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Technical Q&A with Lennard Zinn – Readers ask, Lennard answers

  • By VeloNews.com
  • Published Jan. 13, 2009

By Lennard Zinn

The Dura-Ace 7950 compact crank.

Photo:

Dear Lennard,
I would like to change my Dura-Ace 7800 53/39 crank set for a compact 50/34 set and am considering the new Shimano 7950 but noticed that Shimano states that to run the 7900 crank set, the new 7900 chain and front derailleur are also required. Can I not simply put a 7950 compact crank set on my 7800 system and if so, will shifting be compromised? If indeed the new front derailleur is required, do I then need to change my shifters to 7900 as well?
Paul

Dear Paul,
I’m assuming from your letter that you’re not planning on purchasing the new 7900 STI levers. If you are, you’ll need the new front derailleur to go with the new 7900 shifter, because the cable pull is different. If you’re going to continue using the 7800 shifter, you can use the 7800 FD. If you’re using the new chainrings (7900 or 7950), it will shift better with the new 7900 chain.

You could, of course, run 7800 or Ultegra chainrings on the new crank and there would be no issue. But if you do use the new chainrings and your CN-7801 chain is not in need of replacement, there’s no need to throw it out before its time, unless you experience auto-shifting in the big-big combination. When it’s time for replacement, get the 7900 chain.
Lennard


Dear Lennard,
A cyclist who assisted me with a double flat situation last year told me that side walls of alloy rims wear over time and that a wheel or rim should be replaced on regular intervals to avoid catastrophic failure of the braking surface. I have a set of 4-year-old bullet straight Eurus wheels approaching the 20k mile mark and wonder if this is a valid concern.

I had never heard of this in over 35 years experience on various clincher and sew-up wheel setups. Please clarify that this is or is not the case as I wonder if sidewall thicknesses have been reduced over time to save weight or was this a salesperson’s story to empty his wallet unnecessarily. How would one measure the rim thickness?
Bob

Dear Bob,
Absolutely this is an issue. The EU recognizes it as such and requires rim manufacturers selling within the EU to have wear indicators on their rims. This rule is at least a few years old, so I wouldn’t doubt it if your rims even have them. Inspect them carefully.

The indicators vary, but some are small, blackened-in holes or grooves cut in a prescribed depth into the braking surface. When they disappear, the rim is too worn to continue using. Others are small voids inside the rim wall at a prescribed depth inside. When the rim is worn enough that they are visible, ‘tis time to replace the rim. Mouse over the second to last red and black icon on the row of icons above the wheel photo on this page, where you can find the DT Swiss rim wear indicator.

These rim wear indicators are analogous to car-tire wear bars (a.k.a. wear indicators). These are raised features at the base of the tread grooves that indicate the tire has reached its wear limit. When the tread lugs are worn to the point that the wear bars connect across the lugs, the tires are fully worn and should be taken out of service.

I have certainly seen rims fail from the braking surface becoming worn too thin, and it is not particularly pretty. It usually happens under hard braking, when the pad wears through the final amount necessary – the straw that breaks the camel’s back, combined with the elevated air pressure due to the rim heating up from braking. Then the tire blows right off of the rim as the rim wall folds outward.

A good rule of thumb is to examine (or ask your bicycle dealer to do so) the remaining thickness of the braking surface of your rims (with a micrometer) before your second set of brake pads is worn out. Don’t wait until the rim wall is worn so thin that it fails.
Lennard


Dear Lennard,
Last month (November) I completed my goal for ’08 by riding up Haleakela Crater (Sea Level 0 ft, to the summit 10,023 ft.) on the island of Maui. After pictures, I began to make my decent on the park road that was about 6- 7-percent downgrade. I felt like I was absolutely flying down the mountain.

Was my sensation real or imagined? Does less barometric pressure at altitude create less resistance and hence, faster downhill speed? I ride Southern California mountains regularly, but I’ve never quite had that sensation. Just curious.
Dale

Dear Dale,
Absolutely there is less air resistance at altitude. That’s why bicycle track records are often attempted at altitude. Recall, for example, that the hour records of both Eddy Merckx and Francesco Moser were set in Mexico City’s rarified air. People who regularly ride at altitude and then go to low altitude can’t believe how slowly they go.

I remember when I raced the Tour of Ireland in 1981 coming from the thin air of Boulder (5500 feet), Colorado Springs (6500 feet), and Los Alamos, NM (7500 feet) – the three places I’d lived and trained in the longest – it felt like I could cut the Irish air with a knife, and time trialing seemed painfully slow.

Taken to the extreme, consider the fate of space junk hitting our atmosphere. At remarkably high altitudes there is little air resistance, and these particles, acted upon by gravity, can attain incredibly high speeds. But as these particles travel deeper and deeper through the earth’s atmosphere, they begin to encounter more and more atmospheric molecules and eventually burn up due to friction. That’s why we see shooting stars. They attain speeds much greater at those high altitudes than they would were they to begin their journey in an atmosphere as dense as it is at sea level.
Lennard


Lennard,
I’m wondering what is the best way to check for chain wear on my Dura-Ace 10-speed chain. I’ve used the Park Chain Checker a number of times, but even with a new chain it reads 0.5 to 0.75. Any insight is greatly appreciated.
Tony

Dear Tony,
I greatly prefer either the ProGold chain indicator, or the Rohloff Chain Checker to any other chain-elongation gauge. These are simple go/no-go gauges and are quick and easy to use.
Lennard


Dear Lennard,
Before I made the upgrade to 10-speed from nine, I had been running a front chainring combination of 53/38 (130mm BCD, five-bolt spider). I liked having the slightly smaller gearing that the 38 tooth chainring provided w/out going to a compact crank. My question is whether or not the 38 tooth chainring will work w/ my 10 speed setup? My cranks still have the 130 BCD/5 bolt spider, but is there a spacing issue that would result in the 38 not working?

The crank is the TruVativ Rouleur Carbon. The chainring in question is a “Rocket Ring” 130mm, five-bolt, non ramped 38 tooth.
Luke

Dear Luke,
Probably you will be able to use that chainring with a 10-speed chain and never notice the difference. When crank makers went from nine speeds to 10, they generally did not change the thickness of the tabs against the chainrings mount. Instead, they cut the chainring teeth off-center toward each other from the central plane of each chainring.

But be forewarned that it is possible that you could jam the chain between the rings; check out the letter at the bottom of this column, followed by the four letters at the bottom of my December 23rd column.
Lennard


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. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.

FILED UNDER: Bikes and Tech

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