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Technical Q&A with Lennard Zinn – Bent fork, slipping seatpost

Is repair an option?Dear Lennard,I have had trouble with my car rack not holding the fork properly.Recently, a bike with a Deda Black Magic fork was victimized by this lousypiece of gear. While the bike stayed on the rack, one of the fork tipswas twisted (the other came out of the mount). It was an easy matter tobend the tip back into shape. However, is it safe to ride the bike?JimDear Jim,No, it is not safe. You need to buy a new fork… and should considerbuying a new rack.LennardSlip-slidin' awayDear Lennard,I recently purchased a USE Alien carbon seatpost for my Look KG461(27.2 mm). It is a

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By Lennard Zinn

Is repair an option?
Dear Lennard,
I have had trouble with my car rack not holding the fork properly.Recently, a bike with a Deda Black Magic fork was victimized by this lousypiece of gear. While the bike stayed on the rack, one of the fork tipswas twisted (the other came out of the mount). It was an easy matter tobend the tip back into shape. However, is it safe to ride the bike?
JimDear Jim,
No, it is not safe. You need to buy a new fork… and should considerbuying a new rack.
LennardSlip-slidin’ away
Dear Lennard,
I recently purchased a USE Alien carbon seatpost for my Look KG461(27.2 mm). It is a phenomenally light post and, more importantly, lookskiller. The literature that came with the post emphasized that grease notbe applied when installing the post and to avoid overitightening the binderbolt.  “It is only necessary to tighten the post so that it can sufficientlysupport riders weight — excessive tightness will damage the post.” Notorque values were supplied.I am 175 pounds and found that no matter how much I tightened the binder,the post just would slowly slip down. I brought the problem to the attentionof USE and they suggested I send the post back to them for an evaluation,which I did.Look frames have a wedge style binder bolt to hold the seat post. Iam wondering if carbon may not be advisable on this kind of set up. Iscarbon just too slippery? What’s your advice?
John
 Dear John,
I get innumerable questions about people with carbon seatposts slippingdown. It is a big problem, particularly in a carbon frame. Also, overtighteningthe clamp can cut some carbon fibers and weaken the post.I think carbon may be too slippery for the kind of clamp you have. It’seven hard to clamp them so they don’t slip with standard seat tube clamps.Partly for this reason, Campagnolo came up with a special seat binder clampthat it insists purchasers of Campy carbon posts use. The new clamp, withan angled slot and bolt, provides greater clamping force without pinching.Try a beer-can shim against the side of the tube opposite the clamp.
LennardMore on the topic of White Carbon
Dear readers,
I just got back from the Taipei bike show (as well as from Vittoria’stire factory in Thailand). I continually receive mail about how white carbonis made, so I asked ITM, who uses it extensively in products like thisseatpost, handlebar and stem, which are aluminum wrapped with carbon anda final layer of “white carbon.” I was told that white carbon is indeedmostly fiberglass – 90 percent fiberglass and 10 percent aluminum, to beexact. ITM has discovered that wrapping the center section of an aluminumbar with carbon saves weight, increases strength, stiffness and fatiguelife, and is way cheaper than an entire carbon bar. Same goes for a carbon-wrappedseatpost. This does not apply to the carbon-wrapped stem, however, sincethis is a preformed, snap-on piece of carbon that is popped over the stemshaft and glued on – rather than wrapping on wet and molding at high temperatureand pressure like the seatpost and handlebar. It does not improve the mechanicalcharacteristics of the stem, but it does improve shock damping.When the top layer is “white carbon,” the mechanical characteristicsof the bar and post are reduced due to the lower stiffness and strengthof fiberglass relative to carbon fiber, but ITM assured me that fiberglassis still stiffer and stronger than aluminum, so it still improves thesebars and seatposts.I also want to offer this well-researched letter offering another explanationof what white carbon could be.
LennardDear Lennard,
White carbon is most likely a surface coating by vapor deposition onthe graphite fiber. Which some manufactures claim to provide a better “grip”between the epoxy matrix and the graphite fiber.
 
I have also heard of existence of a white form of carbon molecules,that exists naturally or in man made form. I foundthis on the Internet:
 
 “In 1969 a new allotropic form of carbon was produced duringthe sublimation of pyrolytic graphite at low pressures. Under free-vaporizationconditions above ~2550oK, “white” carbon forms as small transparent crystalson the edges of the planes of graphite. The interplanar spacings of “white”carbon are identical to those of carbon form noted in the graphite gneissfrom the Ries (meteroritic) Crater of Germany. “White” carbon is a transparentbirefringent material. Little information is presently available aboutthis allotrope.”
 
As for the actual use of this form of carbon in the composite industry, it can’t be very practical.

A third possibility is that it may just be plain Boron fibers. I have heard people used that term “White Carbon”, when promoting the use of Boron Fiber composites…. Boron Fibers have some what similar properties as carbon fibers, and is used more extensively in the pre-carbon days. I believe the F-14 sweep-wing gear box is made of that stuff, but it being far more expensive than graphite, it is used less now days.I looked around and also foundthis:
“…. Moreover, this success extended to another field, which is the production of tennis rackets. In Fall‚ 1986, Yamaha International for tennis rackets produced its new model Gold-90. This model was made of a fibersalloy consisting of Ceramics fibers, graphite, boron, and Kevlar material.Wilson company for tennis rackets used the same technology to produce itsnew so-called “graphite” rackets, (because it is made of Boron Nitridewhich is called white graphite) that are characterized with high strengthand resistance to scratching and abrasion. These rackets are well-knownof their ability to withstand shocks.This spread in the use of boron in many domains is attributed to itsoutstanding properties; some boron compounds are six times stronger thansteel and five times stronger than aluminum. However, the only problemis the high price (US $500/Kg). That is why the price of the new Princeboron racket is $400. “
Lawrence


Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides “ Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.