal 205/55 x 16 Continental Premium Contacts, ie not runflats and when I checked the pressures after taking delivery they were set to 30psi.
Jacks
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I habitually run my cars at the specified full load pressures, partially because I tow or fit a tow-bar mounted bike carrier regularly and can't be bothered to adjust the pressures down afterwards, partially because of the potentiality greater economy - I do mostly motorway miles.
However, the car was delivered (a year ago) with the tyres inflated to the maximum pressure and I generally manage to get many more miles out of my (company car) tyres than colleagues do and invariably get comments on this from tyre fitters when they're changed. Against this the tyres do tend to wear out slightly quicker in the middle section but possibly only as fast as the edges would were I to run them 'softer'. you so often see cars with the outside edge of their tyres worn away.
The ride is a little harsher but I've never noticed any handling disadvantages - even in the snow. Mine are 225 section run-flats on a 330d.
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To a first approximation, an increase of 20% in tyre pressure must manifest itself as a 20% reduction in contact area 'twixt road and rubber....
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>> To a first approximation an increase of 20% in tyre pressure must manifest itself as
>> a 20% reduction in contact area 'twixt road and rubber....
there is no way you can possibly derive one from the other in a straight scale like that.
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I have always run my cars tyre pressures at half way between the normal and full load reccomendations. My tyres have always worn evenly and without any problems. My car is usually lightly loaded and used on all types of road.
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>>there is no way you can possibly derive one from the other in a straight scale like that.
Actually, it's not that far wrong.
For equilibrium at the contact patch, pressure ~= force / area
The force is the vertical load, the pressure, the pressure of the air in the tyre, and the area, that of the contact patch.
Yes, there's a bit of fudging because the tyre has some rigidity of it's own rather than being wafer thin like a balloon, but, this local rigidity of the rubber doesn't dominate at the levels of load that are in the contact patch. If the local rigidity did dominate, tyres wouldn't be effective in providing cushioning against bumps in the road.
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so if I increase by 100% I fly?
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>> so if I increase by 100% I fly?
Increase by 100% = doubling
so, no :-)
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but you said "increase the pressure by 20% decrease the contact patch by 20%"
So I if increase the pressure by 100% I decrease the contact patch by 100%
I have no contact patch. I fly.
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I regard the manufacturer's advice as a starting point.
My car wears the outside edges of the front tyres at the recommended 29psi. I'm currently running them at 35psi with no ill effects (other than even wear across the front treads).
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no idea how the fronts wear on the lancer they were new when I got it.
The rears have done 24k miles. They are wearing on the outside edges, the right rear more than the left. The car came from swindon, I blame all the roundabouts there.
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Ah...... Swindon ! That explains it. I was wondering how you managed to find one of those in Surrey.
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>> Ah...... Swindon ! That explains it. I was wondering how you managed to find one
>> of those in Surrey.
>>
Ha ha!
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>> so if I increase by 100% I fly?
>>
>>
No it doesn't quite work like that - BUT, that doesn't invalidate the point being made.
For small changes from nominal, a certain increase in percentage pressure being matched by a corresponding reduction in percentage of contact area is quite true. In a happy coincidence, the relationship stated in the terms of percentages does begin to fall apart once you go beyond about 20% from nominal.
Of course, the physics which governs what's happening, namely pressure = force / area doesn't fall apart.
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Oddly enough as the contact patch gets smaller, the speed at which you risk aquaplaning actually begins to rise.
Physics and tyres have a voodoo-like relationship with so many fudge (I mean confounding) factors as a result of the physical properties of rubber and tarmac under pressure/heat (not including tread patterns, etc).
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>> Oddly enough as the contact patch gets smaller the speed at which you risk aquaplaning
>> actually begins to rise.
>>
Yes, I found this when I got a road bike after years on mountain bikes, concerned over the virtually slick tyres I did some research and found that the tiny grooves on road bike tyres are for ostensibly psychological effect only, the small contact area actually makes them less prone to aqua-planing than knobbly mountain bike tyres.
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>> To a first approximation an increase of 20% in tyre pressure must manifest itself as
>> a 20% reduction in contact area 'twixt road and rubber....
>>
......so if I increase my tyre presure by 100% then I would have no contact and effectivly be floating above the road! That must be even better for fuel economy :-)
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I don't think anyone's asked about the difference it'll really make to fuel consumption.
So, what difference will it make?
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All the "eco" cars run at hire pressures to reduce rolling resistance. This will come from two components:
Smaller contact patch gives lower frictional losses (but less grip)
Higher rigidity gives lower heat build up in the side wall (but less cushioning).
HTH
Joe
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Another bit of tyre voodoo is that a smaller contact patch doesn't reduce grip by a comparable level - as the patch gets smaller the force per given contact area increases, maintaining grip levels to a significant extent.
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Following my OP and considering the answers given and the fact that nobody is screaming "don't do it" I have decided to try out the increased pressures. I'm driven by a mild disappointment in the mpg figures although it's early days for this car, i've only done approx 1000 miles and the engine is still loosening up I imagine.
I've filled up 4 times.
1st tankfull = 36 mpg
2nd tankfull = 46.5 mpg (this covered a 300 mile motorway / A road trip )
3rd tankfull = 41 mpg
The car is a 2.0 diesel Volvo S40 fitted with the PSA 136bhp engine as used in the Focus/309/etc. with a quoted manufacturers mpg of 50.
Jacks
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With today's wretched low profile tyres being fitted as standard to most cars and especially my new Golf. there's no way that I am going to increase the tyre pressures any higher than the manufacturer's minimum figure.
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>> The car is a 2.0 diesel Volvo S40 fitted with the PSA 136bhp engine as
>> used in the Focus/309/etc. with a quoted manufacturers mpg of 50.
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>> Jacks
>>
I had that engine in my Focus, once loosened up (after 10,000 miles) it did 42mpg around town and 50mpg on the motorway. I have never been accused of holding up the traffic. :-)
My current car (KIA Ceed SW3 diesel currently 1,500 miles) is doing about 40mpg, not bothered until it has done the 10,000 mile diesel loosening up period.
Last edited by: Old Navy on Sat 1 May 10 at 09:23
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But how much difference is it going to make to fuel consumption?
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>> But how much difference is it going to make to fuel consumption?
>>
Sod economy. Comfort is my main concern.
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For the past couple of years, I've run my cars at about 12% to 15% above the recommended pressure.
I've noticed no difference in the wear on tyres. Nor have I noticed any difference in grip. My guess - I've never tested this in anything like scientific conditions - is that it probably improves fuel economy by 5% - but it could be closer to 3%.
Increasing pressures by 20%? I've never tried it, largely because I reckon that fuel economy improvement is subject to the law of diminishing returns - but also because I reckon that it might start to have a material effect on grip. (However, if Volvo are recommending it . . .) However, I'd guestimate that you'd get 5%-6% fuel economy improvement.
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In the days when people used to race bog standard cars, or as near as dammit, radically increasing the tyre pressures was usually part of the race tune.
Crossply tyres in particular distort and squirm under cornering pressure, and raising the pressures reduced the sloppy feel of cars that heaven knows weren't usually adapted for that sort of thing. In the dry, provided the dampers were working properly, grip was increased and handling made more predictable, at the expense of a more sudden loss of grip when it finally went. They squealed less when they were hard too.
My impression is that modern radials are less affected by these differences, but they certainly will be affected by them and I would think higher pressures would be more economical. But not very much unless you are one of the very many cars I see with visibly - i.e. very - too-soft tyres. One in five or ten in London at a rough estimate.
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This post just shows how important a subject tyre's are. I think it was the last F1 grand prix and one of the experts said ''how important tyre choice and how you run them are''. There are so many variables to consider in tyre choice and the pressure at which you run them. Over inflate you don't have the traction, i would imagine thats a true statement. But look at the influence the weather plays in a tyres performance. What type of surface are you running over, what temparature is that surface. The talk of manufactures reccomendations are we talking the vehicle or the tyre manufacture. Do we drive and inflate the tyres for safety or economy.
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Not a full answer by any means FT, but, based on my performance prediction spreadsheet, using a mk4 Astra as the example, the rolling resistance makes the following contributions to the fuel consumption;
10 mph 29.3%
20 mph 27.4%
30 mph 24.8%
40 mph 21.9%
50 mph 19.2%
60 mph 16.7%
70 mph 14.5%
80 mph 12.6%
So, this represents the very best change that could ever be acheived using tyres made from magic materials with no losses.
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The EU will be introducing mandatory TPMS (tyre pressure monitoring) shortly as a way of reducing CO2 emmisions (low pressures increase CO2 and many people don't check).
The other suprising way tyres alter mpg is profile. The frontal area of a tyre has a big affect on drag! thats one reason Prius's have skinny tyres.
Joe
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With the cost of fuel in the news recently we are bombarded with ways of saving fuel and money. One of these is to make sure your tyres are at the correct pressure or as on here well above.
How much will you actually save on fuel over the year compared with 4 new tyres, lets say £320 worth, every year. Which one should we go for. The pennies saved on the fuel or the pounds saved having to replace tyres prematurely?
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Since Volvo recommend it for your car, I see no reason to doubt the wisdom of doing it. For my car, I'll continue to stick to the recommendations in my handbook.
Last edited by: L'escargot on Sat 1 May 10 at 07:26
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...I'm driven by a mild disappointment in the mpg figures...i've only done approx 1000 miles...
Jacks,
This is the key to your economy worries.
My CC3 - same engine as yours, I think - does much better mileage at 16,000m than it did at 1,600m.
I get up to 50mpg now compared to 42-44mpg when it was new.
Mix of journeys is the same, so it's a reasonable comparison.
A loosened-up engine is what you need, so get in your nice new car and give it some proper driving.
In other words don't pump up the tyres, pump up the gas. :)
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>> A loosened-up engine is what you need ..............
I agree. My car's economy peaked at 27,000 miles. Since then it's gradually fallen, and at 52,000 miles it's now giving 0.6 mpg lower than the peak figure.
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>> A loosened-up engine is what you need so get in your nice new car and
>> give it some proper driving.
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>> In other words don't pump up the tyres pump up the gas. :)
>>
I'm doing that - following the HJ recommended advice for running in a diesel
as in
Leave factory fill of oil for the first year or 10,000 miles. For the first 1,000 miles do not exceed 3,000rpm, but make sure you reach 3,000rpm regularly. For the next 1,000 miles (to 2,000 miles) do not exceed 3,500rpm, but make sure you reach 3,500rpm regularly. For the next 1,000 miles (to 3,000 miles) do not exceed 4,000rpm, but make sure you reach 4,000rpm regularly. For the next 1,000 miles (to 4,000 miles) do not exceed 4,500rpm, but make sure you reach 4,500rpm at least a couple of times a week. After that, no limit, but make sure you continue to hit 4,500rpm through the gears several times a week.
I've had diesels before, a couple of company Vectra 2.0 models, and didn't notice much difference as the mileage increased right up to 140K.
Parkers Guide had an estate version the V50 with the same engine on long term test and the reviewer commented a couple of times about the (poor) fuel consumption. He ended up averaging 45mpg over 10K miles and he remarked he had to drive quite carefully - sticking to 70 on the m/way - to achieve this.
Maybe the Volvo S40 is heavier than the Focus CC3 etc
Jacks
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Jacks,
Your driving regime looks spot on to me.
It's a pity we can't fast forward to 15K miles to see what the consumption will be then.
Your researches don't look too promising, but some of the motoring hacks are extraordinary lead-footed, whatever they say.
Some will disagree, but I'm a fan of branded fuel.
It might not make much difference, but Ford recommend BP for the CC3, so I use that, or something like Shell or Esso whenever possible.
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Well the last set of tyres on my Octavia tdi, I've been running them at close to 40 psi on the front and back, , after the previous 2 front sets wore out on the edges rather than evenly, after 15 to 17000 miles. This set is still going strong after 21000 miles and I estimate have another 5000 in them max. I shall change them in a couple of months though. When I first started this higher pressure trial, I noticed a slightly harsher, noisier ride and slightly sharper steering I thought. As for fuel economy, I really don't know, too many variables.
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>>Leave factory fill of oil for the first year or 10,000 miles. For the first 1,000 miles do not exceed 3,000rpm, but make sure you reach 3,000rpm regularly.>>
That sounds fine though ...
>>... ... ... After that, no limit, but make sure you continue to hit 4,500rpm through the gears several times a week. >>
... ... ... as for the rest, my Mondeo has really hardly ever exceeded 3500 and not often 3000 because the the torque low down ensures swift progress / overtaking without reving it out regularly.
Still sweet at 140k.
Last edited by: Cheddar on Mon 3 May 10 at 08:11
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SFAIK having your tyres 20% over-inflated will make plod unhappy at any roadside check you might be pulled in for; the permitted error is a lot less than+/- 20%
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Ridiculous. If it's not dangerous, they should mind their own flippin' business.
However, I suspect that your "SFAIK" (AFAIK?) is wrong.
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Increasing tyre pressures by 20% for economy??? - idle_chatterer
