Bike Sizing

I’m far from an expert on anything bike-related.  I have figured out how frame size works for me though.

Excuse my crude drawing below.

This is a representation of my Raleigh Record single-speed bike.  I feel that it fits me the best.  I measured three tubes (top tube, seat tube, down tube).  Each measurement is given in centimeters.  That bike was advertised as a 59cm.

My Surly LHT is almost the same size.  I bought the Surly thinking it was a 56cm, but I had test ridden it, and it seemed to fit.  Based upon geometry numbers from Surly’s Site, I’m now convinced it’s a 58cm frame.  It does have a longer top tube, which explains why I have a shorter stem on the bike.

There’s a lot more to bike geometry than these three measurements, but these three (plus standover, which varies on tire size) are the most important for proper fit.

I figure having the information here will be handy later when doing research for another bike… a bike that will be built from the frame up, so I won’t be able to test ride it.

4 thoughts on “Bike Sizing”

  1. Don’t forget that bottom bracket height from the floor or ground will affect standover height. Seat tube angle (usually measured relative to horizontal) and head tube angle will affect handling and power output. Fork rake is a factor in handling. Chainstay length is a factor in handling and power output.

    Have fun…

  2. Right, but the three I listed affect fit the most.

    Standover height is affected by a few things. My 59cm Raleigh would actually we a tad close on standover if it had the original 27″ wheels and fattish tires (not sure it would fit the fattish tires with 27″ wheels, but that’s a different issue). With the 700c wheels and fattish 700x-32 tires, it had enough clearance.

    A bike with a sloping top tube would eliminate the standover issue (the Surly actually has a minor slope to the top tube).

    Chainstay length and fork rake have an effect on handling, but not on fit. I’m not looking to build a frame, just make sure one that I buy will fit me. 🙂

  3. I know LHTs are famous for long top tubes, but I mostly notice the BB and LONG chainstays. I think what is missing is the nature of angles and how they interact with the measurements. Fit without handling doesnt get you far.

  4. Tim,
    That’s true, but my purpose in looking into this is fit. I’m assuming a bike manufacturer will have the handling dialed in for whatever purpose the bike is for.

    There aren’t many new steel road bikes in stock in the area to test ride different size. I got lucky with the LHT.

    Based on recent discussions, I need my “fast bike” and a “gravel bike”. I’ll use the LHT for a gravel bike, if I need to, but a mountain bike would be better.

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