 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Hubjub technote 001: Spline drive crank primer v1.0 25/1/12
Why you should:
Cheap, stiff, light, tuff
Why you shouldn't:
Massive Q-factor
Funny little bearings
Rust never sleeps
William Gibson described one of his fringe dystopias as 'a deranged experiment in social darwinism'. It was a playground for mainstream society to work out its relationship with upcoming tech. If you're into cycling, BMX serves much the same function.
Every spring, BMX makers come up with shiny new kit. They sell it to BMX bandits, who spend the summer doing their best to break it. Next winter the makers pick up the pieces and do it all again. Any design or manufacturing process that gets through is reckoned a survivor. Its manufacture is passed on to volume factories for bulk sale, perhaps to non-BMXers.
BMX engineering has deeply influenced the mainstream. BMX aesthetics haven't, but that may change soon.
If your antennae are well-tuned, you may have clocked the appearance of 700c streetfixes with 48 spoke wheels, straight forks, and/or reinforced risers. These bikes are built for BMXers making the transition to fixed gear riding, and they're coming soon to a shop near you. It's already too late to tip poison green deep-section rims or fat platforms for the high street, but one particular innovation has been slower to cross over. It's called spline drive. You should check it out.
When you look at the transmission of a modern high-end BMX bike, the first thing you notice is that the cranks are weirdly fat, about the same thickness as steel frame tubes. That's unsurprising, because that's what they're made from.
The second is that the cog and ring are really small. In fact, the ring will likely be so tiny that you'll wonder how it could possibly fit onto a spider. Look closer, and you'll see that it doesn't. It will be sitting right on the spindle of the BB, and may or may not be bolted to the crank.
The third thing will only become apparent if you get your nose right down to the cranks. (Try not to get bitten by the bike's owner.) The BB is a fat cylinder of solid steel, larger in diameter than any squaretaper job and with its ends machined into ridges, known properly as splines. The cranks may have external bolts to clamp them to the BB, or they may push right over it. Either way, they'll be cut precisely to fit.
Tubular cranks evolved in response to demand. BMX riders found that hard landings caused alloy cranks to split around the BB. Tubular steel, being tough, light and stiff, makes a no-brainer replacement.
Splined BBs are a less obvious choice, and their advantage doesn't become apparent until you have one in your hand. A splined BB is not tapered. The crank can be shunted along it ad lib, being held in position by spacer washers and crankbolts. This makes it less subject to wear at the BB interface than its tapered counterpart, and obviates the danger of damage from overtightening.
As tubular/splined cranks have evolved over the last decade or so, splines have proliferated, with 36-spline making a bid to become the effective standard a couple of years back. But the move to 48-spline is a game-changer.
Previous splined BB systems were 'crank drive', that is to say, the chainring was not splined, but floated free on the BB and was secured to the crank by a single critical fastener known as 'the ringbolt'.
48-spline, however, is 'spline drive'. The chainring is splined, engaging with the BB the same way the crank does. This is more significant than it seems. Fixers experimenting with crank drive report that the all-important ringbolt tends to loosen when subjected to the added stresses of fixed gear riding. Spline drive doesn't have this drawback. If you're a fixer, 48-spline rocks.
Further, spline drive is accessible. So-called 'euro BBs' enable the installation of 48-spline cranks on any bike with a standard 68mm BB shell and Brit/BSA threading. Track chainline is doable. Smart makers have identified the potential value of the crossover market and spline drive chainrings are now available in sizes 40t+. In other words, everything you need to put spline drive on your fix is in place.
I'll let that sink in. You could install tubular cranks tomorrow if you wanted. (I've done so myself, and I haven't sprouted horns or anything.)
That being so, the rest of this article lists the pros and cons of going splined.
Pros
Cheap. A good spline drive system (cranks, ring and euro BB) can be had for £150. That may not sound all that cheap, given that you can buy alloy cranks and a matching BB for £70.00, but the spline drive cranks will suck up abuse that would knacker the cheapies. A similarly tough squaretaper setup would cost a lot more than &163;150.
Stiff and light. A 19mm splined BB is heavier than its squaretaper equivalent, but overall your crankset will weigh about the same. And, if you're moving up from a cheap alloy crank, you will notice the increased stiffness.
Funky. Yas, yas.
Cons
Q-factor. As you'd expect with inch-thick cranks, your Q-factor is massive. I moved direct from Campy Gran Sport to Gusset Pygmies, and found myself riding with my legs two inches further apart than I was used to. I thought I'd mind, but I didn't. Maybe you will.
Fitting. Working splined metal cranks onto a euro BB takes buckets of anti-seize and raw heroism. Plus, if you don't get the spacers right first time, you have to start over. Your local bike shop will hate you.
Engineering. The thick 19mm spindle takes up most of the BB shell, leaving no room for bearings. Given the wimpy little balls they have to work with (fnaar!) many designers spec euro BBs with four bearings rather than two, but you should still reckon to swap 'em out frequently.
Corrosion. The BB shell is the dampest part of the bike, and no-one is making stainless euro BBs yet. Rust is a fact of life.
So there you have it, the good and the bad. Tubular cranks, we gottem. Give spline a chance. You might dig it.
|
|
|
|
 |
 |
||
|
|
|||
|
|
|
|