Nerding Out: Why You Shouldn't Worry About Pedal Kickback

At World Champs in 2014, Neko Mulally snapped his chain out of the gate but went on to score a career-best 4th place. The following year, Aaron Gwin snapped his chain in Leogang, and he went on to win. After crossing the line, he could be heard saying "The suspension works so much better without a chain."

How can we explain this? It's obvious, right? Pedal kickback.




In virtually all suspension designs, the rear axle gets further away from the bottom bracket as the suspension compresses - that's because the main pivot is above the bottom bracket. This causes the length of the chain span from the cassette to the chainring to increase, which is known as chain growth. If the suspension is compressed while the rear wheel doesn't rotate, the cranks will be forced to rotate backwards through a certain angle because the cassette moves further away from the chainring. This is pedal kickback.

If the suspension moves through its entire travel without the wheel rotating, the cranks would rotate backwards by an angle that can be as much as twenty degrees or so, depending on the suspension design and the gear selected. Generally, bikes with more rearward axle paths and therefore higher levels of anti-squat will have more chain growth and therefore more pedal kickback.

Hucking

It's often said that pedal kickback leads to harshness, feet being blown off pedals and restricted suspension. But pedal kickback is more of a theoretical calculation than a real-life phenomenon. When riding, the crank doesn't usually rotate back to allow the chain growth; the cassette rotates forwards. As long as the wheel is spinning at a reasonable speed, it's very easy for the cassette and freewheel to rotate clockwise, which allows the suspension to compress without the cranks rotating.

You can see this in the below huck-to-flat video. Here the bikes are going quite slowly and the suspension is compressing pretty fast - quick enough to reach bottom-out - yet the cranks don't rotate backwards. In fact, the chain has enough slack to wave around and sag under its own weight. For the chain to rotate the cranks, it would need to be taut which would make it roughly straight.


There are situations where the wheel may be rotating so slowly and the suspension compresses so quickly that the cassette and freewheel "catch up" with the wheel speed and so cannot rotate any faster, in which case the chain will become taut and apply a backwards torque on the crank. You can feel this if you land a drop to flat while travelling at a very slow speed.

But even in a worst-case scenario - a bike with a lot of chain growth, the suspension compressing at a particularly high speed (five meters per second), and the chain on the smallest sprocket so the wheel has to rotate further to spool out enough chain - pedal kickback is impossible when travelling above about ten miles per hour or sixteen kilometres per hour. Remember that's a worst-case: for most combinations of bike choice, suspension compression speed and gear choice, the minimum speed required to avoid kickback is much slower.

I'm not saying that pedal kickback never happens, but it's what you might call a corner case, a rare combination of circumstances. You certainly can't blame it for repeatedly blowing your feet off the pedals on a rock garden.

Braking


One exception to this is when the rear wheel is locked up (skidding) while riding over bumps. Even for experienced riders, lock-ups are common, especially on rough terrain, and while the wheel is stationary pedal kickback could come into play. But lock-ups usually occur when the wheel is unloaded, for example on the back-side of a bump or step, and the suspension isn't compressing in those situations anyway. When the wheel hits the ground hard enough to compress the suspension by a meaningful amount, the wheel usually has enough grip to start rotating again. Sure, there will be times where the suspension is trying to compress but there isn't enough traction to get the wheel spinning (panic-braking on slippery, rough terrain), but this is another corner case, and you probably have bigger problems in those moments. To be fair, even a slight lag in the time taken for the suspension to start compressing freely could be significant, but based on the many bikes I've ridden I can't say I've noticed the ones with more chain growth feel harsher when braking.

Pedaling

One area where bikes with high levels of chain growth can perform worse is when pedalling over bumps. That's because the change in the length of the upper chain span as the suspension compresses can upset the pedalling rhythm, and because the chain tension caused by pedalling can restrict the suspension from compressing. On the other hand, high chain growth means higher anti-squat, which improves pedalling efficiency up to a point. Bikes with an idler pulley (and therefore minimal or no pedal kickback) do tend to pedal more smoothly over bumps, but the idler compromises drivetrain efficiency which is less than ideal if pedalling is a priority. Meanwhile, many of the other devices designed to minimise pedal kickback (like those pictured above) don't work when pedalling anyway.


Conclusion

The reason why bikes with a high pivot and idler pulley perform better in chunky terrain isn't so much the lack of pedal kickback, but their more rearward axle path. For bikes without idler pulleys (most bikes), having a more rearward axle path inevitably means more pedal kickback, so bikes with more pedal kickback may perform better in the rough. Ultimately, the amount of pedal kickback a bike has is just a side-effect of how much anti-squat it's designed to have and isn't something that affects performance much in itself.

So how come World Cup racers ride faster without a chain? I think the main answer is they don't. Snapped chains aren't uncommon and almost always result in a disappointing race run. Gwin and Mulally are memorable exceptions, and tests have shown that not pedalling can be a good strategy on downhill runs even with the chain on, because the energy saved can be better used on the technical sections. Sure, the chain slapping around wildly could create additional vibration and the derailleur's clutch might impede the suspension movement (very) slightly, but even these effects aren't directly linked to pedal kickback, so fitting your bike with a neutral gear or a slower-engaging hub won't solve them.

---