HOW TO FIT IN A BIKE

Post date: Feb 15, 2015 6:37:43 AM

How to Fit a Bicycle

by Peter Jon White

Copyright 1989, 1998, 2001, 2007 

Overview

Bicycle fitting is a subject most people find quite mysterious. Fitting systems with charts and graphs, computer software, measuring devices and "rules of thumb" make for a lot of confusion. But I believe it's really quite simple. Bicycle fit involves compromises. Compromises between comfort and performance, quick acceleration and handling stability, top speed and "taking in the scenery". 

Your body's position on the bike affects how you ride. It affects how much power you can efficiently deliver to the pedals. It affects how comfortable you are on the bike. A position that is more comfortable may not allow you to put as much energy into moving the bike forward as a less comfortable position might. How do you decide where to position your body on the bike?

Ask yourself, "What do I want to do with my bike?", "Why am I riding?". A track sprinter is not the least bit concerned with how comfortable he is sitting on the bike. During the race, (which may last for less than a minute), he may only be seated for 5 or 10 seconds. A long distance tourist traveling coast to coast across the USA might spend 5 to 12 hours a day in the saddle, day after day. He is probably far more concerned with being comfortable and enjoying the scenery than with going as fast as he can.

This article relates only to traditional road and cross country mountain bicycles. I know next to nothing about recumbent bicycles and have absolutely no advice to offer regarding recumbent fitting. Nor have I any experience using "aero bars", which allow the rider to rest his forearms on the handlebars. 

Let's get started

Your body contacts the bicycle in three areas; your hands, your seat, and your feet. The relative positions of feet, seat and hands determine your comfort and efficiency on the bike. There are several variables that determine these positions; crank length, distance from crank center or bottom bracket to saddle, saddle angle, seat tube angle and saddle offset, distance from saddle to handlebar, relative height of saddle and handlebar, handlebar width, and handlebar drop on road style handlebars. I'll discuss each of these variables.

Crank Length

Crank length determines the diameter of the circle that the pedals move in. The larger that circle is, the more flexion of your knee and thigh muscles will be needed to turn the cranks. Your thigh muscles cannot exert the same force throughout their range of motion. This is very easy to demonstrate. If you squat down so that your knees are fully bent and lift yourself up, say, five inches, it takes a good deal more effort than it would to squat down just five inches from standing straight and then lift yourself back up. At the full squat position, your muscles can't put out the same power as when your knees are just bent enough to drop you down five inches. So if you had to choose between a crank length that had your knees bending through their entire range of motion and a length that only required, say, 20 degrees of flexion at the knee, you would choose the shorter crank. That crank would have your muscles working through a more efficient range of motion. You would avoid having to flex your knees enough to bring you into an inefficient range of motion.

So how long should the cranks be? Well, that's a good question. I wish I had a good answer but I don't. It should be obvious that a 5' 2" rider would not want to use the same length crank arms as a 6' 7" rider unless they somehow managed to have the same leg length (highly unlikely). Some research has been done to determine the optimum percentage of leg length to crank length. I doubt that there is an optimum percentage that would apply to all people. One writer in a major magazine article quite a few years ago claimed that after considerable testing with many different riders, 18.5% of the distance from the top of the femur to the floor in bare feet should be the crank length. You can find the top of the femur pretty easily. It's 5" to 6" below your hip bone, and moves rearward when you raise your knee. After reading this I promptly changed from the industry standard 170mm cranks for road bikes to 175mm cranks. There was an immediate improvement in power and endurance. I began using this formula when recommending cranks to my customers. So far, I haven't gotten any complaints. But of course that doesn't mean my customers wouldn't be as happy or happier with some other length. And I must admit that I have never tried still longer cranks than 175mm for enough time to tell if I would be even happier with them.

The top of the femur measurement ignores differences in legs themselves. Differences in the proportion of calf length to thigh length should affect the optimum crank length. A rider with longer thighs and shorter calves would use a longer crank to get the same flexion at the knee as a rider with short thigh and long calf. Of two riders with the same body proportions, one might prefer to pedal at a faster cadence. That might favor a shorter crank length. And perhaps even two riders with identical skeletal proportions would find after testing that they required different crank lengths to achieve maximum performance due simply to differences in their muscles.

Trying different cranks to find the optimum length would be time consuming and expensive, but I believe it is the only way to determine the correct length for any individual, assuming there is a correct length. It would be nice to have a crank with many pedal threads at various lengths to test. But I know of no such thing being made and I lack the ability to make one! Of course, some riders with multiple bikes report being just as happy on one crank length as another. Go figure! So, for lack of a better system, I'm staying with the 18.5% guide for my customers until something better comes along. It hasn't failed yet.

Note: As of August, 2007, we have here at Peter White Cycles a special crank with adjustable length that we use while fitting cyclists to our bikes. And, if you would like to come by and have it installed on your bike for determining your best crank length, just call ahead for an appointment.

In the United States, it has been difficult and expensive to obtain cranks shorter than 165mm or longer than 175mm. But a French company, Specialites TA has been making high quality cranks in lengths of 155mm through 185mm for many years. In order to offer my customers better fitting bicycles, I've decided to sell these cranks. See my web page; http://www.PeterWhiteCycles.com/carmina.asp for details.

Saddle Tilt 

In order to fit a bicycle, you need a saddle you can sit ON. That may seem too obvious to even mention. But sadly, most bikes seem to be sold with saddles designed by people who have never ridden a bicycle. In order to ease the pain of using these implements of torture, people often angle the saddle with the nose down. This makes it impossible to sit ON the saddle. You tend to slide forward. You end up pushing against the handlebar just to keep yourself on the saddle. Get yourself a saddle you can sit on so that your pelvis is resting on a level surface. For most saddles that would put the nose of the saddle a bit higher than the rear. Women's saddles should be wider than men's since a woman's pelvis is wider. Many women's saddles now have a cutout or low density foam section in the center to take pressure off the pubic bone while allowing a level saddle position. The closer you can get to a level platform, the easier it will be to find the best fore-aft position of saddle and handlebar.

Saddle Height

Once the crank length is determined, (by whatever means), the saddle should be set at a nominal height. There is no objectively determined ideal saddle height for any rider based on leg length alone. Some riders naturally pedal toes down, while others have the foot in a more level position. For starters, sit on the saddle with one leg hanging free and your hips square, (not tilting to either side). Set the saddle high enough so that your other heel can just touch the pedal with your leg straight, and with the pedal at the bottom of the stroke, in line with the seat tube. For most people this results in a saddle height that leaves some bend in the knee at the bottom of the pedal stroke, when you're pedaling with the balls of your feet over the axle of the pedals. It also should prevent you from having to rock your hips through each crank rotation. This gets you close enough to your optimum saddle height that you can go through the rest of the fitting process and fine tune saddle height later. Any later saddle height adjustments shouldn't be enough to throw off the other adjustments other than handlebar height, which is easily changed. 

The Fore-Aft Saddle Position

Now we get to what I think is the most important part of fitting a bicycle, the fore-aft position of the saddle. Once you get this right, everything else is easy. This position is determined more by how you intend to use your bike than by anything else. If you look at a typical bike, the saddle is behind the crank center, or bottom bracket. There's a frame tube (the seat tube) running from the cranks to the saddle, and it's at an angle. That angle partly determines the fore-aft position of the saddle relative to the cranks and pedals. That fore-aft position determines how your body is balanced on the bicycle. Your balance determines how comfortable you are, and how efficiently you can pedal the bike.

Stand up straight in front of a mirror and turn to the side. Look at yourself in the mirror. When standing straight your head, hands, seat and feet are all fairly close to being in line with each other. Now bend over at the waist. Notice that not only has your head moved to a position ahead of your feet, but your rear end has moved behind your feet. If this were not the case, you would fall forward. Your seat moves back when you bend at the waist to keep you in balance.

Your torso needs to be leaning forward for two reasons; power output and aerodynamics. With an upright torso, you can't use the gluteus muscles to good effect. Also, you can't effectively pull up on the handlebar from an upright position. An upright torso is also very poor aerodynamically. When cycling on level ground, the majority of your effort goes against wind resistance. The easier it is for your body to move through the air, the less work you'll have to do. With your torso closer to horizontal, you present less frontal surface to the air and don't have to work as hard to maintain a given speed.

Obviously, the most aerodynamically efficient position may not be the most pleasant position to be in for several hours on a cross country tour. So there's a tradeoff. As you move to a more horizontal position, the saddle needs to be positioned further to the rear to maintain your body's balance, just as your rear end moves to the rear as you bend over while standing. It so happens that racers are more inclined to use a horizontal torso position than tourers, and racers are more concerned with having the handlebars further forward to make climbing and sprinting out of the saddle more effective.

If a bicycle had the saddle directly over the cranks, you wouldn't be able to lean your body forward without supporting the weight of your torso with your arms. Because the saddle on a typical bicycle is behind the cranks, your seat is positioned behind your feet and your body can be in balance. Try this test. You'll need a friend to hold the bike up, or set it on a wind trainer. Sit on your bike with your hands on the handlebars and the crank arms horizontal. If you have a drop bar, hold the bar out on the brake hoods. Try taking your hands off the bar without moving your torso. If it's a strain to hold your torso in that same position, that's an indication of the work your arms are doing to hold you up.

For starters, I like to put the saddle in the forward most position that allows the rider to lift his hands off of the handlebar and maintain the torso position without strain. You should not feel like you're about to fall forward when you lift off the handlebar. If it makes no difference to your back muscles whether you have your hands on the bars or not, you know that you aren't using your arms to support your upper body. If you are, your arms and shoulders will surely get tired on a long ride. But this is a starting position. Remember that bicycle fit is a series of compromises.

So what's being compromised? Power. There's a limit to how far you can comfortably reach to the handlebar while seated. If the saddle is well back for balance, the handlebars will need to be back as well. But to get power to the pedals while out of the saddle, it helps to have the handlebars well forward of the cranks. Particularly when climbing out of the saddle, the best position tends to be had with a long forward reach to the bars. You can tell this is so by climbing a hill out of the saddle with your hands as far forward on the brake lever tops as you can hold them, then climbing the same hill with your hands as far to the rear as you can on the bars. Chances are you can climb faster with your hands further forward. So you need to find the best compromise between a comfortable seated position and reach to the handlebar, and a forward handlebar position for those times when you need to stand. Only an inch or two in handlebar placement fore-aft can make a big difference while climbing. That same inch or two in saddle position can mean the difference between a comfortable 50 mile ride and a stiff neck and sore shoulders!

As you move the saddle forward from that balanced position, you'll have more and more weight supported by your arms, but you'll be able to position the handlebars further forward for more power. The track sprinter has the frame built with a rather steep seat tube angle, which positions the saddle further forward from where the tourer would want it. But again, the track sprinter spends very little time in the saddle.

If you can't move your saddle forward enough or backward enough for the fit you want, don't despair. Different saddles position the rails further ahead than others, giving more or less saddle offset. Seatposts are available with the clamps in different positions relative to the centerline of the post.

So, how do YOU want to balance on YOUR bike? Do you want to emphasize speed and acceleration? Do you care mostly about comfort and enjoying the scenery? The answers to these questions determine how you position the saddle, not some computer program or someone's system of charts and graphs. How your best friend fits his bike should have no bearing on what you do even if he has exactly the same body proportions as you. YOU know why you ride a bike. Only YOU know what compromises you are willing to make in order to achieve your purposes on a bicycle.

You may have a bicycle for short fast rides, and another for long tours. Just as the two bikes will have different components so as to be well suited for their purposes, so might the fit be different. The rider hasn't changed. You are still you. But your purpose has changed. The light, fast bike for short rides will likely have a more forward and lower handlebar position than the tourer. And so the saddle may well be further forward too.

As you move the saddle forward or rearward, you are also changing the effective saddle height, relative to the cranks, since the saddle rails are usually not perpendicular to the seat tube. So be prepared to change the seat post extension as you adjust the fore-aft saddle position; lowering the saddle as you move it back to maintain the same leg extension, and raising it as you move the saddle forward. 

What about knee over the pedal axle?

Most fitting "systems" specify that some part of your knee be directly over the pedal axle at some alignment of the crank, usually with the pedal forward and the crank horizontal. This is pure nonsense. Imagine two riders, almost identical, but one rider's knees are 1 inch lower than the other's. In other words, the thigh bones of one rider are 1 inch longer than the other, and his lower legs are 1 inch shorter. Everything else about these two riders is identical, including overall height, torso length, arm length and weight. If you position the saddle such that the knee is directly over the pedal axle, the rider with the shorter thighs must have his saddle a little under 1 inch further forward of the other rider. It would be exactly 1 inch if his thigh was horizontal at that pedal position, which it isn't likely to be. 

But with the saddle positioned forward, the rider with shorter thighs now has more weight that must be supported by his arms, all because of this arbitrary rule about having your knee over the pedal axle. This makes no sense. What matters is your weight distribution fore and aft, and that's determined by the fore-aft position of the saddle relative to the cranks.

Handlebar Position

Next, where does the handlebar go? Just like the saddle, it all depends on what it is you're doing on a bike in the first place. The further forward the bar, the more power you will have standing and accelerating, the better the aerodynamics and high speed control. The lower the bar, the more you can pull up under hard acceleration and the better the aerodynamics. With the bar closer to you and/or higher, you can sit more upright and take in the view.

I like to use an adjustable stem that my customers can use for a few days to try different positions for a long enough time to be meaningful. But what about a starting point? For riders with drop bars, if you place your hands down in the drops at the forward most position, (the point that allows you to easily reach the brake levers), then bend your elbows enough that your forearms are horizontal, your elbow would be at a ninety degree angle for a good starting point. From there, try moving the bar in one half inch increments forward and back to find the best reach for you. Most people are quite comfortable just with the ninety degree elbow position. But that doesn't mean it's right for you. And of course this isn't a position you'd want to spend much time riding in, except on the occasional banzai descent down a mountain pass! 

Racers generally end up with the handlebar height two to three inches below the saddle height, tourers will often like to have the bar at the same height as the saddle. Mountain bikers usually position the bar a couple of inches below the saddle. The important thing is to take enough time to find the best position for you. If that means setting up a touring bike with the handlebar four inches below the saddle height, so be it. I recommend the longest reach and lowest position you feel comfortable in, (with emphasis on comfortable).

Handlebar width (road & ATB) and drop for road style bars

A few brands of drop style bars come with a choice of how much lower the drop section of the bar is from the top. Unless you are a track sprinter or a criterium racer, you don't need the very deep drop bars. Most bars come in a selection of widths. Most people seem happiest with their hands positioned on the bar at about the same distance apart as the width of their shoulders, so that your arms are roughly parallel when reaching to the bar. When determining stem dimensions, try the different bar widths available, starting with one that's the same as your shoulder's width. Then see which works best for you.

Fine tune saddle height

As you get familiar with the way your bike feels with these changes, go back to the beginning and check your saddle height again. You should be able to pedal through the bottom of the stroke without completely straightening your knees, and without rocking your hips on the saddle. If either is the case, your saddle is too high. Straightening your knee during the pedal's rotation limits how fast you can smoothly rotate the pedals, and causes you to want to use a higher gear than that which would be most efficient. By limiting the extension of your legs you smooth out your pedaling and make higher RPMs possible. That's better for your muscles and joints. If the saddle is too low you'll quickly feel a burning sensation in your quadriceps or thigh muscles.

Stock frame sizing

So what does all this mean when it comes to picking a frame size down at your local bike shop? Stems and seat posts come in lots of different configurations. That means you can choose from several different frame sizes and still get the same good fit.

All other things being equal, a longer top tube will give you a bike with a longer wheelbase, less twitchy handing, better shock absorption, and require a shorter reach stem. Since the down tube, (which connects the bottom bracket with the head tube) will be longer, it can twist a bit more making the frame somewhat less stiff while accelerating, so there is a performance penalty.

A longer seat tube will allow for a higher handlebar position with the same stem and give more room for pumps and water bottles. It can also prevent you from getting as low a handlebar position as you may want. Most importantly though, the longer seat tube raises the top tube and decreases stand-over clearance, something you should give careful consideration to. 

Methodology

Notice that in most of this there is no mention of measuring body parts. And nowhere do I have you dropping plumb lines from knees, positioning handlebars so they block views of front hubs, comparing the length of your forearm to the distance between the front of your saddle to your handlebar, etc. My methodology is quite different from what most people are doing in bike shops. The Fit Kit and other marketed fitting systems are based on the measurements of lots of different riders and their bikes. It assumes that the averages of those measurements are somehow going to result in a good fit for you.

But take the case of two riders; Rider A, and Rider B. Rider A has very little upper body muscle but very strong legs. Rider B is identical to Rider A but has been working out at Ralph's Gym and looks like a body builder. The fore aft position of the saddle will be slightly different for the two riders. The extra upper body mass of Rider B will require a slightly further back saddle position to give the same balance. But that doesn't necessarily mean that Rider B should have his saddle further back. He may prefer the more forward position. Only he knows what his preference is.

Take me for example. I started riding long distances in the mid 1970s. After much trial and error, I arrived at a bike fit that worked for me. I could ride a century without much fatigue. I still have a bike that's set up exactly like the racing style bike I rode back then. I haven't gotten any taller, or shorter. My arms haven't grown or shrunk. But my neck and back are a lot stiffer in 2001 than they were in 1975. The low handlebar position is still great while I'm climbing a hill, but on the flat after 60 miles my neck isn't as comfortable as it once was. A higher handlebar is called for now that I'm older.

Measuring my body wouldn't tell me that the handlebar needs to be higher. But I used to be comfortable in the drops, (the lower part of a road handlebar). Now, I can't see the road ahead of me if I'm in the drops.

Somewhere between the fit of the track racer's bike and the long distance tourer's bike is where most of us want to be. But each of us has to find that point for ourselves. Remember, there is only one expert when it comes to fitting your bike. Only you know how you feel on your bike. Only you know what compromises you are willing to make while riding. You're the expert!

By the way, for reasons that escape me, I frequently get email from folks who tell me that they read my fitting article, loved it, and have a question. They then tell me how long their arms, legs and torso are, proceed to inform me that some bike shop wants to sell them a particular bike, and want to know if I think the bike would fit them well. My reaction is to wonder whether I wrote the article clearly, or if they read it but didn't understand it, or if they just hadn't read it. I sometimes go back and reread my own article, assuring myself that yes indeed I did make my ideas clear, and for whatever reason the person just didn't get it. Oh well...

So I'll take this opportunity to rephrase myself. I don't know if a particular bike will be a good fit for you. Even if I knew every dimension of the bike, and every dimension of you, I couldn't tell you if it's a good fit or not. So please don't write asking me to tell you something that I can't possibly know. Reread the article.

If you need an authority figure to tell you how your bike should fit, then by all means go to some shop that offers the Fit Kit or some such thing, pay them whatever they charge, and do as they say. Some folks need to be told what to do. But I don't want to tell you what to do. I'd rather give you the knowledge you need to fit yourself. Because with that knowledge, I believe you can do a much better job of it than some expert charging you money.

And, by the way, I'm a bike mechanic, not a doctor. So if your knee or back or neck or wrist hurts, and you've set your bike up using the information in this article, well, I suppose you could sue me, but please don't expect me to diagnose your ailment. See a doctor. Then sue me. ;-)

By the Way

This article is for your information only. It's not intended as the jumping off point for a conversation with me about bicycle fit. I'd like to discuss it with everyone who reads it, but unfortunately if I did that I'd have no time to build wheels, which is how I make my living. I don't do bicycle fittings any longer. Nor do I have the time to respond to or even read all of the mail I get regarding this article. So, when you write to me and I don't reply, don't feel as though I've singled you out. I no longer reply to any email on this topic. I just have no time. Sorry. 

http://www.PeterWhiteCycles.com

This article is Copyright 1989, 1998, 2001, 2002, 2007 Peter Jon White. Last revised February 21, 2007

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