# "Semi Compact" Gearing (50/38 Chainrings)



## KillerQuads (Jul 22, 2002)

I bought a bike that had a stock compact crank with the now common place 50/34 chainrings. It also came with a 12-26 10 speed cassette. The 50T big ring makes sense because as the small cogs have shrunk from 14 to 13 to 12 to 11 teeth, we don't really need 53T big rings to have an adequately tall top gear. The 50T ring saves some weight and lets stay on the big ring even against the wind or with moderate uphill grades.

My problem is with the 34T small ring, which for me and my area (rolling hills in central Virginia) is way too low. Shifting either up or down with the FD is too much of a jump. The 34 is a granny gear which I find useless on the flats. It also shifts noisily or worse yet drops the chain completely off the chainrings.

My solution was to replace the 34 with a 38T inner ring for what I call "semi compact" gearing. I stiill have the advantage of the 50T compact ring, but now have a higher range of gears on the small ring. The "jump" when shifting up front is perfect for rolling hills and when stopping and starting at intersections. For example, when I crest a hill on the small ring and start descending, I shift to the big ring and attack the downhill with power. When slowing for a red light on the big ring I shift to the small ring and am ready to start from a dead stop when the light turns green. The 50/38 combo is perfect for this. Shifting performance is much better (even more so with SRAM's new 10 speed chain).

The 50/38 setup would work well for racing and time trialing too. Just swap the 12-26 cassette for an 11-23. For a mountain tour, it is easy to reinstall the 34T without removing the crank.

I have converted a 50/34 110 BCD crank (compact) to 50/38. I have also converted a 53/39 130 BCD crank to 50/38. FSA makes 50/38 chainrings for both sizes of cranks. Both bikes have much better gearing for both commuting and weekend rides.

One problem is that the big rings have ramps, pins, and teeth optimized for shifting with only certain small rings. It took a little trial and error to find the best way to mount the small ring in relation to the big ring for the smoothest FD shifting.

Tips: You don't have to remove the crank to change chainrings. Make sure you don't over torque alloy chain ring bolts, as they are fragile. Lower the FD as needed when downsizing from 53 to 50T rings. Check the FD set screws after changing rings (should need minimal or no adjustment). For me, the new SRAM 10speed cassettes and chains shift better than their Ultegra counterparts. FD height, FD alignment, FD set screws, and FD cable tension are all very important to front shifting performance, and bike shops may not have the time to adjust the FD perfectly.


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## The Flash (May 6, 2002)

What kind of small ring did you use?

Flash


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## Jesse D Smith (Jun 11, 2005)

I've been riding the same 50/38 setup for a few years. TA rings have proven very durable.


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## KillerQuads (Jul 22, 2002)

*50/38 Chainrings*

Very few manufacturers make 38T inner rings. I am using FSA 38T inner rings in both 110 BCD and 130 BCD sizes (two different bikes) with good results. One of the bikes also has a new FSA 50T 130 BCD outer ring that replaced a 53T ring. AEBike.com carries the FSA rings. Just make sure you know whether you need 110 or 130 BCD. It should be marked on your current rings.


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## DIRT BOY (Aug 22, 2002)

*I been doing that for years now...*

My current set up is 51/39T. I use Stronglight CT2 rings with a 11-21 in back or 11-23.

I have also used the follwing combos:
50/39 SL and FSA
50/38 SL and FSA
51/38 SL
I think this works really well for mere mortals who live in flatter lands....

No shifting issues with Campy or Shimano. I use a standard FD as well.


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## Erik_A (Sep 14, 2008)

KillerQuads said:


> One problem is that the big rings have ramps, pins, and teeth optimized for shifting with only certain small rings. It took a little trial and error to find the best way to mount the small ring in relation to the big ring for the smoothest FD shifting.


I have a Shimano 50/39 and just bought an SRAM 38t inner ring. How did you find the best position of the 38t for FD shifting?


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## Agent319 (Jul 12, 2012)

The chainring should have a mark on it to align it to the other mark on the big chainring. My shimano rings do at least.


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## Erik_A (Sep 14, 2008)

That's what I do with my stock Shimano 50/39 but the SRAM 38t inner ring that I bought is matched to the ramps and pins on an SRAM big ring. Other than trial and error, how do you find the best position of the SRAM 38t for FD shifting?



Agent319 said:


> The chainring should have a mark on it to align it to the other mark on the big chainring. My shimano rings do at least.


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## Wadl (Oct 8, 2011)

Erik_A said:


> I have a Shimano 50/39 and just bought an SRAM 38t inner ring. How did you find the best position of the 38t for FD shifting?


Where did you find a SRAM 38T inner ring ?? I am looking for one to upgrade on my 2012 RED compact crank.

Thx


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## Erik_A (Sep 14, 2008)

$17.99 new from "The Pros" on eBay they had "more than 10 available". That is about 50% off retail.

if you Google "SRAM 38t 130mm chainring" it still shows up as a picture at the top:

http://www.google.com/aclk?sa=L&ai=...&adtype=pla&rct=j&q=sram+38t+130mm+chainring+



Wadl said:


> Where did you find a SRAM 38T inner ring ?? I am looking for one to upgrade on my 2012 RED compact crank.
> 
> Thx


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## Wadl (Oct 8, 2011)

ok, well I will thy it with 110mm and will do it at home, here, ebay doesnt get through !!


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## JCavilia (Sep 12, 2005)

Erik_A said:


> That's what I do with my stock Shimano 50/39 but the SRAM 38t inner ring that I bought is matched to the ramps and pins on an SRAM big ring. Other than trial and error, how do you find the best position of the SRAM 38t for FD shifting?


I'd start by comparing the ring with the old 39, and see if there's any similarity in their short and odd-shaped teeth, match them as closely as you can, and position similarly. It may not be perfect, but the teeth on the little ring don't have as much influence on shifting as those on the big ring.


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## Erik_A (Sep 14, 2008)

Thanks-



JCavilia said:


> I'd start by comparing the ring with the old 39, and see if there's any similarity in their short and odd-shaped teeth, match them as closely as you can, and position similarly. It may not be perfect, but the teeth on the little ring don't have as much influence on shifting as those on the big ring.


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## Local Hero (Jul 8, 2010)

*Does it matter if the big ring is ramped/pinned for a 34? *


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## craigmbrannon (Jun 3, 2008)

I know this post is rather old, but I am also trying to modify a Shimano compact crank. I purchased a SRAM 38T 110mm ring and installed it on the Shimano crank with a 50T large ring. If the chain is in the smallest 4 cogs on the cassette, and I shift from the 50T to the 38T, the chain will sometimes ride against the side of the large ring and on top of the small (not engaged with the small ring). I think this is because Shimano does not support rings as large as 38T on the inside and the ramp on the large ring has too large a space when a ring this large is used.


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## Erik_A (Sep 14, 2008)

Try flipping the 38t ring. Also readjust the front derailleur.


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## craigmbrannon (Jun 3, 2008)

Erik_A said:


> Try flipping the 38t ring. Also readjust the front derailleur.


The chainring is not symmetric, and I had it dished toward the large ring so reversing it would make the issue worse. I also maxed the mechanical limit for the FD. I have Di2, but don't think it is limiting the FD travel.


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## wim (Feb 28, 2005)

I understand the desire to keep the 110 BCD crank. But a standard 52- or 53/39 would be so close to a 50/38 that after considering all these issues mentioned here plus ring-versus whole crank prices, I would seriously consider buying a standard 52- or 53/39 crank.

With a three-tooth chainwheel difference about equal to a one-tooth cassette difference and a one-tooth chainwheel difference almost imperceptible under most riding conditions, going back to a 52- or 53/39 standard crank would not require that much of an adjustment on part of a rider.


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## craigmbrannon (Jun 3, 2008)

I took the standard crank off the old bike and it is much better. Looks like I will have to spend a little more money


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## wim (Feb 28, 2005)

craigmbrannon said:


> I took the standard crank off the old bike and it is much better.


Can't argue with that.

I'm a bit of a klutz when it comes to feeling these differences. So with the difference between a 53 and a 50 big ring nulled out with one click of the rear shifter, I just kept grinding along in my standard crank.


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