Bicycle Mechanics - Road stem question

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plain.jim
11-16-04, 06:32 PM
I'm starting to think about what I'm going to want in my next bike, and I have a question about threadless road stems. I see 'em advertised in the Performance Bike catalog, and they have "rise" indicators ranging from -17 to 5. There's a note that says, "Rise measurements are based on the angle of the stem; a -17 stem on a 73° steerer tube would be parallel to the ground."

Well, that's not clear enough for a newbie like me. Does that mean that a -10 would be 7° above parallel to the ground (given that same 73° steerer tube), a -6 would be 11° above parallel, and that one marked 5 would be 22° above parallel? Or have I got this all wrong? (22° above parallel seems awful high... but I have an adjustable stem on my current Giant OCR2, and it's set about as high as it can go, so maybe that's what I'd be looking for anyway.)

T-Mar
11-16-04, 08:05 PM
You have interpreted the catalog data correctly. It's all based on the fact that a rise indicator of '0' means the stem extension is 90 degrees to steering column clamp. The formula to calculate the difference from parallel becomes:

90 + rise indicator - head tube angle = difference from parallel

For the -17 indicator and 73 degree head angle, this becomes 90-17-73=0 (parallel to ground)

For the -10 indicator and 73 degree head angle, this becomes 90-10-73 =7 (7 degrees above parallel to ground)

Negative results would indicate a stem extension that would be below parallel to the ground.

Note that any of these stems could be flipped upside down, provided you don't mind upside down logos and in some cases, clamps. So the indicators are actually +/- X and that 17 indicator stem could be used to obtain an extension that is parallel to the ground or 17 degrees above parallel.

22 degrees above horizontal is not unusual. Some hybrids stems are 45 degrees or steeper above horizontal.

To find the rise indicator of your current set-up, eyeball the centre-lines of the quill and extension of your stem and mark with two pieces of masking or electrical tape. Measure the resulting angle with a protractor and subtract 90 to get the rise indicator. You should be able to get a measurement within a degree or two of the actual using this method.