Simon1892

I'm currently riding a 53/39 Shimano 105 crankset (circa 1995). It was supplied as 53/42, but I swapped out the 42T ring for 39T.

A recent discussion led me to realise that the current 'standard' is 50/34, but 34T won't fit on a 5 bolt, 130mm BCD crank, so I have to look at other options.

I can go for a Claris 8 speed Octalink crankset and change the bottom bracket (68mm shell), plus new cassette and chain. My only hesitancy is the spindle length, which presumably sets the chainwheel position relative to the front derailleur and the chain alignment relative to the cassette.

Is is as simple as measuring the spindle length of the existing square taper bottom bracket spindle and matching that? I suspect not. So is there a way of identifying the spindle length required?

Is there anything I'm missing in terms of other options? Any advice or suggestions will be much appreciated.

JohnDThompson 

Use the spindle length the crank manufacturer calls for.

Bill Kapaun

Is there a compelling reason to switch?
You throw in FDER adjustments/potential problems with the change in ring size & difference in #teeth between the 2.
IF you need lower gears, get a 9 speed "mountain" RDER and a cassette with bigger cogs.

DaveSSS 

I'd move up to a higher level crank with a modern 24mm spindle.

AngelPedaller

Go for a Tiagra 4650, 105 5700 full crank and don't bother changing chainrings. A 34t 130BCD chainring would cost more than a decently used 10sp/9sp compact crank in marketplace....

JohnDThompson 

A 34T, 130mm BCD ring would actually be an impossibility, at least with 1/2" chain pitch. 39T is the smallest 130mm BCD ring. For a 34T ring, you need a 110mm BCD arm, or smaller.

bmalmquist

This is called chainline and is basically as you described it. It is an important factor in wear on a chain, shifting quality, and - in extreme scenarios - whether the chain can physically reach all of the gears.

It's not necessarily as simple as that, but it could possibly be in your scenario. Sheldon Brown has a simple (relatively) breakdown of chainline and a table of manufacturer recommended chainlines. The most accurate answer is to, as someone else said, "use the spindle length the manufacturer calls for", which is 121mm for this Octalink Claris crankset.

I'm with others here in that I'd look for a 2-piece crankset. These have the spindle attached to one of the two arms and use bottom bracket spacers to adjust for chainline. The 2-piece design is the more common style versus Octalink (splined 3-piece). In case you didn't know, you can pretty safely run an 8-speed chain on a 9-speed chainring. That should open up some options in case you were looking at the Claris because it was 8-speed.

Iride01 

So why do you want to switch? The only thing you suggest in your OP sounds like you are doing it because you think everyone else has a 50/34. That's a poor reason.

If you are struggling in all your riding to sprint and get yourself up hills then maybe you do need a 50/34. But if you are struggling to find speed on slight downgrades or -1% or -2% then the 50/34 might do you less good.

IMO the trend for 50/34's peaked about 4 or 5 years ago. Now I see more bikes with the 52/36 or 53/39 of yesteryears. Especially the bikes that are aggressive with their fit.

My two most current bikes are 52/36, but I'm considering going back to the 53/39 as my Raleigh had that and I actually rode well on that except for the steepest of hills near me.

Simon1892

You can’t get a 34T 130BCD chainring, hence the reason for changing. The 105 crank is 130BCD, I would need 110BCD

Simon1892

I was on 53/39 and 12-25 cassette, but haven’t been cycling for some time due to commitment to coaching my son’s football team. I’m now 55 and looking for a more forgiving range of gears, so 50/34 seemed a good option.

I’ve struggled on some climbs, so was thinking about a wider range cassette, but can’t do much more on a short cage derailleur.

From discussion with the local store, I was recommended looking at 50/34 which would enable a wider cassette.