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effect on reach(4 posts)

effect on reachbostonkiwi
Jan 30, 2004 7:03 AM
Q: If I flip a 110 x 95 stem to 110 x 85 I figure I'll get a ~2 cm increase in drop. If I then add 1cm spacer beneath the stem to reduce the increse in drop to 1 cm, assuming a 73 HTA, what would be the net overall effect on the horizontal reach??
a little trig...C-40
Jan 30, 2004 9:07 AM
The horizontal length of the stem in the 95 degree position is 102mm. In the 85 degree position it is 107mm, for a net difference of 5mm.

To calculate the horizontal length, take the stem amgle minus the head tube times the cosine of that angle. For instance, with a 73 degree head tube angle, cos(95-73) x 110 = 102.

If the center of the stem extension intersects the center of the stem clamp, then the drop would be 1.8cm, but the center of the extension usually does not intersect the center of the stem clamp, resulting in a few mm of error.

To determine the exact drop, place a small piece of smooth board or a hard-bound book on a smooth table. Place the the base of the stem clamp on the boardand push down firmly. Measure from the table to the top edge of the handlebar clamp. Flip the stem and take the same measurment. The difference in height is the exact drop or rise when the stem is flipped. A machinist's 6-inch or 150mm scale is handy for this type of measurement.
minus 3 mm from the spacerahaile
Jan 30, 2004 11:19 AM
I just went through this kind of trig the other day trying to figure out a few sizing questions of my own (thanks C-40 for your help!).

As C-40 said, flipping the stem will give 5mm more of extension. Since you said you also plan to add a 1cm spacer, that will subtract 3mm of reach (as you move the stem up the steerer it also moves it back, since the steerer is angled at around 73 degrees). Math for this is cos(STA) * spacers, so cos(73) * 1cm - =.29cm.

Net difference will be 5mm - 3mm = 2mm, almost nothing.

A friend of mine the other day told me that, in order to get more reach, he'd flipped his stem down then used spacers to bring it up to the same height as before. Since it pointed more foward, he thought it would move the bars forward. Actually, it makes absolutely 0 difference, since the spacers exactly nullify the gain in reach of the more foward stem. It's just two sides of an isoceles triangle.
Jan 31, 2004 3:27 PM
I only did half the problem. This is often the case if you compare two setups that produce the same bar height, one with a low rise stem and more spacers compared to a higher rise stem with few or no spacers.

A good example is the now common 84 degree stem. Folks who use 2cm of spacer under it can flip it to 96 degrees, remove all the spacers and get the same reach and bar height.