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How to Choose the Right Frame Size

How to Choose the Right Frame Size

Everyone ought to know a ballpark figure of their frame size. That way if Ralph, the bike sales guy, tries to sell you a 47cm, and you need a 58 or 56 then you can try’n guide them toward the size you actually need, or walk out. Get the new kid out on the wrong sized bike thing happens a bunch, way more than normal bike shops would like to admit. You come in for a bike, they don’t have the size you need so they say they can fit you on whatever they’ve got laying around. It’s a great way to get rid of floor stock. It’s also a great way to make the buyer hate riding. Putting customers on appropriately sized bikes is better all around. The best place to start figuring out your frame size is your pubic bone height (PBH). Learn how to measure that here.  

The next step is to ask yourself: what kind of bike do I want? If you want a road bike, a road touring bike, or a gravel bike, then you want about an inch (2.54 centimeters) of standover clearance based on your Pubic Bone Height number. Standover clearance is how tall the bike is, where you stand over it. Makes sense, right? It’s measured as if the bike was sitting perfectly perpendicular to the ground, which it rarely is. It’s a real number though, and handy when talking about fit.

Quick Sizing Rules Of Thumb

  • Almost all bike frames have their standover number published in inches or centimeters. Convert to centimeters by multiplying the number inches x 2.54.
  • For roadish bikes with level (horizontal) top tubes: Frame Standover should be 0-2cm less than your PBH.
  • Roadish bikes with sloping top tubes: Frame Standover should be 2-3cm less than your PBH.  
  • Off road /ATB bikes: Frame standover should be 3-5cm less than your PBH, generally.
  • Fat bikes should have 5-10cm of standover clearance. That sounds nuts, but when you put a foot down in deep powder right next to the compacted groomed trail, you’ll see why. If you are never going to ride your Fat bike in snow, refer to the Off Road sizing.

Some bikes have totally horizontal top tubes, and that puts a hurt on stand over clearance. This is compounded by the fact that horizontal top tube bikes have short head tubes, which puts the handlebars on the lower side. You gotta go big on these bikes to get the bars up. For those bikes, you can cut that 1” of clearance down to 0”, but no less. That’s because you’re always wearing shoes, and PBH measurements are taken without shoes. So if your PBH is 80cm and the bike in question has a standover height of 80cm, you could ride it because shoes add height, at least a centimeter, and one rarely stands over their bike while holding it straight up. When at a stand still, you tilt your bike a bit or keep one foot on a pedal and really tilt your bike. In that last case, standover barely matters at all.

Look at old touring and race bikes. Not even a fist-full of seatpost showing. What’s that mean? Those bikes HAD to be tilted over to stand over. For that style of riding, where you aren’t off and on the pedals much, that’s okay. Now, I’m not advocating for an overtly huge bike, but a properly sized horizontal top tube roadish bike will have minimal standover clearance. It’s not a big thing. Stop thinking about it.

You want to get on the biggest roadish bike you can, because as a general rule, comfortable riding = high handlebars. The bigger the bike, the longer the headtube, which means a 54 cm frame has a 2cm higher (almost an inch) handlebar than a 52 cm frame, right outta the gate. It’s a myth that taller bikes are longer, and that you should fit to the top tube length, not the standover.  

Let’s look quickly at some geometry numbers to investigate this further. On the surface, a Gunnar Grand Tour 52cm has a 54cm top tube, and with a 100mm stem with 25 degrees of rise, with 4 inches of spacers under, it that bike has a handlebar reach (called the Handlebar X measurement) from the center of the bottom bracket to the center of the bars of 40cm. The Handlebar stack, called the Y measurement, is 75.6cm.  

A 54cm Gunnar Grand Tour has a top tube 1cm longer, at 55cm. With the same stem and spacers, the Handlebar X is…. 40cm. No more reach! The bars are higher, and as bars get higher, they get closer to you.

A 56cm Grand Tour has a top tube that’s 56cm. The Handlebar X, with the same stem and spacers is…. 40.2 cm. But the handle bars more than 3cm higher on the 56! It’s easy to make bars low. It’s hard to make them high enough to be comfortable. Bigger frames allow you to bring the bars up with very minimal reach consequences.

Now, it should be noted that sometimes a larger frame will increase reach a bit, and if it does, you can just fit a shorter stem.   There is a total BS myth that short stems on normal road / gravel / ATB bikes makes handling weird. It doesn’t. You can go with impunity to a 0mm stem, as long as the steerer tube of your bike is steel or aluminum.  

Once you have your pubic bone height number in hand, you can then figure out how high saddle should be. That’s in the PBH article too, but for quick reference: its just your PBH minus 11cm. With your saddle height in hand, you can look at any bike and determine: if you can get bars up high enough either out of the box, or with some basic modifications. ‘High enough’ bars are at a minimum level with the top of the saddle.  This goes for drop bars or alt bars.

We say: fast riders who are super fit should ride with the bars level, semi fast riders or semi fit riders, add a centimeter for road bikes and 2-3cm to the bar height for off road drop bars, and up to 3 inches of bar height for casual riders. Bring a tape measure to the bike shop. Set the saddle height up, just like you would at home, measuring from the center of the bottom bracket to the top of the saddle, along the seat tube. At this point, the sales person is probably gunna stop making terrible recommendations to you, at least about bike fit, unless they are truly clueless. Happens! Once the saddle height is set, you can measure the bar height to the floor and the saddle height to the floor.  Ask if the bars can be raised to get the numbers you are looking for. If they can’t, see if you can stand over the next size up. Lots of contemporary road bikes have loads of slope to the top tube. Sizing up beyond the size you have ridden in the past shouldn’t be a scary thing. IE, if you have always ridden a 56, and you can stand over a 58, chances are the bar reach will not be longer when the stem is raised. That in turn means that your bars are 2cm higher to start.

 Correct, non-racer bike fit works like this: Get the biggest bike you can safely standover for your given height. If the seat tube angle is 72.5-74 degrees, fit it with a 25mm setback seatpost. If it is 74-75 degrees, fit a 32mm setback seatpost. If it is 75+ degrees don’t buy the bike.

Things to consider: carbon steerer tubes can only the bars 80mm above the top of the top of the headset. Sorry, that was awkward to say. Higher than 80mm means you could damage the steerer tube. Steel stems and most aluminum machined stems are not compatible with carbon steerer tubes. This means that most stems that can get your bars high, IE the Discord Creemee, the Discord Peeper, the Velo Orange Cigne Stem, the Gnome Hopper, are not carbon compatible. DO NOT BUY THE BIKE IF: it’s carbon and you can’t get the bars up high enough by raising them 80mm from the top of the headset. DO NOT BUY THE BIKE IF: it has a steel or aluminum steerer tube and the bar height required is more than 165mm higher than the top of the headset. These bikes will never fit well. Get something with more stack height!