Drivetrain Efficiency & Marginal Gains

In recent years British Cycling and Team Sky have introduced us all to a concept known as the aggregation of marginal gains. Put simply, it means looking for small advantages in all aspects of a rider’s preparation, environment or equipment in order to maximise the probability of that rider enjoying successful results. Within this framework no improvement is too small because together they all add up.

Now in this context consider the question of drivetrain efficiency – that is “what percentage of the power going into a riders cranks is being delivered to the road?” Ideally it would be 100% - a perfectly accurate PowerTap (measuring power at the rear hub) would read the same number as a perfectly accurate SRM (measuring power at the cranks). The reality though is somewhat different and there are some real marginal gains to be considered in maximising drivetrain efficiency.

Experimental Findings

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  and Spicer  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. But have Tony Martin & Bradley Wiggins - two of the best time triallists at the time of writing - taken conscious decisions in this regard? This Bradley Wiggins Interview  in which he talks about cadence and "something to do with rolling resistance and with the gears" suggests they have.
  • 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.

Modelling Drivetrain Efficiency

When we estimate elite riders power output, for example in our Pro Race Power Analysis  we assume a drivetrain efficiency of 98%. In other words, the speed they achieve on the road relates to a certain propulsive power going into the road which we scale by 1/0.98 to estimate the power the rider is delivering to the cranks. The rationale behind this number is the high power output involved – see the first bullet point above – and that all pro’s should be using high end chains – Dura Ace, Record, SRAM Red.

All of the experimental data suggest that when modelling amateur riders slightly lower efficiencies may be in order – typically 96-97.5%. Regardless of the baseline number that applies to your cycling the marginal gains mentioned above are very real and very worth considering.