Hi, I'm running a fairly low 38X17t gear at present, and was just wondering how much more life I could get out of a chain by running something like a 52X24t instead? I have a lot of rings laying around, and was just curious....

Also, I'm using a derailleur as a tensioner (vert. dropout) and was wondering if it is generally ok to use cassette cogs instead of bmx cogs for a SS in this instance? I was thinking it might not be as critical to keeping the chain on since the RD is tracking the chain directly onto the cog.

Thanks,

Seems preety much the same ratio. Same torque...I might be wrong

In theory your cog and chainring will last longer the more teeth they have (spreading out the wear over a larger surface area), but I haven't heard about that applying to chain life as well. The opposite might be true for chains, in that they'd be in contact with more teeth per revolution.

Realistically it's probably not going to effect things enough to worry about, but it wouldn't hurt to try it if you have the parts laying around.

I just buy Z-chains $10.00 a piece and replace them when it seems like they need to be.

Chain tension increases with a smaller chainring size, so chain wear will be more rapid. This is one of the reasons that an mtb with a very small inner chainring will see more rapid chain wear and chain failure is more common. So in principle you will get longer drivetrain life (chainring, cog, chain) by using larger chainrings and cogs.

There are a lot of other factors that determine how long your chain will last. However, the added chainlife you will get out of moving from a 38t to 52t chainring is going to be very minor (maybe 10-15%?). Good SS chains are cheap. If you can upgrade your DT for free then go for it, otherwise just ride your current DT til it dies, then replace it.

As the cassette cog wears or your chain tensioner bounces, you might experience chain derailment or slipping as the cassette cog is usually cut at angle to facilitate gear changes. For some this may not be a big deal. In some instances this could cause an injury if you slip, fall or otherwise cause the bike to mishandle as you apply a large amount of force like climbing a hill for example.

A BMX or individual rear cog is preferred for safety and longevity.

If you have the right range of chain tension, the difference between wear rates on larger versus smaller chainring + cog combinations is not onerous. Just buy a new chain when you need to.

How does tension vary with ring size?

It's a matter of mechanical leverage. When you apply a force to the pedal perpendicular to the crank arm, the result is a moment (torque) equal to the product of the applied force and the crank arm length. This applied moment is resisted by an equal and opposite moment which is equal to the product of the chain tension force and the radius of the chainring. The chainring radius is directly proportional to the number of teeth on the chainring, which form the circumference of the chainring. Therefore, the tension in the chain is inversely proportional to the size of the chainring, and a smaller chainring size will equate to a larger tension force in the chain.

Ok, that makes sense. I thought you were talking about chain tension as it is commonly thought of, in terms of slack, as opposed to load while pedaling.

The common terminology used for chain adjustment is really incorrect, as the chain is never really "tensioned". In fact, if it were tensioned, then the adjustment would be incorrect. The idea is to have as little slack as possible, while having nominally zero chain tension. There will always be a small amount of tension in the chain due to gravity, which increases as the chain slack is reduced and the chain pulls itself straighter. However, this tension is very small in comparison with the tension resulting from the rider applying force to the pedals.