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A4 tyres



 
 
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  #41  
Old December 27th 04, 09:00 PM
SteveH
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Peter Bell > wrote:

> In message >
> Glenn Richards > wrote:
>
> > That's not a road I want to start going down. If I can fit the next size
> > up tyres on the existing rims, for another 10-15ukp per tyre it'll give
> > a small but worthwhile improvement.

>
> I guess that the question which must be asked is - what improvement do
> you expect to achieve?
>
> Altering the width of a tyre does *not* achieve more rubber on the road,
> it merely alters the shape of the 'footprint' - unless you are reducing
> the inflation pressure, but you could do that with your existing tyres.
>
> The actual area of tyre in contact with the road is a simple
> relationship -
>
> contact area = (weight carried by wheel)/(inflation pressure)


I'm obviously being a bit thick here, but could you please explain how
a, for example, a 185/60 tyre will put the same area of rubber down as a
220/45?

--
Steve H 'You're not a real petrolhead unless you've owned an Alfa Romeo'
http://www.italiancar.co.uk - Honda VFR800 - MZ ETZ300
VW Golf GL Cabrio - Alfa 75 TS - VW Passat 1.8T 20V SE - COSOC KOTL
BoTAFOT #87 - BoTAFOF #18 - MRO # - UKRMSBC #7 - Apostle #2 - YTC #
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  #42  
Old December 27th 04, 11:14 PM
BillyRay
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The formula in a prior post will give you the contact area but makes no
reference to its shape or the tire's friction force. If your tire load
remains constant (the same car) a wider tire will have "wider" shape which
leads to greater lateral resistance to shift due to its shape alone.

The following equation is perhaps more accurate:
(friction force = (coeff of fric) x (summation of tire forces (Fx, Fy, Fz)

If you want to go around curves faster on dry pavement then a wider tire is
the way to go. You should be aware, however, that widening the tires will
lower the velocity at which flotation on wet or snowy surfaces occurs.

There is also the factor of changing the stress on wheel bearings by the
alteration of the "effective" offset. The change of one size wider and one
size lower profile will probably have a small effect on bearing stress.

More importantly is to make sure that the proposed lower profile will not
allow the tire sidewall to contact and suspension or brake component as it
flexes, jounces or rebounds. There is limited information on this due to the
relative rarity of Audis in the US.

Thirty two years ago when I began racing it was much easier as there was
always someone else that had tried a combination before and, by trial and
error, the acceptable sizes became known. If you wanted to exceed those
conventions we would alter suspension components. Changing the shackles on
leaf springs or "taking a torch" to coil springs were very common ways to
effect the suspension. We knew that this would alter the driving
characteristics but.... well... we were young and stupid, and didn't care.
But then... I wasn't a middle aged man driving a $45K vehicle either.

For what it is worth I have seriously considered going up one size and down
one width for my next set of tires... In my case I would replace 215-55-16
with 235-50-16. The speedometer error introduced would be approximately
+0.2%



"SteveH" > wrote in message
...
> Peter Bell > wrote:
>
>> In message >
>> Glenn Richards > wrote:
>>
>> > That's not a road I want to start going down. If I can fit the next
>> > size
>> > up tyres on the existing rims, for another 10-15ukp per tyre it'll give
>> > a small but worthwhile improvement.

>>
>> I guess that the question which must be asked is - what improvement do
>> you expect to achieve?
>>
>> Altering the width of a tyre does *not* achieve more rubber on the road,
>> it merely alters the shape of the 'footprint' - unless you are reducing
>> the inflation pressure, but you could do that with your existing tyres.
>>
>> The actual area of tyre in contact with the road is a simple
>> relationship -
>>
>> contact area = (weight carried by wheel)/(inflation pressure)

>
> I'm obviously being a bit thick here, but could you please explain how
> a, for example, a 185/60 tyre will put the same area of rubber down as a
> 220/45?
>
> --
> Steve H 'You're not a real petrolhead unless you've owned an Alfa Romeo'
> http://www.italiancar.co.uk - Honda VFR800 - MZ ETZ300
> VW Golf GL Cabrio - Alfa 75 TS - VW Passat 1.8T 20V SE - COSOC KOTL
> BoTAFOT #87 - BoTAFOF #18 - MRO # - UKRMSBC #7 - Apostle #2 - YTC #




  #43  
Old December 27th 04, 11:36 PM
Dan Koren
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"BillyRay" > wrote in message
.. .
>
> There is limited information on this due to the
> relative rarity of Audis in the US.
>



Where in the US do you live?

At least 1/3 of the vehicles in the parking lot of the
company where I work are Audis. It is certainly the
single most popular marque around here...



dk


  #45  
Old December 28th 04, 07:30 AM
BillyRay
external usenet poster
 
Posts: n/a
Default

Mid West US.
The keyword is relative. If you stop at Tires-R-Us and ask what sizes of
tires you can safely fit in the wheel wells of a Ford or GM car you will
easily get an answer.... not so for an Audi. (at least in this area)


What kind of company do you work for and where is it located?



"Dan Koren" > wrote in message
...
>
>
> "BillyRay" > wrote in message
> .. .
>>
>> There is limited information on this due to the
>> relative rarity of Audis in the US.
>>

>
>
> Where in the US do you live?
>
> At least 1/3 of the vehicles in the parking lot of the
> company where I work are Audis. It is certainly the
> single most popular marque around here...
>
>
>
> dk
>
>




  #46  
Old December 28th 04, 06:43 PM
Jay Somerset
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On Mon, 27 Dec 2004 23:09:43 GMT, Peter Bell > wrote:

> In message >
> (SteveH) wrote:
>
> > I'm obviously being a bit thick here, but could you please explain how
> > a, for example, a 185/60 tyre will put the same area of rubber down as a
> > 220/45?

>
> One will have a long thin footprint, the other will have ashort fat
> footprint.
>
> Think about it - it's the air in the tyres which supports the car, and
> the force between the ground and the tyre provides that support. If
> there is 500lbs weight bearing on one wheel, and the air pressure in
> that tyre is 50lbs/sq.in., then it will take 10sq.in. of tyre surface to
> take the weight, therefore the area of the contact patch will be
> 10sq.in. Whether that tyre is one inch wide, or ten inches wide, the
> surface area in contact with the ground will still be 10sq.in.


This is correct theoretically, but assumes that the tire tread and sidewall
are infinitely flexible. Actually, they are not.

Lowering tire pressure will certainly increase the contact patch, but if it
is long & narrow, then the tread will buckle, raising the center line of the
tire and reducing traction. If the contact patch is short & wide, then a
given decrease in pressure will cause less tread distortion.

The net result of all this is that low-profile, wide tires can run with
lower pressures than high-profile, allowing the contact patch to be
maximized without tread distortion, and consequent loss of adhesion.
 




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