Are you setting the right tyre pressure

sgbman

Hello, I have 2 track pumps, a SKS and JoBlow2.
These both start at 0 bars, and when I want to inflate to 7 bar, i pump up to the 6 on the gauge.
The reason being that normal air pressure is 1 bar and if I want 7 bars I need to increase pressure by a further 6 bars(ie go from 0 to 6 on the gauge).
If this sounds wrong, try this- put your finger over the end and inflate to the one, and you can feel that the pressure build-up is higher than normal air pressure(ie feels like double)

Please could people tell me if my reasoning is correct.

diamonddog

You are wrong, in your example the pressure in your tyres should be 7 bar gauge pressure not atmospheric pressure.

gozzy

(Assuming you have a car) Do you inflate your car tyre tyres to 1 bar less than the recommended pressure too?

Ben6899

Manc33, is that you?

rafletcher

sgbman wrote:

Hello, I have 2 track pumps, a SKS and JoBlow2.
These both start at 0 bars, and when I want to inflate to 7 bar, i pump up to the 6 on the gauge.
The reason being that normal air pressure is 1 bar and if I want 7 bars I need to increase pressure by a further 6 bars(ie go from 0 to 6 on the gauge).
If this sounds wrong, try this- put your finger over the end and inflate to the one, and you can feel that the pressure build-up is higher than normal air pressure(ie feels like double)

Please could people tell me if my reasoning is correct.

OK, you're wrong. Happy now? :roll:

sgbman

1 bar =0.986923267 atmosphere, so roughly the same.

Off wiki:
Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."

So I am wrong, it seems that 7 bar pressure is the same as 8 bar absolute. This is never stated in the manuals. :roll:

earthvspace4XTuUWcX

sgbman wrote:

1 bar =0.986923267 atmosphere, so roughly the same.

Off wiki:
Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."

So I am wrong, it seems that 7 bar pressure is the same as 8 bar absolute. This is never stated in the manuals. :roll:

I think because most people would naturally assume 7 bar means 7 bar above ambient air pressure

diamonddog

OP it's pumping tyres not scuba diving so not really that important. If you want to stick to your theory then go ahead it probably won't be life threatening. :evil:

desweller

sgbman wrote:

1 bar =0.986923267 atmosphere, so roughly the same.

Off wiki:
Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word "vacuum" or the gauge may be labeled a "vacuum gauge."

So I am wrong, it seems that 7 bar pressure is the same as 8 bar absolute. This is never stated in the manuals. :roll:

You could have figured it out by dint of the fact that the gauge reads zero when it's at atmospheric.

Matthewfalle

But how accurate is the gauge in the first place? Is 0 really 0?

desweller

Oh god no they're crap ha ha ha. 6 probably ain't 6, never mind 0 being 0.

sungod

it's a conspiracy!

team47b

I live virtually ( ) at sea level should I put more air or less in my tyres as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation?

Bozman

Use your thumb to gauge the tyre pressure.

Matthewfalle

Does this mean that if I pump up my tyres at my house (sea level) and ride up the nearest really, really, big hill (so now not sea level) I have to let out air/pump up my tyres more to allow for pressure changes?

Does this also mean that I have to let out air of my tyres as they get hotter as I ride?

southdownswolf

Is a reading of 6 on the SKS exactly the same as a reading on the JoBlow2 ?

sgbman

Yes 6 on both.
Tyres always feel rockhard on 6, I weigh 75kg, and use 23mm tyres.

veronese68

Matthewfalle wrote:

Does this mean that if I pump up my tyres at my house (sea level) and ride up the nearest really, really, big hill (so now not sea level) I have to let out air/pump up my tyres more to allow for pressure changes?

Does this also mean that I have to let out air of my tyres as they get hotter as I ride?

Give yourself a slow puncture at the bottom of a big climb. As you climb you will lose some air allowing you to maintain a constant pressure. On the descent you need to drag your brakes sufficiently to build up the heat required to maintain air pressure. Obviously this technique stops tyres blowing off rims through overheating as well. It's a win/win situation.
There is of course an exception to this. If you have disc brakes you will die to death as you will not be able to maintain sufficient pressure through heat build up so will crash. At which point the red hot disc will cut your head off.

ElectronShepherd

Good grief there's some right nonsense written on internet forums!

What about the effects of the sun? You've completely ignored those! If you are riding in bright sunshine, the effect will be to heat the tyre, and so heat the inner tube, and eventually heat the air inside as well. This will, of course, affect the pressure. To be on the safe side, it's best to make sure that you ask about the thermal transfer coefficients of both the tread and the sidewall in your LBS when buying tyres, so that you can best determine how much extra heat will be transferred through to the air inside.

I avoid the problem by the simple means of painting my tyres white, and making sure I only ride on cloudy days.

Matthewfalle

If I pull loads of wheelies will this mean my front tyre is running at a lower pressure than my rear tyre as it's less in contact with the road and getting more air cooling?

shmooster

Pump them up to 11

veronese68

Matthewfalle wrote:

If I pull loads of wheelies will this mean my front tyre is running at a lower pressure than my rear tyre as it's less in contact with the road and getting more air cooling?

Yes, of course, but this is a good thing because the slight drop in pressure after a short wheelie means you get a slightly more cushioned landing so reducing the stress on other components. Unless you wheelie too far in which case you will lose too much pressure, get a pinch flat from the impact of landing and crash. If this does happen, you'd better hope you haven't got disc brakes because ...

Matthewfalle

What if I pump my tyres up over here then fly to Australia and ride my bike there? Will being upside down affect the pressures?