FSA OS99 stem - real lengths?

Discuss light weight issues concerning road bikes & parts.
Svetty
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Joined: Thu Mar 26, 2009 11:06 pm
Location: Yorkshire - God's Own Country

by Svetty

I have an FSA OS99 stem which is 12.3 mm from the lip of the stem at the bar clamp end to the centre of the top cap bolt. Is this a 120mm or 130mm model?

duvivr6
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Location: PR

by duvivr6

My OS99 has he length written on it, is that not the case with yours?

5 8 5
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Location: UK

by 5 8 5

What does it measure centre to centre (side-on)?

ipenguinking
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Location: Sunny So Cal

by ipenguinking

I have this stem in three length - 90mm, 100mm and 110mm. (per package) All three measured approximately 10mm longer.

carlcurry
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Location: Sherman Oaks, CA

by carlcurry

I'd be interested in the actual lengths as well. My Easton and FSA SL-K stems match up exactly as I would expect. The OS99 stems don't. I have a couple OS99's.
I've been treating my 100mm OS99 as a 105mm. I've been considering buying a 90mm OS99 thinking it might measure 95mm, which would be perfect for one of my bikes.
Bianchi Infinito CV, Cervelo R3, Giant TCR, Trek Domane SLR, Specialized Allez

DJT21
Posts: 183
Joined: Sun Mar 27, 2011 7:35 pm

by DJT21

Svetty wrote:I have an FSA OS99 stem which is 12.3 mm from the lip of the stem at the bar clamp end to the centre of the top cap bolt. Is this a 120mm or 130mm model?


It's the 120mm model.

Stems with any degree of rise/drop will always measure longer; they have to, due to "Trigonometry"

Fiery
Posts: 412
Joined: Sat Mar 16, 2013 9:21 am

by Fiery

Measuring along the top from the center of the top cap will always yield a longer measurement than the actual center to center length. See the diagram:

Image

On the other hand, there apparently a fair bit of variation in length of forged stems, so it's possible that the actual length is different than nominal. (Pro doesn't actually make stems in one millimeter increments for Team Sky, they just measure and label each stem with the actual length).

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ms6073
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by ms6073

Don't have any images to offer, but when I set 3 12cm stems - a 2016 Enve carbon road stem, FSA OS99, and generic alloy stem - on the workbench with the clamp flush against the wall, the Enve and FSA were ~5-7mm longer than the alloy stem.
Michael - The Anaerobic Threshold is neither...

Svetty
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Joined: Thu Mar 26, 2009 11:06 pm
Location: Yorkshire - God's Own Country

by Svetty

OK, turns out that my stem is a 120mm model. Thanks for the help guys :)

Briscoelab
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Joined: Thu Jan 26, 2006 6:01 pm

by Briscoelab

No they don't.

1. Stems should be measured c-c along the side of the stem. This will give a consistent measurement of length, independent of rise/drop.

2. Some companies don't adhere to this method, confounding the issue.

3. Aluminum stems come out of the forging with a surprising amount of variance in length from unit to unit. I've had 3 stemf of the sam mfg measure from 3mm short to 7mm long, for the same length/model stem.

4. Some companies are notorious for well documented stem length variances. 3T ARX for example, are ~8mm long on their -17 stems. This was likely a mistake in the forging design when someone wanted to account for the angle.

joejack951
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Location: Wilmington, DE

by joejack951

Briscoelab wrote:3. Aluminum stems come out of the forging with a surprising amount of variance in length from unit to unit. I've had 3 stemf of the sam mfg measure from 3mm short to 7mm long, for the same length/model stem.


There's absolutely no way that two forgings out of the same die vary in length by even a single millimeter. Either there is more than one die in the mix and the dies do not match (possible but unlikely for the same manufacturer and model stem unless we are talking about some generic brand like Kalloy) or there is a measurement error on your part.

How did you take your measurements and do you have side by side pics of the stems?

Tenlegs
Posts: 285
Joined: Wed Jul 16, 2008 10:37 am

by Tenlegs

Briscoelab is completely right, stem forgings do vary considerably, stems are not forged in a single die, 3d forging is a multi step process it all comes down to QC,
In this example Salsa sets a tolerance of ±3mm on their stems, after forging they say there can be upto ±5mm difference from nominal size.
It all comes down to where the manufacturer sets their QC limit.

http://www.peterverdone.com/the-stem-pr ... gineering/

joejack951
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Location: Wilmington, DE

by joejack951

Tenlegs wrote:http://www.peterverdone.com/the-stem-problem-and-reverse-engineering/


As a Solidworks users and frequent reverse engineer-er of parts for various purposes, that was really interesting. I hadn't considered that stems were forged in a multi-stage process like that but it explains a lot. I always wondered how stem manufacturers managed to have such a clean parting line along the extension of the stem. As it turns out, they do it by not having a parting line there at all.

Here's one of my recent reverse engineered parts (with some changes from the original) and the actual CNC piece made from it:

ImageTRP HYRD Conversion Arm by joe jackson, on Flickr

ImageTRP HY/RD with short pull conversion arm (Campagnolo or older Shimano and SRAM) by joe jackson, on Flickr

Briscoelab
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Joined: Thu Jan 26, 2006 6:01 pm

by Briscoelab

joejack951 wrote:
Briscoelab wrote:3. Aluminum stems come out of the forging with a surprising amount of variance in length from unit to unit. I've had 3 stemf of the sam mfg measure from 3mm short to 7mm long, for the same length/model stem.


There's absolutely no way that two forgings out of the same die vary in length by even a single millimeter. Either there is more than one die in the mix and the dies do not match (possible but unlikely for the same manufacturer and model stem unless we are talking about some generic brand like Kalloy) or there is a measurement error on your part.

How did you take your measurements and do you have side by side pics of the stems?


Do you really think manufacturers make all stems of a given length on only 1 die? They have multiple 110mm dies for example, and these were initially made with varying tolerances from the start. Then, based on age and usage, they have worn to different degrees. So even out of a single die step, you can get a lot of variance. (Analogous variances happen with extrusions for aluminum rims)

As mentioned, stems are made in a multistep process with each step adding to the tolerances.

Again, all stem measurements should be taken c-t along the side of a stem. It's pretty straight forward, but not convenient to do if the stem in on a bike. So, people get lazy and measure along the top or try to estimate. This adds to the confusion.

joejack951
Posts: 367
Joined: Fri Feb 18, 2005 6:50 pm
Location: Wilmington, DE

by joejack951

Briscoelab wrote:Do you really think manufacturers make all stems of a given length on only 1 die? They have multiple 110mm dies for example, and these were initially made with varying tolerances from the start. Then, based on age and usage, they have worn to different degrees. So even out of a single die step, you can get a lot of variance. (Analogous variances happen with extrusions for aluminum rims)


I've made a few forged aluminum parts and quite a few more injection molded and die cast parts. Die to die, or cavity to cavity, tolerances are an order of magnitude less than the +/-3mm tolerances being discussed. Die wear would be of similar magnitude. You start running into other problems long before the die could ever wear to account for that much variation. Can you imagine an extruded rim coming in 3mm too wide?

Briscoelab wrote:As mentioned, stems are made in a multistep process with each step adding to the tolerances.


With this admittedly new knowledge, the large length tolerances make a lot more sense. I don't know the exact steps used in the multi-step process but given that at least one of those steps appears to be a forging die 'hit' along the axis of the stem's extension, that step likely accounts for the length variation.

Thanks again for pointing me to a source of information that explains the length variation. I've never bothered to try taking exact measurements on a stem's length so I likely would have never bothered to look very far into the process used to make them.

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