Doc Brown: If my calculations are correct... you'd be dead!

"Bullitt Ride" (yesterdayknight) 05/13/2015 at 14:10 • Filed to: back to the future, bttf, dmc, delorean, marty mcfly, doc brown

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Back to the Future is hands down one of my favourite movie series, I’ve seen it dozens of times, and any time I see that it’s on TV I end up watching it... with the commercials... even though I could simply throw in the DVD to avoid the commercials... that must be the definition of lazy haha. As I’m sure it’s been discussed ad nauseam on the interweb, we are all aware that if Doc Brown’s calculations were correct Marty would have got to the line before the lightning struck. Fortunately, the Delorean was reliable enough to die on Marty just before the alarm clock went off...

So I was thinking this morning about Doc’s other “calculations”, mainly about getting up to 88mph in the parking lot of a mall. For the average sports car these days it’d probably be quite easy, however as we all know, the Delorean isn’t exactly a quick car, in fact it’s quite slow by today’s standards. So I wanted to try and do a quick rationalization as to whether a Delorean in real life would be able to reach 88mph in roughly the distances shown in the movie.

I did a quick Google search for the mall that the scene was shot in and I found this diagram that some other fans had already made for tourists showing the paths of the Delorean as well as the Libyans...

The paths on the diagram seem to be relatively accurate, so for the sake of this simple calculation I assumed that they were accurate. Okay, let’s look at Einstein’s drag race. In order to get the actual distance travelled I took a measurement off of Google maps and it was roughly 100 meters.

According to Wikipedia a Delorean will do 0-60mph in 8.8 seconds. To keep things simple I assumed a constant acceleration, up to 60mph this is a reasonable assumption, above 60pmh drag typically starts to take hold and acceleration starts to decrease, so if anything it’s an optimistic assumption. I’m also ignoring the fact the the parking lot in the movie was wet, and that the car would have likely been even slower with the added weight from the equipment Doc added to it. And yes, I’m also assuming that Doc hadn’t done anything to improve the acceleration performance of the Delorean.

The equation for acceleration is A=(V2-V1)/(T2-T1). In this case we know that V1=0, T1=0, V2=60mph, and T2=8.8seconds. Doing a unit conversion we end up with an acceleration of 1.894x10^-3 miles/second^2.

So knowing the acceleration and knowing that the car travelled roughly 100 meters or 6.214x10^-2 miles we can look at how long it would have taken to traverse that distance. For an object starting from a standstill the equation for distance assuming a constant acceleration is D=0.5AT^2. Solving that equation for time gives an answer of 8.1 seconds. What that means is that the Delorean wouldn’t even be going 60mph (let alone 88mph) by the time it reached Doc and Marty since it takes 8.8 seconds for the car to get to 60mph. Doc wasn’t joking about seeing some serious shit... him and Marty would be splattered across the Twin Pines parking lot!

Ok, well what about Marty? Did he have a better chance at making it since he had a rolling start? In the movie just before he rounds the corner and puts the hammer down we see a shot of the speedometer dropping down to below 35mph.

I’m going to assume that once he got the car straightened out from his little moment of oppositelock that he started his drag race at 35mph or 9.722x10^-3 miles/second. Another Google maps measurement shows that he had roughly 120 meters until whatever that little booth was that the Libyans smashed into.

The equation for final velocity assuming constant acceleration is V2^2 = V1^2 + 2A(X2-X1) where in this case X1=0 and X2=120 meters or 7.456x10^-2 miles. Plugging everything into the equation and solving for V2 gives an answer of approximately 70mph. Not that Doc had calculated this scenario... he was dead at this point, but Marty wouldn’t have had a shot at making it either. I’m sorry Jennifer, but if you want I’ll go up to the lake with you instead... and I’m talking about the original Jennifer here, not her lesser replacement.

Arch Duke Maxyenko, Shit Talk Extraordinaire > Bullitt Ride 05/13/2015 at 14:17

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But, you see, that was no stock DeLorean...

RamblinRover Luxury-Yacht > Bullitt Ride 05/13/2015 at 14:21

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It’s only reasonable to assume that somebody building a time machine into a DeLorean would arrange to make it possible to reach 88 (how much does the time machinery weigh, anyway?), but scientists are often unreasonable. “Can the DeLorean speed up, and who cares how it slows down? That’s not my department.” says Dr. “Doc” Brown (with apologies to Tom Lehrer)

Brobalt SS > Bullitt Ride 05/13/2015 at 14:24

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Ah, but you are forgetting that the DeLorean was fitted with a massive supercharger that you could hear whine every time it accelerated!

Sn210 > Bullitt Ride 05/13/2015 at 14:27

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Doc definitely had a I/H/E set up on the Delorean, plus a tune. +55hp easily before the tune. And sticky tires. Plus all the weight savings by removing the rear lourves. Not sure about the added HP from stickers, they probably had to remove them for the movie.

RallyWrench > Bullitt Ride 05/13/2015 at 14:33

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I find the fact that the DeLorean had 32,994 actual, moving under its own power miles on it by 1985 highly implausible. I mean, it’s a PRV put into a half-assed Lotus chassis by some disgruntled Irish.

Nobi > Bullitt Ride 05/13/2015 at 15:48

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Far as Doc’s calculations go, it’s well established in the movie that he’s kind of a quack inventor. He doesn’t really make much that works properly. For him to be off on his timing in 1955 when the lightning was supposed to strike isn’t too much of a stretch. As far as the DeLorean being too fast, Doc made the DeLorean into a nuclear powered time machine, who’s to say the engine was stock? I’ve never heard a DeLorean sound like that(Yes I know that was a sound created for the movie, but still.)

Bullitt Ride > Nobi 05/13/2015 at 16:07

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The engine still ran off of regular 1985 gasoline. The nuclear power was only to generate electricity for the time circuits and the hover conversion (after going to the future). I guess it’s possible that he could have had a hybrid electric drive system to supplement the combustion engine... mad torque for a mad scientist.

bre92ser > Bullitt Ride 05/13/2015 at 18:09

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Marty showed some impeccable slalom skills here. Fact: Libyans can’t slalom.

rbcp > Bullitt Ride 05/13/2015 at 19:10

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The Libyans didn’t run into a bus stop. That was a 1 hour photo booth.

Nobi > Bullitt Ride 05/13/2015 at 19:26

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Yeah, I know that was the “kick” for the flux capacitor, but what I meant was if he did that, i’m sure the gas engine isn't stock.

Tohru > RallyWrench 05/13/2015 at 22:25

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The last transportation vehicle made by disgruntled Irish was the Titanic.

Christian Williams > Bullitt Ride 05/14/2015 at 00:47

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Great post. In case you wanted to go even deeper, I thought I’d point out that the on-screen speedometer isn’t accurate. Per law in 1981, the DeLorean’s speedometer was limited to 85 MPH and it had to be repainted (and probably not recalibrated) for the movie.

KatzManDu > Nobi 05/14/2015 at 06:46

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These engines (PRV V6) were turbocharged and the basis for the Renault Alpine V6 GT, so anything is possible.

delongedoug > Bullitt Ride 05/14/2015 at 10:22

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First Jennifer was best Jennifer

Gusmer > Bullitt Ride 05/14/2015 at 14:01

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Hold on... didn’t Doc Brown start acceleration with the remote controller while the car was still standing still??? I just saw the clip, and he did; and then released the Delorean when it had already accelerated to 65 mph. So the car would only have had to accelerate only from 65 to 88 when it travelled said 100 meters.

Starts at the 3:00 mark.

Bullitt Ride > Gusmer 05/14/2015 at 14:04

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AMG = AH MAH GAHD! > Bullitt Ride 05/14/2015 at 17:59

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Wow. All that trouble and work to make a detailed map with vehicle paths, but end up misspelling “Souplantation” and “T.G.I.FRiDAY’S”!

AMG = AH MAH GAHD! > Bullitt Ride 05/14/2015 at 18:02

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Nope. Elizabeth Shue > Claudia Wells.

1948CJ2A > Bullitt Ride 05/15/2015 at 08:58

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Nuclear power generates electricity by generating heat and applying that to water to create steam and then having the steam turning turbines to generate power. At least that’s the basic principle in power generation. So applying that to the film, he would have needed a reactor, which presumably he did have (where the plutonium goes in). Then he would have needed a water vessel, steam pressurized containment, and turbines to generate the power. He also would have had waste/bi-product from the used plutonium. Uranium 235 could have been substituted as well. Even if all that is plausible, which its obviously not, the power generation would be a steady trickle versus a big burst of power we assume is needed. Even more components would be needed, perhaps a series of giant capacitors (no pun intended) to store power for immediate dispersion.

Food for thought. Yea I know, it’s a movie and purely for entertainment, of which it does a great job. The nerd in me red-flags this kind of stuff thought.

Final thought - I got a kick out of this author when he said “whatever that little booth was that the Libyans smashed into.”. Funny, I can recall seeing those everywhere when I was a boy. In case any of you youngsters are curious, they were places to drop off your camera film for development. You’d often find them in large parking lots. These days the ones that are left are usually the equivalent of a food-truck without wheels.

webmonkees > Bullitt Ride 05/29/2015 at 10:14

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With all the time machine stuff going on, most forget about the other impossibilities.

deekster_caddy > Bullitt Ride 10/21/2015 at 13:54

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Are you kidding me? It’s a TIME MACHINE. Obviously he did an LS swap.

Bullitt Ride > deekster_caddy 10/21/2015 at 14:18

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If the LS existed back in 1985 he probably would have.

deekster_caddy > Bullitt Ride 10/21/2015 at 15:29

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But he has a time machine...

thestumpps > Bullitt Ride 12/27/2018 at 16:37

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Actually, if you toss out the facts about the Delorean and assume that Doc did customize it (since the Delorean didn’t even have a speedometer that went up to 88 mph and the one seen would have been a custom installation, and BBTF3 shows us that Doc definitely isn’t unwilling to modify vehicles to make them go faster), and you assume the tendency for movie time to be at around a rate of 4 or 5 to 1 with reality (since movies like to show multiple moments within a moment, there tends to be a time dilation that causes the run time of an intense moment in an action scene to be around 4 to 5 times longer than the actions in total would have been in reality),   and you take the amount of time that the footage shows the car taking for Einstein’s drag race (~ 18 seconds) , then you get a time of around 4 seconds as the median between the min/max ranges of time slippage in film action scene tendencies (4.05 seconds).

Now, if you assume that Doc is telling the truth about it hitting 88 mph, then the distance that you get for that to be accomplished using d=V 0 t + 0.5at 2 turns out to be around just about 80 meters (79.6 ) . Which, if you look at that map the fans made again, and keeping in mind the red lines mean the time travel initiation sequence (which required 88mph to be reached to be triggered) has begun, so the amount of distance isn’t 100 meters because 100 meters was how long it existed before the car “exploded” into time , but it’s somewhere around ~80 meters for the car to trigger the initiation for the time travel sequence by reaching 88mph ( 39.33952 m/s ) .

Since we see a car initiate the sequence by roughly 80 meters in a bit over 4 seconds (when time is converted...because otherwise, we’re talking about a car that would take over 2 00 meters to reach anything even 60mph due to an 18 second timeline...which is clearly not their reality, but an effect in our reality of multiple scenes within one moment being cut back to back ) , I think it’s fair to assume the car reached the target trajectory ... which I think we all agree a real Delorean could not have achieved, nor can a real Delorean be modified to travel through time, so I think we can assume this is no ordinary build.

Now let’s flip over and look at Marty, on the other hand. Marty also holds up pretty well if you assume that he held the 35mph rate while he was looking in his rear view mirror up until the rocket was pointing at him and he shifts the gear and speeds up. If you assume that then, it looks as though (best as I can tell from looking at the footage repeatedly) that he has passed somewhere around 1 5 meters of the path after he straightens out before switching speeds, which if we compare those paths from the fans then we get this point being somewhere around 8 5 meters away from the final point where the car explodes into time. The amount of time is 17 seconds, or a median of just under 4 seconds (3.825). Starting at 35 mph (15.6464 m/s ) and assuming he achieved 88mph as we were shown, then it would have taken him just around 65 meters to do this (63.33). The total distance path for Einstein was about 100 meters, but only 80 meters of it were required to reach 88mph, and the remainder of the distance was the total distance it took to complete the time machine launch sequence. That means that once you hit 88mph, the distance that it took to completed the launch sequence is around 20 meters (20.4).
Marty reaches 88mph in a bit under 65 meters (63.33), and his total distance was roughly 15 meters less than the path of Einstein’s for attempting to speed to the 88 mph goal, which means that Marty has to hit it in 8 5 - 20 meters or less, leaving him with a target distance of around 65 meters to hit 88mph and have enough distance left to run 20 meters for the remainder of the launch sequence; which fits with the values he appears to achieve.

Basically we have to assume that if Doc owns something, he probably doesn’t leave it stock build (not even his clocks in his house are...nothing in his house appears to be stock, actually).

Cheers

The Stig's graphic designer cousin > Gusmer 09/18/2019 at 13:47

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I’m going to make the assumption that you are being serious. Also making the assumption that it is physical vehicle speed that matters, and not the wheel speed. Otherwise, you’d just put the car on a dyno, or at least keep spinning the wheels until it reads 88mph.

That assumption out of the way, when wheels are spinning like that, the speedometer is showing wheel speed, not the actual vehicle speed. In other words, the wheels are spinning at the same speed they would be rotating at if the car was in fact going 65mph. But the vehicle is in fact, stationary. Which means it’s speed is 0.

When he lets off the parking brake, the car is still doing 0, and just because the tires are spinning at the equivalent of 65 mph, doesn’t mean that the car will instantly accelerate up to that speed. Since the tires have already broken traction, the co-efficient of grip will be very low, so the tires will just keep spinning to a faster equivalent speed, but car will just creep forward at a slow speed.

Now, lets pretend that when Doc lets off the brake, the tires instantly grabbed and was able to accelerate like normal. It still would accelerate up to 65mph. The instant shock to the drive train would like snap something, and if by some miracle something didn’t brake, the woefully under powered engine would just bog down to the point where the engine and drive train were spinning at their appropriate rates for speed/load, so you’d still be accelerating from 0, but that highly abusive start could shave some time off, but it would be in tenths of seconds, not the multiple seconds shown in the movie.

Now, all that is assuming normal DeLorean power. If the plutonium power supply also helped power the wheels through an electric motor, then they may have been able to accelerate in the allotted space/time, but spinning the wheels up to 65 would still be more counter productive than helpful.

The Stig's graphic designer cousin > webmonkees 09/18/2019 at 13:47

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Right, there’s no way a DeLorean could outrun a VW bus.