March 18, 2013

How far can 1x drivetrains go?

As I’ve noted before, I think front derailleurs are dead. There’s still some controversy about this amongst mountain bikers but I think in a few more years it will be rare to see a front derailleur on a high end mountain bike. I could be wrong.

I was recently interviewed (though I don’t think they used any of my gibberish) for BRAIN about whether or not I thought single chainrings would become popular for road/cross bikes as well, which honestly I hadn’t thought much about. But when I did think about it, I’d be surprised if we didn’t see some form of 1x road drivetrain soon, because it makes a ton of sense:

-Let’s assume you want to get at least as much gear range as a typical compact double – for the sake of argument, that’s a 34/50 chainring combo in the front (note that a non-compact crankset is inherently going to have a *smaller* range of gears) and an 11-28 (the widest ratio commonly used) in the back.  That’s a (50/34)*(28/11) = 3.74:1 overdrive (meaning the highest gear is 3.74 times the ratio of the lowest).

-Now assume we don’t want to lose any of the top end, so we need a top gear that’s equivalent to 50×11 (or 4.55:1). If we’re using something like the XX1 cassette (with a 10t as the highest gear on the cassette) then we’ll need a 46t chainring (rounding up to be conservative).

-Let’s also match the lowest gear – 34×28 (or 1.21:1). That will mean our easiest gear will be 46×38 – actually a considerably smaller cog than the existing 42t on the XX1 cassette. So the total range can easily be achieved.

-The downside, of course, is the gaps between gears. With an 11-28 Dura-Ace 11sp cassette, the gearing looks like this:
11,12,13,14,15,17,19,21,23,25,28

So our biggest gaps, percentage-wise, are the 15-17 and the 25-28 which are 13/12%.

Contrast that with an XT 10 speed cassette:
 11,13,15,17,19,21,24,28,32,36

Now, we’re going to add a 10t, and then jump up by 5 teeth after the 28th (so 10, 11,13,15,17,19,21,24,28,33,38) to minimize gaps and still get us the range we need, but we now have some big gaps: 11 to 13, 28 to 33 (18% each, roughly).

For folks who don’t need as big of a range, you could go to a 10-34 or even 10-32/30 depending on your local terrain and needs, but there’s no question that the gaps between gears are going to be bigger if you throw away the front derailleur.

-So the gaps between gears are about 50% bigger through a significant portion of the range of the cassette. For recreational riders or folks who aren’t super cadence sensitive, this probably isn’t a big deal and getting rid of the weight and hassle of the front derailleur is most likely worthwhile. For very serious racers or time trialists, or someone who wants both a big range and tight gears, the front derailleur will probably stay.