Last time I showed you "less than perfect" hanger design, this time let's see one that's more robust.
Whooshboards is a small, EU based, esk8 parts shop. They have some quality motor mounts and this is going to be their first, newly designed, truck.
The truck has very familiar contours which resemble a trusty workhorse, the Caliber trucks.
*I'm not affiliated with Whooshboards.
Whoosh truck is slightly wider than Caliber, and it introduces some cool features that aren't very common with esk8 truck offerings:
- It's made out of 7075 Aluminum, which has superior properties to the widely used 6061.
- It introduces a 3mm rake for additional "dialing" capabilities.
- The axles use axial flange preload for stiffer and more robust construction. They also use end thread rather that shoulder bolts.
The hanger is machined out of 7075-T6 aluminum, the axles are PH17-7 stainless steel. Both of these are a no compromise materials with better properties than typically found in esk8 applications:
I'm taking the "old trusted standard" 120kg board rider.
This gives 60kg per hanger. This time I'll spread it 1/3 and 2/3 between the two wheels (why not). Think about very tight turn...(this might be the worst case these will ever see)
The hanger sits on a 45deg base plate, and together with 30kg motor drag (per side) it will look like this:
This is the first sanity check, and we're getting this cool non symmetric result. With the non symmetric loads it makes perfect sense.
All smooth with no weird jumps - should be your first check.
First view over the equivalent (von Mises) stresses points to a high value of 110MPa at the edge of the bore.
This edge load however is from the axle pressing on the edge. So this is mostly a compression stress, and it tells very little about the integrity of the structure.
Meaning it can go much higher without any structural problem.
In order to understand where to look, it's useful to compare equivalent and the principal stresses. Let's look on the other side of the hanger.
Top - equivalent, bottom - principal:
See what I mean?
The real value to estimate the strength of this hanger is the large middle one in tension ~64MPa.
Compared to the Yield value of the material, the safety factor is more than 6! Not bad.
The low stress level also makes sure there's no problem with fatigue.
What about the axle?
This one is more interesting, let's look at the more loaded one.
Clearly it bends, now however the interaction with the hanger is not just edge load, like with shoulder bolts, but also through axial support of the flange.
The axles show an elevated stress at the root fillet, the one shown might be the worse case scenario, and still with a safety factor of ~2 to Yield.
Actually, I didn't take pretension of the axle into account. This should give more interesting (and real) structural interaction (maybe in one of the next posts). But the values above do give you a good design criteria.
One more thing about the axles: The interaction with the hanger is handled better with this design (in my opinion). The flange is supported axially and laterally, which makes it stiffer, and reduces the stresses. Look at the interaction in bending in the next picture:
Based on proven geometry, simple and robust design with premium materials.
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