Ich hab mal nachgefragt:
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Hi Sascha!
Yes you are correct, there are watts savings in general by running larger rings due to 3 main factors;
1) Less articulation of chain links around larger ring vs smaller ring.
2) Lower chain tension on top span of chain under load. Chain tension is crank torque at pedal divided by chainring radius. So not only are the links articulating less, they are doing so under a lower tension
3) For a given gear inches required, one will be using a larger cog at rear, again requiring less link articulation.
Specifically the difference between say a 32t and 36t has not been tested, Jason Smith did do a cross chain test and as a for instance the same ratio in the big ring on a road bike as running 39 x 11 ratio on small ring yielded a 3watt saving. But that was as change of moving from a 39 x 11 to a 53 x 15 so a more dramatic change.
There are other factors to consider;
Ø Test was at 250w load – the higher the load, the greater the savings. The tension in mtb chains with smaller rings and steep ramps can be at times very high.
Ø The test was on a clean low friction chain. This is often not what one is running in the mtb world. Running just the usual lbs lube that seen a bunch of dirt miles and comparatively running a grinding paste masquerading as a lubricant vs a clean low friction lubed chain in lab – the penalties will be much higher. It is not hard to find 15w is chains at 250w load on mtb’s vs 5 or 6w chains as would have been the case in lab test. A 15w chain would see that penalty be 9w not 3w etc. In a long event even with the best lubes there will obviously be a degradation in chains efficiency performance, so the penalties will get worse as a chains efficiency gets worse vs freshly cleaned and lubed with a top lube chain
But – one possible negative however will be how tough the climbs are and chain lines involved. Ie if the course has lots of tough climbing and with 36t this will mean that most of the climbing will be in two biggest cogs, there will be some loss of the benefits due to a slight increase in efficiency losses coming into play from the more extreme chain lines.
In road cycling the TT specialists typically run a 58 or 60t ring with an 11-32 cassette – big ring for lower articulation and lower chain tension and front, but also bigger cog at rear for given gear inches so less articulation there. By running an 11-32, the 25t cog is 3rd cog down, whereas on an 11-28 it is 2nd cog etc – it typically means that for 80% of the tt depending on the course, the cog sizes they need for the gear inches with those size chain rings will keep chain in a very good chain line. If the course had a lot of 8% grade climbing meaning for the biggest ring options they would often need to be in biggest cog, they may opt for 56 or 54 instead – its all about making the right call for the course / athlete.
But definitely getting it right and once can easily get a few watts for free, sometimes more than a few watts as a harsh conditions event gets hours deep.
Hope that helps! J
Adam Kerin
Zero Friction Cycling
Australian importer for MSPEEDWAX"
