Super compact road double, how low can you go?

[niggle]

Hi, I have picked up an old road bike with little use for cheap, with 2x8 gearing and Sora STIs. Chainset is a compact double 48/34t. I will swap out the narrow range cassette and rear mech with a good condition 11-32t cassette and XT 9 speed rear mech that I have, which will give 118"-29" gear range, but I still want to go lower, ideally to a sub 25" bottom ratio, the lower the better, and don't need anything over a 110"ish top gear.

I have a spare Spa cycles "104" triple chainset which I was considering reconfiguring to a super compact double using middle and inner ring positions. I could use the 104 chainset's current 44t outer ring in the middle position, but not sure if the current unidentified low end front mech will be OK with it. If not a Claris FD-R2000 is rated down to 46" chainring so maybe that will work? If not then 46t 104bcd rings are available from £20ish.

However what about the inner ring? The 22t currently fitted I anticipate will cause the chain to rub on the outer ring, especially if a 46t has to be fitted, plus the fact there will be a need for using some fairly small sprockets with the overlap of ratios between the two chainrings being relatively small. Chain might also hit the bottom of the cage? Has anybody tried a 44t or 46t with 22t, 24t, 26t or 28t inner and got it to work OK?

Another option may be to leave the front end as is and fit a Sunrace 11-40t cassette, but possibly a deralleur hanger extender will be needed, which I am unsure about in terms of affect on shifting across the widely differing sprocket sizes and the gear steps are rather large.

EDIT: seem to recall something about modding a 9 speed 11-36t or 12-36t cassette to 8 speed??? That would just about cut it and be relatively affordable.

[Brucey]

the inner ring can be spaced to the left in order to make clearance for cross-chaining, right up to when the chain will occasionally ride on the tops of the teeth. Whether the chain hits the bottom of the FD varies depending on the seat angle and chainstay angle, as well as the size of the chainring interval and the FD in use. Roughly you need 2mm more clearance per tooth more of interval, so you can estimate what is possible most accurately on your bike, probably.

BTW a 44T outer ring can't always be used in the middle position; reasons include

a) that the teeth are shaped and will only engage with the chain one way
b) that there are shift aid features on the chainring which limit cross-chaining.
c) that the chainring needs counterbores to accommodate the chainring bolts and the ones on the big ring are on the wrong side.

So you will have to suck it and see.

cheers

[niggle]

the inner ring can be spaced to the left in order to make clearance for cross-chaining, right up to when the chain will occasionally ride on the tops of the teeth. Whether the chain hits the bottom of the FD varies depending on the seat angle and chainstay angle, as well as the size of the chainring interval and the FD in use. Roughly you need 2mm more clearance per tooth more of interval, so you can estimate what is possible most accurately on your bike, probably.

BTW a 44T outer ring can't always be used in the middle position; reasons include

a) that the teeth are shaped and will only engage with the chain one way
b) that there are shift aid features on the chainring which limit cross-chaining.
c) that the chainring needs counterbores to accommodate the chainring bolts and the ones on the big ring are on the wrong side.

So you will have to suck it and see.

cheers

Thanks, yes I was pondering the counterbore issue, and that the outer ring will only work well the way round it was intended to go due to ramps & pins, forgot about teeth shaping.

[keyboardmonkey]

EDIT: seem to recall something about modding a 9 speed 11-36t or 12-36t cassette to 8 speed??? That would just about cut it and be relatively affordable.

Assuming your 8- and 9-speed cassettes have individual sprockets (rather than being mounted on a spider - do they even exist for 8/9-speeds?) you just need an 8-speed cassette’s lockring and smallest one or two sprockets with integral spacers.

Break up both cassettes by removing the rivets and use the 8-speed spacers, the one or two sprockets with integral spacers from the 8-speed cassette and your largest six or seven 9-speed sprockets.

If your 8-speed cassette starts with 13T you probably won’t miss the 11T.

[Jamesh]

No one does an 8 speed 36t cassette?

Not even Chinese copies?

Cheers James

[niggle]

No one does an 8 speed 36t cassette?

Not even Chinese copies?

Cheers James

Chinese 11-36t 8 speed cassettes do exist, e.g. https://www.ebay.co.uk/itm/Bolany-11-36 ... SwmMVdn9ub To be fair by definition they are not copies as no well known brand name 11-36t exists for them to copy, they did it on their own. OTOH, quality concerns aside, the ratios are not very well thought out: 11-13-16-20-24-28-32-36 gives gear steps of 18.2%, 23.1%, 25.0%, 20.0%, 16.7%, 14.3%, 12.5%, i.e. a horrible mess of big jumps right where you don't want them.

[Valbrona]

You deserve all the faff that this project is likely to give you … and at the end of it be hankering for a triple.

[niggle]

You deserve all the faff that this project is likely to give you … and at the end of it be hankering for a triple.

If I had a triple left STI, a road triple front mech and a road triple chainset then maybe, but I don't (actually must check the left STI in case it does have triple capability, but I suspect it doesn't).

[slowster]

In your shoes I would weigh up the potential costs of what you are considering doing against the cost of a more 'off the shelf' option, e.g. a Spa Super Compact double.

If you already have various other bits and pieces and spares, it should be relatively cheap to experiment with the chainset you've got and see if you can get it to work. If you haven't, you could end up spending quite a bit of money on extra bits and pieces only to find at the end of all your efforts that it doesn't work, or at least not as well as you want. For example, not only are you considering buying new chainrings, you would presumably need either new chainring bolts (or spacers to use with the existing bolts) to convert the chainset from triple to double. Getting the right chainline may require a new bottom bracket, and you may need a different front derailleur. I presume the chainset is an MTB triple, and I guess that might have an impact on how straightforward it is to get the required chainline: as I understand it, you will need the chainrings to have a road double chainline.

A Spa Super Compact is £69, plus say £15 for the appropriate length bottom bracket. Whether or not your existing front derailleur would be suitable would probably depend upon what chainring combination you chose, i.e. with the smallest 40/24 option some front derailleurs cannot be mounted low enough over the 40t ring without the tail of the cage fouling on the chainstay when changing down. You can use http://www.ritzelrechner.de to compare the gears that different chainring combinations would provide, e.g. this is a comparison of 44/28 with 40/24, using your 11-32 cassette:

http://www.ritzelrechner.de/?GR=DERS&KB=24,40&RZ=11,12,14,16,18,21,24,28,32&UF=2170&TF=90&SL=2.6&UN=MPH&DV=gearInches&GR2=DERS&KB2=28,44&RZ2=11,12,14,16,18,21,24,28,32&UF2=2170

[Nigel]

My partner's bike has 46/29 on the front, using a Shimano Ultegra gear changer - its an 11speed, Ultegra rear. The choice was based on the then available Middleburn short cranks, whilst keeping a narrow Q-factor. Don't know if that information helps on choosing an inner gear which may work.

[niggle]

In your shoes I would weigh up the potential costs of what you are considering doing against the cost of a more 'off the shelf' option, e.g. a Spa Super Compact double.

If you already have various other bits and pieces and spares, it should be relatively cheap to experiment with the chainset you've got and see if you can get it to work. If you haven't, you could end up spending quite a bit of money on extra bits and pieces only to find at the end of all your efforts that it doesn't work, or at least not as well as you want. For example, not only are you considering buying new chainrings, you would presumably need either new chainring bolts (or spacers to use with the existing bolts) to convert the chainset from triple to double. Getting the right chainline may require a new bottom bracket, and you may need a different front derailleur. I presume the chainset is an MTB triple, and I guess that might have an impact on how straighforward it is to get the required chainline: as I understand it, you will need the chainrings to have a road double chainline.

A Spa Super Compact is £69, plus say £15 for the appropriate length bottom bracket. Whether or not your existing front derailleur would be suitable would probably depend upon what chainring combination you chose, i.e. with the smallest 40/24 option some front derailleurs cannot be mounted low enough over the 40t ring without the tail of the cage fouling on the chainstay when changing down. You can use http://www.ritzelrechner.de to compare the gears that different chainring combinations would provide, e.g. this is a comparison of 44/28 with 40/24, using your 11-32 cassette:

http://www.ritzelrechner.de/?GR=DERS&KB=24,40&RZ=11,12,14,16,18,21,24,28,32&UF=2170&TF=90&SL=2.6&UN=MPH&DV=gearInches&GR2=DERS&KB2=28,44&RZ2=11,12,14,16,18,21,24,28,32&UF2=2170

Thanks I was using Sheldon Brown's calculator but I knew something like that one existed, however could never seem to find it.
Re the Spa Super Compact Double, which I was aware of (road triple conveted to a double with a chain guard in place of the outer ring) versus my Spa "104" touring (MTB square taper) triple converted to a double, the possibilities of having to change BB and FD exist with both, I have a bunch of spare chainring bolts knocking about and even a few different shortish BBs to play with, so will start by playing around with what I have including the 104. Even the current compact double is square taper BTW. 44/28 chainrings are probably adequate with the 11-32t cassette and stand a reasonable chance of working with a road double FD.

[Brucey]

IIRC the spa super compact double is essentially a converted triple chainset anyway, so the intention is to get something similar to that by DIY means, which is not an unreasonable objective.

It has not been emphasised in this thread but part of the motivation for a compact double over a triple is often that if you have a double LH STI shifter, and a budget build in mind, a double chainset suddenly seems quite appealing.

If you have a converted triple chainset as a double, you have the same Q value etc as the triple, and the only differences might be that

a) the chainline in some of the gears will be different
b) you have a smaller choice of gears with the double
c) the double setup is about 100g lighter (the weight of the missing middle chainring)

cheers

[niggle]

Re the chainline issues, road double = 43.5mm, mtb triple = 47.5mm or 50mm, with 5mm ring spacing the chainline of two inboard rings of an mtb triple, used as a double, should be either 45mm or 47.5mm (with recommended BB), if it is 45mm then it may well be possibly to fit a 3mm shorter BB than recommended to bring it down to 43.5mm. Will play around with 104 triple and various BBs later.

[Brucey]

IME ring spacing is nearer 8mm than 5mm; unless it is made that way the chain rubs in every small-small combination, i.e. no crosschaining rightwards at the back is possible.

cheers

[niggle]

IME ring spacing is nearer 8mm than 5mm; unless it is made that way the chain rubs in every small-small combination, i.e. no crosschaining rightwards at the back is possible.

cheers

Did a session of careful measuring (the Spa 104 is currently installed on a steel touring frame with unknown bb, with only about 1mm of clearance between the right crank centre and the bb cup)
104 triple chainline is 47.5, proposed chainline at half way between middle and inner chainring positions is 43.75mm, chainring spacing 7.5mm, Q factor = 169mm, offset 1mm to the right.

Current square taper road double chainline is 45.75mm, chainring spacing 6.5mm, Q factor = 163.5mm, offset 3mm to the left.

Front derailleur is a FD 2200*, with the 50/34 road double the cage sits about 18mm clear of the chainstay. 44T chaingring is aprox 10mm smaller radius than a 50T.

Cassette mid point chainline, between 4th and 5th sprocket, 41.5mm.

Looks encouraging apart from what the lack of counterboring on the 44T ring may result in, time to suck it and see...

(*FD 2200 seems to be identical to the Sora FD 3300 8spd apart from the outer link and clamp being black instead of silver.)