Thread: Simultaneous inflation.deflation of tyres.

Originally Posted by trout1105

I personally can't see an advantage fitting one of these inflation/deflation devices because it is just something else that can fail

It's not a device, really, and you don't "fit it". It's a portable air hose that connects to two wheels at a time (same axle) with a central manifold that offers you a pressure gauge, a deflation valve and somewhere to hook up your compressor for inflation. The biggest downside is what Tombie pointed out: it's another somewhat bulky thing to carry.

I disagree because as soon as you get in, the load shifts.
As soon as the fuel starts being consumed the load shifts.
As soon as you park on cambered ground the load shifts.

I’ve run TPMS for years, and never seen a radical shift in balance as tyres are inflated.

More likely volume changes due to cooling of tyres whilst inflating the other side...

As it usually only takes 5-10 min to air up again I can't see that temperatures of the tyres will alter that much in so short a time.

Originally Posted by Tombie

I disagree because as soon as you get in, the load shifts.
As soon as the fuel starts being consumed the load shifts.
As soon as you park on cambered ground the load shifts.

I’ve run TPMS for years, and never seen a radical shift in balance as tyres are inflated.

More likely volume changes due to cooling of tyres whilst inflating the other side...

All fair comments, but I didn't say "radical" (I don't think I did... nail me if I'm mistaken about that!) and I also listed cooling as a reason for ending up with different pressures when working on one tyre at a time.

Doing both tyres on an axle simultaneously with equalised pressure eliminates two sources of difference. That's the point of these kinds of things. Whether they're worthwhile overall is pretty much an individual decision – everybody weighs up the pros and cons according to their personal priorities.

I haven't decided to get one. It's just something I've been mulling over.

Originally Posted by trout1105

As it usually only takes 5-10 min to air up again I can't see that temperatures of the tyres will alter that much in so short a time.

You’d be surprised... if you have a temp gun give it a go...

Originally Posted by trout1105

As it usually only takes 5-10 min to air up again I can't see that temperatures of the tyres will alter that much in so short a time.

Rate of temperature change is proportional to the 4th power of the temperature difference, meaning that things go from really hot to kinda hot way faster than they go from kinda hot to just warm.

Gas pressure is directly proportional to temperature, so the pressure follows the same downwards curve as temperature. Fast drop initially, slowing down as it goes.

I've got a fast compressor and it plugs straight onto the Anderson near my tow hitch. So I can be airing up the first tyre within 30 seconds of pulling over, while that tyre is still really hot. All the tyres are rapidly cooling at this point.

By the time I get to the fourth tyre, it's temperature/pressure will be a lot lower than the first tyre was when I started. I can get it up to the same pressure as made the first tyre, but by this time the first tyre will also have dropped down the steepest part of the curve and is now at a lower pressure than what I'm about to make the fourth tyre.

But hey, we all get by with a bit of experience and judgement and most of our wheels don't fall off. This is not to say there's a problem and everybody needs to start doing something different, it's just explaining why there's some physical reasons that airing two tyres simultaneously has some technical benefits. Are they worth it? Up to you.

Originally Posted by Tombie

You’d be surprised... if you have a temp gun give it a go...

I am thinking that the cooler air that is being added to the tyre when airing up would have more of an effect on the temperature/pressure of the air in the tyre than the amount of heat being radiated off the outside of the tyre.
I could be wrong ( It's not that odd an occurrence ) But that's the way I see it.

Air from a portable 4wd compressor is significantly hot...
Hence why pressure should be checked again later.