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Drop bar diameter vs. Stem diameter

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Drop bar diameter vs. Stem diameter

Old 11-04-18, 01:54 PM
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Drop bar diameter vs. Stem diameter

I have a set of drop bars that are 24.8 mm and a couple of aluminum stems that are 25.4 mm. I tried one and was able to get it to tighten up, but it took a little force-- not excessive though. I saw a couple of 24.8 mm stems on eBay, but they looked like death stems-- which I don't want.

Is there anything I should be concerned about safety wise with the mismatch in stem to bar diameter and having to close the clamp on the 25.4 stem down to 24.8? I figured that's only .6mm or .023 inches, but I'd rather be sure that I'm not over stressing the stem and creating a possible safety issue.
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Old 11-04-18, 01:58 PM
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Diameter difference is 0.6mm, as you say,
can you find 0.3mm thick, shim stock to get the fit spot on?

Or bars that are not undersized..?






...
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Old 11-04-18, 03:55 PM
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Originally Posted by fietsbob View Post
Diameter difference is 0.6mm, as you say,
can you find 0.3mm thick, shim stock to get the fit spot on?

Or bars that are not undersized..?
...
I had a seat post shim that was .3mm thick -- cut it to width and that works much better. Thanks.
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Old 11-04-18, 04:53 PM
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Sounds French. A lot of French bars and stems were 25.0.
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Old 11-04-18, 05:00 PM
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Originally Posted by rickrob View Post

Is there anything I should be concerned about safety wise with the mismatch in stem to bar diameter and having to close the clamp on the 25.4 stem down to 24.8? I figured that's only .6mm or .023 inches, but I'd rather be sure that I'm not over stressing the stem and creating a possible safety issue.
...yeah, there is. Even if you can get it to work, it's a bad idea and can lead to failure at the clamping end. Even 26.0 to 26.4 mismatches can fail sometimes.
It's not guaranteed it will fail, but you're much better off shimming the bar as you've mentioned. MOst of the old bars in that diameter are better off replaced with something more durable anyway. The bar itself can fail after it starts to sag.
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Old 11-04-18, 06:05 PM
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Originally Posted by wrk101 View Post
Sounds French. A lot of French bars and stems were 25.0.
Yep. It's a French bike and the bars are French -- Belleri (BF).

Originally Posted by 3alarmer View Post
...yeah, there is. Even if you can get it to work, it's a bad idea and can lead to failure at the clamping end. Even 26.0 to 26.4 mismatches can fail sometimes.
It's not guaranteed it will fail, but you're much better off shimming the bar as you've mentioned. Most of the old bars in that diameter are better off replaced with something more durable anyway. The bar itself can fail after it starts to sag.
Thanks -- I'm glad I asked. I hope I didn't stress the stem by tightening it down to 24.8. I've had these bars for a long time-- they are in good shape and I wanted to put them to use.

Maybe I should play it safe, toss them both and get new bars and a new stem...
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Old 11-04-18, 06:07 PM
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Originally Posted by 3alarmer View Post
...yeah, there is. Even if you can get it to work, it's a bad idea and can lead to failure at the clamping end. Even 26.0 to 26.4 mismatches can fail sometimes.
It's not guaranteed it will fail, but you're much better off shimming the bar as you've mentioned.
This happened to me with a 26.0 stem that, unbeknownst to me, was clamped down onto a 25.4 bar. After many years, I noticed a crack, figured out the cause, and took the stem out of service.
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Old 11-04-18, 07:33 PM
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DO NOT clamp a too-small thing with a too-big clamp. It creates concentrated stresses that can lead to part failure.

Use compatible parts or an appropriate shim.
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Old 11-05-18, 02:38 AM
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Hey watch this!

Cinelli 1A 30+ years 26.4mm stem on 25.0mm bars then PING!




3TTT Record





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Old 11-05-18, 05:10 AM
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Originally Posted by verktyg View Post
Cinelli 1A 30+ years 26.4mm stem on 25.0mm bars then PING!
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Old 11-05-18, 09:00 AM
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Originally Posted by verktyg View Post
Cinelli 1A 30+ years 26.4mm stem on 25.0mm bars then PING!

verktyg
And there's the proof that it's a bad idea to mismatch bar/stem diameters...
I won't even think about trying that again...
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