Tyre pressure for touring?

[Limitless]

I usually run at 18psi all round but wondered what pressure people run at when carrying more weight ie two people and luggage? Avon motorsport have suggested 25psi all round but thought that might be a tad high.

420r shod with zzs's 185 front & 215 rear. 

 

[SM25T]

I stick with 18psi for all conditions. We tour for 2 weeks at a time, two up with luggage and tools.

[revilla]

I have also found 18psi to be fine for touring. Mine's on 15" Toyo T1Rs though. As I think is Ian's?

[John Vine]

IMO, 25 psi is definitely too high for such a light car.  I have 195/R50-15 ZZSs on my R400D and run 18psi all round in all conditions.  But practically all my mileage is solo.

JV

[AdamQ]

This is something that has always puzzled me.  You see in tintop handbooks tyre pressures for normal and heavily laden conditions, say 22 and 27 psi or something.

I'd always assumed that it was the extra load in the car that caused the increase in the tyre pressure, NOT that one was expected to add air to the tyres when carrying a lot of extra weight.  However, the general consensus seems to be that you ARE expected to pump the tyres up when loading the car heavily.

I can see arguments for both opinions - stick an elephant in the back of a Seven, and I'm sure the tyre pressures will increase, without any air being added to the tyres - stand on a balloon and the pressure in the balloon goes up.  PV = nRT rings a very vague bell as something possibly relevant to this, but that's as far as I can go.

I suspect my take is wrong, but I would be interested in the take of others on this.

[Jonathan Kay]

However, the general consensus seems to be that you ARE expected to pump the tyres up when loading the car heavily.

Yes. That's the recommendation for ordinary vehicles.

Jonathan

[Limitless]

The thing is, I phoned Avon motorsport last year for the same reason and they told me a lower pressure than this year, something like 19 rear and 21 front. Hence the phone call this time as I'd forgotten. I might go 20psi all round as I have around 35 kilos of luggage (camping).

[Wrightpayne]

I was going to say add a couple of PSI.

My Skoda Superb rears go from 2.3 bar normal to 3.0 fully loaded which is a 10 PSI increase!

[Alastair B]

In response to AdamQ...."stick an elephant in the back of a Seven, and I'm sure the tyre pressures will increase"

That is not correct - pressure will not change, pressure is affected by Volume (V) and Temperature (T)....as in the Ideal Gas equation you quote (n- number of Moles and R - gas constant). You might deform the tyre forcing it off the rim, but it would let go through the seal breaking not a pressure overload.

[revilla]

I disagree.

Pressure will change.

Pressure does depend on volume and temperature, neither of which will remain constant. Sit on a balloon and it will burst. Sit on one end of along thin balloon and it may well burst at the other end, nothing to do with the distorted shape but the increase in pressure inside the whole balloon. I'm not saying your tyre will burst, just illustrating the point that the pressure will increase. Place weight on suddenly and there will be an element of adiabatic compression so the temperature will rise too (although not by much, and not for long, and in all practial senses completely irrelevant here).

The point is that the gas laws you quote mean that this increase in pressure comes at the expense of a decrease in volume, and more to the point a change in shape. The tyre distorts. Adding more air raising the pressure still further will tend to restore the shape of the tyre, reducing the distortion.

So yes, adding weight will naturally increase pressure but paradoxically you then need to add more air and more pressure to correct the situation (restore the tyre to its intended operation shape).

[Jonathan Kay]

SNAP!

The model in #9 makes assumptions about how near the behaviour is to that of an ideal gas and about the compliance of the containing vessel.

The former probably isn't too far off but I have no idea about the latter in practice.

Anyone fancy doing the experiment? The adiabatic heating might be a bit difficult, but otherwise it would only take a decent pressure gauge and some convenient mass to measure the effect. Three sentient life forms should do.

Jonathan

PS: I think that there's a previous discussion somewhere...

[David White]

The handbook for my (new) 310S says 23psi all round for Avon ZZS 185's.  Interestingly it came back from the PBC/IVA at 32!

[revilla]

I really wouldn't worry about adiabatic heating I was just being pedantic. It will be immeasurably small and only really something to worry about under rapidly changing conditions; I do wonder though to what extent inflating a tyre rapidly on an airline leaves the within the tyre at a temperature different to ambient and therefore whether the pressure may then "settle" afterwards as the temperature comes to equilibrium. Again probably not a big enough effect to worry about in practice.

Just out of interest, where adiabatic heating effects are very measurable in our cars is when doing a compression test on the engine. My engine has a nominal compression ratio of 10.5:1 so by simple theory should produce a peak pressure of 10.5 * 14.7psi in the cylinder at best, which would read as 9.5 * 14.7psi = 140psi on a gauge (as the gauge reads zero at atmospheric pressure). But because it is compressed rapidly the work done on the gas heats it further raising the temperature and therefore the pressure actually registers closer to 200psi. Compressing a gas more slowly allows time for the temperature to come to equilibrium so the constant temperature assumption applies.

[Jonathan Kay]

I do wonder though to what extent inflating a tyre rapidly on an airline leaves the within the tyre at a temperature different to ambient and therefore whether the pressure may then "settle" afterwards as the temperature comes to equilibrium.

What sort of pressure gauge could you connect to your oscilloscope?

Jonathan

[revilla]

You know I would happily spend all day doing such investigations on my car, but my boss and my wife seem to have other plans.

[Jonathan Kay]

:-)

Jonathan

[aerobod - near CYYC]

As a rule of thumb, one minute per PSI of pressure added is an advisable delay to allow the pressure to equalise and then re-adjust, after adding air.

[Alastair B]

interesting discussion. In large overload situations like the elephant the tyre distortion could result in a small volume reduction with the tyre distortion experienced contained to a degree with the sidewall stiffness, and a corresponding small pressure increase and Revilla in this regard you are correct.

I think in a regular road tyre situation the temperature effect is more powerful and for the size of load increase for the capacity available in a touring scenario the likely pressure increase would be quite small and possibly negligible, .....the test as JK suggests on a 7 would be great to know about.

Meanwhile a video demo here of this very principle on a

[Jonathan Kay]

Meanwhile a video demo here of this very principle on a 

Thanks.

It would have been even better to measure the pressure again after unloading the wheel... following the on effect with the off effect enormously improves the evidence. (See also speed limits in France.)

Jonathan

Updated in #24 below.

[anthonym]

I use 1.2 bar (18psi as I recall) and carry tons of stuff. When I am paying attention I check by altitude because it makes a big difference (doesn’t matter what as my base is 1.2 bar cold). I also use a Longacre pyrometer to control (check) what is really going on. Ambient temperature also impacts as does tarmac temp. However, roughly it’s about being neither under or over inflated and how to figure out what that is on any given day.

and avoid checking when one side is parked in the sun...

Simple really ;-)

anthony

[Limitless]

Wow, this got technical. So basically, I would imagine that it's all about keeping the footprint/contact patch the same? 

Anywho, off to Switzerland and the Alps tomorrow for three weeks!! Thanks for all your input guys. 

[revilla]

I think the simplest answer is just to go with experience. A lot of people seem have gone for 18psi and it has worked for them. Why not just give it a try? It's not going to be far wrong and if it feels wrong on the road you can always add a bit at a petrol station. You don't need to set a pressure now that's fixed for the whole trip.

[anthonym]

I'm in Switzerland in the Alps... call in :-)

[Jonathan Kay]

I think the simplest answer is just to go with experience. A lot of people seem have gone for 18psi and it has worked for them. Why not just give it a try? It's not going to be far wrong and if it feels wrong on the road you can always add a bit at a petrol station. You don't need to set a pressure now that's fixed for the whole trip.

Exactly. But you can beef that up enormously by blinding yourself to the intervention.:

1. Start with something sensible.
2. In advance define the interventions, which will probably be increases in pressure in this case.
3. Label the interventions with something that doesn't disclose what they are.
4. Whenever you want to change pick one at random and get someone else to apply it.
5. Drive for a bit and record the effects, perhaps as Ride, Handling, Other preferably with predefined numerical or categorical scores.
6. When you've finished break the code and relate the effects to what the pressures actually were.

Not too hard with a passenger, but could be interesting with garage attendants or convenient strangers performing 4!

...

Revised version of #19:

Meanwhile a video demo here of this very principle on a 

Thanks.

Good to see repeated measures being made. That helps in distinguishing a genuine effect of the intervention from the background variability. (See also changes to cooling systems, dynamometer plots after tuning etc.) 

It would have been even better to measure the pressure again after unloading the wheel... following the on effect with the off effect enormously improves the evidence. (See also changes to cooling systems, speed limits in France, etc.)

...

Blind testing, with some possibilities for when the driving is over for the day. :-)

Jonathan

[Limitless]

Thank Anthony, will try too.