# Fact Check – Crashing a full-size bronco down a hill into a pond, does the science pan out?

Lunden
I have a scene in a story where a full-size lifted bronco with 35” tires is sliding skewed down a steep sandy hill. The driver cranks the wheel upon seeing a ledge which fails to change the trajectory of the vehicle, now sliding on sandy gravel at about 20 MPH. The rim of the front wheel connects with a boulder that doesn’t budge, stopping the wheel immediately, however the rear of the vehicle pitches and the incline + momentum takes the bronco over a cliff.

The drop is a sheer 10m (30ft) to a pond of standing water only disturbed by the gravel falling in ahead of the vehicle. The pond is 10ft deep at point of impact. The bronco has a hard top and windshield over the front cab, nothing over the rear seats, and no doors. See the example photo: https://www.broncozone.com/uploads/post-4365-1146769033.jpg

This happens at night and with the lights off its pitch black. A driver and passenger knew of the cliff and water only because the headlights reflected off it. Realizing the bronco will land upside down, the two mutually conclude to unbuckle and jump away from the bronco, getting out of the vehicle when it is perpendicular and barely in free-fall from the cliff.

My questions as follows:
• Would the momentum of the vehicle pitching forward have a realistically debilitating impact on the driver and passenger being physically able to jump free from the vehicle?
• Would they be able to get far enough to the side as not to get caught beneath it?
The two know there’s water and roughly how far away it is but can’t see it that well or at all in the dark. It’s a 30’ drop, plus whatever momentum they had already from pitching off the cliff. They’re both high school girls of average fitness, though one is tall and thick boned. Presumably they would hit the water face forward, mostly head down and with arms flailing. They would impact slightly after the vehicle broke the surface.
• What kind of injuries would they more than likely sustain from water tension, and would it be believable the two are in good enough condition to swim after impact?
• Would 10ft of water be deep enough to stop them from hitting the bottom?
• With the front of the vehicle having come to a dead stop against a boulder and momentum pitching the rear of the vehicle over the edge, what part of the vehicle would likely break the surface of the water first, and what impacts would this have on the two passengers falling into the water beside it in regard to surface tension and either pushing them away from or pulling toward the vehicle?
• As it is an open cab, would air have any chance to slow the vehicle or would it plunge like a rock right through to the bottom?
• How long would the headlights remain lit before shorting out, plunging them in darkness?
• Would the engine hydro-freeze before the headlights shorted out? Would this happen simultaneously?
Based on the answers I may have some follow-up questions. The scene doesn’t need to be muddled down with all this plausible science but does need to be more authentic than your everyday Hollywood movie.

Thanks in advance for anyone willing to discuss on making the scene as plausible as possible!

berkeman

Staff Emeritus
What happens with the air bags when the wheel hits the rock?

Lunden
Good question. Full size broncos went out of production after 1996 (until just recently). This is an older model and it stands to reason it would only have a driver side air bag, disabled/removed after all the custom tinkering done on it. I'm actually not sure if an air bag would normally deploy in this situation as the front of the vehicle wasn't impacted. It might not deploy until after hitting the water even in today's modern vehicles.

Mentor
The rim of the front wheel connects with a boulder that doesn’t budge, stopping the wheel immediately, however the rear of the vehicle pitches and the incline + momentum takes the bronco over a cliff.
If the vehicle went straight off the cliff and only pitched forward from that, the two passengers would have an easier time pushing straight out to get away from the vehicle and try to make a reasonably vertical entry into the water. But with it going off the cliff sideways, the outboard rider is going to have to manage to exit while the vehicle is pitching sideways down onto them, and the inboard rider is going to have to exit with their side of the vehicle pitching up at them. Still do-able, but maybe make the girls gymnasts or something to make it more plausible that they manage to exit and push away far enough to avoid the vehicle.

As it is an open cab, would air have any chance to slow the vehicle or would it plunge like a rock right through to the bottom?
There is still the engine compartment and the lower dashboard area that will not fill with water for a couple of seconds, so I'd guess the vehicle would hit and then sink over the next 5 seconds or so...

Fun questions! Do you 'wheel?

Lunden
If the vehicle went straight off the cliff and only pitched forward from that, the two passengers would have an easier time pushing straight out to get away from the vehicle and try to make a reasonably vertical entry into the water. But with it going off the cliff sideways, the outboard rider is going to have to manage to exit while the vehicle is pitching sideways down onto them, and the inboard rider is going to have to exit with their side of the vehicle pitching up at them. Still do-able, but maybe make the girls gymnasts or something to make it more plausible that they manage to exit and push away far enough to avoid the vehicle.
Great points, and I think in this instance the vehicle would pitch pretty much straight over the front, maybe at a just a slight angle. So I think the two would be able to clear the vehicle then.

There is still the engine compartment and the lower dashboard area that will not fill with water for a couple of seconds, so I'd guess the vehicle would hit and then sink over the next 5 seconds or so...
True, nice call, I'd forgotten about the closed engine and dash. Another question then, would enough air be able to fit around the engine and inside the dash to offset the weight, or might the weight pull the bronco below and the air jettisoned from under water?

Fun questions! Do you 'wheel?
It's been a very long time since I've had the pleasure. Used to go with a friend of mine in his bronco, so I have a feel for how the vehicle handles on rough terrain. Makes it convenient if able to write from experience.

berkeman
Mentor
• What kind of injuries would they more than likely sustain from water tension, and would it be believable the two are in good enough condition to swim after impact?
• Would 10ft of water be deep enough to stop them from hitting the bottom?
Imagine doing a belly flop versus a proper dive from a 10 meter high diving board. The first will stop them a short distance down and leave them in severe pain if still conscious. The second might have them hit the bottom at low velocity depending on how they act after hitting the water. If they are jumping out, they should be able to land feet first. In that case, lots of excitement with little injury. Assuming that they are able to swim up to the surface in the dark while underwater. Knowing which way is up under those circumstances is not trivial.

• How long would the headlights remain lit before shorting out, plunging them in darkness?
• Would the engine hydro-freeze before the headlights shorted out? Would this happen simultaneously?
If it's fresh water, the headlights should stay on until the battery goes dead. The engine will hydro-lock when the air intake sucks water, which will stop the alternator, but not short out the battery. The headlights might help them determine which way is up.

As it is an open cab, would air have any chance to slow the vehicle or would it plunge like a rock right through to the bottom?
It will not plunge like a rock. There will be some trapped air, especially if it is upside down. The effective mass will be less than the actual mass, and there will be a large drag area, so it will sink slowly.

Lunden and berkeman
Gold Member
• What kind of injuries would they more than likely sustain from water tension, and would it be believable the two are in good enough condition to swim after impact?
• Would 10ft of water be deep enough to stop them from hitting the bottom?
Diving into a 16-foot pool from 30 feet is "normal":
Official Guidelines

All pools must follow strict guidelines set forth by FINA, regarding the depth of the water when a diving board and stand is installed. At a minimum, a pool with a one-meter springboard must be 11.5 feet deep at a point directly underneath the tip of the diving board. For a three-meter springboard or five-meter platform, the water depth must be 12.5 feet (4 meters) deep, and 16 feet (5 meters) deep for a 10-meter platform. These pool depths are always listed either on the pool deck or on the side of the pool.

The diving well for Olympic diving must be at least five meters deep. This allows it to be used for the 10-meter platform diving competition and the 3-meter springboard competition.

Checking Pool Depth

Those are the minimums, but not every pool is the same. Some may be 15 feet deep, others 18 feet. The point is that when a diver trains in a pool that is 15 feet deep and then trains or competes at a pool with only 12 feet of water, the bottom will come much quicker than what they are used to. It can be quick enough that if the diver does not make adjustments such a strong somersault save, they may find themselves unprepared for the possibility of sustaining an injury.

[...]

Diving Safety

The standards are set taking into account that when diving from a ten-meter platform, a diver in a streamlined position will come to a stop at a depth of between 4.5 and 5 meters. Typically, competition divers roll in the direction of the dive's rotation as they enter the water and come to a stop at about 2.5 meters below the surface of the water.

Hitting the water flat in a belly flop from 10 meters would be very painful and could result in injury, but would result in a stop about one foot under the surface.

what part of the vehicle would likely break the surface of the water first,
Anything is possible but I would say the front end as it is the heaviest part of the vehicle (by far, looking at the picture).

what impacts would this have on the two passengers falling into the water beside it in regard to surface tension and either pushing them away from or pulling toward the vehicle?
I don't think it would change something about surface tension. The water will go away from the truck, except after the truck has entered completely the water. At this point, the water will go back over the truck, so "pulling back" the divers over the truck if they are close by.

Imagine a leaf while you driving. As the car approaches it, it pushes the air away - and the leaf with it - but as the car passes by, the turbulence behind the vehicle will pull back the air and you will see the leaf twirling behind the car.

• As it is an open cab, would air have any chance to slow the vehicle or would it plunge like a rock right through to the bottom?
• How long would the headlights remain lit before shorting out, plunging them in darkness?
• Would the engine hydro-freeze before the headlights shorted out? Would this happen simultaneously?
I second what @jrmichler said.

Lunden
Mentor
Great points, and I think in this instance the vehicle would pitch pretty much straight over the front, maybe at a just a slight angle. So I think the two would be able to clear the vehicle then.
Well, speaking as someone who has gone off a cliff while 4-wheeling (very scary, but I survived thanks to great seatbelts*), the vehicle can go sideways very quickly in the transition. At least the way you described the crash with the vehicle pivoting off the trail and over the cliff, it will start rolling as it goes over. You will need to modify the scenario a bit if you want it to not roll as it goes off the cliff, IMO.

True, nice call, I'd forgotten about the closed engine and dash. Another question then, would enough air be able to fit around the engine and inside the dash to offset the weight, or might the weight pull the bronco below and the air jettisoned from under water?
It will sink fairly quickly. I would guesstimate around 5 seconds on the surface and 5 seconds descending to hit the bottom.

* So in my crash down that cliff (Hollister, CA 4x4 area), I would have likely been killed if I wasn't wearing my lap and shoulder belts. My driver window was down and the sunroof was open (on my lifted Explorer), so if my belts didn't lock down tight and help me stay pinned in the driver's seat, I likely would have been ejected and run over by my rolling vehicle.

So one other story line you might consider:
• The vehicle is fitted with 4-point harnesses for the seats
• The young ladies understand that if you get into a rollover or other impact situation, you leave your 4-point harness in place for the worst impacts to protect you, and then deal with escaping the vehicle
• So the ladies realize that they are leaving the trail and going off the cliff and will impact the water below, so they brace as they go over, and handle the impact well. Then they release their harnesses and abandon the vehicle to get far enough away to not get caught up in the full sinking.
• As the engine compartment fills with water, it will be the heaviest part of the vehicle, so the vehicle will likely sink nose-down (the half-full gas tank has air in it, and offers some upward bouyancy). As JR says, most likely the headlights will stay on long enough for the girls to get away from the vehicle, and try to figure out how to get out of the water.
Edit -- like this:

Last edited:
Lunden
Staff Emeritus
PF doesn't do accident reconstruction; apparently just accident PREconstruction.

Tom.G, BillTre, Lunden and 1 other person
Gold Member
the two mutually conclude to unbuckle and jump away from the bronco, getting out of the vehicle when it is perpendicular and barely in free-fall from the cliff.
For me, jumping from a vehicle is implausible for the most part - so little time, and even less so in pitch black to decide together. I know it's a shameless critique of a draft; unless this is for an action movie scene.

jbriggs444, Lunden and berkeman
Lunden
Imagine doing a belly flop versus a proper dive from a 10 meter high diving board. The first will stop them a short distance down and leave them in severe pain if still conscious. The second might have them hit the bottom at low velocity depending on how they act after hitting the water. If they are jumping out, they should be able to land feet first. In that case, lots of excitement with little injury. Assuming that they are able to swim up to the surface in the dark while underwater. Knowing which way is up under those circumstances is not trivial.
I was wondering if the problem with the dark and headlights on constant rotation wouldn't be disorienting to the point they might not right themselves correctly, especially given the very short amount of time between exiting the vehicle and hitting the water. Pushing out of the vehicle they'd be horizontal to the water, facing down, and would probably need to right themselves immediately. I guess I'd imagine slipping from the diving board at the moment before the jump and the shock has you all rag-doll. No time to think.

All that said, my guess is outside a spread-eagle belly flop their limbs would break the water tension enough not to knock them out or twist anything too much?

If it's fresh water, the headlights should stay on until the battery goes dead. The engine will hydro-lock when the air intake sucks water, which will stop the alternator, but not short out the battery. The headlights might help them determine which way is up.
Great point, this should solve for which way is up issues, as they should be able to see the bottom. Not too deep to begin with.

Diving into a 16-foot pool from 30 feet is "normal":
True may be, however in this scene the water's only 10' deep on a 30' uncontrolled drop. Given all the responses though, I don't believe there is a strong risk of injury unless I feel the specific need to write one into the scene. The girls should plausibly get out of it mostly unharmed.

Well, speaking as someone who has gone off a cliff while 4-wheeling (very scary, but I survived thanks to great seatbelts*), the vehicle can go sideways very quickly in the transition. At least the way you described the crash with the vehicle pivoting off the trail and over the cliff, it will start rolling as it goes over. You will need to modify the scenario a bit if you want it to not roll as it goes off the cliff, IMO.

And that makes sense, a rolling pitch in any direction has a higher chance than straight on I'm guessing. So in further thinking through the scene I'm wondering if only the passenger makes it clear of the vehicle, the driver being taller and bigger in general fails to get the belt undone, and suffers some injuries to her shoulder and chest as a result. The bronco pitches in her direction so a good amount of water rushes in through the open door at the moment of impact, dampening some of the strain on the belt?
I think with that the belt shouldn't be too difficult to undo underwater, especially as it is one of those large push button ones from the top rather than a small button on the side. I can then contend with how she escapes the vehicle after sinking in a plausible number of ways.

* So in my crash down that cliff (Hollister, CA 4x4 area), I would have likely been killed if I wasn't wearing my lap and shoulder belts. My driver window was down and the sunroof was open (on my lifted Explorer), so if my belts didn't lock down tight and help me stay pinned in the driver's seat, I likely would have been ejected and run over by my rolling vehicle.
Sounds like you were quite lucky and happy you're still here!

• The vehicle is fitted with 4-point harnesses for the seats
• The young ladies understand that if you get into a rollover or other impact situation, you leave your 4-point harness in place for the worst impacts to protect you, and then deal with escaping the vehicle
• So the ladies realize that they are leaving the trail and going off the cliff and will impact the water below, so they brace as they go over, and handle the impact well. Then they release their harnesses and abandon the vehicle to get far enough away to not get caught up in the full sinking.
• As the engine compartment fills with water, it will be the heaviest part of the vehicle, so the vehicle will likely sink nose-down (the half-full gas tank has air in it, and offers some upward bouyancy). As JR says, most likely the headlights will stay on long enough for the girls to get away from the vehicle, and try to figure out how to get out of the water.
This would be the smartest way for them to go about it, but for the story they're both quite inexperienced, and while some work has been done on the bronco, its been done on the cheap. Safety wasn't something high on the list as high school kids are invulnerable to injury until they're not.

PF doesn't do accident reconstruction; apparently just accident PREconstruction.
Great part of being a writer is you get away with pre-construction, even when pantsing. Readers will never know!

For me, jumping from a vehicle is implausible for the most part - so little time, and even less so in pitch black to decide together. I know it's a shameless critique of a draft; unless this is for an action movie scene.
Agreed. My idea was they didn't decide together, more they came to the same instantaneous conclusion. The passenger was already inclined to jump even before the cliff, stopping only due to the realization she'd still be sliding and torn up on the rocks. I'm now thinking the driver didn't make it out, so will need to construct what happens to her from the impact within the vehicle.

256bits
Mentor
Imagine doing a belly flop versus a proper dive from a 10 meter high diving board.
I guess I'd imagine slipping from the diving board at the moment before the jump and the shock has you all rag-doll. No time to think.
Clearly it's time for some experiments by the author (you) at the local swim center that has a 10m diving platform.

Sounds like you were quite lucky and happy you're still here!
Thanks, me too.

Lunden
Gold Member
OK, someone already tested a similar hypothesis and filmed it. (at around 3:00, he drops it head first as well):

Also, I just realized the pond is only 10 feet deep, while the truck is 15 feet long. Water may not cushion the truck's fall as much as one thinks if it goes head first.

Lunden
A driver and passenger knew of the cliff and water only because the headlights reflected off it.
Reflection from water would be visible only at 90 degree. In the described scenario I think what will be reflected (and visible) is the dark sky. The pond will be 'visible' as a dark patch only.

Ps.: kind of like here:

Reflections from the other side are visible, but regardless the light source from behind the water is still dark.

Realizing the bronco will land upside down, the two mutually conclude to unbuckle and jump away from the bronco
I don't think there will be enough time for that. For a 10m fall it's < 1.5 seconds. Barely enough to assess the situation, but unless they are some cold blooded stunt artists, they will be rather busy with screaming and instinctively grabbing for anything at hand while trying to maintain balance instead.

Last edited:
Lunden
Mentor
Also, I just realized the pond is only 10 feet deep
10 meters deep. Do you work for NASA?

BillTre
Gold Member
10 meters deep. Do you work for NASA?
The drop is a sheer 10m (30ft) to a pond of standing water only disturbed by the gravel falling in ahead of the vehicle. The pond is 10ft deep at point of impact.

Lunden
Mentor
Oh. Hmm. Time for my weekly Teams meeting with NASA, gotta go...

jack action and Lunden
Lunden
Also, I just realized the pond is only 10 feet deep, while the truck is 15 feet long. Water may not cushion the truck's fall as much as one thinks if it goes head first.
Right, and makes sense. The bronco is nearly 15.5ft long, so looking at the video it does seem it would cut through the water and impact the bottom with a decent hit if it enters vertically. This puts more emphasis on how the vehicle should move through the air after the front left wheel hits a boulder and stops it from sliding, the back pitching forward to pull it off. This should have swung the vehicle past 90 degrees, so a straight vertical drop is unlikely. My guess the truck would hit somewhere on driver side hood and fender, spreading out the impact enough the truck won't hit bottom. I'd also guess it would touch the bottom before the rear sunk, but would be irrelevant to the scene as the point of view character is underwater at this point.

The pond will be 'visible' as a dark patch only.
That's pretty helpful, as it would indicate the two would have no way of knowing just how far a drop it is, negating the impulse to try and exit.

I don't think there will be enough time for that. For a 10m fall it's < 1.5 seconds. Barely enough to assess the situation, but unless they are some cold blooded stunt artists, they will be rather busy with screaming and instinctively grabbing for anything at hand while trying to maintain balance instead.
Excellent point, and accounting for extra time the truck was pivoting still wouldn't be enough for an unexpected reaction that would clear the vehicle. Not that it would have been a realistic option for them to consider if they can't see the bottom of the drop.

So with all the points mentioned, its feasible the two will need to remain in the vehicle, buckled, and the truck lands more or less upside down in the water. The two end up under water pretty near immediately as their heads will be near the roof of the vehicle that's underwater, so now they're drowning until the point they get out of their seat belts and right themselves. If this isn't done in mere seconds, the truck is sinking to the bottom and they need to breach the surface before they breathe again.

I would imagine some bruising from the seatbelts, sore ribs and shoulder. Maybe some impacting against the steering wheel for the driver, but not enough they couldn't unfasten the belts and work themselves out.

My guess is the truck should settle on its hood on the bottom, so both doors are available for escaping and there's not much risk the driver might end up pinned beneath the frame.

Have a good idea how to render the scene then. Thanks everyone for the input! I'll post the scene once completed and let you be the judge on how plausible it comes out.

so now they're drowning until the point they get out of their seat belts and right themselves. If this isn't done in mere seconds, the truck is sinking to the bottom and they need to breach the surface before they breathe again.
Maybe worth to mention that releasing the seatbelts in those circumstances (upside down, underwater, thorough panic) is not something easy or trivial. Even with closed windows it's absolutely a life-threatening situation.
Fortunately, with old-style (switches&buttons) car electronics the lights will remain operational for a few minutes (maybe even longer) underwater. It would help a lot: the water will scatter the light so there will be some visibility.
Though as hitting the bottom everything will become muddy anyway. Also, I don't know if this would work with modern cars (any sophisticated electronics will be inoperational pretty fast underwater).

Last edited:
Lunden
Lunden
Maybe worth to mention that releasing the seatbelts in those circumstances (upside down, underwater, thorough panic) is not something easy or trivial.
Aside from disorientation and panic, I think it'd be easier to unfasten the buckle being underwater than if gravity was pulling your weight into the belt, where you'd need to contend with lifting your body to relieve tension before it could be unfastened. Definitely worth noting, however.

Fortunately, with old-style (switches&buttons) car electronics the lights will remain operational for a few minutes (maybe even longer) underwater. It would help a lot: the water will scatter the light so there will be some visibility.
The same with the headlights I'm thinking. Should scatter quite a bit of light into the cab, maybe enough to orient with if able to stop panic from taking over. Thanks, great points!

Lunden
Here's the scene everyone helped me render, thanks again for all the feedback! :D

#### Attachments

• Wrecked Bronco.docx
16.5 KB · Views: 8