Technical FAQ: Cracked carbon seatposts and more

Lennard Zinn addresses questions about cracked carbon seatposts and the compatibility between drivetrain parts

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A cracked carbon seatpost

Dear Lennard,
I have a cracked carbon fiber seatpost. Photo [above]. This is the second in four months, and this one failed after less than 25 rides. Both cracked in the seat vertical way. The frame is a 2006 custom-sized Serotta titanium that has over 40,000 miles, and the previous seatpost (also carbon) was the original. The questions I have are:

— Any ideas why this might be happening?
— Should I get a replacement titanium or aluminum seatpost? It seems to me another carbon one will fail, too.
— Any considerations where replacing with ti or aluminum?
— Harry

Dear Harry,
Yes, I certainly do have an opinion about why that has happened.

Carbon fibers are very strong, and, while they can be quite flexible when dry, they become brittle when cured with resin into a matrix. In this case, it looks to me like the seat clamp at the top of the seat tube pinched the corners of the seat-tube slot into the fibers. Carbon fibers cannot take being crushed like that, and the carbon layers will crack and delaminate in response.

There are a number of ways to avoid this, all of which are intended to avoid the stackup of the slot and the tabs of the binder clamp together. If your frame has the binder tabs welded on (rather than a separate band clamp), and your bike takes a 27.2mm seatpost, you are out of luck and should look for a different material other than carbon fiber for your seatpost. (I suppose there is one option with this situation, and that is to cut and file off the binder ears so that you can use a removable binder clamp on it, and then follow Option 2, below.)

With welded-on binder tabs there is only one option to use a carbon post — and that is only as long as the seatpost diameter is larger than 27.2mm. However, if you have a removable seatpost clamp that slips over the top of the seat tube, you have other options.

Option 1
If you have a seat tube with an inner diameter of 28.6mm or larger, then you can use a carbon 27.2mm seatpost along with a slotted shim sleeve that will bring it up to the inner diameter of your seat tube. When you slide the seatpost into the sleeve and in turn slide the whole shebang into the seat tube, make sure you rotate the slot of the shim sleeve so it is on the opposite side from the slot in the seat tube. That way, as the binder clamp is tightened and its corners push inward, the sleeve distributes the load away from the high stress concentration at the top of the seat-tube slot that cracked your seatpost.

Option 2
If you have a removable seat binder clamp, rotate it so that its slot is on the opposite side from the slot in the seat tube. In other words, the binder bolt will be in front of the seatpost, not behind it. This way, the binder clamp’s clamping force will be distributed around a large area, and there will not be a stress concentration at the corners of the seat-tube slot. A variation on this is to use a binder clamp with an angled slot; these are made specifically to address this problem and avoid pinching in at the top of the seat-tube slot.

Using Options 1 and 2 together will further decrease the stress concentration at the top of the seat-tube slot.

As for a preference between a titanium and an aluminum seatpost, I don’t have one. Yes, a titanium one could look very nice with your titanium frame. On the other hand, with aluminum seatposts the price is generally lower, there are far more options, and you can pick the post based on your preferred saddle-rail clamp atop the post.
― Lennard

Cutting a carbon seatpost

Dear Lennard,
I have a carbon seatpost that is way too long for my frame size and would like to cut it down because it’s hard to install and perhaps save a little weight.

How much seatpost is recommended inside the seat tube (below my required seat height) to keep it safe for riding?
— Sam

Dear Sam,
Make sure you have at least four inches (100mm) inside the frame. That should be sufficient for most bikes, but ensure that it is also long enough to extend below the intersection with the seatstays and top tube.
― Lennard

Shimano Ultegra compatibility

Dear Lennard,
I have a Shimano Ultegra-equipped Cervelo R3 and my STI shifters are of the old variety (exposed cables not under the bar wrap). My friend offered me his new Shimano 6703 shifters that he no longer needs. Will the Shimano 6703 left shifter (being meant for a triple) be compatible with my compact double (50-34 teeth)? I have upgraded my front and rear derailleurs in the last year to Shimano 6700 due to other reasons and was hoping this would work. What do you think?

Dear Robert,
That system could be adjusted to be rideable, perhaps even raceable, but it can’t work like either system was intended to work, because you won’t be able to trim the derailleur’s position to avoid chain rub in cross gears at both ends. The left ST-6700 double shifter has a trim position on both chainrings, but the left ST-6703 triple lever has no trim adjustment over the middle chainring, which now becomes either the inner-chainring position or the outer-chainring position when used on a double. So whether you attempt to use outer/middle position clicks, or middle/inner position clicks for running it as double shift lever, you will almost certainly not be able to get an adjustment without chain rub on the front-derailleur cage in all cross-chain combinations. That may be fine for you, and the price is certainly right.
― Lennard

SRAM road/MTB compatibility

Dear Lennard,
I stumbled upon an older post of yours regarding 10-/11-speed compatibility. In a response, you mentioned that, “SRAM did not change the cable pull ratio when going from 10-speed to 11-speed, so a 10-speed SRAM road rear derailleur will work quite well with an 11-speed SRAM road shifter.” This sparked my interest! I am running a 10-speed SRAM Rival rear shifter with an inline barrel adjuster, a SRAM X5 type 2 rear derailleur, a SRAM 11-32 10-speed cassette, SRAM 10-speed chain and a Raceface narrow-wide ring on my CX rig as a budget, CX1-style build. In my research, folks who’d set their bike up this way had great luck, but claimed it would only work with a 10-speed setup. I can attest that it indeed has work flawlessly. That said, if the pull between SRAM 11-speed and 10-speed road shifters is the same, with an 11-speed chain and cassette, can a SRAM X5 type 2 rear derailleur be paired up with a Rival 22 (11-speed) shifter?
— Ryan

Dear Ryan,
Yes it can, because SRAM also maintains the same cable-pull ratio on both its road and MTB rear derailleurs, other than 11-speed MTB. So your 11-speed road shifter/10-speed MTB rear derailleur combination will work fine, just don’t even think about trying to pair any road shifter with an XX1, X1, or X01 rear derailleur! This also applies to X01 7-speed and X01 10-speed rear derailleurs; their cable-pull ratio is the same as other SRAM X-Horizon MTB rear derailleurs (which are all 11-speed other than these two exceptions).
― Lennard

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