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Brake Disc Friction

Discussion in 'A3/S3/Sportback (8P Chassis)' started by Karcsi, Jan 4, 2006.

  1. Karcsi
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    Karcsi Member

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    [Jan 4, 2006]
    Has anyone else noticed that the pads seem to grip the discs quite a fair bit even at rest? Both I and my father have noticed that it takes far more effort to manhandle the A3 than his A4 on level ground. It's been 21k miles now, so it's not because the the car is new. Could this account for the low mpg figures some people have experienced compared to the old 1.9TDI 130 A3/A4 models? Or is it just because the discs / pads are much bigger on the A3 TDI 140 than on the A4 (I haven't compared)?
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  3. Eeef
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    Eeef Lord of War

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    [Jan 4, 2006]
    If that was the case you'd have glowing discs when you stopped after a journey.

    It's more likely to be the difference between standard power steering and the new electro-mechanical power steering. Can also be down to a difference in tyre widths (if there is one)
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  4. cosmicblue
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    cosmicblue Member

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    [Jan 4, 2006]
    There a two reasons why the car will seem hard to move:

    1) When you are driving down the road the run-out (would be seen as an almost Imperceptible rotaional wavering) pushes the pads off the discs, not by much but enough to prevent drag; also the seals that surround the pistons in the brake calipers are slightly distorted when the brakes are applied, when the brakes are released the seal seeks to return to its original shape a pulls the piston back fractionally from the pad.

    2) Some tyres have quite a high rolling resistance, especially the wide, low profile variety and that makes the vehicle very hard to roll around bt hand.
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  5. Karcsi
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    Karcsi Member

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    [Jan 4, 2006]
    A3 has R17 225/45 and A4 R16 205/55. So circumference is only about 2.7% more on mine, and 10% wider. Would that really make that much of a difference in rolling resistence? It takes about twice the effort to shift my car.

    My tyres have 32PSI in then, and his 38PSI (he says, but that sounds high for his tyres). But I doubt it's as simple as more PSI equals lower footprint considering they are two difference tyres.

    Sorry Eeef, I don't understand how power steering can effect a car's reluctance to being pushed? I'm just talking about pushing the car to and fro in the garage with the engine off.
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  6. Eeef
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    Eeef Lord of War

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    [Jan 4, 2006]
    [ QUOTE ]

    Sorry Eeef, I don't understand how power steering can effect a car's reluctance to being pushed? I'm just talking about pushing the car to and fro in the garage with the engine off.

    [/ QUOTE ]

    Ah, ok, prob just part 2 of my answer then
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  7. cosmicblue
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    cosmicblue Member

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    [Jan 4, 2006]
    [ QUOTE ]
    A3 has R17 225/45 and A4 R16 205/55. So circumference is only about 2.7% more on mine, and 10% wider. Would that really make that much of a difference in rolling resistence? It takes about twice the effort to shift my car.

    My tyres have 32PSI in then, and his 38PSI (he says, but that sounds high for his tyres). But I doubt it's as simple as more PSI equals lower footprint considering they are two difference tyres.

    Sorry Eeef, I don't understand how power steering can effect a car's reluctance to being pushed? I'm just talking about pushing the car to and fro in the garage with the engine off.

    [/ QUOTE ]


    Check out this article about tyres and rolling resistance: Rolling resistance and silica
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  8. Karcsi
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    Karcsi Member

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    [Jan 4, 2006]
    Thanks Cosmic. Had a quick read. 20% reduction in rolling resistance equates to about 5% saving in fuel consumption? That's a bit drastic? Or is that 5% saved of that lost, rather than of you current fuel consumption? But surely that's no enought to cover a the cost of a set of tyres over their lifetime?

    The rolling resistence on my car seems to be twice that on my Father's!! How much am I losing?!

    What I ought to do is remove the brake pads and see how much different it makes.
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  9. cosmicblue
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    cosmicblue Member

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    [Jan 4, 2006]
    [ QUOTE ]
    The rolling resistence on my car seems to be twice that on my Father's!! How much am I losing?!

    What I ought to do is remove the brake pads and see how much different it makes.

    [/ QUOTE ]

    The link was really to highlight the fact that rolling resistance soaks up a lot of energy - especially when you are pushing the thing.....

    Removing the pads does seem a tad drastic though....

    Back in the days of scary cross ply tyres, a car would roll for ever, then the 1st generation of radials appeared and it all got a whole lot harder......
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  10. Karcsi
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    Karcsi Member

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    [Jan 4, 2006]
    Only joking. The simplest thing to do would be to swop wheels with the A4 and see what difference that makes.
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  11. markdp
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    markdp New Member

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    [Jan 11, 2006]
    no, go on and drive it without the pads for a bit and let us know how you get on, or at least get someone else to let us know what happened and how fast you were going when it did.......
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  12. miketweed
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    miketweed Member

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    [Jan 11, 2006]
    [ QUOTE ]

    My tyres have 32PSI in then, and his 38PSI (he says, but that sounds high for his tyres). But I doubt it's as simple as more PSI equals lower footprint considering they are two difference tyres.

    [/ QUOTE ]

    32psi is low. That will make a whole load of difference. Try pushing a car with a flat - almost impossible.

    I have my 225/45/17's at 39PSI front, 36PSI rear. Thats Audi book for light loading. (heavy loading is 42/42). Makes no end of difference from 32psi.

    When they've gone down to 30 ish it makes a hell of a difference to steering load and rolling resistance.

    Also, my car has an engine. So I don't have to push it.
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  13. Karcsi
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    Karcsi Member

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    [Jan 11, 2006]
    Mark, yep, I'll make sure the coroner notifies you what I found discovered.


    Mike, I don't think they are. Off the top of my head, I think book for light loading is 30PSI all round, and 32 front 38 rear for full load. But I will have a look when I get home.

    I set them to 32PSI because I like to go round corners faster than I should, and it's a bit more protection for the rims when hitting pot holes.
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  14. miketweed
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    miketweed Member

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    [Jan 11, 2006]
    Karcsi,

    When mine are at 32 they look flat and bulge at the bottom edge.

    Regardless of book pressures, the 38psi to 32psi difference will make a lot of difference to the rolling resistance.
    Its 20% less pressure, which means 20% more tyre tread will be in contact with the road to support the vehicle. This means 20% more tyre deformation i.e. probably 20% more tyre rolling resistance.

    I've checked the book and for 225/45/17 it is 39f/36r light load.
    #13
  15. jdp1962
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    jdp1962 Grumpy Old Moderator Staff Member Moderator

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    [Jan 11, 2006]
    If you're really that worried to know whether the brakes are grabbing, why not just jack the wheels up, and turn the bl00dy things?!
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  16. Karcsi
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    Karcsi Member

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    [Jan 11, 2006]
    OK, I've done jd's blindingly obvious suggestion (I hope you feel suitably smug /ubbthreads/images/graemlins/wink.gif )

    Rear wheels are reasonably free - a quarter turn pull results in a further full rotation. The front wheels on the other hand hardly turn at all - quarter turn pull results in about 1/8th turn before stopping (yes it was out of gear!). This is even between the two sides.

    Is this normal considering you are also turning the prop/side shafts connected to the front wheels?


    On the subject of tyre pressures, this is what the plaque on my car says:

    [​IMG]

    30 29 light, 32 36 full load. The 36 33 / 38 39 is for all weather or reinforced tyres.

    As you can see, mine are perfectly fine at 32 all round.

    [​IMG]

    [​IMG]
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  17. miketweed
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    miketweed Member

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    [Jan 12, 2006]
    I suppose they have model specific pressures. I'll take a photo of mine asap.

    But the extra 20% rule i mentioned before still applies from 38 to 32 psi. You can see that the smaller tyre size gives a higher pressure because of its reduced foot print, so it will have a lot less rolling resistance.

    RE wheel friction. Don't forget when turning 1 front wheel with the other on the ground you are also rotating half the differential and the entire output shaft of the gearbox in all its oil so its never going to free wheel.

    Is yours a diesel / dsg or what?
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  18. jdp1962
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    jdp1962 Grumpy Old Moderator Staff Member Moderator

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    [Jan 12, 2006]
    [ QUOTE ]
    OK, I've done jd's blindingly obvious suggestion (I hope you feel suitably smug /ubbthreads/images/graemlins/wink.gif )

    Not smug, just exasperated. And I can't believe you went out after 11pm to check your brakes & take photos of your tyre pressure guide. WTF is THAT all about?!
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  19. Karcsi
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    Karcsi Member

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    [Jan 13, 2006]
    Clearly you're taking it far too seriously. Sorry Officer, I didn't realise the time police were watching.

    It was meant in jest, as I should have thought of that - although perhaps I would need to raised both front wheels, and that I won't be doing after 11pm.
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  20. miketweed
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    miketweed Member

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    [Jan 13, 2006]
    Further to this experiment, I've let my tyres down to 30.

    Now the car won't roll down the drive with the brakes off, which it easily did with them at 39/36 psi.

    Even putting a leg out the door and trying to push it backwards won't budge it!!
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  21. Eeef
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    Eeef Lord of War

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    [Jan 13, 2006]
    er.....guys? please?

    Hobbies? Women? Beer? Sleep?

    Pick one of the above and move on. /ubbthreads/images/graemlins/wink.gif
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  22. mikep
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    mikep Member

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    [Jan 13, 2006]
    You know he's right...........
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  23. miketweed
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    miketweed Member

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    [Jan 13, 2006]
    Oh I thought this was a car forum? not a self help group? Silly me.

    BTW (please don't be offended by this errant car comment) but the car also did 4mpg less this morning with 30psi in the tyres.
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  24. mikep
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    [Jan 13, 2006]
    Mike what are you averaging on the Revo map now? I found the original production map wouldn't do better than 45mpg, I've now installed the new 2 Stage map which uses the original map developed on my car with the more agressive map as Stage 2 and I'm back to 50-55mpg again with very little change in performance.

    cosmicblue had the same problem, except he stuggled to get 40mpg some days, John now has the same 2 Stage map.
    #23
  25. miketweed
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    [Jan 13, 2006]
    MikeP

    Generally 30-32mpg on the DIS on a good run on M-way and A roads. It used to do 34-35 on the same. With cruise on at 70, on the level, the DIS says 42mpg where it used to say 40mpg.

    But I am a bit heavier on the gas now, can't resist that 300lbft feeeeeeeeeeeeeling!!

    I was doing 2000-3000 miles a month, but a week or two after I got the Revo I started a local contract in Manchester, then got banned over Christmas so I havent really done anywhere near the mileage I'd hoped.

    Now I'm walking to work 3 out of 5 days and the wife takes the A3!
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  26. mikep
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    [Jan 13, 2006]
    Mike I forgot you had the TFSi, but those figures aren't too bad for a 260bhp 300lb ft car.

    I know what you mean about the urge to see how fast you can make the cars behind disappear! From 70 to somewhere north of 100 doesn't take long at all.

    I remember you saying about the 104mph? ticket, that may have a nasty impact on your insurance, they don't like any bans no matter how short it was. At least you can walk to work, my closest shop is 4 miles away.
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  27. miketweed
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    miketweed Member

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    [Jan 13, 2006]
    My solictor and I chose to take the 2 week ban instead of the 6 points they offered. Didn't fancy pootling round with 9 points for the next 3 years.

    So now I only have 3 points, but heftier insurance maybe?

    My renewal is due in February, so i'll have to wait and see!
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  28. mikep
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    mikep Member

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    [Jan 13, 2006]
    Shop around carefully, you may find some companies take a more pragmatic view on short term bans.
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  29. Karcsi
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    Karcsi Member

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    [Jan 13, 2006]
    [ QUOTE ]
    Further to this experiment, I've let my tyres down to 30.

    Now the car won't roll down the drive with the brakes off, which it easily did with them at 39/36 psi.

    Even putting a leg out the door and trying to push it backwards won't budge it!!

    [/ QUOTE ]

    I can understand almost 10PSI making that much of difference, but I don't understand why your standard pressures are so much higher than mine.

    I will try putting mine up to the same as yours and see what difference it makes re ride quality, grip as well as rolling resistence. But I can see my traction control light being even busy than usual.
    #28
  30. miketweed
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    miketweed Member

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    [Jan 13, 2006]
    Karcsi -

    Since you showed the picture of the pressure list I must say I never knew what the XL M+S suffix was, thought S meant 'sport' or something as theres no mention in the manual.

    So its looks like i've had mine at Winter tyre pressures! My sticker has 205's and 255's in the centre box and only 225's in the bottom box so I just saw '225' and thought that was it.

    The correct pressures are 33f / 32r for my model. So i've had them 6/4 psi over book, but they wore perfectly evenly on all four wheels and the car felt much more responsive with better feedback at the higher pressures. (quieter too)

    Now I've got them at 30 it feels like i've got jelly in the tyres!
    #29
  31. wilko
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    wilko Top Gear

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    [Jan 13, 2006]
    Can we get some clarification on what pressures we should be running at:

    Mine like the picture above says 30 front 29 back so that's what I run at. And I do recall speaking about this awhile ago and when I noticed that the center of my fronts started to go a bit bare and everyone said I was running them over inflated but they have never bin over 34PSI. In normal UK weather (which yes is ****) but what PSI should A the 3 door and B the Sportback be running at?? Whats the ideal, best for handling, MPG, etc, etc....?? /ubbthreads/images/graemlins/noidea.gif
    #30
  32. miketweed
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    miketweed Member

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    [Jan 13, 2006]
    It seem that they vary between body style / engine type etc

    Originaly mine were at 32 ish (from delivery), but were wearing at both edges (i.e. sign of under inflation). Then I misread the pressure sticker at put them at winter tyre pressures, but they all wore perfectly evenly for the next 20k miles, and it felt better to drive. We also have a lot of potholes in Manchester, so it helped fend those off too.

    My sticker says 33/32 (light load), I think I'll go for 2 over, so 35/33 to try next.
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  33. wilko
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    wilko Top Gear

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    [Jan 13, 2006]
    I suppose being England it's not winter but it still feels like it most of the time... yerh so maybe somewhere in between.

    So for a 2.0 TDI Sport with 17's you looking at 33/31. I might try it... Can't see how 3PSI is gonna make a massive difference tho!
    #32
  34. miketweed
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    miketweed Member

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    [Jan 13, 2006]
    I don't know what model Karcsi has, his sticker says 30/29.

    Try 35 to feel a difference, then make up your mind.
    #33
  35. jdp1962
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    jdp1962 Grumpy Old Moderator Staff Member Moderator

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    [Jan 13, 2006]
    The way I look at it is, the manufacturer's recommendations are a compromise between the various confllcting criteria of ride comfort, handling, fule economy, noise,etc.

    So long as you avoid massive under- or over-inflation (either of which would soon become apparent from the wear patterns), it's then up to the individual driver to vary the pressures up or down within reasonable margins according to their preference.

    I generally run mine at about 2 or 3 PSI above the recommended setting. I find that's just enough to sharpen up the steering without making the ride too stiff or causing the handling to become too nervous & fidgity. The tyre wear patterns are also also nice and even.
    #34
  36. miketweed
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    miketweed Member

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    [Jan 13, 2006]
    Put perfectly.

    35/33 it is then!
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  37. cdb2
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    [Jan 13, 2006]
    I first checked my 225/45 17s after 3500 miles and all were at 23.5psi !!!

    When I checked the first tyre, I assumed it had a slow puncture and the tyre pressure warning system wasn't working! But they were all like it which points to them being set to that at the factory and not being checked at PDI.
    #36
  38. Karcsi
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    Karcsi Member

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    [Jan 13, 2006]
    I have a 2003 3-dr 2.0 TDI Sport, for reference. I set them at 32/32 because I travel a tad faster than I should, and it also protects a little against any unavoidable potholes. It's also not too uncomfortable.

    I think mine were set correctly to 30 all round on delivery. The space save spare only had 30 in it though. I pumped it up to 50 - I was too chicken to go all the way to 60. Then again, my bicycle tyres are (hand) pumped to about 100PSI, so I don't know what I was worried about.

    Mike, I wonder whether standard is higher for the 2.0TFSI as it is a more powerful car, with a much higher top speed (which is more usable, I suspect - in the right legislative environment, of course).
    #37
  39. jdp1962
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    jdp1962 Grumpy Old Moderator Staff Member Moderator

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    [Jan 13, 2006]
    [ QUOTE ]
    Put perfectly.

    [/ QUOTE ]

    Apart from the school-boy typo, that is /ubbthreads/images/graemlins/blush.gif
    #38
  40. bacardi
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    [Jan 13, 2006]
    [ QUOTE ]
    I first checked my 225/45 17s after 3500 miles and all were at 23.5psi !!!

    When I checked the first tyre, I assumed it had a slow puncture and the tyre pressure warning system wasn't working! But they were all like it which points to them being set to that at the factory and not being checked at PDI.

    [/ QUOTE ]

    This is similar to my wife's Mini, checked them once a month and they all dropped quite considerably and the tyre pressure warning thingie never said a thing, The manual or stealer (Not sure which, we got rid of it!) said it will only tell you when there is a difference between the tyres, i.e. if one was lower pressure than the others which would happen if you had a blow out.
    #39
  41. d3fy
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    d3fy Active Member

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    [Jan 13, 2006]
    Same as on the Audi it works of the ABS I think and detects a change in one tyre compared to the rest
    #40

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