Triple chainring spacing.

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toontra
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Joined: 21 Dec 2007, 11:01am
Location: London

Triple chainring spacing.

Post by toontra »

My audax bike has an Ultegra 6703 10-speed triple groupset, currently with 52/40/28 chainrings and 11-28 cassette. I spend 80% of my time in the middle ring.

I'm considering replacing the chainrings with 50/40/26 - 50 as I rarely need anything beyond that, and 26 to help on the steep stuff when loaded.

Any thoughts on whether this spacing would pose any shifting problems with my current setup?
PT1029
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Joined: 16 Apr 2012, 9:20pm

Re: Triple chainring spacing.

Post by PT1029 »

What front gear mech are you using? (part number useful). A lot of modern Shimano front gears are designed for a 13t difference between the middle - big ring (I have not checked 10s for this). Varying from this might affect crispness of geat shifts if using indexed levers.
Using a middle - big gap of less than 13t means you have to mount the mech too high for the big ring (lowering the front mech often causes it to foul the middle ring).
Having the mech too high increases the chance/risk of unshipping the chain onto the crank.
So it might work, it might not work quite so well, it might be very fiddly to adjust to work.
If you made all 3 rings 2t smaller you would stand a better chance, though I don't know if you could get a 38t middle.
Sometimes you have to try it and see.

Going to a 26t inner might make the down shift less crisp (or even hesitant), especially if you cannot lower the mech as mentioned above.

If you are using bar end shifters, you'll probably be fine.
toontra
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Joined: 21 Dec 2007, 11:01am
Location: London

Re: Triple chainring spacing.

Post by toontra »

Thanks for the reply!

6703 Ultegra front mech. STI levers - not bar end.

As you say, it's the shift from middle 40 to small 26 I'm wondering about. I could get a 38/39 middle but I spend so much time on it I would have to get used to more regular upshifts.
peetee
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Location: Upon a lumpy, scarred granite massif.

Re: Triple chainring spacing.

Post by peetee »

I believe the 7800 group is of the same design as the 6700 group. I have a 7800 group and I have made a similar modification on mine. I have replaced the 53 tooth ring and now have a 48/39/30 set-up. Because of the position of the inner cage plate of the front mech I could not lower the mech to provide the optimum spacing. Nevertheless with careful set-up of angle and cable length I have excellent shifting without suffering over-shift through the gap between the big ring and mech. It has been working faultlessly on and off road for about 500 miles.
If you zoom in on this photo you can just about see the gap.
A3F324E8-0A2D-49E0-B103-BB7F63496E96.jpeg
toontra
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Joined: 21 Dec 2007, 11:01am
Location: London

Re: Triple chainring spacing.

Post by toontra »

Good news. So the 50 - 40 rings should be OK. Just the 26 to think about.
pwa
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Joined: 2 Oct 2011, 8:55pm

Re: Triple chainring spacing.

Post by pwa »

I did a similar thing with a triple some years back and I did put a 39 in. I didn't regret it. It maintained something like the angle of travel the mech was designed for, and the difference in riding experience between 39 and 40 is tiny. I suppose it means you have to move to the big ring a tiny bit sooner on descents, but you can stay on the middle a tiny bit longer at the start of climbs.
peetee
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Joined: 4 May 2010, 10:20pm
Location: Upon a lumpy, scarred granite massif.

Re: Triple chainring spacing.

Post by peetee »

toontra wrote: 13 Jan 2022, 10:14pm Good news. So the 50 - 40 rings should be OK. Just the 26 to think about.
The restricting factor in relation to chainring options is the depth of the inner cage plate. The mech has to be set up so the bottom edge of the plate misses the middle ring when changing up to the big ring (Shimano’s set up instructions usually refer to the gap between the outer plate and the outer chainring. When changes to chainrings occur this interaction sometimes has to be overlooked).
Once that is established you can find tune the angle and stops to get the best shift.
bgnukem
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Joined: 20 Dec 2010, 5:21pm

Re: Triple chainring spacing.

Post by bgnukem »

I had big problems with a 10 tooth outer chainring difference using a Deore XT front mech (model no. FD-M761) on my commuting bike, and had to change to a small middle ring to increase the difference to 12 teeth, which worked fine.

The problem was that the upshift from the middle to the outer ring became very difficult, with the mech positioned so the inner cage plate just cleared the middle ring. I think the problem was that the inner surface of the iner cage plate is profiled to assist the upshift (i.e. it is not flat) and the contour did not match the smaller chainring spacing.

If I were you, I'd go for a 50/38/26 set-up, the 38T is I think the smallest 'ring that will fit on a traditional road bike PCD (130mm) and preserves a 12T gap with the smaller outer 'ring.
iandriver
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Joined: 10 Jun 2009, 2:09pm
Location: Cambridge.

Re: Triple chainring spacing.

Post by iandriver »

I went with 50, 39, 26 on my Ultigra with 11-34 on the back. Seemed to work fine on my bike
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toontra
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Joined: 21 Dec 2007, 11:01am
Location: London

Re: Triple chainring spacing.

Post by toontra »

Excellent advice all - many thanks. I'll try a couple of variations with spare/old rings & see what works.
NetworkMan
Posts: 720
Joined: 25 Aug 2014, 11:13am
Location: South Devon

Re: Triple chainring spacing.

Post by NetworkMan »

I've done this sort of thing twice myself and never had a problem.
In each case I went from 48/38/38 to 48/38/24 - both 7/8 speed.

I agree that 48/38 is the smallest you can go with a 130 mm BCD road chainset.

On my Spa Audax I'm about to go from 48/38/28 to 48/38/24 again, this time 9 speed. I wondered about 26T but it's just 7.7% lower - only about 1/2 a gear in my book whereas 28->24 is 17% lower and well worth having.

On another bike I went from (IIRC) 52/42/30 to 48/38/28 using the same FD after reading that it's a good idea to maintain the same spacing between the two largest rings though if you go from 12T spacing to 10T you will probably be OK and you can drop the FD a little.

If you are gong to lower the FD you need to be sure that the bottom of the cage will clear the chainstay. You can estimate this by allowing 2 mm per tooth, so if you go down 2 teeth you need 4 mm clearance etc.

You can generally use anything you like for the small chainring; chainging down you just knock the chain off and it lands on the small ring. Changing up you just press the chain against the middle ring. It's the change between the two big rings that's the tricky one.
There is a chance the chain could rub the bottom of the cage when on small front and small(ish) rear. You can check, again using the 2 mm rule.

Of course you need to be sure you have enough capacity in the RD or the chain may go slack in the smal/small(ish) gears. Of course that may not be a problem if you never use those gears anyway

Good luck!
toontra
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Joined: 21 Dec 2007, 11:01am
Location: London

Re: Triple chainring spacing.

Post by toontra »

Excellent advice NetworkMan! Will experiment accordingly.
peetee
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Location: Upon a lumpy, scarred granite massif.

Re: Triple chainring spacing.

Post by peetee »

There is another limiting factor on some front mechs and that is the position of the bridge-plate at the trailing end of the mech. With some cassette combinations the chain can drag across the plate when on the small chainring even when restricting the number of cogs you use with the small chainring to, say, the largest four or five.
Valbrona
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Joined: 7 Feb 2011, 4:49pm

Re: Triple chainring spacing.

Post by Valbrona »

toontra wrote: 13 Jan 2022, 7:33pmAny thoughts on whether this spacing would pose any shifting problems with my current setup?
I doubt it.

Some FDs need a minimum jump between middle and big to work, or else the cage contacts the middle ring when the chain is on the big ring.

Your system is compatible with Shimano 105 (5703) front derailleurs which have greater cage curvature to them and can cope better with more tourist oriented triples. I know for a fact that a 5703 FD will work with 48/36/26 rings and Ultegra 6703 LH lever because that's what I use. But I guess that would require a new (110 BCD) chainset.
I should coco.
toontra
Posts: 814
Joined: 21 Dec 2007, 11:01am
Location: London

Re: Triple chainring spacing.

Post by toontra »

Thanks all. I'm thinking I'll probably try a 50 outer and test if there are any out/mid problems and swap the inner for a 26, so will end up with 50/40/26.
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