November 19, 2009

BB30 and seat mast thoughts, also hoodie update

First, quick news: Brent thinks the hoodies will be ready in about 2 weeks (much faster than I thought) and we’re doing an XXL size (it’s been added as an option to the paypal link).

Now, on to BB30:

I got a great question from Steve about the BB30 bottom bracket/crank system and integrated seat masts – both of which he wanted to know if I’d do.

I rudely said, “no,” and Steve followed up with (being much more polite than he had to) – “Why?”

Long boring rambling follows. You’ve been warned.

Steve –

Sure thing. The whole point of BB30 is that you can do the final alignment of the BB shell (as the bearings are press-fit) AFTER the frame is built. This solves a MAJOR problem for building with carbon fiber, since the shell has to be bonded into the frame, and the threads can’t be re-cut afterwards to be in alignment. The BB30 solves this problem by using an aluminum shell with a lot of extra material, then having the final boring of the bearing surface cut after the frame is finished – allowing the BB bearing alignment to be done after everything else is done. That’s huge for building carbon bikes, and cuts a load of cost. And it’s a bit lighter, since you’re replacing a pretty thick aluminum shell (plus bearing cups) with just the shell.

Fast forward to steel – alignment can be checked at every stage of the process and kept correct, since the frame is built in pieces (I do alignment checks at 6 different points in the process). So there’s no alignment advantage.

And the shell is much different – instead of a 1 3/4″ aluminum shell, we use a 1 1/2″ steel one – which weighs about 85 grams for a 68mm (pre threaded) model. An equivalent BB30 shell in steel (you can look it up at Paragon) weighs 140g. Even after subtracting the weight of the bearing cups (about 30g) you end up with a heavier setup. The BB30 axle is a tiny bit bigger, so in theory it’s stiffer – but I don’t hear anyone complaining about how stiff their external bearing cranks are, so I can pretty confidently say that the extra stiffness isn’t useful (or even noticeable outside of a laboratory) unless you weigh 400 pounds or something.

Another downside is corrosion – if you get water into the shell, either the shell or the bearing races can get nice and corroded, and lock themselves together. You can grease the crap out of everything and prevent this, of course, but if something does start rusting – that’s the end of the frame, since you can’t easily machine the bearing seat *smaller* again.

So bottom line is that BB30, for steel, is heavier and has some major downsides. It’s also a huge pain to build with, since you’ve got to put the ENTIRE FRAME in a milling machine and use a boring head to machine the bearing seats when you’re finished. Screw it up, even a tiny bit, and that’s the END of the frame.

Great stuff for building carbon bikes in a mass production facility – I’m guessing that BB30 is saving manufacturers a nice chunk of change in costs. And it doesn’t have any downsides for carbon or aluminum bikes, really. Weighs about the same, should perform fine.

But not steel, unfortunately.

The seat mast is likewise something that doesnt’ end up making sense. If we assume we can use .049″ 4130 cromoly for the mast, and it extends, say, 150mm above the toptube (a reasonable estimate for a normal non-compact road bike) then we’ve added 120g of steel to the frame (and that’s being generous – you’d actually have to add more to make the seat tube/seatstay/toptube cluster strong enough – call it another 50g). So we’re 170 over the normal frame weight. If we figure the seat clamp is 30g or so (I’
m being generous again – I bet it’s more like 50g), we’re at an even 200.

Contrast that to a standard collar (10-20g) and decent quality (WCS, Thomson Masterpiece) post that weighs 160-180. Best case scenario, we break even on weight. Worst case, we add 30-80g to the frame.

Second problem is corrosion. If we want to clamp the seat clamp to the steel, we either have to clamp on top of the powdercoat (slips, wears off and corrodes, etc) or to bare metal (corrodes). We could use 304 or 306 stainless to get around this problem, but that would need to be thicker to be strong enough – say .058″ or so. So now the extra steel is 140g.

And I haven’t even gotten into the downsides of the mast if you cut it too short, or decide you want to ride a different saddle and need to raise it, or decide you want to loan the bike to a friend, or (heaven forbid) sell it. Being stuck at one saddle height and then having to cut the mast down to go lower is, IMO, not very useful when you can move a conventional seatpost up and down as much as you want.

It just doesn’t end up working out well. There’s a reason most steel double triangle frames look pretty similar to each other – they just work well. In other materials, other configurations can be better, but not in steel, really.