A-Class Tyre Puzzle

[rhud]

In my thirst for knowledge (now sadly diminishing with advancing years) I have been trying to get to the bottom of a curious difference between my former A Class (A150 Classic SE 2007) and my new one (A-Class A160 Avantgarde SE Blue Efficiency 2010). As follows :

Old: 15" steel wheels; 185/65/15 Continental Eco Contact tyres.

New: 16" alloys; 195/55/16; Continental Premium Contact tyres.

Old filler flap recommended tyre pressures: 29psi all round,

New filler cap recommended tyre pressures: 32psi front,38 rear.

Given that old and new cars are more or less the same weight - why the big difference in tyre pressures?

Mercedes Customer Service say : 'We do not have precise information on tyre pressures - but what it says on the filler cap will be correct.'

Continental Tyres say: 'Pressures are set by the car manufacturer,not by us'.

Kwik-Fit tyre pressure search engine says (when I enter reg no of new car): 195/55 tyres; 29 psi all round).

My theory is that that MB - anxious to demonstrate that their Blue Efficiency initiative makes a real difference, along with stop-start etc - are recommending higher pressures to reduce the rolling resistance of the tyres and further marginally boost mpg.

Alternatively, all these Blue Efficiency models are supposedly fitted with low rolling resistance tyres - and it may be one of the characteristics of low rolling resistance tyres that they are designed to run at significantly higher pressures.

In the scheme of things,none of this is a big deal. It's just that I don't like things for which no-one seems to have an explanation.

Any ideas anyone?

 

[Sp!ke]

You'd think the bigger the contact patch the less weight per square inch and the less pressure the tyre would need.

Low rolling resistance tyres are a bit of a myth in my book. I've never seen any difference in fuel consumption between tyre changes.

I suspect you are on the right track and that MB's Blue efficiancy models just have higher tyre pressures with little else changed.

 

[Bellow]

My theory is that that MB - anxious to demonstrate that their Blue Efficiency initiative makes a real difference, along with stop-start etc - are recommending higher pressures to reduce the rolling resistance of the tyres and further marginally boost mpg.

Only a guess, but I'd go with the above.
In Autocar a couple of weeks back, they reported on how an engineer with Ford had broken into components where all the energy from the fuel is expended.
Shockingly, the rolling resistance of tyres accounts for 33%!

Elsewhere I came across a comparison of energy efficiency between a haulage truck and a rail locomotive. Those iron wheels have their benefits...

(Perhaps this is what BMW really have in mind with 'run flats'!)

 

[Bellow]

You'd think the bigger the contact patch the less weight per square inch and the less pressure the tyre would need.

.

Guessing again, but I think it's more to do with sidewall stiffness and flexing absorbing energy.

If it's true that that's where a third of the fuel is going, I suspect this will be hotly debated topic soon.

 

[grober]

I think you are somehow misreading the figures? Continental's own site give the following recommendations.

NORMAL LOAD [depicted as 1 person] 2.0 BAR [29psi] FRONT AND REAR
FULL LOAD [Depicted as 4 people] 2.2 BAR [32psi] FRONT 2.4 [38psi] REAR

the pictograms inside the filler cap can be confusing?? Unless Mercedes are now only quoting fully laden figures for some reason as you suggest?

http://www.conti-online.com/generat...ownloadarea/download/air-pressure-2010-en.pdf

The diesel engined cars run slightly increased pressures due to slightly heavier engines??

 

[Rory]

You'd think the bigger the contact patch the less weight per square inch and the less pressure the tyre would need.

That would be true *if* the pressure was lower.

The contact patch stays the same size for a given weight and pressure regardless of the size of tyre used (as long as its not a vastly different size tyre).

Normally it's recommended to use the same pressure whatever size of tyre is on the car.

Low rolling resistance tyres are a bit of a myth in my book. I've never seen any difference in fuel consumption between tyre changes.

I suspect you are on the right track and that MB's Blue efficiancy models just have higher tyre pressures with little else changed.

Certainly in fuel economy tests in the US they reduced tyre pressures significantly and found little difference in MPG.

 

[rhud]

I think you are somehow misreading the figures? Continental's own site give the following recommendations.

NORMAL LOAD [depicted as 1 person] 2.0 BAR [29psi] FRONT AND REAR
FULL LOAD [Depicted as 4 people] 2.2 BAR [32psi] FRONT 2.4 [38psi] REAR

the pictograms inside the filler cap can be confusing?? Unless Mercedes are now only quoting fully laden figures for some reason as you suggest?

http://www.conti-online.com/generat...ownloadarea/download/air-pressure-2010-en.pdf

The diesel engined cars run slightly increased pressures due to slightly heavier engines??

It wouldn't be the first time I have misread some instruction or other - but not this time.

What's slightly interesting about the fuel filler flap figures on my new one is that the pictogram offers no variants as between lightly and heavily loaded - ie there are no pictures of two little passengers/four little passengers etc. Just a straight diagram of a car and one set of figures - 32psi front, 38psi rear. Which, as you remind me, were the maximum figures for my previous model.

I think this lends weight to my theory that MB are trying to get the maximum mpg advantage from the Blue Efficiency scenario.

I shall experiment with lower pressures (but not all in one go). I am not bothered about the 1mpg extra which higher pressures might give. But it could be that the ride might improve further with lower pressures.

 

[rhud]

Further to the above - I have just been reading a thread on Honest John about this very subject - ie tyre pressures for low rolling resistance tyres. A man with a Vauxhall Corsa says that his handbook has a section with 'ECO pressures' in it for those wishing to minimise rolling resistance. These pressures are some 10psi higher than 'normal'.

Consensus of other posters is that manufacturers are doing this so that 'best mpg' figures can be quoted in these times when there is a powerful marketing drive for fuel efficiency.

Consensus also is that ride suffers and people are best off sticking to normal pressures.