Technical FAQ with Lennard Zinn: Rim weight and slipping seatposts
VeloNews technical writer Lennard Zinn is a frame builder, a former U.S.national team rider and author of several books on bikes and bike maintenance.This is Zinn's regular VeloNews.com column devoted to addressing readers'technical questions about bikes, their care and feeding and how we as riderscan use them as comfortably and efficiently as possible. Readers can sendbrief technical questions directly to Zinn. We'll try to print a representativesample of questions regularly.First, some follow-up from previous columns:Comment on rim weight:Froma July, 2002 column,"As an aside- the old wheels
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First, some follow-up from previous columns:Comment on rim weight:
Froma July, 2002 column,
“As an aside- the old wheels are all 32 spoke tubular, with rim weights of 280 (Mavic GEL), 290 (Fiamme gold label), 330 (Mavic) or 390 grams (Mavic).Why are all the modern rims so heavy??? All but the 290s seem pretty bulletproof for a 160 lb. rider.
I have been pedaling since 1974 and I do have a opinion on this. So many of the old 300gram range tubular rims have fallen out of fashion due to the current fashion of the lowest spoke count for “lower aero dynamic drag.” When the number of spokes are reduced, the strength of the rim must make up the difference along with much higher spoke tension. This results in a wheel that is heavy where it should have the lowest mass for rapid changes in wheel speed (aka; sprints). The weights of wheels have definitely increased in recent years due to the dominance of clinchers over tubulars and “aero section aluminum rims.” IMO, the older 300gm. tubular rim wheel built with 36 or 32 spokes are lighter and stronger than a clincher rim wheel of equal weight made of the same materials. Marketing and fashion has made these wheels part of cycling history.
I would agree with this assessment with a caveat, namely that the aerodynamic rim sections are not necessarily marketing fluff. There have been a number of studies I have seen that showed that, other than in cases of a steep climb or a race with frequent accelerations like a criterium on a short course, the rider is often faster (or consumes less energy at the same speed) with a heavier, more aerodynamic wheel than with a lightweight, non-aero’ wheel. Lance Armstrong is certainly a student of these studies, and he and many of his teammates often have used deep-section aero’ wheels like Mavic Cosmics on easier road stages in the Tour.
On shimmy problems:
I have a 2001 model Lemond Zurich ( 57 cm) with the “relaxed geometry,” in a criterium I was coming out of a corner at approx. 23 mph and hit a small obstruction in the road. This causes a “shimmy” in the front wheel which caused me to lose control and crash. Is this “relaxed geometry” given to instability? I cannot understand why the front wheel began to shimmy after hitting an obstruction that should have been no big deal? I would appreciate any opinion you may have regarding this matter. A little nervous about riding this same bike until I know what happened.
Here is the site that gives the geometry
It is probably not the “relaxed geometry.” It is probably the torsional stiffness of the front triangle of the bike. If the down tube and particularly the top tube were larger in diameter and perhaps thicker wall, it is my opinion that you would have a greatly reduced or nonexistent shimmy. It is also a function of your weight and its position relative to the wheels, a heavier rider with a higher center of gravity tending to have a bigger problem with this.
Please check past Tech FAQ columns here on velonews.com. There is a lot of discussion of shimmy problems, particularly of Lemonds with shimmy problems. Also see the July 1, 2002 VeloNews for more on shimmy problems.
Rene Herse and Alex Singer had another way of coping with front-end shimmy. They brazed the top and down tubes inside the head tube. They were the only builders that I know of to do this. This helped increase the rigidity of the connection between these tubes and the top tube. The Old French size Reynolds tube sets also came with slightly oversized top tubes and slightly undersized down tubes. The oversized top tubes also helped reduce the possibly of the shimmy occurring. Any comments?
I imagine it is a subtle difference. I would guess that this would only help with bikes that would otherwise have only very moderate shimmy problems.
Can you define the terms rake and trail, and explain their function with differing forks and with differing frame head tube angles? Example: If a particular fork was fitted to a frame with a 73 degree head tube angle, and the same fork was fitted to a frame with a 75 degree head tube angle, does the rake description of the particular fork take on a new “angle”? And what happens to the particular fork’s “trail” measurement?
Very briefly, “fork rake” is the offset distance of the front hub from the center of the steering axis, measured perpendicular to said axis.“Fork trail” is the distance between the wheel contact point on the road and the intersection of the steering axis with the road. Generally, greater trail means greater stability, and it is dependent on head angle, fork rake and wheel diameter. In answer to your question, increasing the head angle from 73 to 75 degrees with the same fork does not change the rake. It does, however, decrease the trail and hence the bike’s stability. Trail is also reduced by increasing the fork rake with a given headangle. Please see the Tech Report article in the July 1, 2002 issue of VeloNews. I explain all of this in great detail there, along with illustrations to further clarify it.
On slipping seat posts: Alec wrote in saying:
Your book on road bikes does not address a new and specific question I have run into. I recently supplemented my Trek 5900 with a DeRosa NeoPrimato (read: steel). I tried using a Thompson seat post but in kept slipping. Tried reducing and finally degreasing. Still slips. I torqued to 175 inch pounds. Still slipped. I torqued to 250 inch pounds and the Campy Allenbolt gave up. I was told by Colorado Cyclists (since they stock Campy seat clamp bolts) that this is a common problem. I now have switched over to a Campy record carbon post, I’ve degreased the seat stay and re-torqued to 175. The new post seemed to fit “tighter.” I will ride after work tonight and hope this is the end of this. But if it not, what suggestions do you have?
I responded that Alec should:
Try a beer-can shim as I describe on page 170 of the road book.
To which Alec responded:
Being a commercial pilot and working with engineers may have led to this discovery that was different than either of us expected. And, by the way, you were right but for the wrong reasons! It seems DeRosa uses a Campy double Allen-headed seat binder bolt. And it’s the bolt’s design that turned out to be the problem. Here’s the techno geek stuff: As you know, the Campy binder bolt consists of an Allen bolt that threads into an Allen-headed sleeved nut. When the binder bolt is completely turned into the sleeve bolt the over all length is 20mm. Eureka! The De Rosa seat binder’s inside bolt “seat” to bolt “seat” measurement is, are your ready, drum roll please . . . 19mm.
What was happening, as the binder bolt was tightened and torqued, it would just snug up the seat post as the Allen bolt contacted the sleeved nut. So, although the proper torque was applied, the seat would slip slowly down into the seat tube . . . about a half inch an hour. Any additional torque provided the same slow seat post travel into the seat tube. I thought that I had better go on a diet to be able to ride the red wonder!
Fix: I took 2mm off the length of both the nut and bolt and everything now works perfectly. If I would have inserted a shim, this would have worked also. But the reason I did not was that both the Campy record seat post and the Thompson seat post I tried (unsuccessfully until the above was uncovered) were so snug into the seat tube before the binder was tightened that 1.) that did not seem to be the problem and, 2.) I could not get a small piece in there when I tried because of the already tight fit. If I would have gotten a shim in, it would have worked . . . and the real culprit would have waited later, if ever, to be discovered. I appreciate your book and your prompt response. I have both your mountain bike and road bike editions and have learned a lot from both of them.
I bought your book with many questions in mind. Most were answered. One annoying one wasn’t. I’m hoping you can help.
I have Mavic UST tubeless rims. One of the rims is leaking air around the spoke nipples. I tried tightening the nipples but it didn’t do any good. A local bike shop said they contacted Mavic and they said it was impossible for the air to be leaking around the spokes (I don’t trust what the bike shop says).
What can be done to stop the leaking?
The only thing I can think of is that air is getting around the rubber foot of the valve and into the hollow between the two walls of the rim. If air got in there, it certainly would get out around the spoke nipples, since those are not intended to be airtight. I would tighten the valve stem more, and if that did not work, especially after cleaning up any burrs that there might be around the valve hole in the rim, I would get a new valve stem and tighten it in very well.
VeloNews technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books including the pair of successful maintenance guides “Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance,” and “Zinn & the Art of Triathalon Bikes.”