4 ms·

Twitter thread on the physics to go 150mph in this tunnel: https://twitter.com/Model_4_/status/1075270290978963458?s=19 https://twitter.com/Model_4_/status/107

Twitter thread on the physics to go 150mph in this tunnel: https://twitter.com/Model_4_/status/1075270290978963458?s=19 https://twitter.com/Model_4_/status/1075270290978963458?s=19 Note that air resistance is a much bigger problem in a cheap, narrow tunnel.

npunt 8y ago
I imagine it’ll also probably be hell on tires. You’d need W- or Z-rated tires to do that and those are $$. Perhaps smooth concrete will mean lower rolling friction along with lack of sudden maneouvers. Still, going that fast is expensive in all sorts of ways.
- carlivar 8y ago
  And it's best not to think about what happens if a car stops for any reason, or worse, catches fire. I do not believe any escape hatches were part of the demo and cost calculations.
  - npunt 8y ago
    Tunnels looked wide enough to get out and walk thru. They’ll need ventilation and will probably get maint / esc hatches for regs. Dunno about ADA. Didn’t seem like a detail worth hashing out in a debut tho.
    - carlivar 8y ago
      Agreed not necessary in the demo, but should acknowledge this in the cost information.
      - npunt 8y ago
        True, lots of info yet disclosed & some impt questions raised. Seems like an Elon intro where it’s vision first and then lots of details sorted out later and/or changed last minute (lookin at you BFR).
  - King-Aaron 8y ago
    Devil's advocate chiming in here... What happens if you're in an enclosed tube, like in say a commercial airliner, and it stops suddenly or catches fire?
    - carlivar 8y ago
      For starters, other vehicles don't get backed up behind you.
    - youngtaff 8y ago
      There's escape options - whether you can use them depends on a number of factors Challenge with Elon's tunnel is you're stuck underground with no escape options, and maybe nowhere for the smoke to go As well as escape options most tunnels in EU, also have smoke extraction, fresh air insertion options which help make it easier for passengers to evacuate. Tunnel fires (even miles from anywhere) are survivable with the right design - https://en.wikipedia.org/wiki/1996_Channel_Tunnel_fire https://en.wikipedia.org/wiki/1996_Channel_Tunnel_fire
    - SmellyGeekBoy 8y ago
      Volatile materials (eg lithium) are heavily regulated on commercial airliners. Not to mention the maintenance schedules and other safety regulations.
- konschubert 8y ago
  People go 250 kmh on German highways regularly (unfortunately). It doesn't seem to be unpractical.
  - maxxxxx 8y ago
    Not impractical but these tires are very expensive.
  - Balero 8y ago
    In a confined tunnel pushing the air out of the way is more difficult than in open air. There's just less space for it to all go.
  - emptyfile 8y ago
    And whats the speed limit in German tunnels? In case you don't care to find out: 80-100 km/h (50-60 mph)depending on the state.
    - konschubert 8y ago
      Yes, but my point was whether the price if the tires is a show stopper or not.
  - deleted 8y ago
    [deleted]
- dsfyu404ed 8y ago
  Heat kills tires. Flexing heats up tires. More load, more (rotational) speed and less pressure all increase the amount a tire has to flex. High speed automotive tires are expensive because they need to withstand all the flexing that comes from running at high speed with the kind of inflation pressures that deliver what the owners of the high dollar cars they go on consider a reasonable ride. They have to be fairly soft/flexible tires for the same reason. A hard, stiff tire would not deliver the kind of traction people expect from a German driving machine. Basically you have speed, weight capacity and ride quality and you only get to pick two. So long as you can compromise somewhere everything is easy and cheap. A boring old light truck tire will live a long life at 150mph so long as you run it at a very high pressure and light load. If you want to see this principal in action look at small trailer tires (like snowmobile and jet ski trailers). They carry a ton of weight (for their size) and run at very high speeds (a sub 20" tire is really spinning fast at highway speed) by running very high pressure with a stiff sidewall and the kind of rubber that makes butter look like a high friction surface. So yeah, tires are the least of their technical issues.
tlrobinson 8y ago
Would the constant movement of cars in the tunnel induce a wind, reducing the pressure differential? I guess that depends a lot on the density of cars and topology of the tunnels (a simple loop would be ideal, I think).

npunt 8y ago

I imagine it’ll also probably be hell on tires. You’d need W- or Z-rated tires to do that and those are $$. Perhaps smooth concrete will mean lower rolling friction along with lack of sudden maneouvers. Still, going that fast is expensive in all sorts of ways.

carlivar 8y ago
And it's best not to think about what happens if a car stops for any reason, or worse, catches fire. I do not believe any escape hatches were part of the demo and cost calculations.
- npunt 8y ago
  Tunnels looked wide enough to get out and walk thru. They’ll need ventilation and will probably get maint / esc hatches for regs. Dunno about ADA. Didn’t seem like a detail worth hashing out in a debut tho.
  - carlivar 8y ago
    Agreed not necessary in the demo, but should acknowledge this in the cost information.
    - npunt 8y ago
      True, lots of info yet disclosed & some impt questions raised. Seems like an Elon intro where it’s vision first and then lots of details sorted out later and/or changed last minute (lookin at you BFR).
- King-Aaron 8y ago
  Devil's advocate chiming in here... What happens if you're in an enclosed tube, like in say a commercial airliner, and it stops suddenly or catches fire?
  - carlivar 8y ago
    For starters, other vehicles don't get backed up behind you.
  - youngtaff 8y ago
    There's escape options - whether you can use them depends on a number of factors Challenge with Elon's tunnel is you're stuck underground with no escape options, and maybe nowhere for the smoke to go As well as escape options most tunnels in EU, also have smoke extraction, fresh air insertion options which help make it easier for passengers to evacuate. Tunnel fires (even miles from anywhere) are survivable with the right design - https://en.wikipedia.org/wiki/1996_Channel_Tunnel_fire https://en.wikipedia.org/wiki/1996_Channel_Tunnel_fire
  - SmellyGeekBoy 8y ago
    Volatile materials (eg lithium) are heavily regulated on commercial airliners. Not to mention the maintenance schedules and other safety regulations.
konschubert 8y ago
People go 250 kmh on German highways regularly (unfortunately). It doesn't seem to be unpractical.
- maxxxxx 8y ago
  Not impractical but these tires are very expensive.
- Balero 8y ago
  In a confined tunnel pushing the air out of the way is more difficult than in open air. There's just less space for it to all go.
- emptyfile 8y ago
  And whats the speed limit in German tunnels? In case you don't care to find out: 80-100 km/h (50-60 mph)depending on the state.
  - konschubert 8y ago
    Yes, but my point was whether the price if the tires is a show stopper or not.
- deleted 8y ago
  [deleted]
dsfyu404ed 8y ago
Heat kills tires. Flexing heats up tires. More load, more (rotational) speed and less pressure all increase the amount a tire has to flex. High speed automotive tires are expensive because they need to withstand all the flexing that comes from running at high speed with the kind of inflation pressures that deliver what the owners of the high dollar cars they go on consider a reasonable ride. They have to be fairly soft/flexible tires for the same reason. A hard, stiff tire would not deliver the kind of traction people expect from a German driving machine. Basically you have speed, weight capacity and ride quality and you only get to pick two. So long as you can compromise somewhere everything is easy and cheap. A boring old light truck tire will live a long life at 150mph so long as you run it at a very high pressure and light load. If you want to see this principal in action look at small trailer tires (like snowmobile and jet ski trailers). They carry a ton of weight (for their size) and run at very high speeds (a sub 20" tire is really spinning fast at highway speed) by running very high pressure with a stiff sidewall and the kind of rubber that makes butter look like a high friction surface. So yeah, tires are the least of their technical issues.

tlrobinson 8y ago

Would the constant movement of cars in the tunnel induce a wind, reducing the pressure differential? I guess that depends a lot on the density of cars and topology of the tunnels (a simple loop would be ideal, I think).