Helium in tires.

Nitrogen doesn't expand at all under temperature. Its used instead of normal air because there's no humidity/moisture factor to deal with, which is where the expansion and condensation with temperature changes come into play. Its these changes that annoy auto racers because it can alter how the tire behaves on the track over the course of just a few laps. Bad enough they have to deal with how the rubber tread behaves at different temperatures. They don't wanna have to deal with their tire pressure changing 10% as well. As to helium, its problem is the size of the molecules vs the porousity of the butyl/rubber tubes. Its only of a practical application on track bikes at the Pro/Olympic level... bikes which only have to be used for a few laps at a time, and can immediately be inspected/serviced by a mechanic after each race. You might try it as a pro DH-racer, because your race is over in 6 mins anyways, but its all the waiting around before each time down the mountain where you'd have the issues.

Anyways, this is a REALLY old subject on mtbr, its come up at least once every six months for the past 9 years.

A tire internal volume is about 0.14 ft3.

Air at 60 psi and 60 f weighs 0.38 lbs/ft3.

Air in a tire weighs 0.0532 lbs or 24 grams.

He would weigh 2/28.9*24 or 1.7 grams

The wieght savings is about 22 grams per tire.

Yes it would leak out faster but He balloons last at least several days.

And Dee eight N2 versus air (air is 71% n2) have almost exactly the same pressure/temp relationship so the diffenernce is in the humidity.