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Unread 03-10-2013, 09:59
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Re: Durability in FRC

Quote:
Originally Posted by nathannfm View Post
After building FRC robots for years it still amazes me how cars, that only cost 4 times as much can stand up to 100's of thousand of miles of driving over years and still hold up reasonably well when our robots are always barely hobbling along by the last off season.
Nathan,
High volume production cars (50,000+/yr) work as well as they do for as long as they do due to testing, refinement, and good processes. Over the years, as problems arise, they build new tests to run cars through. For instance, several decades ago, there were a lot of suspension failures in a particular area of Mexico... So the proving ground engineers brought Mexico to Chelsea Michigan. Here is a link about them "Fixing the broken road" http://blog.chryslergroupllc.com/blo...=entry&id=1303
Cars will go through various "life tests" that attempt to simulate the life of a vehicle in a short timeframe. Sometimes these are referred to as "HALT" or Highly Accelerated Life Tests. In the short time span of developing a new car, they will run a couple iterations of early product through these tests in order to verify that they would be good for customers. For a car, there are thermal test, vibration tests, electrical interference tests, several different body/chassis durability tests, several component-subsystem-vehicle level powertrain tests....

They also had us (when I worked there), design for 150,000 miles. What that used to mean was for a normal distribution of failure modes, but the average part life was 150,000 customer miles. This typically meant few failures under 3 years/36,000 miles, but due to normal distributions, you would frequently have some issues creeping in around 90,000 miles. This is in part why a used car looses so much value around the 100K miles.

The final round of testing on "production" cars typically runs into the first 4-6 motnhs of production. That is why you will often hear about a new car getting service bulletins to add or modify a brake line clamp and a wire harness rub shield. It takes that long to get those final cars made in the plants and tested, and reviewed. Guards and clamps will be added to reduce the risk of failure due to rubbing through (hose, wire insultation or ...)

I find it interesting the "issues" taht sometimes arise with customers. For instance, My father-in-law had a trunk seal wear out on his car with only about 40K miles on it... I did a quick estimate on the number of opens/closes I would think a trunk would see in the course of 150K mile vehicle life. 2/open/close cycle on 50% of trips with an average trip around 10 miles thus about 15,000 open close cycles/150K of mileage. I then asked him about his usage. He opens an closes his trunk to put his lunch box in the trunk. this means 4 open/close per day. He only uses that car to commute to work, thus he was at about 4X my estimated usage... or another way to put it, he had 160K worth of open and closes in 40K of driving... (sometimes you get lucky with your math). After that discussion he felt better about his seal wearing out so quickly...