Sub-Compact Double chainsets

[NATURAL ANKLING]

Hi,
That's a cracking view and looks a fantastic climb.
I'd love to live near there.

What's the like on the way down, my heaven.

[NickJP]

What's the like on the way down, my heaven.

The descent involved lots of braking, the road being narrow and winding for most of the ~12km descent. Here's a photo I took part way down when I stopped to let the rims cool off.

[tooley92]

Here’s mine Ultegra 6604 cranks with 38/24 rings

The shift upto the 38 ring is slightly hit & miss due to the cage being so high, have ordered an XT 786 like the one in CJs pic to swap out the Dura Ace mech.

[CJ]

Here's the double-ised 105 triple on my carbon gravel/road bike. This is a 42,26 (sometimes 42,24) with a laser cut 'guard ring' to my own design. It drives an 11-36 10-speed cassette.

Double-ised 105 triple

You may notice I've modified the front mech bracket, so it attaches lower on the frame. It's attached directly by the lower screw, via an extra hole I drilled between the orginal two holes in this bracket, and indirectly by the upper screw in an additional plate I made to fit very snugly against and overlapping the top edge of this bracket, that I filed at 45°. There's also an agled slot in the lower edge of this plate to locate the front of the bracket. I've also made and attached a deflector plate via the mech's mounting screw, to ensure that the chain never misses the inner ring. This device together with the guard ring compensate for the mismatch between the curvature of a 42-tooth outer and the size of ring this mech was designed for!

I feel it's worth going to a bit of trouble to get just the range of gears I want, with a close enough chainline that all ten sprockets can be used with the outer ring. In easy country it's just like having a 1×, but when I'm at home in the Peak District that inner ring gets a lot of use! It cannot be used with the top two sprockets but it doesn't need to: those combinations are too 'cross-chained' for efficient use and beyond the tension capacity of the rear mech, so the chain grating against the back of the outer ring is a handy reminder to shift up a ring and down two or three sprockets, for a similar ratio that runs nice and smooth!

[pwa]

Here’s mine Ultegra 6604 cranks with 38/24 rings

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The shift upto the 38 ring is slightly hit & miss due to the cage being so high, have ordered an XT 786 like the one in CJs pic to swap out the Dura Ace mech.

And the reason you can't lower the mech is that it would then hit the chainstay?
Being a bit thick, it has just dawned on me that it is the mech strinking the chainstay that must prevent high end MTB doubles, with their small inner rings, sharing the short bottom brackets of road groupsets with larger inner rings.

[CJ]

Do you not end up with a large Q factor with the Spa solution though? In order to keep a reasonable chainline you need the bb as long or even longer than that required for a triple? I ask as I don’t have a Spa triple or a Middleburn Incy that I could measure. Some aren’t concerned by Q factors but I am and have never liked triples for that reason.

Sorry for the late reply, but I've just located and updated my notes of the Pedal Track (so-called Quack factor!) and chainline for most of the cranks I've used.

Narrowest at 143mm pedal track is the Avocet/Ofmega triple on my old King of Mercia touring bike. The inner ring misses the chainstay by a gnat's whisker. If I doubled that one I guess the chainline would be less than 40mm.

My Spa Ti Tourer's original Stronglight Impact triple chainset on a 110mm Stronglight axle was more than 2cm wider at 164mm. I don't have a note of the chainline and never reduced that one to a double. I swapped it for...

The Middleburn Incy double at the top of this thread, originally on the same 110mm Stronglight axle and now a Shimano 107mm unit. Both those BBs result in a pedal track of 146mm and chainline of 44mm - when the Shimano 107 is installed with a 2mm washer on the right. I would install the Shimano unit without a washer, resulting in the same pedal track and a 42mm chainline, except that the XT mountain-bike double mech I'm using won't shift inwards that far. As supplied it won't even shift close enough for a 44mm chainline. I persuaded it to do this nevertheless, first by filing small amounts of metal off the parts of the mechanism that prevent closer movement by bumping into one another, and then by filing away at the 1/8in thick shims intended to adapt it's 1+3/8 clamp onto a 1+1/8 seat-tube, so they feather away to zero inches thin on the right-hand side, shifting the mech 1/16in (1.6mm) leftwards on this frame's 1+1/4in titanium seat tube. Those are the lengths - or short-cuts more like - I'll take to get a nice chainline on a sub-compact double!

So to answer another question, that's the reason for chainline and pedal-track bloat when you go to shift a sub-compact double with a MTB mech that's designed for rings that small: the mech is designed for a 50mm chainline and won't shift in much further. And it's not much better with the mechs designed for gravel bikes, where doubles seem to have standardised on 49mm chainline rather than 43.5mm road bike standard. Spokespeople for Sachs and Shimano justify this by prattling on about wider tyres. Sorry, but that does not stand up to any amount of examination.

See my latest post with a picture of the carbon gravel bike with a double-ised Shimano 105 triple. That has a reasonably narrow pedal track for a triple crankset at 157mm, which is a good thing because the integral axle of its 'Hollowtech II' design allows no alteration. Reduced to a double this has an even narrower chainline than a normal road double, only 42mm but thankfully not too much narrower for the corresponding mech. This very close chainline however, causes no problems at all on this genuine gravel frame, with fat carbon chainstays spread wide enough to accommodate tyres up to 47mm section. Gravel bikes clearly COULD have exactly the same chainline as road bikes. Why component manufacturers don't at least permit that option is a mystery.

The average chainline a 10-speed cassette on a 135mm hub, by the way, is 45mm. I like my sub-compact doubles to be a bit closer than that because I'll be using ALL of the sprockets with the outer ring (like a 1×), but only the middling to big ones with the inner 'crawler gear'.

[NickJP]

A while ago I measured the Q-factor on some of the bikes in our garage. To avoid comparing apples and oranges, I only measured those bikes that had 135mm rear spacing and which were setup with two chainrings and 43-44mm chainline. All these cranks except the last one were setup as sub-compact, with a large chainring of 42t or smaller.

Middleburn RS7 mounted on a Phil Wood 103mm BB: Q=158mm

Deore XT M737 - a low profile 94BCD Shimano MTB crankset from the mid-90s, mounted on a Phil Wood 98mm BB to get the correct chainline: Q=153mm

Sugino PX - a TA Cyclotouriste copy, but with a low profile to give both derailleur and heel clearance. Mounted on a 107mm BB: Q=149mm

Sugino OX801D installed in a BB386 EVO frame using one of the Wheels Manufacturing angular contact screw-together BBs: Q=148mm. Not sure how this compares with the Q I'd get if using the supplied external bearing cups in a 68mm threaded BB shell - on their web site Sugino claim a Q of 145mm with a 43.5mm chainline.

Ritchey Logic 94BCD - nice forged crankset actually manufactured by Sugino. Also mounted on a Phil Wood 98mm BB to get correct chainline: Q=146mm

TA Cyclotouriste, mounted on a 118mm BB: Q=142mm. This crank could have gone on a shorter BB (maybe 115mm), but the 118 was needed for the inside of the crankarms to clear the chainstays on this particular frame, which has S-bend chainstays to provide fat tyre clearance.

Ritchey Logic 110BCD - this crankset is 110BCD only, no provision for mounting a 74BCD granny ring. Another badge-engineered Sugino crankset mounted on 107mm BB: Q=140mm.

I find the limiting factor for me is that I ride toes out/heels in, and cranks that don't flare out at all (such as the TA Cyclotouriste) require me to use longer pedal spindles to avoid the heels of my cycling shoes striking the crankarm as I pedal. With the TA cranks I'm using Speedplay pedals with +12mm spindles, so the width of my stance with those cranks is actually the widest of any of the cranks, because I can use all the others with normal length pedal spindles.

[jimlews]

I've read through this thread several times and am still not clear what advantage the compact double brings.

My triple C/S comprises 24 inner, 38 mid and 44 outer. 11 - 34 rear sprocket cluster.

Unlike CJ, I find that the triple changes just fine, but I use old front changers; from before the time that Shimano decided to impose smaller middle chainrings (i.e. 22/32/44 and the like).

It seems to me that my 24/38/44 setup does everything a compact double does, but with the option of an occasional overdrive.

So, apart from "retail therapy", what is the point of the compact double?

[Cugel]

I've read through this thread several times and am still not clear what advantage the compact double brings.

My triple C/S comprises 24 inner, 38 mid and 44 outer. 11 - 34 rear sprocket cluster.

Unlike CJ, I find that the triple changes just fine, but I use old front changers; from before the time that Shimano decided to impose smaller middle chainrings (i.e. 22/32/44 and the like).

It seems to me that my 24/38/44 setup does everything a compact double does, but with the option of an occasional overdrive.

So, apart from "retail therapy", what is the point of the compact double?

If you can find a decent triple, there's no real advantage to a compact double. The trick is to find a decent triple if you have a modern BB wanting a 24mm diameter shaft on the chainset. Otherwise, Spa Cycles triples with a square taper axle are very good indeed - functionally, price-wise and even aesthetically.

I have two bikes with triple 24mm diameter Shimano chainsets - an older Dura-Ace triple 7803 and the slightly more modern Ultegra 6703, both operated upon with 6703 STI LH levers and front mechs. They're easy to set up and the indexing with trimming allows very good gear changes. The 30-39-52 rings give you a racing chainset set of ratios but with the addition of a bail-out gear. One works with a 13-34 10-speed cassette and t'other with a 14-28 11-speed cassette. (11-speed chain works on the 10-speed Ultegra chainset teeth).

You can still find the 6703 stuff for sale new but the 7803 seems to have all gone apart from worn-looking e-bay items.

Both the Spa and the Shimano 6703 triples allow a close ratio cassette and a wide range of gears, including very low gears. Personally I really dislike wide ratio cassettes with 20 inch leaps between ratios in the high end.

Cugel

[scottg]

So, apart from "retail therapy", what is the point of the compact double?

If you are using a rod operated front mech, a double is it, there is no 3rd position
on a clanger. If you are using a floating chain, a double front can be shifted
easily with a stick stored in the end of handlebar, a triple shift would be quite
the party piece.

[jimlews]

So, apart from "retail therapy", what is the point of the compact double?

If you are using a rod operated front mech, a double is it, there is no 3rd position
on a clanger. If you are using a floating chain, a double front can be shifted
easily with a stick stored in the end of handlebar, a triple shift would be quite
the party piece.

Have you been talking to Vernon Blake again ?

[Tigerbiten]

So, apart from "retail therapy", what is the point of the compact double?

A compact double has no real benefit unless you like a lot of high gears or your frame can take a cassette starting 13t/14t.
But a sub-compact double is much more useful as it steps the whole gear range down.

A rear derailleur has a max capacity of ~48 teeth.
So it's how you split it between front and back.
Do you go 24t/24t front/back or 16t/32t front/back.
If 24t/24t then a triple is the way to go.
If 16t/32t then a sub compact double is the way to go.
If you work it out then a 24/36/48 triple and 12-36 9 block has almost the same range as a 44/28 double and 11-42 11 block and almost identical gears.
And a sub compact double is easier to setup than a triple with indexed shifters.

Luck .........

[scottg]

So, apart from "retail therapy", what is the point of the compact double?

If you are using a rod operated front mech, a double is it, there is no 3rd position
on a clanger. If you are using a floating chain, a double front can be shifted
easily with a stick stored in the end of handlebar, a triple shift would be quite
the party piece.

Have you been talking to Vernon Blake again ?

Full Marks.

[PH]

So, apart from "retail therapy", what is the point of the compact double?

The point is it's a mechanically easier shift than the middle to inner on a triple, it's usually noticeably smoother and quicker. There's also no indexing issues, you're basically shifting to the stops in both directions.
If you're happy with what you have, there is no point changing. Though a knowledge of the options might still be useful for future reference.

[TheBomber]

Sorry for the late reply, but I've just located and updated my notes of the Pedal Track (so-called Quack factor!) and chainline for most of the cranks I've used.

Thanks for the posting and also thanks to NickJP - both very comprehensive answers to this question. Looks like I could have tried a Middleburn triple spider with just 2 rings to get the ratios I was after. Effectively the ‘Spa solution’ with a lower Q factor chainset. As what I have on a 94mm BCD spider (30T inner ring) is running so well currently I don’t believe I’ll change. Just have to keep pushing a little harder than desired on the steep stuff.