[Healeys] Tyre Pressures

[Bob Spidell]

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.
>
>
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> 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
>
>
>


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Bob Spidell           San Jose, CA            bspidell at comcast.net

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