The smaller the 2x crankset chainrings, the larger the gear change (jump)

As gravel drivetrains move to smaller and smaller chainrings the (almost?) standard 16t difference between the two chainrings becomes a larger and larger gear ratio jump.

Some chainring pair examples...

52/36 = 1.44 so there is a 44% increase in gear ratio when shifting up from the 36 to the 52.

50/34 = 1.47 a 47% increase

46/30 = 1.53 a 53% increase

So SRAM's 43/30, with a 43% jump, is nearly identical to the common 52/36 road crankset.  While it would appear the top gear ratio of a 43t chainring and 10t cog (43/10) is much lower than a road bike, consider that the 40 mm gravel tire is about 5% larger than the common 25 mm road race tire so the final drive ratio of the 43/10 with 40mm tires is identical to a 50/11 with 25 mm tires. 

As gravel bikes evolve to wider tires the diameter and final drive will increase. If one wants to maintain the same gearing then  smaller chainrings and/or larger cassette cogs will be needed.

FWIW the smaller the chainrings and cogs the greater the stress in the driveline.  For that reason I use big cogs rather than small chainrings for my lower gears.  And for higher gears the wrap and chain angles on very small (10T and 11T) cogs also increase wear. 

stu wrote:FWIW the smaller the chainrings and cogs the greater the stress in the driveline.  For that reason I use big cogs rather than small chainrings for my lower gears.  And for higher gears the wrap and chain angles on very small (10T and 11T) cogs also increase wear. 

Yes exactly!  Lennard Zinn (Velo News) published some drivetrain efficiency test results showing friction increases with cross chaining - no surprises there.  But what was surprising was how much more friction resulted in using the smallest cassette cogs.  The two worst (i.e. highest friction) gear combinations were with the small chainring and two smallest cassette cogs. The higher friction was attributed to greater articulation of the links and the higher tension on the chain.

I’ve always been partial to Shimano, but I’m really drawn to the 43/30 SRAM. I was looking at a bike with GRX 48/31, but it would be way over-geared for the riding I do. 

GSPChilliwack wrote:I’ve always been partial to Shimano, but I’m really drawn to the 43/30 SRAM. I was looking at a bike with GRX 48/31, but it would be way over-geared for the riding I do. 

Using a larger small cog will give you the same effect as using a smaller large chainring.  For example, 44/11 and 48/12 and 52/13 will all give you the same ratio.

I never ride fast any more so I'm fine with a 48/15 high gear.  Of course I understand that's far from the norm, some folks would ride that gear on a fixie.

stu wrote:Using a larger small cog will give you the same effect as using a smaller large chainring.  For example, 44/11 and 48/12 and 52/13 will all give you the same ratio.

I never ride fast any more so I'm fine with a 48/15 high gear.  Of course I understand that's far from the norm, some folks would ride that gear on a fixie.

Yup and using the larger chainring/cog combinations reduces wear.  For gravel I very seldom find myself using the 48/11 of my GRX drivetrain. On the spirited road rides there are times I'm all spun out. Guess the high cadence work is good for me!

Since the big gravel tires already increase my fastest gear and since I despise the Shimano 11-34 arrangement (starting cogs at 11-13-etc which makes a giant gap when spinning downhill) I always replace the 11-tooth cog with a "first position" 12-tooth cog (which you can get cheap on eBay). Makes for a nice, smooth gear curve....
For hard climbing on road bike, I use the SRAM 11-36 which starts with 11-12-13 for downhills and goes 28-32-36 for the climbs

shmuelyosef wrote:Since the big gravel tires already increase my fastest gear and since I despise the Shimano 11-34 arrangement (starting cogs at 11-13-etc which makes a giant gap when spinning downhill) I always replace the 11-tooth cog with a "first position" 12-tooth cog (which you can get cheap on eBay). Makes for a nice, smooth gear curve....
For hard climbing on road bike, I use the SRAM 11-36 which starts with 11-12-13 for downhills and goes 28-32-36 for the climbs

Interesting idea replacing the 11 with a 12 on the 11 speed Shimano 11-34 cassettes, but don't you still end up with a big gap from 13-15? I have always found sub-compact 48/32 rings with the 11-34 cassette to be ideal for drivetrain efficiency with my modest power output, but the gaps on the smallest 3 cogs are annoying.

Really, really hoping Shimano doesn't screw up GRX 2.0 (12 speed), and just uses the new 105 road cassettes with the 2x version, because those would be PERFECT for gravel.