I'd say titanium or alominium which is pretty expensive but the lightest bmx ever is the grim reaper by eastern hope this helped
The lightest bmx bike is owned by Samuel Powell. His bike ways 18.3 pounds and is built custom from the ground up. It totalled to about $2,300.
4 lbs. 1 oz.
The lightest bike for adults is probably a redline or a hyper but hypers are kind of expensive
Eastern titanium grim reaper 19 lbs
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the lightest bmx bike is from KHE in Germany they make a bike that weighs lighter than the eastern grim reaper. The bike from khe weighs in at about 19.6lbs. The bike is the Maceto Pro 2012 you can see the bike at khebikes.com
Shadow ravenger conspiracy only £17.99
probably the haro prolite, I'm warning you they are about 800 to 900 bucks Canadian
The Lighest BMX frame is Lopez-lt weighing in at 3 pounds 8 ounces. Although it is light it has a tendancy to crack at the headtube. Otherwise it is an amazing frame.
i have one of these they arnt the lightest in the world but i think its about on a scale to 1-10 about 6.5-7