Good question (one I've pondered for a while).
First thing is, are your sure your pressure gauge is accurate? In order to use
absolute pressure values, you have to
have the gauge calibrated. I have an expensive dial gauge which is very
sensitive and has high repeatability, but I
believe it reads approx. 3psi low at around 30psi (but I've never had it
calibrated). The cheap 'slider' type gauges
seem to be more accurate but less sensitive.
Also, ambient temperature affects measured pressure greatly--about 0.75psi per
10degF (by my observation only--I haven't
checked a chemistry book lately). So, I would ask, at what temperature are you
taking your readings?
Assuming an accurate gauge, the desirable pressure is (usually) determined by
vehicle weight; i.e. heavier cars need
more pressure in the tyres. Manufacturers' recommended pressures are based
somewhat on weight, and some by mileage
and noise considerations (generally, I've found that recommended pressures are
2-3psi higher than the pressure that
gives best handling and wear characteristics).
IMO, the 'best' pressure is the one that produces the largest sideways contact
patch (low pressures will produce higher
longitudinal patch, but may be too low for other reasons). The front/rear
differential, if any, can affect
over/understeer, and is both personal preference and road/track requirements.
I watch the tread for wear patterns; if
the tyres appear to be wearing more in the center, I reduce pressure a little,
and vice versa if they are wearing more
on the edges.
I've found, overall, that about 30 rear and 31-32 front--allowing for my gauges
inaccuracy--gives best handling and wear
for my Mustang (recommended is 35psi). I use about the same or slightly lower
for my BJ8, depending on how loaded-up
the car is (passenger, baggage, etc.). Both these cars understeer a little;
the front/rear differential seems to help
offset this a bit.
I think your pressures are a bit low. Running too low can cause excessive heat
buildup, possibly contributing to
failure of the tyre. Also, your mileage is probably lower than you could get.
The racers probably know this topic better than anybody; hopefully, they'll
weigh in.
Bob
On 8/6/2011 4:59 AM, Tim Ward wrote:
> Hi List
>
>
>
> Please give me the benefit of your collectives wisdom again.
>
>
>
> On my BJ8 I have Fulda Y2000 Tyres. They are 185/70R15 89V.
>
>
>
> Any views as to the idea pressure to set these tyres for speeds up to 80mph.
> Currently they are 22 psi at the front and 20.5 psi at the back.
>
>
>
> Your views will be gratefully received.
>
>
>
> Thanks
>
>
>
> Tim
>
>
>
>
>
> Tim Ward
>
> Warwick House
>
> 12 Mill Road
>
> Kislingbury
>
> NN7 4BB
>
> Tel: 07855 388 751
>
>
>
--
*******************************************************************
Bob Spidell San Jose, CA bspidell@comcast.net
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