🚀 go-pugleaf

In article <2096@dutoit.UUCP> ssp@dutoit.UUCP writes:
>
>Can someone explain what difference wheel/tire size makes to a car's
>	
>	acceleration
>	top speed
>	economy

None, unless the overall diameter is different.  In that case your
speedometer will be off as well.

>	road noise
>	ride

Better performance often means more road noise and a rougher ride.

>	road holding
>	handling 

Here the big differences.  The improvement comes from a number of
sourrces:
	1. Wider wheels provide more support for the tire
	2. Lower tire sidewalls don't flex as much
	3. Stiffer sidewalls don't flex as much
	4. Better compounds provide better grip
	5. Aggressive tread provides better grip (not in snow!)

>Also what the different numbers mean:eg. 185/60 HR 14 80 H tires
>on 6 J x 14 wheels, alloy/steel etc.

185 / 60 H R 14
 |  |  | | |  |
 |  |  | | |  --- Rim diameter
 |  |  | | --- 'R' means radial.  Bias ply has no indication
 |  |  | --- Speed rating.  V  > 210 km/h
 |  |  |                    H <= 210 km/h
 |  |  |                    U <= 190 km/h
 |  |  |                    S <= 170 km/h (the last 2 from memory)
 |  |  --- Aspect ratio.  100 * (overall height) / (overall width)
 |  --- A slash.  Indicates that this is a tire.  :-)
 --- Overall width.  != tread width

The /60 or whatever is optional; eg. 165SR13 is valid and indicates a
165/78SR13 or 165/80SR13 tire (depends on manufacturer).

>And are there any physical limitations on the wheel/tire choice for
>a given car?

Yes.  If it doesn't fit, you can't use it.  :-)  As in, if the bolt
pattern is wrong, or the tire is too wide to clear suspension and
fenders.  You should also retain approximately equal overall diameter.
This can be calculated as follows (in millimetres):
	OD = (rim size) * 25.4 + (overall width) * (aspect ratio/100) * 2
This is only approximate, though; the actual aspect ratio of a given
tire will vary with the rim width (wider rims will 'spread' the tire
and give a lower aspect ratio, lower OD, and better handling).  An
example of various tires with same approximate OD:
	175/70SR13 on 5.5J13
	205/60HR13 on 6J13
	185/60HR14 on 6J14 (known as 'Plus 1')
	195/50VR15 on 6J15 (known as 'Plus 2')
As you go down the list, both price and performance increase...


				   \tom haapanen
				   watmath!watrose!haapanen
I'm all lost in the Supermarket
I can no longer shop happily
I came in here for that special offer
Guaranteed personality				 (c) The Clash, 1979

> >Also what the different numbers mean:eg. 185/60 HR 14 80 H tires
> >on 6 J x 14 wheels, alloy/steel etc.
> 
> 185 / 60 H R 14
>  |  |  | | |  |
>  |  |  | | |  --- Rim diameter
>  |  |  | | --- 'R' means radial.  Bias ply has no indication
>  |  |  | --- Speed rating.  V  > 210 km/h
>  |  |  |                    H <= 210 km/h
>  |  |  |                    U <= 190 km/h
>  |  |  |                    S <= 170 km/h (the last 2 from memory)
>  |  |  --- Aspect ratio.  100 * (overall height) / (overall width)
>  |  --- A slash.  Indicates that this is a tire.  :-)
>  --- Overall width.  != tread width

[]
   In this example, the 185 is the aspect ratio, the 60 is the tire 
width, and the H is a designation of the tire sidewall height...I
have a set of sport radials that are supposed to have a speed rating
of 180 *mph* (they are Swedish tires too) and there is no other
leter before the R that means radial...

-- 
     Adrian Zannin
     SUNY at Buffalo Computer Science

..{bbncca,decvax,dual,rocksvax,watmath,sbcs}!sunybcs!ugzannin
CSNET:    ugzannin@Buffalo.CSNET
ARPANET:  ugzannin%Buffalo@csnet-relay.ARPA
BITNET:   ugzannin@sunybcs.BITNET

In article <2924@sunybcs.UUCP> ugzannin@sunybcs.UUCP (Adrian Zannin) writes:
>> >Also what the different numbers mean:eg. 185/60 HR 14 80 H tires
>> >on 6 J x 14 wheels, alloy/steel etc.
>> 
>> 185 / 60 H R 14
>>  |  |  | | |  |
>>  |  |  | | |  --- Rim diameter
>>  |  |  | | --- 'R' means radial.  Bias ply has no indication
>>  |  |  | --- Speed rating.  V  > 210 km/h
>>  |  |  |                    H <= 210 km/h
>>  |  |  |                    U <= 190 km/h
>>  |  |  |                    S <= 170 km/h (the last 2 from memory)
>>  |  |  --- Aspect ratio.  100 * (overall height) / (overall width)
>>  |  --- A slash.  Indicates that this is a tire.  :-)
>>  --- Overall width.  != tread width
>
>   In this example, the 185 is the aspect ratio, the 60 is the tire 
>width, and the H is a designation of the tire sidewall height...I
>have a set of sport radials that are supposed to have a speed rating
>of 180 *mph* (they are Swedish tires too) and there is no other
>leter before the R that means radial...
>
>     Adrian Zannin

NO, the first message is correct.
Jan

In article <2924@sunybcs.UUCP> ugzannin@sunybcs.UUCP (Adrian Zannin) writes:
>> 
>> 185 / 60 H R 14
>>  |  |  | | |  |
>>  |  |  | | |  --- Rim diameter
>>  |  |  | | --- 'R' means radial.  Bias ply has no indication
>>  |  |  | --- Speed rating.  V  > 210 km/h
>>  |  |  |                    H <= 210 km/h
>>  |  |  |                    U <= 190 km/h
>>  |  |  |                    S <= 170 km/h (the last 2 from memory)
>>  |  |  --- Aspect ratio.  100 * (overall height) / (overall width)
>>  |  --- A slash.  Indicates that this is a tire.  :-)
>>  --- Overall width.  != tread width
>
>[]
>   In this example, the 185 is the aspect ratio, the 60 is the tire 
>width, and the H is a designation of the tire sidewall height...I
>have a set of sport radials that are supposed to have a speed rating
>of 180 *mph* (they are Swedish tires too) and there is no other
>leter before the R that means radial...
>
>     Adrian Zannin
>     SUNY at Buffalo Computer Science
>

Mr Zannin is totally wrong, and the '>>' poster is mostly correct. I just
wanted to squash this before somebody goes out and buys the wrong tyres...

Some corrections:

Speed rating in imperial measurements:
	'V': >130mph 'H': <= 130mph 'S': <= 113mph '': <= 90 mph
where '' means no letter. Speed ratings in this form are only
valid for radial tyres; I forget what the crossply tyre rating
system is. I never heard of a 'U' rating, but see below.

The maximum speeds are for "sustained high-speed running". The
ratings are really a form of temperature rating, although most of the
good tyre manufacturers build tyres which if they have good speed
ratings generally do most other things well, like acceleration,
braking, handling, &c.

If the aspect ratio of a tyre is not given it defaults to 85.

Rim diameter (actually the nominal diameter of the rim the tyre is
supposed to fit) is measured in inches, while the overall width is
measured in millimeters, even for British tyres.  However, some newer
tyres (Michelin TRX for example), are designed for cars which have
metric rims and then the rim diameter is given in mm as well.
Typically this is ~25 times any reasonable inch measurement.

Some further information:

This system outlined above is changing, in two ways. One change is
that the speed rating is given *last*, so that the example above
would be 185/60R14H. I think that this answers Mr Zannin's question.
The second way is that there is a new speed rating system which has
more bands, and which uses different letters (this may be the 'U'
rating mentioned above). I don't know what these are of course.
Adherence to either of these changes is spotty at best.

The 'V' rating, although just defined as >130mph, in fact only goes
up to ~150mph. (I do not remember the absolute limit - it could be
10mph more - or less).  Tyres rated above this are built specifically
for the car, so that (for example) Pirelli makes a P7 in size X for
the back of a Porsche 911, and a different P7 *in the same size X*
for the front of a Lamborghini Countach (whatever the makes are). No
standard tyre is rated at 180mph. Of course if you have a set of race
tyres then this doesn't apply, but then you can't use them on the
street (at least if you obey the law).

Certainly if you are planning to drive fast (let's says regularly
over 75mph) then you need tyre advice from a professional. If you
throw on a set of Sears Worst covers and then go beat up the nearest
stretch of road, then you are a menace to everybody else on the road
even if you do have the ability to be Jacky Ickx Jr. A serious race
driver regularly chooses between >twenty slightly different sets of
tyres for every race. You only get to choose once - make sure it's a
good selection.

-- 
Jonathan Clark
[NAC]!mtune!jhc

My walk has become rather more silly lately.

> I have a set of sport radials that are supposed to have a speed rating
> of 180 *mph* (they are Swedish tires too) and there is no other
> leter before the R that means radial..

  Get out of here! Unless you own a Countach or a Testarossa with
  special Pirelli P7's (normal P7 are only worth 150 mph) your
  tires are NOT rated for 180 *mph*. A speed rating of 180 kph 
  (~110 mph) is an SR rated tire, which is quite possible.
  
						    Tim Takahashi