I apologize in advance. I will be committing heresy: 30mm spindles aren't better than 24mm cranksets from Shimano or FSA MegaExo cranksets, 25mm cranksets from Campagnolo, and 24/22mm GXP cranks from Sram. They're not worse, or bad, they're just not better.
The statement often goes like this; "30mm spindles are better than 24mm spindles. Everybody knows it."
Then why does Shimano, and up until recently and I'm sure grudgingly, Campagnolo, with their unfortunately named 30mm OverTorque bike crank, refuse to acknowledge this obvious fact? It's not because these firms come from insular cultures, and are therefore hard headed and set in their ways, or anything of the sort. It's because these companies are run by engineers FIRST, and not marketing people. Let's not forget that it was Shimano and Campy that gave us electronic shifting, so the set-in-their-ways argument regarding these two pioneering firms stops here.
Sram? Sram is run by pragmatists. They'll make anything you'd buy. Not a criticism, just an observation, which is funny seeing that I'd argue that GXP is the best bike crankset format for optimal bearing performance, thanks to Sram.
Here's how the 30mm bike spindle came about: Somebody wanted to make a "lighter" crank set---because low weights are an advertising boon---and they realized the spindle was a great place to cut weight. The only place, actually, because anything you can do to arms and spiders and rings on a crank to lighten it would be applicable to any other crank. If you didn't shed grams at the spindle you had nothing special.
They were smart enough to realize that a 24mm aluminum spindle would be too weak, unless they left it a solid piece, which would've negated the weight loss, and so they researched, and bench-tested, and field tested, and soon realized 30mm would be best...
No. That's not how it happened.
Actually, they went over to a bearing table and found that the next "step" in bearing size from 6805 (37x25x7) was 6806 (42x30x7) and that after 6806 everything just got stupid big, like 47 x 35 x 7 big; ridiculous for a human powered piece of equipment.
Bearing availability made their decision.
The dirty little secret in any engineering endeavor is that you design-to a constraint. In our business, it's the bearings. You don't just start making a new bearing standard. Bearings are for plug-and-play industrial applications that go way beyond the scope of the comparably tiny bicycle industry's needs.
So the 30mm spindle was born, not based on any proven shortcoming in performance of a 25/24mm spindle, but based solely on a desire to shed weight. The "Less flexible" and "Gives the rider more heel and ankle clearance" arguments came afterward. None of this was a reason. And the old saw about BB30's superior ankle clearance doesn't explain BB386Evo, which has straight arms and the identical stance of a Shimano or Campy bike crank. The spindle flexibility rationale is silly. You---a mere mortal---are never going to realize a torsional loss due to a steel spindle that is the same size as the input shaft of a transmission for a 1.6L engine.
But is 30mm better?
Only if you think every gram kills you. If you're weighing tires and water bottle cages then stick with the 30mm spindles.
This brings us to the reason for this entire blog: Stop buying 30mm spindle cranks for your BB86 and BB92 bikes. Call it PressFit41, or some other moniker, but you all know what I mean. Quit doing it. It is a terrible idea, so bad, in fact, that Shimano conceived of, and now pushes the BB86/BB92 "standard" specifically to make the use of a 30mm spindle impossible. I bet they thought, "Nobody in their right mind is going to try to stick a 30mm spindle in hole that is only 41mm to begin with. There's no room for the bearings if all you're left with is a 6mm tract of real estate for a set of races AND bearings AND a housing to occupy."
And yet many try it. Inexplicably bicycle companies actually spec 30mm spindle cranksets in bikes that are BB92. They do this because on paper it should work. Someone makes an adapter BB, and so it must work. Yes?
No. It's dreadful, and most of all, dreadfully unnecessary when all anyone had to do was use the right part from the beginning. A BB86/BB92 frameset gets a Shimano, Campagnolo UltraTorque, or a GXP bike crankset. Period.
I hope some company out there never finds out that wearing cycling shoes one size too small is good for 2 watts due to reduced drag.
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