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165 vs. 170 crank length

big-ted

Danced with A, attacked by C, fired by D.
Sep 27, 2005
1,400
47
Vancouver, BC
Definitely. That's why BMXers/4X riders often run 180s. More leverage = more force at rear wheel for gates etc. Is this important for DH? Popular consensus would suggest not as important as being able to pedal through rock gardens/corners without fear of clipping a pedal.
 

CBJ

year old fart
Mar 19, 2002
12,860
4,154
Copenhagen, Denmark
Yes, but BMX is single speed. What happens when you add gearing to the equation. I know this has been discussed before just can't find the thread.
 

In8Racing

Monkey
Jul 5, 2006
292
0
Trying to find some skillz...
The ability to turn any gear is dependent on the length of the crank - even if you have 1 vs. 9.

Shorter cranks = Lower maximum power.

That being said, you could run 180's but if you can't turn them it wouldn't matter anyway...
 

SuspectDevice

Turbo Monkey
Aug 23, 2002
4,155
355
Roanoke, VA
There is absolutely no correlation between crank length and power.

Power is force X angular velocity

Leverage does not equal force.

Let's point out that the two most "power-centric" events in the world (Kilo and match sprint) are normally won on 167.5 cranks.... It is about legspeed and neruomusuclar recruitment, not muscle-car style displacement.
 

skate

Chimp
Aug 19, 2006
84
0
There is absolutely no correlation between crank length and power.

Power is force X angular velocity

Leverage does not equal force.

Let's point out that the two most "power-centric" events in the world (Kilo and match sprint) are normally won on 167.5 cranks.... It is about legspeed and neruomusuclar recruitment, not muscle-car style displacement.
This is the most ridiculous thing I have ever hear.
 

ChrisKring

Turbo Monkey
Jan 30, 2002
2,399
6
Grand Haven, MI
power = force * crank length * RPM [fixed this]

Anyway, back to the original question. I don't notice any power, efficiency, top speed, etc difference with 165mm cranks. I do notice that I hit rocks less with 165s than 170s. I would have never predicted it based on that it is only 5mm.
 

sanjuro

Tube Smuggler
Sep 13, 2004
17,373
0
SF
There is absolutely no correlation between crank length and power.

Power is force X angular velocity

Leverage does not equal force.

Let's point out that the two most "power-centric" events in the world (Kilo and match sprint) are normally won on 167.5 cranks.... It is about legspeed and neruomusuclar recruitment, not muscle-car style displacement.
Uh, match sprints and the kilo are based on super high cadence, not just power outage.

While this is not relevant to this discussion, what length crank arms do pursuiters use?
 

iridebikes

Monkey
Jan 31, 2004
960
0
seattle
there is a bit of torque lost with going with shorter cranks, however it is easier to have a faster cadence with shorter cranks. 165mm cranks on a bmx are sweet! 165's on a dh bike work out well because the faster your cadence the less bob and the more clearance you have so you clip rocks less. less bob+less pedal clipping+faster pedaling= faster riding...


On my 4x bike, hardtail, and bmx I run 170's because 175's are too long, I run 165's on my dh bike because of clearance. I prefer 170's because it's a good median between high cadence and high torque
 

big-ted

Danced with A, attacked by C, fired by D.
Sep 27, 2005
1,400
47
Vancouver, BC
There is absolutely no correlation between crank length and power.

Power is force X angular velocity

Leverage does not equal force.

Well, you were doing ok until the last bit, and I can see what you're getting at. You're ight, I should have pointed out I was talking about force, not power.

The angular speed with which one can spin the cranks is reduced for longer crank lengths, such that power output should be constant regardless of crank length. Of course, this neglects the issue of ergonomics, and people with longer legs are generally more efficient with longer crank arms, but no-one seems to care about that in DH anyway.

However, my post pointed out that FORCE at the rear wheel is increased by using a longer crank arm, which it is, by basic Newtonian levers. Acceleration being Force/mass, one is able to accelerate better with a longer crank, hence BMXers run longer cranks for gates etc. Which is what I said.

And yes, the track events you mention depend on maintaining power output at high cadence, hence shorter cranks are used such that the rider is more efficient at higher cadences.
 

SuspectDevice

Turbo Monkey
Aug 23, 2002
4,155
355
Roanoke, VA
Uh, match sprints and the kilo are based on super high cadence, not just power outage.

While this is not relevant to this discussion, what length crank arms do pursuiters use?
167.5 All trackies for the most part run the short cranks for two reasons, ground clearance and optimal cadence, so do prefer longer cranks, hell Indurain ran 180s for his hour.

All track, bmx, mtb or road cycling events are based on power output, and the best way to maximize power output is to get peak contractile velocity of the atheletes muscles at the same time or as close to the same time you are getting peak recruitment.
Time and time again, for ~80% of the athletes I have examined sprint files with from a power meter, peak power output occurs at 110-140 rpms. If you back calculate gear inches from that and look at their speeds you'll see that they are also using relatively small gears to make their peak power.

Hence perhaps another reason Hill ran the 34 so much last season?
 

Zutroy

Turbo Monkey
Dec 9, 2004
2,443
0
Ventura,CA
All the studies I've seen published only found in increase in power output once you went to insainly long cranks like 220mm, and even then it wasn't that much higher than a 170.

Also don't forget you can switch gears on a DH bike, that's a bigger deal than the crank length.

The length really only has an effect on power when it's on a fixed system like a track bike or BMX.
 

Repack

Turbo Monkey
Nov 29, 2001
1,889
0
Boston Area
I went from 175 to 165 on my DH bike. What I noticed more than anything was that the bike felt more stable due to my feet being almost an inch closer together. I also don't catch as many rocks.
 
Feb 13, 2002
1,087
17
Seattle, WA
I ran a 175 on one side and a 170 on the other for a full year before I noticed. (warranty sent me back mismatched arms)

I can tell the diff from the 180s on my bmx. I reckon for a tall gent like myself, using short-ass cranks doesn't allow for the most efficient use of my stature.
 
Oct 8, 2003
202
0
I have spare 170 holzfellers and 175 holzfellers and I just tried each on my 4x, dirtjump bike ( it is a HT and has a 9 speed 12-16 cassette and 36t up front) and when going from 0 to say 10 or 15mph I could feel a small diffrence when starting off in 4th gear. the 175s were slightly easier. When starting in 2nd and sprinting and grabing a gear or 2 couldn't really feel any diffrence.
 

ChrisKring

Turbo Monkey
Jan 30, 2002
2,399
6
Grand Haven, MI
The angular speed with which one can spin the cranks is reduced for longer crank lengths, such that power output should be constant regardless of crank length. Of course, this neglects the issue of ergonomics, and people with longer legs are generally more efficient with longer crank arms, but no-one seems to care about that in DH anyway.

However, my post pointed out that FORCE at the rear wheel is increased by using a longer crank arm, which it is, by basic Newtonian levers. Acceleration being Force/mass, one is able to accelerate better with a longer crank, hence BMXers run longer cranks for gates etc. Which is what I said.
However, my post pointed out that FORCE at the rear wheel is increased by using a longer crank arm, which it is, by basic Newtonian levers. Acceleration being Force/mass, one is able to accelerate better with a longer crank, hence BMXers run longer cranks for gates etc. Which is what I said.
BMX bikes are typically single speed. DH bikes have multiple gears. Force at the wheel is the same if you shift.
 

offtheedge

Monkey
Aug 26, 2005
955
0
LB
power loss doesn't mean sh*t if you are clipping pedals and for Dh racing pedal/ground clearance will be more relevant than cadence IMO.
 

dsotm

Monkey
Jul 21, 2006
151
0
WRJ, VT
There is absolutely no correlation between crank length and power.

Power is force X angular velocity

Leverage does not equal force.

Let's point out that the two most "power-centric" events in the world (Kilo and match sprint) are normally won on 167.5 cranks.... It is about legspeed and neruomusuclar recruitment, not muscle-car style displacement.
That being said, Torque is equal to Radius times force (in this situation atleast). However, there is still the question of whether or not the increase in actual torque is more than gearing can compensate for and whether or not the ability to pedal more often is a greater advantage. I'd say that gearing can make up for it as well as the ability to pedal in more places, but that's without actually calculating any numbers.
So, since the length of the cranks is only one of several factors in the actual amount power and torque produced, short cranks can be easily compensated for by other factors such as gearing.
 

pdawg

Monkey
Feb 27, 2006
310
0
Espoo, Finland
power = force * RPM

Anyway, back to the original question. I don't notice any power, efficiency, top speed, etc difference with 165mm cranks. I do notice that I hit rocks less with 165s than 170s. I would have never predicted it based on that it is only 5mm.
That's good to know... I'm considering a similar thing going from 175 to 170. I was concerned that shorter cranks would be like pedaling a children's bike.
 

heikkihall

Monkey
Dec 14, 2001
882
0
Durango, CO
I ran a 175 on one side and a 170 on the other for a full year before I noticed. (warranty sent me back mismatched arms)

I can tell the diff from the 180s on my bmx. I reckon for a tall gent like myself, using short-ass cranks doesn't allow for the most efficient use of my stature.
I am the same way. I can almost never tell the difference between crank arms, even if they are mis-matched arms on the same bike. The one time that I did notice a significant difference was when I rode my street bike down to the shop with a pair of 175s on and rode back from the shop with a pair of 170s on. It felt a little funny for about 5 minutes. But bike to bike I can never tell any difference.

As it was mentioned, when gears are involved the difference is made up simply by changing the gears. Force has almost nothing to do with this argument when you consider that there are gears on the bike. Power is the only base. Force will only matter for the first foot out of the gate; which is probably the equivalent of that 1/4th revolution from your starting position to the bottom of your leading foots pedal stroke.

Fact is, if you do not have enough leg speed/ power to match that force you will be dropped on the first straight.
 

Acadian

Born Again Newbie
Sep 5, 2001
714
2
Blah Blah and Blah
The one time that I did notice a significant difference was when I rode my street bike down to the shop with a pair of 175s on and rode back from the shop with a pair of 170s on. It felt a little funny for about 5 minutes. But bike to bike I can never tell any difference.
That's how I am...when I go from my XC bike (175) to my FR bike (170) or DH bike (165) I can't definitely notice a difference. but only for a first few miles.
 

SPDR

Monkey
Apr 21, 2006
180
0
Engerland
I'd go shorter than 165 if I could, hell I don't pedal much anyway and I'd much rather spin to win than grunt my way down even if I did.

Don't BMXers run 175+ for the leverage out of the gate for the holeshot, being as that is 90% of the race, even though they sacrifice ultimate cadence, they gain by being in front? Not really relevant for DH in any way. Hi RPMs and ground clearance rule.
 

PaulE

Chimp
Feb 7, 2003
99
0
Sheffield, England
If you want to go shorter than 165 mm, look into some flatland BMX cranks - they tend to run 165 as a maximum length. I think you can get Profiles in 162.5 and 160....
 

Huck Banzai

Turbo Monkey
May 8, 2005
2,523
23
Transitory
There is absolutely no correlation between crank length and power.

Power is force X angular velocity

Leverage does not equal force.

Let's point out that the two most "power-centric" events in the world (Kilo and match sprint) are normally won on 167.5 cranks.... It is about legspeed and neruomusuclar recruitment, not muscle-car style displacement.
Turning it into an excercise in relative vocabulary doesnt take away from the fact that longer cranks provide more torque allwoing you to push a higher ratio with the same effort.

Other factors play into specific applications, but even a novice engineer wouldnt and couldnt refute this basic principle.
 

Boxxer

Monkey
Jul 18, 2005
856
2
Dirty South
Turning it into an excercise in relative vocabulary doesnt take away from the fact that longer cranks provide more torque allwoing you to push a higher ratio with the same effort.

Other factors play into specific applications, but even a novice engineer wouldnt and couldnt refute this basic principle.
Amen to that!:clapping:
 

SuspectDevice

Turbo Monkey
Aug 23, 2002
4,155
355
Roanoke, VA
Right, longer cranks allow you to push a higher gear, but they DO not allow you to make more power, and for lots of people get in the way of making peak instantaneous power. I am faster out of a BMX gate on 165's than I am on 175's, and lots of other people would be as well if they took the time to think about what was really going on.
 

HearNOevil

Chimp
Aug 25, 2003
62
0
Nor- Cal
So I wonder why most pro bmxers run 180s??

What do you think about the new Truvativ OCTs not coming in 165s??

I was hoping to make the trasition to 65s this season and wanted to run the OCTs, so I'm bummed. I have been running 70s for years though so not that big of a deal.

I never heard of anyone runnin' 65s on a bmx. How tall are you? Does your local track have a steep start?
 

SuspectDevice

Turbo Monkey
Aug 23, 2002
4,155
355
Roanoke, VA
While I think that 180's on BMX bikes might work for some folks, what I am getting at is that the dogma of cranklength, and gearing for that matter is just that, dogma, and not really based on anything except tradition. I have a fella who I am helping get into the 26" scene after a decade of bmx racing. His cruiser has 190mm cranks!

here is a little post from our local BMX board that goes into more depth than I feel like getting into right now

The number 1 thing I always see in every cycling discipline is people sprinting in too large of a gear.

Power=force (pressure applied to pedals) x velocity(angular rotation)
A picture of what it takes to create peak power looks like the below...

Not even taking fiber type and individual metabolic characteristics into account, no rider creates their peak power output with a big gear, as the peak amount of force you can apply to the pedals is very minimal. Being a big meathead doesn't help, as the force you can put into the equation is extremely limited when compared to the amount of force that can be expended on the squat rack. Spriniting a bike is a very "specific" motion, and actually requires a good deal of technique.

Working on sprint technique and emphasizing leg speed is the way to be faster, period. Super huge cranks aren't going to give anyone a significantly better gate or first straight, and are going to hinder someone pedaling through a technical element.

I have reams of annecdotal evidence, as well as gigs of power-meter files from dozens of riders that show extremely large improvements in peak power output by dropping gears down and focusing on the technique.

This chart below has w/kg values that are good indicators of form and performance for road/track racing. I haven't done enough work with BMX riders to come up with a good idea where the amateurs are, but top adult experts and up seem to be in the top 3 strata for 5 seconds through 5 min as most road and XC racers. Fitness!

I work with one XC mtb racer in Georgia who went from a 900 watt sprint to a 1600 watt sprint over the course of 2 years. He also lost 30 pounds in process (going from 6' 180 to 150#) That is a big enough leap in power to weight ratio (measured in watts/kg) that he is now at 23.5 watts/kg.


I accomplished this by using a powermeter that gave him feedback on how much power he was really making and analysing the power-velocity curves post-hoc to see where his "sweet spot" is in terms of velocity. We then played with gearing until he was hitting that velocity with the peak amount of force, and hence the most power he is capable of producing. Now that he is making that kind of power in the sprint, we are slowly turning him into a track racer, where he can use his aerobic fitness along with his excellent sprint to win the endurance events at a world-class level. If it wasn't for being analytic about things, this kid would probally have stopped riding bikes a long time ago...


Spinning lighter gears doesn't mean you have to sacrifice track speed for gate speed either, you just have to modify your technique.
My freind and MTB teammate Walter is pretty much 100% better than me at BMX, but when we both race cruiser on our MTB's I can hang with him pretty well. He runs a monster gear (42-18 on 26" wheels!) and 180's and I run a tiny gear (36-21) and 165's. He beats me because he is wiley and just makes more power/weight.

Little gears spun fast with short cranks make just as much, or more power than big gears in all situations, and they are much easier to spin up to speed out of corners. As bmx tracks get less technical, shorter cranks also allow riders to pedal more over and through straights...

I predict with the very imminent introduction of power meters that are suitable to bmx use, and the fact that bmx racing is now an olympic sport we will start to see significantly more top pros, and especially upcoming riders using gears and equipment that are significantly more thought out than what we see now.


This really doesn't relate at all to DH crank selection any more, so sorry about the derailment, but long cranks have been one of my pet peeves for a really long time.
 

Zutroy

Turbo Monkey
Dec 9, 2004
2,443
0
Ventura,CA
The other problem you run into, is depending on the length of your legs and particularly the length of your femur, if your crank of too long or two short, that can put your leg in a less anatomically advantageous position to produce power.

I agree with Mickey there is an certain RPM/Force "sweet spot" on everyone for max power output and everyone is different.