Quill stem shim problems

[Brian73]

I have a pair of 25.4mm bars

The quill stem I bought was a 3TTT Status described as having a 26mm clamp

I bought a Velo Orange 25.4mm-26mm shim to fit the bars to the stem.

I've measured the bars with a digital vernier at 25.4 and the shim measures 0.3mm each side, but the clamp measures 25.8mm with the clamp bolt backed off fully

The stem will clamp up tightly squashing the rubber pad without the shim, I can only just squeeze one side of the shim into the stem using a plastic tyre lever to prize the clamp open, the clamp will tighten but the rubber pad flaps around.

I'm guessing this stem is actually a 25.8 clamp so should be OK for 25.4 bars as this seems to be a compromise the Italians reached in the 1990s

[CREPELLO]

Perhaps some .2mm brass sheet would be the thing to use
http://www.ebay.co.uk/itm/Brass-sheet-0 ... 4641bc5fc9

[gaz]

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[gaz]

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[Brucey]

I have a pair of 25.4mm bars

The quill stem I bought was a 3TTT Status described as having a 26mm clamp

I bought a Velo Orange 25.4mm-26mm shim to fit the bars to the stem.

I've measured the bars with a digital vernier at 25.4 and the shim measures 0.3mm each side, but the clamp measures 25.8mm with the clamp bolt backed off fully

The stem will clamp up tightly squashing the rubber pad without the shim, I can only just squeeze one side of the shim into the stem using a plastic tyre lever to prize the clamp open, the clamp will tighten but the rubber pad flaps around.

I'm guessing this stem is actually a 25.8 clamp so should be OK for 25.4 bars as this seems to be a compromise the Italians reached in the 1990s

No-one ever said the rubber pad had to be a specific fit in the slot. Remember that you are at the mercy of several different tolerances here; the ID of the clamp, the OD of the bars, the ID of the shim, the OD of the shim. Any parts that anything other than a perfectly smooth finish and/or are anodised will give extra clearance tolerances that will have to be allowed for and/or can vary. In addition any of these parts may not be round (especially if you have tightened the stem down onto a 25.4mm bar....).

If you go + 0.050mm (about the thickness of anodising) on just three surfaces then this will make the overall diameter +0.3mm and the clamp will have to spring ~1mm more than it would do otherwise.

In an ideal world when the pinch bolt is tight, the gap in the stem where the bolt is should be about the same as when the pinch bolt was slack (and before it was ever tightened onto the wrong diameter). Often this means that the clamp has to be sprung slightly when the bars are fitted; shims often just mean that you need to spring the clamp a load more. If the shim finish and sizing is a bit dodgy, often it can help if you resize them (eg fit the bars and the shim into another stem with a slacker fit to it, and settle them into one another a bit), or lightly sand the finish of the shim and/or the stem.

cheers

[Brian73]

The stem was from Planet X, but they can't measure anyway as I bought a pair of 42cm bars from them which turned out to be 40cm despite them saying that 'they measure all of the bars centre to centre'

It didn't come with a 25.8 - 26mm shim, does that size shim actually exist? I cannot find one anywhere on the internet. I think they have confused the rubber wedge in the clamp with a shim.

According to Sheldon Brown 25.8 was supposed to fit 25.4 and 26mm.

Anyhow, I've ordered a matching set of 25.8mm bars. Most of the older 3TTT stems and bars are 25.8mm.