Subcompact gearing - 42-28 possible with T47 BB?

[NHAero]

I know we had a long thread on subcompact gearing. The best options all seemed to have square taper BBs. As I contemplate a frame with the T47 BB, is there a crankset that I can run 42-28 on?
Thanks

[R3awak3n]

yersir there is

White Industries - http://www.whiteind.com/t47

I think for that kind of gearing that will be your only choice as of right now.

Personally though, I think I would prefer something like 46-30 and you will have some more choices on that. I think if you pair that with a 11-34T in the back you can pretty much climb anything.

[wkeller79]

Originally Posted by R3awak3n

yersir there is

White Industries - http://www.whiteind.com/t47

I think for that kind of gearing that will be your only choice as of right now.

Personally though, I think I would prefer something like 46-30 and you will have some more choices on that. I think if you pair that with a 11-34T in the back you can pretty much climb anything.

This is exactly what I'm planning as well. I'm currently in queue for a build from Horse Cycles, and Thomas suggested the T47 / White Industries crankset. Will be matching it to Campagnolo Record drivetrain.

Everything but the bottom bracket will be lugged

[Mark McM]

T47 BBs are available for 24mm and 30mm spindles, which are the most common integrated sizes for cranks with integrated spindles, so there are many models of cranks that will fit. Some of these cranks will fit subcompact chainrings. Here are a few:

Sugino OX series cranks. These aluminum cranks arms with 24mm spindles have a 110mm outer BCD and 74mm inner BCD. Chainrings in a wide range of sizes are available, including 42-28.

SRAM XX 2x cranks. These carbon cranks are actually intended for MTBs, but the Q156 version may work okay for road bikes as well, due to their 156mm Q factor. These are available with either 24mm or 30mm spindles, and have a 42-28 chanring option.


I'm currently setting up a bit of a "Frankenbike" drivetrain that will allow a huge range of chainring sizes (anything from 34-22 to 53-39) with a low Q-factor road crank. As it happens, SRAM uses the same 3 bolt spider mount on several of their road and MTB cranks, and spiders are available in a wide range of BCDs, including 130mm, 110mm, 120mm/80mm, and 104mm/64mm. I'll be using a Force22 road crank (carbon crank with a Q factor of 146mm) as the base, and a variety of front derailleurs depending on chainring size (actuated with Campagnolo Ultra-Shift shifter, which is front derailleur agnostic).

[NHAero]

Mark, are you saying you can get a Force22 road crankset and mount a 104/64 spider on it? And if so, does SRAM make the 42-28 rings to hang on it?

Originally Posted by Mark McM

T47 BBs are available for 24mm and 30mm spindles, which are the most common integrated sizes for cranks with integrated spindles, so there are many models of cranks that will fit. Some of these cranks will fit subcompact chainrings. Here are a few:

Sugino OX series cranks. These aluminum cranks arms with 24mm spindles have a 110mm outer BCD and 74mm inner BCD. Chainrings in a wide range of sizes are available, including 42-28.

SRAM XX 2x cranks. These carbon cranks are actually intended for MTBs, but the Q156 version may work okay for road bikes as well, due to their 156mm Q factor. These are available with either 24mm or 30mm spindles, and have a 42-28 chanring option.


I'm currently setting up a bit of a "Frankenbike" drivetrain that will allow a huge range of chainring sizes (anything from 34-22 to 53-39) with a low Q-factor road crank. As it happens, SRAM uses the same 3 bolt spider mount on several of their road and MTB cranks, and spiders are available in a wide range of BCDs, including 130mm, 110mm, 120mm/80mm, and 104mm/64mm. I'll be using a Force22 road crank (carbon crank with a Q factor of 146mm) as the base, and a variety of front derailleurs depending on chainring size (actuated with Campagnolo Ultra-Shift shifter, which is front derailleur agnostic).

[BikeNY]

Originally Posted by NHAero

Mark, are you saying you can get a Force22 road crankset and mount a 104/64 spider on it? And if so, does SRAM make the 42-28 rings to hang on it?

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.

[NHAero]

Thanks much, I have some research to do!

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.

[d_douglas]

Interested in this too, but not sure what gearing I would want. It would be for an 11spd di2 bike.

Could I track down an XTR M985 10spd triple crankset and just drop the small ring, making it a 42x32 setup?

[NHAero]

Are these SRAM chainrings compatible with 11 speed drivetrains?
And what is the intended chainline measurement?

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.

[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".

[Kirk007]

The new Easton cranks have 46-30, 47-33, 50-34, 52-36, 52-39 - chainring combos all of which are interchangeable on the same crankarm and with multiple bb options. Crazy light to boot

Sent from my Pixel 2 using Tapatalk

[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.

[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.

[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.)