Get the right size bike: Ignore the sizing names!

 When you walk into a bike shop and begin to browse the bikes you'll notice that the bike sizes are often noted on the price tag or on the frameitself.  These sizes are how we're supposed to figure out which bike will fit us, and it seems logical that to get the right size bike we'd need to pay attention to these.Road bikes have historically been named in centimeters -- 50 cm, 52 cm, 54 cm frames, etc -- while mountain bikes were often named in inches -- 16 in., 18 in., 20 in. frames, etc.  Some road and mountain bikes are still named in this way, but more and more often (and mostly since the advent of carbon bikes and fewer sizes overall) they are all increasingly being named with "Small", "Medium", "Large" monikers.You'll often hear of people referred to by their bike size...you might walk into a bike shop and after seeing how tall or short you are they'll tell you, "You're a 54..."  And if a bike shop does this just by looking at you, you should turn and run away from them screaming and never shop there again, because it turns out.....

The sizes don't really mean anything.

No really.Why?  Well the nominal sizes are worthless for a few different reasons:

Agree to Disagree

There is very little agreement between manufacturers on sizing.  If you take a 54 cm road bike from Manufacturer A and compare it to a "54" from Manufacturer B, they will usually have very different geometries.  In fact, not only are you likely to find sizing disrepancies between two manufacturers, you'll find them within the same manufacturer.  I once carried a brand of bikes and in one model I fit the "54 cm" frame while in another similarly priced model I got the exact same fit from the "51 cm".Here is just a taste of the sizing discrepancies we see from similar models among three large manufacturersSpecialized Tarmac "54" -- 54,8 cm effective TT; 140mm head tubeTrek Emonda "54" -- 54.4 cm effective TT; 120mm head tubeGiant TCR Advanced "Medium" or "47.0" ?? -- 55.5 cm effective TT; 148mm head tube

You want a fitted dress shirt?  Let's get your waist size and go from there...

The primary reason the names of the sizes are a waste of all our times is because they're measuring the wrong thing.  Bike makers predominantly still measure their bike sizes via the seat tube measurement....from the center of the bottom bracket up to the top of the seat tube...

• Or sometimes to the center of the junction of the seat tube and the top tube.
• Or sometimes to the top border of the junction of the seat tube and top tube.
• Or sometimes to an imaginary point on the seat post that corresponds to the height of the top of the head tube.

I am not making that last one up...The reason this is the wrong thing to measure, ignoring the fact that the usefulness of the measurement must be pretty thin because they can't seem to agree on where they're measuring to, is that the seat tube measurement is a throwback measurement.  Twenty years ago, when bikes were predominantly made of metal, they used to make a lot of sizes for each model -- sometimes as many 12 or 13 sizes in 1 cm increments.  Now with carbon frames, where in order to make another size you need a whole new mould -- and moulds cost money -- the number of available sizes has been cut down to 3-6 depending on the manufacturer.  This has not been an improvement for the public.Around the same time, compact frame designs -- where the top tube is dropped and isn't horizontal to the ground like in the old days --  became more mainstream.  Why is this important?  Because back when top tubes were horizontal, which is sometimes referred to as a "traditional" frame design, the standover of the bike was an important consideration, and the length of the seat tube was one of the parameters that determined how high up in the air that top tube lived.A bigger bike meant a taller seat tube which led to a higher top tube which meant a taller standover.This is where we get the antiquated bike sizing trick of straddling the frame and pulling up on the frame to make sure you have two inches of clearance between the top tube and your crotch.  Sizing a bike this way, while certainly not perfect (or even good really), at least provided some information about the bike with traditional, non-sloping, geometry:Can't straddle it comfortably?  Then you have to go to the next size down (with the shorter seat tube -- lower standover) and this should be sized more appropriately for you.Today with most road bikes being compact designs, still sizing via the seat tube measurement we get zero information about how the bike is actually going to fit.  And to add insult to injury, I still run across instances of bike shops having customers straddle the (compact) bike and making sure they have the clearance to their crotch.  Never mind that a 5'6" person could probably comfortably straddle a XL compact bike.  With today's sloping geometries, still using this method for any sort of sizing is beyond worthless and is another scenario where if the shop employs this method, you should immediately dismount the bike and run screaming from the store.So now we know that bike makers are measuring the wrong part of the bike, and not even agreeing on how they're going to measure it, but then what is the best way to size a bike?

1. Frame Stack & Reach

Many manufacturers are listing frame stack and reach, which measures the horizontal and vertical distance between the center of the bottom bracket and the center of the top of the head tube, but most bike shops don't use this information at all and in fact many don't know how to use it.  It's also not the easiest thing to measure without an X-Y tool.  So stack and reach is useful, but not used enough.

2. Effective Top Tube

The simplest way to size the bikes, in my opinion, is through the effective top tube measurement.  It's very easy to measure with just a simple tape measure and it gives us an idea of how long the bike is going to be.  From even a rudimentary fitting, we can determine what length of bike is optimal for a rider, and working backwards from there we can get to the effective top tube.

A rubber-meets-the-road example:

Let's take a client's existing bike and assess it's setup.  For instance if a client has a "52 cm" bike that has a 53.7 cm effective top tube, an 80 mm stem on it (which is shorter than average), and the saddle is slid all the way forward on the rails of the seatpost, which are all changes aimed at making the reach of the bike shorter.  We can tell pretty easily that their next bike ought to have a shorter effective top tube than this one -- maybe closer to 52.5 cm or even 52.0 cm.  This isn't rocket science.If we just listed the size of the bike as the effective top tube, this would at least give us some actual information about how big the bike is and act as a starting point.  So now we know we're looking for a 52-52.5 cm bike (as measured via the effective top tube or "eff. TT")

Head Tube Length

The next determination we would need to know is how high or low the bars are likely to be on this bike, and the major variable in this is the length of the head tube.Let's go back to our client on the "52 cm" frame -- let's assume their 80mm stem on their current bike also has 30° of rise and they have 30 mm of spacers under the stem.  If the head tube length on that bike is only 110 mm we can pretty easily say that we'd love the head tube on the new bike to be longer than that because this would make it easier to achieve the higher bar position that this rider has gravitated towards.So our new bike quest has a pretty laser focus now -- we're looking for a 52-52.5 cm (measured via eff. TT) with the longest head tube possible - perhaps in the 125-140 mm range.  We can safely say the "longest possible" because it's unlikely we'll find one with too much head tube length.  Finding a bike with that short of an effective top tube and a head tube length over 150mm is exceedingly rare.I help clients every day find the right size bike for them.  I teach them all of these things, and even show them how to properly measure effective top tube on their own.  All manufacturers list their geometry on their websites, so with a little digging you can unearth this information.So go in armed with information on your next bike purchase -- know what you're looking for, or at least understand the vocabulary and what it all means.  It will greatly increase the chances you leave with a bike that will leave you smiling after every ride.Any questions or comments about this topic?  Start a conversation below, or reach out to me via email