Technical Q&A with Lennard Zinn

By Lennard Zinn

Dear Lennard;
I remember a while back you mentioned some auto part glue that worked well for glueing on sew-ups. What is it and any tricks to using it? –Ignacio

Dear Ignacio;
The glue is 3M Fast Tack.Except on Continentals (which have no coating over the base tape), scrape the base tape (instructions in “Zinn and the Art of Road Bike Maintenance”). Layer the glue on the rim and tire, letting dry between coats. After the final coat on the rim, stick the tire on. Fast Tack can be problematic with Continentals, as it has a solvent in it that can soak through the base tape and loosen the glue holding it on.There is more on Fast Tack in the December 11, 2000 issue of VeloNews (Gadgets section).–Lennard

Dear Lennard;
Because of your experience building Big bikes I was curious of your opinions on crank length. Should 175 cranks be used by riders of a certain height, or leg length? Is there anything to be aware of (like knee pain) when switching from 172.5 to 175 cranks? –Mark Dear Mark;
The short answer is that crank length should increase as the rider’s leg length increases, but there is probably no predicting the one that will work best for you. To get the ideal length for yourself, you would study it over time with a power meter and heart monitor (and maybe while surrounded by mirrors with plumb bobs over your bike – see below), using various lengths.In general, for people over 6 feet 6 inches, I often use cranks of 195mm-200mm, sometimes even 210mm or longer (custom-made). A 175mm normally fits a rider between perhaps 5 feet 8 inches and 6 feet.I think a possible rule of thumb for crank length is 20 percent of inseam length. That is a good place to start anyway.Usually, you will not see a knee pain problem with a crank length increase, especially one of 2.5mm, if there is not already a knee problem with the rider. A longer crank will make the knee bend at the top of the stroke sharper. If a rider has a problem with chondromalacia, which is the deterioration or inflammation of the articular cartilage on the back of the kneecap, sharper knee angles will increase the pressure on the back of the kneecap and hence the pain and inflammation. A higher seat and shorter crank are usually what the doctor orders for this condition.Now for a longer answer
There is also reason to believe that changing the crank length can even out asymmetries in a rider’s bike position. In other words, I have seen examples where you can move a rider who is sitting over to one side of the bike to a centered position and even move them over to the other side of the bike by adjusting the crank length. This sort of asymmetry was heretofore always corrected with cleat shims and cleat fore-aft position changes to one foot, asymmetric chainrings, asymmetric crank lengths, asymmetric pedals, and other methods designed to alleviate leg-length discrepancies. I am interested enough in this that I plan to do more testing with it for a future VeloNews article.Another advantage of a long crank for a tall rider is that you can improve the handling performance of the bike. Assuming the rider has a custom frame designed to accommodate the longer crank (built with a correspondingly higher bottom bracket), the frame size can be smaller. In other words, if the rider uses a 200mm crank rather than a 170mm crank (or a 210mm rather than a 180mm) and has a 3cm (30mm) higher bottom bracket, the seat tube can be 3cm shorter. The rider will still have the same seat height with the same amount of seat post extension, and the center of mass of the bike and rider will be the same height above the ground.However, the smaller frame will tend to twist back and forth less. This is a huge advantage for a tall rider, as tall frames are notorious for their tendency to shimmy – i.e., build up a back-and-forth shaking oscillation that increases in amplitude rapidly, especially when riding at high speed with the hands off of the bars.If you keep back issues of VeloNews (and everyone should be doing that, right?), here are some articles I did on the subject of crank length:
#12 July 23, 1990
#6/April 10, 1995
#7/April 29,1996
#11/July 1, 1996
#3, 3/1/99
Thanks and good luck. –Lennard

Dear Lennard;
Do you sell or know where I can find a replacement 222mm crankset in aluminum or other lightweight durable material?I have experimented with steel crank arms by cutting and welding to the point of finding what fits me best and 222mm is the magic number for me. A 220mm or even a 215mm would be acceptable I suppose.It would nice if I knew someone who could broach the tapered square hole in some billet aluminum blanks then I could drill and thread them where I like and have them age hardened as needed. Or if I could buy unfinished arms with the square hole already broached. –Mike

Dear Mike;
You can just buy nice custom aluminum cranks without having to drill or broach anything. Try High Sierra Cycle Center at: http://www.hscycle.comOr email at: : [email protected] — Lennard

Dear Lennard;
After living in Boulder for several years as well as keeping track of you in VeloNews, I know you to be not only an avid cyclist, but an avid tall cyclist. This is why I thought you might be able to help.I am almost 6 feet 5 inches tall and have been cycling in some capacity for years now (decades, I guess). After a several year hiatus, I have recently thrown my leg over the top tube again. I am riding a 2 year old 63 cm Schwinn Paramount (lugged 853 built by Waterford) that is “square” in that the top tube is at 62.5cm. My Selle saddle is slammed the whole way back and my pedal stroke feels about right in that my leg is almost extended and my hips don’t seem to be rocking.The problem is that I am experiencing pain during and after my rides in my left knee (on the inside edge of the kneecap). I’ve never had pain like this on my old bikes (Cannondales mostly) and am wondering about my setup.I know this is impossible to diagnose from 1800 miles away, but I thought you may at least have some ideas of where to look for the problems (tall cyclist/bike builder that you are). The answer of, “you’re just out of shape, give yourself a chance to work the knots out” is also a reasonable response. Just thought you may have some quick ideas.–Ian

Dear Ian;
Well, in that location, my first guess would be a saddle too low (chondromalacia–the deterioration of the articular cartilage on the back of the kneecap, is often caused by this), although you seem to feel it is high enough. Since you say it is on the edge of the kneecap, it could also be a cleat rotation problem. Make sure you are riding within the float range of your pedals. Otherwise, that is all I can do from this distance. (At least I didn’t say “you’re just out of shape, give yourself a chance to work the knots out.”) –Lennard

Dear Lennard;
Having read your article regarding shimmy, I thought I’d ask you about a peculiar shimmy problem my cousin is having (the only thing that’s typical about it is the badge on his frame).The bike is a 2000 LeMond built of 853 steel that is equipped with a carbon fork and Rolf Vector Comp wheels. He had a bad crash while descending at 45 mph when he lost control due to front wheel shimmy.Since the crash, the wheels have been trued and his LBS confirmed that both the frame and the fork are straight. Since he is small and light (5 feet 8 inches and 150 pounds) and rides a 53cm frame, what could be causing the shimmy, which still occurs whenever he exceeds 35mph?Meanwhile, I am 6 feet 2, 170 pounds and ride a 60cm Bianchi XL Boron steel frame with carbon fork and Zipp 303s, and I’ve never had it shimmy, even at 50mph. Go figure. –Bob

Dear Bob;
It’s just the resonant frequency of that frame. The oscillation will build and build when the vibration induced by the road is the same at which the frame resonates. It’s a bummer for sure. Changing forks, wheels, tires, etc., will change it some, although it will move the resonance to a higher or lower speed, not eliminate it. You might look back in last year’s strings on shimmy on this site.–Lennard

Dear Lennard;
I noticed something strange in a Graham Watson photo close-up of Lance Armstrong on last year’s Mt. Ventoux. On Armstrong’s super hi-tech carbon Trek, with super hi-tech carbon rims, and I’m sure a lot of other super hi-tech stuff, the left side shifter for the front chain rings appeared to be a down tube-mounted friction shifter. This was confirmed when I looked at the left brake lever, which appeared to be an old-school brake lever without any shifting mechanism. Is there a technical reason why Armstrong used this set up? –John

Dear John;
It saved him a quarter of a pound. We’ll see if he does it again this year with his new 10-speed Dura-Ace system.–Lennard

Dear Lennard;
I seem to remember some months ago that you posted tips on how to change the return springs on Campy Ergopower levers. Is there any chance I could get a copy of those instructions? –Eric

Dear Eric;
Sure. They are in my book, “Zinn and theArt of Road Bike Maintenance,” which is available from VeloGear.comamazon.com (and probably amazon.com UK, since you are in Switzerland) or any book store or bike store in the USA.
Thanks, –Lennard

Note: Finally, several readers pointed out that I had my head in the sand when I said you could not run two shifters to the same rear derailleur. (The question was about the two levers on Tyler Hamilton’s Tour de Romandie TT bike.) Below is one sample. The fact that so many people wrote me about it leads me to believe that it must work as well. — LZ Hey Lennard!
You can in fact run both a STI and bar-end shifter together. Profile has a nifty adapter bracket that allows this setup. It attaches to the shifter boss, two cables go in, and one comes out. It’s pretty damn slick. Check it out at Profile-Design.com. –Bruce

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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 including the pair of successful maintenance guides “ Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.” Zinn’s VeloNews.com 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 each Tuesday here on VeloNews.com.