Maybe I’m late to the party, but I’ve been noticing a lot more “S� and almost “S� shaped downtubes recently. Giant has begun to use them on their Trance & Reign models after their 2007 models just had the conventional straight downtube.
2007 Giant Trance with a conventional “straight� downtube
2008 Giant Reign with a new almost “S� Shaped downtube
Specialized, too, has been doing this for a while now to their full lineup although some are a little more pronounced (think Enduro) than others.
Yes, it’s a dual crown fork so clearance isn’t an issue, but if a conventional downtube had been used, the shock and unconventional top tube would have meant no water bottle cage
This particular design has been around the past few years but now it seems that more people are using it and with good reason: it helps with fork crown clearance and allows you to keep a water bottle cage in FS bikes. In the 29er world the “S� shaped downtube should be used more judiciously than the 26″ world because of the taller wheels.
With the 29er wheels being slightly taller than 26″ wheels, standover height starts to become an issue. To get a lower standover height, you need to lower the top tube but, lowering the top tube means having to lower the downtube too. This in turns can cause an issue with the fork crown clearance.
The “S� shaped downtube solves this problem though. The “S� starts at the headtube. By taking advantage of the flatter angle for the top of the downtube, you can still lower the toptube and have the fork crown clear the downtube in case you’re in an accident and the front wheel turns underneath the bike. Rather have that happen than having the fork crown damage the bike frame or vice versa, right?
Another dilemma ensues if you have a FS with a shock in the front triangle. When you lower the toptube and use a traditional downtube you run the risk of not being able to fit in a water bottle cage on the downtube. But with an “S� shaped downtube, the middle portion of the “S� downtube drops at a steeper angle than a conventional downtube allowing you to still put in a water bottle cage even if a shock takes up a good portion of the space available in the front triangle. This works because the “S� then flattens out at the bottom before reaching the bottom bracket allowing the shock to still have its space.
Brand new Stumpjumper 29er with a very pronounced “S� downtube
Lenz Sport uses an almost “S� shaped downtube for many of their bikes including this 29er, the Leviathan
