Bike Chill-Factor (C-Factor)

Bike sizing by a person’s height is bullshit. Some more enlightened types will use pubic bone height (PBH), which is a step in the right direction. But I think it’s still bullshit. I haven’t cracked this yet, but I’ve got one more arrow in my quiver.

This angle… remember it. It’s the C-Factor. Thanks to Colin for the diagram!

It’s not only leg length that determines fit, but also torso length, arm length, posture, flexibility… the list goes on. So why is it that we try to measure bikes by listing one arbitrary measurement of of the frame?! It drives me crazy.

For a while now I’ve been playing with stack and reach and I think they are the most important measurements on a frame, but it gets difficult when you try to compare the relative fit of a bike across multiple sizes. So what I needed was a way of looking at the average ratio of stack to reach across all sizes of a particular frame. Just doing a simple stack÷reach calculation was a step in the right direction but then a fellow bike nerd with a bigger brain and a more than elementary understanding of math reached out to me and suggested a really cool way to do this:

“Obviously, there are many different styles of geometry. Modern endurance road bikes advertise their upright riding position which can make a long ride more comfortable, and modern mountain bikes are all about getting “long and low”. To be able to tell where a bike falls along this spectrum, I measure the angle between a flat line projecting forward from the bottom bracket and the Pythag BB -> HT line and call this, rather brilliantly, “Angle of BB -> HT”.

Angle of BB -> HT = ARCTAN( stack / reach ) in degrees

The larger this number the more comfortable a bike’s fit should be, all other things being equal. For example, the comfort oriented Fargo has an angle of 59 degrees, while XC racer SC Tallboy has an angle of 51 degrees.” – Alexander Sollie from The Earth Remains

So we’re dealing with angles, and that’s cool because we can simply average those up for all sizes of a frame or even all bikes from a particular manufacturer. Finally we can rank bike manufactures by who has the chillest ride (stay tuned for more on this)! I think it’s a profound way to think about bike geometry.

So without further ado, here’s the ranking of frames by c-factor (most chill at top):

  • Alexander Sollie

    This is fascinating!

    1) I love “Chill Factor”. I think you’ve coined the official name for this metric.
    2) It is amazing how big of a spread some of these have, and how some bikes get more chill as they get bigger, and some get less. This means that you really can’t assume the feel of a bike will be comparable if you’re reading a review written by someone riding a different size of the same frame.

    • hobocross

      Haha, I hoped you wouldn’t mind that silly bit of terminology. I guess we could call it C-Factor and then people with a long reach can consider it “Cramped-Factor” 😉
      That’s a really good point about the range of angles and part of the reason that I really want to expand the data set to include the full range of sizes. Because it’s generally people on both ends of the size spectrum that get screwed here. And there’s some surprises like with the new Surly Karate Monkey 27.5 where the XS-M sizes all have the same stack (I guess they realistically couldn’t lower it any more) while the reach gets progressively shorter. You basically have 3 different bikes at that point.
      Again, thanks for sharing your work and being open to collaborating on this. Once I can get your list of bikes loaded in to the DB I can out together some views for the tall cyclist crowd that should really complement your work.

  • Kevin Rasco

    I have been doing similar calculations on drop bar all-road type bikes. I love that you have so many analyzed the same way. For me comparing 5 bikes with all the different ways mfg’s measure stuff was hard enough. Kudos! I tried to add a Soma Fogcutter… not sure I did it right.

    • hobocross

      Hey I just manually refreshed and the Fogcutter is in there. Yeah, I now what you mean about the seatpost thing. I have a whole post on the same thing that I ended up simplifying to this. I think a super tall seatpost is usually a sign of poor fit and sometimes it’s entirely unavoidable for a particular frame. As a long-legged guy, this is my bane.

  • kimbo305

    A diagram at the start of the article for BB-HT angle would do wonders to help grasp the chillness.

    e: also, it occurs to me that a slack seat tube angle, assuming normal saddle offset, would counteract chill angle. Maybe “rotate away from effective chill angle” is more precise.

    • hobocross

      Yes! I know I’ve been holding this up because I wanted to make a diagram but I have no idea how to do that… maybe I can snap a picture of a napkin drawing. Good point about seat tube angle, although I feel like I have some more studying to do on that front really understand this. I always followed the KOPS method of bike fitting which seems to basically disregard seat tube angle… but recently I got an old StumpJumper with a ton of setback and it feels so good. I know Grant Petersen of Rivendell likes to put you farther back from the BB too. Interesting stuff.

      • kimbo305

        > basically disregard seat tube angle

        No, that’s correct — each person has some natural setback from the BB that works for them. Though each frame’s STA “implies” some expected setback, you can usually dial around that with seatposts and sliding the rails. Still something to consider as a design intention, given that manufacturers have mostly free rein to set that angle.

  • Matt Larrabee

    Added specs for Black Mountain Cycle’s Monstercross frameset, how do I see it on the chart now? Cool idea!

    • hobocross

      Thanks Matt- charts usually refresh once a day but I just did it manually and it’s in there now. Thanks for contributing!

  • TP

    Have you cracked the nut on how to compare flat bar geometry to drop bar geometry? Like in the Fargo to Tallboy example, it’s not an apples to apples comparison due to the effective hand position of the Fargo being lower and more forward that the stack/reach suggests. Amirite or amidumb?

    • hobocross

      Yeah you are absolutely right about the flat-vs-drop thing and the best idea I have yet is to give you the orange/blue color coding and a filter to distinguish here. It’s definitely an apples-to-oranges comparison but since I know there are a lot of people trying weird stuff with the “wrong” bar types, I think it’s still useful to be able to compare drops and flats in the same space.

      Maybe one day I’ll do a comparison of the % change in reach/stack across same sizes of drop and flat bar bikes within a company to establish some general guidance like… MTB bikes are x% longer in reach.

      It sounds like you and I sort of have the same fit issues- I had a long post about this and about the whole assumption that you can just correct with a taller steerer/stem. It just looks bad and you have to take into account what the headtube angle does to the reach and steering. In my opinion, just because your bike is smack in the middle of what is considered “normal” does not mean that you can make it work. I think fit is a really complicated subject- I’m just dipping my toe in- but I hope we can figure it out. And I want to get a hardtail MTB so if you see any bikes out there that look interesting, get ’em in the database!

  • Colin
    • hobocross

      Yes! Thanks Colin! Man… a picture is worth a thousand words. Can I steal this and post it up in the article?

      So you can sort of see how this angle should ideally stay consistent as you take a particular frame up in size. This is something I’ve been looking at lately with respect to road vs. mountain bikes and the differences there are pretty interesting. I need to revisit this now that we have a lot more bikes in the database.

      • Colin

        Please steal! let me know if you need a higher res version. Was trying to figure out if there’s a way to animate it to match an actual bike when you select it, but I’m not sure I know how to do that.

        • hobocross

          Yeah I was thinking about getting some pictures off of BikeCAD. Might do that when I update this. Can you send a higher res version to my gmail account (bikecounterculture)?