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I've found myself in a ridiculous situation where to rebuild the CX bike would require butchering two other bikes! (wheels and cranks of gravel bike, brakes off singlespeed commuter).
Then had a brainstorm, I've always wanted to convert the commuter for CX, it's one of those garish pink Charge Plugs from ages ago and will actually build up lighter than the CX bike 😂 Might eventually then put together the 'proper' CX bike and then have the luxury of a pit bike, look proper pro 😎
What's the consensus on a good ratio, is there consensus? I was always a 32:16 sort of guy on my 26" MTB, what's the CX equivalent?
Ta
39x17 was my sweet spot racing cx. Plentiful supply of front rings 39/130bcd 👌🏻
I've got 39-18 on my Plug (less garish black version), it's not raced, though we do do 1hr CX rides from work one lunchtime a week. Usually alternating between "gravel" and "laps of the woods till you vomit including dismounts, barriers and carries".
I think like most SS, it's faster to be under geared unless there's a really long road section.
What’s the consensus on a good ratio, is there consensus? I was always a 32:16 sort of guy on my 26″ MTB, what’s the CX equivalent?
It's the magic 52 gear inches you want isn't it.
36/19 I reckon based on zero scientific analysis...
I think mine’s either 40:18 or 38:18, can’t remember.
Whichever I went with it seems to be enough to get through anything technical and can just make it up climbs. Not sure I’d like to try any proper climbs on it though 😂
It’s the magic 52 gear inches you want isn’t it.
Not for CX racing.
57" got me mid pack finishes when I used it.
Just checked, mine is currently 38:16, so at the higher end of the above suggestions which is not where I really want to be (expecting plentiful mud and being susceptible to tedious over-use type injuries).
I have a 39, a 38 and a 36 tooth chainring that will fit the cranks, might start with the 36 then just to give the knees a more gentle introduction to the idea... Possibly invest in a new freewheel but was quite enjoying doing this on the cheap.
At least the 36 tooth is a posh Absolute Black oval ring, maximum traction and pimpness 😎
57″ got me mid pack finishes when I used it.
Well yeah, but that's you the OP has basically stated he's used to 52" and what's 5 inches less? it might be the ability to get up a couple more climbs in a lap rather than dismount and carry...
If you can OP the sensible choice would probably be to pick a chainring between 36t and 40t, use a cassette hub and have a choice of sprockets (they're not too expensive) that give you some ratios between 1.9 and 2.1-ish and just have a play, see what the trade off is in terms of spinning on the (muddy) flat Vs getting up the odd short, sharp climb..
Edit: It definitely looks like the right tool for the job, but I'd change those pedals 😉
Edit: It definitely looks like the right tool for the job, but I’d change those pedals
Haha, yeah, and while I'm at it I might even tape the bars 😉
What inches are you guys using for a 35mm tyre on a 700mm rim? I've calculated 27.25" but don't normally talk in gear inches...
I was assuming CX inches might be higher than MTB, if only because I wouldn't expect to be riding any long climbs anyway, but I guess by that same logic I won't really be worrying about pedalling top speed on the downhills...
If you want to be super precise take the drivetrain ratio lets say 36/16 = 2.25 that's your gearing ratio.
Then multiply by the wheel diameter (inc tyre) in inches (622mm +(2x35) = 692/25.4 = 27.24") so 27.24 x 2.25 = 61.29 'gear inches' call it 61 rounded.
TBH I tend to just us 27" as the wheel dia, fits best if you're using ~32mm tyres.
Ultimately you're just comparing ratios, to me 61" sounds on the higher side, but wouldn't be un usable.
Is the hub flip-flop? could you bung a bigger FW on the other side?
EDIT: Actually what am I saying, that's a 104 BCD crank you should be able to find loads of chainrings all the way down to 32t and a chainring swap would be less bother than a Freewheel change. a 34t would give you ~57" as suggested above.
I have a 39, a 38 and a 36 tooth chainring that will fit the cranks, might start with the 36 then just to give the knees a more gentle introduction to the idea… Possibly invest in a new freewheel but was quite enjoying doing this on the cheap.
I'd look into getting a 17t* and 19t**. That way you've got a sensible spread of options (I'm guessing that it won't need as much fine tuning as track gearing where it's common to have 3x chainrings and 3x sprockets because changing sprockets 1t is ~6" and chainrings 1t is ~2". At the moment I've not taken it out in SSCX mode for a while as ever things so bone dry and the grass is dead it's almost as quick as riding on the road. The track ends should cover that range, might even get away with a 19t.
* slightly draggy courses
** get out of jail mud gearing
Well yeah, but that’s you the OP has basically stated he’s used to 52″ and what’s 5 inches less?
I was 52" on a MTB, 57" on a CX.
The extra 5" on the CX works well as the CX bike is far more energy efficient & there's generally less hills in a CX race than a MTB ride. But yea it'll be trial and error.
possible option? it's for 3/32 chain but would work with a 1/8" chain and gets you in the ballpark of 57"
I'm "WFH" today so just has a look in the garage thinking I had a 34t ring spare, but it's a 32t (velosolo), probably a bit too low.
I use 39/19, which is about right for most courses in the London League until it gets muddy and then it's just easier to run when you lose traction.
Looks like I'll end up with 32:16 again at this rate 😂
I realised the AB chainring on the 29er is a 32t and would also fit, so I have a 32, 36, 38 and 39 to play with.
Will start with 36/16, there's a few evening coaching sessions being held nearby and first races are mid-Sept, so hopefully won't be much in the way of heavy going in the next few weeks, good opportunity to experiment.
52" is too low - anything you gain on the climbs is lost again later in the lap as you get re-overtaken on any long flat grass sections (and there are always long flat grass sections somewhere).
I'm seriously trying 3 speed Sturmey Archer for this season 🙂
I reckon a Duomatic (kickdown 2 speed) would be hillarious for CX...
Retro-direct would be even more amusing (pedalling backwards for one gear).
If you're doing a season, I don't think there is a single right gear. What's good for a drier, faster course is going to be horrific when it gets claggy. I did do it once on a too large gear on a muddy race, might have been 42/18 because that's what the bike came with. Horrible, stood up grinding for 45 mins and really slow. Other times, SS 29er was 30/16 was too slow on the straights but proper grip meant I made back the places lost on the twisty bits.
I'd have a 38, 36, 32 teamed with a 16 sprocket and swap it based on the conditions.
I’d have a 38, 36, 32 teamed with a 16 sprocket and swap it based on the conditions.
Yep, think this is how it will be, 36:16 felt OK on a tiny wee test spin there but can't see me pushing it through an hour (not a Vet till next year 😉) of mud.
Hadn't realised how cheap singlespeed chainrings were (have gotten used to Shimano road chainring prices) can afford a 34t to complete the set
That actually looks great!
Run your chain with two split links and a short section of chain in-between. You can then have a few different lengths of the joining piece to suit different ratios. Changing in steps of 4 teeth is approx one full pair of chain links.
Minor Thread resurrection:
I'm looking at pulling together a SSCX bike for next year myself now (just acquired a cheap frame)...
What did you end up using OP, how did it all pan out?