What to buy. Part 2 Bike sizing and fit

Having Fits. A discussion of bike sizing.

Bikejohn’s second rule of buying a bike: Buy one that fits. If it doesn’t fit, you won’t ride it.

How should a bike fit? Very well thank you. But let’s start with some discussion of the basics: bike sizing, fitting, top tube length, standover height (and why you should ignore it). If you know the basics you can make an inform decision. If you rely on the advice of others–especially the sellers, of which many are ill informed themselves–you probably will be disappointed.

Identificaton of major measurements

Bike sizing.
The proper bike size is based on two things, how long your legs are, and how long your torso is. It really comes down to that. But unfortunately, unless we have a bike custom fitted and made just for us, we have to choose from premanufactured models, then adapt them to us. So to begin with we need to get a frame that gets close to what our legs are and our torso length.

Bike size.
This has to be one of the most least understood things about bicycling. Look at any list of bike ads on craigslist and you see: 26″ bike, 24″, 20″, 32″ standover, “I’m 5’5″ and it’s too big for me”, 52cm, and more. Confused? You should be because of all those numbers only one or two might be correct in describing bicycle size. It just depends.

Most of the numbers seen above are things like wheel size, standover height, wild guesses, and possibly “I don’t know but I’ll take a chance that you won’t either”. For example, a 26″ bike, any way you measure it would be huge. Maybe a few pro basketball players–somebody 6′ 5″ or taller –that sort of huge. Huge. So many people think bikes are sized by their wheels. If you find a bike advertised 26 inch and you know it’s a bike that has 26″ tires, you need to ask the seller to measure it correctly before you waste your time going to see it. With experience you might be able to guess a bicycle’s correct size from a photo, but many times you can’t. Call before you go, and if necessary let them know that you won’t come unless you know the correct size. If the seller is put out by your call, doesn’t want to help, then look for another bike.

seat tube measurements

The correct way to determine a bicycle’s size is to measure the distance from the center of the bike’s cranks to the center of the top tube (also called “c to c” or center to center) or to the top of the seat tube (“c to t” center to top tube). Either can be correct. Roadbikes will usually be measured c to c, and most others will measure c to t. And then there’s the small frame bikes that don’t rely on either of these measurements, but if you understand the basics you’ll understand these, too.

seat tube measurement center to top

Why do we measure frames like this? Because the seat moves (is adjustable), you really can’t measure middle of the cranks to the top of the seat because you can adjust a seat 1, 2, or maybe 9 inches. With the c to c or c to t you have a fixed, universal, and understandable measurement. You know that this measurement is a start.

But the real dimension is the distance from the top of the seat, where your butt or tailbone is, to the bottom of the crank stroke (pedal all the way to the bottom) without going to full or over extension of your leg. This, for all practical reasons, roughly about 1-4 inches longer or shorter than your inseam (which is why inseam isn’t a good measurement, either). And this is really just a start as your exact fit is unique to you. But many factors will enter into the equation, the thickness of the soles of your shoes, even the seat style can make a difference.

leg bent
Notice the slight bend at knee

To simplify this, when you are sitting on the seat you want your leg to reach the pedal at the bottom of the stroke (but not the ground–we’ll discuss this later) and you want a slight bend at the knee. How slight is up to you, but if your knee is straight at the bottom of the stroke or if the foot relaxes from or even leaves the pedal, then the seat is too high. Ideally you can achieve the correct proper leg extension by adjusting the seat post up or down.

full  leg extension
Fully extended or hyperextended

If the seat and post is bottomed out and your leg is fully extended, the frame is too big. Say “thanks” to the seller then go look for another bike–you cannot make it fit you. Should you decide this is the perfect bike and you’ll never find another deal as good as this one, remember this little gem: you’ll only ride it once. Why? Because if you can’t support your weight at the bottom of the stroke because your leg is overextended, you will rock back and forth over the top of the saddle, stretching body parts that weren’t designed to be stretched. I don’t know that the damage is permanent, but it hurts like heck for up to a week or more. You will learn to loathe your bike within just a couple of miles.

I installed a new saddle on a friend’s bike once. The bike was a little too tall for me but I could reach the pedals, and the seat was a little wider, but hey, it was for a short test ride. It actually turned into a 10 block or so ride, just a few minutes. All was good till that evening and I got out of the easy chair to go to bed. The pain subsided–about 4 days later, but the worse part was the ridicule from my boss who took great glee in any suffering in her employees–especially the self inflicted type. So anyways the moral of this is: don’t ride a bike where you overextend your legs. Things other than your “bits” may be damaged.

Now if the seat post is all the way out to the limit line (seat posts have a mark that is the limit of their extension–extending the post beyond this mark is dangerous, and foolish) and you cannot fully extend your leg, the frame is too small. For many bikes you can get longer seat posts but this can cause other issues. (I’ve got a story about this too–riding a smaller (52 cm–I ride a 58cm) bike. It too involves pain–I couldn’t walk the next day. My wife thought that was funny. What’s with women thinking guys in pain from riding bikes is funny?).

So why is the leg extension thing so critical? After all you see people riding down the road looking like circus clowns on tiny bikes all the time. One problem is efficiency. Unless the legs are mostly extended with only a slight bend at the knee, you’re not pedaling at your best efficiency. If of course you have a nickname like “Cuddles” or Whizzer” or even “Skank” — “the Clown”, then you’re not too worried efficiency because you’re only going to do a lap or two around the center ring.

But for the rest of us, lets illustrate this: riding a bike is like climbing stairs, you place one foot on a step, push down hard enough to raise your body level with the step. lift the other foot to the next step, push hard enough to raise your body, and so on. On a bike you alternate pushing down with your feet, just like climbing stairs.

Now, stand on a level floor with your feet about a foot apart, then lower yourself by bending your knees and hips until you are in a deep squat. How deep is up to you, but get that butt down there. Now, slowly raise yourself back up to about half the distance to your full standing position–then stop. Now back down into the squat, then back up, then back down, then up. Do this a few times.
Doin the squat
Climbing in the squat

Now back to the bike. Compare the motion to climbing the stairs. Which is easier, climbing with the legs extended or climbing in a squat? Riding is essentially climbing, and since your legs are considerably stronger near their full extension, you want the bike to fit properly to be efficient. If not, you won’t be comfortable, you’ll run out of energy sooner, and quite simply you won’t enjoy riding–because you can’t.
Upright extension

Top tube length
The top tube measurement is the distance from the head to the seat tube along the top tube–if there is or were one. Obviously woman’s bikes, many atb’s, compact roadbikes, and others don’t have a measurable top tube, so the distance from the front of the saddle to the top of the head tube is used. This is where your torso length comes into play.

In a word, you don’t want to be too far or too close to the bars. The type of bike has a lot to do with this, on a road bike you “lay out” to increase aerodynamics, on a cruiser you sit upright. The road bike’s bars are further away from the seat and the cruiser’s are closer. At first glance one would think that the roadbike would simply be uncomfortable, but this isn’t necessarily true.

Personally, I prefer a roadbike over any other style. Other people prefer the upright style, but cruisers don’t tend to be comfortable for long periods of time–you have no back support. Chances are you will at some point change your ideas about what makes a comfortable bike. But in the meantime sit on as many bikes as you can, test ride every one you can.

So while top tube length is a critical measurement, you may not have a lot of selection with used bikes. Actually you don’t really have much of a selection anywhere, anymore. Manufacturers are making two or three frame sizes and if you don’t fit, well buy some parts. But there are some things to be aware of.

One thing is certain, the smaller the frame the shorter the top tube (unless you’re talking compact road frames). If you don’t fit the “ideal” human proportion (whatever that is), this can become a fit issue. Let’s say you have short legs and a longer torso, based on the above fitting advice you choose a smaller frame based on your leg length. But the frame’s top tube measurement is probably going to be too short. The converse is true, legs are long but the torso is short, then a larger frame may have length issues as well.

This means that the handlebars will be too close or too far away. This can be corrected somewhat with different bars and stems, moving the seat, but it also may mean looking for a different brand of bike or even a different style of bike. It’s certainly all right to purchase a bike that fits very close then change stems, bars, seats, etc to perfect the fit. But don’t buy a frame that’s too large or too small in the top tube, either.

In extreme circumstances for some, an off-the-shelf bike will never fit. But there are hundreds of frame builders that can help fit a bike for almost anyone, and sometimes at a surprisingly reasonable price. You may pay a little more initially buy you may never need to buy another bike.

Standover height
Lastly, the measurement that everyone thinks is important, standover height. This is the distance from the ground to the top of the top tube.

Applying usually to roadbikes, this has nothing to do with fitting or sizing a bicycle. If you can straddle the top tube with both feet flat on the floor, the bike is probably too small for you. And realistically this measurement can’t apply to anything but a roadbike, when you think about it. It can’t apply to slope-tube mountain or ATBs, women’s, mixte’s, or any other style.

But Wait a minute! you say–if I come off the seat for some reason I don’t want my (privates? bits? you can fill in the blank here) to hit the top bar! It hurts!

Yep, it will.

If for some reason you come off the saddle and hit the top bar, unfortunately the pain of hitting the top bar will only be secondary to the other problems you’re going to have. Like hitting a car. To get you off the saddle onto the bar will be a biggie. At that point, putting both feet down will likely be low on your list of things to worry about.

In reality you probably will never place both feet on the ground simultaneously when actually riding. Not even when you’re stopped. Correct riding, when starting and stopping means keeping one foot on a pedal (say, your right one) and standing on the right pedal as you come to a complete halt, then leaning left on your outstretched left leg.

When standing with your right foot still on the pedal, you shouldn’t be touching the top tube, if you do the frame is probably too big.

Starting is simple, while stopped, rotate the pedals to a 3 and 9 o’clock position, and stand on right to accelerate. The right foot will stop at the bottom of the stroke but you will have enough momentum to allow you to get your left foot on the left pedal. Practice starting and stopping and it will soon become second nature. Of course this doesn’t apply to coaster brakes, but the standover height advice still applies.

Another reason standover is a useless number is because the height of the bottom bracket above the ground differs with almost every bike. And how do we correctly measure a bicycle? From the bottom bracket to the center of the top tube or top of seat tube. If the bottom bracket is higher one one design, the top tube is going to be higher and if the bottom bracket is low, the top tube will be lower. And yes, different manufacturers place the bb at different heights on different models of bike.

But remember that the distance you are concerned about is the distance from the center of the bottom bracket to the top tube (or top of the seat tube). The standover height means nothing. It isn’t a method for determining bike size.

So how do I know if it fits? Before you buy any bike you should ride as many different bikes as you can, in your size. Look at bikes at garage sales, bike shops, bike sales, borrow one from your friends, your parents, your ex–but ride as many different bikes as you can. And ride them as long as you can. At least up and down the street a few times, but for ten or fifteen or thirty minutes if you can. Yes, the dealers don’t want you going out of sight so this is a good reason to borrow one. But get as much saddle time as you can in different bikes.

I suggest taking some tools with you when you go shopping. I’ll cover this in more detail in a later section of this missive, but invest in a couple of wrenches and an allen key set or bike tool set so that you can raise or lower the seat, and adjust the handlebars. Most sellers won’t care, especially if you look like you know what you are doing (this will help with finding problems as well). I’ll give you a detailed list of what to take in an upcoming post. In fact I’ll devote a whole article on a pre-buying inspection.

But next let’s talk about a couple of things that will should influence your buying knowledge, gears and quality of components.

Next: What to buy. Part 3 Gears

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