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

You can go bigger. You'll need to change the chain and the derailleur. You might want to try a combo that is within the capacity of your existing derailleur and chain; a 12-27 will probably work fine for example. Also you could swap out the crank for a compact.

Welcome, Brent.
Do you struggle up hills in your lowest gear? Do you sometimes ride so fast you can't keep up with your highest gear? If neither, keep the ratios you have. The gearing on your cassette is perfect for a relatively flat place.
If the derailleur on your bike is a conventional short cage road one, you shouldn't go higher than 28T on your cassette. You'll need a longer chain too if you decide to go larger.

any idea what is the correct size chain I should be getting if I stay in the same range? 116 link?

thanks

I'm told there aren't many choice to replacing an 8 speed Shimano derailleur. Is that true? I have a Shimano 105 now.

The chain length is determined by how it fits on your bike. Frame size contributes to this. Here's info on how to size a chain to your bike.
If you're keeping the same size cassette as current, and the chain fits, you want to size the new on the same.

You'll need a longer cage derailleur if you're going above 28T. They're not hard to find. Remember that tighter ratio cassettes with short cage derailleurs (like you have now) work better. Going for a big cassette will only improve your ability to grind up steep hills.

Where are you located? What bike do you have? This will help, as there are many different 105 rear derailleurs. The more info you give, the better will be the recommendations.

Now, what are you trying/wanting to accomplish with your ride?

I live in a rural suburb of Buffalo. I have a combination of hills and flat rides. Most rides are about 300-500' elevation but nothing crazy like when I ride in Adirondacks. I have a Cannondale 2004 R600 8 speed 13/23 cassette. All Shimano components. I just ordered new front rings 53/39. SO I need a new cassette and chain as well.

Right now the shifting isn't that smooth. Just put on a new hanger as it was slightly bent. I'm looking for a smoother shifting experience with hopes this will improve my riding.

thanks

Stay with the ratios you have, or go up to a 25T if you want one extra climbing gear. Smaller increments on the cassette make for better shifts.
If the new parts do not improve your shifts, have your new hanger's alignment checked. It's important.

any recommendation a chain make? I have 116 links and was looking at Shimano HG91.

The chain on your bike is 116 links? That's about the size they come in before you shorten them. I've never installed a new chain without shortening it first. If your current chain is too long, it may explain your shifting problems.
The HG91 is a decent chain. I always used SRAM for 8sp chains because I liked that they came with a quick link.

True, but not really.

A 10 speed 105 derailleur will work just fine on an 8speed cassette. It only does what the shifter tells it to do.

BTW, Poor shifting is often caused by gunked up cables or shifters.

on the cassette I did find the 13-23 ratio. If I want the 1 extra climbing gear can I use 12-25 or 13-26?

The first number is the small cog and is what you use when you're going fastest or downhill. If you don't spin out on your current cassette, stay with the 13T. The second number relates to the cog you use when climbing the steepest grades. 26T is a 4% lower than 25T and is therefore a little better for climbing steeps.