Trucks fatigue… Oh NO! Not again…

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We know that Calibers sometimes break. And we also know this is due to “fatigue issues”.
Still there aren’t any numbers that support this knowledge.

I want to generate these numbers here.

Calibers is the test case and a well-known base line. Because of this, it can be useful for any new designs out there. If we know how these fail, it can help all the new designs out there.
And maybe we’ll have less new hangers that break.

I did this many times for things that fly, and now I’ll try to make this here, with your help.
The idea is to share all the steps, from loads, through stresses, to fatigue life estimation.
But I need the community help to make it much better and usable.

Any input is welcome! Seriously, I’m a noob in the skateboard world.

Hope there will be interest in this.

Alternative title for this might be: Why did Brian’s trucks break?
Brian’s @b264 numbers will be our base line (he agreed for me to share these) together with some data from my own board.

(I’ll try this thread not to derail as that other one…)

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Starting with the loads

Assuming 260lb (120kg) is riding on the deck, rider and gear.
Resulting in 30kg vertical reaction force (W) on each of the wheels.

In order to move, the motor must overcome the friction force (Fr) which is µ times this vertical force. µ being the friction coefficient, ranging from 0 to 1.
To tackle the worst case, let’s assume µ=1.

Now, the motor can overcome this force by applying belt tension (Fb), through the pulley.

The picture is of my own board, note while the mount is at ~30deg angle, the belt is almost horizontal. Your setup can vary, but it’s probably somewhere close.

This force is always tangential to the pulley. The balance between these forces is through different diameters:

Fb*(Dpulley)=Fr*(Dwheel)

My wheels are 90mm
36T pulley diameter is 56.1mm
40T pulley diameter is 62.5mm

For my wheels and 40T pulley we’ll get: Fb=43.2kg

So, the difference between regular truck and truck with working motor is this ~40kg load.

What do you think so far? Is this making sense?

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Well, it does something.
You’re still moving

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you’re moving when you kick push a skateboard too? :thinking:

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Just add in temperature fluctuations, because brian rides in the snow.

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He also rides into boxs

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“I fought the box and the box won”…

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Video video video

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I punched it.

It was a really, really bad idea. For some reason I thought it’d just fly off the roadway. What happened involved flying and roadways, but not by a box.

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At times, the 120kg is spread on two wheels, or even on one. That’s skating. Bumps and curbs. So maybe we shouldn’t consider the base loads, but the peaks.

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Thank you all for the replies

Well, the bearings are supposed to take loads. What load will or won’t pulverize them?
There’s a load. (Thought of this a little more) The one I calculated is going from a stand still, during ride it’s probably much lower.

The trucks are much stiffer than the mount, it does more to it than to the truck.

You’re right. It was a clean snap, as fatigue failures are. You never see plastic failure as a result of fatigue. I believe that repeated bending initiated the crack.

That’s a valid point, we’ll add it to the mix.

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You need to consider all of them, but this is a bit later

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Quite a few people pulverized rear bearings that where ceramic, so there is that.

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Probably due to the fact that ceramic is more brittle than stainless steel, and our wheels are under more weird loading than push boards.

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I’m also on the train of thought that what really kills them in the millions of cycle that the vibrations impose on the truck, they won’t break it by themselves, but weak to the point that a peak load finally does the job

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