Zap Hassellhoff

Hey all, I searched with no luck.

I currently have an new bike with an FSA 50/34 double paired with a Shimano 105 10-speed 12-27
cassette.

I find that I really dislike the higher gears of the 50 tooth chainring and rarely use them. I like the low
and mid-range gears of the 34 tooth ring.

I am thinking of swaping out the 50 and replace it with a 48 or 46 tooth ring in order to gain a little more "useability". With this in mind, would the 105 front derailleur cover the decrease in range?

This assumes that all derailleur height adjustments are made correctly.

This might be over simplistic, but is it correct to think that a decrease of 2 to 4 teeth (-2 -4) on a chainring would add 2 to 4 (+2 +4) to the cassette range?

Sorry, my mechanical theory is lacking at times.

joejack951

Swapping the 50 for a 46/48 makes a lot of sense. FSA makes cross cranks with 48/38 (or 48/36, can't remember) chainrings so sourcing that size ring won't be an issue.

In terms of shifting performance, it should be a wash between the two setups. You'll gain some smoothness by using closer spaced rings but you'll lose a little due to the smaller ring not matching the curve of the 105 derailler as well. I think it will shift just fine though. I've run a few front deraillers 2-4 teeth outside of their spec and none have given me problems.

In terms of gearing, you can do a quick gear comparison using a calculator. Just divide the chainring teeth by cassette teeth. This will yield a unit-less number that is only good for direct comparisons (all other factors held equal) but can be useful for making quick gearing decisions.

For example, your current high gear is a 50/12. 50 divided by 12 equals 4.16. Your new high gear will be 48/12 (4) or 46/12 (3.83). 50/13 is 3.84 so the 46/12 combo will be like losing your current top gear. The 48/12 will be in between your current two top gears. Make sense?

HillRider

Harris Cyclery has both 46 and 48T 110 mm bcd chainrings set up for outer position use on your crank. Look here:
https://www.sheldonbrown.com/harris/chainrings/110.html

You have to calculate percentage difference to see the effect of gearing changes, not just tooth difference. Substituting a 48T for your 50T will lower gearing by 4% and a 46T will lower it by 8%. That's the same as increasing your 12T small cog to a 12.5T (obviously not physically possible but the concept is valid) or to a 13T cog.

Yes, your 105 fd will work fine with the smaller chainrings if you position it correctly.

Zap Hassellhoff

Joejack, thanks for the crash course in gear calculation.

Hillrider, I'll use your info as a re-enforcement of Joe's argument.

I just ordered an FSA 46t chainring at Niagra Cycle Works thru Amazon for less than $30 shipped.

Thanks again you two.

I'm over this thread.

Al1943

You could have a problem moving the front derailleur low enough if it is the braze-on type.

DieselDan

You may need to remove a link or two from the chain too.

davidad

You may not have to lower the der. Try it first. This helps with the gear selection. https://www.panix.com/~jbarrm/cycal/cycal.30f.html

barturtle

My Fantom CX came with 50/36, the gears got replaced with a 46/34 as soon as it came out of the box. Lower the derailleur. Shifts great.

rustybucket

Another question while we are on the topic of chain rings. Are all 110mm BCD chain rings compatible with other brand cranksets? Eg. could I put a SRAM 110mm BCD chain ring on a 110mm BCD FSA crankset? Thanks.

roberth33tiger

simple answer- yes

HillRider

The answer is yes......but.

Almost every maker's 110 bcd chainings will work on any other makers crank with one exception. Campy made their higher-line "110 mm bcd" chainrings and compact cranks with one bolt offset slightly so their chainrings will not fit other cranks and thir cranks will not accept other maker's chainrings.

Zap Hassellhoff

Once again all, lots of good info here. My derailleur is a clamp on so, if needed, I can lower it a bit. Diesel, good point on removing a link or two. I'm betting I'll have to.

I'm really looking forward to this "upgrade".

DinoShepherd

I run a 46/34 with an 11-23 on my road/race bike.

No problem keeping up with the fastest dudes on the downhills. The front shifting is great.

-Z

RT

I had considered swapping out the 46t chainring on my new cross bike (FSA Gossamer cranks) for a 50t ring, but was also concerned about FD height. As far as I can tell, the 105 FD is a braze-on, so wouldn't adjustment up or down be impossible? My reason was that at 46t max in front and 12 in back, that I would spin out on long hills. My 48/12 on my MTB comes close on hills right now.

Al1943

All of the braze-on type FD's I've seen have a slot in the hanger for up and down adjustments. But sometimes the slot isn't long enough. On many road bikes the slot doesn't go low enough.

tiggermaxcocoa

I also run this setup with an 11/32 9 speed cassette. I love it. The spacing is great and I never have a problem with spinning out except on the really steep downhills. Probably helps to have an 11t cog, though. I had no issues with the front derailleur, which is ultegra.

DinoShepherd

As far as spinning out on downhills, that is a yes and no.

If you are by yourself, you can spin out.

If, however, you are in a pack, no problem drafting behind someone.

-Z

mikael17128

Dino (or anyone with an opinion),

Hi, I saw your post and want to run the same setup. Do you have any experience with brands? Most brands want to sell 46/36 now-a-days. I'd be more interested in your 46/34 configuration with an Ultegra 11-28 cassette in back for climbing: The 34/28 will help to grind the 10K+ days and the 46/11 will work for the downhills, as it's essentially the same as a 50/12 (which is enough gear for me).

Thank you for any help!!

Mike W.

tFUnK

Adjusting (lowering) the FD could present a problem, as mentioned. Also, if your bike has really beefy chainstays, the FD cage might not clear. When I made the switch from 50t to 46t, shifting on the stand seemed a bit crunchy, but on the road, it's quick and crisp.

Chain length, I don't know... I set my chain length based on the small-small combination, which, if using the same cassette, remains unchanged.

rydabent

This is not a big deal at all, of course it will work. As mentioned you may need to lower the FD some, and depending on how long the chain is now, a link or two may need to be removed.

balesmachine

I love hearing folks that say they want a smaller big ring on their road bikes. I am a "seasoned" rider with almost 50 years of road riding, both racing and touring. For the first 20+ years, my high gear was a 52-14, which equals about 100 inch high gear. I raced, and toured, and I never remember wanting more, and I was certainly stronger then. Some folks had a 52-13, which made a 108" gear. Along came the popular 39/53 chainwheel setup, and the smallest cog went to 12 teeth. So now we have a high gear of 53-12, or about 116". I, for one never needed it, and certainly don't need it now. So I recently took my 53 tooth FSA chainwheel and replaced it with a 46 tooth. My high gear is 46-12, or 101", which is still bigger than what I rode and raced on back in the 70's.