How do you know what size of bike to get?

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The question is often asked, “What size bike should I have?”

In traditional road bikes the answer is simple. That is, road bikes with a traditional diamond frame, having a horizontal top tube. For a very long time, I have observed riders and the fit on their bikes. The short answer is this. Get the tallest bike you can comfortably straddle with your shoes on.

Here is the rest of the story.

With a road bike, there are several elements which contribute to overall performance. There are rules of thumb:

  • You should have a handful of seat post? – Why
  • You should be able to just raise the front end off the ground when barefooted? – Are you going to ride barefooted?
  • How tall was your last bike? – Does it matter?
  • Get one just a little smaller than your husband?s? – Huh?
  • You should be able to pass two fingers under your crotch when standing over it? – What for?
  • You should just be able to touch the floor with both feet when on the seat? – Why?

All of these Rules of thumb are fine, I suppose, but more cute than useful. The question is really, “what size frame will best suit my road riding needs?”

The Best Fit

Let’s take a look at the needs of a road rider. We need to be able to sit on the saddle with a comfortable leg extension, ball of the foot on the spindle of the pedal, and when need be, step to the ground for maneuvering. As a practical matter, this will give a bike with a small seat post exposure and a comfortable reach to the bars.

It could be more complicated, but what’s the point? If we simply get a frame that we can stand over, then the other adjustments fall readily at hand. If the frame it too tall or too short, then the other adjustments rarely work out. Ok, so let’s assume for the moment that the basic frame size has been found to be 56 cm, as measured from the center of the bottom bracket (the crank spindle) to the center of the joint between the seat tube and the top tube. That puts the rider standing, with no discomfort and minimal clearance, over the top tube. By minimal clearance, I mean that the rider can feel the top tube intimately, but there is no pressure. That is the right place to start.

Modern frame builder have realized that a person 76″ tall is not simply a 66″ person scaled up. The additional height is generally in the limbs, and not so much the torso. So the frame can be taller, for sure, but not a whole lot longer. For example a 56 cm frame might have a 57 cm top tube, while a 60 cm frame might have a 59 cm top tube. As the frame gets taller, it does get longer, but to a lesser degree. The reason is simple really. The distance from the saddle to the pedals is spanned by the legs. The distance from the saddle to the bars is spanned by the torso and arms. The legs grow more proportionally than do the torso and arms combined.

Ok, so the frame size is selected. Now, what about the saddle location and handlebars? Well, as luck and frame builders would have it, there is plenty of adjustment for both, once the frame has been selected. The saddle rails allow for about 4cm of adjustment fore and aft. The bars, are mounted on a stem. The stems are available in a variety of lengths and angles. A higher stem has a similar effect to reducing the length.

There has been a recent movement toward smaller frames, or so-called “compact frames” in the pursuit of performance. The proponents say that a smaller frame is lighter, stiffer, more responsive, etc. The other side of the coin is that they also have shorter seat stays, for a harsher ride. A harsher ride leads to a more abused rider who is more tired from the ride impact. 700c x 23 or 700c x 25 tires at 115psi roll like there is no tomorrow, it is true. They would probably roll better yet if they were totally aluminum with no air cushion involved at all. In fact they would be quite energy efficient, but who could stand to ride them? There has to be a balance drawn between performance equipment and the rider’s needs. Over the last couple decades, the elements, which provide the comfort to the rider, have become more efficient at transmitting energy from the rider to the road. Well, it works the other direction as well. The road will send the high frequency shock waves right through the high-pressure tires, “unobtanium” spokes and high tensile frame directly to the saddle. Ti frames, carbon frames, carbon seat stays, butted tapered curved seat stays and chain stays etc. are all attempts to separate the road shock from the rider. The frames are very well designed, and beautifully built. The notion of a shorter seat stays is just making it harder for the frame to feel good. It means that a shorter section of tubing has to absorb more energy.

So get a tall frame. The tallest one you can straddle with your shoes on, because it rides better. When the top tube is as high as possible, the need to have elevated stems and bars is reduced. Fitting the bars and brake handles becomes easier. The reach is adjusted by the stem length and the seat position on its rails. When the bars are elevated slightly, the angle between the torso and legs opens a little. There is more abdominal room to breathe, the lungs can get filled more fully and the ability to process oxygen is improved. When we sit up just a little, we can make more power. As an added bonus, we get a better view of the road and our neck, shoulders and back thank us.

Regarding the seat position on its rails, for most riders, the position can be used to adjust the reach to the bars. The exact angle of the fork is very carefully worked out and critical to the handling of the bike in all manner of road conditions. It is far more exacting than the location of the saddle. The exceptions to this are time-trialers, who are making fine tuning adjustments of 3 to 5 mm and everybody on the Tour deFrance.

There is an arcane ritual involving a plumb bob and the kneecaps. Dropping a plumb line from the knee cap, with the pedal at 3 o’clock to see on which lace on the rider’s shoe the plumb-bob falls, (as if cycling shoes still had laces). Using this to determine the fore and aft location of the saddle is rather like letting the reach to the radio in your minivan determine the position of the driver’s seat. People have different size feet, shoes, leg proportions etc. The plumb line method may work for some custom builders, or people just looking for some additional way to make a decision, but it is simply not necessary.

So get a bike with a tall enough frame to ride and handle well. The best frames are made of Titanium, Carbon fiber, Chrome-Moly Steel and Aluminum. Any of the materials can be wonderful. Which one is best for you? That depends on several personal choices and your wallet.