Burning out in a car and then accelerating

[jtvang125]

This is taken from the scene in Back to the Future where the Doc is testing the machine for the first time. He does a burnout and you can see the speedometer climbing but the car is stationary but I believe his actual speed would be zero, correct?. I can't remember at what speed he launches the car but lets say it's 40mph. As the car starts rolling forward wouldn't the speedometer keep climbing from 40 on while the car is physically starting from 0?

Or would the speedometer stay at 40 and as the car continued accelerating its velocity would finally catch up to the 40 and then increase from there?

 

[Cogman]

Most speedometers measure how fast the axle is spinning to determine the speed of the car (especially older cars). Assuming he keeps a constant pressure on the gas pedal, the speedometer should appear to slow down as the wheels take grip (because the axle will slow down) It is possible for the car to go all the way up to 40 MPH though without being able to see a noticeable change in the speedometer.

Either way, the change in the speedometer needle will be pretty hard to notice.

 

[L00PY]

I agree with what Cogman said except I suspect the change in speedometer needle will be very easy to notice. The needle would noticeably jump down from burnout speeds once they gripped the road. You can see the needle jump similarly if stuck on ice / snow, or if you hydroplane. This assumes no ESC.

On a related note, putting tires and wheel combinations of a very different size than intended for the car can have a large effect the speedometer.

 

[Minerva]

Speedometer readings are derived from actual power train speed. If you've ever driven in snow and lose traction while accelerating you see the needle jump. If you need true speed over ground a GPS would show the correct speed.

EDIT: This is why you must recalibrate your speedo if you change tire size significantly. Larger tires mean the speedo is showing a slower speed than you are actually going.

 

[VeryCharBroiled]

something else to consider on open axles is if only one tire breaks loose and the other doesnt the speedo will register only 1/2 of the spinning tires rotational speed when the cars not moving. so if it says 40, the tire thats loose will be rotating at 80. this is due to where the speedo gets is speed info from (the transmission output shaft) and the way open axles transfer RPM when one tire has grip and the other doesnt. the tire thats spinning gets the "extra" unused RPM of the tire that has grip and is not rotating, so the loose tire spins twice as fast. as the other tire gets rolling the RPMs will equalize.

if you have lockers or a good posi both tires will roast and then the speedo will read the actual tire (not vehicle) speed as both tires will spin at the same RPM.

 

[KIAman]

Nobody ever got a smog check? Never been to a dino?

A wind gauge and visual gauges can be defeated.

The only true method to determine speed is by GPS (and even this can be defeated).

 

[bobsmith1492]

Originally posted by: KIAman
Nobody ever got a smog check? Never been to a dino?

A wind gauge and visual gauges can be defeated.

The only true method to determine speed is by GPS (and even this can be defeated).

Many places don't require "smog checks." What's a dino?

Why would you want to defeat your speed measurement device?

 

[KIAman]

Originally posted by: bobsmith1492

Originally posted by: KIAman
Nobody ever got a smog check? Never been to a dino?

A wind gauge and visual gauges can be defeated.

The only true method to determine speed is by GPS (and even this can be defeated).

Many places don't require "smog checks." What's a dino?

Why would you want to defeat your speed measurement device?

One of the benefits to living in California. Rolling smog check requires you to pass smog at idle, 15mph and 30mph.

You can't obviously tow the smog machine with you so they have rollers that free spin your wheels (front, back or both for AWD) and the smog technician can determine your speed by looking at your speedometer, all the while the car is not moving from it's location.

A dino is short for dynamometer and is a device to measure a car's horsepower and torque at the wheels by using the same mechanism as the rolling smog check machine.

http://en.wikipedia.org/wiki/Dynamometer

As to defeating speed measurements, I don't think anybody would WANT to defeat the readings, but I was simply pointing out the fact that you cannot have an accurate speed measurement other than through external means.

IMO, this should all be common sense. Your boat could be going 50mph against a river current going 60mph the opposite direction and your net speed would be 10mph backwards but your speedometer will clearly show 50mph.

Same concept for cars, bikes, airplanes, space ships everything that moves.

 

[geno]

Originally posted by: bobsmith1492

Originally posted by: KIAman
Nobody ever got a smog check? Never been to a dino?

A wind gauge and visual gauges can be defeated.

The only true method to determine speed is by GPS (and even this can be defeated).

Many places don't require "smog checks." What's a dino?

Why would you want to defeat your speed measurement device?

A dyno (dynomometer), or a stationary roller (or rollers) which you drive the driven wheels onto. The dyno / car's wheels spin and the car stays stationary, basically recreating a burnout (wheels spinning / car stationary).

 

[bobdole369]

Usually the speedo gear is located within the transmission on the output shaft - which is effectively on the axle before any differential. It's the same concept whether mechanical or electronic speedo - a gear spins, the needle reflects how fast its spinning At least on GM and Chryslers it is.

Thus when doc brown locks up the nondriven wheels with the brakes and floors it and the driven wheels spin out - he gets a speedo reading. Unknown whether he used the E-brake or service brakes, but it won't matter if your engine is big enough - you just end up with cherry red rotors that need replacing if using the service brakes.

It all depends on how quickly the tires grip. If theres enough torque when the brakes are released it will continue to spin the tires for a bit. Once the tires slow down because they catch road the speedo will slowly (relatively speaking) drop to actual road speed. If theres not enough torque to keep em spinning the car violently bucks and the speedo drops like a rock.