Do you really need the through axles on your trucks?
It lowers the stresses, and it doesn’t really matter...
I love my Calibers, they are basic, wide spread in the esk8 diy community, and they're excellent.
These introduced me and many others into the world of esk8 diy.
These are also gravity cast and have through axle (single unit from side to side).
New Offerings
Recently we see lots of CNC machined trucks. These have their axles bolted in (or otherwise attached) form the sides.
None are doing it with the through axle.
It was strange to me, and required resolution.
So this post is all about answering this question, from the structural point of view.
Do you really need the through axle?
Will the ones without it be weaker and break sooner?
Let's find out, by comparing the stress levels of course.
Dry Facts First
Calibers are made out of cast A356-T6 Aluminum. Which is a very popular alloy for casting. These also use Chromoly axles
Machined trucks can be made from higher grade aluminum, like 6061 or even 7075.
Comparing material properties first (MatWeb):
At first glance, cast trucks use inferior aluminum. Can the stronger steel axle provide the advantage?
Comparing with Calibers
CAD model of Caliber truck can be found on GrabCAD.
How are the stresses compare with and without the axle?
You can do this easily with Finite Elements
Because we are comparing in the linear elastic region, we don't care about the exact loads.
Let's do a simple 120kg rider spaced evenly between 4 wheels. Each side will have it's own 30kg of vertical force on 50deg inclined truck.
Standard truck with steel axle inside
Not bad, with slightly elevated stresses on the edges, the most significant is the middle at 26.4MPa.
Same truck, bulk aluminum with side axles only
We don't care about the actual numbers, but the ratio is impressive. Removing the steel axle added around 6% only!
What's going on here?
That's much lower than I initially anticipated.
Thinking about it further, the axle adds stronger and stiffer material, but near the center of the cross section.
Remembering the formula for the moment of inertia, this adds very little.
So, yes, through axles reduce stresses, and the trucks are slightly stiffer.
But in an almost unnoticeable way.
And if you go with better alloys for the machined trucks, this becomes even less significant.
Also, adding thickness to the hanger will reduce the effect even further.
So, what are the advantages of through axle?
I believe the main one is the easy alignment during casting, see here.
The other might be not dealing with end bearing stresses and assembly of the end axles. But that's another story.
Enjoy,
Dani
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