Used 75% on road. Can find a 38 or 42T 104bcd chainring but what size rear sprocket to stop me spinning out on the flat?

i use 38/18 on my drop-barred SSCX that’s ridden on all sorts of everything from tarmac to hills to CX races, so work from there really

there’s no real definitive answer, you have to suck a gear and see, it depends on how hilly it is where you live, how fit you are and what you define as ‘spinning out’

I’ve found anything from 32-18 (rough XC type riding) all the way up to 42-16 (road commuter) works depending on riding type on my old SSCX

flat bar SSCX

Didn’t that used to be called a Hybrid? 🙄

Yp, spinning-out is relative to the rider, so start with a 16 sprocket and see how you get on?

Didn’t that used to be called a Hybrid?

Hybrids generally have slick/road tyres

They have hybrid tyres, something suitable to both.

I use 45:17 on the road and the equivalent of 34:18 off road.

It’ll be a massive compromise for both. I’d go for a 42 or 44 as you could get a 20t on the back for a low off road suitable gear or go down to a 16 if it is too low for road use. Rear sprockets are cheap, front ones not so much. For more choice try spa cycles. They have pretty much every option.

Disregarding any hills, I’d start with something in the 38-16, maybe even 15.

My old Tricross Singlecross was 42-17 iirc, more to get me back up Chessel Avenue’s moderate Incline ~6% 1km.

On my Wazoo, I’m using 38-15/17 as my cruise ratios on flat segments, with the FatNotFat wheels and 40c Marathon Cross tyres.
https://www.strava.com/athletes/17697777

I use 42 16 ratio on my drop bar Pompino, not too hilly on my commute, and can get into late twenties mph if my Cateye Strada is to be trusted.
I am thinking about getting a flat bar set up on it for a change, but then I like it being completely different to my mountain bike.

42×16 is geared for 16 mph at 90RPM
42×15 is geared for 18 mph at 90RPM
42×14 is geared for 20 mph at 90RPM

How fast do you want to ride, and how many hills do you want to climb? Most SS bikes come with 42×16. You’ll find this a good gear for a cross bike with knobbles and high rolling resistance.

Switch to decent road tires and you’ll want 42×15 but curse a big headwind.

I ride 42×16, and that includes a medium paced club chaingang (Average speed 21 mph) on my fixed road bike. A tired morning into a headwind sees me cursing and turning the cranks over in the 70s rpm.

I used 32/14 at Hengrove CX today. Was quick the first few laps, held my own in the middle of the race, thought my legs were going to fall off in final couple of laps.

Course was 98% grass, flatfish over parkland banks and hillocks. I came 24th out of 48, 46 of the other bikes were geared.

Off road I ride 32×16 and raced CX 32×15 on 26″ wheels. This is far too spinny for the road though.

I’ve used 42:20 in the past for SSCX which was fine off-road with hills. Not great for the roads.

Cheers all, some good advice there