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stem rise vs. reach(7 posts)

stem rise vs. reachmainframe
Jan 2, 2003 5:33 AM
I have two Thomson stems, each is 120mm. One is an 80 degree(-10),the other a plus 5 degree. All other factors of geometry being equal, will reach change by any appreciable amount going from the -10 to the plus 5? I understand the implication of drop change, just am not sure about the aspect of reach.
Reach differenceNessism
Jan 2, 2003 7:27 AM
Unless I have it wrong here, you could caluclate the difference in reach using the following equation:

Reach = Stem length X Cos(angle relative to horizontal)

To ues this equation, a 90 degree stem is 17 degrees from horizontal. Thus:
-10 deg. stem = 7 deg. from horizontal
+5 deg. stem = 22 deg. from horizontal

Based on this equation,
120/-10 stem Reach = 119 mm
120/+5 stem Reach = 111.2 mm

The reach does seem significant to me unless I figuring something wrong here.

Reach differencemainframe
Jan 2, 2003 8:39 AM
Your formula assumes an HTA equal to 73 degrees (73+17=90). In my case, the HTA is 73.5 degrees. How would that change the result? Math is not my forte. Thanks.
Reach differenceNessism
Jan 2, 2003 8:55 AM
The HTA difference you note is not significant.

Regarding reach in general, one thing to think about is reach of the handlebars. Many people are careful when selecting a frame to get the proper top tube length but they don't think about the reach difference between different bars. For example, Deda anatomic bars have a full cm more reach than 3T anatomic bars. Just food for thought.

Reach differencemainframe
Jan 2, 2003 9:39 AM
Good thoughts. I am surprised that fifteen degrees of rise provide for that much (nearly 8mm)difference in reach. My real issue was to decide, while going to the plus 5 (to eliminate spacers)whether to go with a 130mm or remain at 120mm in an attempt to preserve the present reach. Your calcs provide the answer. Thank again!
More info...Nessism
Jan 2, 2003 10:30 AM
If you remove spacers the stem will more forward slightly since the steerer tube is angled down and forward. The difference in not all that much but something to consider. For example, removing 10 mm of spacers will move the stem forward 2.9 mm (assuming a 73 deg. head tube angle).

Good luck.

More info...mainframe
Jan 2, 2003 12:51 PM
If the change in rise (-10 vs.+5) will reduce reach about 8mm, yet eliminating 25mm of spacers will increase reach by perhaps 6+mm, it appears staying with the 120mm stem would match (or nearly so) my existing reach. Again, thanks for your help.