PhoenixBiker 

The time has come in my gravel-bike build-up to decide on a cassette. On my old Trek 520 frameset, I'll be running a NOS Deore LX 48-36-26 crankset with a 105 front derailleur and a Deore LX MTB rear derailleur. I'm looking for an 8-speed cassette that will provide me with as low a granny gear as possible (I'm 63, a Clydesdale, and I live in West Virginia, The Land of Ridiculously Steep Road Grades). I was leaning toward a cassette with an 11-34 range, but in my online shopping I ran across one with a 40T low cog. So here's my question: what is the largest cog that's still compatible with this crankset/derailleur setup? Will it handle a 40T cassette, or is that too broad a gear ratio for the drivetrain to handle? Thanks in advance for your advice.

HTupolev

What particular derailleurs do you have?

Your rear derailleur will definitely have trouble getting onto a 40T cog unless you lower it with a derailleur extender. And you'll almost certainly have dangerous cross-chain combos unless you give up some teeth from your bigger chainrings.
An 11-34 cassette might work out fine, depending on the exact rear derailleur model.

PhoenixBiker 

I'll have to do some research on that. I purchased the bike originally in 1999, and I don't know the original Shimano part numbers for them. All I know at present is that the front derailleur is for a 105 road triple, and the rear derailleur is a Deore LX mountain bike spec. I just found the original cassette that ran on the bike. It was an 11-32, and it originally ran with a 52-42-30 triple road crankset.

mstateglfr 

The combo of chain rings and 11-40 cassette would be too much capacity for any DX rear derailleur ive seen before.

I get your concerns regarding age and hills, and I am usually one of the few to push for even easier gearing than most, but a 26/34 ratio will be super easy gearing for most all situations. Assuming 40mm tires, that gives you a 21 gear inch granny.
if you want even lower, perhaps see if a 24t small ring works for the front.

the problem with a super wide range cassette and 8 speed is if you go too wide, the jumps between cogs are pretty substantial. Better to drop the chain ring size.

PhoenixBiker 

Thanks! I thought that might be the case. I'm currently running 52/42/26 x 13-34 combos on both my 9-speed road bikes. I'll just go with something similar on the 520.