Change of crankset - chainlength?

[RussellZero]

I’ve got a 46/36 front crank (105) on my CX bike and thinking of changing it to be more like road bike gearing - maybe 50/34. I can test it out by swapping the crank from my road bike (ultegra) and I have a spare ultegra left crank. Question is do I need to change the chainlength as a result? If I was making this permanent I’d put a new chain and cassette on anyway, but just wondering before I waste a load of time messing around!

thanks

 

[Cycleops]

Ideally you should put a new longer chain on but you could try the existing.
Should be big ring to big ring (omitting the derailleur) plus two links.

 

[Sharky]

What are you looking to achieve? A higher high gear or a lower low gear?
The difference between a 46 and a 50 is equivalent to one tooth difference on the cassette, so might be easier to change the cassette if not currently with an 11 cog.

 

[wafter]

Ideally yes, but it depends on the current chain length and rear derailleur capacity. Note that you'll be asking the RD to accomodate an extra six teeth of potential range (46-36=10t, 50-34=16t). As such you might encounter problems at the extremes of the RD's travel and depending on what you're running on the back you might need a new longer-cage derailleur or a drop-link / spacer fitting to the existing one.

Also, be aware that if you're looking at a new 105 chainset the chainring spacing is different so it may not work well with your current FD.

If I've done my maths correctly (can somone please peer-review the following statement!) for every additional pair of teeth on the chainring you should need one extra link pair (outer and inner link) in the chain as the effective length of the chain should be affected by half the change in the number of teeth on the crankset. When moving from 46/36 to 50/34 and assuming your current chain is correctly-sized and not excessively worn I reckon:

- To retain the same RD position on the small/small gear setting you'd be looking at losing one link pair from the chain.
- To retain the same RD position on the big/big you'd be looking at adding two link pairs to the chain.

Assuming the RD has the capacity to accomodate the extra range, the ideal chain length will be somewhere betweenadding one link pair and removing two; so you may just get away with it as is..

There's a good youtube video by Park Tool about speccing the length of a new chain that should give a good idea of what to look or. You could also split the chain and use a chain holder (like the one pictured below) to effectively lengthen and shorten it which should give you some idea of what's going on

 

[RussellZero]

What are you looking to achieve? A higher high gear or a lower low gear?
The difference between a 46 and a 50 is equivalent to one tooth difference on the cassette, so might be easier to change the cassette if not currently with an 11 cog.

Higher high gear - at the moment I spin out too much on the down hills.

 

[Sharky]

Higher high gear - at the moment I spin out too much on the down hills.

What's the little cog on your current cassette?
A 46x11 would be a pretty big gear.

 

[RussellZero]

What's the little cog on your current cassette?
A 46x11 would be a pretty big gear.

I think I have 11-28 on both bikes. I’ll check tomorrow. But definitely notice the difference with the 46 vs the 50 when swapping bikes. (And I miss the smaller gear when climbing on the cx bike vs the road bike)

 

[Cycleops]

You can make a chain holder out of an old wire coat hanger like the one @wafter posted, very useful bit of kit.

 

[RussellZero]

What's the little cog on your current cassette?
A 46x11 would be a pretty big gear.

But you’re right. When I look on the gear calculator I’d have to be spinning a lot to be going that fast. To be honest it’s not a massive problem I’m mainly curious and thinking about making tweaks here and there. Was thinking about going up to a 52/36 on the road bike but then I don’t spend very long in the biggest gear at 90+rpm so probably wouldn’t benefit much.

 

[Sharky]

Have a think about what gear you use when bombing along on the flat. With a 50/34, I was finding that the comfortable gears were in the crossover range. That is in the big/big combination or small /small. To change up or down involved changing the front and rear gears simultaneously. Got fed up with this and with a deep breath, swapped the double for a single 40t and removed the front changer. Now a single click up or down is all that us needed. Have lost the top and bottom extreme gears, but works for me.

 

[Ajax Bay]

If I've done my maths correctly (can somone please peer-review the following statement!) for every additional pair of teeth on the chainring you should need one extra link pair (outer and inner link) in the chain as the effective length of the chain should be affected by half the change in the number of teeth on the crankset. When moving from 46/36 to 50/34 and assuming your current chain is correctly-sized and not excessively worn I reckon:

- To retain the same RD position on the small/small gear setting you'd be looking at losing one link pair from the chain.
- To retain the same RD position on the big/big you'd be looking at adding two link pairs to the chain.

for every additional 4 teeth on the chainring you should need one extra link (one inch - outer and inner link) in the chain as the effective length of the chain should be changed by half the change in the number of teeth on the crankset (because the chain is only engaging half the ring init) and each extra tooth requires an extra half inch of chain.
Small/small 'doesn't matter' - the RD can either handle it or not. And if it can't then it will 'tell' the rider, who can then shift into the large chainring.
Large/large matters. Assuming the chain is the correct length to start with, going from 46 to 50t needs the chain to be 1inch longer.

do I need to change the chainlength as a result? If I was making this permanent I’d put a new chain and cassette on anyway, but just wondering before I waste a load of time messing around!

For the OP trying it out, it doesn't matter - use the current chain, but consciously avoid large chainring to largest two sprockets during the trial.