Kawasaki Sportsbike Riders Club - Australia - Online Forum

...for track or otherwise?
If so, got any pointers on initial pressure to try e.g. 110% of usual cold air pressure?

Wouldn't bother unless your racing WSBK.....

I run 78% nitrogen in mine.

I've used nitrogen in race tyres. The main reason is that nitrogen increases the tyre pressures at half the rate of compressor air. That is, if while out riding, your compressor air filled tyres rose by 12 psi due to ambient/road temperatures, load, riding, grip level then your nitrogen filled tyres would increase by 6 psi in the same road, bike, load, temperatures, conditions and riding. (Comparing identical conditions).

However, the biggest problem is moisture.

Moisture can occur in both air and nitrogen filled tyres and is the thing that creates the greatest pressure variation.

The issue with compressed air is that atmospheric air has varying percentages of moisture in it (humidity) which when compressed in a compressor for use in inflating your tyre creates a water trap that gets pumped into your tyre.

To test this, just start up your compressor and point the nozzle onto the concrete and watch the water mist out.

The higher the atmospheric humidity the more moisture is created in your compressor and forced into your tyre.

Good tyre shops have extremely good dry air systems to filter out the moisture. Bad tyre shops don't know and don't care and don't drain their tanks much at all.

However, using nitrogen isn't exactly perfect either. Moisture still gets trapped in the tyre and this causes pressure rises.

Just to demonstrate the issue with moisture problem with race tyres we would fill a tyre with nitrogen and then use a vacuum machine (modified and adapted from a hospital medical cabinet vacuum machine) to suck out all the nitrogen. We would then refill with clean new nitrogen. Vacuum again. Refill, vacuum etc. Each time we would use a hydrometer to measure percentage moisture content in the tyre. Once we had the lowest possible moisture reading, the tyre would be bled down to desired starting pressure.

The thing is even after doing all this, when the tyre was measured after the session the pressure would have still increased depending on track ambient/track temps and grip levels and believe it or not, the moisture content increased dramatically.

To this day I have no idea how or why the moisture increased. I have no idea how it manufactured water in a nearly perfectly dry atmosphere!

I'm not a a scientist and I have asked scientists to explain it however their suggestion was to experiment under lab conditions to find the cause.

The point is: if you are so concerned with pressure rises on your road bike then your best approach is to know very well how much you pressures rise under various ambient/ road temperatures, loads, grip levels and riding characteristics and adjust start pressures to suit.

Mister_T wrote:...for track or otherwise?
If so, got any pointers on initial pressure to try e.g. 110% of usual cold air pressure?

No. Most of the teams racing in Australia don't bother with it, so unless you are running magnesium wheels, factory suspension and pushing for lap records, I would think there are much better things you could be spending your money on. Just make sure the compressor you use to pump your tyres up with isn't full of moisture and you will be fine.

Tack wrote:I've used nitrogen in race tyres. The main reason is that nitrogen increases the tyre pressures at half the rate of compressor air. That is, if while out riding, your compressor air filled tyres rose by 12 psi due to ambient/road temperatures, load, riding, grip level then your nitrogen filled tyres would increase by 6 psi in the same road, bike, load, temperatures, conditions and riding. (Comparing identical conditions).

However, the biggest problem is moisture.

Moisture can occur in both air and nitrogen filled tyres and is the thing that creates the greatest pressure variation.

The issue with compressed air is that atmospheric air has varying percentages of moisture in it (humidity) which when compressed in a compressor for use in inflating your tyre creates a water trap that gets pumped into your tyre.

To test this, just start up your compressor and point the nozzle onto the concrete and watch the water mist out.

The higher the atmospheric humidity the more moisture is created in your compressor and forced into your tyre.

Good tyre shops have extremely good dry air systems to filter out the moisture. Bad tyre shops don't know and don't care and don't drain their tanks much at all.

However, using nitrogen isn't exactly perfect either. Moisture still gets trapped in the tyre and this causes pressure rises.

Just to demonstrate the issue with moisture problem with race tyres we would fill a tyre with nitrogen and then use a vacuum machine (modified and adapted from a hospital medical cabinet vacuum machine) to suck out all the nitrogen. We would then refill with clean new nitrogen. Vacuum again. Refill, vacuum etc. Each time we would use a hydrometer to measure percentage moisture content in the tyre. Once we had the lowest possible moisture reading, the tyre would be bled down to desired starting pressure.

The thing is even after doing all this, when the tyre was measured after the session the pressure would have still increased depending on track ambient/track temps and grip levels and believe it or not, the moisture content increased dramatically.

To this day I have no idea how or why the moisture increased. I have no idea how it manufactured water in a nearly perfectly dry atmosphere!

I'm not a a scientist and I have asked scientists to explain it however their suggestion was to experiment under lab conditions to find the cause.

The point is: if you are so concerned with pressure rises on your road bike then your best approach is to know very well how much you pressures rise under various ambient/ road temperatures, loads, grip levels and riding characteristics and adjust start pressures to suit.

Quite a lot of moisture is crap sucked in from the lube used to seat beads, that'd be my guess. Over lube tyres to fit them onto rims and that's a great way to end up with huge pressure increases as you boil the water off when it gets stinking hot. Otherwise... not such a problem. You can also get moisture absorbed back through the tyre itself, partial pressure of H2O vapour being smaller on the inside of the tyre, so it'll diffuse through the tread still... maybe. (Technically I am a scientist though not in gasses, but I have chatted to some peeps who are about this).

Nitrogen does not have half the change in pressure compared to compressed air. For one, compressed air is 78% nitrogen... so that would mean pure oxygen would have 9x the expansion of nitrogen and it doesn't. Air is not an ideal gas but neither is nitrogen.

The supposed benefits of nitrogen:
1. Less reactivity with the tyre, which means less diffusion (means it holds pressure better)
2. Less diffusion due to molecular sizes (means it holds pressure better, as well)
3. Less corrosion in wheels
4. Dry gas compared to the moisture and potentially oil from a compressor
5. Slightly less flammability for tyres and especially for magnesium wheels.

5 is why aircraft use it, and traditionally it was used in racing. 4 seems to matter a bit, moisture changes the temperature capacity of the air a little but generally this just means pressure spikes at tyre temperatures over about 105 degrees when all the moisture turns to steam under the typical tyre internal pressures. 1-2 are why some people recommend it for the road - better fuel economy due to more stable pressures. But the uncertainties on that are such that there's no conclusive evidence it maintains pressure much better than atmospheric compressed air.

Quite a lot of race teams use it entirely cause it's easier to buy bottles of N2 and use them to power air guns and redo shocks and stuff at tracks rather than carting compressors around.

In short, I'm not sold scientifically it's worth the time or effort for most people.

My guess would be no5. From memory the manifold packs sent to the tracks is industrial nitrogen. If they were worried about moisture content it would have been high grade or medical grade to reduce impurities