Size matters, at least it does with 1x drivetrains. If you're anything like me, you spend a lot of time thinking about your bike's gearing. Maybe you want a low gear that makes it easier to crawl up steep climbs. Or maybe you want a high gear that will help you achieve higher average speeds and stick to a fast group of riders.
Picking the right tooth-count for your 1x chainring is important because it gives you the right gear range to for your local terrain and riding goals. With the right chainring you'll be able to comfortably handle your toughest climbs and fastest descents. But how do you figure out what size chainring you need? Gear up with this easy guide.
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What Are Standard MTB & Gravel Chainring Sizes?
1x chainrings will come in 2-tooth increments and many 1x mountain bikes and gravel bikes will come from the factory with a “standard” size chainring. These sizes became standard because they mirror the size of the middle (and most used) chainring in older 3x drivetrains. The good news is you can easily change your ring to suit your needs.
Mountain bikes: 30t vs. 32t vs. 34t chainrings
32t chainrings are generally considered "standard" and most bike companies will outfit their mountain bikes with 32t chainrings. XC bikes made for racing may come with a slightly larger 34t chainring while a beefier trail and many enduro bikes are equipped with 30t chainrings.
The assumption is that XC riders will want more top-end gearing for fast terrain. Enduro riders, on the other hand, might find themselves grinding up steep climbs aboard heavier bikes, so they'd appreciate an easier gear. Plus, on many of the rough descents that aggressive trail and enduro riders favor, you don't pedal all that often.
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Gravel bikes: 42t vs. 40t chainrings
Chainring size is a little less predictable on gravel bikes. Many brands opt for 42t rings. However, some choose a 40t for a slightly easier climbing gear. In general, gravel bikes designed for more rugged terrain — verging on mountain biking — often come with a smaller chainring. A bike meant for high-speed riding and racing is likely to have at least a 42t chainring, if not something bigger.
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When should you try a smaller chainring?
The Queen of Pain, Rebecca Rusch, said she sized down her chainring to conquer the tough climbs at her namesake race. Photo by: Wyatt Caldwell / Redbull Content Pool
If you need easier gearing, just swap to a smaller chainring. This is good for riders who struggle with climbing, regularly ride steep terrain, or carry extra weight with bike bags.
On a mountain bike, the small change of swapping from a 32t to a 30t chainring gives you a low gear that is 6.7% easier.
For gravel, going from a 42t to a 40t provides 5% easier gearing.
That could be the difference between conquering a steep climb and being forced to get off and walk. We like riding bikes, and successfully cleaning a climb is always more fun than hiking.
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Mountain bike chainrings are often available in smaller sizes all the way down to 26t, though I recommend that most riders searching for an easy climbing gear should try a 30t or 28t chainring first.
For most gravel riders, I wouldn’t recommend going any smaller than 36-38t if you want to maintain top-end speed on flat roads and downhills. With smaller chainrings, you may find yourself spinning out on fast downhills.
That said, personally, I think the risk of spinning out is always preferable to being overgeared on climbs. Easier climbing gears keep my legs fresh and allow me to tackle ambitious rides. For regular riders, saving energy and preserving your knees through spinning, and completing tougher routes will improve the overall experience of riding.
Extra nerdy tip: On my full-suspension mountain bikes, I also like how the smaller chainring increases anti-squat, which firms up the rear end for efficient climbing. 30t is my go-to chainring size for steep Colorado climbs.
When should you switch to a bigger chainring?
Pros like Nino Schurter (left) and Mathieu van der Poel can mash big 38t chainrings up climbs. We probably shouldn't copy them though. Photo by: Bartek Wolinski / Red Bull Content Pool
Bigger chainrings are great for riders who need more top-end speed or ride mostly flat terrain. When using my 29er hardtail as a gravel bike, I swapped to a 36t chainring to keep up with drop-bar gravel bikes on flat roads. Look at pro MTB and gravel racers and you’ll see many using bigger chainrings. Many top pros use 36-38t chainrings on XC mountain bikes and 46-48t chainring on gravel bikes.
There are two main reasons racers upsize chainrings. First, they are fitter and faster than the general population. Thus, they need to sustain higher speeds and have no problem climbing with harder gears.
The second reason is more nuanced; 1x drivetrains are simple and reliable, but one drawback is increased chain angle at the extremes of the cassette. When the chain isn’t straight, there’s more friction, which leads to a slight efficiency loss. Choosing a chainring that allows you to stay in the center of the cassette for most of your riding maximizes drivetrain efficiency. Bigger chainrings also reduce friction because the chain doesn’t articulate as much when wrapping around it.
My advice: Unless you’re a pro or in the top 10% of riders, those small efficiency increases probably don’t matter. You’re better off prioritizing climbing gears and using the smallest chainring possible.
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Do You Need to Shorten or Lengthen Your Chain When Swapping Chainrings?
As a general rule, you can usually change the size of your chainring 2 teeth bigger or smaller without changing your chain length. If you go more than 2 teeth smaller, you will need to shorten your chain. More than 2 teeth bigger, and you'll need to add chain links or get a new, longer chain. 95% of the time, your rear derailleur will have enough b-gap or b-tension adjustment to compensate for a 2-tooth change. However, there are always edge cases.
You'll know your chain is too short if it pulls your rear derailleur into an extreme angle in the lowest gear. The rear derailleur will be pulled so tight that it is near horizontal and can no longer articulate. On full suspension mountain bikes, this can lead to poor suspension performance and even broken parts.
You'll know your chain is too long if it has too much slack in the chain when in the hardest gear. It will be loose, causing it to bounce around and hit your frame.
If you're unsure, check your chain length using the tips in our chain length/replacement tutorial.
Also, both of the above cases assume that your b-gap/b-tension is properly adjusted. You should check and adjust the b-gap/b-tension of your rear derailleur after changing the size of your 1x chainring. Fortunately, it's super easy.
Choosing the Best Chainring Size with a Gear Calculator
Gear calculators are a great tool if you want to compare different chainring options. To calculate the difference in gearing between 32t, 30t, 42t, and 40t chainrings, I used Sheldon Brown’s classic gear calculator. Seeing the difference as a percentage convinced me to try racing with a 30t chainring on my quiver-killer mountain bike.
Another favorite of mine is this HTML gear calculator. It provides a visual comparison of different drivetrains.
In the above example, I compare a 1x drivetrain with a 42t chainring and 10-42t cassette to a compact road drivetrain. For gravel, I know a 42t chainring will work because it has a similar range to the compact while providing an easier low gear for climbing. If you have another drivetrain as a point of reference, this tool is great for finding the optimal 1x chainring for your needs. It also allows to figure out what your top-speed will be with different chainrings, which can help you decide if a chainring is too small for your needs.
In general, stock 32t and 42t chainring sizes are standard because they're pretty good for most riders. However, I think many riders would enjoy riding more with a smaller chainring. Don’t be swayed by macho racers pushing pie plate-sized rings. It may require some experimentation to find the perfect chainring for your bike set-up. Luckily they’re not too expensive, so try a few and find what works best for you!
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