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Replacing 50/34 chainrings with 46/30

Old 06-23-23, 12:07 PM
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Replacing 50/34 chainrings with 46/30

I've got some store credits at a local bike shop, so I'm thinking about swapping my 50/34 chainrings with 46/30 ones. I have the following questions regarding what's needed to do it.
  • My assumption is that I can just change the chainrings without changing the crank itself. Am I correct?
  • I also assume I need to adjust the chain length. Am I correct?
  • What parts other than the actual chainrings do I need, if any?
  • Do I need to adjust the front derailleure afterwards?
Thank you in advance for your help.
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Old 06-23-23, 12:16 PM
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If you have a typical 5-bolt road crankset with a bolt center diameter of 110 mm, the 34-tooth chainring is as small as you can go.

Can you picture why? The 30-tooth ring just isn't big enough to fit the bolt hole pattern. The bolts would be wanting to go where the teeth are.

But maybe you have a different MTB crank or are running a triple crankset with the large chain ring turned into a guard or something more unusual.

If so, you usually can make a 4 tooth chainring swap without having to shorten the chain, but you will have to move your front derailleur down and adjust it.
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Old 06-23-23, 12:19 PM
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- As long as the bolt circle/pattern is the same for the chainrings, you won't need to replace the crankarm.
- You might need to shorten the chain. Or, you might not.
- You probably won't need any other parts.
- The front derailleur will need to be moved down further on the seat tube or mount to suit the smaller size of the new chainrings. This will also require resetting the front derailleur cable.
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Old 06-23-23, 12:22 PM
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Originally Posted by ljsense
If you have a typical 5-bolt road crankset with a bolt center diameter of 110 mm, the 34-tooth chainring is as small as you can go.

Can you picture why? The 30-tooth ring just isn't big enough to fit the bolt hole pattern. The bolts would be wanting to go where the teeth are.

But maybe you have a different MTB crank or are running a triple crankset with the large chain ring turned into a guard or something more unusual.

If so, you usually can make a 4 tooth chainring swap without having to shorten the chain, but you will have to move your front derailleur down and adjust it.
Thank you! That had never occurred to me, but you're right. I can see why a 30T chainring would be too small for a 110mm BCD.

For the refrerence, my current crank is the Praxis Alba M30 (5 arm bolts, 110 mm BCD).

Now, since my main concern is the big (50T) chainring too big for me, is it feasible to just swap that with, say, a 46T chainring, making it 46/34T together instead of 46/30T? I've got 11/34T in the back.

Thanks again!
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Old 06-23-23, 12:24 PM
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Originally Posted by daihard
Thank you! That had never occurred to me, but you're right. I can see why a 30T chainring would be too small for a 110mm BCD.

For the refrerence, my current crank is the Praxis Alba M30 (5 arm bolts, 110 mm BCD).

Now, since my main concern is the big (50T) chainring too big for me, is it feasible to just swap that with, say, a 46T chainring, making it 46/34T together instead of 46/30T? I've got 11/34T in the back.

Thanks again!
Yes. A 46T ring should work fine. Again, you will have to adjust your front derailleur position.
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Old 06-23-23, 12:29 PM
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Yeah, no problem going to a 46T big ring -- makes it more like a cyclocross double. Should help the front shifts a bit.
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Old 06-23-23, 12:37 PM
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Originally Posted by daihard
Thank you! That had never occurred to me, but you're right. I can see why a 30T chainring would be too small for a 110mm BCD.

For the refrerence, my current crank is the Praxis Alba M30 (5 arm bolts, 110 mm BCD).

Now, since my main concern is the big (50T) chainring too big for me, is it feasible to just swap that with, say, a 46T chainring, making it 46/34T together instead of 46/30T? I've got 11/34T in the back.

Thanks again!
I feel your pain. I live around the corner from Alba Road (for which your current crank is named), and the gearing is not low enough for me to climb it.

I bought a GRX 46/30T crankset for $150. It has a 24mm spindle, so you will have to get a new $20 bottom bracket. An adjustment of the front derailleur H limit screw was all that was needed. I also use a 11-34T or 11-36T cassette in the back.
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Old 06-23-23, 12:47 PM
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Originally Posted by Eric F
Yes. A 46T ring should work fine. Again, you will have to adjust your front derailleur position.
Originally Posted by ljsense
Yeah, no problem going to a 46T big ring -- makes it more like a cyclocross double. Should help the front shifts a bit.
Thank you both!

One (hopefully) last question. I'd assume it's okay to have two chainrings from different brands. Am I correct?
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Old 06-23-23, 12:54 PM
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I bought this Shimano 46/30 recently to ride in hilly regions, and am very happy with it.
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Old 06-23-23, 01:16 PM
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Originally Posted by daihard
Thank you both!

One (hopefully) last question. I'd assume it's okay to have two chainrings from different brands. Am I correct?
It shouldn't be an issue.
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Old 06-23-23, 01:37 PM
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If they like each other
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Old 06-23-23, 01:49 PM
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I've gone both ways with this. Replaced a 50t with 46t for a 46-34 and currently have a GRX 46-30. The reasonable price of GRX cranks and the high cost of single chainrings means you can focus on the gearing. I initially found the 30t to be less useful for road riding than the 34t but I need the 30t for climbs so I am adapting. I've never missed the 50t, I can't spin out a 46-11 unless I'm going downhill.
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Old 06-23-23, 02:11 PM
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On a Specialized Roubaix it's just a matter of switching out your chain rings. You may need a new chain but no biggy as they are one of the most disposable items on your bike.

For an older square tapper crank bike with a freewheel you could consider my my Ravel Bike set up...
bikinGreen Classic 170mm Crankset with 46/30T BCD 110mm Silver chainring Vintage
SHIMANO Tourney MF-TZ500- 6 or 7 Sp Multiple Freewheel 34T Copper/Black
Suntour Long Cage Derailuer
KMC X8.99/X8 Bicycle Chain (1/2 x 3/32-Inch, 116L, Silver

old school
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Old 06-23-23, 02:22 PM
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Keep in mind that going from a 34 ring to 30 equates to like a tooth and a half in the rear.
So going to, say, a 36T as your largest cog will give you lower gearing. Could save you money, too.
Also, check if your braze-on front derailleur (if equipped as such) will slide down enough to accommodate a 46.
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Old 06-23-23, 02:30 PM
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Just to add to the sound advice already posted, if your crankset has a direct mount spider, you can replace that and get a direct mount setup to run 46/30. If your cranks have a traditional spider (non-removable) then 34t is the smallest you can go.

Example of direct mount crank where the entire spider can be replaced.
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Old 06-23-23, 06:16 PM
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The 46/30 BikingGreen combo mentioned in post #13 is available as a chainring set that works on 110BCD cranks. I have that on two of my bikes (and love it); it's around $90 on ebay, pricey but worth it for my broken down body.
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Old 06-23-23, 07:33 PM
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Originally Posted by daihard
I've got some store credits at a local bike shop, so I'm thinking about swapping my 50/34 chainrings with 46/30 ones. I have the following questions regarding what's needed to do it.
  • My assumption is that I can just change the chainrings without changing the crank itself. Am I correct?
Yes, provided the bolt-circle diameter of your crank allows a 30T ring.
  • I also assume I need to adjust the chain length. Am I correct?
It will likely have to be somewhat shorter, but you may luck out.
  • What parts other than the actual chainrings do I need, if any?
  • Do I need to adjust the front derailleure afterwards?
You will want to lower the mounting position of the front derailleur to keep it withing a couple millimeter of the large ring's teeth. If you have a "braze-on" mount for the front derailleur, you may find it cannot be lowered enough to provide optimum shifting. Depending on your front derailleur, you may find that the tail of the front derailleur cage will collide with the chainstay before you get it low enough for opimum shifting.
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