Hello all,

I'm new to upgrading bikes. I'm looking at potentially purchasing a bike with
FD-2400 STI
RD-3500 Sora GS
30x39x50 crankset

But might want to replace the CS-HG50 8spd 11-28T with an HG51 8spd 11-32T.

Shimano lists the stats of the RD-3500 Sora GS as:
Maximum Sprocket 32T / 14T
Minimum Sprocket 25T / 11T
Maximum Front Difference 20T
Total Capacity 41T

If I understand the "total capacity" correctly, the RD can handle 41T, so the (50-30) + (32-11) = 41 should be allowable - but how much more do I need to be aware of before changing cassettes? I assume the Max/Min sprocket specs indicate that 11-32 is okay, right?

It also might just happen that I am perfectly fine with the 11-28T ...

Thanks!

Yes, your RD seems to have enough capacity, so there's no major obstacle.

If the chain is relatively new, you need to check it's length. You need 1" more chain to wrap an added 4 teeth (1/2 way around wrap), so shift the big/big combination on the existing drivetrain, and see if you can pull 2" forward on the lower loop (1" for minimum room to spare, and 1" for the added teeth). If so replacing the chain is optional, if not it's necessary.

Lastly, you'll need to readjust the RD for the new cassette, this will probably include the limits and trim, but will almost definitely include adjusting the "B" screw to push the RD lower.

Francis covered it well-

TOO long of a chain could result in "sag" when in the smallest ring/cog, but you shouldn't be using those anyway.

The downside of the 11 - 32 is bigger ratio gaps. I would not make the change unless you are sure you need the 32. If you do change cassettes It may well make good sense to go with a cassette that starts with a 12 unless you plan to race down steep hills. If you start with a 12 the lower speed ratios will be closer.

If you decide to keep the existing chain when changing to the larger 32T cassette it is imperative that you make very sure that it is long enough by confirming the ability to easily shift into your big ring-big cog combination without straining anything.

Do this by pedaling by hand slowly, with the bike on a stand or upside-down; STOP if anything gets tight. If all is well then make your B-screw and other derailleur adjustments.

Thanks for all of your quick replies ...

Al1943, according to a reviewer of the cassette I'm considering, the gears are as follows:

11-13-15-18-21-24-28-32
11-13-15-17-19-21-24-28

The spacing appears to be the same at both ends, except for the 15-18-21 vs. 15-17-19-21 in the middle - would that ease your mind at all regarding the ratio gaps?

And to clarify - it HAS a 28 ... I'm considering going to a 32 if I need to. I have a hilly commute for which I'd like to have those low gears - but OTOH, it might just be the case that having a bike that is 25 years newer and 15 pounds lighter (28-38-48 with 13-25 rear cogs) might make up the difference those extra 4 teeth would give me ...

Given the circumstances, I suggest you do the 2" test to know your options and estimate the total cost of the change. That tells you what you can do and at what price.

Then test ride the commute and see hoe it goes, and what gears you actually use, and if more range is actually needed. Then it's a question of balancing the benefits of the lower low end vs. the loss of some mid range tightness to decide what you should do.

IME- often, people often fret over things they can't do, but once they find they can decide not to anyway.

OP, Al1943; If there is a 14-32 to fit, that would make a good setup, depending on what other cogs he can get for the middle. I have found that 11 to 12T cogs wear out too fast for me and a 13T seems to have less use that a 14T. But to each his own.

Ultimately the rider will be the best judge of what gearing is needed. I start all of my cassettes with a 13, but I have a 53 big ring, the OP has a 50, and a 30 granny ring. And my cassettes are 9 and 10 speeds, the OP has an 8-speed. My recommendations take all of this into account as well as trying to stay close to what he already has.

I meant to recommend that you not make a change until you are sure you need the 32. I have made the correction to my post #4 .
On the 11 - 32 the jump from 15 to 18 is really big for a road bike. But if you need a 32 you need it, and you'll be the best judge of that. If you're riding on fairly flat terrain trying to draft with a group of riders, a close ratio cassette becomes more important. This is especially true on longer rides or into stiff head-wind. Most experienced riders agree that the best pedaling efficiency is found in the 85 - 105 rpm cadence range. A closer ratio cassette can help you stay in that range. And unless you are racing down hill, having an extra cog in the middle will be more useful than having the 11 cog.

My Globe (hybrid) originally came with that cassette.
I found the 18-15 shift way too wide.
I had a different cassette on it the next day.
I later went to 9 speed and run a 12-23 with much smaller rings.