• Rotor System cranks
• RS-4 Ti $865 (about £480); RS-4 Cr-Mo $665 (about £380)
• Rotor

Remember Biopace? It was Shimano’s late-80s attempt to smooth out our pedal strokes and take the pressure off our knees by using non-round chainrings to vary the gear ratio during each crank revolution. You could hardly buy a bike without it for a couple of years, but gradually the non-round rings got more and more circular (having started off with corners on them) and some time in the early 90s the whole idea was quietly dropped.

There’s something behind the thinking, though. Unlike an engine, it’s hard to get a smooth power delivery out of a pair of legs. And with conventional cranks you’ve got Top Dead Centre to deal with – that point when one crank is straight down, the other one’s straight up and you’re perched on top desperately trying to get the leading crank to go down.

This is where the Rotor System crank comes in. It’s been around for a few years, but the Spanish manufacturer doesn’t have the marketing muscle of the big boys so you may not have heard of it. The original concept was developed by a Spanish university, and a company formed to produce and sell it. But what does it do?

It’s a simultaneously simple and sophisticated concept. Conventional cranks have a top dead centre because both crank arms are aligned at 180°ree; to each other. In an impressive bit of lateral thinking, the Rotor does away with that and articulates the drive side crank on a pair of linkages. When the left-hand crank reaches a vertically downwards position, the right-hand one has gone past vertically upwards and is poised at the top of its power stroke ready to go. As the each crank approaches vertical, the linkages accelerate its opposite number forwards over the top of the stroke.

It looks completely bonkers when you turn the pedals on the workstand. Of course, you’ve got the get them on to a bike before you can experience this for yourself, and you’ll need to forget a lot of what you know about fitting cranks. The Rotor comes with its own bottom bracket assembly and the right hand crank and linkages are attached to this out of the box. Fitting involves locking out the linkages with the supplied bolt and threading the whole shooting match in to the bottom bracket shell. Not forgetting to mount the inner chainring, of course. You’ll have to do a bit of measuring to get the chainline right, and the actual orientation of the BB in the shell is important – get it wrong and the cranks’ll be articulating in the wrong places. The final steps are putting the lockring on the non-drive side, mounting the left-hand crank (it’s a regular square-taper item) and mounting the chainrings. The instructions are reasonably clear, and it’s not difficult. Don’t expect to chuck ’em in and go straight out for a ride, though – the system relies on Loctite to keep the BB in the right position so you need to allow a few hours for it to set.

What you can do is turn the cranks by hand and boggle at the action. With the extra bearings and cam-action cleverness going on, it’s not an endlessly free-spinning arrangement such as you may be used to – turn the cranks and the whole thing feels stiff and, well, lumpy. But we don’t ride bikes attached to workstands by pedalling with our hands…

We had a preconceived idea that the Rotor cranks would feel weird to ride. This is partially from looking at them, partially from spinning them by hand but mainly because Rotor says they will. Different riders take different amounts of time to get used to it, we’re told, anything from a few hours to a couple of months. And they did feel distinctly odd when we first set off. To achieve a variable alignment of the two cranks, inevitably they can’t spin at a constant speed – each crank accelerates towards the top of the upstroke so it passes through TDC faster than the other crank passes through bottom dead centre. This initially gives the impression that you’re riding a bike with square wheels. As soon as we got out of the saddle, though, the point of the Rotors started to become clear.

On low-rev standing climbs we quickly felt at home. You wang the leading crank down, get ready to make that boot-scraping action at the bottom of the stroke to help the other crank over the top and then notice that said crank is already over the top and you’ve got another power stroke in. In situations where you’re effectively pushing one pedal, then the other, you don’t really notice the cranks moving at different speeds because you’re not really thinking too much about what the current non-driving foot’s actually doing. You just notice that it’s a lot easier to turn the pedals. And we mean a lot – we were initially impressed that we seemed to be running one or two sprockets higher at the back, and then noticed that the stock middle ring on the Rotor crank is a 34 rather than the 32 we’re used to.

That sort of pedalling is one thing, but what about spinny stuff? One of the reasons that Biopace never caught on is that a lot of opinion-forming experienced cyclists found that it interfered with their spin. They’d spend years training their legs to pedal in circles at high revs, “staying on top of the gear”, perfecting their souplésse and all that. Non-round chainrings seriously messed that up, but once we’d got used to the Rotors (which we were pleasantly surprised to find took all of ten minutes) we could happily spin away at 90+rpm. We were even more pleasantly surprised to find that we could happily chop and change between Rotors and normal cranks without any trouble at all.

Rotor Systems has a huge wodge of studies and research showing that its cranks offer all sorts of impressive benefits – 16% more power output, 15% reduction in lactic acid, 5% reduction in pulse rate and so on. We’re not in a position to verify (or otherwise) these claims, and we’d be very surprised to see quite such clear-cut results in the real mountain bike world – on mixed surfaces and obstacle-laden trail there are a lot of other variables affecting performance. But we’re confident that they do work – being able to turn considerably bigger gears by pedalling at the same speed looks like a good thing to us. The question is, do they work well enough?

If there were no downsides that’d be an easy question to answer. But you rarely get something for nothing, and the Rotor cranks are no exception. There’s an inevitable increase in complexity with cunningly-articulated crankarms, and while we haven’t experienced any issues with the test set there is more here to wear out than on a regular chainset. Having the drive-side crank and BB as one piece makes them rather fiddly to work on (especially now that we’ve been spoiled by the ever-so-easy Hollowtech II and similar systems) but they are user-serviceable.

The square-taper spindle is a couple of steps back along the crank/spindle interface technology path, but plenty of people have never found it lacking and probably never will. Of more concern is the weight. The current Rotor offering is the RS-4, a lighter, sleeker system than the RS-3 pictured, but even in its titanium-spindled incarnation you’re looking at the best part of an extra pound over a lightweight conventional crank. Depending on your attitude to such things, that’s quite a chunk. But adding weight to increase performance is by no means unprecedented in mountain biking – suspension forks are a lot heavier than rigid forks but these days few would argue that the extra weight isn’t worth it.

The real stumbling block, though, is price. You’re looking at at least 50% more cash than XTR – twice as much for the Ti version. You could pick up four sets of XT Hollowtech II cranks for this kind of money. Which puts the Rotor firmly in the high-end performance category – you’re going to have to be very keen to get faster to splash the cash.

One thing’s for sure, there’s a lot of potential here. We’re looking forward to future generations of Rotor crank – it’s hard to see the weight ever getting near XTR levels but we’d like to think that the cost could come down over the next few iterations. And really it’s the price that’s the main obstacle to this system reaching wider acceptance.

Positives: Do what they claim, well-made, reliable

Negatives: Heavy, expensive, actual crank arms a bit low-tech

From our standpoint as reasonably brisk recreational mountain bikers, we’re pretty convinced that Rotor cranks work. It was pretty obvious that we were pushing bigger gears without any noticeable extra effort, and that’s got to be a good thing. On the other hand, even if you get as big an advantage as Rotor claim, you’re paying big money for it. Unless you’re taking speed seriously and you’re already very highly-trained, a Rotor crank is unlikely to be a worthwhile purchase – most of us would see bigger gains from hiring a coach for a few months. But in the context of high-end race kit, these cranks are only a bit expensive and if your body’s already giving all it’s going to give, Rotors are well worth a look.

Performance: 4/5
Value: 2/5
Overall: 3/5