This is from a thread on the framebuilder’s forum on mtbr. If you want to read it, click here.
That DH bike is all custom. Not just the front end. And it’s all steel. Why didn’t I build it from aluminum like most other downhill bikes?
A) If something cracks or dents, I can easily fix it without resorting to having to re heat-treat the frame or other similar shenanigans. I fully expect to be riding this frame in 20 years, if I still want to at that point.
B) It’s not really any heavier in this application. Frame/shock are about 10#, whole bike is 37 or so. Same as any other DH bike. Aluminum isn’t necessarily lighter for building abuse-proof bikes, it’s just a lot easier to machine for making crazy gussets and swingarms and stuff. So you see aluminum DH bikes because they’re cheap to make and most DH guys expect to kill their frames after a season or two and replace them anyway, so there’s no need for them to be easily repairable.
C) Stiffness is mostly a function of tube diameter (this one has a 45mm downtube) and pivots (this is a BMX bottom bracket modified to attach to the swingarm). It’s way stiff. You might be able to make it stiffer in aluminum with even bigger pipes, but it’s plenty stiff as it is, so I’m not worried about it.
D) I like working with steel and suck at welding aluminum.
So there you have it. If steel was a little easier to machine, you would see a ton of steel DH and FR bikes. But it’s prohibitively expensive to mass produce a lot of DH frames with steel, and the frames are basically expected to fail due to crashes and/or abuse. So cheap (and aluminum) rules the day.