Understanding chainline; Shimano GRX and SRAM "Wide" cranksets

You may have noticed an increasing number of drivetrain products - notably, Shimano's GRX and SRAM's "Wide" (XPLR, Force, etc.) series components marketed as "gravel specific". These cranksets are designed for bikes that demand large tires and short chainstays, while still retaining compatibility with front derailleurs - i.e. modern gravel and cyclocross bikes.

Let's take a look at how these "wide" cranksets came to market, and whether you should purchase a bike/frame that uses them.

A gravel bike geometry problem

As riders migrated to these new off-road, drop-bar bikes, a problem emerged - they wanted wider gravel tires for comfort and control on rough surfaces, but there simply wasn't room to get wider without compromising frame geometry. There are just too many parts - tire knobs, front derailleur cages, and chainstays competing for the same space. Interference problems developed between the inside of the crankarms and the chainstays, or between the large chainring and the drive-side chainstay. In some cases, the front derailleur cage, when shifted to the innermost position, could even touch edge knobs on the tire! Less than ideal.

One easy solution is to simply increase the chainstay length. This moves the axle of the rear hub further from the "problem area" where the side of the tire, the chainstay, the chainrings, and the front derailleur cage all come together. It's an imperfect solution, because while increasing chainstay length can create more tire and chainring clearance, it also changes the handling of the bike.

Bikes with longer chainstays tend to feel slower, and more stable (for this reason, longer chainstays are common on bikes for loaded touring, for handling purposes. They also create more space for rear panniers, reducing the chance of the rider touching them with their heel.) Slower and more stable aren't characteristics that riders tend to like on their race machines, though!

You've also no-doubt seen frames with a "dropped" drive-side chainstay, where the chainstay doesn't go in a straight line from the dropout to the bottom bracket, as seen on bikes like the 3T Exploro - another creative way to get the chainstay out of the way and create a few millimeters more space. 

Enter the "wide" crankset to solve this problem

To address these clearance concerns, brands like Shimano and SRAM moved the cranksets further outboard from the centerline of the bike - which has the effect of moving them further away from potential interference between the crankarm and chainstay, or large chainring and chainstay.

The "magic" number they choose was 2.5mm. For example, Shimano's GRX line sits 2.5mm further outboard compared to a Dura-Ace crankset. Doesn't sound like much, but that's 2.5mm of extra clearance, on either side of the bike, which can be just enough to create enough space while still keeping the chainstays short.

What is chainline anyway? 

These "wide" cranks can be identified by their bigger chainline number. Chainline is the distance from the centerline of the bicycle to the center of the chainring - on a 1x bike with a single chainring, the measurement is taken at the center of that one ring - and on a 2x bike with two chainrings, the measurement is taken at the midpoint between the two chainrings.

This measurement on the GRX (about 46.9mm) and SRAM Wide chainlines (47.5mm), as expected, is roughly 2.5mm wider than the 44-45 mm on common road cranksets.

What is Q-Factor?

I think I first heard about "Q-Factor" in the Bridgestone Bicycles catalog when I first started riding seriously in the late 1980's and early 1990's. Q-Factor describes the distance between the pedal mounting surfaces on the outside of each crankarm, i.e. "how far apart do the cranks put your feet?"

A Shimano road crankset has a Q-Factor of 146mm, so GRX with the +2.5mm design on either side pushes that out to 151mm; i.e. the rider's feet will be 5mm further apart when riding a GRX-equipped bike.

Wider Q-Factor has some downsides:

  • reduced pedaling clearance when cornering. The larger the Q-Factor, the higher the likelihood of pedal strikes at extreme cornering angles.
  • Some riders, like Bridgestone's Grant Petersen, describe riding a bike with a large Q-Factor as "pedaling like a duck" and favor narrower designs.
  • An argument can also be made that narrower Q-Factor setups could be more aerodynamic.

So these "gravel specific" or "wide" cranks increase that Q-Factor number, which is often seen as a negative. EXCEPT - Shimano makes an XTR pedal with a "-3mm" option on either side, effectively negating that difference and providing the same (slightly improved, actually) Q-Factor as a road crankset when the -3mm XTR pedal is used with GRX. Man, Shimano thinks of everything! 

Not everyone uses Shimano SPD pedals though, and riders of other makes/models might be out of luck. Should this worry you? Probably not. The industry has moved away from different length bottom bracket spindles, which means that you largely don't have a choice in Q-Factor anyway.

Downsides of "wide" cranksets

  • limited cross-compatibility of front derailleurs
  • possible loss of pedaling efficiency or ergonomics
  • additional noise in the big-to-big gear combos

Because the chainrings on a "wide" crankset sit further away from the centerline of the bicycle, a matching front derailleur is required. That means if you're planning to run a 2x drivetrain, your mix-and-match ability is limited.

On the Shimano side, a GRX crank needs a matching GRX front derailleur, and on the SRAM side, you'll need to match the "wide" labeled cranks with "wide" front derailleurs. It's not like a traditional road setup, where Shimano 11 speed cranksets, front derailleurs, and shifters could all be interchanged between Dura-Ace, Ultegra, and 105 components.

In addition to limited cross-compatibility, some riders also believe that wider cranksets are less efficient than their "road" cousins, because placing the pedals more outboard affects rider pedaling performance. An argument can be made that the most natural pedaling motion would be one in which the distance between the feet approximated the same distance between the feet when walking. See the "Q-Factor" discussion, above. Different pedals can partiallly mitigate this.

Finally, recall that the relationship between the cassette and the bicycle frame did not change, it's still in the same spot. But the chainrings are now 2.5mm outboard compared to their position on traditional road cranksets! The cassette and rear derailleur did not move 2.5mm outboard to match.

While 2.5mm is a small amount, this can create some extra drivetrain noise; for example, I notice it on my own GRX-equipped bike (2x double crankset) that feels slightly louder when I use the big chainring in combination with the two largest cogs on the cassette - especially when the chain is due for a wax or fresh lube. Such "cross-chaining" is less than ideal, of course, but those gears get used occasionally nonetheless.

Bikes that require "wide" cranksets

As you shop for frames, be aware of an increasing number designed as "wide crankset specific" - they'll only work with Shimano GRX, SRAM "Wide", or others with similar designs that create extra space at the chainstays by adopting a wider stance.

If you prefer to outfit your bike with a "road" style crank, you may find that there is insufficient space on these frames - the inside of the crankarm may come too close to the chainstay, or even touch it (unrideable obviously) in extreme cases. This can also be a problem on some power meter cranks that have fittings or battery doors on the inside face of the crankarm, which can put them too close to the chainstay for comfort.

If a complete bike includes a "wide" crank like Shimano GRX or SRAM XPLR Wide as stock to accommodate modern, wide gravel or cyclocross tires, you can't count on retrofitting a road-type crank to it later - ever.

What if I don't want a "wide" crankset?

Generally, you aren't going to have a choice when buying a complete bike, because you'll get the crank the manufacturer has spec'd. However, some brands are recognizing that riders want choice.

Mosaic, for example, uses the "Wide" crank requirement on their GT-1 and GT-2 gravel bikes by default, but also recognizes there are other ways to create more tire clearance without resorting to a wider crankset - like dimpling the drive-side chainstay. Upon request, they'll make this tweak to their custom frames, enabling the same tire clearance and 2x drivetrain capability with "road" cranks, like Campagnolo Ekar or Cane Creek's lovely titanium eeWings. A nice compromise.

If you're building your own bike from a bare frame, recognize that selecting one that accepts a non-wide "road" crankset might create other limitations, notably tire clearance. This might not be a big deal on a cyclocross bike where you plan to use a 33mm tire, but could definitely be a concern on a gravel bike where you want to go much wider.

Although GRX has become increasingly common in cyclocross, if you look carefully at pro bikes, many of them are using Dura-Ace cranks, not GRX... perhaps for weight savings, or a rider preference to a narrower Q-Factor.

Wide cranks are likely the best choice for many riders, despite the negatives

Despite some of the downsides of wide cranks, for riders who want modern cyclocross and gravel bikes with short chainstays and wide tire clearance they're likely still the best choice at this time.

Your next bike might have a dropped chainstay, "wide" crank, short chainstays, AND a 1x, single chainring drivetrain! The good news is, it will also come with massive tire clearance that allows you to ride a larger variety of terrain on a single bike than ever before.


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