Saturday, July 9, 2011

Q&A: Internal-hub gear-inch calculating, routing cables through frames & spoke lacing

Happy Tour de France month! I hope you race fans are enjoying the action. With the high cost of gas, lots of people are taking to commuting by bicycle and hunting for that ideal city bike is part of the process. So, I provide an excellent online resource in my answers today to help you compare the gearing on a derailleur drivetrain to an internally geared one (many town bikes come with these drivetrains). Speaking of le Tour, the second Q&A deals with internally routed cables that are so popular on today's racing bikes; and that make me feel for the poor race mechanics who might have to replace them on several bikes after a bad crash (and this year's race has been a crashfest so far). Lastly, I help a new wheelbuilder identify the 2 common types of spoke lacing.

Good rides!

Q: Hello, Jim,
I have a 1993 Bianchi road bike and have worked out my gear inches so that I know that my lowest gear is 27 gear inches, which as I understand it, means that one turn of the pedals moves me 27 inches, which is a really low gear (nice easy gear for climbing hills).

I’m now considering buying a Specialized Globe city bike with a Shimano Nexus 7-speed hub (the gears are inside the hub), and the bike shop does not seem to know how those 7 speeds translate into gear inches, and I don’t see anything on the Shimano or Specialized bicycle websites to help me figure it out.

Not that I need a 27-inch gear, but if I could figure out the gear inches for the Nexus 7-speed drivetrain, I would at least have a reference point to compare to my Bianchi and know how low the gears should be. Do you know of a way to get a comparison between internal hub gears and derailleur gears?

Thank you,

A: Hi Glenn,
If you visit this page on the late great bike guru (screenshot, right), Sheldon Brown’s website, you’ll see a section called Sheldon Brown’s Gear Calculator.

Use the drop-down menus to select the correct choices, and answer the questions it asks about the bike you’re considering buying. You might have to call the bike shop and ask them for the information. They may need to count the number of teeth on the front and rear sprockets (chainring and cog) and give you the number (sometimes this information is provided on the specifications chart for the bicycle on the company’s website, however, Specialized doesn’t appear to provide it for this bike).

Where Sheldon’s calculator asks you for the cassette cogs is where you put the number of teeth on your rear cog. You only have one cog (sprocket) so you only put that one number in the first box. You don’t put any number in the other boxes in the Cassette field. Once you’ve entered the bike’s numbers and made your selections from the drop-down menus on the calculator, hit Calculate and it will give you your answer. Be sure to select Gear inches next to Gear Units.

Note, that when you hit the calculate button the results will come up in a new browser window. If you have a pop-up blocker in your browser you won’t see the results unless you turn off your pop-up blocker. If you can’t get the calculator to work, give me the information about your drivetrain and I’ll run it and give you the results because I can get the calculator to work.

Note, that In an ideal world you would always test ride a bicycle before buying it. That way you could pedal it up the hills you need to get up to make sure the gearing works for you. This is helpful because the gearing is one thing that determines the effort to get up a hill, but a bicycle that weighs 30 pounds is harder to climb up a hill on than a 20-pound bike, even if they have the same gearing. That’s why a test ride would be the best thing before buying a bike and why gearing is only part of the puzzle.

Happy gear calculations,

Q: Jim,
I am presently approaching my wit's end over this problem. I have a TT bike with internal cable runs using plastic sleeves. The sleeve for the front derailleur got chewed up by a frayed cable. Long story short, the sleeve is history. I have been able to run a cable backwards through the frame and have inserted a flexible sleeve from the head tube to approximately where the cable exits on the seat tube. My problem is how to connect the new cable to the cable I have run through the frame so I can pull the new one through. I've tried super-gluing jewelry wire to both ends but it pulls apart at the exit opening which is not much bigger in diameter than the cable.

Any suggestions?

A: Hi Charles,
I have 3 bicycles with internal cable routing and none of them are easy to install new cables in. The photo shows the cable port on my Cervelo P2. In theory you poke the cables in the top of the down tube and they come out the port. Not.

They go everywhere but the port and the only way to get them to come out the port is to keep poking and fishing and hoping, or shove a stiff piece of cardboard up into the port to make the cable bump into something and follow it out the port.

At least your bike has liner cables, which usually make the job of feeding cables into the frame tubes easier. These are usually Teflon housing sections inside the frame that guide and direct the cable when you poke it into the frame. Since yours got damaged on the end you can see, maybe your best bet is to replace the liner cable first and then you’ll be able to simply feed your cable into your frame and it will follow the liner and come out.

If you don’t have any of this liner, but you have some Teflon-lined cable housing, you can unravel the housing to get at, remove and use a piece of the Teflon lining from the cable as shown below.

I’ve tried feeding a cable (with the head cut off) backwards and then tried to attach it to the new cable by trimming both cables down to a single strand at the end of the cables and tying the two individual strands on each cable end together. But, as soon as you start pulling the new cable through, the connection wants to separate. It might be possible to solder the individual cables together and get them to stick better, but I haven’t tried that.

On some bikes if you take the crankset off and remove the bottom bracket from the frame you can see the cable’s path in there. If you can, you would insert the cable(s) at the top and then when it comes down into the bottom bracket you would be able to feed it into the hole that leads to the derailleur(s). It’s only takes about 5 minutes to take off modern cranksets with external cups so that technique might work, depending on the internal cable setup.

At the factory or in the shop, the mechanics usually just feed the wire through and if it doesn’t go where it needs to they’ll pull it out and poke it in again and again until it finally finds home and comes through. But, if that doesn’t work they’ll try to disassemble things like I suggested with the crankset.

I hope something here helps and if not, tell me more about your TT bike and maybe I can do some research on it and come up with another suggestion. Or a reader might be able to offer a good trick. You also might visit the company’s website, find their contact information, call them and ask to speak with some one providing technical support. If you can talk to the right person, there’s a good chance they can tell you how to get a cable back into your TT bike without too much more difficulty.

Good luck,

Q: Hello Mr. Langley,
I recently built my first wheel by following the extensive wheelbuilding directions on your website. I do have one question for you. I started on the drive side of my rear hub and 3x laced everything exactly as described on your site. The first spoke to the left of the valve hole and with the head on the inside of the hub. Then I flipped the wheel over and did the non-drive side exactly like the drive side so the 2 sides are mirror images of each other. The first spoke on the non-drive side is to the left of the valve hole with the spoke head on the inside of the hub.

I then looked at another set of track wheels that I have and it appears the non-drive side of the wheel is exactly opposite of the drive side, again, relating to the spoke head-in/head-out orientation. Is this going to cause problems for my wheel down the road? Is this what is referred to as a "symmetrically laced" wheel?

Thanks in advance,

A: Yes, Bruno, one pattern is symmetrical lacing, the other is asymmetrical lacing. Please see the illustration I’ve provided drawn by Karen Lusebrink and used from Robert Wright’s excellent book Building Bicycle wheels, which you can still find sometimes online or in stores selling used books.

There are many theories on which is better and why, however, wheelbuilders have done it both ways forever and I have never seen any proof that one way or the other produces a significantly stronger wheel – or more importantly – causes any problems.

The pattern isn’t what determines the quality of the wheel so much as the trueness, roundness and the even and correct tension of the spokes. That’s all far more important. In fact, many production wheels have been built all wrong and they still hold up fine in use as long as they’re true, round and tight.

Hope this helps and congrats on becoming a wheelbuilder. Good job!


Jon Paul Baker said...

Gear inches means the diameter of a direct-drive wheel that would be the equivalent of the gears selected. It isn't how far the bike moves with one turn of the pedals. If the bike moved 27" with one turn of the pedals, the gear inches would be around 8.6.

Gear inches = diameter of rear wheel & tire * # of teeth in the chainring selected / # of teeth in the cog selected.

Or, for an internal gear hub like a Nexus:

Gear inches = diameter of rear wheel & tire * # teeth on chainring * gear ratio of the gear selected / # of teeth on the rear cog. The gear ratios of the gears are available on the websites of the hub manufacturers.

While the definition doesn't really matter as long as the calculations are the same so the bikes can be compared, I suspect Glenn has made an error in his gear inches calculation. This will render the comparisons with the Globe meaningless, as Sheldon Brown's calculator returns what I described above.

The only way for a normal (around 27" rear wheel diameter) bike to have a gear selection that corresponds to 27 gear inches is to be in a 1:1 gear (the number of teeth on the front and rear gears selected are the same). A Bianchi road bike won't have a gear this low. A Bianchi mountain bike will have a gear this low, but will also have many gears lower than this.

What Glenn describes would be "inches development". I suspect his bike doesn't have a low gear of 27 inches development, though. The inches development of a nominally 27" diameter bike wheel in a 1:1 gear is around 85". To get 27 inches development, he'd have to have a 1:3 low gear. Even a mountain bike doesn't go that low.

If he'd like to calculate inches development, he can use Sheldon's calculator to calculate meters development and multiply the results by 39.37.

Jim Langley said...

Thanks for the additional and very helpful information on calculating gearing, Jon Paul!