Tuesday, September 28, 2010

Techie Tuesday

Finally! My new 39T middle chainring arrived, and of course you know you're going to get the best pics on the web right here. I'm very happy with the lightening fast shifts, and the installation was familiar, if not easy. Yes, I had to pull the crank again.

The chainring nuts CAN be backed out without removing the Salsa 28T 74 BCD granny, but I couldn't get the old 38T TA Alize middle ring off over the spider. Fortunately, my crank is one of the 'exo's, FSA MegaExo to be exact, so pulling the crank only requires a 5mm hex wrench and a few minutes. The new steel chainring nuts and bolts made for a worry-free swap, although I did use an informal torque sequence when doing the initial tightening of the bolts to make sure I didn't bend either ring. 

The red background yielded pretty true colors in spite of the halogen lamp. Photoshop did the rest. This ring is darker than the outer ring, and in looking up the new 6700 crank on Shimano's site, the middle ring, I believe is this same very dark, anodized ring. The anodizing makes the metal 2-3X as hard. 

I also couldn't help but notice that this is very close to the same color as Mavic's ceramic rims. Those have a 'ceramic' coating of aluminum and titanium oxides welded onto the braking surface with a plasma torch. The very heavily profiled teeth on these rings make them very narrow, and having such a treatment, said to be as much as 30X as hard as aluminum, would be a huge benefit. Perhaps that will be an option on DuraAce rings soon.

(My first encounter with this kind of ceramic coating was doing research into the Soviet experience in Afghanistan, where their helicopter turbine engine blades were being eaten up in less than a thousand hours by dust. They began using titanium oxide coatings to solve the problem. The USMC's new SuperCobras' engines are getting the same treatment, as I believe are all of the new US military turbine engines. Scaling up the technology for this use may well explain why the cost has plummeted.)

I had some reservations about mounting the new ring, as it is a 39T, not a 38T, and 52/39 rings don't make good half-step gearing. Half step gearing is an arrangement where the gaps between gears the big ring creates are filled in by the middle ring. The Granny's role is then just to provide 3-5 gears below the lowest the middle ring makes. 53/39 makes better half-step gearing, and on Compact cranks, 48/34 and 50/36, but not 50/34.

Here are the gear charts my Excel program spit out for the various gear combos mentioned. Pay special attention to any two gears that are the same or close for the large and middle chainrings. They are duplicates, and a waste of a gear.

As you can see, the 52/39 combo has a lot of duplicates, including a disaster at 21.1 where the first gear on the middle ring is a duplicate and adds nothing. If you are pulling shallow grades, touring at altitude, or bucking variable winds with changes in road surface, you'll sorely miss the granularity that half-step gearing affords you. It's not too bad with a 12-23 out back (9-speed gearing, which is 12-25 for 10-speed gearing), but with a 12-27 or 11-28 you'd really notice the big gaps between gears.

When flying downhill your cadence will pick up a bit, and when grinding up a steep grade, your cadence will slow down some. I used a factor of 95% for the adjustments, which has my base 77 rpm maxed out at 82 on the high end and 60 on the low end. This is a recent enhancement to my gear chart system, along with dynamic titling for the cadence.

Note how close together the line of gears is for the outer and middle ring with 46/38/24 gearing, and how the 1st gear on the middle ring starts helping with big-ring granularity after only its 3rd gear. Something to look for in half-step gearing. These are the smallest gears you can mount on a 130 BCD triple, and are pretty close to standard cyclo-cross gearing if you are looking for a sweet touring setup.

I didn't modify the cadence on the remaining charts to help detect duplicate gears. I also blocked out gears that can't be reached or used due to cross-chaining. I'm running the 28T granny because the chain-drop with my 24T is pretty bad. Kik Armstrong's Chain-Minder is a good option if you have a braze-on front derailleur. (like you could braze anything onto carbon - a terrible misnomer)

No comments: