In this blog we have a quick look at some of the many problems to consider when designing bike rear suspension. Then look at one unintended consequence of that design: Pedal Kickback.
What is it?
Can it be solved by fitting a hub with slow engagement? (In short NO it can't!)
Can you do anything to reduce it?
Some rear suspension design issues
Let's start with a brief look at just some of the many issues when designing bike rear suspension. There is a great deal going on other than just bumps making the back wheel go up and suspension pushing it down again. The rear suspension is also affected by a list of other things. Here are just a few:
Braking (Anti Squat)
Pedalling (Anti Bob)
Chain (path) Growth (during suspension compression.)
Chain Tensioner and clutch (resitance to suspension compression.)
Of course its not just the items above that affect suspension movement but it also works the other way around. Suspension movement affects other items too. One unintended affect is called Pedal Kickback.
We'll start by looking at one specific case: When braking has locked the rear wheel, the rear tyre is sliding and the rear suspension is compressing as the rear wheel hits bumps. This is generally when Pedal Kick is felt most accutely.
What is pedal kickback?
Pedal kickback occurs when the rear wheel is not turning but the rear suspension is compressing. If the rear axle path tries to tighten the chain (this is often called chain (path) growth) while the rear wheel is stationary (i.e. brakes on) then the chain (stopped from moving by the ratchet) will pull the pedal backwards giving the rider's feet a 'kickback'.
'Snake Oil' Kickback Claims
There have been some dubious 'Snake Oil' sales claims that some hubs reduce Pedal Kickback. Those trying to sell 'slow' hubs sometimes claim that they reduce pedal kickback. This is misleading. Slow hubs make this sort of kickback more unpredicable.
The teeth of slow hub are much further apart and depending on where the wheel stops there could be a long way to move before the pawls 'drive' or kickback could be instant.
Fast hubs with ratchet teeth much closer together are far more predictable. You are likely to get close to the same feeling each time.
What is a fast hub?
The feeling of riding a fast hub is even better than it sounds. But what is a fast hub? or more accurately fast ratchet?
The term 'fast ratchet' refers to how quickly, from freewheeling, the forward motion of the pedals causes the ratchet to grip and start to push the back wheels. To put it another way: with the bike stationary if you rotate the pedals backwards on a slow hub there will be a large pedal movement between clicks of the ratchet. With a fast hub this movement will be much less or there will be far more clicks for the same angle of rotation.
The range across different hub manufacturers is massive, some hubs have fewer than 20 clicks for each revolution of the back wheel. There is no clear line or limit as to the number of clicks that makes a ratchet slow or fast but as a rule of thumb a fast hub has more than100 clicks per revolution of the back wheel.
Compared with less than 20 clicks, a 100 click hub when averaged would be more than 5 times faster in engagement pick up.
The Xeno Hub is unique with its Disc Side Drive ratchet. It allows for a larger diameter ratchet large ratchet teeth and still 120 clicks per revolution.
What are the advantages of a fast ratchet?
After all does it really matter if my pedal moves a few inches before my effort starts to drive the back wheel? The difference is most noticeable when riding up something technical where you have to keep rapidly adjusting you pedal position by stopping or backpedalling to miss rocks or roots. Getting the power back on quickly makes you far more likely to get up the climb.
The feeling of riding a fast hub is even better than it sounds. Once you have ridden one you will not want to go back.
'Snake Oil' Kickback Claims
In truth there is nothing good about riding 'slow' hubs when mountain biking. However, that's not stopped some pretty dubious 'Snake Oil' sales claims.
Those trying to sell 'slow' hubs sometimes claim that they reduce pedal kickback. At best this is misleading but to see why probably requires a quick explanation of what pedal kickback is:
What is pedal kickback?
Pedal kickback occurs when the rear wheel is not turning but the rear suspension is compressing. If the rear axle path tries to tighten the chain (this is often called chain (path) growth) while the rear wheel is stationary (i.e. brakes on) then the chain (stopped from moving by the ratchet) will pull the pedal backwards giving the rider's feet a 'kickback'.
A slow take up hub would give a far less predictable feel depending upon what position the pawl and ratchet stopped in when the rear wheel is held by the brakes. On some occasions you might feel little kickback when the pawls had a long way to move before engaging. On another occasion, if the pawls had just dropped you might get full kickback. The issue is that the amount of kickback would be random depending on the position the ratchet stopped in. The faster hub would be far more predictable at transmitting kickback never having the chance of long movement in 'neutral'.
It's surprising that 'unpredictable kickback' could honestly be sold as an advantage but, there it is, snake oil claims can be a bit slippery.
Disconnect the chain?
The chain clearly has to part to play in the problem since it is connected to the pedals. Of course most of the time it is used to transmit motion from pedals to drive the rear wheel. That's how many of us get up hill. However the opposite is also true: Any movement of the chain, however it is caused, will try and move the pedals. This can be felt through the feet as feedback.
The pedals can be decoupled at least partially, from the chain, by use of devices such as the O Chain. This allows the chain a bit of back and forth movement at the pedals (The range can be adjusted and end stops are flexible) without moving the pedals much directly. Of course there will be a lag between starting to pedal again and take and up of tension in the chain (like a slow hub) though some reviews suggest the soft take up is better than a slow hub where the take up can be harsh. A Pinkbike review of the O chain device can be read here.
If you ride technical climbs you probably don't want this disconnect or delay between moving the pedals and power to the back wheels.
Still worried about Kickback?
In certain areas of the cycling web a lot of people continue to worry about pedal kickback. Some claim it happens when the back wheel is still turning. All our testing proves this is extremly unlikely to be caused by the ratchet:
If you watch a video and hear the rear hub is clicking when landing a jump, hitting braking bumps or whatever, then pedal kickback is definitely not being caused by the hub ratchet. The clicking proves the hub is free wheeling. (The maths also confirms you would have to be rolling incredibly slowly or backwards for the ratchet to cause kickback.) The point is that if the rear is freewheeling then the ratchet in the hub is clearly innocent of pulling the chain and thus is not the cause of any kickback sensation!
This high pivot frame has virtually no kickback with the chain in this gear. (It would be a better test if the rear brake was fully stopping rear wheel from rotating) but look how far the rear mech has to travel each cycle.
Other kinds of 'kickback'
We are certainly not claiming that rear suspension or feeling through the pedals feels the same with and without the chain. Just the chain slapping about can probably be felt through the feet.
Moreover, if the axle path tries to lengthen the chain (even if just on the lower mech side) then the chain tensioner spring in the rear derailleur has to compress as the chain tensioner rotates to lengthen the chain. This movement is often deliberately further resisted and damped by means of clutches and other devices to try and reduce chain slap. This resistance to tightening of the chain will inevitable affect the rear suspension performance with or without an O chain device.
The rear mech spring and clutch is nearly always a lot less high tech than the rest of the suspension so it's very unlikey to improve suspension performance. Imagine, you go to all that work on the rear suspension geometry and damping - maybe even get it near perfect and then somebody goes and adds an abritary spring and clutch which totally mucks things up. Designing rear suspension must have its frustrations.
Kickback to the future
What about the future? While removing the chain continues to makes the rear suspension work better there will always be opportunities for enhancements to suspension mechanics, chain path and how chain tensioners work.
You'd be better off asking somebody else to predict the future but I think the rear mech will be with us for a while.