Improving the Efficiency of a Derailleur: Enlarging the Cassette Sprockets to reduce Friction

Question

For reasons that I have not yet understood, the smallest sprockets with only 11 teeth have prevailed in derailleur cassettes of road bikes, which leads to a very strong deflection of the individual chain links when passing the sprocket.

However, the stronger the chain links are deflected and the closer the force is applied to the pivot point, the worse the efficiency of the entire drivetrain (friction). With a sprocket with only 11 teeth, this problem is particularly pronounced.

Question: Would it be technically possible to install larger cassette sprockets? Of course, the chainrings would then also have to be enlarged in order to maintain the same gear ratio.

Is there anything you know that would be technically feasible and commercially available?

Answer 1

Back in the 9s and 10s eras, my recollection (which could be mistaken) is that cassettes starting with 12t were common, although gearing was 53/39. Professionals used cassettes starting with 11t. A 53/12 gear was regarded as more than sufficient.

In the mid 2000s, we started to see compact (50/34) cranks. Those were paired with cassettes starting with 11t cogs. As compact and subcompact (52/36) cranks started becoming more common, my recollection is that most cassettes eventually migrated to 11t starting cogs; a 50/11 is the rough equivalent of a 53/12. Why not provide more cassettes with 12t starting cogs? I've wondered this myself, as I'd like to pair a 52/36 subcompact with, say, a 12-28 or -30 (11s) cassette. However, for Shimano, 12/28 cassettes are only available in Dura Ace, and Campagnolo may have one Centaur-level 12-xx cassette. As to the question of why all cassettes seem to start at 11t, I have a feeling that this is because most amateurs have probably shifted to 50/34 chainrings (or the equivalent in SRAM's AXS gearing, which I think is 46/33 with 10t starting cogs).

You are correct that the smaller the sprocket, the more the drivetrain friction. I believe the primary contributor to friction is actually how much the chain is articulated around the cog. The impact of cross-chaining is smaller, if I remember correctly. If you wanted to investigate this, you'd have to dig up reports from Friction Facts (some of which may be available on the CeramicSpeed website; remember that CeramicSpeed bought out Friction Facts and its owner, Jason Smith). However, we only started to pay attention to drivetrain friction recently. I suspect path dependence is part of the reason why they haven't shifted to larger cogs and chainrings. In political science and other social sciences, this is the notion that past decisions constrain future decisions as institutions and technology become adapted to the choice - in this case, a lot of it would be the tooling, and no small part would be consumer expectations.

As to the question being asked: if you're asking if it's technically possible for consumers to build their own drivetrains with 13/14t starting cogs and commensurately larger chainrings, it could be. But you'd be talking about contracting with someone to make the cogs and rings. Given the patents around the shift ramps, you'd probably be talking about unramped cogs and chainrings, which would worsen shifting. Also, consider that while you do pay a friction penalty for being in small cogs, it's most prominent in 11t or smaller cogs. Most amateurs are rarely in these gears. If they are, it's most likely downhill. The wattage penalty to an equivalent 53/12 gear is probably small, and also you are probably at the speed where it may be preferable to just coast (since the power to overcome aerodynamic drag is proportionate to velocity cubed).

Answer 2

Commercial cassettes are available starting from 13 or 14 teeth. However, for any given system, particularly road groups, the largest sprocket/lowest gear is limited by the derailleur design as well as materially by a wish to keep weight to a practical minimum.

Competative cyclists do not require particularly low gears, even in hilly areas but may opt for a 13 or 14 tooth small sprocket with large chainrings and a low sprocket of limited size in order to get small differences between gear changes and thus stay at their optimum cadence.

Most cyclists who ride recreationally, for pleasure or for excercise (as well as those who ride for practical purposes like commuting etc) generally require a wider spread of gears (the market proves this by providing the wide range options for bikes that did not traditionally have them i.e. road/race). The wide spread of gears is provided by larger sprockets than normal for the low gears and smaller sprockets than normal for the high gears (down to 10 or even 9 on the latest systems) while maintaining a "normal" size chainring at the front that does not interfere with the design of the bike (chainstay clearance, tyre clearance).

Most riders do not spend the majority of their time in the highest gear, however. It should be overgeared and the crusing gear should be a little closer to the middle of the cassette where the chainline is better. When we think about our riding, what proportion of each ride is spent at maximum speed in top gear? I don't believe it is so much of the time even for the very best riders. The best riders may ride a higher gear than me, but they will probably also have bigger gears overall.

Chainrings are available with 60 teeth or more and cassette sprockets are produced with over 52 teeth or more nowadays so there is no technical reason why the gears couldn't be made bigger other than a lack of necessity.

Answer 3

It should be possible to construct a cassette with a bigger smallest sprocket from off the shelf parts. Although it may require buying multiple cassettes to cannibalise pieces to construct your finished product.

Here is the shimano data sheet for the R8000 11 speed cassette.

You can use the diagram and the list of which sprockets fit into which places to see what you would need. I guess the smallest 6 sprockets off the 14-28 junior cassette mated to the biggest 5 sprockets of the 11-32 cassette would be close to what you had in mind.

Note: I just picked R8000 at random. Other cassettes have a similar type of construction and might be able to give an even better range of possibilities.