Determining the Correct
Matt Russ - The
is one type of bicycle that is extremely hard to fit-- a
bicycle that is the wrong size for the athlete.
I sometimes have cyclists coming from out of
state for a bike fitting so I like to do my homework
before they make the trip.
This means ensuring they are on the correct frame
size and it can be a little tricky.
height used to be a good method of frame sizing for a
traditional diamond bike frame.
About 0-1 inches of clearance between the top
tube and crotch (bare foot) was a decent way to
determine if the bike was the correct size.
With today’s non-traditional frame geometries
and frame types, stand-over height goes out the window.
There is no frame sizing that is considered
“standard” and each manufacturer may measure their
frame size in a different way and from a different
brands measure from the center of the bottom bracket to
the top of the top tube or to the center of the top
tube, while others measure to the top of the seat tube.
Compact frames may use a “virtual top tube”
and even this point of measurement may vary from
manufacturer to manufacturer.
Some frames now come in just three sizes; small,
medium, and large and rely heavily on componentry to
correctly fit the rider.
This sizing range is great for manufacturers as
they only have to produce three frame sizes, but it does
leave gaps in sizing and makes getting a good fit more
technical and difficult.
net of all this is that the 51 cm bike you are riding
now may be different from a 51 cm bike in another brand,
and that it
is important to define the sizing method used.
Manufacturers generally post sizing guidelines on
their websites, or at the very least geometry
good bike shop that carries multiple brands will look up
the correct frame size for your inseam (sizing is based
on inseam measurement) and not “eyeball” it.
determine your inseam measurement, stand with your back
against a wall (bare foot) and place the spine of a
one-inch thick book against your inner leg and snug
against your crotch.
Make sure the book is flat against the wall
forming a 90 degree angle between the wall and top of
the book, then measure from the top of the book to the
you used inches, convert the inches to centimeters by
multiplying the measurement by 2.54.
For a road bike with a conventional frame, the
frame size will equal approximately 2/3 of your inseam.
Now take your inseam in centimeters and multiply
it by 0.67 to determine your frame size in a traditional
generally have shorter torsos and may need a relatively
shorter top tube. This
is where “woman-specific” designs come in.
These bicycles may be appropriate for certain men
with shorter upper bodies or riders who prefer a more
upright position. Top
tube length can be adjusted to an effective length using
different size stems but this can affect handling.
It is important to be in the correct frame for
your body type and the type of riding you will be doing.
Manufacturers size their frames to accommodate
the normal proportions of reach to leg length.
If you are outside of these proportions, finding
the correct frame or bicycle to fit you will be more
your height and divide it by your inseam. If the value of this is more than 2.2, you will need a
bicycle with a longer reach or top tube.
If the value is less than 2.0, you have longer
legs and will need a shorter reach or top tube.
Generally speaking, bicycles with a steeper seat
tube angle will have a longer effective top tube length.
you are buying a new bike, I recommend doing your own
due diligence before purchasing it by checking the frame
sizing on the manufacturer’s website.
If you are on the cusp between two sizes, ride
them both to determine which one feels better after each
has been adjusted correctly.
Realize that the shop may not have the correct
frame size for you in stock and that they are in the
business of selling bicycles.
A reputable shop will not sell you the wrong size
bicycle, however, I have run across athletes who spent a
lot of money on a bicycle that was the wrong size.
you are purchasing a used bike, again, go to the
manufacturer’s website and look up your model.
If it is no longer listed, you can send the
manufacturer an email or take it to a coach or an
experienced bike fitter to determine if it is right for
great deal you got on a used bike will not be such a
bargain if it is uncomfortable and you can not produce
Russ has coached and trained elite athletes from
around the country and internationally for over ten
currently holds expert licenses from USA Triathlon, USA
Cycling, and is a licensed USA Track and Field Coach.
Matt is head coach and owner of The Sport
Factory, and works with athletes of all levels full
time. He is
a free lance author and his articles are regularly
featured in a variety of magazines and websites.
for more information or email him at firstname.lastname@example.org