Subcompact gearing - 42-28 possible with T47 BB?

[NHAero]

I have an update on this, because these compatibility things are really vexing (at least to me!) and I hope that this info helps the next person. I bought the North Shore Billet 104 spider, and figured out that the best gearing for my use was 40-28 instead of 42-28, and found that Shimano made an XT crankset with the 4 arm 104 BCD and a matched 40-28 chainring set. I got both in the mail today. The Shimano rings just barely don't fit on the NSB spider - they aren't symmetrical but rather have a slight angle to where they meet the "lands". So they don't fit. It looks as though it will be very little work with a file or dremel sanding drum to remedy this - the portion of the chainring that interferes is very thin. So it's going to work, but it's just an example of how non-standard things have gotten. I'm excited to be getting close to the solution (cranks come Friday, GXP BB early next week) and again very grateful to Mark McM for all the info making this possible.

Originally Posted by Mark McM

I've been looking into this quite a bit recently, so just to add a bit more:

Snip
Another feature feature of the North Shore Billet crank is that it uses a standard diameter for the chainring "lands", so that any 104mm chainrings will fit - SRAM spiders use a larger diameter for the chainring "lands", so only SRAM (or SRAM-specific aftermarket) chainrings will fit. .

[Mark McM]

Ugh, that sucks. I've used SRAM, Truvativ and FSA chainrings on that same spider, and these fit without modification. It's kind of annoying how frequently manufacturers make deviations from industry norms only for reasons of preventing intercompatibility.

[NHAero]

Yup!
You can see how thin the material is where it contacts the lands, so I think a bit of modification will do the trick!

Originally Posted by Mark McM

Ugh, that sucks. I've used SRAM, Truvativ and FSA chainrings on that same spider, and these fit without modification. It's kind of annoying how frequently manufacturers make deviations from industry norms only for reasons of preventing intercompatibility.

[donevwil]

NHAero and Mark McM,

I'm pursuing a similar mod of a SRAM S900 GXP crankset and am a 4 bolt newbie. I'm shooting for 2x with 44 (or 42,46) outer x 28 (or 26, 30) inner and am struggling to find an appropriate chainring set while ideally avoiding NHAero's needed mods. Since you've both already done a lot of the legwork am I best going 120/80 (appropriate SRAM spider) or 104/64 (North Shore spider) for the sake of chainring availability?

Are rings labeled "outer" suitable for both inside or outside mounting, or intended only for the bash guard/3x set-up mounting location outside of the crank.

Your assistance is appreciated and thanks for all the excellent detail earlier in this thread that convinced me to give it a go.

[Mark McM]

If you plan to use no smaller than 26 tooth (for the inner chainring) and 46 for the outer chainring, then 120/80 chainrings might be best option. The SRAM 'standard' 2x10 chainrings for 120/80 are 42/28 and 39/26. Aftermarket chainrings are available in those sizes, plus 30 tooth (inner), 44, 45, and 46 tooth (outer). If you want to use smaller or larger chainrings, then 104/64 would be required. For the standard SRAM sizes, you can find chainring + spider assemblies fairly cheaply on ebay.

One difference between 120/80 and 104/64 is that 120/80 is used for double (2x10) cranks only, but 104/64 is used in both double (2x10) and triple (3x10) configurations. The double and triple 104/64 spiders are essentially the same, with the 2x10 crank mounting chainrings in the "middle" and "inner" positions, and the "outer" position can optionally be used to mount a bash ring. Most larger (42 tooth and bigger) 104 chainrings are intended to be used in the "outer" position of a triple, and the chainring holes are counterbored on the outer surface. There are a few larger chainrings specifically designed as the big chainring for a double crank (with chainring bolt holes counterbored on the inner surface), but in this case make sure any 104 chainrings labeled as "outer" are also explicitly labeled "2x10".

The one exception is that SRAM made a special 2x10 set of 104/64 chainrings in 48/32 tooth (the 32 tooth fit on the 64 BCD), but while the 32 tooth chainrings are still available, I have not found a source for the 48 tooth 2x10 chainrings. However, the 32 tooth chainring is larger in diameter than the 104 BCD, which will allow you to use a triple "outer" chainring in the "middle" position to be used in a double crank. Although the chainring bolt holes are counterbored on the wrong side and the heads of the chainring will protrude on the inner face of the chainring, the 32 tooth chainring is large enough to prevent the protruding bolts from interfering with the chain.


Also make sure any spider you get is specifically for X0/XX cranks. These will fit the profile of road cranks. X9/X7/X5 cranks are narrower, and the spiders for these cranks have a narrower inset for crankarm, and won't fit road cranks. Also make sure the spider is for GXP MTB cranks - spiders for BB30 MTB cranks have a different offset, and the chainrings will be located too far outboard.

[donevwil]

Originally Posted by Mark McM

If you plan to use no smaller than 26 tooth (for the inner chainring) and 46 for the outer chainring, then 120/80 chainrings might be best option. The SRAM 'standard' 2x10 chainrings for 120/80 are 42/28 and 39/26. Aftermarket chainrings are available in those sizes, plus 30 tooth (inner), 44, 45, and 46 tooth (outer). If you want to use smaller or larger chainrings, then 104/64 would be required. For the standard SRAM sizes, you can find chainring + spider assemblies fairly cheaply on ebay.

One difference between 120/80 and 104/64 is that 120/80 is used for double (2x10) cranks only, but 104/64 is used in both double (2x10) and triple (3x10) configurations. The double and triple 104/64 spiders are essentially the same, with the 2x10 crank mounting chainrings in the "middle" and "inner" positions, and the "outer" position can optionally be used to mount a bash ring. Most larger (42 tooth and bigger) 104 chainrings are intended to be used in the "outer" position of a triple, and the chainring holes are counterbored on the outer surface. There are a few larger chainrings specifically designed as the big chainring for a double crank (with chainring bolt holes counterbored on the inner surface), but in this case make sure any 104 chainrings labeled as "outer" are also explicitly labeled "2x10".

The one exception is that SRAM made a special 2x10 set of 104/64 chainrings in 48/32 tooth (the 32 tooth fit on the 64 BCD), but while the 32 tooth chainrings are still available, I have not found a source for the 48 tooth 2x10 chainrings. However, the 32 tooth chainring is larger in diameter than the 104 BCD, which will allow you to use a triple "outer" chainring in the "middle" position to be used in a double crank. Although the chainring bolt holes are counterbored on the wrong side and the heads of the chainring will protrude on the inner face of the chainring, the 32 tooth chainring is large enough to prevent the protruding bolts from interfering with the chain.


Also make sure any spider you get is specifically for X0/XX cranks. These will fit the profile of road cranks. X9/X7/X5 cranks are narrower, and the spiders for these cranks have a narrower inset for crankarm, and won't fit road cranks. Also make sure the spider is for GXP MTB cranks - spiders for BB30 MTB cranks have a different offset, and the chainrings will be located too far outboard.

Thanks so much. How about chainring brands to focus on? Appears I can mix and match to some degree, but matched sets, or even same mfg'r might be a challenge. Small tooth count 64 or 80 rings < 30T are a dime a dozen, ≥ 30T a bigger challenge and 44+ in a 2x (and inner position for 104) very hard to find. Blackspire has options, but isn't too clear on 2x or inner v. outer mounting position, most other aftermarketers make primarily 1x rings or < 42T 2x. TA has potential. Should I be looking into OEM rings?

Edit: OK I now see FSA lists larger 104 rings under the "Road" tab, these must be the ones you alluded to earlier.

[Tony]

Originally Posted by Mark McM

Just a few more tidbits I picked up along the way:

For 120/80 BCD spiders: The largest 120/80 chainrings that SRAM/Truvativ appears to make is 42 (120BCD) and 28 (BCD). Also note that SRAM uses special 10mm chainring bolts for 120BCD chainrings that screw directly into the chainring. TA and Stronglight, make chainrings that also have the bolts threaded directly into the chainring in sizes up to 45T. FSA makes 120BCD chainrings in 46T, 48T, 50T that use standard chainring bolt holes, and these can be fitted with standard chainring bolts/nuts. The largest 80BCD chainrings I've been able to find are 30T - but this should allow one to make the popular 46-30 chainring combination. A further caveat about the chainrings with threaded holes: The special bolts from SRAM are a fixed length, just long enough to thread through the spider into the chainring. If the chainrings need to be shifted inboard for derailleur/crankarm clearance, the bolts may not be long enough to insert chainring spacers. However, WickWerks makes aftermarket 10mm chainring bolts which are an extra 2mm long, which should allow a small amount of chainring offset. (the 80BCD chainrings use standard 8mm chainring bolts, and these bolts are available in a variety of lengths.)

104/64BCD: It appears that at one time, SRAM/Truvativ made a 48-32 chainring set for a double crank in these BCDs. While the 64BCD 32T chainring is still available, I have not been able to find the 104BCD 48T outer double chainring anywhere. Normally, the outer chainring in a 104/64BCD double crank mounts to the inner face of the spacer, and on a 104/64BCD triple crank, the middle chainring mounts on the inner face of the spider and the outer chainring mounts on the outer face of the spider. There are some 104BCD 48T chainring available that are meant as the outer chainring of a triple, which I have been experimenting with mounting on the spider inner face double crank, but I can't yet report results. When mounted this way, the chainring offset may be different, and their are no countersunk holes for the chainring nuts, which may protrude out far enough that they might interfere with the chain in some gear combinations. However, a 32T inner chainring is large enough that it completely covers the chainring nuts, so I'm hoping that there will be no interference.

And if you're wondering why I've been experimenting with tiny chainrings on a road bike, I've been setting up a bike for the New England hill climb series events. These are races up some seriously steep mountains. In particular there is Mt. Washington, which is not only the highest mountain in the Northeast of the US, but has one of the steepest continuous grades in the world - it averages 12% for 7.6 miles, with extended sections at 15%-18%. For this event I'd prefer to use a crank with the low Q factor (and low weight) of a road crank, but with a chainring as small as 22T. This will also allow me to use a standard road rear derailleur and cassette, for a compact, lightweight, and relatively simple drivetrain, and one that can be easily swapped back and forth between "regular" and "super-low" gearing with the swap of crank spider/chainrings. (Okay, I'll have to swap different length chains, too, but that's easy with a quick link, and also readjust the front derailleur.)

This is all great info, thanks! Yes, I was wondering...good luck!

[NHAero]

Mark is the font of knowledge on this, I'm just a minion experimentalist :-)
I'm at the point where I've just switched Shimano 11 speed hydros onto the bike (finished the brake install last night.) A Wolftooth Roadlink DM shows up today. The set-up will be:
- SRAM Force22 crankset with North Shore Billet 104/64 spider
- Shimano XT 2x10 40-28 chainring set, the large ring filed to fit on the spider
- Shimano 10 speed CX70 top pull front derailleur
- Ultegra R8000GS rear derailleur (works but hoping the Roadlink is an improvement)
- XT 11-40 11 speed cassette
- Shimano ST-RS685 shifters/brake levers
- Shimano BR-RS785 calipers
- Whisky No. 7 12F bars
- 26" wheels running Conti RaceSport SpeedKing 26x2.2 tires

Brakes are great. Front shifting on the stand is fine - still not sure if I will have some cage rub in an extreme gear, if I do, it will be set so it happens on the 28/40, which won't get a lot of use! In fact, the scheme is essentially a 1x, because low gear is 40/40, which on the road is fine, but I have a whole second lower range for trails. Rear shifting is OK, but expecting improvement from the Roadlink DM. Will report back when it's all working!

[Mark McM]

Originally Posted by donevwil

Thanks so much. How about chainring brands to focus on? Appears I can mix and match to some degree, but matched sets, or even same mfg'r might be a challenge. Small tooth count 64 or 80 rings < 30T are a dime a dozen, ≥ 30T a bigger challenge and 44+ in a 2x (and inner position for 104) very hard to find. Blackspire has options, but isn't too clear on 2x or inner v. outer mounting position, most other aftermarketers make primarily 1x rings or < 42T 2x. TA has potential. Should I be looking into OEM rings?

Edit: OK I now see FSA lists larger 104 rings under the "Road" tab, these must be the ones you alluded to earlier.

OEM chainrings tend to target specific standard usages (i.e. road or MTB), so the chainrings tend to be either large (road) or small (MTB), whereas a sub-compact tends to be somewhere in the middle, so OEMs may not always have the chainring sizes best adapted to a sub-compact. Aftermarket chainring suppliers tend to offer a wider range of sizes than OEM, so they may provide a better alternative. In particular, TA Specialites and Stronglight offer a good range of sizes, and offer matched pairs. The TA Specialites US distributor is Peter White Cycles, who carries a wide range of sizes. Stronglight has less availability in the US. Peter White Cycles also carries some Stronglight, but European on-line retailers often have a wider range of both TA Specialites and Stronglight chainrings than US retailers (and usually at better prices, too). Two European retailers with a good variety of Stronglight and TA Specialites chainrings are Starbike in Germany and XXCycle in France. A number of UK retailers also carry these chainrings.

As far as the larger chainrings from FSA, they have two varieties: Their 104 BCD chainrings are much like other brands 104 chainrings, where the larger chainrings (42+ tooth) are intended as the outer chainring for triples MTB cranks. However, some of the double road cranks use 120 BCD for their outer rings, and these can be mounted to an SRAM 120/80 MTB spider (using standard two-piece chainring bolts for single chainrings). FSA makes 120 BCD chainrings in 46, 48, 50, and 52 teeth. Sadly, FSA does not make 80mm BCD inner chainrings to match the 120mm BCD outers, but instead uses their proprietary 90mm BCD (smallest chainring size 30 tooth).

[NHAero]

Mark, are there conditions in which an outer triple is not a good fit for the inner-mounted position of the 2x spiders we have been discussing? I'm not thinking of the counterbore, but rather that how the teeth are located laterally on the thickness of the ring.

One apparent advantage of the inside mounting location on these spiders seems to be that the large ring could be shimmed inwards to reduce the spacing (11 speed is 0.5mm narrower?) and also to help with the cage rub issue. Do you know a good source for a variety of these washers? I expect they are in the 0.5 to 1.5mm range.

Originally Posted by Mark McM

OEM chainrings tend to target specific standard usages (i.e. road or MTB), so the chainrings tend to be either large (road) or small (MTB), whereas a sub-compact tends to be somewhere in the middle, so OEMs may not always have the chainring sizes best adapted to a sub-compact. Aftermarket chainring suppliers tend to offer a wider range of sizes than OEM, so they may provide a better alternative. In particular, TA Specialites and Stronglight offer a good range of sizes, and offer matched pairs. The TA Specialites US distributor is Peter White Cycles, who carries a wide range of sizes. Stronglight has less availability in the US. Peter White Cycles also carries some Stronglight, but European on-line retailers often have a wider range of both TA Specialites and Stronglight chainrings than US retailers (and usually at better prices, too). Two European retailers with a good variety of Stronglight and TA Specialites chainrings are Starbike in Germany and XXCycle in France. A number of UK retailers also carry these chainrings.

As far as the larger chainrings from FSA, they have two varieties: Their 104 BCD chainrings are much like other brands 104 chainrings, where the larger chainrings (42+ tooth) are intended as the outer chainring for triples MTB cranks. However, some of the double road cranks use 120 BCD for their outer rings, and these can be mounted to an SRAM 120/80 MTB spider (using standard two-piece chainring bolts for single chainrings). FSA makes 120 BCD chainrings in 46, 48, 50, and 52 teeth. Sadly, FSA does not make 80mm BCD inner chainrings to match the 120mm BCD outers, but instead uses their proprietary 90mm BCD (smallest chainring size 30 tooth).

[AngryScientist]

there is some excellent info disseminated in this thread, linked with the one i started a few months ago here:

https://forums.thepaceline.net/showt...ht=big+waiting

Though, the lengths of the threads and the paragraphs in those threads only proves the issue - more and more people are interested in non "standard" (53/39 or 50/34) gearing these days, and these home brew solutions are cumbersome at best. scouring the internet and UK suppliers for suitable parts may be OK for us bike nerds, but too cumbersome for most people to want to bother with.

i'm reasonably certain that shimano and/or sram will see the light and offer some expanded gear ratio offerings (at the chainrings, not just the cassette) soon.

please, and thank you!

[R3awak3n]

Originally Posted by AngryScientist

there is some excellent info disseminated in this thread, linked with the one i started a few months ago here:

https://forums.thepaceline.net/showt...ht=big+waiting

Though, the lengths of the threads and the paragraphs in those threads only proves the issue - more and more people are interested in non "standard" (53/39 or 50/34) gearing these days, and these home brew solutions are cumbersome at best. scouring the internet and UK suppliers for suitable parts may be OK for us bike nerds, but too cumbersome for most people to want to bother with.

i'm reasonably certain that shimano and/or sram will see the light and offer some expanded gear ratio offerings (at the chainrings, not just the cassette) soon.

please, and thank you!

I love that you don't mention campy because you know they are not going to do it.

[Mark McM]

Originally Posted by NHAero

Mark, are there conditions in which an outer triple is not a good fit for the inner-mounted position of the 2x spiders we have been discussing? I'm not thinking of the counterbore, but rather that how the teeth are located laterally on the thickness of the ring.

One apparent advantage of the inside mounting location on these spiders seems to be that the large ring could be shimmed inwards to reduce the spacing (11 speed is 0.5mm narrower?) and also to help with the cage rub issue. Do you know a good source for a variety of these washers? I expect they are in the 0.5 to 1.5mm range.

Well, the chainring bolt counterbore might actually be the biggest problem. If the heads of the chainring bolts stick out, they might snag the chain in certain gear combinations, causing the chain to skip. The smaller the chainring size (a smaller difference between the teeth and the bolt diameter), the bigger the problem this may be.

As you say, the teeth are usually offset from the chainring faces. Outer triple chainrings are intended to mount to the outer face of the spider, but when used as the outer chainring of an MTB double, they mount to the inner face of the spider. There's no standard on chainring thickness - some are machined out of thick plates, others are stamped out of thin plates. This can result in varying amounts of offset to the teeth. But as you say, chainring spacers might be able to help to adjust the offset of the chainrings. Wheels Manufacturing makes chainring spacers in a variety of sizes, although oddly the smallest increment is 0.6mm (0.015").

[AngryScientist]

Originally Posted by R3awak3n

I love that you don't mention campy because you know they are not going to do it.

correct.

campy is still very much a traditional road racing oriented company. it is what it is.

[donevwil]

Originally Posted by AngryScientist

there is some excellent info disseminated in this thread, linked with the one i started a few months ago here:

https://forums.thepaceline.net/showt...ht=big+waiting

Though, the lengths of the threads and the paragraphs in those threads only proves the issue - more and more people are interested in non "standard" (53/39 or 50/34) gearing these days, and these home brew solutions are cumbersome at best. scouring the internet and UK suppliers for suitable parts may be OK for us bike nerds, but too cumbersome for most people to want to bother with.

i'm reasonably certain that shimano and/or sram will see the light and offer some expanded gear ratio offerings (at the chainrings, not just the cassette) soon.

please, and thank you!

All exacerbated if riding 177.5s and 180s. I fortunately found a used set of White Industries 180s a few years ago and have now decided a second set is warranted, but with used being so scarce and both new WI and Rene Herse solutions being so expensive I'm attempting a 177.5 SRAM S900 retrofit. If any one part (crank, spider, chainrings) has to be sourced new that'll pretty much blow the financial viability. Side benefit being the reasonable Q factor of these SRAM cranks vs WI.

I really do appreciate both threads, worst case scenario I run a less than ideal ring pairing until the right rings materialize. This GXP solution is my alpine swap for the Potts.