Friday, May 15, 2009

Big in Japan: Part 1

This is the #2 interesting frame this week (though it's hard to beat Eric's bizarre FR-29er). Derek is looking for a 29er for (mostly) commuting when he moves to Japan.

No problem! Buy a bike when you get there!

Er, no. Derek is almost 300 pounds and 6'8" tall. They don't sell Godzilla size frames much in Japan.

When building bikes for really big folks (to me, this is >250 pounds) you really have to start throwing conventional tube choices out the window, if you want the bike to survive rough treatment from a really big person (and in this case, the bike will also be carrying loaded bags a lot of the time).

So tube choice is critical all around. Here's what went into the front triangle (pictured) and a few words about why I chose what I did.

The head tube is 1.2mm wall OX platinum. I often use 1.0mm wall head tubes, but for anyone over about 180 pounds, I always use these 1.2mm jobbies. Yes, that's still thinner than a lot of head tubes (it's not uncommon for builders to use 1.3mm or thicker wall 4130 head tubes) but it's also OX Platinum, which is a darn sight stronger than your garden-variety 4130 head tube.

As a side note, these head tubes are unpopular with custom builders because if you make them distort even a little, it's VERY hard to ream/face them for headset installation. I use heat sinks, backpurge the welds with argon, and weld carefully to keep them pretty close to round, so it's not a problem for me.

Paragon machine works makes a thicker BB shell than the one I used here, but honestly, even their lightweight shell is more than beefy enough for anyone. I've never had anyone encounter a problem with a BB shell (except that one I put in backwards 5 years ago, right Brad?) so this is the one part of the front end that is pretty much bone stock.

Seat tubes are usually 28.6mm (1 1/8") diameter, externally butted to fit a 27.2 post. Derek bends 27.2 posts like Uri Geller, so something beefier was called for here. His seat tube is a 9/6/9 double butted 31.8mm diameter top/downtube (no joke, this would be a moderately beefy road bike downtube for a normal person) which will accept a 30.0mm Thomson post. Of course, I've lugged/sleeved the seat tube to beef it up, and if he does manage to break the 30.0 post (or just doesn't like it) it can easily be shimmed down to 27.2.

The top tube is also pretty odd. In this case, it's a Nova 35mm 9/6/9 29er DOWNTUBE (I usually avoid Nova tubing, as it's cheap 4130 and the butting isn't to my liking, but it was the best choice here). Yes, this is a downtube that a lot of builders would use for a pretty big dude on a 29er. Thanks to Nova's weirdly long butts (80mm at one end, a ridiculous 220mm at the other) it can also be used as a toptube in this case (to be fair, Derek's frame has a 65cm toptube, so it's pretty long). Beefy!

Finally, the downtube. I really wish there was a better way to do this, but the only tube strong enough (IMO) is a 44.5mm OD 1/.7/1 Supertherm BMX downtube, and in this case, it's not nearly long enough (TT, make some longer versions!). So I was forced to use straightgauge .049 x 1 5/8" 4130. It's plenty strong, though quite heavy, and there's loads of room for 2 sets of bottle cage bosses. It's a bit of a shame not to be able to use a butted tube here, but I like to err on the side of caution when building bikes for really big folks - and Derek is aiming to ride around Japan, not win world cup races.

All in all, a tremendously overbuilt front end for a tremendously big dude (note that Derek is not even CLOSE to the biggest guy I've built a frame for, though - that honor goes to a fellow who is 7' tall and 400 pounds!)

Next time (probably Monday, since Paragon didn't ship me the dropouts I needed) some words about the rear triangle. There's unusual/interesting stuff going on there too.

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