I've stumbled across this kickstarter project, which promotes an original shape for cranks:
They claim a wider arc of useful push during rotation. While their main point is rendered moot by clipping systems, I remain unsure about their claim in general.
Can these really have a positive impact on power/torque? Is there any risk due to increased stress on some body parts?
If the aluminum is sufficiently stiff it makes zero difference -- the crank could be any shape (a disk, an S shape, etc), but the relationship between the two contact points would still remain the same, and that's all that counts.
The only effect the crank could have is adding a bit of spring to the crank, which might be good or bad for effective cranking. But aluminum makes lousy springs, and if it flexes much it will soon fatigue and fail.
The risk to your body parts is getting cut by sharp edges when the crank fails.
The comments on the Kickstarter project have a few good explanations of both why the design is effectively identical to a straight crank, and why the plan to make carbon-fiber versions is dangerous.
Now leverage: if you tried to push down on the pedal (as shown in the video) when it was exactly top dead center and stopped, it doesn't matter if it is a normal crank or the Z-Torque, it will not want to move. They are two rigidly attached points in space, so it doesn't matter WHAT you use to connect them, 'physics' just treats them as though connected by a straight bar.
Simple proof: If you move the Z-torque back just 2 degrees before top dead center and apply a force (you could try with a weight rather than a foot to make it scientifically valid), you will see that it doesn't move FORWARD as would happen if your crank worked as you argue (advancing the leverage because of the bend), but still BACKWARDS the exact same as a normal crank.
It looks different but it works THE SAME.
..and why it is dangerous:
2) You absolutely cannot take a design that is optimized for machining "mold them out of carbon fiber". Woven composite strength is highly anisotropic. If anything, your design shown in your picture is weaker, not stronger, than the machined part, if molded out of carbon fiber (and by that I assume you mean, "molded out of cast polymer with a carbon fiber veneer on the front surface).
A true carbon fiber crank would have the fiber direction laid out (often by hand) in such a way that the fiber direction (or grain direction) aligns with the force exerted on the piece. So all that ladder structure that you've molded into the part,would actually make it a LOT weaker. It would also have machined metal components laid into the part and bonded to the composites for a metal-on-metal mating surface. See http://www.zipp.com/support/identify/carbon_cranks.php for an example of the construction technique - note the use of an aluminum spider that the actual crank is bonded to.
[...]
5) Finally, and the most worrying point: You are dealing with an important piece of a bike's drive train. The failure mode on this is likely going to be the a cyclist cranking away and your crank giving way and breaking off. This is the makings of a serious, and possibly life altering accident.
Non-destructive testing and examination of composite materials is no joke. I can assure you that the folks from Shimano and RaceFace has years of experience building, validating and testing composite structures that they built. The failure modes of composite materials is also very different than traditional metal - there is very little to none inelastic deformation.
The mold and setup you show is good for producing cosmetic composite components (like my buddy's "Carbon fiber" gas and brake pedal). A mission critical drive train piece, not so much.
The complete comment is worth reading.
This is just a rehashing of a very old and horrible idea. See PMP Cranks et al.
Edited for additional information:
RE: PMP cranks
A moment's thought shows a straight crank and an L crank always have the same relation between pedals, chain, and bottom bracket. Thus, there is no advantage to L cranks. And an L crank always has more material than a straight crank, so is always at a disadvantage for weight, strength, stiffness, and/or cost.
The P.M.P. cranks have a spider bolted to the right crank, with the square taper formed from both pieces. The square taper is loaded heavily, and making it in pieces, especially with a few light bolts to hold them together, is likely to increase loads greatly and thus lead to premature failures. So it may be unwise to ride these, even for art or humor value.
RE: Z Torque cranks
The Z Torque crank manages to take a bad idea and make it worse: the crank spider is not simply bolted to the right arm. Instead, it is about 1/3 of the square taper, while the arm is the other 2/3. This puts a joint in the middle of a highly-stressed joint — so highly stressed that hardened steel spindles sometimes break here.
Z-Torque also has much less ground clearance than a straight crank, much less even than a P.M.P. crank.
And then there is this from their own video:
Participants achieved similar maximal oxygen consumption, peak power outputs and gross efficiencies with the Z-Torque and normal crank configurations (Table 1). In addition, ratings of perceived exertion (RPE) at 150 and 200 W, heart rate (HR) at peak power output, 150, and 200 W, and cadence at 150 and 200 W were not significantly different. However, participants perceived their effort to be significantly lower at peak power output with the Z-Torque crank.
Even their own study and material showed no significant difference between the Z crank and a normal crank. The "perceived" notion is flawed because it is not a blind/double blind test.
Unfortunately this doesn't help. The example pictures demonstrate misunderstanding elementary classical mechanics and more specifically, statics. Moment, a.k.a. torque, is defined as
M = F * d
where
F = the force applied
d = the perpendicular distance from the axis to the line of action of the force.
The shape of the crank has not effect on either. F is the force coming from the leg/foot, and d is defined solely by the distance between pedal and hub, multiplied by sine of the angle between F and the vector defined by the pedal and the hub.
In the second example picture,
the pushing hand is misleadingly positioned forward so that d is increased, and that will increase moment. However, the pedal is straight above the hub, so pushing downward still creates a zero moment.
If the normal crank is A, do you think adding arms B and C and would make a difference (except adding mass)? Remove A, and you have a "Z-shaped" system.
It just adds extra weight and probably is less reliable due to increased flexing. The "lever arm" for mechanical advantage is the same as it would be for an arm that went straight to the pedal. This is like those adverts for "male enhancement".
This design adds nothing new, many cranks similar to this have been developed during the 100+ years of cycle design and all have failed to achieve the supposed power advantage and become mainstream.
Buy this if you like the look and feel of flex at the pedal spindle buy it.
No.
You have less room for the front wheel to turn when the longest part of the crank is horizontal to the ground.
You have less ground clearance when the longest part of the crank is closest to the ground.
Flex always loses power, and even if these were perfectly stiff, there is no mechanical advantage since force is not applied on the end of the Z but instead in the same place as a traditional crank.
These cranks are however perfect for cranks (eccentric people) or scrapers that want some bling and don't care about safety or efficiency.
