Lots of folks have asked us whether we are going to make titanium spindles. Most of our competitors do, so it might seem like a logical move. Our reasons for avoiding ti spindles are a little technical, but bear with us.
A pedal spindle has to be strong because it is stressed as a cantilever: the load is at one end of the spindle (your foot); the spindle must transmit force to the other end (the crankarm). If you use plain sleeve bearings (aka bushings) and/or loose ball bearings like Ritchey, Shimano and most Taiwanese pedals, strength is pretty much all you need to look at. But cartridge ball and needle bearings require that that the pedal body stay precisely parallel to the spindle. That requires the spindle to be stiff as well as strong. This is where most titanium spindles fail Engineering 101. To understand why, you need to look at a few key properties of steel and titanium and understand a couple of basic engineering concepts.
Engineers have a buzzword for stiffness: modulus. It is a convenient, quantifiable way of comparing the rigidity of different materials. In terms of modulus, the differences between steel, titanium and aluminum are not very interesting. Titanium is half as stiff as steel, and half the density. Aluminum is one-third as stiff, and one third the density. (The buzzword for stiffness as a function of a material’s density is “specific modulus.” All three materials have essentially the same specific modulus.) So a titanium spindle that is the same size as a steel spindle will weigh half as much, but will bend twice as much. Even if that bending doesn’t eventually cause the spindle to break, the constant bending will cause rapid bearing failure (just ask SRP, which recalled all its Ti spindles a few years ago for that reason).
OK, here is the part you don’t hear so much about. You may have read about the modulus of materials before, but even more important is the inherent stiffness of different shapes and structures, sometimes called “section modulus.” This is probably the most important (and misunderstood) concept in bicycle frame design. A lot of folks have been talking lately about the “typically stiff aluminum frame ride." This is hooey. Aluminum is only 1/3 as stiff as steel, and anybody over six feet tall who ever tried to ride an early Vitus frame knows it. Folks like Klein and Cannondale make mondo stiff frames by taking advantage of the section modulus of a cylindrical tube. Here’s the basic deal: when you double the diameter of a tube, its stiffness increases by roughly eight times. So the stiffness of the basic structure of Cannondale’s 2 1/2” down tube is about 8x stiffer than a normal 1 1/8” down tube. Aluminum is only 1/3 as stiff as steel, but because of the vastly increased section modulus, you still end up with an aluminum tube that is more than twice as stiff as the smaller steel tube. So by trading off section against material you can end up with any degree of stiffness you want -- with almost any material. So talk about the “typically stiff big-tube frame ride” and you can impress your friends even if they have plastic pen pockets.
You’ve probably guessed that a titanium spindle will work fine -- it just needs to be bigger than the steel spindle it replaces. Unfortunately, that approach creates a number of problems. First of all, the crankarms, bearings, pedal body and a bunch of other parts will have to be completely redesigned. Second, making the spindle larger generally means significantly increasing stack height, which we think is a bad idea.
There is one more problem with titanium spindles: hardness. Most pedals, including ours, use the spindle as the inner race for at least one bearing. To function properly, this inner race must be very hard. It is easy to make steel hard enough, but almost impossible with Ti. (Those fancy coatings, like titanium nitride, help somewhat, but if they are much harder than the underlying metal, they tend to separate and flake off.) Some other manufacturers solve this problem by pressing a steel sleeve over part of the spindle, but this reduces the diameter of the structural portion of the spindle, which we think reduces stiffness and requires that the overall diameter of the spindle be increased still further.
We aren’t saying that a good Ti spindle is an impossibility. Like a number of very smart people in this business we’ve spoken to about this subject, though, we’ve concluded that they are a bad idea as direct replacements for steel spindles. So for now, we suggest you ignore Ti spindles and go with the lightest all-purpose pedal system you can buy -- which happens to have a very stiff, strong and light steel spindle.
Bebop's way is to find a better steel -- our exclusive Aero-S ultra-high strength stainless steel. Aero-S may be the most advanced steel alloy available. Originally created for things like landing gear for navy jets and aerospace fasteners, it is nearly twice as strong as cromoly. It isn't any stiffer than ordinary steel, but because it is stronger, we can use less of it. So you get a spindle that is twice as stiff as any Ti spindle, and only a couple of grams heavier than most of them. Oh, and it it is also super hard, which makes it nasty to machine but great as a bearing surface.
And finally, unlike our competitors who think it is just fine to sell more
expensive parts that are not as strong as their cheap stuff, we don't put arbitrary
weight limits on our Aero-S parts. I mean think about it -- when they say 170
pound rider weight limit, do they mean a 170 pound roadie profiling past Starbucks,
or 170 pounds of human cannonball ripping down a rocky 20% chute? Do you want
to bet your insurance deductible on your answer?
(Dr. Andon has recently announced that he will donate $10.87 to the Sierra Club's
"Ban bikes from singletrack and non-interstate roads" fund every time some knucklehead
claims that aluminum is a "stiff" frame material. Help keep the roads
and trails safe for bicycles -- smack the next guy who says it.)