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Old 03-07-08, 10:18 PM   #1
bellweatherman
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Is there an online bike stem angle calculator out there somewhere?

I have an 120mm old quill stem that has a rise of 0 degrees when inserted into the head tube, ie, the stem angle is -17 degrees approximately. I want to buy a new road stem. All I'm trying to do is figure out what length stem a comparable 80 degree stem I would have to order to get the same type of length (ie reach) as my old quill stem.

...so, is there some sort of online calculator that will allow you to enter...
1) your old stem angle & length
2) and also enter in the new stem angle,
3) and allow you to calculate what length of new stem you need to buy
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Old 03-07-08, 10:28 PM   #2
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Maybe too low-tech for what you're asking for, but you can try:
http://www.habcycles.com/fitting.html
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Old 03-07-08, 10:31 PM   #3
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There may be one somewhere, but a bit of intuition and the knowledge
that stems come in 10mm increments suggest that a 7deg change in
angle is not going to change the stem reach enough to get around
the 10mm increments.
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Old 03-07-08, 10:41 PM   #4
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Quote:
Originally Posted by sch View Post
There may be one somewhere, but a bit of intuition and the knowledge
that stems come in 10mm increments suggest that a 7deg change in
angle is not going to change the stem reach enough to get around
the 10mm increments.

Yeah, you're right. Who cares? But, I'd still like to find some sort of calculator somewhere so someday if I switch to a stem with a large angle change, a calculator would just tell me the length stem I need to order to get the identical length (reach) as my old stem. Just convenience.
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Old 03-07-08, 10:50 PM   #5
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Perhaps you can write the software yourself.
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Old 03-08-08, 12:11 AM   #6
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A stem that makes a 90° angle with the steerer tube (also called a 0° stem) actually rises at about 17° from the horizontal. That's because the head tube angle is right around 73° (73+17 = 90). An 80° stem rises at 10° less, or 7°. If t is used to represent the angle that the stem makes with the horizontal and r represents the extension of the stem, then the forward reach is given by r·cos(t) and the vertical reach is given by r·sin(t). So if you have an 83° stem, you'd subtract 73 from 83 to get 10, which is your value of t. Just subtract 73 from your stem angle to get t. If you measure your stem angle in terms of deviation from 90° (for example, I see stem angles quoted as ±6°) then your value of t is given by 17±6. That means if you flip the stem up, you'll get t = 23° and if you flip it down, you'll have t = 11°. If your actual head tube angle is not 73, then replace 73 in the above discussion with whatever that angle is and replace 17 in the above discussion with 90 minus whatever your head tube angle is. Clear as mud?

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Old 03-08-08, 01:08 AM   #7
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Quote:
Originally Posted by soma5 View Post
A stem that makes a 90° angle with the steerer tube (also called a 0° stem) actually rises at about 17° from the horizontal. That's because the head tube angle is right around 73° (73+17 = 90). An 80° stem rises at 10° less, or 7°. If t is used to represent the angle that the stem makes with the horizontal and r represents the extension of the stem, then the forward reach is given by r·cos(t) and the vertical reach is given by r·sin(t). So if you have an 83° stem, you'd subtract 73 from 83 to get 10, which is your value of t. Just subtract 73 from your stem angle to get t. If you measure your stem angle in terms of deviation from 90° (for example, I see stem angles quoted as ±6°) then your value of t is given by 17±6. That means if you flip the stem up, you'll get t = 23° and if you flip it down, you'll have t = 11°. If your actual head tube angle is not 73, then replace 73 in the above discussion with whatever that angle is and replace 17 in the above discussion with 90 minus whatever your head tube angle is. Clear as mud?

-soma5
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Old 03-08-08, 01:11 AM   #8
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Quote:
Originally Posted by JiveTurkey View Post
Maybe too low-tech for what you're asking for, but you can try:
http://www.habcycles.com/fitting.html
+1

This chart works just fine.
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Old 03-08-08, 01:52 AM   #9
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Quote:
Originally Posted by soma5 View Post
A stem that makes a 90° angle with the steerer tube (also called a 0° stem) actually rises at about 17° from the horizontal. That's because the head tube angle is right around 73° (73+17 = 90). An 80° stem rises at 10° less, or 7°. If t is used to represent the angle that the stem makes with the horizontal and r represents the extension of the stem, then the forward reach is given by r·cos(t) and the vertical reach is given by r·sin(t). So if you have an 83° stem, you'd subtract 73 from 83 to get 10, which is your value of t. Just subtract 73 from your stem angle to get t. If you measure your stem angle in terms of deviation from 90° (for example, I see stem angles quoted as ±6°) then your value of t is given by 17±6. That means if you flip the stem up, you'll get t = 23° and if you flip it down, you'll have t = 11°. If your actual head tube angle is not 73, then replace 73 in the above discussion with whatever that angle is and replace 17 in the above discussion with 90 minus whatever your head tube angle is. Clear as mud?

-soma5
I think you are simplifying things too much. You are forgetting about the variable of the rider. Wouldn't the location of the shoulder of the rider also affect the reach/stem equation? I might be wrong, but I think your equation would only work if the rider's shoulder is on the same horizontal plane of the stem base. If the shoulder is located on the same plane, then a 6° stem would have the same reach either way you flip it. But if the shoulder is above the horizontal plane, then the reach would change if the stem rises or lowers, less of a reach if the stem is angled up and a longer reach if angled down.

On second thought, wouldn't only the reach desired of the rider decide the length of the stem? For example, say I want a reach of 30" and a 80° stem. Then where the arc of 30" from my shoulder intersects the plane of 80° angle meet be the length of the desired stem? But, I will assume that the desire of a higer stem angle is to put the rider in a more upright position, not just to raise the hands. Then the shoulder location will also change as will the arc.

Too complicated, just take an educated guess or use one of those stems that allow the change of angle and length until you get the position you want.
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Old 03-08-08, 10:23 AM   #10
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I think you are simplifying things too much.
You might get some disagreement on this.

All I'm addressing is the horizontal reach of the stem, from the point of attachment to the steerer tube to the point of attachment of the handlebars.

The other method of doing all of this is to ignore all of the math, trial fit a stem to your bike and see how it feels. That also works very well, in my experience.

-soma5
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Old 03-08-08, 10:27 AM   #11
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http://mathforum.org/library/drmath/view/53925.html
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Old 03-08-08, 03:21 PM   #12
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Quote:
Originally Posted by bellweatherman View Post
I have an 120mm old quill stem that has a rise of 0 degrees when inserted into the head tube, ie, the stem angle is -17 degrees approximately. I want to buy a new road stem. All I'm trying to do is figure out what length stem a comparable 80 degree stem I would have to order to get the same type of length (ie reach) as my old quill stem.

...so, is there some sort of online calculator that will allow you to enter...
1) your old stem angle & length
2) and also enter in the new stem angle,
3) and allow you to calculate what length of new stem you need to buy
See if this one helps out any..... it has to do with bike steering geometry.

http://www.kreuzotter.de/english/elenk.htm
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Old 03-08-08, 03:30 PM   #13
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Quote:
Originally Posted by soma5 View Post
A stem that makes a 90° angle with the steerer tube (also called a 0° stem) actually rises at about 17° from the horizontal. That's because the head tube angle is right around 73° (73+17 = 90). An 80° stem rises at 10° less, or 7°. If t is used to represent the angle that the stem makes with the horizontal and r represents the extension of the stem, then the forward reach is given by r·cos(t) and the vertical reach is given by r·sin(t). So if you have an 83° stem, you'd subtract 73 from 83 to get 10, which is your value of t. Just subtract 73 from your stem angle to get t. If you measure your stem angle in terms of deviation from 90° (for example, I see stem angles quoted as ±6°) then your value of t is given by 17±6. That means if you flip the stem up, you'll get t = 23° and if you flip it down, you'll have t = 11°. If your actual head tube angle is not 73, then replace 73 in the above discussion with whatever that angle is and replace 17 in the above discussion with 90 minus whatever your head tube angle is. Clear as mud?

-soma5
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Old 12-15-09, 07:44 AM   #14
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found this as one of the main searches on google, and thought people in the future might like:

http://alex.phred.org/stemchart/Default.aspx

sorry for reviving an old thread.
cheers
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Old 12-15-09, 08:04 AM   #15
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The angles of 80 or 84, compared to a 73 are only 1-2mm shorter due to the slight rise. It's simple trig. The 80 degree has about a 7 degree angle with the ground. Multiply cosine7 (.99) times the stem length to get the horizontal length.

The other issue that has not been mentioned is stems are now measured differently than they were in the old days when virtually every stem had a 73 degree angle. Back then you could lay a scale along the top and get the correct length, like 110 or 120, but a modern Ritchey stem in a 110mm length and 73 degree angle will meaure about 115mm if you lay a scale along the top. That's because the measurement is really taken along a line parallel to the extension, but through the center of the bars.

Last edited by DaveSSS; 12-16-09 at 03:46 PM.
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Old 12-15-09, 08:09 PM   #16
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I've found this interactive app to be pretty useful when comparing stem/spacer combinations

http://alex.phred.org/stemchart/Default.aspx
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Old 01-05-10, 05:43 PM   #17
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I've seen questions similar to this in a number of places, so I created this little tool to do the math for you: http://www.brightspoke.com/t/stem-calculator.html

For example, a 120mm 0 degree rise stem on a 73 degree head tube is here - http://www.brightspoke.com/t/stem-ca...&sl=120&hta=73 - with a reach of ~115 mm and a net 17 degree angle to the ground.

You'd need to play with it a bit, but a 10 degree angle stem (80/100) of the same length (120mm) would give a reach of ~107/119 mm and a net stem angle of 27/7 degrees: http://www.brightspoke.com/t/stem-ca...&sl=120&hta=73

If my math is right then 130 mm is close to what you want: http://www.brightspoke.com/t/stem-ca...&sl=130&hta=73 Actually, 128.8 mm is more exact, but I assume you won't find a stem of that exact length.

This is just a first draft, I'm very interested in any feedback you may have about this tool.
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Old 01-08-10, 03:51 PM   #18
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If my stem is 10degrees, spacer height is 40mm, stem length is 90mm and head tube angle is 73.5degrees. What do I lose in vertical height and reach if I flip the stem over.

Also, what if I put a 7degree, 100mm stem on there, what do I gain in reach and/or height?

Thanks.
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Last edited by Jed19; 01-09-10 at 02:38 PM.
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