Category: Resources

I want to build my new Torpedo Automatic hub into a wheel I can use on my Ross (frenchified touring bike), and my Quickbeam (british-style rough-stuff bike). I realized that replacement cogs are a lot cheaper than replacement chainrings, and easier to install. The same three-spline cogs work on Sturmey-Archer, Sachs (F&S), and Shimano internal gear hubs, and seem readily available: $8 some places, $15 from Harris, who has a comprehensive selection of sizes (14 to 23 teeth in both chain widths).

A 36 tooth ring would work perfectly with the 19 tooth sprocket the hub came with. Luckily it’s 3/32″ width, as well, rather than the 1/8″ bullshit BMX/track width. However, the chainring setup I have is perfect for each bike, and getting a 36 tooth chainring would almost necessitate a new bike. Or building the hub into a 26″ wheel, which just isn’t going to happen.

Since the rings on both bikes work great with the 3 fixed wheelsets I already have, one of which I JUST built up… the Torpedo cog was up for re-consideration.
My existing wheelsets have 15t cogs and Surly “dingle” cogs (really unfortunate name – I say ‘dingle,’ you say… ‘berry’). The double-fixed cogs work great on the Quickbeam for road and steep singletrack with the 32 and 40 tooth rings, and also on the Ross, with 40 and 44 tooth “Rocket Rings” from Harris Cyclery. You can see from the chart above the gear inches I can use.

After a lot of cogitation, mathematics, and plugging numbers into Sheldon Brown’s gear calculator, I decided that the best cog for the Torpedo Automatic… was the one it came with! A 19 tooth 3/32″ dished cog.
I think that will give me the most fun and versatility with each bike. The ‘road gear’ on the Quickbeam is crazy-low at 62 inches, but it also usually has larger tires (35 to 42 mm), which knocks it up an inch or two. On the Ross, the high gear is 77 inches, and the low gear is 57″, both higher than I prefer, but very usable.

There’s no way I would be doing this right now and considering it fun, if it weren’t for Sheldon Brown. Viva Sheldon!

I got some use out of my Surly “Dingle” cog today, riding up Peavine Road. Peavine is remarkably low-traffic, even around 5:00.

This Dingle cog has a 17 tooth cog and a 21 tooth cog. The Quickbeam’s 40 and 32 tooth chainrings make a large gear of 63.9 i nches and a low gear of 41.4. I have my usual 15t cog on the other side of the wheel for a 72.5″ gear. Your exact development changes depending on your tire size and crank length, of course; I’m running 35mm Paselas and 170mm cranks.
Dingles are also available with 18, 19 or 20 tooth big cogs. I chose this one because I’m an extremist. Also, I’m remarkably weak, and I wanted something that would take the pain away.

I rolled around town a little, cutting through the park to get onto 2nd, which took me out to Hill Road. This was the highest traffic leg of the loop, or maybe it just felt like that because the shoulder disappears halfway to Peavine and I was passed by a big rig and a dump truck that was tailgating him.

I rode out Peavine, and switched to the low gear when it got a little steep. There’s a nice shaded turnout there at just the right spot, but of course I went another hundred feet up the road and changed gears across from a dirt driveway on a blind corner.

Of course, there was fender-fussing, where I had to brute-force a little more slack from one side and finesse the quick releases about halfway before the fender wouldn’t rub the wheel in its new position. The Dingle makes the low gear axle position about half as far back as with just the two rings and a 15t cog.

The 32×21 is a nice low gear for that road. There were a couple times when I spun out on some slight downgrades, but the sustained climbing felt really good.

When I got to Power House Road, though, I switched back to the 40×17 and navigated the steep washboard gravel road. Too much speed and you just start bouncing around! The trick seemed to be to stay off the rear brake. The 35mm tires seemed to handle the gravel fine, though.
There was one big black pickup that came up the road, throwing a lot of dust. I took a deep breath and half-closed my eyes to get through that roiling cloud.
There’s also a house with two enormous Rottweilers. They came running out the driveway, but I said “HEY!” and they let me go. I’m glad Power House Road is a lot shorter than I thought. About the time I started saying “this road sucks,” it was over.

Right turn on Baker Creek, and I’m almost home. I stopped at Ed Grenfell park for some water, and because I always like to make a circuit of the park and cross the bridges.

Update 7/2/2007
I’ve extended the graph a bit, because I intend to gain a bit of weight this summer. Now I can use it to figure my tire drop when I weigh 275 with a 25 lb bike (180 lbs on the back wheel).

Now if someone could just figure out the line for 45mm and 54mm tires!
The latest issue of Bicycle Quarterly had a short article (by their standards) about how to optimize your tire pressure to get the ideal 15% ‘drop’ front and back. The drop is how far your tire deforms when you get on the bike.
If your tires are too hard, they bounce on road irregularities and rob you of speed. Also, it’s less comfortable. Too soft, and there’s too much drag.

The BQ tests corroborated a Frank Berto test that indicated 15% is the ‘sweet spot’ between too soft and too hard, and included Mr. Berto’s chart.

Unfortunately for me, it didn’t have a line for 35mm tires, which is what I currently run. I found two places where the gap between 32 and 37 evenly divided into fifths, and measured me a new line.

I even weighed my bike, with me on it, with the tires on a doctor’s scale and a decorative cinder block (which was the perfect height) to get the weight biases fore and aft. It came out 40/60 with some random (but light) stuff in the basket.

So I plotted the tire pressures, made sure it was also a 40/60 ratio, and pumped up the tires to 55 front and 75 rear with my hugely aggravating SKS Kompressor pump that probably needs a new red rubber part to grab the stem.