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Stuff Beginners Should Know (But Almost Never Do): Basic Bike Sizing: Road Bikes

Posted by bikezilla on August 6, 2010


Originally post at Bikezilla.blogspot.com

UPDATE**

Consider this information road bike specific, not CX, TT or Triathlon bikes.

I’ve been pestering Eric from BikFit about proper bike sizing.

Eric and I agree that 1. Most cyclists have no idea about the proper way to size a new bike and many, if not most, local bike shops aren’t much better informed.

Because you may not be able to rely on your local bike shop to properly size and fit you, your best bet in finding the bike that you’ll be most comfortable on and the one that you’ll perform your best on is to become educated in sizing and fit yourself.

The first things I learned are that the issue of bike sizing is more complicated than “what size frame should I buy” and that bike sizing is just the first step in proper bike fitting.

So, we’re just gonna give you a starting point, here, then provide some links for you to get additional information.

As Eric says: “. . .frame sizing is a fairly complex issue, and is really the key to obtaining a comfortable reach to the handlebars . . .”

Eric: “Most people I fit are on bikes that are simply too long for them.  Most often it’s because they “got a good deal.”  I’m amazed that someone will shell out 6K for a new bike, but was never really sure if it was the right size.  Most people think if they get in the right ballpark, a different stem is all that is needed to get them “dialed in.” 

Did you see that? It isn’t just schulb’s who haven’t been riding seriously before or for long who screw this up, it’s guys who’ve been riding bikes long enough that they’re ready to invest in top end gear. Why? Because, we’re just not taught this stuff as we go along or before we get heavily involved with the sport. We’re taught to make alterations by buying parts that adjust different aspects of our bike’s fit, instead of how to first buy a bike that fits us best.

What is the most important measurement to consider when attempting to find the proper frame size? The top tube (actual or, in the case of sloping top tubes or compact / women’s frames,
“effective” or virtual) measurement.

How do you find the correct top tube measurement?

Eric:

“The really simplified rule on frame sizing is the same one that has been used by bike fitters and frame builders since the dawn of time – multiply your inseam by 2/3, and that will give you your correct center-to-center seat tube length (on traditional level top tube bikes, or the virtual center-to-center measurement for sloping geometry).  Provided your inseam is in proportion to your torso (see the attached Fitting Guidelines for further guidance), then the top tube should also be the same length (again, for sloping geometry this would be for the virtual, or effective, top tube length). It really doesn’t matter what size the manufacturer may label the bike; it’s the effective center-to-center top tube length that is more important than some arbitrary sizing convention like Small, Medium, and Large.   
 
The above rule applies to those who meet my “5Fs” rule: (More on Eric’s “5Fs” later).

So, you first need to measure your inseam, and no, it’s definitely not the same as your pant inseam.  

— First, you will need an assistant.  Make sure you are in socks and cycling shorts.

— Next, set your feet about 9-10”(23-25 cm) apart and straddle a 2-3 foot (61- 91 cm) carpenter’s level that has about a 2-3” (50-76 mm) thickness.

— Pull the device firmly into your crotch while facing a wall (leave enough space between you and the wall so you can hold both the front and back of the level, and your helper also has room to mark the wall).  Make sure to use enough pressure to simulate the pressure you would feel when sitting on your bike.  Make sure your level is perpendicular to the wall.  

— Have your helper mark the spot on the wall at the top of the level.

— Measure from floor to this mark and you now have a fairly accurate measurement of your inseam. “

If you walk into a bike shop it’s unlikely they’ll so much as mention top tube length.

Talking to a bike shop tech it’s all about stated frame sizes (usually listed in centimeters, but sometimes in inches) and sometimes it’s about “standover height” and you’re really lucky if anything else is even mentioned.

For instance, I was told by a dealer that for an MTB you want one inch standover height clearance “and for a road bike you want at least two inches.”

Hence, by standard bike shop wisdom, you begin by choosing a frame with the proper standover height and all else should fall neatly into place.

Is there any truth to that at all? Or is it nonsense? Should “frame size” or “stand over” be used as a guide at all? Should it be entirely ignored?

Eric:

“First, a little background. Back in the days when all top tubes were level on road bikes, frames were sized by the length of their seat tube, which was usually measured from the center of the bottom bracket to the center of the top tube (sometimes it was to the top of the top tube).  If you selected a frame that was about 2/3 the length of your inseam, standover height wouldn’t be a problem, and generally you’d have about 1-2” clearance over the top tube. Usually the top tube lengths would be within about a centimeter of the seat tube length. Bikes were also usually built in one centimeter increments,and it wasn’t uncommon to find a range of 12-15 sizes from any given builder.    
 
Now, flash forward to today and almost nothing has a level top tube.  Compact geometry and sloping top tubes rule the world.  Most bike models will come in a range of 4-6 sizes. Strangely, the same sizing conventions are now used today – bikes are still often measured by their seat tube length, and that is given as the stated size. As an example, a 56 cm bike with compact geometry might only have a actual seat tube length of 49 cm, yet the “virtual” or “effective” seat tube length is 56cm.  In other words, if the bike had actually been built as a traditional diamond frame design with a level top tube, then the seat tube would have measured 56 cm. Some bike companies have opted to simply label their bikes as XSmall, Small, Medium, Large, and Extra Large. (this is the way mountain bikes have always been sized).  What does this tell us about the top tube length?  Not a whole lot. Most people can probably comfortably standover 3 or 4 of these sizes, so the top tube length now becomes ever more important in determining correct frame size. 
 
The moral of the story, is that a bike should fit you both vertically and horizontally. If your inseam and torso are proportionate, then a bike with the correct seat tube length (actual length for a level top tube, or virtual/effective length for sloping/compact geometry) is going to get you in the right ballpark, and standover height will then take care of itself.     
 
P.S. Mountain bikes should have a top tube that is about 4-6cm longer than a correctly sized road bike, as more weight needs to be over the front wheel. I’d use the same seat tube length requirements”

Now for some basic adjustments to your desired top tube measurement (this is where Eric’s “5Fs” come in):

Eric:

“Sometimes static measurements alone aren’t the best way to ultimately determine your correct
top tube length. Measurements and formulas are a good starting point, but your level of
fitness, type of riding, flexibility, age, and injury history should also be taken into
consideration. I use the following “5 Fs” to help determine if the top tube length should be
further shortened:

Fit – How often does the cyclist ride?

No Deduction – Cycles at least 3x per week.

.5 cm deduction – Cycles 2x per week (preferably with one mid week ride).

1 cm deduction – Cycles only once per week.

Fast – What consistent speed can the cyclist maintain on relatively flat ground, while by
themselves, for at least an hour (group rides don’t count)?

No deduction – approximately 18+ mph (27+ kph)

.5 deduction – 15-18 mph (23-27 kph)

1 cm deduction – less than 15 mph (23 kph)

Fairly young – How old is the cyclist?

No deduction – Less than 40 years old

.5 cm deduction – 40-60 years old

1 cm deduction – 60+ years old

Flexibility – Can the cyclist touch the ground when bending over at the waist, feet together,
and knees locked?

No deduction – cyclist can easily touch the ground

.5cm deduction – cyclist can get to about ankle height

1 cm deduction – cyclist can only reach to shins

Free of pre-existing conditions – Has the cyclist had injuries/pain/accidents/medical
conditions (includes obesity) that would affect fit?

no deductions – none/never

.5 deduction – in the past, but not currently an issue

1 cm deduction – current/present (within the past year)

Overall, I find that most “roadies” tend to need about a 1 cm reduction from their starting top
tube length, thus the vast majority of them are on bikes with top tubes that are a bit too long.”

There’s much more that may need to be considered in getting the proper size and fit. Here are some recommended links to help you figure it out:

Visit Eric’s BikFit website and sign up for his newsletter. Once you do that you’ll get a link to an article with a lot more detail about bike sizing and bike fitting.

You might also look at:

Neuvation’s Bike Geometry Page. There’s some information about “reach” that you might want to understand a little better, there.

and

Fit Kit for a location of a bike shop that provides Eric’s recommended bike fitting system.

*UPDATE: Eric just put up a little more detailed article on bike sizing.

He’s also offering a new service that I enthusiastically recommend for those seeking help outside of his area:

You can also get in touch with me. I have a bike buying program for which I charge $75 for all of the following:

— a telephone interview,

— emailed instructions on how to take your anatomical measurements,

— determination of your ideal frame geometry,

— help with locating a bike/frame or custom builder within your budget,

— and if you want, I’ll even try to locate a bike fitter in your area who can help with fine tuning your final position and pedaling biomechanics after you get the bike.

If you live in my coverage area and want me to perform the final bike fit, I’ll credit the cost of this program to my bike fitting services.

Eric Bowen (858) 414 -7093 or bikfit@yahoo.com

It may be the best $75 you invest in your bicycle.

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