In our drivetrain analysis, we used δ to measure how efficiently the drivetrain was able to transform PedalingPower to CyclingPower. Estimating δ is a bit of an art and is dependent on a number of factors.

The following was taken from a post by CyclingPowerLab. [1] which included a number of excellent points, along with the δ values used in their modeling analytics.

CyclingPowerLab Post of Cycle Resistance Efficiencies

A number of researchers have studied bicycle drivetrain efficiency using test rigs in a laboratory setting where cadence, gearing and power could all be tightly controlled. The works of Wilson[2]  and Spicer[3]  are often quoted resources. Some of the findings which may be of real value to riders considering marginal gains have been:

• Drivetrain efficiency of a modern bicycle (i.e. derailleur system with typical road gear range) peaks at about 98% in optimal conditions however variations of as much as 5% (down to 93%) are possible at realistic power outputs.
• As power output increases efficiency increases because frictional losses become a smaller part of total input power. Typical best-case efficiency of a drivetrain in the 200 – 300 watt range is 96-97.5%. Above 300 watts typical best-case efficiency is 97-98%. Read on to understand what we mean by “best case”…
• Efficiency is higher when using larger sprockets because the chain benefits from a less extreme radius of rotation – chain links going around corners cause greater frictional power losses. The take away here is that if you can achieve the same gear using a large chainring + larger sprocket combination than a small ring + smaller sprocket combination it is a worthwhile consideration. The efficiency difference between an equal gear that involves the 24 sprocket and the 13 sprocket can be worth 1-2 watts when riding in the 200-400 watt range.
• “Cross Chaining” (riding with the chain at angles between the chainring and rear sprocket) really hurts efficiency – it’s pretty obvious that frictional losses increase in this scenario. You may intuitively avoid this scenario because it can be noisy and to avoid dropping a chain but efficiency of power transfer is another key consideration.
• Efficiency falls in lower gears (riding at higher cadence) and improves with higher chain tensions (riding at lower cadence). Adusting ones cadence for this goal alone is a risky decision to take – the total efficiency of a rider and bike is more complicated than simple drivetrain efficiency.
• On a perfectly clean chain in a laboratory environment choice of lubricant makes little difference to efficiency. The real value of lubrication is to fill the gaps that would otherwise be filled by dirt and grime – things that do increase friction and decrease efficiency. Lower viscosity lubricants maximise efficiency. Friction-Facts  suggest savings of 1-3 watts just by thoroughly cleaning and re-lubing new chains with thinner oil and sells “Ultrafast Chains” which have benefited from this process.
• Brand new chains are less efficient than chains that have been run-in. The greatest efficiency gains are made in the first hour of use but gains continue throughout the first several hours of use. Don’t use a brand new chain for a key race unless you want to give up half a watt.
• Cheap derailleur pulleys compared to high end equipment can cost 1 watt. Upgrades with ceramic bearings work but the marginal gain is tiny compared to the flagship offerings from Shimano and Campagnolo.
• Cheap pedals with cheap bearings – technically a little higher up the power transfer chain than the drivetrain per-se – can also cost 1 watt.

[1] http://www.cyclingpowerlab.com/drivetrainefficiency.aspx

[2] https://mitpress.mit.edu/books/bicycling-science

[3] http://www.ihpva.org/HParchive/PDF/hp50-2000.pdf

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