On 16/6/19 3:46 am, Clare Snyder wrote:
On Sat, 15 Jun 2019 14:24:27 +1000, Xeno
wrote:

On 15/6/19 10:30 am, Clare Snyder wrote:


You NEVER mix types from axle to axle - for MANY reasons - one of
which is legal.

Yes, I am well aware of that but it used to be legal, within limits, and
some unknowing individuals still do it.
OI t was illegal already back in 1969

It wasn't illegal in this country back in 1969. There were specific
limits to what you could do but the only legal requirement, as I recall,
was the tyres had to be the same on the same *axle*.

I found, even though you could in that era have different tread patterns
front to rear, you needed to ensure the best *handling* tyre was at the
rear. This was not always easy to determine as I found out when I
couldn't buy tyres to match the tread pattern on a car I owned and
bought two tyres which looked very similar tread wise to the existing
pair. Both types were textile radials and I fitted the new ones to the
front. The rear tyre tread depth was very close to that of the new front
tyres. What I found however was that the new tyres were way too precise
with very low slip angles. When in even moderate cornering situations,
the front would spear into a corner with minimal understeer
characteristics whilst the rear displayed tail happy behaviour. The car
felt *pigeon toed* as if the front wheels had extreme toe in settings. I
couldn't leave it like that because my wife was the primary driver of
that car so I reversed the set and it made the car stable once more. Not
long afterwards I bought another 2 tyres and matched the existing new
ones to give much more balanced handling and used the originals on a
trailer. Whilst it appeared only to be a tread pattern difference, it
soon became obvious that handling characteristics were vastly different
between the two types and probably involved much more than the tread
pattern alone. It wasn't possible to deduce this from merely looking at
the tyres, it required a run on the road to discern the characteristics.

When I first entered the automotive industry, crossply tyres were king
and radials were just entering general use. It was the recommendation at
that time, if mixing types, to place crossplies at the front, the
radials at the rear. As steel belted radials entered the scene, it was
recommended that, if mixing the two radial types, that the steel belted
radials were fitted to the rear and textile radials to the front. The
reasoning back then was simple - cross ply tyres had the greatest slip
angle characteristics, textile radials had lower slip angle
characteristics and steel belted radials had the lowest slip angle
characteristics of all. It was always mandatory to have tyres matched on
the same axle according to roadworthy laws but the front to rear
placement was Ok if you followed the above guidelines. As I found
however, it wasn't always that simple and there were situations which
were a case of try it and see. Since that time I have always bought
tyres as a set of four at a minimum, fitted them all at the same time
and followed tyre rotation procedures.

As for the legality of different tyres front to rear, some new vehicles
would fall afoul of that since they have a larger rim size at the rear,
usually in width but occasionally in diameter. Some small vans have a
larger diameter wheel at the front, a smaller on at the rear for a
flatter cargo space but, in these cases, the rear are *duals*.

--

Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)