Hacker News new | ask | show | jobs
by jacquesm 944 days ago
What is the cause? Thermal expansion?

I have a room in my attic that can make really loud bangs on hot summer nights (enough to scare the crap out of me when I am concentrating on something and do not expect them) when it contracts again after a day in the heat. The root cause there is stick slip: the thing doesn't move for a long time due to its weight so tension builds up and then suddenly it will shift a little and the whole structure amplifies the sound from the shockwave passing through it.

Your 'pops' remind me of that (and supercharging must have some thermal effects).
1 comments
Kirby64 944 days ago
Similar mechanism as your attic noises. There's a giant metal plate on the bottom of the thing, so all the heat generated while it's supercharging (in a very cold environment) causes that 'snap' which makes a loud noise. Same thing happens if you've ever heated up a thin metal baking sheet. It'll 'snap' from one position to another and make a horrible racket.

Same thing apparently happens if you are heavy on regenerative braking when descending a mountain pass. Cold air, plus 50kW+ of regen braking is similar to just supercharging the car.

link
flutas 944 days ago
Something else interesting when supercharging in the cold...LOTS of steam(?) at times.

Shocked me the first time it happened. Cold (~5F outside), low and hot battery (just went from 80 to ~5%). Legit thought the car was on fire at first.

link
Kirby64 943 days ago
Never experienced the steam myself, although I can't say I've ever tried supercharging the car at 5F. Coldest I've ever done it was probably in the 40s, which is plenty cold enough to cause that popping sound when it's pulling 250kW.
link
flutas 942 days ago
Oh yeah, the popping was crazy loud as well.

Thinking back to it, the battery must have been extremely warm at the time too, since that 5F was after two hours through a 2 lane mountain road (crossing the range).

link
alliao 944 days ago
thanks! I've heard the pops during my winter trips but never associated with thermal expansion! Always thought I'd hit some animals or debris on the road.
link
jacquesm 944 days ago
Oh, that last one is an interesting one.
link