jmud-ancien

I am trying to lower the gearing on a friends bike without spending too much money, so going to a triple is not an option. The current setup is an older 8-speed shimano 105, 53-42 on the front 12-23 on the back. I have two main questions:

1.) If I add a smaller chainring(s) and a bigger cassette, what would be the lowest combination possible?

2.) If I switch to a compact double, will the derailleur cage be adequate? Is this something that is normally changed as well? Are there any other issues with converting to a compact double?

Any other advice is appreciated.

KevinF

Your current rear derailleur should be fine if you switch to a compact double. You might have a "fun" time trying to locate a square-taper compact crank in this day and age (unless you want to get a new bottom bracket as well, but you said you're trying to do this cheaply). E-Bay will be your friend.

These are the 8-speed cassette combos: https://sheldonbrown.com/k7.html#8cassettes. Again, I imagine E-bay will be helpful to find them anymore.

As for the smallest chain-ring you can fit on your current crank... A modern double crank will only fit a 38-tooth ring. To get the "for sure" answer for your specific setup, grab a ruler, measure the distance between two of the chainring bolts, and look here: https://www.sheldonbrown.com/gloss_bo-z.html

joejack951

Your cheapest option for a significant gearing change will be a MTB cassette, MTB medium/long cage rear derailler, and a new chain. If you shop around, you can have all the parts for $50 plus shipping.

derailler: https://www.universalcycles.com/shopp...66&category=72 $23
cassette: https://www.universalcycles.com/shopp...08&category=42 $20
chain: https://www.universalcycles.com/shopp...11&category=61 $12

Make that $55.

A 39 tooth chainring would hurt either for another $25: https://www.universalcycles.com/shopp...&category=2629

One more thing, for the compact crankset, my experience and that of others has generally been that standard road deraillers work just fine when used with a compact crank. Until you start getting below 48 teeth, you shouldn't have any issues with achieving perfect shifting.

DannoXYZ 

How much lower gearing do you want?
The least intrusive/costly option is to put a 38t inner-chainring on your existing crank. This will give you one gear lower than what you have. Then a 12-28t cassette will be about maximum that your RD can handle. These two would give you about 3-gears lower than what you have now or 26%.

urbanknight

Correct. Also a warning: I once destroyed an 8 speed rear derailleur by cross shifting into a 50/28. Be careful on the chain size and don't cross shift!

jmud-ancien

Thanks for the quick replies, and especially the great links.

Here's another thought. If I let my buddy test-ride my bike with a triple (53-42-30, 12-25) which combination should he be in to simulate the lowest gear on a bike with a 38 or 39 tooth chainring and 28 tooth rear cog?

Jeff Wills

39/28 = 1.4
30/21 = 1.42

Small chainring, 3rd cog down.

joejack951

FWIW, a 39/32 (1.22) is about the same as a 30/25 (1.2). A 38/32 is slightly lower at 1.19.