Building a Disc CX - what rotor sizes, and going 1 x 10

mattrixdesign2

I am building up a disc CX, its a mix of new parts (frame/forks/wheels/disk brakes) and gearing from my Tricross.

I will be going cable disc, I think Shimano CX77 (any good?). The plan is to keep the weight down, where possible (it will be 1x10), its got me thinking about the rotors.

Is 140 too small to be effective? Is 160mm OK? What about 140/160 combo? I am no heavy weight, the bike will be used for commutes, the odd CX race, and winter road riding.

Also, my current bike is running a 105 triple, that should give me a perfect alignment for going single up front.

For now, is it a valid option to keep the current middle ring and just use a chain guide to hold the chain in place? Or should I go for N/W (if I can find one that fits a road chain set!).

deano79543

Have you got a frame & forks?

I have a Felt 65X (size 55cm) for sale. Included in the sale price are (Felt Superlight) handlebars, stem & seatpost, (2*) Elite bottle cages, Sram Rival rear mech (10 speed) and Sram Apex front mech.

All are in excellent condition.

Cheers

Dean

Ber Nard

mattrixdesign2 wrote:

Is 140 too small to be effective? Is 160mm OK? What about 140/160 combo? I am no heavy weight, the bike will be used for commutes, the odd CX race, and winter road riding.

You may not be able to go as small as 140 depending on the disc mounts on the frame and fork.

tgotb

Ber Nard wrote:

You may not be able to go as small as 140 depending on the disc mounts on the frame and fork.

This. I tried the same thing, and also found that 140mm rotors are harder to get hold of, and that some Centrelock rotors foul the brake itself. For the marginal weight gain, I wouldn't bother. If you're that worried, just get a lighter 160mm rotor.

mattrixdesign2

Thanks, will just play safe with 160.

Any thoughts on going 1x10 using a 105 triple. On a budget at the moment so no change of the chain set. Do I keep the centre ring with a chain guide?

Monty Dog

Fit a narrow-wide chainring like Race Face in mid-position, no need for chainguides

jomoj

Monty Dog wrote:

Fit a narrow-wide chainring like Race Face in mid-position, no need for chainguides

I am waiting for delivery of one of these to convert my Crosslight to 1x10 so will post pics once it's done.

Have you thought about gearing choice? A single set up means some compromise at top or bottom. I'm fitting a 38 with an 11-32 cassette for what it's worth.

Regarding brake choice, I have the Shimano R517 which are the same as the cx77 but with a different finish. They are a bit ugly and take some fettling to get set up initially but are effective brakes with a decent feel. The most annoying thing is that they need 3 different sizes of Allen key to adjust so I taped a 3&2.5 together to make a diy tool.

Some other things:

Get some compressionless outer cable, it makes a big difference
Fit some inline cable adjusters so you can tweak them on the fly.
Make sure you are using the newer model Shimano levers with the concealed cables, the older ones don't pull enough cable.
Don't faff about with 140mm rotors, stick with 160.

mattrixdesign2

I have gone for 160mm Rotors, and for the moment Sora spec calipers (only £18.89ea), until I can afford some top class cable pull or even hydro.

Are inline cable adjusters really needed, they are built into the brakes.

Really not sure on the gear setup now, I currently have 3x10 (105), I very rarely use the biggest ring, I have a few options.

Keep 3 x 10 (everything works as it should)
Go 2 x 10 (drop the big ring, limit the mech, no real gain)
Go 1 x 10 (drop the inner and outer, save weight, but have to spend more on N/W ring - also limit the use of the bike on road)

What chainset have you got, what NW ring are you buying?

jomoj

hi

if the calipers have cable adjusters then no, you probably don't need them. The R517 / CX77 don't have these, just adjustments for each pad.

I got a Raceface ring and have SRAM apex cranks with 105 shifters and an XT cassette. I have a 105 short cage mech that is rated up to 30T so I'm hopeful I can get it to fit the 32 sprocket as well. Will also fit a dog fang chain catcher as extra insurance.

re: your comment about limiting the bikes use on road - yes, you will lose some top end gearing but you will still have a gear that you can turn up into the mid 20 mphs so are only likely to be freewheeling on longer descents. If you still think that is a problem then probably a single setup isnt for you - or you can skew the gearing towards the higher end with a bigger chainring and use a wide ratio cassette... but then you'll need an MTB mech and will have some big jumps in gearing.

If you are doing this primarily as a weight saving exercise then really I wouldn't bother as you're only losing a few grams. The benefits are less stuff to collect mud and grass, simpler shifting choices, shorter chain so it doesn't flap about + the shifts are crisper and the side effect is a neat appearance

Whichever way you do it there is a compromise somewhere. However I ran a 1x9 with a 36T and 11-28 block for commuting and some racing and only rarely missed the higher gears (not that I'm particularly fast). i rode a 2 day C2C with the 11-34 cassette from my MTB on for a slightly lower gear and just freewheeled the big descents. That setup had a chainguard and a dogfang and never dropped the chain, I'm hoping the NW chainring will mean the chainguard isn't needed either.

mattrixdesign2

Its not just weight saving, its the simplicity. Recently made the change on my HT MTB, gone up some tricky ascents and never really thought "I wish I had a granny ring to bail me out"... it did save weight, but its also much easier to clean, and no annoying rub with the mech and no place for mud to sit.

This bike I am building up is for CX type riding, I may do some races, but it will also be used for the commute (no need for 2x 3x) but also winter rides (I still won't need the extra gears, as the will more likely be local 40/50 mile rides over winter), but also thinking the bike may be useful for longer hauls/tours, that's when the chain rings become handy.

The issue now is I have spent quite a few quid here and there getting the bits together, so really don't want to splash out on a NW chain ring yet... choices are limited for my 105 chainset.

jomoj

Cool. Well you can get a chain catcher and outer chain guard to hold the chain on and just use your middle ring but you may not save a lot of money over a specific chain ring. Or a chain guide of course.

mattrixdesign2

Well I thought about a cheap chain guide, but I know I would just prefer N.W.

I guess the sensible and "free" option now, is just to wack the triple and front mech on

mattrixdesign2

Just looking for N/W rings, my triple 105 is 130bcd, and I think I would want 38tooth... they seem rare and expensive!

jomoj

The issue there is that 39t is the smallest standard ring you can fit reliably on 130bcd crank so it may be that it's not cost effective to make them.

I'm half way through fitting mine, should be finished tonight

mattrixdesign2

Oddly just read that - one company offers them, but warns you may need to file/wear down the tabs... I may have to re count the teeth on mine, I am sure its 38t for the middle ring!

jomoj

annoying isn't it? Never quite as simple as it appears.

jomoj

Fitted all the bits and just need to remove the front cabling now. All seems pretty good and the rear mech has no problems with the 32t sprocket as far as I can tell.
With the chainring in the outer position it lines up with the 5th sprocket so that places it well for the whole block.

some photos here:

https://drive.google.com/folderview?id=0B-BsGotPhOBKaFFnZVl3YjExbEk&usp=sharing

please ignore the DIY chainstay guard, it's not a permanent feature :oops:

oldbazza

Where did you get the Sram crankset from? I'm looking to do a similar thing(ie single chainring) with my old Trek hybrid although it'll be road/trail going only.

jomoj

oldbazza wrote:

Where did you get the Sram crankset from? I'm looking to do a similar thing(ie single chainring) with my old Trek hybrid although it'll be road/trail going only.

Ribble or Merlin I think, can't quite remember but it was on offer. It's an S550 compact double, pretty much same as Apex.

oldbazza

jomoj wrote:

oldbazza wrote:

Where did you get the Sram crankset from? I'm looking to do a similar thing(ie single chainring) with my old Trek hybrid although it'll be road/trail going only.

Ribble or Merlin I think, can't quite remember but it was on offer. It's an S550 compact double, pretty much same as Apex.

Ta.