Subcompact gearing - 42-28 possible with T47 BB?

[Mark McM]

Originally Posted by BikeNY

For the SRAM cranks, the spider that runs 42/28 is not 104/64, it's 120/80. And yes, it will mount to any SRAM crank that uses the direct mount standard. Actually there are 2 different 120/80 spiders, one for aluminum crank arms and one for carbon crank arms. I'm not sure about the difference between GXP and BB30 cranks though.

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

As noted, the 120/80 spider normally is the one to use for 42-28 chainrings. However, there are a number of manufacturers who make 104/64 BCD chainrings, and there are some 42-28 chainrings in this BCD.

Aluminum vs. carbon crank spiders: SRAM's Carbon cranks have a larger profile than their aluminum cranks, so the relief cutout in the spider for the carbon crank-arm is larger than for aluminum cranks. Aluminum cranks will work with the "carbon" spiders, resulting in gap in the relief cutout around the crank; but carbon cranks will not work with "aluminum" spiders because the relief cutout is too small to fit the carbon crankarm. SRAM road and MTB cranks have the same profiles, so a "carbon" MTB spider mates perfectly with a carbon road cranks (vice versa), and an "aluminum" MTB spider mates perfectly with aluminum road cranks (and vice versa). There is a caveat for the 104/64 SRAM MTB spiders, however: On these spiders there are two small raised nubs on the face that mates with the crank where the 64mm threaded holes are. MTB cranks have corresponding divots to accommodate these nubs, but road cranks do not. However, the nubs can be ground down to mate with road cranks, with no affect on function. It should be added here that North Shore Billet makes aftermarket SRAM compatible spiders for 104/64 that do not have these nubs, and mates perfectly with road cranks without modification. 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. North Shore Billet 104/64 SRAM compatible spiders come in different versions for GXP and BB30 (see below), and different versions for carbon and aluminum crankarms (labeled "X0" and "X9", respectively). for my SRAM Force 22 cranks, the GXP XO version is the correct version.

GXP vs. BB30 spiders: SRAM's BB30 MTB cranks are narrower at the spindle than their GXP MTB cranks, so the spider mounting surface is more inboard. Consequently, in order to maintain the same chainline, the BB30 spiders have offset the chainrings a few millimeters more outboard of the spider mating surface than the GXP spiders. SRAM road cranks use the same width between cranks for both GXP and BB30, so the road (130mm and 110mm BCD) spiders have the same chainring offset for GXP and BB30 cranks. For double chainring spiders, the GXP version of the MTB (120/80 and 104/64) spiders have almost exactly same chainring offset as the road spiders, so substituting an MTB spider on a road crank will give nearly the same chainline.

11 speed: SRAM MTB spiders are available in a 2x10 version, with a C-C dimension between chainrings of about 8 mm. SRAM Road 2x11 spacing is about 7.5mm C-C between chainrings. In most cases, this will be close enough.

Some caveats: Like most cranks, SRAM cranks splay outward from the spindle to the pedal. This means that as the front derailleur is lowered (for smaller chainrings), there will be less clearance between the derailleur cage and the back of the crank arm. When lowered for the smallest chainrings, the derailleur may actually scrape the crank. To mitigate this, chainring spacers may be used to shift the chainrings inboard. The practicality of this solution may vary, depending on frame clearances, front derailleur movement range, affects on chainline, etc. I have tried this on one particular crank, chainring, frame and front derailleur combination with some amount of success, but I can't guarantee that it will work with all chainring, frame, drivetrain and derailleur combinations. I should also note that I am using Campagnolo Ultra-Shift shifters, which are micro-indexing and adaptable to wide range of front derailleurs - with other shifters, you may be stuck with one specific type of front derailleur, which may not be compatible with subcompact chainrings.

For more information on some of the dimensions and offsets of SRAM cranks and spiders, see the SRAM Frame Fitting Specifications documents. These can be found on the SRAM website, or by googling "SRAM Frame Fitting Specifications".

[BikeNY]

Originally Posted by Mark McM

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

As noted, the 120/80 spider normally is the one to use for 42-28 chainrings. However, there are a number of manufacturers who make 104/64 BCD chainrings, and there are some 42-28 chainrings in this BCD.

Aluminum vs. carbon crank spiders: SRAM's Carbon cranks have a larger profile than their aluminum cranks, so the relief cutout in the spider for the carbon crank-arm is larger than for aluminum cranks. Aluminum cranks will work with the "carbon" spiders, resulting in gap in the relief cutout around the crank; but carbon cranks will not work with "aluminum" spiders because the relief cutout is too small to fit the carbon crankarm. SRAM road and MTB cranks have the same profiles, so a "carbon" MTB spider mates perfectly with a carbon road cranks (vice versa), and an "aluminum" MTB spider mates perfectly with aluminum road cranks (and vice versa). There is a caveat for the 104/64 SRAM MTB spiders, however: On these spiders there are two small raised nubs on the face that mates with the crank where the 64mm threaded holes are. MTB cranks have corresponding divots to accommodate these nubs, but road cranks do not. However, the nubs can be ground down to mate with road cranks, with no affect on function. It should be added here that North Shore Billet makes aftermarket SRAM compatible spiders for 104/64 that do not have these nubs, and mates perfectly with road cranks without modification. 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. North Shore Billet 104/64 SRAM compatible spiders come in different versions for GXP and BB30 (see below), and different versions for carbon and aluminum crankarms (labeled "X0" and "X9", respectively). for my SRAM Force 22 cranks, the GXP XO version is the correct version.

GXP vs. BB30 spiders: SRAM's BB30 MTB cranks are narrower at the spindle than their GXP MTB cranks, so the spider mounting surface is more inboard. Consequently, in order to maintain the same chainline, the BB30 spiders have offset the chainrings a few millimeters more outboard of the spider mating surface than the GXP spiders. SRAM road cranks use the same width between cranks for both GXP and BB30, so the road (130mm and 110mm BCD) spiders have the same chainring offset for GXP and BB30 cranks. For double chainring spiders, the GXP version of the MTB (120/80 and 104/64) spiders have almost exactly same chainring offset as the road spiders, so substituting an MTB spider on a road crank will give nearly the same chainline.

11 speed: SRAM MTB spiders are available in a 2x10 version, with a C-C dimension between chainrings of about 8 mm. SRAM Road 2x11 spacing is about 7.5mm C-C between chainrings. In most cases, this will be close enough.

Some caveats: Like most cranks, SRAM cranks splay outward from the spindle to the pedal. This means that as the front derailleur is lowered (for smaller chainrings), there will be less clearance between the derailleur cage and the back of the crank arm. When lowered for the smallest chainrings, the derailleur may actually scrape the crank. To mitigate this, chainring spacers may be used to shift the chainrings inboard. The practicality of this solution may vary, depending on frame clearances, front derailleur movement range, affects on chainline, etc. I have tried this on one particular crank, chainring, frame and front derailleur combination with some amount of success, but I can't guarantee that it will work with all chainring, frame, drivetrain and derailleur combinations. I should also note that I am using Campagnolo Ultra-Shift shifters, which are micro-indexing and adaptable to wide range of front derailleurs - with other shifters, you may be stuck with one specific type of front derailleur, which may not be compatible with subcompact chainrings.

For more information on some of the dimensions and offsets of SRAM cranks and spiders, see the SRAM Frame Fitting Specifications documents. These can be found on the SRAM website, or by googling "SRAM Frame Fitting Specifications".

Wow, Mark, lots of great info in that post! I'm saving this for future reference, as I like to play around with this stuff as well.

[AngryScientist]

agreed, i've been thinking of experimenting with a different set-up next year, thanks to Mark for compiling that info.

[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.)

[monarchguy]

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.)

Mark,

Is this hillclimb series an official set of events and, if so, where would I find info on them? Thanks,

-- Dan

[Mark McM]

Originally Posted by monarchguy

Is this hillclimb series an official set of events and, if so, where would I find info on them? Thanks,

For the BUMPS series (Bike Up Mountain Point Series):

https://www.facebook.com/BikeUpTheMountainPointSeries/


For individual hillclimb races:

https://www.bikereg.com/events/Hill-Climb


It's a bit early in the season, so I don't think there's much cast in stone for this year's series yet.

[monarchguy]

Originally Posted by Mark McM

For the BUMPS series (Bike Up Mountain Point Series):

https://www.facebook.com/BikeUpTheMountainPointSeries/


For individual hillclimb races:

https://www.bikereg.com/events/Hill-Climb


It's a bit early in the season, so I don't think there's much cast in stone for this year's series yet.

Great, thanks. I did Whiteface last year and would like to try some others. Appreciate the info.

-- Dan

[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!

[bikinchris]

Yes, you can get T47 bottom brackets with bearings to fit GXP spindles from Wheels Mtg. I run a SRAM XX carbon double crank with 42/28 rings is what I use with the 156q factor and GXP spindle, but it's available in BB30 also.

[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.