Thursday, February 05, 2009
The grand scheme
Ok, this is a rant about ceramic bearings on bicycles. First off, keep in mind that I think ceramic bearings are actually a pretty great idea for bikes. I just don't think they're great for the same reasons as the people trying to sell them. More on that in a minute.
Here's some typically gushy prose from the bicycle press:
"Made from pure Silicon Nitride material with extreme high density from uniform compaction (3.25 g/cm3), the micro-structural development of this material during manufacture is second to none. These high precision Grade 5, SI3N4 pure ceramic balls are 5/1,000,000" from exactly round in sphericity. 60% lighter weight, and 7 times harder than steel, friction is reduced to near zero. Resistance to heat is 8 times greater than that of steel while there is simply no comparison for corrosion resistance."
Ok, step by step:
-"Exactly round in sphericity"? Ok, so they're _exactly_ round in roundness. Nice. Just a quibble, really, but is the word "sphericity" (if it's even a word) really needed here?
-"60% lighter weight". This is a penny wise/pound foolish kind of statement. Sure the actual BALLS weigh 40 or 50% less (according to Wikipedia, anyway) than steel ones. But how much do all the actual balls in the bearings, in, say, your bottom bracket weigh? After all, the whole assembly (bearings, seals, races, cups, etc) is well under 100 grams for an external bearing setup. Giving the benefit of the doubt to the bearings themselves (since I don't want to pull apart my bottom bracket today to find out) I'd say that, at most, they comprise 50% of the weight of the BB. Most likely it's a lot less, but for the sake of argument, let's say it's 50g of bearings. Subtract 40% from that and you have a mighty 20g of weight savings, and you've paid $100-150 for it. If you're the type of person who goes out and buys titanium bolt kits and shaves knobs off their tires with a knife at night while your friends are out drinking beer, then more power to you. For the rest of us, 20g isn't worth $150, methinks. Heck, you could hire a coach for a month or two for that price - I'm pretty sure that would make you a lot faster.
-"Friction is reduced to near zero." Well, I hate to tell you this, but conventional steel bearings ALSO reduce friction to "near zero". Even a *crappy* steel ball bearing set will give you efficiency well in excess of 99.8%, and it's safe to say that with any kind of decent bearings at all (ie, what you'd find on any quality bicycle) you're wasting no more than a tenth of a percent or so of your effort on bearing resistance.
If .001 of your total effort going to waste really chaps your hide, I'd suggest improving your fitness by the same tiny fraction might be a better use of your time than spending a ton of money on bearings. But what do I know?
-"Resistance to heat is 8 times greater than that of steel". When was the last time you were riding 120mph and fried your hub bearings? Don't you *hate* it when that happens?
I'm sure if I was running a 150,000 rpm centrifuge or something, this heat resistance would be great. On a bike, not so much a big deal. And before you say "Repack", let me remind you that those guys were riding BEACH CRUISERS. Not a great counterexample.
-"there is simply no comparison for corrosion resistance". NOW you're going somewhere. This is actually worthwhile - steel bearings *do* corrode and have trouble in wet (enough) conditions. This is a benefit worth having on a bike!
But here's the thing - I *only* want ceramic bearings for corrosion resistance. Lower grade, lower "sphericity" bearings would be *just fine*, as long as they weren't any worse than steel bearings. I'd be happy to pay, say, an extra $10 for ceramic headset bearings that wouldn't corrode. Ditto for everywhere else on the bike. What I don't want, though, is to spend $800 to get a bunch of minute (in fact, with a rider onboard, probably not even measurable) performance benefits from aerospace-grade bearings.
Since bike snobs are constantly replacing perfectly good bikes with this year's latest thing anyway, I'm guessing the manufacturers aren't too worried about longer-lasting bearings, though, and they see the potential to profit from people who don't understand that the performance benefits they're touting are a couple orders of magnitude away from being even perceptible.