Technical Q&A with Lennard Zinn – Stress, carbon and unusual bodies

By Lennard Zinn

Spacers and stress
Dear Lennard,
Regarding the GeorgeHincapie spill at Paris-Roubaix, I always wondered why that happenedand then something similar happened to my brother recently. His alloysteerer tube broke in half flush to the top of the headset. He waslater told that a threadless stem should be placed onto steerer with noneor very few spacers over it. The rep told him that adding spacers mightcreate a stress point on the steerer and was perhaps reason for the failure.My brother is fine and is getting a free replacement fork, but you seethe issue here. If people buy bikes and are not informed, there is a potentialhazard out there.

If what my brother was told is true, the information is not posted onthe product itself in the form of a sticker or mentioned as a safety concernon the manufacturer’s web site or manuals included with any of the forksI own.I think most all new bikes come with a good 25 to 40mm of spacers overheadset. I adjust my stem height at times during the year. Is this a safetyissue?We could venture other guesses as to the cause of steerer breakage relatingto handlebar torque, the star nut position relative to the handlebars,etc., but in my brother’s case he had about 40mm of spacers overstem (to allow for later adjustments) and no spacers under stem.
EricDear Eric,
I decided to ask the manufacturer instead of trying to answer thisone myself.
LennardAnswer from Trek:

The number and placement of spacers does have an effect onthe fatigue life of an aluminum steerer tube. Running less than 5mm ofspacers under the stem can concentrate or “point load” a great deal ofstress in one spot on the steerer. Therefore, Bontrager recommendsa minimum of 5mm, and a maximum of 40mm of spacers between the stem andthe headset to maximize the fatigue life of the steerer.
Scott Daubert
Trek Bicycles

Slip sliding away
Dear Lennard,
My carbon seatpost keeps slipping down in my (carbon) frame. I am notusing any grease on the post. How can I stop it from sliding?
GeorgeDear George,
You might try the carbon assembly paste that appeared at the Interbikeshow this past fall. Sold variously under the Tacx, Syntace, FSA, and RitcheyLiquid Torque brands, the orange-colored paste contains small spheres thatallow the part to be slid, rotated and adjusted without scratching, yetit will hold firmly due to pressure of the spheres on the carbon, evenwhen clamped at a lower clamp-tightening torque than you were using tostave off the slippage.
LennardAnd, from Craig Calfee:

Seatpost slippage seems to occur more often on carbon seatpoststhan on aluminum posts. There are two main reasons for this: under-sizeand under-stiffness.Size matters: Many carbon seatposts are undersize and not round.Using the common 27.2 seatpost size as an example, it should measure noless than 27.15 mm. Otherwise, it may slip. The non round seatpostsare hard to measure and their lack of roundness means it will have lesssurface area to properly contact the inside of the seat tube (which isprobably quite round as a result of being reamed to 27.2 mm).Stiffness matters: And then there are the slipping seatpoststhat are round and do measure between 27.15 and 27.2 mm. These postslack stiffness in the hoop direction and are unable to resist the clampingpressure of the seat binder. They slip more slowly, hour-glassinginto the seat tube. Some of this lack of stiffness may be from damagesustained by the seatpost as a result of prior over-tightening. Checkfor cracks. If you find a crack, replace the seatpost. Otherwiseit definitely will get worse and break suddenly at a very inconvenienttime.There are other more obvious reasons for slippage, like the seat binderbottoming out or the bolt threads being damaged or otherwise unable toreach the full tension when torqued. Some people question whetherthe seat tube is bored out to the correct size. It’s hard to measurethe seat tube ID accurately unless you have a bore gauge. It’s alsodifficult if there’s a sleeve because the sleeve expands slightlyto fit the ID of the seat tube. Testing the fit with a round, 27.15- 27.2 seatpost should result in no looseness at all. If it is loose,ream the seat tube to the next size up or have the manufacturer installa new sleeve.
Tricks to keep a seatpost from slipping (in order):Put the bolt side of the seat binder in front of the seat tube and grease the binder bolt threads.De-grease the seatpost and seat tube.Lightly sand the seatpost (just where it inserts into the seat tube) with circumferential sanding strokes.Use a little green Loctite.There are more tricks, but if the above don’t work, you really need a new seatpost or binder or seat tube bore.
Craig Calfee

Feedback regarding my December19 column
Dear Lennard,
That was a great article on the technology being used to assess themotion of cross-country skiers. I wonder how much variation thereis among the elite level skiers, and to what extent the form predicts function.

Surely we can recognize what is “bad form” but we can probably also findpeople with good form, but low aerobic capacity or other limiters. We may also find people with unorthodox form that perform at a very high level,because their form fits their unique physiques or “wiring.”

I get a kick out of seeing some of my taller friends trying to emulate the spinning of Lance Armstrong. Such a deal was made of it that people actuallythought perhaps Ullrich could have beaten Lance if only he’d have learnedto use a higher cadence. In basketball, we see very different styles insuccessful jump-shooters, and we see players who use their unique strengthsand gifts in ways that are instantly recognizable, “signature moves.”

Occasionally, with certain writers (you among them, perhaps because of your size) I see acknowledgment that “your results may vary because of body type,” but many articles offer the this-works-for-everyone! whiz-bang sales pitch. To what extent do you believe physiognomy limits the application of training principles, bike position, cadence, and other factors believed to determine performance on the bike?Thanks for your scientific approach to cycling!
RichardDear Richard,
You raise several important points. Certainly, the form a successfulrider or skier uses is adapted to his or her body type and strengths andweaknesses. And you won’t get any argument from me on the importance ofbody size and shape – after all, I build custom frames and custom-lengthcranks just because of it. I know it is an advantage to have equipmentdesigned for the person.

That said, there are often people in many sportswho are frustratingly fast while displaying what looks like terribleform. But often, closer examination reveals that they understandexactly what muscles to use and when to maximize propulsion and are unconcerned(and rightly so) about extraneous motion that comes at low physiologicalcost. They may look terrible, but the major motions are in the properdirection.
Lennard

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Technical writer Lennard Zinn is a frame builder (www.zinncycles.com),a former U.S. national team rider and author of several best-selling bookson bikes and bike maintenance including Zinn& the Art of Mountain Bike Maintenance, which is now availableas a 4-hour instructional DVDZinn& the Art of Road Bike Maintenance, and Zinn’s Cycling Primer:Maintenance Tips and Skill Building for Cyclists. Zinn’s regular columnis devoted to addressing readers’ technical questions about bikes, theircare and feeding and how we as riders can use them as comfortably and efficientlyas possible. Readers can send brief technical questions directlyto Zinn. Zinn’s column appears here each Tuesday.