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Fitting a 'sub-compact' road chainset to BB90 bottom bracket?

Old 11-27-19, 11:08 AM
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Fitting a 'sub-compact' road chainset to BB90 bottom bracket?

Anyone got an 'easy' option (an cheap!) for fitting a sub-compact chainset (e.g. 48-32) to my Trek Domaine SL6 which I believe has BB90 press-fit bearings for Shimano Ultegra.
I will want to revert back to standard 50-34 again so I don't want to have to change the bearings if at all possible (I realize now that not getting a bike with threaded bearings was a mistake!).
I have looked at the Praxis Works Alba M30 48/32 chainset but it looks like this would require new bearings to be press-fitted: I would have to get this done and it practically rule out swapping between chainsets.
I've also looked at Absolute Black oval rings but they are expensive and a bit 'different'; anybody got any experience of them or got them for sale?
Secondly, is there a noticible difference between 32 and 34 (I am 64 and I can get a 34 on the back).
And finally, what distance would you practically get away with between the deraileur and ring - I think I am a bit limited here.
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Old 11-27-19, 06:49 PM
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I'm not sold on those oval rings. The effective gearing, when you're actually applying force to the pedals, is higher than during the dead spots of the pedal stroke. So I don't consider that a true 32T ring, even though it may have 32 teeth on it.

I'd just get a cassette that goes lower.
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Old 11-27-19, 09:57 PM
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I am not totally confident in this but I think that if an ultegra will fit that a GRX will fit. Could be wrong, best to buy from an lbs that knows what they are doing.

I assume you have a front derailleur mount on the frame. It's not too hard to find the distance spec for smaller chainrings. Once you have about 5mm space between the derailleur and the big chain ring, it starts to be a real problem
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Old 11-28-19, 01:06 AM
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I was just thinking of a 48-32 today, but I didn't realize there might be issues like needing to change bearings? Or is this specific to BB90? My bike's a PF30, and like the OP, I would also be interested in a solution that requires minimal tweaking so that I could easily go back to my current 53-39 whenever I like to.
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Old 11-28-19, 05:35 AM
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Originally Posted by atwl77 View Post
I was just thinking of a 48-32 today, but I didn't realize there might be issues like needing to change bearings? Or is this specific to BB90? My bike's a PF30, and like the OP, I would also be interested in a solution that requires minimal tweaking so that I could easily go back to my current 53-39 whenever I like to.
I like the GRX suggestion, if it would work ok??
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Old 11-28-19, 07:57 AM
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If your bottom bracket works with Ultegra it will work with GRX. You may or may not need the GRX front derailleur as the chainline is a little different.
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Old 11-28-19, 11:42 AM
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Originally Posted by shelbyfv View Post
If your bottom bracket works with Ultegra it will work with GRX. You may or may not need the GRX front derailleur as the chainline is a little different.
Yes, I've been looking into this further and I'm told that the rings are a little over 2mm further out. I'd like to hear from anybody who has tried it.
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Old 11-28-19, 12:36 PM
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Originally Posted by tomgdaly View Post
Yes, I've been looking into this further and I'm told that the rings are a little over 2mm further out. I'd like to hear from anybody who has tried it.
I answered in your other post, but briefly, it works great on my bike.
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Old 11-28-19, 08:11 PM
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I was looking at the Shimano website and the relevant part is the FC-RX810-2? Looks interesting. I have a Super Randonnee coming up in Febuary that I'll have use for...
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Old 11-29-19, 11:51 AM
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Originally Posted by atwl77 View Post
I was looking at the Shimano website and the relevant part is the FC-RX810-2? Looks interesting. I have a Super Randonnee coming up in Febuary that I'll have use for...
there is also the FC-RX600-11 which weighs a bit more

my gravel bike has a mountain crank on it with a road derailleur (CX-70). It works, even with a 2.5mm spacer behind the bb bearings. The GRX cranks are about that far outboard. The GRX or a CX-70 front derailleur would shift better on a sub-compact than a regular road derailleur, even ignoring the extra space.
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Old 12-01-19, 10:19 PM
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Originally Posted by unterhausen View Post
there is also the FC-RX600-11 which weighs a bit more
Saw that one too, but it's a 46-30 though.
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Old 12-04-19, 11:55 AM
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I put a 46 on all my road bikes. I used to have a 54-44 crank, and the 44 was great to cruise around in. So when I found out it was easy to get 46 tooth big chainrings, I moved to that. My gravel bike has a 42 tooth big chainring, and I am only rarely spun out in a downhill sprint by someone that has a 53. So I figure a 46 is fine. If I'm going to spin out a 46-11, that means I should be in a tuck or slow down.

Of course, there isn't much difference between the 31 tooth chainring with the 48 and the 30 tooth chainring with the 46, and it might be easier to adjust the front derailleur for good shifting.
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Old 12-04-19, 02:11 PM
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I'm seriously considering going sub-compact (like 42/30 or thereabouts) next time I have a crank that can do it natively. Other folks (I miss Homeyba) can make good use of monster gears for randonneuring and commuting, but they're wasted on me.
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Old 12-04-19, 09:20 PM
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46-11 is a really big gear. When the 11 tooth cog was first introduced, it was intended to be used with much smaller front chain rings. For example, many classic bikes came with a 52-14 top gear. 42-11 is a higher gear than that. I'm not sure I'm going to end up using it, but I have a spider that fits mountain chain rings to SRAM road crank arms. The thing I like about a 42/28 crank is that I can use an 11-32 cassette, and I almost never have to stand on steep hills.
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Old 12-04-19, 10:50 PM
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For me, I think 48-32 hits the sweet spot.

My bike was originally fitted with 52-36 with an 11-28 cassette. The 52 is great for the fast Sunday club rides, but I noticed on hilly audax rides (or one with plenty of rolling elevation) I tended to stick to the small ring. But I kept cross-chaining 36-11 as a result so kept shifting between big and small rings. That got annoying.

So I switched to a 53-39 in front and an 11-32 at the back, which roughly retains both high and low ends. I figure for randonneuring I would be able stick onto that 39 better rather than keep alternating between the big and small rings. In practice, it only helps a little.

In retrospect it was a bit of a stupid/rash decision to just jump into 53-39 and 11-32 on a hunch or feeling. I got smarter and now did my proper research - by plugging numbers into https://www.bikecalc.com/speed_at_cadence to see what the actual differences are.

Based on the results, I figure 48-32 is gonna be the sweet spot for me, the 48 being sufficient for top end speed on the fast club rides as well as the easier-paced randonneuring and audax rides. And then as an added bonus I'll have 32-32 for the really nasty hills that our local audax club seems to love throwing at us these days.
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Old 12-04-19, 11:53 PM
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My LBS is recommending a Croder Gearmate, so I'm gonna bring my bike over this weekend to have a look.
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Old 12-05-19, 07:12 AM
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I am trying to figure out how that works on a Shimano. The spider would seem to be in the way.
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Old 12-05-19, 12:50 PM
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Originally Posted by unterhausen View Post
I am trying to figure out how that works on a Shimano. The spider would seem to be in the way.
Since both rings are mounted on the back of the spider, they can do it a couple ways.



The chainring bolts aren't hollow, so I imagine they're using narrower ones than usual to keep then from interfering with the chain.
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Old 12-05-19, 01:35 PM
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the chainrings on that crank are always mounted to the back of the spider? Because otherwise, it would be quite tricky to fit the crank on a real frame
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Old 12-05-19, 01:50 PM
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Originally Posted by unterhausen View Post
the chainrings on that crank are always mounted to the back of the spider? Because otherwise, it would be quite tricky to fit the crank on a real frame
The stock rings would mount on either side, like normal. Perhaps the fact that the inner ring is 30T or 32T helps keep it from interfering with the chainstay. In my book, it's still a hack, when a smaller BCD would be the right tool for the job.
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Old 12-05-19, 09:33 PM
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The LBS said the only thing I needed was an FD extender, so I guess we'll see how it fits tomorrow
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Old 12-06-19, 12:32 AM
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I used the Absolute Black 46/30 chainrings to ride two hilly double centuries over the summer (Terrible Two and Alta Alpina). The low gearing was great, and I can't say I noticed the eccentricity (there was definitely a different feel for standing climbing in the beginning).

I just bought a GRX 48/31 crankset and will be switching to it in next few days. The Absolute Black chainrings and bolts will be for sale as soon as I can remove them from the spider (and I've got the original cardboard sleeves for them so they won't poke any holes in a shipping envelope).

Of note, the Absolute Black chainrings shift the chainline _IN_ while the GRX shift it _OUT_. I didn't have any problems adjusting the limit screws on my Ultegra Di2 front derailleur to work with the Absolute Black's and I'm guessing it'd work fine with the GRX chainline as well, though I have bought a GRX front derailleur just in case (and a whole new frame to go with it).
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