TomK wrote: ↑Fri Nov 10, 2017 7:35 pm
I'm not sure though that the e46 subframe issue is such a great analogy for the forces involved here? I do though agree that getting access to fix any spot welds etc issues around that area is going to be an issue.
Its a fine example to show that load bearing issues are not always apparant in the first1-2 years or 5k miles.
And also a fine example of the possible extend of damage.
Its not said that the first signs of damage will be a small crack in the top of the shock perch easily welded up. It can just as well be a row of spotwelds somewhere down low where the side/shockperch is welded into the bottom plate which can be really hard to get to.
Is it also really a potential issue that a load is subjected constantly as you suggest? I would have thought that would not be a problem as long as it is within tolerance?
Well, the tolerance is just the issue isnt'it? What is the tolerance?
In the stock configuration the rear shock tower doesnt get any constant load. Its just there to hold the shock, that dampenes motion.
The shock doesnt resist the force of bumps and such; thats the task of the spring. The shock slowly dissipates energy.
In the normal situation the weight of the car sits on the spring perch underneath, not on the shock tower.
About the tolerance thats very very hard to predict. I'm sure bmw makes a computer model that stress tests all welds etc but based on the stock situation. If the coilover situation holds and doesnt break immediately that doesnt mean in a few years there wont be any issues.
I dont know if you know what a Wöhler curve is, but thats basically a curve that dictates that increased loads will accelerate stress fails. It's a life cycle curve (used in material sciences studies). And its logaritmic/exponential, so a linear increase in load will have an exponential accelerated failure rate.
But that makes it also very difficult to predict. There might be failure in 3 years or 30 years. It's a personal decision if one is willing to take that risc. There is no real user experience. There is no one saying: hey, I installed that kit 7 years ago and did 60k miles since then without any problems. People who buy those kits are low mileage users. Going to shows and trackdays.
But there are plenty of people who daily their car for years on normal coilovers like kw/hr/etc that did serious miles.
That is the only information you can go on really. Failure rate of others who've done serious miles.
Someone who's selling the product assuring its ok only has a value if he can give a no questions unlimited warranty on shocks and chassis for a representative time in relation to how long you want to hold on to the car.
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