Help me understand stem angles

[Dan Le foot]

Hi gang.
I have a 84 degree (-6%) 130mm stem. In order to get the bar to the correct height I need to place 2cm of spacers on the top of the stem. Looks like a stove pipe.
I guess I can cut the stem but I would rather not for a possible future sale.
If I replace the stem with a 73 degree stem can I drop the bar the needed 2cms? (I don't have one to try) In other words how much of a drop will I get by going from a -6% to a -17% stem? Is there a formula that can be used?
Thanks
Dan

[Kirk007]

there is a geometric formula, but I don't have it on me; I found it though by searching the net. The short answer to your question is probably. IMy recollection is that a 10 degree change in angle is about a 2 cm difference in rise.

[Simon Q]

http://www.zinncycles.com/stemFit.aspx

[Don Ventura]

http://www.habcycles.com/fitting.html

[Louis]

Soh Cah Toa

[mosca]

By my calculation, this stem swap would move your bars about 2.5cm down and 0.25cm forward, based on a 130mm stem length.

I should add that if you then put the 20mm of spacers below the stem, your bars will be about 5.5mm lower and 3.5mm farther back.

[Dave]

It seems like the simple solution is to remove 2cm of spacer and cut the steering tube shorter. Whether that's a bad idea depends on how much spacer is left.

There is a simple formula for the rise and the horizontal reach. Using 84 degrees, the common road bike designation is a lot easier, instead of -6. In this case, the head tube angle is not relevant, but subtracting a nominal HTA of 73 degrees from 84 gives you angle with the horizontal (11 degrees). The rise is just the sine of that angle times the stem length. In this case, the rise would be sin11 x 130 = 25mm. The reach is cos11 x 130 or 127.6mm.

A -17 degree stem is a 73 degree in road stem terminology, so the angle is zero, or hoizontal and the comparable rise is zero. The horizontal length should be a full 130mm.

If you want the bars 2cm lower then you need to remove only 15mm of spacer, although this assumes that both stem have the same clamp height and distance from the base of the stem clamp to the centerline of the extension. That might not be the case.

Bottom line, buy a 73 degree stem, put it on with no change to the spacers and measure the bar height change from the floor to the top of the bars. It might be a bit too low, if the 84 degree is 2cm too high.

The best solution might be the compromise angle of 80 degrees, which would drop the bars 9mm. Then remove 10mm of spacer and either leave it or cut the steerer shorter.

[capybaras]

every one is different :banana: :banana:

[Ti Designs]

It's January.

[capybaras]

It's January.

not for long! :banana:

[MarleyMon]

I like the excel ss linked here:
http://serotta.com/forum/showpost.php?p=323263&postcount=7

[Ti Designs]

not for long! :banana:

My point was that it's not mid season, your bike shouldn't be set up in your mid season position. My fixed gear is set up about 1cm closer and higher than my early season road position, which also gains a bit of length later in the summer. What I have for flexability changes with 5 layers of winter clothing and an additional 200 miles a week. Why anyone would spend countless hours dialing in their winter position is beyond me - sit up a little and put in the base miles!

[gman]

What a novel concept! Wish I'd thought of that.

Score a big "duh" for me.

This explains why I feel like I just ate thanksgiving dinner everytime I reach for the drops during the cold months.

How is yours setup differently from your "summer" position? How far off?

Good tip.