There are two major factors - fatigue (think bending a paper clip repeatedly) due to driving over tons of small rocks, and the the mechanics of having six wheels contributing additional damage to the front and middle wheels. Think of pushing a rolling suitcase versus pulling one - when hitting an obstacle it tends to dig in instead of easy popping up. The front and middle wheels are in front of their pivots, and so tend to get driven in to rocks by the other wheels.

In terms of metal fatigue, aluminium doesn't last very long at all. That's one reason why you want electric cables made of copper, not aluminium.

True! The actual induced strain is not particularly high (the wheels do not visibly deform) but it's the cycle count that will get you. The chevron-shaped grousers also act as stress concentrators, which is why the cracks are all starting at those tips.

So the rear wheels are on trailing arms, but the front and middle are leading arms, which increase the incidence angle by the angle of the arm in regards to ground? So impacting a 45degree slope would have the force of say 55 degrees, as the arm is ten below level? The trailing arm would then impact that same slope at effectively 35 degrees. Just trying to understand their conclusions.

I used to think the same thing, it looks pretty intense for just rolling over some rocks! But then I learned that in between the "treads" the aluminum is "half the thickness of a dime" (as the article puts it).

So if you think of it like the lid of a can of soup or something, it's easy to see how it could deform and break like that when rolled up against sharp rocks with a ton of weight on top of it.

I understand that, which is why I said it never seemed strong enough to begin with. I get you need to conserve weight, but doing so on the wheels, seems, crazy? The most important thing of an entire car is the tires. Without good tires, a race car can't do anything, no matter how well engineered the rest of it is.

I think it's a gamble. If they made them thick enough to never break, they could have blown the whole weight budget. But if they make them too thin, the whole project is a waste.

I mean the wheels are probably among the heaviest single "things" on the rover, so adding even a small amount of thickness will really increase the weight, and that would mean cutting other things.

It's a tough call, and I'm sure as hell glad I wasn't the one that had to make it, but I think all things consitered they did a good job! It's been running for 5 years and even though these look bad, apparently it still has a lot of life left in it!

Surprisingly the wheels only weigh a few pounds each! (I still can't think in metric and I don't have a real number.)

They were primarily constrained by packing volume during cruise (https://mars.nasa.gov/msl/mission/spacecraft/cruiseconfig/) and then by shock loads as the securing pyrobolts blew during landing (https://youtu.be/Ki_Af_o9Q9s?t=4m14s).

I'm curious how you came to the conclusion that the wheels are the most important part of the rover.

Without wheels, the rover can not rove. Pretty much makes it useless if the wheels are broke, correct? Just like any vehicle, the wheel/tire is the most important piece regarding performance. Without good contact, all that fancy machinery won't do a thing.

Definitely, but couldn't you argue the same about other important components? For example, without an engine/motor, even the fanciest wheels will get you nowhere. I think it's somewhat of a paradoxical argument.

So the way I see it is that all of the major components are equally important, but you want to optimize your overall design to be both light and reliable.

Of course you could, that's why you don't skimp on those things. You can get by with a crappy interior, but things like wheels/tires/transmission/engine are high priority. That's my whole point here, it doesn't look like the wheels were over engineered enough, but I'm not a rocket scientist.

That wasn't clear from your original comment though. In case of the rover, other important things include comms/nav, movement controller, power source, and sensors.

I would think that given the lack of water temperature variation based weathering is dominant on Mars. If this is true the rover is essentially driving on a surface similar to a pile of freshly smashed glass.