What factors determine stem length ?

Question

Recently I went to a local bike shop to get a bike fitting advice. After taking body measurements, I was given a frame size recommendation. According to that, seat tube size, top tube size and stem length are 500mm, 520mm and 45mm respectively. Since the stem length of 45mm is unrealistic, I was suggested a bike that have 500mm top tube and 65mm stem length so that the total length remains the same. My question is

(1)What factors determine stem length ?

(2)Can shorter top tube compensate longer stem length without any harm ?

Answer 1

Bicycle size is determined first by frame size, which is top tube length, seat tube length, and some more minor geometries like head tube and seat tube angle. The primary determining factor for comfort on the bike is your reach measurement.

Although it is affected by various factors, like skeletal flexibility and the type of riding you do, ideal reach is generally considered to be the distance from your seating position, (where you actually rest on the saddle), to your hand position, (where your hands fall when placed in a neutral position on the bar), while your back and arms are relaxed and at roughly equal positions approximately 45 degrees each. Balance is what you are looking for here.

The stem length is a portion of the reach measurement. That is, reach = saddle setback + top tube length + stem length + bar length.

Any of these factors can be altered to some degree to produce a correct reach measurement. But there are other factors which affect this as well. If your stem is too short, your steering will be too quick to react, as small movements of you body and hands will move your wheel too far, too fast. This will make the bike difficult to control. If you go the other way, with too long a stem, then your bike will handle sluggishly, and you might not be able to react quickly enough to an obstacle in the road.

Based on the measurements that you were given, I would expect that you are approximately 5 foot tall to 5 foot 2 inches. I think that the shop is likely offering to adapt a bike in their stock to your size, rather than getting you the correct size. If I am correct about your approximate height, you should be looking for a 47 or 48cm frame which will have between a 65 and 80mm stem on it. If I am not correct, post your height and riding style, and we can give you more general advice on the correct size. Best though is to find a shop which specializes in bike fitting, and get a second opinion. I am good at bike fit, but is difficult to accomplish well without having you in my studio.

Answer 2

I have recently assembled a bike from used parts, and the last bit of adjustment was precisely the stem length.

Two criteria were key to decide the right length, and they didn't have anything to do with what I call "The Numerology of Bike Fit". Instead, I prefer functional criteria, that is:

• How is the bike HANDLING? I thought I would need a stem of a given length, to get a similar reach I had in another bike. After going out for a test ride, I noticed the handlebar was too responsive, and it was difficult to keep a constant radius during a cornering, due to wobbling. Also, any bump tended to unstabilize the bar, and the overall behavior of the bike was "nervous".
• How are my torso/shoulders/wrists FEELING? I felt the bars were too recessed, so my shoulders felt pushed behind by my arms (strange, isn't it?), and my head felt a bit unsupported. I could ease this a bit by putting the saddle back (a bit more than the sweet-spot).

THEN, I decided to abandon what common sense would say, I switched to a longer bar (3cm more forward), even with the final result being a longer reach than I had in the previous bike. Immediately, the bike felt much more stable, and my shoulders became unstressed.

The conclusions I get are the following (and this is a very personal theory):

• The RIGHT reach depends on each bike's characteristics, mostly the balance between upright/aerodynamic position determined by handlebar height, and the handling characteristics determined by top tube length and steering geometry;
• There is an effect on steering caused by the rider torso's weight being applied by the hands on the bar, and THE SIZE OF THE LEVER represented by stem length, that creates a self-centering force on the front-wheel. This will counteract the tendency for the wheel to fall to one side at low speeds, and the caster effect at higher speeds. By this principle, the final steering behavior depends on stem length, fork trail, and head tube angle.
• For a given bike-rider combination, each possible handlebar HEIGHT will have a sweet-spot on REACH, and usually a change in height would require a DIFFERENT STEM to get the proper new reach, because of the steering tube trajectory not being coincident with the required trajectory of ideal positions.

So, the advice I could give you is this: - With either a large frame or a small frame, there would in principle be a stem length that would give you a satisfactory position; - A longer wheelbase at cost of a longer top tube COULD mean a short stem to get a good reach. That short stem, and its correspondent short lever on the steering, could make the bike unstable and thus make you tired during long rides. - As opposed to what common sense would say, a smaller frame with a larger stem could give you the same reach, but with a longer stem-leverage, thus making the bike more stable, requiring less upper-body input during long rides. - Handlebar height has an effect on all this, since the lower the handlebar, the more weight you will apply over it, and more pronounced the effect of this stem-leverage.

Hope this helps, and hope this wasn't too confusing :o)

Answer 3

The top tube length, seat position, stem length, head angle, and bar design together determine "reach", which is the primary criterion here. Stem length also has an effect on stability.

Top tube length and seat tube length generally are tied together (in a non-custom frame), so a shorter top tube implies a shorter seat tube (ie, a "smaller" frame).

You presumably have arms and/or torso that is short relative to your legs, hence you need a shorter reach. Back problems (or just excess avoirdupois) could also reduce one's ability to "reach".