Hi,
I’m picking up a second hand Genesis fortitude ss on Tuesday and would like to order a 20t sprocket now so it’s here on time but not to sure what I need. Any help would be much appreciated.

Cheers

Wide base ones are easier on your cassette body. Something like Surly, On-One, etc. I’ve got a 5 year old On-One sprocket that’s been on multiple bikes and is still going strong.

If it’s got the standard wheels on it, may well be a screw-on free wheel, not a sprocket type. Charlie the Bikemonger has a good range from dead cheap to oh my god, how expensive? I usually go with the Sturmy Archer ones as I have that tool for removal. The pawls and bearings tend to fail before the teeth wear, but for about 12 quid it’s no biggie.