Friction Facts: How lubricants and seals affect cartridge bearing friction

Friction Facts is a lab based in Boulder, Colorado, that specializes in quantitative mechanical friction testing of bicycle drivetrain components. While some may dismiss some of the lab’s findings as trivial, racers who seek every last possible advantage might be surprised at where they might find some extra speed — and how much is actually sometimes available. Friction Facts’ latest study looks at how different lubricants and seals affect cartridge bearing friction.

Time trial racers have long resorted to extreme measures when it comes to eking out every last bit of speed from their bikes, and I’ve even witnessed bikes at the Tour de France being run without bearing seals and with little-to-no bearing lubrication. In fact, Evelyn Stevens recently broke the women’s hour record on such a custom-prepared bike. Such a setup obviously doesn’t bode well for longevity but when components only need to run optimally for a single event, such pragmatism sometimes gets tossed out the window if there are seconds to be gained — but exactly how many seconds are we talking about here?

Friction Facts founder Jason Smith did a comprehensive bottom bracket bearing test three years ago, coming away with a few key conclusions. One was that while there are some gains to be made here, they’re quite small with just 1.35 watts of frictional losses separating the best pair from the worst samples that were evaluated. Another, however, was that lubricants — and possibly seals — played a bigger role than Smith anticipated.

For this follow-up study, Smith re-tested three of the top-performing BB/PF30 bottom bracket bearings, including the Enduro Bearings ZERO (0.45W lost due to friction), CeramicSpeed’s non-coated cartridges (0.42W), and Gold Race (0.29W). Stock lubricants were then fully flushed out using an ultrasonic cleaner and solvent, and then iteratively replaced with seven different lubricants: a dry-powder molybdenum disulfide, Avid Slip R/C bearing oil, 3-in-1 electric bearing oil, Phil Wood Tenacious Oil, Shimano ‘Premium’ grease, Finish Line Extreme Fluoro grease, and CeramicSpeed TT grease.

Smith evaluated each lubricant’s performance at two different ‘fill’ levels, and also tested two of the lubricants with and without bearing seals.

All tests were performed with 52kg/114lb of radial load per bearing set, which Smith says correlates to a 79kg/175lb rider pedaling at 95rpm and generating 250W.

THE RESULTS
Prior to collecting data, Smith expected the test results to roughly correlate with lubricant viscosity, with the dry lubricant yielding the lowest friction values, followed by the oils, and then the greases. However, that didn’t turn out as expected — and some speed-minded racers and mechanics may want to reconsider their accepted practices.

The speediest lubricant actually turned out to be CeramicSpeed’s ‘TT’ grease, whose foam-like consistency supposedly behaves more like a low-viscosity oil under load and dropped the average total friction of the three bearing sets to a scant 0.13W — essentially nothing, and just one-third as much friction as stock CeramicSpeed non-coated hybrid ceramic bearings with standard ‘all-purpose’ grease.

Coming in at a close second place was the ultra-light Avid Slip R/C bearing oil (0.15W), followed by the Finish Line Extreme Fluoro grease (0.21W), and then the 3-in-1 bearing oil (0.23W). Almost without exception, all of those lubricants recorded their best value at lower fill rates — 25% for greases and a single drop for oils.

Shimano’s notably thick and sticky grease performed as expected with 0.39-0.46W of friction, depending on fill rate, but the results of the Phil Wood Tenacious Oil were surprising. Five drops increased the average measured friction of the high-end hybrid ceramic bearings tested to a (comparatively) whopping 0.79W per pair — roughly on par with an inexpensive Shimano 105 bottom bracket — while a single drop still performed worse than the factory-applied lubricants with 0.47W of drag.

Smith also noted some unusual instability in the bearing’s friction performance while testing at the higher fill rates.

“A condition called ‘churning’ can happen when too much oil is in a bearing,” he told CyclingTips. “With churning, it seems the balls can actually slide against the races rather than roll.”

Meanwhile, the molybdenum disulfide dry lubricant results were surprisingly middling with 0.58W of frictional losses on average.

Smith’s findings on the effects of seals were more straightforward. While seals added more friction when the bearings were lubricated with grease than oil — due to viscous drag between the inner race and seal lip — they contributed little to the overall drag. On average, seals added just 0.02W of friction for bearings lubricated with the Avid Slip R/C bearing oil, and 0.11W for grease-lubricated bearings.

IMPLICATIONS FOR HUB BEARINGS
Smith limited his study to BB/PF30-sized bottom bracket cartridge bearings but based on previous experiments, he was able to extrapolate how similar changes in lubricants might affect friction levels in hubs — in particular, ones built with a pair of cartridge bearings up front and four cartridge bearings out back.

According to Smith, such hub configurations generate roughly 5 1/2 times as much friction as a pair of bottom bracket bearings alone. Using that ‘5.5x’ multiplier, Smith contends that replacing factory bearing grease to CeramicSpeed TT grease throughout both hubs and bottom bracket could save as much as 2.5W.

TAKEAWAYS
Again, we’re not talking about huge numbers here but Smith’s study suggests that racers — and especially time trial racers — can find some free speed for not only fairly minimal effort but also very little cost. Even the most expensive high-performance option tested — the Finish Line Extreme Fluoro — retails for just US$15.

However, those friction reductions will reduce long-term durability, especially in harsher weather conditions. Even with the seals in place — which Smith recommends, based on his data — replacing thicker factory-applied grease with lower quantities of a lower-viscosity alternative will decrease the bearings’ ability to resist water infiltration. In addition, none of the high-performance lubricants tested are designed to be used for extensive periods of time. CeramicSpeed, for example, recommends that its TT bearing grease be reapplied after just 1000-2000km.

Even so, 2.5W means nothing to the average cyclist. It’s far too little to be felt and most of us aren’t going to care if a buddy beats us to a coffee stop by a bike length. Racers in peak form can train for weeks to make those gains physiologically, however, Smith’s theoretical 250W rider can gain roughly six seconds over a flat 40km time trial by saving a single watt of friction — and for a few bucks and some elbow grease, that isn’t half bad.

To download the complete report and detailed test protocol, visit www.friction-facts.com.

Source: Cycling Tips