Re: How safe are our tyres?

[Richard Beels <beels@technologist.com>]

The Speed Rating for tires describes the top speed for which a tire is
certified.  It does not indicate the total performance capability of a
tire.  In the old system, V was the highest speed rating.  Eventually, many
tires were produced with an "Unlimited V" speed rating, so V was capped at
149 and Z was the unlimited rating.  But, tire technology increased again
and W & Y were added as higher speed categories. New tires now use a
different system than was used in the recent past.  Also, note that
retreading a tire invalidates the original speed rating.  The new ratings are:
	V - 149 mph
	W - 169 mph
	Y - 186 mph
BTW, an H rating is for 112 mph.  The ZR rating refers to tires above the V
rating.  The speed rating is molded on the tire with the load index*
(together these represent the service index of a tire) after the size.  ZR
tires will have a W or Y in the service index and if it's a W or Y tire,
the ZR _may_ appear in the size.  If the max is over 186mph, the ZR _must_
appear in the size.  In these cases, you should consult the tire
manufacturer for max speed info.
	* Load Index: Number that ranges from 0 to 279 that corresponds with the
load-carrying capacity of a tire.  It's not a linear scale and I don't know
how it was derived.  Common Audi-specified values are:
	85 = 1135 lbs
	88 = 1235 lbs
	91 = 1356 lbs

Now remember, that for high-speed driving, you may need to raise the
pressure, lower the load or Plus-1/2/3 the tire.  The European Tyre and Rim
Technical Organization (ETRTO) recommends the following:
	Speed Rating up to V
	Top Speed	Pressure Increase	Tire Load
				Bar		PSI		Capacity
	100		0		0		100%
	106		.06		1		100%
	112		.12		2		100%
	118		.18		3		100%
	124		.24		4		100%
	130		.30		5		100%
	136		.30		5		97%
	143		.30		5		94%
	149		.30		5		91%

	Speed Rating of ZR: W and Y
	Top Speed	Pressure Increase	TL Cap	TL Cap	
				Bar		PSI		"W"		"Y"
	118		0		0		100%	100%
	124		.1		1.5		100%	100%
	130		.2		3.0		100%	100%
	136		.3		4.4		100%	100%
	143		.4		6.0		100%	100%
	149		.5		7.5		100%	100%
	155		.5		7.5		95%		100%
	161		.5		7.5		90%		100%
	168		.5		7.5		85%		100%
	174		.5		7.5		--		95%
	180		.5		7.5		--		90%
	186		.5		7.5		--		85%

So, the short answer is "No, not if you have properly rated, inflated and
loaded tires in good condition".

At 05:01 PM 12/5/1998 ,  Philip Ross was inspired to say:
>  A report in Top Gear magazine tells about two people being killed on
>  the Silver State Classic race because they used “road tyres” which
>  blew out at 170mph on a Testarossa, a Mustang blowing a tyre at 170
>  and a Pantera blowing a tyre after hitting a kink in the asphalt at
>  165.  This left me wondering how safe our “road tyres” are.  Are the Z
>  rated tyres on various Audis really safe or are they liable to pop at
>  150?



Cheers,
	Richard
	88 90Q "Hannu" - K+N, new vac hoses, still 0.0 bar....
	88 Golf GTi - PRO Rally