While today’s gravel bikes see a range of component and gearing options, ranging from compact road to subcompact wide gravel, this was not always so. Just a few years ago, gravel road bikes all included a “compact” 50/34 crankset that was the same crankset as a dedicated pavement road bike had. In 2017 Praxis Works introduced a crankset offering 48/32 chainrings to help gravel road riders get a lower climbing gear. Today, most manufacturers have introduced road 2X cranksets offering even lower gearing with a 46/30 combination.
In a previous article I did a brief analysis of what I felt are the advantages and considerations of subcompact 2X crankset gearing of a 46-tooth large/30-tooth small chainring. For many riders, especially those who spend a lot of time on gravel, the 50T chainring doesn’t serve much purpose. Maybe a handful of riders can use a 50-11, or even a 50-12, combination on gravel roads. In my view, in addition to the wider chainring position that allows for wider tire clearance, the lack of functionality for the highest gearing and the desire for lower gearing on gravel in particular is the main reason why this subcompact combination is becoming so common.
I also cannot ignore the impact of Shimano’s release of its two gravel specific GRX drivetrains, one with a 46/30 option, the other with a 48/31. Many cycling websites, including this one, have written about the Shimano GRX groupsets, and I really cannot add anything of value. Suffice it to say GRX has been praised for expanding the gearing options riders have. Shimano is not alone in providing such gearing. Other manufacturers are offering the 46/30 chainring combination that is quickly becoming commonplace. FSA’s SL-K and Rotor’s Aldhu cranksets are two options. This article discusses these two cranksets in more detail.
Why I Wanted Subcompact Gearing
My wife is an excellent climber. However, given the loose, rough surfaces found on some rather steep climbs on Vermont’s dirt roads, and the fact that we are aging, she wanted a bit lower gearing on her gravel road bike than offered by her “compact” crank. As for me, I have always climbed like a stone, regardless of my physical condition, so a lower gear to tackle such climbs is very appealing to me. I therefore began to investigate the various alternatives for subcompact gearing for our gravel bikes.
My wife’s gravel bike has a traditional threaded BSA bottom bracket shell and internal Shimano Di2 wiring, and the bike was built with a Shimano crankset with a 24mm crank axle. Without some major modification to the Di2 wiring (that would be severely frowned upon by Shimano) there is no room for a 30mm diameter crank axle and three e-Tube wires. This limited the choices for her bike to cranks with a 24mm diameter axle. Other than Shimano’s fine shifting, but somewhat heavy, GRX cranks there are few manufacturers offering a 24mm diameter crank axle with 46/30 chainrings.
Rotor Aldhu 24 DM Subcompact Crankset
One 24mm equipped axle alternative is Rotor’s Aldhu 24 DM. The Rotor Aldhu DM line of cranks is one of the most versatile on the market. DM stands for “Direct Mount”; the chainrings do not mount to a spider, such as on a Shimano crank. Instead, the chainrings and spider are a single piece machined out of a block of 7075 aircraft-grade aluminum. This combination then mounts directly to the splined crank axle. This is one of those situations where the picture may be worth a thousand words, so check it out…
Possible chainring combinations on the Rotor Aldhu 24 DM are 46/30, 48/32, 50/34, 52/36, and 53/39, in ovalized Q-Rings or round chainrings. The Rotor Aldhu cranks come in millimeter lengths of 150, 155, 160, 165, 170, 172.5, and 175. You can select from a 30mm or 24mm diameter axle. Talk about flexibility! You can mix and match these to get the exact crankset you want. Want a 155mm crank arm with a 53/39 gear combination with a 24mm axle? No problem. How about a 175mm crank arm with a 46/30 ovalized chainring combination with a 30mm axle? Of course.
Rotor Power Meter
Want a power meter? Rotor offers them, but the options are different depending on whether you are using the 30mm or 24mm version of the crankset. On the 30mm version, there is an available axle based power meter system called INPower. INPower is available in single sided or two sided versions. While there is not a spindle mounted power meter available for DM (Direct Mount), Rotor does offer INspider. INspider takes the place of the DM system and replaces it with a spider that houses a power meter and takes standard 110×4 chainrings.
From crank length to gearing and chainring shapes, to spindle options and power measurement, Rotor may offer more options in their modular array than any other crank company.
Rotor Aldhu 24 DM Subcompact Crank Weight & Cost
I weighed the Rotor Aldhu 24 DM crank arms, crank axle and chainrings, and they totaled 653 grams. This is the actual weight, not a manufacturer’s claimed weight. At the time of writing, the total cost with round chainrings is $575. Oval Q-Rings are an extra $24.
Considerations on the Rotor Aldhu DM Crankset
The only potential functional drawback that I see on the Rotor Aldhu DM is that if you wear out a chainring, you must replace the whole spider/chainring unit; the chainrings are a single piece. This being said, if you keep your chain lubed, replace the chain in a timely fashion before it is thoroughly worn out, and generally keep your entire drivetrain clean, you should get many years of riding out of the chainrings.
Otherwise, the Rotor is significantly more expensive than the Shimano offerings, but provides a lengthy list of features and combinations that Shimano does not in exchange for the additional dollars.
Rotor Aldhu 24 DM Installation
Installation of the Rotor Aldhu 24 crankset was relatively straightforward. However, unless you are experienced in such matters, and have the proper tools, it is better left to a qualified mechanic.
I ran into a big piece of good fortune when the Rotor Aldhu 24 DM crank axle fit the bottom bracket cups on my wife’s bike perfectly in all regards. Not needing to replace a bottom bracket can simplify everything. After installation of the crankset on the bike, I needed to adjust the left crank arm bearing pre-load, lower the front derailleur, and fine tune the derailleur stops.
Going from a 50/34 combination to a 46/30 also meant that the existing chain was too long and needed to be shortened. The chain still had a lot of life so I simply removed one inner and one outer link.
FSA SL-K Carbon Modular Adventure BB386 Evo Subcompact Crankset
Unlike my wife’s bike, the bike I wanted to put a subcompact crankset on for myself can accept a 30mm crank axle. There are more choices of 46/30 cranksets for the bigger 30mm axle. While the Rotor crankset was tempting for my use as well, I ended up purchasing the FSA SL-K Modular Adventure BB386 EVO crankset.
FSA may have far more words in the name of their crank than Rotor, they do not offer nearly as many options as Rotor on other fronts. For example, the available FSA SL-K crank lengths are 170, 172.5, and 175 and there is no 24mm crank axle alternative. Chainring options are 46/30 and 48/32. The outer chainring is of a direct mount configuration, while the inner chainring bolts to the outer. Power meters are available with this gearing and crank arm length combination if that is what you want. The FSA crank features hollow carbon fiber crank arms in an attractive matte carbon finish. Some may be hesitant about FSA carbon cranksets, remembering way back in the 2000’s when FSA carbon cranks had a reputation for the pedal thread inserts coming loose. FSA fixed that problem a long time ago so I was not worried about it, and you shouldn’t worry either.
FSA SL-K vs. Shimano Subcompact Crankset Weight Comparison
This is an apt time for a very quick specification comparison. According to Shimano, the Shimano GRX FC-RX600, with 46/30 2X gearing, weighs in at 816 grams. At $150, the GRX-600 crankset is a financial bargain, but it comes at the expense of weight. The GRX FC-RX810 Ultegra level crankset, with 48/31 gearing, weighs 722 grams, again according to Shimano, and costs $225. This is also a lot of performance for the quality and performance offered.
On my scale, the FSA SL-K Modular Adventure crankset with 170mm crank arms and 46/30 chainrings, and including the required pedal washers, came to 636 grams. The FSA crank cost $420. Which is significantly more than the Shimano options, but also loses a fair amount of beneficial rotating mass.
FSA SL-K Crankset Installation
Installing the FSA SL-K Modular Adventure BB386 EVO crank on my bike was a similar process to my wife’s Rotor, except that I needed to replace my bottom bracket bearing cups. FSA offers compatible bottom bracket cups for threaded bottom brackets, but other manufacturers, such as Enduro and Wheels Manufacturing, do so as well.
After installing the crank I went through the same process as on my wife’s bike, adjusting the left arm bearing preload, lowering the front derailleur, adjusting the low and high stops, and shortening the chain.
FSA SL-K and Rotor Aldhu Shift Quality
Gearing combinations and gear inch numbers are all well and good but meaningless if the crank does not work well. What good is a lower gear if you cannot properly shift between chainrings? My time working on a myriad of bikes at Fit Werx proved that not all cranks shift equally. Based on this, I was interested to see if switching away from a Shimano crank (often considered the standard bearer of front shift quality) would result in shifting compromise.
I was happy to find that my wife’s bike with the Rotor Aldhu 24 DM shifts quite well. The Rotor may not have quite reached the shifting precision of the Shimano Dura Ace crankset she had on the bike, but the Rotor was very good. The Rotor feels stiffer than average and the shifting is at the same level or above that of many other well-regarded cranksets on the market. I could not feel any type of flexing, even when out of the saddle and my wife did not notice any difference in how the Rotor crank performed compared to the Shimano.
My comments about the performance and shifting of the FSA SL-K crank on my bike mirror those for the Rotor Aldhu 24 DM. Shifting was excellent. There was no hesitation, particularly when shifting to the large ring, and there was no discernible flexing of anything. Simply stated, no issues, qualms or complaints.
As for the gear ratios, it feels as though it is encouraging me to spin, which I like, not grind. I really, really like this crank and the 46/30 gearing combination. The 30-tooth ring does make a difference when trying to get up those seemingly common 15% (or greater) pitches found on the dirt roads in Vermont. This change provides her with gearing that is significantly easier to pedal than she had with a 50/34 crank.
In summary, if you are you looking to retrofit your existing road bike (gravel or pavement), equipped with a 50/34 crankset and want lower gearing, a subcompact crank is the way to go. If you want to save some weight over Shimano’s offerings or want a much wider array of crank lengths and gearing options or power meter, consider the Rotor Aldhu DM.
If your bike can accept a 30mm crank axle, the FSA SL-K BB386 EVO offers excellent performance, comparatively light weight, carbon crank arms and reduced price when compared to the Rotor.
Fit Werx specializes in solutions. Contact us if you have questions or want to schedule a time to buy and have a new subcompact crank installed.