The image shows Wattshop's new Cratus aero cranks for SRM power meters

Mid-90s aero is still fast today: The five-watt aero gain in narrower Q-factors

WattShop's new Cratus cranks bring back narrow Q-factor aero gains for SRM power meters.

WattShop, the British watt-weenie brand founded by Dan Bigham, unveiled some new cranks last week. The new Cratus SRM Origin Crankarms are, as the name suggests, aftermarket crankarms designed specifically for the SRM Origin power meter. Manufactured from 7075-T6 aluminium, anodised black, and weighing in at 581 grams a pair (not including chainrings or spider) they aren’t ticking any obvious boxes, but, factor in the aero profiling, adjustable length all the way from 145 mm to 175 mm, and the super-narrow Q-factor (133 mm with track spindles, 137 mm with road), and they start to make a lot more sense.

Q-factor is the distance between the outside faces of a bike’s crank arms, effectively setting the pedal width and cyclist biomechanics. The Q-factor on a standard SRM Origin sits at 145.5 mm, right in the range of many road cranksets. The new Wattshop cranks offer a narrow Q-factor crankarm option that fits those off-the-shelf SRM PM9 Origin power meter spiders by swapping out the SRM crank arms.

The new Cratus cranks are CFD-optimised and mount directly to the SRM Origin power meter spider for what appears to be a truly aero-optimised power meter setup while retaining the Origin’s compatibility with 30 mm, 29 mm (DUB) or 24 mm spindles. 2X compatibility is frameset (and perhaps even front derailleur) dependent.

A brief history of narrow Q-factor

Now, while the new Cratus cranks are very much a late addition to my Santa list, it is Wattshop’s aero claims that really piqued my interest. According to the WattShop website, the 8.5 mm Q-factor reduction equates to a “rider-dependent saving of 0.0009 to 0.0017 m2,” or, in more relatable terms, a 1.5-3 W saving at 50 km/h and a 3-5 W saving at 60 km/h (“assuming a standard air density of 1.225 kg/m3“).

All that said, having tested various stance widths using pedal spacers etc. Dan Bigham told Escape Collective this week that a 10 mm drop in Q-factor could result in a 1% drag reduction. Taking a rider with a .200 CdA, thats a .002 reduction, which Bigham suggests could be as much as 7 (to 10?) W at team pursuiting speeds, or as he says, “not nothing.”

Listening to Bigham explain how “Q-factor has largely been forgotten as an optimisation variable”, I’m reminded of the old saying: there’s nothing new under the sun; It was front and centre in the mid-’90s when the likes of Graeme Obree and Chris Boardman traded the Hour Record back and forth. Boardman and Obree had taken very different routes to achieve the narrow Q-factor and stance width gains. Big-budget backing and the Lotus Type 108’s single stay and offset rear wheel helped Boardman get narrow, while Obree had, as Bigham put it, cannibalised his washing machine to the same end result.

But these understandings and gains were lost over time, initially with the introduction of the UCI’s Lugano Charter and later cemented by, as Bigham sees it, the groupset and frame manufacturers settling on a geometry they are content with, “be that bottom bracket width, chain line, chainstay length, rear dropout width etc.”

It was this background that motivated WattShop to set about developing an aero and narrow crankset. Initially, that R&D culminated in the £890 Cratus Aero crankset (without power meter), with its wild 124.5 mm Q factor first used by Ashton Lambie to break the four-minute barrier for the 4 km individual pursuit (3:59.93, a world record at the time), then Bigham himself and later Filippo Ganna for both of their successful world Hour Record attempts (55.548 km and 56.792 km). The crank looked insanely aero, and the results seemingly backed that up, but with many wanting power on their cranks and the SRM still amongst the most popular on the track, WattShop needed another option. Hence the new Cratus SRM-compatible cranks.

A top-down view on a rendering of the new Wattshop Cratus SRM Origin Crankarms
Narrow Q-factor and narrow profile – a top-down view of the new WattShop Cratus SRM Origin Crankarms.

To the best of my knowledge, this is the first time someone has put a claimed wattage-saving figure on the potential aero savings of a narrower Q-factor. Three watts at 50 km/h might not seem like that much, and even the top-end 5 W saving might seem pretty irrelevant given that’s at 60 km/h, but 1) these cranks are specifically intended for use in time trials and track races where those speeds are achievable and 2) the vast majority of these savings aren’t actually from narrowing or profiling the cranks, but rather from narrowing the rider.

What good is that if one needs the cranks to achieve that narrow Q-factor required to unlock the narrow stance width (the distance between a rider’s feet on the pedals)? An especially relevant question when you factor in the ~£1,400 you’ll need for an Origin power meter, plus the £290 for the new Cratus arms. But, that’s the thing. With most of the savings coming from narrowing the rider, there are options to reduce the stance width with your existing setup.

As mentioned earlier, the new Cratus SRM cranks are effectively a younger sibling of the original Cratus Aero crankset and benefit from much of the CFD work done in that development cycle. WattShop had looked at the somewhat asymmetrical aero characteristics of a rotating crank resulting from the positive flow in one direction and negative flow in the opposite direction. As Bigham explained, the net average is a very minimal positive flow, but even that is very difficult to measure and varies considerably with various yaw angles and how interactions with the rotating pedal while moving forward, not to mention the rider’s foot, shoe, chainring, and down tube. In other words, most of the gains are coming from narrowing the rider rather than the crank.

If you’ve gotten this far, you might well be a time-trialling or track-cycling nerd. As such, you might already own, or at least be open to the idea of, cranks with narrower Q-factors (Miche Pistard Air cranks with a 138 mm Q-factor spring to mind), but perhaps you run some spacers between the crank and pedal spindle to normalise the stance width across bikes. Removing these spacers will save you 2 mm in stance width alone.

The same might be true of other, more-typical-Q-factor crank options, which are also supplied with these spacers as they are required with some pedal interfaces (think power meter pedals especially). That said, many pedals run fine without the spacers and, as such, may offer a narrower position.

Again, if you are a TT nerd you might well be running Speedplay pedals. While Wahoo discontinued the narrower 50 mm spindles, some aftermarket manufacturers do still offer these, and even narrower options seemingly also exist.

Long story short, there are ways to reduce your existing Q-factor and stance widths, and according to Jamie Lowden (aerodynamics engineer at WattShop), that narrow stance width is trainable for most riders. While it may feel unnatural at first, WattShop has found most riders can adapt to the new, narrower position.

Xavier Disley of Aerocoach wrote a thesis on Q-factor in cycling: Kinematic and physiological effects and demonstrated that the self-selected Q-factor is usually close to the optimal Q-factor in relation to gross efficiency, but with most commercially available cranks sitting within a very narrow range, it’s unlikely riders can achieve their optimal and self-selected Q-factor.

But as Bigham explains, those afraid of narrower stance widths needn’t be and there might actually be other benefits. “It’s difficult to generalise, but narrower Q-factors tend to reduce knee abduction moment, which could be an injury prevention positive,” he said. “Your aerodynamics won’t adapt to a stimulus, but your physiology will, so really, it comes down to creating a strong, stable platform to adapt to such changes.” Bigham also pointed to riders hopping between mountain bike and road bike Q-factors without issue.

Dropping from mountain bike to road Q-factors is one thing, dropping down another 8.5 mm to the road-going version of the new Cratus cranks won’t be for everyone, as evidenced by Wahoo’s decision to discontinue the 50 mm spindle width when it released its take on the Speedplay pedal. That said, the two years since that launch is a long time in cycling tech. Might the demand for those narrower spindles and narrower Q-factors generally look very different today?

You might say only time will tell, but I dare say the demand for an SRM-only alloy aero crank is still one for the nerdiest of aero nerds. With that in mind … Dear Santa, I’d like two cranks and all the pedal inserts, please.

A side on view on a rendering of the new Wattshop Cratus SRM Origin Crankarms
The crank length is adjustable with flip-chip-esque pedal inserts. With the cranks available in short or long versions, plus the various pedal inserts, the crank length is adjustable from 145 right the way up to 175 mm.
A rendering of the new Wattshop Cratus SRM Origin Crankarms
The spindle makes up part of the SRM Origin package and is replaceable, meaning wider compatibility.

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