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OSPW pulley cages: how many watts you really gain, and whether the weight is worth it

transmisión6 min readBorja Alfaraz

Drivetrain

≈ 2 W

Typical saving of an OSPW cage versus stock pulleys, measured at 250 W (Friction Facts / ZFC).

Its weight penalty (~30 g) costs under 0.6 s even on a climb: the friction saving dominates, but it is small.

Abstract. This article quantifies the real benefit of oversized pulley wheel systems (OSPW) on drivetrain efficiency and sets it against their weight penalty. A physical model estimates the net balance on the flat and on a climb, and places the result against lower-cost alternatives such as optimising the chain lubricant.

The commercial argument for an OSPW is direct: larger pulleys articulate the chain through a smaller angle, reducing friction loss as it passes through the derailleur. The effect is real and measurable on a test rig. The relevant question is its magnitude and its opportunity cost.

The balance: watts versus grams

Independent testing (Friction Facts, Zero Friction Cycling) places the saving of an OSPW at around 1-2.5 W at 250 W relative to clean stock pulleys. Taking 2 W and a 30 g weight penalty, the AthletePro physics engine yields:

ScenarioSaving from +2 WCost of +30 gNet
Flat · 40 km · 300 W−8.0 s+0.06 s≈ −8 s
Climb · 10 km · 6% · 300 W−9.1 s+0.5 s≈ −8.6 s

Two conclusions. First: the weight penalty is irrelevant —even on a climb, those 30 g cost half a second— so the “watts versus weight” framing is wrong: the friction saving always dominates. Second: the absolute saving is small, of the order of 8 s over 40 km.

Opportunity cost. An OSPW costs €400-600 for ~2 W. Immersion-waxing the chain delivers a comparable or larger saving (2-5 W versus a factory-greased chain) for a fraction of the cost. In €/watt, wax is one of the best drivetrain investments; the OSPW, one of the worst.

When it makes sense

  • Elite level, with everything else already optimised and budget is not the limiting variable: every second counts and the OSPW adds up.
  • Everyone else: before the OSPW, wax the chain, tidy up maintenance and check you are not riding permanently cross-chained. They will give more watts for less money.

AthletePro models drivetrain efficiency as a function of lubricant and chain type, so you can see the effect of waxing before spending on pulleys.

References

  1. Friction Facts (2013-2016). Derailleur pulley and chain efficiency testing. Serie de informes independientes (posteriormente adquiridos por CeramicSpeed).
  2. Zero Friction Cycling (2020-2023). Drivetrain efficiency and chain lubricant test protocols. Bancos de fricción a 250 W.
  3. Spicer, J. B., et al. (2001). Effects of frictional loss on bicycle chain drive efficiency. Journal of Mechanical Design, 123(4), 598-605.