Technical FAQ with Lennard Zinn: Corrosive sealant, springs in series and valve stem ticking
Q. Dear Lennard,
I have a couple of questions about the sealant in cyclocross tubulars.
I’ve grown fond of the Tufo tires over the years for our rough Colorado courses and have dabbled with Dugast for nice grassy courses when I travel. Last year I put some Stan’s in the Dugast despite hearing that something in the Stan’s was not so good for the glue on the base tape. … well, now I’ve learned as the base tape has peeled off. Is there something in the Stan’s that causes this?
A. Dear Jon,
As I said last week, I put Caffélatex sealant in all of my ’cross tubulars (and in road ones that have a slow leak). And other than using Tufo sealant in Tufos and using evaporated milk in tubulars with slow leaks back in the day (late 1970s and early 1980s), I have never tried anything else as a sealant in a tubular.
That said, I still may have an answer to your question. I got this out of a Cantitoe Road newsletter:
“Dugast, producer of fine tubulars, are very concerned about the chemical interaction of sealants with their lightweight latex inner tubes. Over the years they’ve received warranty claims for tubulars where the latex inner tube had been “cooked” by the ammonia found in natural latex based sealants.
After testing Caffélatex, Dugast reports: ‘Racers have extensively field tested Caffélatex with our tubulars. The result is excellent, it’s the only sealant we recommend to our customers.'”
I know that the only latex in Caffélatex is in the name, so its properties are different.
Q. Dear Lennard,
In his recent column, Nick seemed to have completely missed the point on the rider’s question, namely: “Nick, cyclocross tires are often run at extremely low pressures, sometimes so low that the bike can squirm around noticeably under the efforts of the rider. At the same time, manufacturers are trying to make ‘cross bikes stiffer, with huge bottom brackets and head tubes now being the norm. Doesn’t the low tire pressure just cancel out any of the efficiency gains made through a stiffer drive train? — Derek Hardinge.”
I somewhat agree — a mega bike that is really stiff may be largely negated if your tire sidewall is flexing a cm with each pedal stroke.
Great question. Do you have an answer?
A. Dear Alan,
You’re right; it’s a very good question. The answer to Derek’s question is (and I’m prepared for the onslaught of mail telling me I’m wrong): yes, for the most part, it does.
The reason can be seen here, under the heading “Multiple springs.” The illustrations and the derivations show that two springs in parallel increases the stiffness of the system, while two springs in series makes it less stiff than the least-stiff spring in the system. The spring constant k is the measure of a spring’s stiffness, and if the springs are in parallel, their spring constants are additive, while if the springs are in series, the inverse of their spring constants are additive. And as the number of springs in either case increases, the relationship remains the same. (In other words, for n springs in parallel, ktotal = k1 + k2 + k3 + k4 … + kn , and for n springs in series, 1/ktotal = 1/k1 +1/ k2 + 1/k3 + 1/k4 … + 1/kn .)
An example of springs in parallel on a bicycle is a suspension fork with springs in each fork leg. Take, for instance, an old-school suspension fork, like an original Manitou without a damping cartridge. There is a stack of elastomer springs in each leg. If you have a stack of elastomers in one leg and none in the other and you apply a force F on it, it compresses a distance d. Now put an identical stack of elastomers in the other leg (i.e., the spring constant k of each spring stack in each leg is the same). The stiffness (the spring constant) of the combined springs is kcombined = k + k = 2k , and applying the same force F on it now results in a deflection only half as large, or d/2 (i.e., the fork uses half as much travel for the same force applied).
On the other hand, if the springs are in series, you get a very different situation. The individual elastomers inside the above-described fork are all in series, and adding more of them will make the fork softer, not stiffer. For instance, if you had enough space in the fork leg to put a second, identical elastomer stack into it above the first stack (this is possible in some forks that have hard spacers taking up some of the space), then put two spring stacks, each of spring constant k, one above the other in the leg. Let’s use the first example, above, namely with springs in only one leg. Now, 1/kcombined = 1/k + 1/k = 2/k , so kcombined = k/2 = 0.5k. Hence, the deflection d under applied force F is now twice as large (i.e., the fork uses twice as much travel for the same force applied). So it matters where you add the springs — adding the same spring in series into a single leg made the fork twice as soft, whereas adding the same spring in parallel into a second leg made the fork twice as stiff (kcombined = 2k if the identical springs are in parallel and kcombined = k/2 if they are in series.).
So what does this tell us about tire pressure? Well, an example of springs in series on a bicycle is the frame, fork, tires, wheels, stem, handlebar, seatpost, saddle. And if you look at this equation for springs in series in detail: 1/ktotal = 1/k1 +1/ k2 + 1/k3 + 1/k4 … + 1/kn , you will see that the total spring constant (stiffness) of the combined springs can never be greater than the spring constant of the softest spring (smallest k) in the system.
This means that if you run the tires really soft, then the stiffness of the entire bicycle (the system of springs in series with them) is softer than that of the tires! So even though I go to great trouble to make my frame, cranks, wheels, stem, and bar really stiff, the fact that I’m riding my tires at under 30psi in most ’cross races means that I can’t actually feel any of that stiffness. And anyone who has sprinted up a paved hill on a stiff ’cross bike with 25psi in the tires knows what I’m talking about.
Now, of course, we’ve seen that in braking, the lower tire pressure does not cancel out the effects of a stiffer fork. That’s because once the ground has taken all of the flex out of the tire when the brake is applied, the fork stiffness comes into play. The stiffness of the fork’s crown, upper legs (above the brake bosses) and steering tube is critical in preventing a fork from chattering when braking on a bike with a front cantilever brake and the cable hanger above the headset, as you can see in another recent column.
RE: Dear Lennard,
Regarding a reader comment on valve stem tick against a deep carbon rim … another suggestion: loose electrical shrink wrap, and a little bit of heat to secure in place. In my experience it usually does an effective job of dampening the stem. Otherwise a small o-ring on the stem and a short piece of shrink wrap to secure the o-ring.
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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 numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”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. Zinn’s column appears here each Tuesday.