Akerue.Design

I'm sitting down to do some structural analysis before finally sending them out for quote and manufacturing. Its overkill as I've already built and tested them under real conditions, but I'm a nerd and this is fun. This gives me insight into where the high/low stress points are and therefore where I can add/remove material to increase the stiffness/strength while reducing the weight.

One of the trickiest things about structural analysis of a fast, dynamic event like a skater dropping off a roof is understanding the impact force. The amount of energy dissipated during impact is easy to calculate. Kinetic energy is 1/2*mass*velocity^2. Downward velocity can be easily calculated: v=square root(2*gravity*height).
Worst case scenario, a 100kg (220lb) skater jumping the leap of faith 5.7m (18'8") drop. Downward only velocity = 10.6 m/s (24 mph).
Downward energy=5618 Joules, for comparison a 100mph baseball delivers 140J, 9mm bullet 467J, and a 12 gauge shotgun blast=4453J.
So the skates/skater must absorb more energy than a shotgun blast to land the leap of faith. We've now got all the info we need right?

Wrong. I need a force value to apply (in pounds or Newtons). To get that I need to know how much time was utilized to dissipate that energy. It's less than one second, but is it closer to 0.05s or 0.5s? That could vary the force by ~10x.
This is where the skill of a professional skater comes in. They can stretch out the time duration of the impact as much as possible to decrease the impact force. Sadly this is an unknown to me. I'd need some high speed video footage or some accelerometers on a skater to measure this impact time.

Another way to look at the problem is: how much force can the human body endure? It turns out there's lots of good research on this for airplane ejection seats and parachutes. That indicates the upper limit is around 40Gs before severe spinal injury occurs. Given the known velocity of 10.6m/s, 40G=0.27s of impact time, or a 20G = 0.53s.
To be conservative I'll use 40G. F=m*a; F=100kg*40G= 4000N.

There we finally have our force input to the analysis :) @ San Francisco, California

I've been dragging my feet in putting the finishing touches on these frames. I've been saving some of the least fun things for last.
The recent thing I've been working out is deciding on the final tolerances of the critical features on the frames. Tolerancing is one of the most challenging components of mechanical engineering and is probably the most underappreciated component as well.
Most skaters probably take for granted the fact that generally skate parts just fit together nicely. Take for example installing bearings into wheels. That's generally always a nice press fit. The bearings aren't loose in the wheels and it isn't so tight you can't get them in. That means that both the outside diameter of the bearing, and the inside diameter of the wheel are extremely tightly controlled dimensions. The bearing OD typically varies from 21.991 mm to 22.000 mm. That's a maximum variation of only .009 mm or .0004 inches. That about 1/10th the thickness of a human hair.

One of the tricky aspects of designing parts like frames is deciding just how tightly to control each dimension. Tighter tolerances means that the parts are more difficult and more costly to produce. So the engineer must try to design the parts in such way that allows for as much variation as possible in the parts to minimize cost. That's difficult when it comes to moving parts subjected to large forces like skate wheels. If tolerances are too loose, things will rattle and not perform properly. If the fits are too tight, the part may not be able to be assembled together, or they may get stuck together.

I ran into a snag when determining the frame hole diameter tolerance for the frame spacers. To try to keep the cost down as much as possible I'm using off the shelf axles and frame spacers rather than having my own made from scratch to my own specifications. I measured 64 frame spacers and realized the variation across them was too large to allow for a press fit into my aluminum frames. The plastic frames they are usually used with are more forgiving. I've got a few options for a fix, but need to do some research to figure out what the most cost effective solution is.
Stay tuned for more updates.

Unfortunately life got busy and this project stalled out for too long. However I'm happy to report that I'm back at it drawing up the next version of frames. They'll be very similar with some subtle but important improvements. Shown here is a screenshot of my rough notes of the key dimensions on the January 2019 version. This is roughly the amount of measurements to consider when modeling these up from scratch. Lots of little details to ponder and prefect...

If you'd like to learn more about the frames, give this podcast a listen. Thanks to Brian Krans and Levi Sebastian for making it happen!

#131: Nate Herse (Akerue) – blade or die Nate Herse is an engineer and a rollerblader who made some frames called Akerue. They look like aluminum skeletons. He tells Levi Sebastian, Brian Krans and Chris Bjerre about his thought process behind their design and construction and what it will take to make them available to the rest of us.

Great video on the frames from Law.

What Are Those? Akerue Frames - FIRST IMPRESSIONS I had the privilege of trying out the Akerue frames today. Designed and made in California, these things are machined out of a solid block of aluminum. Super...

Some great feedback on the frames from Rob:

Akerue Designs Aggressive Frame - First Impression In this episode of Konjure Kast, Rob discusses his experience testing out the new Akerue Designs prototype aggressive frames. Rob's Skate Setup: USD VII / G ...

How Its Made - 1405 Skateboard Wheels How Its Made Season 29 episode 1 Skateboard Wheels