lolitsJeff

I have a motobecane fantom cross uno with a 20T rear freewheel. I currently have a 38T chainring that I feel will be too low for a typical cyclocross race. What size should I upgrade to?

Oh, and I've never raced cyclocross before, I use mine primarily on fire roads but I'd like to convert it to something I can race with.

Andy_K 

I think 38x20 sounds about right, depending on the course, especially if you're new to CX racing.

I raced singlespeed a couple of times last year with a 38x18 setup. On two completely flat courses, that was about right for me. On one mildly hilly course it nearly killed me.

droobieinop

When researching the same question I found that a 2:1 ratio was popular.
I did 3 training rides and ended up with a 36x18 and found myself wishing for a 34.

good luck

M_S

I guess it depends on your course. I've only done two races singlespeed and used around 63 gear inches, 39 x 17. Seemed about right, especially on the flatter and faster of the two courses. I brought an 18t cog each day to swap if needed, but didn't. It also would depend on how fast you can comfortably spin or how competitive you are. I'm more of a masher and was trying to at least be kind of competitive against people with gears so I didn't want to go too low. I think 55 gear inches would be as low as I would consider, and it would have to be a supper muddy, techy course to go that low. For me. I absolutely had to do a fair amount of standing out of corners, but that's fine for me, especially in a 45 minute race.

thirdgenbird

subscribing.

erichkopp

It will definitely depend on if you like to spin faster or mash a harder gear, as well as your fitness. 39x20 seems to work for me when hills are involved, 39x18 if it's pancake flat.

lolitsJeff

what do you prefer, erich?

erichkopp

I'm a lot more comfortable spinning. I don't have a lot of weight to throw around to power through a big gear.

flargle

42:16

https://t.co/xwTd9FB

https://www.cyclingdirt.org/coverage/...-Championships

mack_turtle

that might be good advice for someone who races and wins, but not so much for an entry-level racer.

Cynikal 

SS gearing is such a personal choice that it can be hard to give advise on. I use between a 38 to 42 chainring and from 16 to 18 on the rear. So far so good.

Alan@TreeFort

From my experience a lot of people start with a 2:1 ratio and change it from there. I use a 36:16 and find it to be a great combination. Can get up basically all hills and never spin out on the flats.

Mr Pink57

A shop owner who has won the SSWC and does SSCX suggested 39x17 (which he runs) most of the time here.

thirdgenbird

61.9 gear inches... then maybe my 63 gear inches (42x18) it about right.

Andy_K 

That depends on your legs, I guess. If I read that I was running a taller gear than a guy who has won SSWC, I'd probably think I needed to gear down. But that's just me. I'm weak.

thirdgenbird

i dont plan on racing at that level. the bike will be a fun/commuter rig that will see some races for fun. if i geared it specifically for a few cross races a year, riding it every other time wouldnt be as fun. if the 42x18 goes poorly, i will just buy a 2nd freehub.

edit: also, depending on tire size, we may be even closer than 1.1 gi. i am currently on 700x30s so i am actually a little less than 63

Andy_K 

For fun/commuting 42-18 is definitely no problem (unless you're riding fixed and skidding). I used that exact combination yesterday for my commute. It might even be a bit low for that kind of riding. The thing is, you need much lower gearing for CX than for riding around the streets. The difference is dramatic enough that you really need to change parts. IMO, swapping in a smaller chainring is probably better than changing the freewheel, because it makes for a bigger changes.

thirdgenbird

i find 42x16 w/ 700x23 tires is best for fixed riding/fast commuting


i am expecting i will need lower gearing than 42x18 but the two cross racers above use 39x17 and 42x16. 42x16 is much higher than what i have and 39x17 is very close. what i have may not be optimal, but it cant be too far off given the success of the two racers above.

you have that backwards, swapping freewheels makes a bigger difference. one tooth on the back is WAY more than one tooth on the front.

fietsbob

Plan to shoulder the bike and run with it, for the low gear.

thirdgenbird

this is not a problem. i am a former xc runner and 400m sprinter.

Andy_K 

Yeah. I don't know what I was thinking.

M_S

42-18 should be a good place to start for general riding around with mixed dirt/road. Really you can analyze this stuff over and over but gear ratios depend on a lot of things and it's best to just get some ride time in on the courses or rides you'll be doing and go from there.

thirdgenbird

well i ran into a new campagnolo 39t ring for $15 so i bought it to give me a second option. in the process i discovered and interesting setup. i fully expect that some may laugh:




now for the explication. my rear hub is a white industries eno.

fixed chain line = 43mm
ss chain line = 47.4mm

this gives me a 42x16 fixed for road riding and a 39x18 for cross riding and a close chain length for each. funny looking yes, but great flexibility for $15

Rumpled

Am I getting that right? Your 42t is your inner?

thirdgenbird

correct. 42x16 fixed, 39x18 single speed. it would look more traditional with the 42 on the outside but that is not conducive the the chain line on the hub (unless you want to ride fixed off road)

the chain will be protecting the outer ring when i am off road. the inner ring will be used for fixed road riding. it is not like there are derailleurs involved. this is a traditional turn the back wheel around setup.