New bike build - front chain ring size help

Equipment
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I’m ordering a Mosaic GT-1 45 this week and need to decide on the component choices within the next few weeks.

This is going to be my gravel/trainer/mild road bike, to replace my 2021 lower tiered diverge. I had my fit last week and looking forward to my significantly more aggressive geometry. Living in the gulf coast, my riding is typically inside on the trainer with a long outside ride every other weekend (around 60-70 miles). Will be outside a little more as it cools down. And some casual evening rides pulling my 3 year old in her trailer.

The main goal will be gravel racing, but I’ll also be doing some general group paved rides.

I’m going with the new SRAM Red XPLR 13 spd. What I’m unsure of is what size front chainring I should go with. My Diverge has: a 46/30 up front and 11-34:11-13-15-17-19-21-23-25-27-30-34 in the rear.

The SRAM 13 speed has the following sprocket sizes: * 10,11,12,13,15,17,19,21,24,28,32, 38,46T

On my typical weekend rides, I never leave the 46 tooth front sprocket and when it’s flat, usually hop between the 17 and 19, but also can vary a bit based on the hills. That’ll be similar to if I have a 42 front chain ring and stay between the 15 and 17.

I used the following chart which is largely supporting my decision for the 42. I hesitate to go much bigger as it’ll be me higher into the cassette where the jumps get larger, but I’m admittedly pretty uninformed on the topic. I do plan some larger gravel races next year (mid south, gravel locos, etc) and probably one mountainous Colorado race, so have also considered getting 2 sizes, like a 40 & 46/48. Just not sure if necessary.

Thanks for any input -

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Choose the size where you’ll do the vast majority of your riding/racing. If you go out west and into the mountains, you’ll likely need a different front chainring and/or cassette. Many use a MTB cassette when getting into the mountains for bailout gears and much longer hills.

For Michigan gravel riding/racing, which is rolling hills, can be quite punchy but only last a few minutes, I use a 42 x 9-45. I am rarely in the 9 or 45. I’d say you’d be good with a 42 or if you tend to spin out then up to a 44 front chainring. If I were to buy the 13 speed today I’d likely get a 44 front.

When I was in the Smoky Mountains (TN) this past spring on vacation my 42 x 10-44 (Xplr cassette) was too tall. I needed at least a 40 front and really wanted a wider ranged MTB cassette.

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I replaced a 2x11 setup with a 1x12 drivetrain on my road bike. I have a 42-tooth chain ring up front and that has been working very well. It gives me a top gear that is essentially identical to 50:12 or 46:11 (as you found out). I top out at 60–65 km/h (at 110–120 rpm), although I am rarely in the 10-tooth sprocket. Typically, I am somewhere between the 13- and 15-tooth cogs on flat, fast terrain.

Personally, I’d start with the 40- or 42-tooth chain ring and look how you get on with it. I’d prioritize climbing gears over gears at the top end.

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The 42 sounds like a solid starting choice, but I agree with the idea of multiple chainrings. You didn’t mention your FTP, but that’s a big factor along with the terrain you expect to be riding.

And I’d echo some of @MI-XC 's comments on gearing challenges with XPLR. I’d personally consider going with a MTB Transmission RD and Cassette over the 13 speed xplr stuff (you can still run the new red levers). Yes, bigger gear jumps with MTB group, but significantly more range. My gravel bike came with xplr running a 40 chainring with 10-44 cassette. I immediately went to a 42 ring and that was OK (but not great) on top end but the 10-44 cassette wasn’t enough range for climbing on long hilly courses. For reference, my FTP is about 4w/kg and I’m a decent climber, but I’m not a fan of grinding up hills at low cadence. I pulled my eagle axs RD and cassette off my MTB and planned to use it for any course with steep climbs, it was so much better. I switched to a 44 chainring as well, better top end and still have a 44/52 climbing gear. My xplr RD and cassette has been sitting in my parts bin for almost 2 years after making that switch. Even for fast/flat road rides, there just isn’t any significant downside to the MTB stuff for me. Wider gear range trumps smaller gear jumps for me and it’s not even a close competition. And I used to be very sensitive to gear jumps and cadence, but I don’t even notice it any more and I think the bigger jumps have actually made me a stronger cyclist.

Anyway, just sharing some perspective. Obviously, I’m not a fan of the xplr gearing for gravel racing. But it works for many. The good news is that it’s a 10 minute deal to swap RD and cassette/chain if you want to go to a MTB setup in the future. The bad news is the RD and cassettes are expensive (chains as well if going axs transmission), so having extras sitting around feels a bit wasteful.

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My FTP is 3.6 w/kg but i’m a relatively new cyclist (only started actual training/regular riding in March). Barring injuries, I should be around 4.0 next spring.

You have a good point on the climbing gears which is a concern (for rides outside the gulf coast)

When I was in Colorado in July, I did just over 200 miles over 9 days, including a 65 mile race with 5500’ of climbing. My current gearing was sufficient over all my rides, although there were certainly some climbs where I was holding at 210-220 watts to maintain a decent cadence, when I would have preferred 180-190 in that part of the ride. Not a deal breaker, but something to keep in mind.

During my race, there was an 1800’ climb over the course of about 45 minutes and I was able to reasonably ride it all (but again, would’ve preferred to do it at around 20-30 watts less).

Good information and something to definitely think about. I do think that the tighter gearing will be more applicable to 90% of my riding than the larger range, so maybe two front chainrings might be the answer.

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Unless you are really strong or want to do low cadence drills on the flats or rollers you’ll probably never miss the larger chain ring but be grateful you have a smaller one instead. But I myself do prefer doing workouts outside and often ended up spinning out but I went to the Alpes and Ardennes and really missed my smaller chain ring

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That’s a very good point. I’m not sure I’d mind going from 35 to 33.5 mph top end at 90 rpm for cadence, vs having a little extra on the climbing…

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Do you spend more time climbing or at 33+ mph? :wink: IMHO the choice is easy.
Even on a road bike, my 10-tooth cog is mostly used as an overdrive gear. My 11-tooth cog doesn’t see much use either, much less than my 10-tooth cog used to (I now live in a very flat area).

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Good suggestion. Thinking back to my races, way more time is spent on the easiest gear climbing than the hardest gear down hills. Looking like the 40 will be the best bet. Thanks, all!

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I run 12 speed 1x on my “all-road” bike 42t x 10-44, it’s perfect.

I also run 11 speed 1x on my “gravel” bike 38t 10-42. 38x10 does spin out on long paved descents, but whatever, it gets me well over 30mph at ~100 rpm.

If you want the low gear, 38t might be big enough up front.

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