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Technical FAQ with Lennard Zinn: Chainring flex, mid-foot cleats, bottom bracket drilling and more

Chainring flex, mid-foot cleats, bottom bracket drilling and more

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Fabian Cancellara can flex chainrings like few others

Q.Dear Lennard,
I have a question that seems like it would have a simple answer. However, I am having a hard time explaining it. People talk a lot about “Chainring flex,” for example, when you are pedaling hard and you look down and see your chain line moving back and forth in your front derailleur gate. The best explanation would be a noodley bottom bracket.

However, I have a new S-works Tarmac SL3 with a BB30 that is renowned for its stiff bottom bracket. When I recently switched the spider from a 130mm to a 110mm BCD and added a 53-tooth chainring, I started tossing the chain to the outside of the big ring when I stand up in the twelve. The 53-tooth that I added is lightweight and I am sure that it is the problem. But why?

I can see the chainline move to the outside of the front derailleur gate when my drive pedal is going down, which would tell me that the arm is flexing the spider, which is flexing the chainring. But, that never happened with the 130mm BCD spider and a heavier chainring. So, if you can trust that the bottom bracket isn’t flexing and the spider isn’t flexing, what forces are acting on the chainring to move it from side to side in the front derailleur gate? And, why would a heavier chainring make a difference?

Chainline is straight under low watts. It only starts to wobble under load. Another bit of info is that this happens most often when I am already going fast (typically downhill) and jump in the twelve (my smallest cog/biggest gear). The chain pops off to the outside and I hit my left knee on the handlebar. The 53-tooth chainring is only three months old so I don’t think that the teeth have worn to the point where the chain is not seating fully. As you can imagine, this is very frustrating and my knee is taking a beating.

I am confused as to where the force is coming from that is bending the chainring from side to side? It seems like the only suspect would be the chain. When the chain is in the 53×12, the 12 is outboard of the 53 so it makes sense that the chainring would be pulled outboard. But, it is only pulled outboard when the drive pedal is stomped on. When the drive pedal goes upward, the chainline returns to its happy position. If the chainring were flexing, wouldn’t it be flexing on both pedal strokes?
— Nathan

A. Nathan,
First of all, running a 53T on a 110mm BCD spider, especially a lightweight one, as I glean from your remarks it must be, will flex much more than a 53T on a 130mm BCD spider. The chainring with the smaller inner diameter without the support of longer spider arms is obviously going to be less stiff, even without being thinner in cross section as well.

And you’re not getting much flex from the left pedal, because on that side, you’re actually improving the chainline when your pedaling force flexes the chainstays and moves the bottom bracket over to the right, better lined up with the smallest cog. But when you stomp on the right pedal, you push the bottom bracket to the left, thus increasing the chainline offset to the outside on the 12T cog, giving the chain more of an angle to come off of the chainring.
— Lennard

Q.Dear Lennard,
I am hoping you can answer a question for me. For a variety of reasons, I’ve started to use a mid-foot cleat position. However, the only pedal system I’ve found that allows this is Speedplay, which offers an (expensive) adapter that allows you to move the cleat rearward. This has meant I had to abandon my much-loved Look Keos. I am wondering if, for 2011, any other pedal manufacturers are offering adapters to move to a more mid-foot clear position, or if any shoe manufacturers are offering shoes drilled for mid-foot cleat placement.

I know some people suggest drilling shoes yourself, but I’d rather stick with factory specs for this sort of thing.

A. Dear Tod,
Letting the shoe manufacturer drill holes in your shoes sounds prudent to me.

I know of no pedal manufacturers other than Speedplay making it easy to set up a midfoot cleat. On the shoes, however, you do have a good option: D2 makes custom shoes with the cleat mounting holes wherever you want them. I have a pair myself with mid-foot cleat mounting (as well as standard mounting — two three-hole mounting positions).
— Lennard

RE: Dear Lennard,
I have seen you recommend drilling drain holes in bottom bracket drain holes several times now. I was wondering if that recommendation also applied to ’cross and mountain bikes? I ask because my Specialized road bike came with a drain but my Specialized mountain bike and Ridley ’cross bikes don’t have them. Should I add them to those bikes or is the chance of dirt and mud getting in the hole a problem for those bikes?

I decided to put a hole in my cross bike bottom bracket shell. What tipped me over the edge was that I use the bike for commuting in rainy weather all winter and the other day I poured some water out of the frame. I figure that if I am worried about mud getting inside in ’cross races I’ll just plug the hole with a plug of some sort. Or, if I feel particularly redneck, tape it over with duct tape.
— Dave

A.Dear Dave,
That’s a perfect way to deal with it.
— Lennard

RE:Dear Lennard,
I wanted to follow up a reader’s comment who recommended VM&P Naphtha over Acetone for removing glue andcleaning tubular rims. I used it for the first time on a set of carbon tubular rims and it works fantastically. I have been able to remove much of the unwanted glue residue with VM&P Naphtha and a rag alone. My hands were also fine with standard nitrile gloves. But, I found the fumes to be much worse than those from Acetone. I would recommend a fume respirator for anyone using VM&P Naphtha; I worked with a fan on and all the doors and windows to my work room open and when my girlfriend stopped in she commented on how strong the fumes were. Just thought I’d add a testimonial, much better than Acetone!
— Nathaniel

RE:Dear Lennard,
I wrote to you in June 2009 about my carbon tubulars, which seemed to be out of balance. I recently noticed that a Zipp rep had responded to my question as well with a different answer than you had offered. I wanted to give you decisive feedback on my solution.

I found (on Zipp’s web site) instructions for balancing the wheel by injecting epoxy into the anti-dependent section of the rim. I removed the tires and stripped the glue from the rims. The wheels still pulsated when spinning. Therefore, I don’t believe my tire mounting was at fault. I stuck playdough on the anti-dependent rim until the wheel no longer had a preferred resting point. Then I weighed the playdough. For the rear, it was nearly 30g; 18g for the front. I under-corrected both wheels by injecting about 70% of those weights in epoxy inside the rims opposite the “naturally” dependent valve stem hole. I remounted the tires. No pulsation! Great ride.
Thanks for your advice.
— Rich

Q.Dear Lennard,
I decided to retire my six-year-old helmet and am searching for a new one. When shopping at local bike shops, I have come across several “new” helmets that have a manufactured date of 3-4 years ago shown by the sticker on the inside. Is this OK or should I search for one that is manufactured in the last year? In other words, what is the shelf life of a helmet for sale in my local bike shop?
— Mark

Answer from Specialized:
Actually, there’s no evidence that there is any deterioration in performance over time. I am attaching below the response we typically send. Deterioration is generally a function of use, and so helmets in warehouse storage should be fine after several years. However, while the standards and improvements over time can be significant, it makes sense to shop for newer models.

The Snell Memorial Foundation recommends the following:

Why should you replace your helmet every five years?

The five-year replacement recommendation is based on a consensus by both the helmet manufacturers and the Snell Foundation. Glues, resins and other materials used in helmet production over time can affect liner materials. Hair oils, body fluids and cosmetics, as well as normal “wear and tear” all contribute to helmet degradation. Petroleum-based products present in cleaners, paints, fuels and other commonly encountered materials may also degrade materials used in many helmets possibly degrading performance. Additionally, experience indicates there will be a noticeable improvement in the protective characteristic of helmets over a five-year period due to advances in materials, designs, production methods and the standards. Thus, the recommendation for five-year helmet replacement is a judgment call stemming from a prudent safety philosophy.

Personally I would recommend a helmet manufactured in the last one year or so as you will be getting the most modern design/functionality and then in another 5 years you will be ready to replace it again.
— Michael Grim
Specialized Helmet Manager