question on size of cranks (chainset)

feck sake lads

quick question for the tech lads, i have a chance of getting a triple chainset at a good price , but the cranks are 175 i usually ride 170 will this make a Hugh difference .
i dont want to but if it's going to be completely wrong crankset.
thanks all in advance.

Keep_Her_Lit

I’ve swapped back and forth between 170 and 175 a couple of times without any problems. The longer your legs are, the less difference it will make. I’m about 180cm (5’11”) tall with approx. 84cm (33”) inside leg.

You should drop your saddle by 5mm to maintain the same knee/ankle bend at the bottom of the pedal stroke, since moving from 170 to 175 will lower the pedal by this amount. Conversely, your foot will then be 10mm higher than it is currently at the top of the pedal stroke.

As far as spinning is concerned, you will produce slightly more torque with 175mm cranks but your feet will have to travel further to do it, since you will be working on a 350mm circle instead of 340mm. This would suggest that you might sometimes choose to push a higher gear at lower cadence. In practice, I have never really noticed any difference.

In summary, if you have typical leg length or longer and you aren’t prone to knee/ankle problems on the bike, I’d say go for it. If not, you should probably research it a little further.

feck sake lads

keep her lit thanks a million for that ,i'm only a little fella 5.6 but i think i will go with dropping the saddle trick.i'm just trying to build up my old but fantastic raleigh 753 into a light touring bike (trailer);)
anyway thanks for the info much appreciated.

Raam

I think you should stick to what you are used to. Typically shorter folk go for shorter cranks. I'm 6'3" and ride 175. I have used 170 and 172.5 and I really don't like them. Now, if I had started off on 172.5 then maybe I would like them, but my first bike came with 175 so that is what I am used to.

feck sake lads

yes i understand but i could be getting them at a good price and the funds are pretty low these hard times;)

Raam

Physios ain't cheap

lennymc

fsl - i have a sora double that 175mm. its currently on a bike that i'm doing some work on, but if you wanted a loan of it to get a feel for the 175mm crank if you like.

Only take 2 minutes to whip off.

what bottom bracket do you have?

http://www.chainreactioncycles.com/Models.aspx?ModelID=43207

feck sake lads

Raam wrote: »

Physios ain't cheap

good point i should listen more thanks;)

feck sake lads

thanks lenny i'll talk to you in the morn,
my b/b would not work on that.

lennymc

feck sake lads wrote: »

thanks lenny i'll talk to you in the morn,
my b/b would not work on that.

ok. talk to you then.

amjon.

I'm not sure it makes any difference. Apparently when Boardman was preparing for one of his shots at the hour he did a session with cranks 7.5mm longer than normal by a mistake and there was no difference.

feck sake lads

i have just measured my two other bikes center to center wait for it :o175mm. so it looks like i have been riding 175 all the time ,jeez i can't believe this i will have to measure then again what an idiot.

edit
double checked it's 172.5 .

Keep_Her_Lit

FSL, if you’re confident that your measurement of 172.5 is accurate (the crank length isn’t stamped anywhere on either crank? – sorry if that’s a daft question), then an increase to 175 isn’t drastic. For your height, it’s probably a change in the wrong direction but it’s only a small change and IMHO you would be unlucky to suffer any ill effects as a result.

Caveats:

1. If you’ve suffered any previous injuries relating to bike set-up or are carrying any known problems or weaknesses in this area, then yes, you should be careful, since a small change could trigger problems which remain below the surface with your current set up.

2. If you intend doing serious mileage on your renovated 753, you should take that into account also. It seems very unlikely that an increase of only 2.5mm in your crank length would cause any immediate problems. But if you’ll be turning those cranks many thousands of times in one day, then something akin to an overuse injury could crop up. So if you have plans for longer sportives and all of the training that goes with it, proceed with caution and “listen” to your knees during and after each ride.

3. If you’re thinking of doing something really crazy … like cycling all the way around Ireland … without stopping … then I’ll defer to Raam on this one.

My hunch is still that you’ll be fine ….

feck sake lads

KHL thanks for that i think i should be ok ,when i do eventually get this bike on the road i will be swapping between bikes .
i will be doing short over nighters on the 753 pulling my trailer:D:D
on bigger tours i will be using my thorn touring bike plus panniers.
anyway lads thanks for all the advice much appreciated.

J Madone

Just to add to this that I have 172.5mm on one race bike and 175 on my other race bike, no noticeable difference for me.
Have raced past two weekends on both
And I'm 6ft 2 for what it's worth
One trek I have is a 58 and the other a 60 cm
The shorter bike happens to have the shorter cranks. Had 175 mm cranks on it prior to my new groupset was only available as 172.5mm at the time of purchase. All good so far
And hello to Caroline, met her at the start of the A3 race today.

feck sake lads

just curious but how did you get on today at the race, where was it.

J Madone

Was spectating in newbridge today,had my girls with me so no racing.
Still thawing out from yesterday in Clonard

C3PO

I have 170mm on my roadbike, 172.5mm on the cross bike and 175mm on the mountain bikes ....... don't think it makes any difference except for the obvious fact that you need to alter the saddle height slightly if you measure from crank centre!

doozerie

I have 170mm on my road bikes and 175mm on my MTB, just because I couldn't get shorter cranks for the MTB at the time. I do perceive a difference when riding the MTB on the road, but I couldn't swear that the perception is not entirely in my head (MTB bottom bracket is wider too, so crank length is not the only thing influencing the difference in feel). By choice I'd go with nothing longer than 172.5mm on the MTB, and probably even 170mm as it spends all of its time on the road these days rather than off-road. I am 5'7" but in reality height alone is a very simplistic basis for deciding on crank length - length of upper and lower legs, and perhaps even foot size, are better guides.

The general thinking is that longer cranks give greater leverage so better suited to riding off-road where you may not want to be spinning gears a lot i.e. you can turn a higher gear with longer cranks for similar effort to turning a lower gear with short cranks. Greater leverage might also be of benefit when pulling a trailer, but the same kind of effect can be achieved anyway by your choice of gear and cadence. "Ideal" crank length is actually (yet another) topic of contentious debate in cycling, with people arguing from all camps about whether longer cranks make no difference, a positive difference, or a negative difference. The arguments usually get into boring detail about torque, leg speed, etc., very quickly but there does not appear to be a winning argument amongst them so it largely boils down to personal preference at the end of the day.

J Madone

Heard of a guy who rode the Ras on a borrowed set of cranks, wasn't noticed til after that one side was 170 and the other 175