Bike stem sizes and styles - what size should I buy?

I am fitting butterfly bars to my MTB, to create a sort of off-road tourer and to help eliminate the shoulder pain I got on my last long ride. I am hoping the choice of positions offered by the trekking bars will keep my shoulders moving on my rides (and the days after).

So I bought the bars, specifically these ones from VeloSolo. And then I went about fitting them to my bike. That's when I encountered problems.

Now I assumed the slightly larger part of the handlebars would rise to fit the stem that I have, like there was some universal number that all parts adhered to, the cyclist's 42. I was a numpty. I should have checked.

Since my little batch of silliness, I have learnt some invaluable lessons which I am going to pass on to you lot now. In all fairness most people probably know this, but I hate to assume knowledge and I like passing things in.

Stems come in two styles. A quill stem is a one piece stem, which is normally found on road racing style bikes or the singlespeed retro affairs. It rises out of the steering tube, contains a bend, and clamps to the handlebars. A threadless stem attaches to the steering tube, attaching the handle bars.

Here are pictures for illustrative purposes.

Quill stem

Threadless stem

Threadless stems are the newer of the two types, and are the only ones I've ever had dealings with, so the rest of this article will focus on them. Sheldon Brown has an excellent piece on quill stems, if that's what you need more information on. You can find it here.

So threadless stems have three different measurement, which have to be considered when purchasing. The first is the steerer tube diameter, and this will be measured in inches. The most common sizes are 1 1/8th inch and 1 inch. This is the size of the clamp which will fit to the steering tube, and so needs to be a snug fit, tightened up well,  or you are going to come a cropper.

The second measurement to consider is the handlebar clamp size. This is going to be measured in millimetres. The most common size is 25.4mm (which is actually 1 inch), but other common sizes include 26.0mm for a road bike and 22.2mm for BMX bars.

Now I have tried to hunt down a reason why the standard way to measure mixes metric and imperial measurements, but I can't for the life of me find a good reason. It is what it is, and must have evolved as bike components have evolved. If you know the answer I would love to know.

So, once you have those measurements then it is good to consider the final two measurements you'll need. The first is the length of the stem. This will be in millimetres again, and can be anything up to 140mm. That's 14cm, or 5.5 inches, so pretty long.

The final measurement to consider is the angle. This will be from 0-40°, and this can obviously be mounted normally or inverted so the angle points up or down.

In summary, that's what you have to remember when buying a threadless stem -

• Steering tube diameter
• Intended handlebar diameter
• Length from steering tube
• And, angle of the stem

And now we've all learnt a valuable lesson, and I am off to purchase this stem so I can get back on my bike ASAP.