Triple chainset question - Road and MTB

[Fatal Swan]

I've just repurposed a mountain bike that I wasn't using for use as an occasional road and probably less-occasional 'gravel' bike (it's basically this one: viewtopic.php?f=21&t=249295&start=50 but now sporting flat bars & bar ends, Panaracer Pasela slicks and a rigid carbon Trigon fork). It's comfortably under 8kg :shock:

Just given it a first ride out and it's very (very) nice and it's given me a couple of ideas for what to tweak. The biggest issue is that predictably the 44t chainring of the compact M970 chainset holds it back on the road - the cassette is 11-27. It's not a huge issue but I want to consider what options I have. My current thinking is:

• Triple chainset that takes a 48t top ring. But as far as I know there aren't many decent quality options for Hollotech II style, are there? I know there's an XT model of this era with 26/36/48 but they must have been rare - I can't find used models any on ebay etc (the new ones are £220!).

• Triple road chainset with a big ring of say 50t. Seem to be more options (they're totally out of fashion and so they're available second-hand). I can check if the 50t chainring would clear the chainstay easily enough but I've imagine there's something about road triples that makes them incompatible with an MTB front mech/shifter, right?

Any experience with this or any options I'm overlooking?

 

[ishaw]

Re:

You can fit a 48t chain ring on the m970, the only issue will be aesthetics due to the design of the spider.

Same goes for xt or others as it's just 104 bcd.

You may need to look at the ring spacing, but that's easily corrected with washers if needs be.

Middleburn did after market rings that fitted the spider as the original rings did, as did strong light, no idea if they did 48t rings though, or if they are easy to come by.

If you are bothered by the aesthetics, I have a set of custom made adapters that were designed to match the spider contours, but allow me to run 2x, you could probably file these down to fill the gap a normal.ring would leave, though might be wafer thin in places?

 

[ishaw]

Re:

I've also got a set of ta specialties rings with 50t outer. Iirc they are 104 bcd.

I've got some m970 cranks I could put them in to see how they look/fit.

 

[hamster]

44/11 is a 108" gear - you will spin out at 28mph at 100rpm cadence. Apart from a few downhill bits that's plenty for the road - it's typically one cog off top gear on a 52T with 12-27 sort of cassette.

I doubt that the 48T ring will get used so much at the very top gear?

 

[Fatal Swan]

44/11 is a 108" gear - you will spin out at 28mph at 100rpm cadence. Apart from a few downhill bits that's plenty for the road - it's typically one cog off top gear on a 52T with 12-27 sort of cassette.

I doubt that the 48T ring will get used so much at the very top gear?

Interesting - I felt like I was starting to spin out at between 25 and 26mph, though that was what the Garmin said and the live speed reading isn't always that accurate.

I hadn't thought to do the maths until this message. So looking up gear inches on the first hit on google (https://www.bikecalc.com/gear_inches), for the wheels I'm riding at the moment (26" x 1.25), 44/11 comes out as 98.11". That's the equivalent of between the 13 and 14 sprockets on my 700c road bike's 50t chainring. It's a compromise of a bike in lots of ways but it'll be more versatile if I can add a bit more to that bottom gear...

 

[niggle]

Re:

Stronglight £34 up to 46T: https://spacycles.co.uk/m2b0s149p487/ST ... cral-Outer

Specialites T.A. £42 up to 50T: https://spacycles.co.uk/m2b0s113p2754/S ... -104-Outer

EDIT Road Hollowtech II triple has a closer chainline (45mm, MTB triple Hollowtech II = 50mm) so even if the rings do clear the chainstays the FD may not be able to get down to the granny ring. A road FD has a different cable pull, you can get a flat bar road shifter e.g. Sora but you will have paid out a lot for no real advantage.

You could get a road triple chainset out to an MTB chainline by going square taper and fitting a 10mm wider than standard BB, e.g. this https://spacycles.co.uk/m2b0s109p3052/S ... cral-Rings with a 122.5mm one of these: https://spacycles.co.uk/m2b0s110p2905/SHIMANO-UN55 but then you will be spending money and adding weight to the bike at the same time, plus aesthetic issues.

 

[Fatal Swan]

Time for the update on the resolution of this one...

In the end I decided to try and keep it simple by replacing the M970 chainset with the best thing that I could guarantee wouldn't potentially cause a string of compatibility problems. Road chainsets most likely would need new front mech and hence shifters, possible issues with chainstay clearance on big chainrings too. And changing the rings on the M970 cranks would also look hideous.

So I got lucky and turned up a very rare set of 26-36-48 M760 XT chainrings, in really superb condition to boot. A cheap set of M760 cranks later and we're ready to go. The weight penalty in the switch from the XTR is not quite as bad as I thought - 93g in total, and much of that will be the larger chainrings I guess.

The change from 44T to 48T will be a modest 9% increase in my top gear length. The 44T chainring gave me an equivalent gear length to between the 13 and 14 sprockets on my road bike (which has a compact 34/50T chainset), but the 48T now gives me a gear length very close to the 12 sprocket. That should be noticeable on the road and while not perfect, it's a fair compromise given it's just a straightforward chainset swap out, plus a few more links in the chain.

 

[Dickyboy]

44/11 is a 108" gear - you will spin out at 28mph at 100rpm cadence. Apart from a few downhill bits that's plenty for the road - it's typically one cog off top gear on a 52T with 12-27 sort of cassette.

I hear this argument quite a lot, but despite not being a particularly fast cyclist always found 44t to 11t was not enough for a road based 26" wheeled bike.

Another option out there is you can get eccentric front mech spacers so the mix of mtb & Road chain lines can potentially be overcome