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Digital speed in DIS

Discussion in 'A3/S3/Sportback (8P Chassis)' started by navnayyar, Sep 22, 2009.

  1. NHN
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    Yes they do Jordanian

    Also the speed sensors are actually the abs sensors IIRC, which are of course on the hubs themselves & you have to actually also set the size of wheels on the car for brakes & its possible this also affects the speedo, only possible mind you but given abs seems to control a fair bit like speed sensing, braking systems & the tpms, it wouldnt be impossible for this to also be linked to what we're discussing here.

    Question is what wheels were on originally, which I would guess a 17 inch & you now have 18 inch but no change in coding to the larger wheels, but on some cars there isnt an option to set 18inch, there seems to be a possible requirement to put a bridge in wiring on the abs controller to be able to set a larger wheel fitted, as I have tried the coding on mine, given I have the larger brakes & wheels & it wont accept my coding as yet, so I will be looking at the bridge method 1st then test coding again.

    Will report back when I check it & for the record I have a mk60 abs controller, maybe with vagcom you can check abs coding & see if yours gives you an option for 18inch.

    This may or may not be relevant but something to bare in mind huh.
    #41
  2. VWAddict
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    VWAddict Member

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    Ummm....

    Has everybody forgotten about the TYRES?

    All this talk about wheel size determining the speedo is 100% complete and utter, unadulterated, vacuum-sealed, glow-in-the-dark bullcrap.

    It's the tyre's rolling circumference which affects the speedo. -You can increase OR DECREASE tyre size with abzsolutley ZERO influence on speedometer or odometer, so long as you decrease or increase (respectively) the tyre's wall-height to maintain the same (tyre + wheel) radius and diameter.

    Wheel rim size in and of itself has ZERO influence on speedometer reading.

    Tyre wear has an influence: as tyre tread depth wears down by approx. 9mm from new to replacement-time, the overall effective diameter therefore reduces by 18mm. Since effective circumference is pi x the effective diameter, the circumference will be reduced in the order of 57mm from new tyres to old tyres.

    Vehicle manufacturers are forbidden under British law from selling a vehicle with a speedometer which UNDER-displays the speed. This is perfectly reasonable, since if you were caught speeding and your speedometer told you that you were NOT speeding, it would be entirely the manufacturer's fault.

    Therefore, all manufacturers make their speedometers VERY SLIGHTLY over-read, when fitted with with factory-recommended tyres. As the tyre wears, the tyres rotate FASTER when travelling the same actual speed, because the effective circumference is reduced, so it takes more rotations to cover the same distance... and as a result, the speedometer over-reads by an even GREATER amount.

    So if you've changed wheels and tyres from factory, it's NOT Audi's fault if your car speedometer can't magically determine what speed you're traveling at.

    But I'm sick of hearing that wheel size proportionally affects speedometer readings... it DOESN'T.

    For vehicles where the factory offers a CHOICE of factory-supplied wheel sizes, SOME OF WHICH RESULT IN DIFFERENT OVERALL CIRCUMFERENCE FIGURES, the manufacturer may include the means to select between different 'divide by' factors, so that the speedometer can stay fairly close. -But if YOUR 18" rim + tyre combination results in a different circumferential number to the factory's combination, the simple fact that they both use 18" bead-diameter is utterly irrelevant.

    Keith
    #42
  3. Shades
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    Shades Member

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    Before VWAddict went all mathematical fantastical on us and started talking about food ("pi", geddit? Thank you I'm here all week, try the veal) what I think he means is something like:

    [​IMG]

    • Any combination above the original size of your rims/tyres combo will result in a higher than indicated speed.
    • Any combination the same as the original size of your rims/tyres combo will result in no change to your actual speed.
    • Any combination below the original size of your rims/tyres combo will result in a lower than indicated speed.
    Last edited: Oct 16, 2009
    #43
  4. VWAddict
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    VWAddict Member

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    Yes... -SORT OF.

    that chart is actually wrong. It's subtracting one inch of sidewall height for every increase in rim diameter.

    In fact it should subtract a HALF-INCH of sidewall height for each additional inch of rim diameter, because a HALF-inch of sidewall height equals a FULL inch of additional diameter, since it's counted on BOTH sides of the rim...

    But apart from that, yes. that's what I'm talking about.

    Keith
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  5. NHN
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    Did I or did I not say "This may or may not be relevant, but something to bare in mind" & I dont recall saying it was audi's fault at any point.

    Was merely pointing out something that may or may not be relevant to the systems overall when fitting bigger wheels/tyres.

    Also I wont disagree with what you're saying Keith & no I didnt forget tyres, I naturally would expect people to include tyres when we're discussing change of wheels to larger sizes, unless they want to try there luck with stretching there old ones, lol, I dont do euro look thanks.

    Also to me, yes if we're talking solely wheel size then yes it wouldnt affect, but wheels need tyres unless I'm mistaken & I think you would agree, fair number of the aftermarket wheel/tyre upgrades probably dont match the original oem spec, so affect the thing in discussion, makes my comments about wheel sizes relevant & most people dont checks these things, they really dont, its just "oh it looks great, cool I'll have them"

    Anyway its not an argument, just a discussion, your point is correct if in the right context, if wheels were the sole item involved, buts it clearly isnt is it, tyres as you have said do make a difference & common sense would prevail in this discussion these were included aswell, maybe I should of said tyres just to please everyone.

    Anyway to see if the coding affects this whole point of discussion, I guess I need to test the speedo with another speed device before & after changing the sizes in the abs controller & see if it makes any differences if you get me.
    #45
  6. Shades
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    Shades Member

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    Doh, of course! :keule:

    I've updated the table above, is this now correct?
    #46
  7. VWAddict
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    No worries Nigel, I wasn't having a go at anyone in particular... just wanted to get the point made that the wheel size per se has no bearing on speedo readings. There seems to be a number of people "on the internets" in general, who overlook this and go with the simplistic assumption that wheel size (rim diameter) affects speedo reading... it doesn't, plain and simple. Tire circumference is the ONLY thing which has an effect.

    I meant people 'in general' associate wheel rim size with speedometer accuracy and CERTAINLY not yourself in particular... -you're much too smart a cookie! ;)

    To make things even MORE complicated, setion height is quoted in metric terms, whereas bead diameter (and therefore rim diameter) is in imperial terms...

    So... if anyone still reading this wants to calculate their overall circumference, the formula is as follows:

    Metric (in mm)
    (2 x [Section width x (section height ÷ 100)]) + (Rim size in inches x 25.4) x 3.1415926535....

    So for a 225/45/17 tyre, the section width is 225, the section height is 45, the rim size is 17.

    Brackets first: 2 x [Section width x (section height ÷ 100)]... with the numbers inserted becomes 2 x [225 x (45/100)] which is 2 x [225 x 0.45], or 2 x 101.25, which is 202.5.

    202.5 + (17 x 25.4) is 202.5 + 431.8 which is 634.3 (which is the effective diameter in mm).

    Multiply the effective diameter by pi (3.1415926535....etc) and you get the effective circumference, which is 1992.7mm.

    Imperial(in inches)
    (2 x [(Section width ÷ 25.4) x (section height ÷ 100)]) + Rim size in inches) x 3.1415926535....

    For the same 225/45/17 tyre

    Brackets first: 2 x [(Section width ÷ 25.4) x (section height ÷ 100)])... with the numbers inserted becomes 2 x [(225/25.4) x (45/100)] which is 2 x [8.85 x 0.45], or 2 x 3.9825, which is 7.965.

    7.965 + 17 is 24.965 (which is the effective diameter in inches)

    Multiply the effective diameter by pi (3.1415926535....etc) and you get the effective circumference, which is 78.43 inches.

    So if anyone inserts their tyre numbers into that set (which I used as an example because it's the factory numbers which came with my particular A3) then they can see what their effective circumference is, and see how far off their car is likely to be.

    Please note that you CANNOT get an accurate effective circumference measurement by simply removing a wheel and using your mum's dressmaking tape measure to measure around the circumference... the ACTUAL sircumference of an inflated -or UNinflated- tyre is NOT its effective circumference.

    The effective circumference will be LOWER, because the ACTUIVE RADIUS is less than the actual radius. -To SEE this in action, you can look at a parked car, and look at the amount of 'squish' of the tyre section at the lowest point, where the vehicle weight compresses the tyre, compared to the top of the tyre, where 'un-squashed' tyre section means the radius measured from the highest point of the tyre to the dead-centre of the hub will be [/u]larger of the same measurement from the LOWEST point of the squashed section to the centre of the hub...

    Anyhow, that's all detail. Suffice to say that if you maintain the same effective circumference, there will be no difference in speedometer/odometer readings, and the forumlae above give anyone who wants to the ability to compare factory and 'upgraded' circumferences.

    Keith
    #47
  8. NHN
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    Keith man, its friday, couldnt you have added that formula at least at beginning of the week mate, lmfao

    Anyway no worries all good here & as for smart cookie, hey I'm as smart as anyone who makes an effort to learn, you certainly seem very knowledgable & there are smarter people than me on this forum I can assure you, maybe I just make a little more effort than most to actually know what I'm talking about, maybe not, lol, but one tries ones best, anyway learning can be fun, kudos to you & all that try the impossible aswell, for me, nothings impossible, it just takes time & money, time I have & money I can earn anyway as the next person huh.

    What you doing stateside anyway, vocation wise?

    Kop or Everton?
    #48
  9. VWAddict
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    This picture contains both answers... ;)

    [​IMG]

    Definitely Kopite, although I've been to more Everton matches in the last decade then Liverpool games! (All my mates seem to be Evertonians), and whenever I go over there it ALWAYS seems to be that the 'Pool aren't playing at home... in fact the same thing is happening THIS year -again- -I'll be there next month, right when Liverpool take a 2-week break from playing at home! :keule: -Looks like I'll be going to watch Everton with my mates again, since ANY decent footy game beats what they have on offer for sports over here. :footy:

    As for work, 'vocation' is indeed the perfect word. I'm head of the technical engineering support at a large technical university here in tourist-town. I work mainly with British manufacturers though, so I get Hobnobs and PG tips smuggled inside the crates which bring the heavy stuff over for installation. (-And don't think for a moment that I'm kidding, either! :yum:)

    If they could only learn to make drinkable beer over here, and drive cars round corners, I reckon they might have the makings of a decent civilisation! :beerchug:

    Keef
    #49
  10. beanoir
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    beanoir Sharpest Tongue in the West

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    you're making the assumption that everybody in life puts the right size tyres on their new bigger wheels, i've heard of people sticking A6/A8 wheels on A3s but using the tyres that came off the bigger car, quite often the overall diameter will be bigger and will affect the speedo reading, not to mention fouling the arches, steering etc....but people, normally idiots granted, do do this! :wacko:
    #50
  11. VWAddict
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    ...Plus...

    I'm a grumpy old ba$tadh! ;)

    :motz:

    Keef
    #51
  12. beanoir
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    beanoir Sharpest Tongue in the West

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    it's the only way to be mate! comes with age i'm noticing, so my misses tells me anyway!

    And it must be age if I find the idea of putting bigger wheels on a car silly now days..."the car was designed with 18" wheels, so thats what will always work best on it"...god I sound like my dad more every day!
    #52

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