rangerdavid

Ive been riding a 170 crankset. I am upgrading my group, and have a chance to get a really good deal on a new group, but it comes with a 172.5 crankset. i guess my question is, how much of a difference is that really going to make? Couldn't I just lower my seat a little bit, and micro-adjust my fit and make that crankset work? Or, is that just stupid to buy a size that I have never ridden to save a few (actually several hundred) dollars.

Silvercivic27

I ride 170 and 172.5 on different bikes and can't feel any differences.

MrTuner1970

Same here.

I could see where it might make some difference if your legs were really short, and 170mm was borderline for you. And, it would make a slight difference in cornering.

But after all ... it's only 2.5 mm.

datlas 

No significant difference.

I say buy it.

Syncmaster

I went from 165mm cranks on my fixie to 175mm cranks on my road bike and it didn't take long to adjust (the donor bike I used for parts onto a new frame was a much bigger bike than I would ride, hence the longer cranks). As long as the length doesn't hurt you when you ride, I think you'll be fine.

fstshrk

Go for it. I ride 170, 172.5 and 175 on three different bikes and I don't even think about it.
You may have to adjust your saddle down and backward (a bit) once you get the new crank.

adclark

You probably will barely notice the difference if you even do.

chi-james

I think you'll have to RAISE your seat a bit with the 172.5 cranks.

rangerdavid

thanks guys..... i really didnt think it would make much of a difference. think I'll go for it!!

FrenchFit 

IMHO, it makes a difference going the other direction. If you normally ride a 175, a 170 gives you cramps.

datlas 

If anything, longer cranks require the seat to be lowered. Assuming you want to keep the same position in full extension you will need to lower the saddle. Also may need to move saddle forward by 2.5mm.

But with only 2.5mm you may not need to do anything, it's pretty small.

MajorMantra

I rode 170s for a while and now have 172.5s because I got a great deal on a carbon crankset. Didn't notice the difference, apart from saddle height. Personally I can detect a change in saddle height of 2mm or more, but some people are less bothered.

david58

2.5mm = 0.098"

Tain't much...

hybridbkrdr

Can you say how long your inseam is?

mollusk

It is different and how much it will matter depends on you. If you have invested 100k miles perfecting your stroke on 170's for racing, then changing to 172.5's will feel like a huge change. If it is for just riding around, it isn't a huge deal.

rangerdavid

I'll have to check the data from my fit calculator, but it's around 31 or 32.

sethco

I have 175 on one bike and 172.5 on another. I *think* that I can tell the difference but it is minor and I'm happy with either. I can spin the shorter crank just a little faster and the longer crank feels a little smoother and more powerful at lower cadence. Or I could be imagining it or it could be some other fit or geometry differences between the bikes

Worst case is that the new crank doesn't work for you and you can sell it and buy a different one and still save money overall.

ancker

I agree. At the bottom of the pedal stroke you have flexion in the ankle to absorb the (small) increase or decrease of the crank arm length. At the top of the pedal stroke the angle of your knee is much more important. If you bring your knee up too far (caused by too low of a seat) you'll end up with really nasty pain where your biceps femoris tendon (hamstring) attaches to your fibula. Ask me how I know this.

I have one bike with 172.5 and another with 170 crank arms. I have the 170 one's saddle set a bit lower (center pedal spindle to top of saddle) than the 172.5 one.

rogerstg

+2. I don't notice any difference. YMMV if you're a "princess and the pea" type though.

rat fink

I upgraded cranks several years ago when I got a deal. All the cranks I got happened to be 172.5 while the ones I had before were 170. I definitely noticed a difference then, and I still notice a difference now. The next time I upgrade my crank, I'm holding out for the 170 arms. You could easily call me a 'Princess and the Pea' type, though.

...For what it's worth, I rarely think about this when I'm riding, since I've adapted over time, but if I get on a bike with 170s, it feels better. If I get on a (road) bike with 175s, I feel miserable.

ussprinceton

more torque on longer cranks

david58

I suppose the Noob in me just can't see a major difference - less than 0.1"? My socks - summer thin versus winter wool - can make that much of a difference.

oldbobcat

I'm amazed at how much advice can be given without asking one very important question--how long are your legs?

Campag4life

Pi * D = Pedal distance per revolution...so its 5mm diameter difference which btw may not be life altering...but an experienced cyclist can feel the diff and to me it matters.
OP..over time if you have long legs, consider 175mm cranks. Short to average length legs = 170-172.5. I believe Zinn may take crank proportion to leg length to an extreme but a 7 foot tall man probably shouldn't be on 170mm cranks.