Here is a thing that has confused me for some time, so maybe some mathematically inclined person here has some insight on this. From what I read on the forums Q-factor is defined as the orthogonal (to the cranks) distance of the pedal mounting points. This definition doesn’t seem fully correct for several reasons:

It’s a distance, not factor (i.e. a number you multiply another number with)

With “zero” q-factor the number isn’t zero at all.

There is no word starting with q in the definiton.

Somehow I can’t believe that this is just a badly chosen name, there must be more to it than what I understand at this point.

(Feel free to link to other threads if this has been discussed previously…)

I suspect it is a badly chosen name, considering it is short for “quack factor”.

From Wikipedia: " The Q Factor of a bicycle is the distance between the pedal attachment points on the crank arms, when measured parallel to the bottom bracket axle.[1] It may also be referred to as the “tread” of the crankset. The term was coined by Grant Petersen during his time at Bridgestone Bicycles.[2] The “Q” stands for “quack”, a reference to the wide stance and waddling gait of ducks."

Zero Q cranks would not actually give you a zero Q factor. They just don’t contribute anything to it. Bicycle cranks almost always have a bit of a Q as they have to flare out to clear the rear stays.

I totally agree, that the q-factor cannot be zero.

Normally the q-factor is about 180mm (muni with disc brake inside the frame) or 205mm (muni with disc brake outside the frame). Race or freestyle unicycles may have lower q-factors.

In my opinion people talking about “zero” q-factor cranks are actually speaking about the shape of the crank, if the outline of the crank is curved towards the end (non zero q-factor) or not (zero q-factor).

It’s a prejudicial name. Let’s consider the “Q” referring to “quack” and further to the wide stance and waddling gait of ducks. This already implies a negative effect for higher values in the naming.

It’s a biased name. While both a too wide and a too narrow stance width can have negative effects, it only accounts for one side.

Great, now I’m disappointed. I spend hours on the unicycle thinking about what the actual definition could be such that the word factor actually applies and now it turns out that someone just chose the wrong word because they didn’t bother to check what it actually means…

Welll, at least I can now think about something else when unicycling.

Considering the number of bikes he’s designed, I’m pretty sure Grant Petersen knew exactly what factor meant and didn’t give a damn when he coined the phrase. He’s kind of a curmudgeon that swears bikes were at their zenith when they had lugged steel frames, leather saddles and friction shifters.

There is a meaningful, alternative definition of Q-factor which is actually a factor (i.e., a ratio): Q₂ = w/h, where

h is the distance from the top of your seat to the hub (or length of legs), and

w is how much further out the pedals force you to place your feet, in relation to where you would naturally place your feet (say, when standing up).

So if Q is the usual Q-factor and your feet are a distance d apart when standing up, then Q₂ = (Q–d)/(2h). A Q₂ factor of 0 would mean your gait on the bike is the same as when standing up. If my 10-year old son had the same Q₂ factor as me, then he would look like a miniature, zoomed-out version of me.

Of course, this definition depends entirely on the physiology of the user, so I don’t expect it to catch on anytime soon.

Indeed I can try. Like every emoji or abbreviation that indicates some form of laughter, (ahem “lol” ahem), nowadays it basically just means that “this-short-sentence-is-not-aggressive-and-here’s-an-emoji-or-abbreviation-to-prove-it.”

In this case it means I’m not preaching or being too serious.

But I always hate it when someone gives in a thumb-up in a text message (on the phone). It always feels like “whatever” to me. Even though I know it most often just means “got it” or “understood”.

Now this takes the discussion into the direction I hoped for. The one definition I considered was the orthogonal distance between pedal mounting points divided by axle width. The problem with it is that it is 1 at zero Q, not zero.

Now that I see your idea I would suggest another alternative that can be calculated solely from the unicycles geometry without relying on the physiology of the rider:

Q_3 = (w_p - w_a)/w_a

where

w_p is the distance between the pedal mounting points measured orthogonally to the cranks.

w_a is axle width (the full width, not between bearings.)

The downside of this definition: The numbers tend to be quite close to zero. But zero Q is zero Q.

Even worse, it takes two easy to understand and work with numbers, and turns them into a useless one…

I’d work the opposite side of the problem and just call it: “Pedal offset” (for the definition that only involves the crank) or “tread”/“pedal spacing” (for the total distance).

Q_3 = 0 means that w_p = w_a, but that’s not the same as saying that you don’t need to widen your stance when sitting on the unicycle. Take, say, a mega axle with w_p = w_a = 50 cm.

I think a simpler, rider-independent, but still meaningful quantity would be S = Q / h
(“stance factor”?) where Q is the usual Q-factor, i.e. distance between pedals, and h is the vertical distance between axle and seat. Then S would never be zero, but it would give a meaningful measure of how much a rider with seat height h would have to “quack” when sitting on the bike.

Thread spacing means something different, but I like “pedal offset” and “pedal spacing”.