Compact road chainset advice please

[Dead Rats]

I recently bought a rather nice Truvativ Elita compact chainset from CRC with GXP BB for under £30 posted. BARGAIN eh? However, when fitted as per manual it offers no chainstay clearance for the arms whatsoever.

I'll keep hold of this chainset as it's worth it just for the rings, but what to do?

The frame I have is a Cube LTD which is a 700c hybrid, hence has quite wide tyre clearance (38mm+) and originally came with a triple 'trekking' chainset. Rest of my cobbled together set-up is 9 speed with 11-34 cassette, XT rear mech, Ultegra shifters and Sora front mech.

My question is will any other compact road chainsets offer more arm/chainstay clearance? Been looking at Tiagra etc. Really don't want a triple on this bike - ideally I want 34t inner ring and 46-50t outer - but fear I'm going to have the same problem with brands other than Truvativ due the width of my chainstays, in which case I may have to go back to an easier-to-tinker-about-with square taper set up.

Your thoughts are appreciated

 

[JPB08]

If your using a compact chainset that uses outboard cups, you can space the cups out to give you more chainstay clearance....Campag 9v cassette spacers work a treat as BB spacers....
If your using a square taper BB then you could try one with a wide axle spacing...127mm ??...
Or - a MTB double Chainset should have tapered crank arms to allow for chainstay clearance....a 42/28 used with an 11 up cassette will give you almost the same gear size as a 50 x 13...

 

[Dead Rats]

If your using a compact chainset that uses outboard cups, you can space the cups out to give you more chainstay clearance....Campag 9v cassette spacers work a treat as BB spacers....
If your using a square taper BB then you could try one with a wide axle spacing...127mm ??...
Or - a MTB double Chainset should have tapered crank arms to allow for chainstay clearance....a 42/28 used with an 11 up cassette will give you almost the same gear size as a 50 x 13...

Good point, but in order for the Truvativ to clear the chainstay I'd have to put approx 5mm spacer on the driveside :shock: and the GXP bb supplied is designed for no spacers - so am pretty nervous about that, I don't think the LH arm would bolt down far enough.

I do have a triple XT HT2 chainset for which I have 48/36 rings, but getting the chainline right using the 3 x 2.5mm spacers right means I'm quite seriously offsetting my arms. As you say I could instead get 42/28 rings, mount them on the middle and inner positions and fiddle about with the spacers.

 

[wynne]

I have experienced this before. A lot of road chainsets do not clear the stays on hybrid and some modern touring bikes - most of which are designed for running a MTB triple. You can move the spacers around but you should not add extra spacers on external cups for, as you say, you won't get sufficient engagement on the axle/crank arm.

You probably know this but you'll need to measure the Q-factor required for the frame - which is the horizontal distance between the crank arms necessary to clear the frame.

 

[Dead Rats]

I have experienced this before. A lot of road chainsets do not clear the stays on hybrid and some modern touring bikes - most of which are designed for running a MTB triple. You can move the spacers around but you should not add extra spacers on external cups for, as you say, you won't get sufficient engagement on the axle/crank arm.

You probably know this but you'll need to measure the Q-factor required for the frame - which is the horizontal distance between the crank arms necessary to clear the frame.

Cheers for the comment. Is Q-factor commonly shown by manufacturers of cranks? I never had this issue when I used a compact double on my old Planet X Uncle John frame which had big rear tyre clearance, obviously that had a lower Q-factor measurement than the Cube.

Latest set-up is as I mentioned above, my 4-bolt XT triple with 44t in middle position and 28t granny. The front mech seems fine with the 16t difference on short test run today, though it won't go as low on the s/tube as I'd like due to the pesky lower bottle cage boss! :|

I have seen a set up where rider put a 40-something outer ring in middle position, then used XL chainring bolts & some spacers to connect it to a 30-something middle ring, therefore not using the granny tabs at all. Wondering if I might try this as then I could run my preferred 48/36 or 48/32 rings, but there's undoubtedly something slightly bodgy about it.

 

[Dead Rats]

Quick update for anyone interested, after a fair bit of Googling and experimentation.

I've dialed in the chainline for the 44/28 set-up on my triple XT using one spacer on the RH side and two on the left, so the big ring lines up with 6th cog best (70" ratio) and runs fine across the whole cassette bar the 34t cog. Chain cut as short as possible and rm 'B' tension screw to minimum. The granny is happiest in the biggest five cogs for long climbs.

I'm going to ditch my FM cable roller as I've bought an Ultegra CX70 top pull front mech from CRC which apparently works fine with 9 speed. Think this will be a big improvement over my Sora triple fm with its grooved cage. I chose the braze-on version of the CX70 as I've already modified my aftermarket braze-on clamp to allow it to sit a little lower, despite the lower bottle cage mount.

Job done, I think

 

[jonnyboy666]

although you seem to have got it working judging by your above post i thought i'd throw in my 2p.


main thing is simply a case of mixed parts, road chainset on a hybrid is not ideal unless the bike was designed for it in the first place, the issue is that there have been alot of cross over bikes in recent years, first of all it was normal hybrids, they used mostly mtb gearing, so simple, then came the more roadie hybrids, these used more road components and i think it created problems as people didn't know what was best for their frame hence the mix up you have had.

road chainsets on mtb based hybrids are unlikely to work as the BB is 68mm on the chainset but is designed for a frame with a narrower rear end so the chainline is less, and as such usually the crank arms are therefore narrower and the Q factor is less, which is the exact problem you have had. specifically with the GXP chainsets you can't space them because there are no spacers so if you put one in you then can't tighten the axle up, there are no spacers because road bottom brackets are only available in 68mm so there are no spacers because road bikes have no 73mm shell option (exception italian threads at 70mm).

the other thing is the front mech, running a triple mech on a double chainset or vice versa, sometimes it will work, sometimes it won't, a combination of the shape of the mech cage, parallelogram throw on the cage, the shifter (being 2 or 3 speed) and being road or mtb specifec can all cause issues, not to mention mixing 8/9/10 speed systems.

your system sounds really quite mixed now, if you have ultegra shifters (designed for triple, your original chainset) running on a mtb triple chainset modified to double with a road throw front mech designed for double it may still have issues, the triple shifters for road have a different amount of indexed positions for trimming the front mech, this may help but it may also hinder the set up.

hopefully it will all gel together though.

 

[elPedro666]

^ That's how he likes it JB, keeps him off the streets with the endless tinkering...

The long bolt solution would scare me as you'll put massive shearing force through the bolts.

I can't believe there hasn't been a single 'get a square taper you modern infidel!' comment before now!

 

[wynne]

Job done, I think

If there's one thing I think we retrobikers know, it's that the job is never done.

 

[elPedro666]

Ain't that the truth!