Subcompact gearing - 42-28 possible with T47 BB?

[owly]

Originally Posted by 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

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I'm doing a 10 speed 2x using the Easton EC90SL crank arms and spindle, with the Race Face 2x 'cinch' spider (104/64) using a 42/27 and 11-28.

[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

[NHAero]

Cool!
What is the q factor of that set-up?
Whose chainrings?
Is 11 speed possible with this set-up?

Originally Posted by owly

I'm doing a 10 speed 2x using the Easton EC90SL crank arms and spindle, with the Race Face 2x 'cinch' spider (104/64) using a 42/27 and 11-28.

[NHAero]

What a load of experience and knowledge! Thanks folks - trying to digest it all!

[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

[d_douglas]

Originally Posted by owly

I'm doing a 10 speed 2x using the Easton EC90SL crank arms and spindle, with the Race Face 2x 'cinch' spider (104/64) using a 42/27 and 11-28.

So, to my earlier question, would this setup work on an 11spd drivetrain?

[owly]

Originally Posted by NHAero

Cool!
What is the q factor of that set-up?
Whose chainrings?
Is 11 speed possible with this set-up?

149mm q-factor.

Rings: any to suit bcd, I suppose. At the moment looking at a few options on the r2-bike site.

11speed?
http://imageshack.com/i/plsOlSw2p
^ SpeedyChix setup with 38/28. Can't quite make out from the pic if 11speed rings.
Edit #2:- 10speed rings in that pic. However the builder thinks they also did the same on another bike with 11speed rings.

--------------------------------------------------------------------------------------------------------------------------------------------

Edit: Ended up going with 10/11sp Chinook rings in 40/26.
(Note: their new 42t ring has two options for spider-tab-width).
Easton EC90SL crank with RaceFace 2x(10sp) spider.

I prefer Wolftooth chainring bolts as they have hex fittings on both ends.
The flat head/end of a set sits down nice and flush with the countersunk bolt holes on the inner side of the rings.

[NHAero]

Trying to put all this info together:
- On 135 and 142 OLD hubs, an 11s cassette has a 44mm chainline
- Looking at the SRAM Frame Fit PDF pointed to by Mark McM, Sram Red and Force road cranks have a chainline about 45mm (diagram is dimensioned to the inner face of the chainrings so I added 1 mm).
- Changing these cranks to a 120/80mm spider allows mounting of a 42T/28T pair that is a stock combo on SRAM 2x10 (either from SRAM or aftermarket)
- If current Shimano road double cranksets also have a +/-45mm chainline, it's possible that the Ultegra 11s front derailleur and Shimano 11s shifter (RS685, R8000, etc.) can shift this chainring set-up, which has a 14T differential, since Shimano makes the Ultegra crankset in 50-34, 52-36, 53-39, and 46-36, a range of 10T to 16T differential. The chain guide shape may not be optimal for the small diameter 42T chainring, and the bottom of the chain guide may hit the top of the chainstay.
- Getting the SRAM crankset with a GXP BB allows it to be fit to a 68mm BSA BB shell.

Did I get this right?

[jtbadge]

With a T47 shell, you would also be able to run a BB30/PF30 SRAM crank.

The Shimano FD should work with that chainring gap - if you can mount it low enough to align properly above the big chainring.

[NHAero]

Thank you. I'm generalizing the question/solution to other, older bikes. One of the main reason I have been looking at the Hakka MX is my frustration with my efforts to get the low gearing I want on the BSA BB bikes I own, both of which has threaded 68mm BB shells. The SRAM cranks come in the GXP variation and with 30mm spindles so it seems generalizable across multiple platforms, and indeed Mark's example of his own bike was GXP.

Originally Posted by jtbadge

With a T47 shell, you would also be able to run a BB30/PF30 SRAM crank.

The Shimano FD should work with that chainring gap - if you can mount it low enough to align properly above the big chainring.

[Mark McM]

A few notes:

It should be noted while generally all the Force 22 cranksets have removable spiders, not all the Red cranksets do. In particular, the Red Exogram cranksets have one piece crankarm-spiders.

If your frame uses a braze-on front derailleur, you may not be able to get the derailleur low enough to shift a 42-28. However, you might be able to get the derailleur lower with a WickWerks Fit-Link, which allows the front derailleur to be mounted up to 2 cm lower.

Depending on derailleur shape, you may have an issue with clearance between crankarm inner face and the derailleur cage out plate. Because the crank arm is angled outward from spindle to pedal, there is less clearance between chainrings and crank arm as the chainrings get smaller. Both chainrings are mounted to the inner face of the 120/80 crank spider, so you may get more room by installing chainring spacers, to offset the chainrings inward. This will also have the affect of reducing the chainline offset. The outer 120 mm BCD chainrings are held by bolts that thread directly into the chainring, so you may need longer bolts to accommodate the thickness of the chainring spacers. Longer 120mm BCD bolts are also available from WickWerks.

Since this gets into the territory beyond which the crank/derailleur was originally designed to work, the results may or may not be satisfactory. I've gotten a Campagnolo shifter/derailleur to work adequately with a 44-29 crankset, but I've never tried the combination you're suggesting. My experience with 42 and 38 outer chainrings has thus far only been on the stand in the work shop. It seemed to work, but I haven't tried shifting under load yet.

[Mark McM]

I just had another thought, about dimensions you might want to verify: The inner chainrings on a road double often come very close to overlapping the BB shell. With standard road BCDs (110mm, 130mm), there's plenty of space for the chainring to overlap the BB shell and external bearing cups. But with a 80mm BCD chainring, how space is there with the outer diameter of a T47 BB shell and bearing cup? With a 64mm BCD chainring, it might very well be a no-go (I've only tried a 64mm BCD chainring with a standard 68x1.370 ISO BB shell).

What's the actual outer diameter of a T47 BB shell, and what's the offset of the outer face of the BB cup?

[NHAero]

Thanks so much, Mark, much appreciated.
Because my Anderson frame has a braze-on tab, I own the Wickwerks Fit-Link. There's a 44T ring on there now, on the outer and middle positions of a Sugino AT triple, and the CX70 10s FD shifts it reasonably well even though it is mounted too high. The 44T is too large for the Fit-Link, but a 42T will allow its use.
I noticed the variation on the Red cranks, thank you.

At some point I know I have to bite the bullet and try this. The 20 year Litespeed I have uses a top pull clamp-on FD so I need to find info on cable pull on the Shimano 11s front shifter and its compatibility with various FDs on the market. Do you know where that cable pull info, for SRAM and Shimano, can be found? I can find it for RDs.

Originally Posted by Mark McM

A few notes:

It should be noted while generally all the Force 22 cranksets have removable spiders, not all the Red cranksets do. In particular, the Red Exogram cranksets have one piece crankarm-spiders.

If your frame uses a braze-on front derailleur, you may not be able to get the derailleur low enough to shift a 42-28. However, you might be able to get the derailleur lower with a WickWerks Fit-Link, which allows the front derailleur to be mounted up to 2 cm lower.

Depending on derailleur shape, you may have an issue with clearance between crankarm inner face and the derailleur cage out plate. Because the crank arm is angled outward from spindle to pedal, there is less clearance between chainrings and crank arm as the chainrings get smaller. Both chainrings are mounted to the inner face of the 120/80 crank spider, so you may get more room by installing chainring spacers, to offset the chainrings inward. This will also have the affect of reducing the chainline offset. The outer 120 mm BCD chainrings are held by bolts that thread directly into the chainring, so you may need longer bolts to accommodate the thickness of the chainring spacers. Longer 120mm BCD bolts are also available from WickWerks.

Since this gets into the territory beyond which the crank/derailleur was originally designed to work, the results may or may not be satisfactory. I've gotten a Campagnolo shifter/derailleur to work adequately with a 44-29 crankset, but I've never tried the combination you're suggesting. My experience with 42 and 38 outer chainrings has thus far only been on the stand in the work shop. It seemed to work, but I haven't tried shifting under load yet.

[NHAero]

I don't know all the answers to your good questions. Ibis publishes a short guide to the Hakka MX that tells which T47 BB goes with which crankset:

https://s3.amazonaws.com/ibiscycles....ka_T47_BBs.pdf

Some require "innies" and some work with external cups.

Originally Posted by Mark McM

I just had another thought, about dimensions you might want to verify: The inner chainrings on a road double often come very close to overlapping the BB shell. With standard road BCDs (110mm, 130mm), there's plenty of space for the chainring to overlap the BB shell and external bearing cups. But with a 80mm BCD chainring, how space is there with the outer diameter of a T47 BB shell and bearing cup? With a 64mm BCD chainring, it might very well be a no-go (I've only tried a 64mm BCD chainring with a standard 68x1.370 ISO BB shell).

What's the actual outer diameter of a T47 BB shell, and what's the offset of the outer face of the BB cup?