Hi,
I currently have an 8 speed Shimano cassette with 13-23 cogs. The front rings are 39/53. Can you tell me if I should keep the same size cassette or can I go bigger? ie 11-30?

thanks
Brent

That depends on your rear derailleur. Most Shimano road rear derailleurs have a maximum cog rating of about 27T but you can often exceed it somewhat. You will probably need a new chain because your current one is probably not long enough. BTW, can you really use a 53x11 high gear or will you effectively have a 7-speed cassette?

I agree with Hillrider's questioning the usefulness of an 11 T cog for most of us, especially with a 53 T chain ring. My bike has Shimano 50 x 13 which will take me to way faster than I can sustain for any reasonable length of time. If I could find an end 14 T, I would exchange it for the 13 T because I would rather have close ratios in the middle of the range, where the most time is spent.

+1. Try a Shimano 13-26 or SRAM 12-26 first. If that's not low enough, I'd swap to a compact crank before moving to an MTB rear derailleur and cassette.

My 8 speed dura ace set up works with a 28t large cog, just barely. In the big/big combo there is very marginal chain rub, safe enough for a few pedal strokes if I accidently shift there.

Free- will .. no hills you can't climb, as it is? just replace worn parts with new of same..

As others have said, there is no "should"... it's a matter of what you need or want.

Shimano compatible 8-speed cassettes are not as common as 9-speed, but they are still out there. They are generally cross-compatible among brands (except Campy). The limiting factor is whether you want to change your rear derailleur: your existing derailleur can probably shift to a 28-tooth large cog, but you probably need a "mountain" rear derailleur if you want to use a larger (30 to 34-tooth) big cog. Changing the rear derailleur will not require a different shift lever.

A 53-11 high gear is quite high and generally useless unless you live in an area with extended downhills. That's the high gear I have, but I only get to use it on a continuous 3 to 5 percent downgrade. (It is kind of fun to pedal past other riders who are spun out and coasting.)

Some possible sources of 8-speed cassettes:
https://harriscyclery.net/product-lis...-8-speed-1243/
https://www.interlocracing.com/cassettes_steel.html
https://www.interlocracing.com/cassbreakdown.html
https://www.biketoolsetc.com/index.cg...settes-8-Speed

Note that you will almost certainly need a longer chain if you install a larger cog. Replacing the chain when you install a new cassette is a good idea in any case.

I just put the 13-16 8 speed cassette on my bike, and I believe the front gearing is very similar to yours. I find myself on that 26-tooth gear on many hills but I haven't failed to get up any yet! As others have said, I'm not enough of a He-man to miss those 11 and 12 tooth gears

You haven't said which Shimano shifters you have. If 8-speed Dura-Ace you will need to stay with the 8-speed Dura-Ace rear derailleur.

I had my store do the install of my e-bay parts. I went from a 12-25 to an 11-32 8-sp Shimano cassette from Fibica/Ebay online. No problems whatsoever with the rear derailure. The front shifting was problematic, and it was apparent to me, a novice that my chain, which was replaced at this time, was way too short. Bit the RD was no problem, and I don't understand why it would be. Shimano 8-sp 2200 STI shifters. I then replaced the front chainset, again, with new and inexpensive Shimano Tiagra from FleaBay to a 50/34 and everything is fine and as it should be. The rear derail is a Sora and I would venture to say it's the "long" one.

what size chain would I need with a 26T cog then?

The classic chain length determining is to place the chain in the big/big and add 1 or 2 link pairs. I've always tended to go longer if all works well, as you can always shorten the chain easily, if needed. Actual length is a factor of cog/ring size, der, cage design, pulley wheel size, hanger length, chain stay length AND the rider's ability to stay out of cog combos as needed. Andy.

+1.5

I would emphasize that the chain needs to be long enough to go around the big cog and big chainring, with enough slack to allow the rear derailleur to shift into this combination. A too-short chain can cause the the derailleur to jam (at least) or tear it completely off the bike (at most). A chain that's too long will hang slack in the little-little combination. Embarrassing, but not a real problem.