Bike-fit geometry relationships

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My flickr friend Jimmythefly drew a set of bikes to show how, if you keep the saddle to bottom bracket (BB) relationship the same, seat tube angles can make bikes with the same dimensions (top tube and seat tube lengths) can fit very differently.

Because of the interrelationship of frame angles influencing tube lengths, two bikes in different “sizes” (seat and top tube lengths) can fit exactly the same.

It’s non-intuitive. It’s weird. It needs a picture to really see how it works. For me, I needed to see the drawings overlayed, which is why I made this GIF. Thanks, Jimmy, for the clear illustration and explanation.

Comments

9 responses to “Bike-fit geometry relationships”

December 9, 2011

jimmythefly

Nice gif -works much better than my basic illustration!

Reply
1. December 10, 2011
  
  philip
  
  I couldn’t see what was happening until I layered it up. Sometimes being dense makes me look smart…
  This is a seriously cool explanation of a very non-intuitive concept.
  
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2. December 14, 2011
  
  apfrancis
  
  This is really interesting. I would be curious to know just how important it is to keep that saddle to bb relationship constant. I remember reading on Peter White’s page how having the saddle farther behind the crank spindle will take more weight off the hands (like a cantilever). So my question really is: If you kept the saddle setback constant (and thus the reach to the bars), would the 75 degree seat angle put noticably more weight on the arms?
  
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  1. December 15, 2011
    
    philip
    
    I don’t see how the steeper angle would change your weight distribution as long as you kept the saddle-BB relationship the same.
    
    Reply
December 15, 2011

Thomas

Don’t forget, while the bikes may “fit” the same, they will have very different ride and handling characteristics.

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1. December 16, 2011
  
  philip
  
  Why? Steering geometry’s the same. Weight balance is the same. The two bikes with the same length tubes and different angles are going to ride a little differently (longer front center, less weight on the bars), but I don’t see the handling changing in the scenario with dissimilar bikes with the same weight balance and steering.
  
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December 15, 2011

jimmythefly

Yeah, really seat tube angle affects only one thing* fit-wise: Can you actually buy a seatpost and saddle combination that will allow you to sit in the right location?

Think of the Trek Y-bikes, or Softrides, or many full-suspension mountain bikes. Essentially the saddle is floating in space, there really isn’t such a thing a “Seat tube angle” on some of those frames.

I have a few friends who hoard high-setback posts because they need to get further rearward than most seat tube angles allow for. Getting a saddle more forward isn’t as much of a hassle since one can always flip a normal post, and because there are triathelet and TT posts with forward “setback”.

*It also affects how much tire clearance there is between the tire and seat tube. Also clamp-on front derailleurs are designed to work for a specific narrow range of seat tube angles.

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March 9, 2012

Jim

Cool!

My only quibble is that 70 and 75-deg are pretty extreme seat angles. Most are 72-73deg, so the variance in fit with any given nominal frame size is much less than what’s shown here. On the other side of the token, the seat angle correction factor (term I just made up) will not allow such a wide range of nominal sizes to fit the same.

My opinion is that bike fit is a seriously misunderstood topic. Most humans are flexible enough to fit a variety of fit geometries in a comfortable, efficient position.

Reply
1. March 10, 2012
  
  philip
  
  I think my Bontrager Privateer has a 74 degree seattube, so they’re out there.
  I definitely agree that most humans can fit a range of bike geometries.
  
  Reply