Learn:  
Here you are: Learn > Bike Fit
Choosing a Frame Size

Choosing a Frame Size

(Our bikes, and most non-Italian bikes, are measured from the center of the crank (or bottom bracket) to the top of the seat tube. In the old days this was called the English Method, to distinguish it from the Italian Method, which measures to the center line intersection of the seat and top tube (think, "center of the seat lug".)

In this section, In General is about bike sizing in general, and is good to read if you don't know about it, or just want to read what we have to say. So it's sort of a primer, but it's not one of those primers that has such basic introductory information that everybody already knows it and you're wasting your time reading it. It's a better primer than that.

And then Sizing Rivendells talks about how we size the frames we design and sell--custom Rivendells, Atlantis, Sam Hillborne, Hunqapiller, Betty Foy, Bombadil, and the earth-rattling A. Homer Hilsen.

In General
People ride bikes that are too small. If you go into a bike shop or ask the local fast riders about frame sizing, you'll likely hear comments such as, "Smaller bikes are lighter, stiffer, more maneuverable, and more efficient."

To that we say this:
Small has to be lighter, but we're talking ounces, and let's not talk about ounces until your belly is so ripped that you're regularly mistaken for one of those guys in the Dance Theater of Harlem. A bike shouldn't have any unnecessary weight, but if the weight comes by means of a larger frame that fits you better, or stronger wheels that stay truer, or a safer frame that doesn't fail catastrophically, then we wouldn't consider that "unnecessary" weight. It's weight that carries its own.

Here's the way it works, in any given tube on a bike. In your left hand is a downtube that's 31.8mm in diameter. The butts (tube wall thickness as the ends of the tube) are 0.9mm thick.

Less than a millimeter, about 1/28th of an inch.

To look at it, you'd think it was nothing--that the tube was on the verge of collapse. The _belly_of the tube (mid portion, between the butts) is 0.6mm thick. Fortunately you can't see that, because that's even scarier.

Now, in aluminum or carbon or titanium or anything other than strong CrMo steel, these dimensions would be instantly foolish and dangerous, but it works in good steel, because steel has the right balance of strength, toughness, and rigidity. This tube is 650mm long and weighs 10.6 ounces.

In your other hand is a lighter version of the same tube. Its butts are 0.8mm, it's belly is 0.5mm, and it weighs 8.8 ounces. The decrease in butt thickness is 12 percent, the decrease in belly thickness is 16.7 percent, and there's a proportional decrease in durability, if we consider resistance to fatigue and denting to be "durability issues." The weight savings is less than 2 ounces.

As a designer/builder, what do you do? If you're building for longterm hard use by heavy men over rough terrain, you reject both of them and pick a tube with 1.2mm butts and an 0.9mm belly, even though that tube weighs 14 ounces. You figure what's five or six ounces? I'm crossing Russia with a load of pigs and canned food and carrying water for a week, and I need this thing to last!
-----
Small also has to be stiffer, but marginally so, and it has not been proven to mean beans in a bike frame. Certainly a frame should be stiff enough to be safe and controllable and to feel halfway normal, but if one frame is so close to another in stiffness (as is likely to be the case when you're talking about two sizes of the same frame), the difference in stiffness is not worth talking about.

"More maneuverable/efficient"? A bike that's more maneuverable sounds good, until you look at the other side of the coin: If you can cut around a dead raccoon that you didn't see until the last millisecond, or dive between two other riders in a race around a tight corner, then it's you, not the bike, that's doing it. In any case, a "highly maneuverable bike" is easily maneuvered by a sudden gust of wind, an unseen pothole, and any other of a dozen or more forces outside your body that act upon it. Or a sneeze. So forget getting a smaller frame because it's "more maneuverable."

Efficiency.
Bikes are already efficient enough. Efficiency is one of those concepts that sounds universally desirable, but in the context of a humble rider wanting to go have fun and get some exercise on a bicycle, is totally overvalued. Once your bicycle has smooth bearings, true wheels, a lubed drivetrain, brakes adjusted so the pads aren't rubbing on the rims, and an efficient engine (that would be you), it is efficient enough.

Too-small bikes are not comfortable
Every day we talk to folks who bought a $5,500 titanium this, or a $6,500 carbon fiber that, and now that the honeymoon is over, they realize it's not comfortable. Sore neck, sore lower back, sore hand---and almost without exceptions it's caused by a frame that's too small and doesn't let them raise the bars high enough to cure these ills. Understandably, they're feeling foolish and bummed out.

Handlebars too low cause 90 percent of the discomfort people suffer. And buying a frame too small guarantees that the bars will be too low.People often size bikes by the top tube length. It doesn't make sense. Since the reach to the bars is so obviously important, it's OK to be concerned about the top tube length. But don't let it lead you around by the nostrils. If the top tube is in the right ballpark, you do the fine-tuning with stem length. Also, there's a good deal of misunderstanding about the effect of top tube length. Scroll down a bit and you'll see how a shallower seat tube angle and higher handlebars can make a bike with a 59cm top tube feel shorter (in the reach) than one with a 57cm tt. Don't go there yet, though.

Sizing Trends
If you look at old racing photos or drawings, you'll see bikes with "a fistfull of seat post" showing. That was the rule --- a fistfull of post. You bought a frame size that, when the saddle was set at the right height for you, exposed a fistfull of seat post! If in order to get the saddle at the right height, it required much more than a fistfull of seat post, then the frame was too small. These days, "a fistfull of seat post" sounds quaintly stupid, charmingly naive, cute but dumb, stay away from me with your dangerous folk medicines!

And yet, riders back then were a lot more comfortable. We aren't suggesting that you go by "a fistfull of seat post," but that simplistic approach was (and still is) successful because it allowed the handlebar to be close to the height of the saddle. So it resulted in a fit that took weight off your hands, and strain off your neck and lower back. (It also allows sufficient standover clearance. In other words, when you straddle your bike, your genitals may rest on the top tube, but your pubic bone will easily clear it -- as you'll notice if you grab a handful of genitals and pull up. Apologies if this is too graphic for you.)

In those days, most saddles were leather, and most leather saddles (of any vintage) sit higher above the saddle rails than do modern plastic saddles. So, on a modern plastic saddle, the equivalent rule might be "seven fingers of post." Of course, fingers vary in fatness. Fitting and sizing are not sciences.

How to Size any Bike, Including Ours
Want some sort of a concrete recommendation for sizing a road bike? Okay. You have to know your saddle height. If you know your saddle height, read the chart below. If you don't know your saddle height, take off your shoes, stand on a hard surface with your feet 10-inches apart, and measure between your legs (the tape measure should be right in the middle) from the floor to your pubic bone. Not your genitals. Hit the bone. Figure out how to do this using a thin, hardcover book and a metal tape.

Your floor-to-pubic bone measurement is your pubic bone height.

Example: If you are 5 feet 9 1/2, your pubic bone might be 85cm. Your saddle height will be about 75cm.

Once you've determined your saddle height, you have a simple subtraction to determine a good frame size. "A good frame size" doesn't mean it's the only size for you. The whole purpose of sizing is too give you a comfortable riding position, and for most people that means getting the bars level with, or within a couple centimeters, of the saddle height. The lower the number you subtract, the higher the bars can be. In France or England in the '40s, you'd subtract about 15cm. In the case above, that would have that 5-foot 9 1/2 inch rider on a 60cm frame.

If that same rider got sized in 10 different bike shops, probably 5 of them would suggest a 54 to 55. One would say a 53, two would say a 56, and one would say a 57. The more expensive the bike, the more likely the size is to be small.

If you're psychologically uncomfortable with a frame so big, instead of subtracting 15 from your saddle height, subtract 16. If you're a tall guy and have long arms, go 17--but be prepared to use a stem with an upslope, or a long quill, because on a typical modern road bike with a level top tube, a small-stack headset, and a short-quilled stem, a 17cm difference between saddle height and frame size will put them bars too low (for comfort).

Sizing Rivendells (the bikes we design) -- frame sizes measured center of crank to top of seat tube
When you come to us already owning two or three or half a dozen or more bikes, and I recommend a size two to five centimeters bigger than the bikes you already own and have spent lots of money on, your brain tries to reconcile what you have (and have spent lots of money on) with what I've just recommended.

Sizing chart for Riv-designed frames by Pubic Bone Height (PBH) & Saddle Height (SH - measured from the center of the crank to the top of the saddle parallel to the seat tube). For 700c, 26" mtn, and 650B wheels. All measurements in metric (cm).



This chart is a guide, and the numbers are based on extensive experience with several thousand riders over the past decade or so. Variances will be minor, but no chart can account for personal preferences or extreme crank lengths, and so forth.

For any given size, the standover heights (height of the top tube) are lower, because the bottom bracket is closer to the ground and the seat tube angle is shallower (less vertical). "Lower top tubes" is not the goal in itself, it is just a result of the frame design. But it is a key reason you can straddle a bigger one-of-our-bikes than one-of-theirs.

Standover clearance, though, is highly overrated.

You need to be able to straddle the bike when waiting for the light to turn, but you don't need oodles of clearance.

And you pay for extra clearance with lower handlebars and less comfort, so at some point you have to ask: "Am I getting this bike so I can stand over it with a fist and a half of air between the top tube and my crotch, or do I want to be comfortable when I ride it?"

The Consumer Products Safety Commission requires an inch between top tube and crotch, but doesn't define "crotch." To us it means pubic bone. Everything we do here, frame-sizing-wise, revolves around pubic bone height (PBH). When you get a Rivendell, you get at least an inch of clearance, and usually more.

Every builder has, or at least ought to have, a bias to his frames. Our bias is comfort. From comfort comes efficiency, strength, endurance, control, and fun. The best way to achieve comfort is with higher handlebars, and the first step toward higher handlebars is a larger frame size.

A Good Position For Many Riders
When you're in your riding position with your hands on the hoods, you should be able to put your hands behind your back without your torso plopping down onto the stem. For most riders, that means a back angle of 50-to-65 degrees.


We're less adamant about the knee position relative to the pedal, but mention our preference here only to get you thinking. We like it behind the center of the pedal, because that way, the downstroke helps you maintain a rearward position on the saddle. If it's directly above it, you tend to scooch forward more. In any case, it is not easy to achieve this position with a normal, off-the-shelf bicycle and conventional sizing methods.

Here are some other things related to fit that 99.999 percent of the experts don't know, haven't considered, and don't talk about:

-- As the handlebar gets higher, your arm becomes more horizontal, effectively getting longer.

-- As the bar gets higher, it also retreats toward you. How much? On a bike with a 73.5-degree head tube, raising it 4cm brings it back 1.5cm.

-- Getting a shorter stem without also raising the bars can have a self-cancelling effect. And if you raise the bars, you may even need a longer stem.

-- Top tubes of a given length tend to feel shorter on bigger frames than on smaller ones, so if you currently ride a 56cm bike with a 55cm top tube, but you know you can fit a 58cm frame, don't be scared off it just because it has a 57cm top tube.


If you aren't sure whether your saddle is set at the right height, or if you just want another opinion, measure your pubic bone height. With your bare feet ten to twelve inches apart, measure from the floor up to your pubic bone. Hook a metal tape through a thin, hardcover book or a record album cover, and push up until you smash into the bone. Have a friend take the reading on the floor. This distance minus an inch (25.4mm) is nominally the highest top tube you should have. Sole thickness affects it, too.



Was this helpful?

Comments? Suggestions?
We read 'em:
 

related articles
 > Fit, Sizing and Position
 > PBH & How to Measure It
 > Common Setup Mistakes