changing from 170 to 172.5mm crank

fago

I've the original 170mm length crank currently on a small giant racer.

I've got a 172.5 ultegra crank for nothing which I'd like to swap in.

My inside leg length is 30 inches, which I know is too short for 170 according to the crank length calculator I've seen.
However it hasn't caused any issues up to now, no knee or hip pain on 100 mile cycles.

If I change to 172.5 can that 2.5mm (or 5mm in a sweep I suppose) make a dramatic difference?
Would you advise against it. I'll probably pay the LBS to change it, so I won't even bother trying it, if there's a high probability of issues.

ullu

I have the same inside leg length, 170 cranks on my commuter, 172.5 cranks on my proper road bike and have never had any issues with discomfort.

gmoorewest

I changed from a 170 to 172.5 back in feb. No problem. Also changed it myself which got me out of the "Oh I better get someone who knows", which gave me the confidance to get back to doing the work on the bike myself.

Quigs Snr

Those fit calculators are rubbish. I ride 170 usually, but I ride 172.5 on some bikes. I don't really notice much of a difference to be honest. Certainly nothing detrimental.

blorg

Presuming it is a new Ultegra crank and you have external bearing BB cups in there already it is a very simple job. A torque wrench is good for the pinch bolts though.

fago

Thanks for all the info. I've changed everything else before except crank, so maybe I should give it a go.

Here's whats currently on it:
CRANKS TruVativ Touro 2.0 36/50
BOTTOM BRACKET TruVativ Power Spline

Do you loosen this with an allen key at the crank?
I assume I'll have to change the bottom bracket aswell.

Do I need any special tools, or shall I just youtube for some example

Mr. Skeffington

Hi Fago,

The Ultegra chainset that you have wont work with the powerspline bottom bracket you have. Depending on the vintage of the Ultegra chainset you will need to replace the powerspline bottom bracket with either:

Hollowtech bottom bracket cups
Octalink bottom bracket
or on the really old ultegra square taper

Id say its a newer style Hollowtech II chainset. In any event there will be very little difference in the extra 2.5 mm on the crank length, virtually un-noticable probably not worth the effort.

penexpers

Mr. Skeffington wrote: »

In any event there will be very little difference in the extra 2.5 mm on the crank length, virtually un-noticable probably not worth the effort.

+1 .

Here's a good article by Sheldon Brown on crank length

http://www.sheldonbrown.com/cranks.html

fago

I'm pretty happy 170 v's 172.5 isn't going to make any odds.

I was just going to change the crank since I have everything else upgraded on the bike, and assumed ultegra is better quality etc.

I have the BB cups aswell for the ultegra crank. Is changing the cups aswell alot of effort?

penexpers

fago wrote: »

I'm pretty happy 170 v's 172.5 isn't going to make any odds.

I was just going to change the crank since I have everything else upgraded on the bike, and assumed ultegra is better quality etc.

I have the BB cups aswell for the ultegra crank. Is changing the cups aswell alot of effort?

Not a huge amount of effort but you'll need some tools

- 8mm hex key to remove the crank bolts
- crank extractor to remove the cranks
- bb tool for powerspline to remove the bb
- hollowtech II bb tool to fit the new bb
- torque wrench with 4 or 5mm hex key to tighten the pinch bolts on the non-drive side crank correctly.

Lumen

fago wrote: »

I have the BB cups aswell for the ultegra crank. Is changing the cups aswell alot of effort?

No, if you have the tool and as long as it's not badly seized. Slip-slop with the anti-seize and in goes the new one. It's one of the more fun jobs IMO. Assuming you have a traditional threaded BB shell, not one of the press-fit ones.

Bottom brackets wear out eventually anyway. If the current cranks/BB are fine (i.e not causing you any problems) you could just leave it for now.

doozerie

Crank length certainly is one of those subjective areas. To start with though, make sure you are accurately measuring your inseam - it's not the same as the measure you use when buying trousers, your actual/cycling inseam is typically longer than that. For example, I buy trousers with a 30" leg but my inseam is actually 31.9" or thereabouts.

Here is yet another debatable source of advice which suggests (based on a measure of 20.5% of your inseam) that an inseam of 30"/76.2cm needs a crank length of 160mm. It also suggests 165mm for my inseam but I've been happily using 170mm for years (except on my mountain bike, where its 175mm cranks seem a bit ridiculous on the road). It is slightly interesting to see where some notable riders appear on their graph, though it wouldn't encourage me to move away from 170mm.

Homme Enigmatique

The idea of a longer crank is that you have more leverage & hence you might ride a bigger gear whick equals a slightly lower cadence. Remember that a lot of Pro's who may use 172.5 cranks as oppossed to 170 cranks will be doing races anything from 4 to 7 hours.
Consider the number of pedal revolutions say 42,000 for a 7 hour race if the Pro rode at an average of 100 rpm for a race like Paris Brussels.
You would expect the Pros to have more strength from all their cycling to be able to use a longer crank & hence a bigger gear.
If you use a longer crank your jump for sprinting will be compromised.
In theory you should also lower the saddle slightly as your foot will go lower & there will be also be an effect on your hip as you will lift your knee higher too. Remember that there will be a slight increase in the strain on your joints. Tendonittis often arises when cyclists go to longer cranks & start using bigger gears.
At a guess I would say that you will do just fine staying with your 170 cranks unless you really want to try a slighly longer crank but remember if it is'nt broken why try to fix it.

fago

Thanks for all the advice. Its probably best just to wait till there's enough wear to justify changing the BB.

From the tools listed its debatable whether it's worth my while investing in them for such an seldom job.