· By Austin Duggan
Our Titanium Cranks
Grade 5 titanium has an incredibly high strength-to-weight ratio. That makes it especially promising as a material in BMX. But it's also expensive and difficult to machine. That's why very few manufacturers are willing to hassle with it. Nick and I are crazy enough to try it anyway. Here's a short blog about the cranks we're developing.
Crank arm design: Austin's basic idea
I began with a core structure different from the ones I had seen on the market. I imagined an I-beam with a pedal boss on one end and a spindle boss on the other, thick enough all around to support a rider's weight under normal riding conditions. That was my skeleton. I then thought about what sort of reinforcements would be needed within the I-beam structure to make it stiff enough and resilient enough for the repeated hard landings of BMX.
In one of my first sketches, I imagined slats running in one direction on one side of the I-beam, and slats running in the opposite direction on the other; essentially forming an "X" pattern. That's what I workshopped with Nick Nieminen, the Mechanical Engineer at Chimera, and the mind behind Subframe.
Crank arm design: Nick's model
In my mind, you wouldn't need all that many slats - the original sketch had just seven of them on a 170mm crank arm. Nick wisely pointed out that we'd need more for the kind of stiffness we wanted, especially against torsion. What he expertly modeled was ten 60° support slats on one side of each crank arm, and ten 120° support slats on the other side of each crank arm. This creates the tall X-cross pattern throughout the core I-beam structure, which you can see appear in the video below.
First round of testing: simulations
We ran a lot of FEA analyses to help refine the initial design. We simulated repeated heavy pressure on every direction of the crank arms; hard landings at 3 o'clock, 6'oclock, at the very end of the pedal where the most cantilevering would occur, etc. As you can see in one of the early screenshots of a static analysis below, we did this for all sorts of small variations; cranks with a thicker center wall, ones with bigger fillets, ones with a skinnier pedal boss, ones with and without bolt-drive bosses, and so on.
Our FEA analyses showed a high degree of strength in all directions for pretty much every variation on the design that was within the ballpark of 215-230grams per crank arm. Anything less than that was sketchy. We decided to pull the trigger on a set of prototypes with a bolt-drive option for maximum compatibility with all of the Rennen Design sprockets we have in the shop (they're a company we've been purchasing from throughout the prototyping process).
Our first set of prototypes turned out well. Here are some photos.
Conforming to standards
We decided to conform to the same 22mm/48 spline spindle standard that's been on the market for a while. It's a tried-and-true system in BMX, and there's been enough riders on titanium spindles of this size for us to trust the level of durability. Also, that allows our spindle to be compatible with other 22mm 48spline cranks and sprockets, for example the excellent products you can get from Profile.
The 160mm wide spindle we prototyped for our bikes is just 131g/4.6oz. A standard 143mm wide BMX spindle should land around 117g/4.12oz.
We're working on some slight revisions to the crank arms to make them a little sleeker, most notably some fillets inside the cutouts and smoother transitions into the pedal and spindle bosses. We're also getting rid of the bolt-drive. It adds unnecessary weight and additional complication, and spline-drive sprockets tend to be more durable. We're developing our own spline-drive sprockets as well.
FEA analyses are helpful, and the ones we've done are very promising. But they're not everything. The sort of abuse found in BMX is difficult to simulate. So right now we're in the process of manufacturing a round of more prototypes to do as much real-world testing as we can with riders who can put a lot more stress on the cranks than me or Nick.
Plans for production
If all goes well, as we think it will, we'll be bringing this product to market. They'll certainly be available on our Founder's Edition Electric BMX. Right now, it looks like the specs and options for these cranks will be:
Material: 6Al-4V "Grade 5" titanium
Spindle: 22mm 48 spline
Spindle widths: 143mm (BMX) or 160mm (MTB)
Bolts: M17x1.25, 8mm hex
Drive: Spline-drive only (compatible with Profile splined sprockets)
Crank arm lengths: 165, 170, or 175
Total weight: 590g/20.8oz with 170mm arms including Ti bolts and 143mm spindle.
MSRP: $540 with Mid or Spanish bearings and black aluminum cones/spacers. For an additional cost we have PF30 cups as well.
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