I want to replace my old -but still good- 52/42 Shimano 105 crank with a 50/34 Shimano Sora (cheap), because i need an easier gear.

is it worth it?

Will it effect the bike speed/performance?

The 50/34 Shimano Sora i want to buy :https://www.decathlon.es/es/p/pedali...937?mc=8511779

My bike:

It will work but you’ll need a new bottom bracket

There’s one issue you ‘might’ encounter going from a 42 down to a 34 with the small ring. The smaller circumference of the 34 changes the angle of the chain back to the gears and could cause it to snag on the large ring. I wanted to change from a 52/42 setup to a 53/38. Once re-assembled, everything ran OK in the middle gears. However, the angle of the chain from the new/smaller ring back to the smallest gear caused it to snag on the large 53 ring. So I changed the small ring to a 39 and that increased the circumference jus enough so that it doesn’t snag on the large ring. Given that you’re also going down to 50 on the large ring ‘might’ allow clearance. But it’s and issue you might run into.

Edited to add a simple drawing to help explain. Obviously not to scale, but just to show where I had the problem.



Dan

Current 50/34 (aka "compact") cranks are so common that the chain interference issue has been long dealt with. Yes, a 53/34 crank can have snagging problems but not a 50/34 the OP is asking about.

Who the HELL uses small/ small?? There is ZERO NEED.

Assuming your 52-42 is 130 bcd, you could go to a 38 tooth small ring, the smallest that will fit on that bcd. Match that up with a cassette with lower gears to get even more low end. In regards to replacing the whole crank set, as stated above, likely going to need a new bb, only you can decide if it is worth it to you. What you are proposing is not that much cost, and relatively easy to do. I would do it if I truly thought it was going to be a plus to riding the bike.

It has been "dealt with" in the sense that Shimano tells us not to do it. For example, with the latest 2x11 drivetrain, Shimano dealer manual expressly states that there would be chain rub and noise while riding in the small chain ring with the four smallest cogs.

some FD’s have a trim / ‘intermediate’ position that can reduce or eliminate the chain rub (when riding small ring with the smallest cogs)

Depending on which RDER you have, you may be able to accomplish lower gears with a simple cassette change.
OR cassette & RDER.

The Shimano dealer manual describes, from furthest out to furthest in, these four FD positions: Top, T-trim, Low, and L-trim. In my experience, shifting from Low to L-trim at most only allows the additional use of a single cog (e.g., the fourth smallest cog) without chain rub or noise.

thank you for pointing that out!
Thank you so much for the drawings, with my english it is hard to understand. I agree there is a high probability of a problem like that.

Where i live there is very few options.. i would prefer a 52/36.
Yes its 130 bcd.. There is some climbs that i like, are hard to do with a 42. thank you
What is a RDER ?

RDER = Rear derailleur

more people than we bike lovers like to admit... and people with poorly adjusted front derailleurs, stuck or broken cables, or shifters with old dried out grease. ... that's "how many".

the cross-chain test is part of every multi-gear setup in my shop.
and if the chainline is good, there is no major issue to remedy... a bit of der. fiddling and out the door it goes.

Hollowtech II bottom Bracket... https://www.theproscloset.com/produc...ttom%20Bracket

Tool to install/remove Hollowtech II BB... https://www.rei.com/product/799545/p...53b9ffd53989e3

these are images to show the Part and tool.. You should shop for the best deal in your area.. or go to your Local Bike Shop for parts and/or installation

Have you checked that the current crank is just a double? The 52/42 rings are standard on a triple for this 105 (FC-55??) and your smaller ring looks that it has shift pins for shifting from a third small chainring. The actual part number should be stamped on the back of the crank arm, right next to where the pedal mounts.

Your RDER is also a longer cage (GS) version of the series normally used for a triple crankset.

Regarding your RDER, it should be rated for a 27 teeth largest sprocket on the cassette. Most people report going a tooth or 2 higher is possible.

I think you'll find the 50-34 more useful than what you have and the new style bottom bracket will be much easier to deal with in the future. Sounds like a good plan.

its a triple "Shimano SG A-52-42-30", the previous owner removed the 30.

next thing to do is price a 30t small ring of the correct bolt pattern, price a left side triple shifter that matches the one on your bike, and price a front derailleur that is made to do a triple shift if the one on the bike is not a "triple".. the part number for the Front derailleur is on the back of the chain cage... post that part number here, or look it up on the internet, to see if it's a "triple".... once all the costs and availability is figured out, then you can make the correct decision for your riding needs...

The Crank swap will be the easiest, and best choice for lowest price total, eh?
the "triple" install will have other hidden costs too... possibly shift cable, cable housing, bar tape(depending on Shifters used), and possibly a rear derailleur...

installing a hollowtech II crank is easy.. remove pedals, remove old crank and bottom bracket, CLEAN everything, put a dab of grease on the BB threads, install Bb, slide new crank in place, side new Left crank arm on, tighten clamp screws, adjust chain length and front derailluer,.. then go ride!

Yup. I'm late to the party but that was my first thought. I ride small-small fairly often. I love to climb. But on a long climb, I dislike doing double shifts. So when the road levels up mid-climb I go to the small cogs until the road ramps up again. Cranksets with chairings with less ramps and no pins allow more use of the smaller cogs at the expense of poorer front shifting. With friction shifting you can pull the chain away from the larger chainring and extend the range of usable smaller cogs in back. Yes, you will hear the rub and eventually wear out the derailleur cage but if getting to the top of the hill as fast as possible is important, those consequences may be acceptable. (And you'll be breathing hard enough to not hear the rub.)

Edit: Ways to get past the FD-shifter compatibility issues - go friction in front. Keep your brifter to match the left but use either a downtube shifter for the front (might be a challenge to mount on your Ridley) or install a bar end shifter on the handlebar. That will allow you my FD cage adjust trick and at least an extra usable cog in back.

What Shimano is probably warning about is the chain rubbing on the front derailleur cage. What I'm was talking about, and in reply to _ForceD_'s posting, was the comment that the chain could catch on the big chainring when in the small-small. I stated that won't happen with a 50/34 crank. Rubbing on the fd cage is another matter entirely.

Replacing the 30T would be a low cost option. If the bike has the front derailleur and shifter that was part of the original 105 9 speed group, it is triple compatible. The shifter works with 2x or 3x shifting.
From the picture it looks like the front derailleur is the triple version of the series, the outer cage has a ridge along lower edge that was only on the triple version.

A 30T inner ring would give you great gearing range and also maintain close ratio.

Find another 30T ring I presume you still have the 3 speed L shifter?
It should be Shimano PN# Y-16X 30010
The "road" guys could probably tell you what else would fit.

How many teeth on the largest cog in back?
if 23T, going to a ??-28T cassette would give the same low gear ratio as changing your 42T ring to a 34T.

I don't know him.. i searched for the 30T that fits, but I couldn't find any + L shifter has only 2 shifters (i would replace it if necessary)
in the back there's a 9-speed Cassette 13-23.

Why not a subcompact double 44/28 or 42/26

The rear derailleur cage is long so replace the cassette w/11-34 or 11/36

That 9 speed RD, even though a GS, won't take more than +/-27. BITD we replaced them with MTB RDs to use a 32 or 34.