Dual Hole Muni?

[unicyclist.ca]

I was wondering if the dual hole cranks are significantly weaker than regular cranks. I have 150/125 dual hole cranks for my 36er and they are great. I wanted to get some 165/137 cranks for my muni so I can use 137 for XC and 165 for DH/Freeride. Has anyone broke the dual holes in the second hole or is there another area that would malfunction first?

Thanks

[skrobo]

i have not heard of anyone breaking the holes off, and as long as you are not doing 8 foot drops i would not worry about it.
i've done multiple things on my unicycle that should have wrecked the hub/cranks IMO, but it is fine as the day i got it (well i do have tons of scratches... but oh well)

[kington99]

never heard of someone snapping a double crank acorss the hole. Bending stresses vary linearly along the crank so the site of the second hole may well still not be the point on the crank most likely to fail.

[skrobo]

Originally Posted by kington99

never heard of someone snapping a double crank acorss the hole. Bending stresses vary linearly along the crank so the site of the second hole may well still not be the point on the crank most likely to fail.

i didn't even think about that, but i doubt there is a spot more likely to fail. the hole seems too big for that.

[kington99]

Originally Posted by skrobo

i didn't even think about that, but i doubt there is a spot more likely to fail. the hole seems too big for that.

on 137/165s the bending moment right at the hub wil be about 5.5 times that at the 137 hole. The hole is pretty large and i think the crank also tapers but I doubt there is less than a fifth of the material cross section at the hole (actually should be considering I not area but they're roughly analogous).

[unicyclist.ca]

I think i am just going to use the 125/150s for muni for now and just put different cranks on my 36er until the 110/137 cranks come out. So no one out there has broken the cranks on the second set of holes? How many people out there are using them?

[skilewis74]

I think it'd prob be fine, esp if you don't do drops. But Kris doesn't recommend it.

[unicyclist.ca]

well i am not going to do ridiculous drops, just 3-5ish feet to a tranny usually. I am mainly wondering if anyone else is using them for DH muni or if anyone has ever had a problem (or knows of anyone who has had a problem) with them.

[skrobo]

Originally Posted by kington99

on 137/165s the bending moment right at the hub wil be about 5.5 times that at the 137 hole. The hole is pretty large and i think the crank also tapers but I doubt there is less than a fifth of the material cross section at the hole (actually should be considering I not area but they're roughly analogous).

wouldn't the moment at the hub be equal to that of the pedal's funky twisting moment at the end of the crank... just in an odd... more dynamic rotational direction?

[skilewis74]

The length of the crank acts as a lever, greatly multiplying the force on the axle.

For a dual hole cranked Muni, I think the closer hole would definitely be the "week link in the chain", and I bet it'd bend first at that hole if the longer position was used.

[kington99]

Originally Posted by skrobo

wouldn't the moment at the hub be equal to that of the pedal's funky twisting moment at the end of the crank... just in an odd... more dynamic rotational direction?

my previous analysis only considered the bending in the plane of the end of the wheel. The fact that the loading isn't at the end of the crank but half way along the pedal axle causes a torque about the crank axis that is constant along the body of the crank. This means you have a more complex failure mode than simple failure in compression/tension due to beam bending.

I'm so bored that I did a quick bit of FEA on this out of interest, this shows siginificantly higher stresses near the root of the crank than around the extra hole.

[joemarshall]

Dave - do you take into account the inserts (which are a bit bigger than the crank holes themselves) , or do they act like solid parts of the crank once they're in there?

One other thing - I'm sure this isn't the case for the moment cranks, but I know one of the custom cranks Roger made up before RTL was double drilled, and I think broke at the inner hole while the outer hole was being ridden. Presumably this is because those cranks aren't designed originally around double drilling?

Joe

[kypa]

I have a question.

What do you think about the length of the cranks for muni?

I brought a KH24 with crcank of 150mm, before i rode a qu-ax with 170mm.
For me there is a big diference betwen 150 and 170.
Now i'm thinking to buy other cranks but i don´t know if choose 165 or 165/137double hole crancks.

What do you think?

Sorry about my english, is not too good.

[tholub]

Originally Posted by kypa

I have a question.

What do you think about the length of the cranks for muni?

I brought a KH24 with crcank of 150mm, before i rode a qu-ax with 170mm.
For me there is a big diference betwen 150 and 170.
Now i'm thinking to buy other cranks but i don´t know if choose 165 or 165/137double hole crancks.

What do you think?

Sorry about my english, is not too good.

In the long run, 150mm is better than 165/170mm for most people. Ride more, you'll get used to it.

[kington99]

Originally Posted by joemarshall

Dave - do you take into account the inserts (which are a bit bigger than the crank holes themselves) , or do they act like solid parts of the crank once they're in there?

One other thing - I'm sure this isn't the case for the moment cranks, but I know one of the custom cranks Roger made up before RTL was double drilled, and I think broke at the inner hole while the outer hole was being ridden. Presumably this is because those cranks aren't designed originally around double drilling?

Joe

I didn't consider the inserts actually, I guess when you got to crank snapping force there would be some interaction between the body and the insert which would weaken the overal piece at that point.

Do you know what Roger based his cranks on? it's possible if there was enough taper along the length and/or the distance between the two holes was larger (or indeed if both the lengths were shorter so the distance between was a larger proportion of the total length) then the weak spot would indeed be at the second drilling site. It's possible that something about the machining process, such as leaving sharp edges, created sites for crack propogation which lead to failure.

Looking at the Moments the flanges on the back do taper significantly but other than that they're a fairly regular cross section, indeed the pocket out of the front face means they might possibly have a larger cross-section down at the pedal end.

Can you tell I'm starved of engineering at the the moment?