rocks in head 

I've got a rigid, carbon 29'er frame that I am building up (Origin8 Paladin). I would really really like to fit it with a Suntour NCS suspension seatpost, which only comes in 27.2

How do I go about shimming a carbon tube? The Origin8 manual says it recommends a 200mm insertion depth for the seatpost, and the longest seatpost shim I can find is the Problem Solvers at 100mm. They cannot make a longer one, I asked.

How is the experience of people who have used these shims? Can I take the dimensions to a machine shop and have them mill me one out of an aluminum rod? Does the gap in the shim tube taper to create even pressure? If so, it would have to taper from the point opposite the gap, which is quite a bit more complicated for the machinist.

Once I find a shim that is 200mm long, do I put carbon paste between it and the seat tube? Between it and the seatpost? both?

All advice accepted, except "just use a suspension fork" because I already have the rigid one and this isn't going to see very heavy duty trails.

fietsbob

Machined Seat post sizing shims have 2 numbers 1 is the outside diameter of the seat post

The other is the bore inside the seat tube .

there are some already Made In 2 common seat post diameters , 25.4 and 27.2
Yes, one supplier QBP Problem Solvers

But I note they are saying not compatible with Carbon posts I'm Not sure about carbon frames

(NB: A 7000 series aluminum* + a carbon post can produce an electric current)

so I recommend You Buy another seat post that precisely matches your frame If you want one in Carbon

or an aluminum post if you want to shim up mis matches .
Or, if you do have the shim made: out of carbon tubing as well..
Or fiberglass, glass being an insulator.



* 7000 means it has a Zinc Content (old dry cell batteries are stacks of carbon and Zinc)

Cane Creek thud-busters are made in more than just 27.2, and are aluminum ...

I have them in steel frames


If any thing leave the frame and seatpost with the machine shop so if your measurements are off

You wont waste their time and Your Money, with the supplied data error .

rocks in head 

Thanks Fiets Bob... the frame is sized for 31.6 - oversize seatpost, but relatively common in non-suspension posts. My research was pointing toward the Suntour and away from the Thudbuster, but if it's going to be an issue I don't want to stress the frame.

Re: 7000 series aluminum + carbon... this is a relatively well known phenomena, and I was told that carbon paste - the grippy stuff they put on seatposts when assembling - is insulative and should prevent galvanic corrosion of the aluminum.

Grand Bois

phenomenon

ltxi

Phenomenon is plural. rocks in head is/was correct. Back to English class you should go for remedial training....

On the other hand..."I've got" vice "I have" (initial post) is, though common, pretty crappy as well.

I should sic my dead mother English teacher on both of you.

3alarmer 

... Phenomena is most def the plural form. Think Latin. #phenomenal

On the seatpost shimming business, shimming a seatpost is always a dicey practice, and I'd avoid it with a carbon bike if I could.

ltxi

You are correct. Dead mom appeared to me in a vision and beat me about the head and shoulders, severely, immediately after I posted. My apologies.

joejack951

Probably the easiest way to make a long shim like that would be to buy a 31.6mm seatpost longer than 200mm, cut a 200mm section of the tube and bore it to size (a good machine shop will have a reamer for this or maybe do it on a lathe). You'll need to split the tube along its length as well so that it will clamp onto the seatpost.

If you are concerned about slippage, you have two options:

1. bond shim to carbon fiber frame (Loctite makes some good epoxies for this)
2. bond/press fit shim to seat post (JB Weld would probably work for aluminum to aluminum, just get everything clean and rough up the surfaces to be bonded)

This could all be very bad advice but it seems worth a shot in my opinion (the second option being the less expensive mistake if it proves to be a mistake).

joejack951

As for galvanic corrosion between the carbon fiber frame and seatpost, as long as you use an anodized or otherwise non-bare aluminum post and/or use grip paste (grease with little beads in it) you'll be fine.

fietsbob

actually the grease Like people should always apply to aluminum seat posts in steel frames is sufficient.
Just occasionally remove and replace the post, add grease if none is Present.

its the grease more than the Grit in the 'carbon paste'.. that slows down the galvanic exchange.

Moe Zhoost

joejack951

The grit adds grip but I believe it also creates a tiny gap between the frame and seatpost. Without contact there can't be galvanic corrosion. You use tiny glass beads in epoxy for bondingcarbon fiber to aluminum for similar reasons.

FBinNY 

There are two issues here, not counting grammar lessons.

Seatpost depth must pass both of two test to be adequate.

1- the post must extend at least 2 to 2-1/2 diameters to ensure maximum transmission of bending moments to the frame, and not cam out of the tube. It's like setting fence poles, too shallow and they fall over. The exact amount of overlap needed depends somewhat on the materials and wall thickness involved, but 2-3 diameters is pretty typical and safe for all applications.

2- the post must extend 2 diameters below the top of the top of the cross bracing tube, or at least one diameter below the bottom. Failure to do this puts undue stress on the extension of the seat tube above the cross brace, and will lead to failure there.

IMO- 200mm overlap is far more than necessary, but may have a hidden reason of preventing the use of posts that extend too far out. The taller the seat post, the greater the bending moments on the frame, so the maker may be trying to prevent the use of short frame/tall post combinations.

So, there' s probably some wiggle room if you're not looking to have the post stick out 300mm above the frame. Use some common sense, and if you're neither tall, nor heavy, you can probably do very well using a 4" long shim.

rocks in head 

Thanks FBinNY for the rule of thumb about the seatpost diameter. It makes sense... I thought that 200mm was a lot, since it would require a fairly long seatpost for somewhat standard standoff distance. I am a pretty average 6' rider at 165, the frame is 19" or large, which on my road/cross bikes puts my seatpost at 150mm or so out of the seat tube.

Thanks all for the clarification regarding what part of the grease / "carbon paste" insulates the seatpost from the frame.

I now consider myself properly schooled in grammar, and will henceforth ask your dead mother to review all my internet forum posts to prevent any subsequent brouhahas.

FYI: This is the seatpost section from the Origin8 Carbon Frameset Owner's Manual:
SEATPOST
Refer to your seatpost owner’s manual prior to installation. Origin8 frames have a 31.6mm seatpost diameter. The
included seatpost clamp’s inner diameter is 34.9mm. Origin8 carbon fiber bicycle frames are precision machined
at the seat tube and seatpost interface so there is no need to machine them again. Recommended torque is 5Nm
(44in-lb). Do not exceed maximum torque of 7Nm (62in-lb). Exceeding this limit can result in damage to the frame
and/or seatpost. Origin8 recommends the application of carbon assembly compound (or carbon paste) between
the seat tube and seatpost to increase friction.
WARNING! Origin8 recommends that the seatpost be inserted at least 200mm in to the frame’s seat tube.
Inserting less than 200mm of the seatpost in to the frame can result in failure, causing serious injury or death.

FBinNY 

That seems clear enough to call for investigation. It may be a typo and they mean 100mm. Or the frame's seat tube is especially underbuilt and really does call for the added buttressing of a post inserted deeper.

I'd call the company and ask them to clarify, (in writing) just to eliminate warranty cop out if anything goes wrong.

Worse comes to worst, assuming the post is long enough, you can buy 2 shims. Then cut or file the flange off one and using it as an extension so you have 200mm of lined tube. You'll need to find a way to keep the post from pushing it lower when you insert it.

If the company confirms that they're not kidding about the 200mm, come back here and I'll offer a few suggestions for how to make a 2 shim system work.