Tuesday, June 17, 2008

Q&A: Remove Crankarm, space-age gearing, chains & cogs

Q: I purchased a 70's vintage Puch mostly to tinker and learn some bike repair. The Puch has a crankset with a square-spindle on the bottom bracket. The arm is not attached by a bolt but by a nut which is threaded onto the end of the spindle. The threading protrudes to the extent that the crankarm removal tool I have will not allow me to thread it into the crankarm. Do you know where I might get a tool that will do the job? Other advice?

A: It sounds like you need a different crankarm remover, Mike, one where you can remove the threaded plunger that pushes on the end of the bottom bracket spindle. Most bicycle shops should have these tools for sale or you could try www.bicycletoolsetc.com But, in the meantime, the easy way to get your crankarm off is to remove the nut and simply ride around your neighborhood. Don't go any farther than you want to walk home - and pedal gently. You want to loosen the arm, not have it fall off. If it gets too loose and you keep riding on it, you can damage the soft aluminum inside the crankarm because the bottom bracket spindle is steel and it will cut into the aluminum under strong pedal pressure.

Usually, a little riding will make crankarms loosen up quickly. If not, you could speed the process by spraying a penetrating lube like WD-40 in between the crankarm and the bottom bracket axle. That should do it!

Q: Jim - I was wondering why bike manufacturers haven't produced a rear cog (gears) that expands and contracts as opposed to one that shifts from small cogs to large cogs. It seems such a design would save a lot of weight since there'd only be one cog and maybe even allow for greater acceleration since the chain would always be in contact with the same cog.

Just a thought,

Q: Thanks for the question, Dan. Actually, drivetrains like you describe have been invented over the years. The most recent design like this that I know about is on a bike made by Ellsworth. The ingenious drivetrain is called NuVinci. Here's a link where you can read all about the technology behind the drivetrain: http://www.fallbrooktech.com/NuVinci.asp Note that they use balls instead of a cog but the principle is similar. I've ridden one of their bikes and it's a unique experience and very fun. This design adds a lot of weight which is often a problem with these designs, but it's sure cool, and it works nicely.

Q: Enjoy your site, Jim. I have a quick question. I've read that every time you change your chain, due to stretch, that you should also change out the cassette. Is this true or could I make the cassette last longer by changing out the chain more often (1/16" stretch vice 1/8").

A: Thanks for the question, Robert,
The cassette will definitely last longer if you replace your chain more frequently, in effect, well before it wears enough to wear the cassette too much. What happens is as the chain wears it also wears the cassette, and the more the chain is worn, the worse the wear is on the cassette cogs. So, if you install a new chain every 1,500 miles or so, you'll get more mileage out of your cassette.

I'll let you do the math, though. The decision is whether to simply ride the chain and cassette out together and replace both at the same time paying for the chain and cassette; or whether to install several chains before replacing the cassette. With the price of certain chains you may decide you'd rather just buy one chain and cassette as needed rather than going through lots of chains to extend the life of your cassette. Keeping the chain and cassette nice and clean and lubed will help a lot, too.

Some argue that the shifting is better and the chain is stronger and less likely to break if you keep it new and that may be true depending on how hard you ride or race. On my training bike I generally ride the cassette and chain out together and don't worry much about it. On my race bike where I want every possible advantage and where I run an expensive Dura-Ace cassette, I change the chain earlier to try to get a few more miles out of that way-expensive cassette. On my training bike I go with the much cheaper 105 10-sp cassette so I'm not that worried about it.

So, I agree with both schools of thought but exercise them according to which bike we're talking about.

Also, I don't agree with the blanket statement that every time you change a chain you need to change the cassette. It all depends on how worn the cassette is. Assuming the chain and cassette are both worn, then yes, you may need both. But, if you have a bike with a worn chain but a cassette that's not that worn, you can sometimes get away with a new chain. And, if you put a new cassette on a bike with a worn chain that will work, too. Not perfectly, but often good enough that you wouldn't know the difference. I just mention that because I don't like it when mechanics insist that you have to buy more than you really have to.

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