5 ms·

I mean regardless of measurement, the gearbox ratios, engine RPM, and wheel/tire size plainly dictate maximum possible speed. Why even use GPS for this, as opp

I mean regardless of measurement, the gearbox ratios, engine RPM, and wheel/tire size plainly dictate maximum possible speed. Why even use GPS for this, as opposed to a known distance and a clock, or an on-site laser (or whatever electromagnetic wave) based system? Seems like using satellites is unnecessarily complex and error prone(esp. civilian GPS).

Avshalom 6y ago
because you don't know where in the run you're going to be at max speed or how long you can maintain it
- Animats 6y ago
  The official land speed record uses a measured mile. You start as far back as you need to and then go through the traps and get timed. The Southern California Timing Association sets up time trials on the Bonneville Flats frequently. They routinely time speeds over 300MPH. These SSC guys need to go to a place like that and do it with timers and spectators, or shut up.
aunty_helen 6y ago
Tire size changes with speed and weight, weight changes with downforce, downforce changes with speed, winds, air temperature. Maximum theoretical speed on a dyno, sure.
- bigiain 6y ago
  > weight changes with downforce Your high school physic teacher is on line 1. Wants to talk to you about what "weight" means...
  - bfuclusion 6y ago
    Mass does not change. But the effective pressure on the tires is as if the weight (which can be defined as the force required to support something) is indeed higher.
  - brokenmachine 6y ago
    My high school physics teacher begs to disagree - weight does change with downforce. Weight is the force the earth needs to provide to support the car. Mass remains constant except for burned fuel loss.
- theincredulousk 6y ago
  yes, I was referring to theoretical, mechanically limited speed. It's not like the tires can expand in diameter 200% though. I'm sure that Michelin has parameterized tables for the tire's mechanical behavior that they are confident about by now. They aren't exactly new to tire engineering...
gameswithgo 6y ago
tires change in size with speed
- theincredulousk 6y ago
  you could use a maximum value from a parameterized table that I'm sure Michelin has for their top of the line super-car class tire.
pengaru 6y ago
Plainly wrong re: maximum possible speed. Agree re: GPS, on-site laser traps timing an accurately measured distance is obviously preferable. I presume laziness prevails.
- theincredulousk 6y ago
  Sorry maybe I wasn't clear - it is a mechanical system with gear ratios directly coupled from the wheel to the engine. The maximum RPM of the engine, propagated through the lowest gear ratio, gives a hard limit regardless of the "real world" variables like wind resistance and friction. The point was really if the mechanical system can only "turn" to a maximum theoretical speed of e.g. 300mph, then they clearly and obviously didn't exceed it, no matter what, unless the car transforms into an airplane at some point.
  - pengaru 6y ago
    Sure, if you can quantify the maximum tire diameter at max rpm and top gear, you should be able to compute the maximum possible speed. I don't personally know how much those tires deform at such speeds. My personal experience with sportbikes approaching 200mph has been it's not even enough to make up for the ~5% speedometer error that still indicated higher than actual around 190. But that's a much narrower and lighter tire, and 300mph is a different world.
myself248 6y ago
Deploying ground-based infrastructure is difficult, especially in a place as inhospitable as salt flats. GPS (really, multi-constellation GNSS these days) is not only easier to physically set up, it also gives you more data, and likely more accuracy. You don't save the PVT output of the receiver on the vehicle. (I mean, you could save that too, I guess.) You save the raw observables, and post-process along with data gathered at the same time by stationary reference stations. (Either run your own base, or use data from networks like unavco.) This gives you a remarkably precise picture of the vehicle's position, motion, and acceleration, in 3 dimensions, throughout the entire run. If they want to be transparent about it, they'll publish a multiconstellation RINEX file of the whole shebang, so anyone can analyze it themselves. There might be fewer internet sleuths who know how to sling rtklib or gpstk than youtube-dl and audacity, but the ones who can, will produce a wealth of data. For bonus points, put two receivers on the car, a few feet apart. This allows moving-baseline so you also get an orientation solution. 1: http://rtklib.com/ http://rtklib.com/ 2: https://gitlab.com/sgl-ut/GPSTk https://gitlab.com/sgl-ut/GPSTk

Avshalom 6y ago

because you don't know where in the run you're going to be at max speed or how long you can maintain it

Animats 6y ago
The official land speed record uses a measured mile. You start as far back as you need to and then go through the traps and get timed. The Southern California Timing Association sets up time trials on the Bonneville Flats frequently. They routinely time speeds over 300MPH. These SSC guys need to go to a place like that and do it with timers and spectators, or shut up.

aunty_helen 6y ago

Tire size changes with speed and weight, weight changes with downforce, downforce changes with speed, winds, air temperature. Maximum theoretical speed on a dyno, sure.

bigiain 6y ago
> weight changes with downforce Your high school physic teacher is on line 1. Wants to talk to you about what "weight" means...
- bfuclusion 6y ago
  Mass does not change. But the effective pressure on the tires is as if the weight (which can be defined as the force required to support something) is indeed higher.
- brokenmachine 6y ago
  My high school physics teacher begs to disagree - weight does change with downforce. Weight is the force the earth needs to provide to support the car. Mass remains constant except for burned fuel loss.
theincredulousk 6y ago
yes, I was referring to theoretical, mechanically limited speed. It's not like the tires can expand in diameter 200% though. I'm sure that Michelin has parameterized tables for the tire's mechanical behavior that they are confident about by now. They aren't exactly new to tire engineering...

gameswithgo 6y ago

tires change in size with speed

theincredulousk 6y ago
you could use a maximum value from a parameterized table that I'm sure Michelin has for their top of the line super-car class tire.

pengaru 6y ago

Plainly wrong re: maximum possible speed. Agree re: GPS, on-site laser traps timing an accurately measured distance is obviously preferable. I presume laziness prevails.

theincredulousk 6y ago
Sorry maybe I wasn't clear - it is a mechanical system with gear ratios directly coupled from the wheel to the engine. The maximum RPM of the engine, propagated through the lowest gear ratio, gives a hard limit regardless of the "real world" variables like wind resistance and friction. The point was really if the mechanical system can only "turn" to a maximum theoretical speed of e.g. 300mph, then they clearly and obviously didn't exceed it, no matter what, unless the car transforms into an airplane at some point.
- pengaru 6y ago
  Sure, if you can quantify the maximum tire diameter at max rpm and top gear, you should be able to compute the maximum possible speed. I don't personally know how much those tires deform at such speeds. My personal experience with sportbikes approaching 200mph has been it's not even enough to make up for the ~5% speedometer error that still indicated higher than actual around 190. But that's a much narrower and lighter tire, and 300mph is a different world.

myself248 6y ago

Deploying ground-based infrastructure is difficult, especially in a place as inhospitable as salt flats. GPS (really, multi-constellation GNSS these days) is not only easier to physically set up, it also gives you more data, and likely more accuracy. You don't save the PVT output of the receiver on the vehicle. (I mean, you could save that too, I guess.) You save the raw observables, and post-process along with data gathered at the same time by stationary reference stations. (Either run your own base, or use data from networks like unavco.) This gives you a remarkably precise picture of the vehicle's position, motion, and acceleration, in 3 dimensions, throughout the entire run. If they want to be transparent about it, they'll publish a multiconstellation RINEX file of the whole shebang, so anyone can analyze it themselves. There might be fewer internet sleuths who know how to sling rtklib or gpstk than youtube-dl and audacity, but the ones who can, will produce a wealth of data. For bonus points, put two receivers on the car, a few feet apart. This allows moving-baseline so you also get an orientation solution. 1: http://rtklib.com/ http://rtklib.com/ 2: https://gitlab.com/sgl-ut/GPSTk https://gitlab.com/sgl-ut/GPSTk