I have french threaded stronglight cranks that i have been able to get off. i wanted to switch them out so i could get some different gear ratios. something between a 40/16 and a 52/16, the original chainrings that came on the crank. and finding a chainring for these cranks is difficult due to the weird BCD and because it has 3 arms. and recently, i broke the smaller chainring because a stackbolt fell out and it bent. so as i am unable to remove the crank or get new chainrings, i was wondering if there are any advantages/disadvantages to having an 18T or 20T cog in the back and a 52 chainring in front. thanks.

Well, other than the slight increase in weight of the larger cog and longer chain, there is no real downside. The advantage is that a larger cog will last a lot longer and will run a bit smoother.

Why is this?

Pedaling force is being applied to a larger surface area. 20 teeth carrying the same amount of force that 16 teeth will last longer.

A larger cog engages more links on the chain, so it distributes the load to more teeth on the cog, reducing the load on individual teeth so they wear slower. It runs smoother because the chain is bent more gradually around the cog.

huh. i think i am just going to do this. i do not mind the extra weight and it is the cheapest solution to my problem. thank you very much.

Thanks for the info. Makes perfect sense.