Crank lengths

rob39

Renewing my current chainset which has 175mm cranks. Just wondering is there any pro's/cons to getting 170mm over 175mm. Or is it just personal choice?

The Northern Monkey

I put 170mm on my Canyon instead of 175mm and didn't notice any difference. I bought them because they were £10 cheaper

cooldad

Guarantee you won't be able to tell what's on without looking.

omegas

rob39 wrote:

Just wondering is there any pro's/cons to getting 170mm over 175mm.

5mm less chance of you grinding your pedals out

Northwind

Putting 165s on my dh bike meant I no longer knee the top of the forks when pedalling out of the saddle!

But on all my other bikes,I can't feel any difference. I've got a set of 170 and a set of 175 XTRs, no clue which is on which bike.

Hacker60

I've been looking at getting more ground clearance for getting over rocks, logs that kinda thing,
how small do cranks go, and what's the smallest you can get away with ? If 175 is the average I'd think
150 would make a decent difference ? Can you get much small Chain rings, I was thinking of changing
my Triple for a 2 x 10 as I want a bash guard. I'm a bit of a clumsy rider, so need all the protection I
can get... any help... anyone..

1mancity2

Hacker60 wrote:

I've been looking at getting more ground clearance for getting over rocks, logs that kinda thing,
how small do cranks go, and what's the smallest you can get away with ? If 175 is the average I'd think
150 would make a decent difference ? Can you get much small Chain rings, I was thinking of changing
my Triple for a 2 x 10 as I want a bash guard. I'm a bit of a clumsy rider, so need all the protection I
can get... any help... anyone..

Just improve your technique, alot of the time you can judge were its going to happen, going twin and bash will help but won't stop you hitting your pedals only technique will stop that, odviously shorter cranks will help.

1mancity2

rob39 wrote:

Renewing my current chainset which has 175mm cranks. Just wondering is there any pro's/cons to getting 170mm over 175mm. Or is it just personal choice?

Only time I noticed different crank lengths was on a bmx which had 180mm, just felt too long, going shorter never noticed it.

.blitz

Each to their own I guess but I have 175s/170s/165s on three bikes and they all feel different. There is noticeably less leverage on the smaller cranks especially when grinding climbs it feels as though my legs are turning in itty-bitty circles like riding a child's bike.

On the plus side there is quite a bit more ground clearance with the shorter cranks esp on a FS where there's a lot going on at both ends.

The Rookie

165mm is the shortest you'll normally get.....besides many are the same arms with the pedal hole drilled in a different place!

If you're hitting cranks, learn to position them, any length will still hit if you have a pedal down rather than having them level.

It's easy enough to go froma triple to double and bash, yes.

Simon

Hacker60

Cheers for the tips

I think I will save my pennies for now, looking to put some Crank Mallet 2's
on over the next week, and a chain guide as I'm trying to tighten up my kit. Getting to grips
with riding an FS and different type of riding. I think I need to really work on my technique
"as mentioned" - or something is going to break, probably me rather than the spesh.

Scott Scale 60 - Stock
Spesh FSR XC - Custom

rockmonkeysc

Better technique will help more than new cranks and it won't cost £100+.
Positioning the cranks for obstacles help so does manualing or jumping through stuff. For rock guardens & roots speed helps a lot.
Most downhill bikes have very low bottom brackets & 165/170mm cranks and they are ridden hard on the ugliest trails without pedal strikes being a huge problem with the right technique

Scalpel_79

longer arms.weight[force] = bigger momentum:
Mmax.=F x r
where:
F- Force [your weight]
r- radius

Build two bikes, while one with 2.5mm either longer or shorter arms than on the other and ride same trails- YOU WILL see the difference. I cannot take same trails with same gear on both [one with longer arms makes everything a bit easier]- BUT!!!, IT IS ONLY VISIBLE WHEN RIDING SAME TRAILS. If you know how to work your arms, you will not be hitting anything [it "only " makes difference on your way up]

Northwind

Scalpel_79 wrote:

Build two bikes, while one with 2.5mm either longer or shorter arms than on the other and ride same trails- YOU WILL see the difference.

Swapping cranks on the same bike will achieve the same. I didn't, sorry, DID NOT see the difference.

Scalpel_79

got two + ...:
one on extralite E-Bones 175mm
and
XTR FS M-970, 177.5mm
NOT SWAPPING ANYTHING

cooldad

In a word, bollocks.

Scalpel_79

cooldad wrote:

In a word, bollocks.

what is pal?

Northwind

Scalpel_79 wrote:

what is pal?

This is:

"YOU WILL see the difference"

Scalpel_79

and what is wrong with that?- i have got two hts,- similar weights, but all I take on with the one that has got longer arms is easier

Scalpel_79

no rocket science there- Statics!

cooldad

Scalpel_79 wrote:

and what is wrong with that?- i have got two hts,- similar weights, but all I take on with the one that has got longer arms is easier

Can't argue with your 'science'.








Oh wait. I can.

Scalpel_79

cooldad wrote:

Scalpel_79 wrote:

and what is wrong with that?- i have got two hts,- similar weights, but all I take on with the one that has got longer arms is easier

Can't argue with your 'science'.








Oh wait. I can.

That is what I thought you have got.
Same goes for 3/4 of your comments. Spread your arrogance and stupidity someplace else, will you?

Torres

Scalpel_79 wrote:

longer arms.weight[force] = bigger momentum:
Mmax.=F x r
where:
F- Force [your weight]
r- radius

My first issue is that force*distance is a torque (where a larger torque will make it easier to spin the cranks), secondly the force is NOT just your weight; sit on the saddle and spin, you're still applying force to the pedals but supporting little weight on them. Your legs will produce a downwards force as you pedal (putting numbers on this is difficult.)

For arguments sake lets say you exert 600N of force on one pedals (pulled this figure out of the air, but it's roughly my weight force). T
he torque produced on a 170mm crank arm is
0.17*600=102 N/m.
On a 175mm crank the torque produced is
0.175*600=105 N/m

Thats less than a 3% increase in the torque produced. I call shenanigans on anyone who can notice this in a blind taste test.

Scalpel_79

Torres wrote:

Scalpel_79 wrote:

longer arms.weight[force] = bigger momentum:
Mmax.=F x r
where:
F- Force [your weight]
r- radius

My first issue is that force*distance is a torque (where a larger torque will make it easier to spin the cranks), secondly the force is NOT just your weight; sit on the saddle and spin, you're still applying force to the pedals but supporting little weight on them. Your legs will produce a downwards force as you pedal (putting numbers on this is difficult.)

For arguments sake lets say you exert 600N of force on one pedals (pulled this figure out of the air, but it's roughly my weight force). T
he torque produced on a 170mm crank arm is
0.17*600=102 N/m.
On a 175mm crank the torque produced is
0.175*600=105 N/m

Thats less than a 3% increase in the torque produced. I call shenanigans on anyone who can notice this in a blind taste test.

I was taking into an account the very same person [of the very same weight----at the time] using first- bike with crankset where arms are longer and the switching to the very same [or similar weight bike], BUT with shorter ones DOING THE VERY SAME TRAIL [just as you put it in your calcs]- 3% [you mentioned] would be someplace there- IS SOMETHING YOU FEEL [I DO]- EVEN THOUGH IT IS THIS SMALL

Torque=Momentum

---

just different terminology [studied engineering abroad- that is why]

Scalpel_79

different way

one rider=same weight= same distance
an hour race
3% advantage over longer crank arms
60mins/100%= 0.6min x 3 [%]= 1.8min. advantage=============(LONGER ARMS)

it is all down to the detail at the end [when i.e. competing]

ddraver

Ignoring the above, how tall are you? I'm 171cms (plus I reckon I ve got short legs too) and struggled with 175mm cranks, I found 170mm much more comfortable over a long day. To get my saddle in the right height with one leg extended, it feel like i was kicking myself in the chin with my knee...

If you re a normal height though then it does nt make any difference

For the above - Cervelo (that road bike maker) did a study on this as varying crank length can help get a better TT position. They found that no matter what crank length people used, they changed gear to compensate so they were producing the same power. I.e, if they shortened the crank 10mm, they just used one cog lower

Torres

I see what you're saying, however this model isn't perfect. There are many flaws:
1) Force is not applied at a constant angle to the crank arm
2) The force is not uniform due to shifts in the riders weight and the variable force exerted due to pedaling (not just weight)
3) Flex in the rider/crank interface will alter the force exerted.

3% is only a marginal increase anyway; and this requires a perfectly replicated situation; which in the real world just doesn't happen. One could gain more than 3% extra torque by paying close attention to techneaque; and power output will depend on food/rest etc.

As a result of all these factors i PERSONALLY do not think a benefit will be notice. We may have to agree to disagree.

Torres

Scalpel_79 wrote:

different way

one rider=same weight= same distance
an hour race
3% advantage over longer crank arms
60mins/100%= 0.6min x 3 [%]= 1.8min. advantage=============(LONGER ARMS)

it is all down to the detail at the end [when i.e. competing]

Not strictly true, as this would assume you're pedalling constantly over the duration of a race. Possibly more applicable in road racing, but for mountain biking i would imagine (and i'm purely speculating) that the improvement would be less than a minute [this is again assuming a perfectly replicably situation as I mentioned above.] Personally my lap times can vary by that amount purely on how I'm feeling and on the conditions on the day (variations in the wind, course deterioration etc.)

Scalpel_79

Torres wrote:

I see what you're saying, however this model isn't perfect. There are many flaws:
1) Force is not applied at a constant angle to the crank arm
2) The force is not uniform due to shifts in the riders weight and the variable force exerted due to pedaling (not just weight)
3) Flex in the rider/crank interface will alter the force exerted.

3% is only a marginal increase anyway; and this requires a perfectly replicated situation; which in the real world just doesn't happen. One could gain more than 3% extra torque by paying close attention to techneaque; and power output will depend on food/rest etc.

As a result of all these factors i PERSONALLY do not think a benefit will be notice. We may have to agree to disagree.

yes, but I am still talking about one----the very same guy using different lengths of arms, same weight, strength, movement,...etc- just the way you put it. Put one different on the very same bikes, BUT with different lengths of arms and he WILL get THAT EXTRA THE WHOLE THING IS ALL ABOUT

Different example:
undo your pedals with standard [whatever] key
I give YOU two very same keys to undo the very same crank, BUT!!!! ONE KEY SHORTER than the other- there will be a point [it could be calculated EASILY] where you have no problems undoing your pedals, BUT!!!! you will NOT be able to do the very same thing with the shorter one

---

EVEN!!!!!!!!!!!!!!! if only by 1, 2, 3mm SHORTER- it is math- YOU WILL NOT MAKE IT!!!- NOT A CHANCE!!!

Scalpel_79

[quote="...to compensate so they were producing the same power. I.e, if they shortened the crank 10mm, they just used one cog lower[/quote]


And there you have it?

Point is- do the very same trail using different cranks' lengths
Which one is going to be easier using?----the one with longer arms? or the one with shorter?

ddraver

^ the one that fits.....