Fellow nerds,
Since I'm a believer in pedal kickback now, I have a new problem to solve. There are too many pedal kickback devices to test, and it would be nice if there were a way to quantify what I'm feeling, or even better yet go around my feelings entirely and quantify what's happening at my rear hub, chain, and cranks whether I'm feeling it or not.
If you wanted to test a pedal kickback device accurately, how would you go about measuring the force of pedal kickback? I included a poll below, asking where on the bike and what force you would measure if you could only measure one thing (obviously, in a perfect would you'd measure all of these forces in all of these places).
I'm including this video because it's the only example I've seen of someone measuring a PK force on their bike. It's a pretty wild setup, because he's computing the equation for chain growth in the upper chain (you could do this using readily available pedal kickback graphs, but homie calculated it from scratch the hard way), the instantaneous position of the rear axle relative to the main triangle (using a potentiometer), and the rotational acceleration of the rear wheel (using a buncha magnets) so he can calculate the rear wheel impacts that we would expect to "chase" the freehub faster than the wheel is spinning and create PK force.
I'm not nearly smart enough to replicate this setup... so I'm wondering if any of you bright people have any suggestions for a better/simpler/easier to replicate method of measuring PK force. Lemme know below.
You have to measure at the crank as opposed to at the cassette because we aren't sure how much is just the chain flapping about and not PK at all.
what's the SI unit for measuring kickback?
Just plug it into Linkage like a lazy person lol. Cute graph and everything.
instead of measuring forces, why not displacement at the pedal spindle (rotational/angular distance)? that's essentially the riders contact point with the system.
Here's an angle nobody has mentioned yet: isn't pedal kickback a little bit like free acceleration?
Suspension compresses = chain "tightens" = assuming no movement from the rider's foot = cassette turns forward.
I’m not sure pedal kickback force is the right metric for quantifying the rider experience.
I think frequency or duration might be a better metric to correlate to what the rider feels on trail.
High speed video would be my first choice to see what’s going on. Find a section of trail or a set of stairs or something repeatable and see what’s happening.
Why not ask the manufacturers for their pedal-kickback values? Or do you actually want to measure it via instrumentation?
Would need a standardized model. What style of suspension, how much travel, length of crank arm, gearing...
Also PBK is typically communicated in degrees so easiest to do this in the stand is: remove shock, lock the brake up, level(ish) the crank arm and preload it by hanging a weight off it, attach an inclinometer to the crankarm and record the difference between starting and ending angles.
PBK is dependent on the exact instance that the chain speed exceeds the wheel/FH speed (meaning full FH engagement) and does not include chain forces caused by bumps. Also note that the above PBK method doesn't exact take into account the full range of degrees if the values drop mid-travel cycling. You'd want to observe the live readout to see if there's a peak value before the end.
the main items to standardize would be ring and cog size used. Crank arm length would always yield the same degree value HOWEVER shorter cranks will travel a smaller distance vertically but a longer distance horizontally. I'm not sure how important that horizontal measurement is in terms of feel, but that vertical distance sure is. If someone is running a 155mm vs 175mm crank arm on a bike with 25 degrees of PBK, that's about 9mm difference vertically.
Have you considered reaching out to Matt at Cass Labs? Last I saw him he was running an ochain and said he was hoping to do a follow-up to that video you posted. Pretty approachable guy.
In my mind I'm not sure if your 'you can only choose one' directive could work, I don't see how you could isolate the cause to be pedal kickback rather than some other input without multiple measurements.
My 'other' suggestion would be weight sensors on the pedals, looking for an abrupt increase in weight on the leading foot and a decrease in weight on the trailing foot. This is what I think my experience of PK is, and is the thing I'd be hoping to reduce with one of these devices.
I wonder if the Garmin pedals would give you the readouts you are looking for.
The simplest thing I can think of would be to just put a powermeter on the cranks. Pedal kickback is essentially just torque on the cranks, which is exactly what a powermeter measures.
I imagine a crank-based strain gauge or power meter would be your best bet, they normally use something like an IMU to detect if the crank is rotating so you could separate out pedalling related torque from pedal-kickback. Most power meter products transmit data at a fairly low sample rate so you might need something a bit more custom which would be easier to combine with braking and suspension data at a higher logging rate too. Guys like BrakeAce (their technology is basically a powermeter) or syn.bike might be best to supply something.
The challenge with that would be filtering the noise.
Angular speed vs shaft speed, but I watched that video too! You'd want at least a button, or some kind of event for your "human" feel of pedal kickback to correlate to see what you see in graphs.
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