change cassette and der

Quoted message said:

I wish to change to a Shimano Megarange cassette (11-34) (knee
problems dontcha know) on my road bike. How do I check whether it will
fit? Also I'll need a long cage der. What is the simplest part to
substitute for the Sora 8 unit. Advice please. TIA

If you've got an 8spd shimano-compatible hub now and you're putting on
an 8spd megarange cassette, it will fit. If your chain is the correct
length now, it will need to be lengthened/replaced (replacing the
chain along with the cassette is generally good anyway).

With some exceptions you probably won't encounter (Saint and Hone),
any current or recent Shimano SGS (long cage) derailer can be swapped
for your Sora. Closest comparable quality would be Alivio. Note that
the Shimano mountain sgs rear derailers with the big pulleys have the
same capacity as normal looking long-cage derailers, they just go
about it differently.

If you have a mid-cage (gs) rear derailer and a triple crank now,
understand that all that happens when you exceed it's capacity is that
you get chain slackness/rub in the small/small combos. But if like
most riders you only use your road bike's small ring with the biggest
couple cogs, it's not an issue, and med cage is actually somewhat
preferable.

If you have a typical double (up to 16t difference), you'd only need a
med cage rd, not long.

Quoted message said:

I wish to change to a Shimano Megarange cassette (11-34) (knee
problems dontcha know) on my road bike. How do I check whether it will
fit? Also I'll need a long cage der. What is the simplest part to
substitute for the Sora 8 unit. Advice please. TIA

Deore long cage rear derailleur..get a new chain whileyou are at it as
the old one is probably too short..least expensive compatible chain.

The cogset, an 8s, will fit onto the hub fine.