Crank Arms 165mm vs 170mm
 

I need some crank arms. Is it really that important to have 165mm vs 170mm arms? On Sheldon Brown's they hype 165mm for fixed riding. However, I can get some 170mm arms for much cheaper at a LBS. What length do you all run?

165mm no pedal strikes

I think this depends on how high the BB is?

On some road conversions you can have a frame with a 8cm BB drop (range is 8cm-6.5cm). On a dedicated track frame you are talking about a 5cm BB drop. That's a 15-30mm difference (which affects crank length maximum at the bottom of the pedal stroke)

Agreed. E.g. sugino 75 cranks (and others) run from 165 to 175 in 2.5mm incements. What you get I think is dependent on your bike. If you have a real track bike with a raised BB, the difference between 165 and 170 shouldn't really matter as far as the pedal strike issue goes. If you're riding a conversion, maybe go with 165.

Conversions really beg for 165mm cranks... My Maserati conversion started with 27" wheels with big 1 1/4" tires. I switched to 700c wheels, which dropped the BB a little bit, and then 23mm tires, which dropped it a little farther still. I actually pedal strike quite a bit, and its a terrifying thing to have happen. I think the stock cranks are 172.5mm. I'm definately putting on a set of 165mms as soon as I can find a set for a price I want to pay. ;)

peace,
sam

My conversion is currently bolted to a pair of ultra-cheap 170s. If I take a corner in town with any sort of speed, I get a LOT of pedal strike. So, I either slow down for corners or scrape the heck out of my Looks. Either way, I'm saving for a decent pair of 165s.

one thing that might be helpful (it is for me) is to lean your body more than your bike when you take turns on conversions. it often requires getting out of the saddle to do it much, but it'll allow you to take sharper turns.

other than pedal strike and clearance between foot and front wheel, are there spinning/pedalling advantages to 165 cranks?

Shorter cranks means your legs don't move as far forward and backward, which means you can spin faster. For two riders with the same gain ratio and different crank lengths, the one with the shorter cranks will be able to go faster, since they'll be able to spin their legs quicker.

peace,
sam

i run 175's on my conversion. so far so good. just slow for the corners. The Q of your pedals is going to make more of a difference in cornering clearance than 5mm on your cranks.
my track bike is going to have 170's on a 42 degree baked track.

Good point. Using something like an eggbeater would probably give you more clearance than toe clips for example.

the smaller the crank, the less distance your feet must travel per revolution (shorter radius gives smaller circumference). also, your feet will be traveling slightly slower for the same wheel speed on a shorter crank (probably not noticable with a 5mm difference)

What you lose with shorter cranks is leverage which you may notice when climbing or starting a sprint. A longer crank gives more mechanical advantage.

As for pedaling power in general, I heard somewhere that in a study of crank lengths between 110mm and 200mm, the really short cranks gave the most power overall (balancing leverage power with spin power). But I think that this study was really old and may have had some serious flaws.

so, in the end, I think the overall impression is that shorter cranks are better for spinning, longer cranks are better for mashing. You may not notice the difference between 170 and 165mm or it may make all the difference in the world.

tim

yup, platforms are bulky. funny thing... I was in the LBS a while ago talking to their track frame designer (former dutch national champion - designs frames for Cramerotti) He specs a 7cm BB drop on their frames since "modern pedals are so much narrower than the things [he] raced on".

eggbeaters are wider than you think. my cornering clearance actually dropped when i switched to eggs from cheapo SPDs.

oh yeah, I run 170's but only b/c I can't afford another crankset. They're on a fuji track and I've only had pedal strike once or twice.

tim

They didn't adjust seat position at all when they changed crank length. That throws the whole study out the window IMO.

When I go from 175mm cranks to 180mm cranks I have to change seat position or else I lose power with 180s. That's just a 5mm difference. The range of crank lengths in that study was huge.

P.S. I'm not an advocate of "proportional cranks" or anything like that. Too long of cranks can in fact hurt your cycling.

I converted a Centurion Le Mans 12. I had 175mmranks on it for a while with 40x14 ratio.

This LeMans has a higher BB by 1" over my new Lemond Poprad. (Don't remember off hand what the drop or height actually is)

I never had strike when I had 175mm cranks, but always worried about it and cornered conservatively. I changed cranks to Sugino 170mm (and changed ratio to 48x17), I opted for 170 over 165 as I am 6'2' and LBS strongly suggested 165 would be too short for me. I now corner much more agressively and still have no strike yet.

I mention the ratios above because one thing I found counter to the idea one can spin easier with shorter cranks is that with the 175x40x14 I could spin faster (130rpm) consistently than now with 170x48x17 where 120rpm is my steady state limit. Note these overall combined ratios end up being only about 2% different.

Al

i bet those 165mm fanatics don't even know how small 5mm is...

5mm isn't much, but a 165mm crank has 3cm less travel per revolution than a 170mm, and that is a noticable amount.

Also, a bike with 70mm of BB drop, and reasonably long platform pedals (shoe to shoe outer measurement of around 378mm) will strike at 25.6 degrees with 175mm cranks, 26.8 degrees with 170mm cranks and 28 degrees with 165mm cranks.

velocity = sqrt(9.8 * radius * tan (lean angle)), in meters, seconds and degrees.

That puts your max turn speed around a 12m radius at 16.75, 17.24 and 17.69 mph for 175, 170 and 165 mm cranks, respectively. Sure, 5mm isn't much, but that is a meaningful change in max turn speed, nearly 1mph between 175 and 165, all else being equal. You'd have to lower your Q by 20mm to equal the change achieved by lowering crank length by 5mm. Both methods will have the same effect, but crank length or BB drop gets you more turning speed per mm of change than Q.

*puts away his engineering calculator*

peace,
sam

My frame has a high track-style BB and i run a 165 mm which is a nice combo because you're practically guaranteed to avoid scrapin your pedals.

165 are good because there is more room to avoid a pedal strike and there is more room to avoid toe overlap. However I chose 170 because I like the extra arm leverage. I think using 165mm ranks on a conversion is a good idea because of the lower bottom bracket height. On my track frames i use 170.

I use 171's. They're purty.

i got a 170mm 105 crank for $29 shipped. not complaining. besides, the iro frame has a high BB

170s on a road convert--73.5 & 73.5... no overlap and havent struck ground yet... though sometimes during quick cornering pixies next to my head yell "WATCH OUT" and i uncontrollably understeer wide

So you went with the IRO frame, huh? (I think that one has a 5.0cm BB drop)

Have you decided on parts?

This is what I do. I also know the cornering limit of my bike pretty well. On fast, tight corners I end up leaning the top tube against the leg that is at the outside of the turn to guarantee that the inside pedal won't scrape.