Rise of the machines: PRIMO Cycles STT 3D motion capture bike fit review 

Cycling has long been a numbers game – but can motion capture give you a better bike fit?


veryone who watches professional racing admires not just the speed, athleticism and instinct, but the form of the riders, too. The supplesse, as it’s known. That effortless combination of rider and bike that hangs on the road like art in a gallery.

While it’s exact definition is widely interpreted, you generally know supplesse when you see it. It’s form that comes from riding a bike obsessively and insistently, but also through fit to a bike that makes pedalling near perfect to watch.

Today the bits and bytes we generate while cycling is bringing that level of bike fit to more people than ever before (the first part of getting that form is still up to you). And as it gets interpreted by a new breed of bike fitters – people with race backgrounds, a deep understanding of human physiology and most importantly, passion – it just might change the way you sit on your bike, forever.

Is this the most precise, aerodynamic and comfortable bike fit ever? Or just another data set in a sport that already relies heavily on them? In a relentless search for perfect pedal strokes, we test the STT 3D motion capture system to find out.

Everything starts with fit. 

In the modern peloton, there is perhaps no other rider that better exhibits proper bike fit than Danish professional, Jakob Fuglsang. That’s the opinion of Maurizio Bellin, general manager of Full Speed Ahead – a company whose purpose in cycling is to exact better fit by making lighter, stiffer, better cranksets, power meters, stems, bars – anything that will somehow help a rider perform. While Bellin believes being fast on a bike has more to do with the individual rider than the bike (or its parts), he explains how modifications to fit can unquestionably help performance.

“Bike fit is critical – everything on a bike comes down to that important first step,” he says during an exchange over email. “Teams are using the high-end of everything because there’s no compromise between weight and stiffness, it is pure performance product, but also because it is part of an exact fit that helps a professional perform their best.”

So how does an average rider miles away from pro racing get a similar fit? Ben Hallam, the head fitter at PRIMO Cycles, a Cambridge-based performance bike shop, has some ideas. He thinks the way most cyclists get fit is “wholly outdated.” A method called KOPS (knee over pedal spindle) – which would probably be familiar to most cyclists. It places riders over top their bottom bracket and their knee in line with their pedal spindle to get fit on a bike. An objective way to get a mass audience on a frame and according to Hallam, an imprecise one, too.

“Hip mechanics and body weight balance – these are far more important for determining fit than where your knee lands as it rotates over the pedal spindle,” he says, sitting in front of two Matrix-style monitors, a Silca tool set and a spindle of half-inch velcro stickers. “What most traditional fitters neglect are position changes from hoods to drops and its fit implication. With 3D motion capture, we account for all of it.”

Three-dimensional motion capture – while you may not have seen this in your LBS, you soon might. It tracks pedaling at a very high speed (one hundred frames per second, for you sticklers), and processes every bit of motion to produce data on angles, velocities, positions, distances and trajectories measured when you pedal. STT, the company that spent two years perfecting the tool used at Primo say it yields “a powerful visualization and dynamic dashboard” that is then able to comment on rider form.

But is that data set enough to exact the perfect fit? At Hallam’s office I start my first-ever STT measurement to see if our legs, lungs, arms and chest have an answer. It is ten minutes of discussing with Hallam goals, injuries, riding style and what I want from cycling. Minutes later I am watching a skeleton of myself pedalling.

Data, data everywhere.

First motion capture assessment. 

When part one of the assessment ends, previous fits in my cycling life seem like time I could have spent doing hill reps. Sure, body types change. Weight goes up and down. Goals change. But two dozen motion capture stickers don’t lie. Neither does a 3D image of your skeleton. And placed in the right hands, what it says about you as a rider – bike geek or not – is revelatory.

My hips move a lot while pedaling – the first harsh attack of motion capture reality. Not uncommon in a lot of cyclists, but mine rock up and down when I pedal and aren’t helping my Koblet-style smoothness. My knees also make tiny circles when I ride. No biggie, but news to me, and most likely related to a lifetime of inflexibility, otherwise known as hugely inefficient for going fast.

I see numbers measuring leg, hip, foot, and upper body movement relative to casual, competitive and race profiles. A bunch of others that make absolutely no sense to me, too. This is where Hallam inputs the experience, excitement and interpretive abilities of a DS. We spend the next twenty minutes making sure I grasp what I’m seeing and more importantly, how we can make use of the numbers.

Inflexibility is something I’ve dealt with for years but never seriously addressed – by having stretches suggested and motion analyses done on a massage table – in two previous fits. The hunched position I’ve adopted to accommodate a dropped stem and elevated saddle height – pro, after all –  have also not doing me any favours with respect to my range of movement limitations.

As the fit process then moves off the bike, we pour through all of critical contact points, specifically cleat and saddle position, and consider what adjustments would meet my “profile” as well as my (in)flexibility, range of motion, injuries and aspirations.

Something’s different here.

Outcomes from my fitting: obvious to some but in no way does that diminish their importance to proper fit.

A change to stem length (from 100 to 110mm) has created more room between knees and elbows and more comfort riding in the drops. My saddle is moved forward to bring the seatpost and rear rails in alignment which gives me much better contact between my Antares saddle and sit bones. My core actually feels engaged after rides (does that mean I’m finally using it?). My cleats are also moved way back to eliminate hot spots and generate better leverage in each pedal stroke.

Minor adjustments? Personally I thought these had been dialled during previous fits and didn’t need further adjustment. Exacting those crucial contact points however is a guessing game (at best) without the benefits of a powerful triumvirate: motion capture, data and a trained eye to interpret it. (No one would have ever made saddle changes like these in past because the rocking in my hips and knees would have been impossible to spot.) (No more circles, less wasted energy).


One hundred frames per second (versus Retul’s 18) don’t lie. Nor does two hours of sitting with an individual who has the experience, passion and knowledge to pour through irrefutable data to help you perform your best on the bike.

Like I said earlier, even if you’ve had a fit before – bodies change. And so do cycling goals. The subtleties revealed in the STT HD cameras are not only a window into becoming a faster cyclist and an investment in injury prevention, it’s an investment in form equal to the cost most spend on energy gels in a season. Entirely worth it.

But the data and the machines producing it is only one part of proper fit. And sure there are app-based fit tools that might get you close. However the fitters working with 3D motion analysis have in their hands what is maybe one of the wealthiest data sets available about you, the rider. Add in the ability to interpret it and that sought after pedaleur de charme moniker is closer at hand than ever before.

A STT 3D motion capture bike fit at PRIMO Cycles costs from £200. You can book in or contact Ben by emailing bikefitting@primocycles.co.uk or just pop in and see him, he and the rest of the guys in store are really friendly.

PRIMO Cycles is not just about bikefit or overall performance -It also to acts as a popular meeting/socialising place for Cambridge’s cyclists running regular rides on weeknights during the summer. It’s also a dealer for Trek & Cervelo bikes (alongside some other stunning brands), along with parts and accessories, and has a busy repair shop – and it’s not all high-end stuff, as on my visit there was a large number of ratty commuter bikes awaiting attention.

About the author

Mark Cohen has held strong opinions about style and sport since the start of his writing career. This has worsened over time. He is a contributing writer to various sites currently living in the U.K. Connect with him on Instagram @mcohensays.

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