Shimano Di2

| Comments (1) | Gear Sports
OK, this is incredibly cool: Shimano has what looks like a viable electronic shifting system. Most people don't think a lot about bicycle shifting, but this a topic of real interest to people who do a lot of riding: mechanical shifting kind of sucks and a working electronic shifting system would be quite nice.

As background, the way bicycle gearing works is this: on the front you have either two or three gears ("chainrings") directly connected to the pedals ("cranks"). On the back, you have between 5 (if your bike is ancient) and 10 (if your bike is new and expensive) gears. They're connected to the rear wheel by a ratchet so that when you pedal faster than the wheel, it drives the wheel but if you stop pedalling while you're riding the gears spin freely with respect to the wheel so you can coast (this is called a "freewheel" or "freehub" depending on how it's put together.). The gears are connected with a chain. Anyway, the amount of mechanical advantage you get is determined by the ratio between the front and back gears. If they're the same size then every turn of the pedals turns the wheel once: the bigger the front gear the harder it is to pedal (but the faster you go with each pedal stroke); the smaller the front gear, the easier it is to pedal but the slower you go. The opposite is true for the gears on the back.

Roughly speaking people choose the general range of the gearing with the front gear and the fine-grained gear selection with the back gears, which are closely spaced. So, for instance, if you want to climb a hill you'd choose the small gear in the front. On the other hand if you're on flats or the downhill you would choose the big ring. Incidentally, while it's natural to think of the gearing as being sequential, it's not. Let's denote a given gear configuration X-Y as being ring X on the front and Y on the back, with "harder" numbers bigger. So, 1-1 is the easiest gear and 2-10 is the hardest. However, 1-10 is almost always harder than 2-11 and there's actually quite a bit of overlap. This means that you can get a fair amount of gear flexibility without changing the front gear, which is good because front gear shifts are very clumsy compared to back gear shift.

Why Electronic Shifting Is Promising
Mechanical shifting has a number of drawbacks, mostly connected with the front derailler (the gizmo that does the shifting):

  • Because all the linkages are mechanical you can only really have one set of shifters. This is inconvenient for triathletes and time trialists who tend to use aerodynamic handlebars a lot of the time. You can put the levers so you can shift conveniently in the aerodynamic position or the upright position but not both.
  • The front derailler doesn't shift well under load, so if you suddenly find yourself on an uphill (which means you're putting a lot of load on the chain) it can be hard to downshift, which is inconvenient because this is exactly when you need to shift. In the worst case, the chain can come off and then you're really hosed.
  • Because the rear has a lot of gears stacked on top of each other, there's a lot of displacement of the chain even as far as the front. This means that you need to "trim" the front derailler to stop it from rubbing; a configuration at the front that works with the biggest gear in the back will cause rubbing with the smallest gear in the back and vice versa.

It's easy to see how electronic shifting can solve the first of these: you can run as many wires as you want so you can have shifters in any location you want. This has been obvious for quite some time and there have been a number of stabs at electronic shifting but they've never worked well. Early reviews suggest that Shimano's does.

Another advantage of Shimano's system is that the front derailler is self-adjusting: this means that you don't have to trim and that allegedly it shifts well under load. The downside is that it's $4k for now, but this is the kind of thing that comes down in price.

1 Technical note: gears are denominated in the number of teeth. For instance, a 20-speed bike might have two front chainrings with 53 and 39 teeth, and 10 back cogs with teeth ranging from 11 to 25 teeth. With our above notation: the ratio for 1-10 is 3.5 and the ratio for 2-1 is 2.12, so there's a very substantial amount of overlap.


"mechanical shifting kind of sucks"

I have been excited about this since I heard about this a couple of years ago. Mechanical shifting blows on every level. Having to manually tune my bikes just annoys me. Most get joy out of it - I just get annoyed.

But this does add complexity and its more difficult to fix on a 50 mile bike rid. Oops - battery died - tough luck. So there are different challenges to this system. With a minimal set of tools I can still get home with the old system.

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