Does the weight of a car affect people inside?

Posted by thatgymdude@reddit | cars | View on Reddit | 73 comments

I have physics question for you all after a pants-shitting experience in real life that happened to me. How much does the weight of a car affect the occupants of a vehicle especially during emergency stops? I was riding with my friend in a Rivian R1S doing around 40 mph in traffic when a stupid driver pulled out in front of us without looking which caused the SUV to do an automatic emergency stop and it was one the most violent experiences I have ever felt in a car. The SUV went from 40 to 0 before my friend driving could fully react to it, and I thought from the force we hit the vehicle in front of us but we actually missed hitting him from under a foot. It injured him (sprained his shoulder and did something to his collarbone) and it knocked the wind out of me and after he had to see a doctor. I swear nothing has as much force as that thing during an emergency stop, it didnt dive much at all shockingly, and almost no tire squeal and just crushed us then lurched backward after we stopped, and then we realized what happened. I have been in emergency stops before in other cars including my truck and full on the brakes in a track car, but this was different. Was the weight of the car a factor in it or just how sudden the stop was? Don't get me wrong I am thankful we didn't hit the other driver, but I am surprised a stop like that injured him bad enough to need a doctor and I am concerned the ever increasing weight of some of these vehicles is playing a part in it. Thanks!

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73 Comments

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EnvironmentalUse2498@reddit

This is all caused by inertia. Since you have mass you will resist acceleration or a change in velocity/speed. However, the weight of the car is irrelevant to your acceleration.

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VINdotORG@reddit

Yeah, weight absolutely plays a role — but not in the way most people expect. When something as heavy as a Rivian does a full emergency stop, all that mass wants to keep moving forward. Modern EVs can brake insanely hard because they’ve got huge tires, low centers of gravity, and crazy-advanced stability systems. So instead of the car diving and absorbing some of that energy over time (like older or lighter cars), it just slams all of it into your body in a fraction of a second. Think of it like stopping a bowling ball vs. stopping a baseball. Both might be going 40 mph, but only one of them tries to rearrange your spine when it comes to a halt. The Rivian didn’t hurt you because it’s “unsafe” — it hurt because it’s so capable. It can go from 40 to 0 faster than your brain can process, and the heavier the vehicle, the more your body feels that deceleration. So yeah, the weight + how instantly the brakes hit = why that stop felt like getting punched by physics.

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National_Beginning_7@reddit

and that's why I have a dash cam and a car without auto brakes. you wanna be a fuckass congratulations you're buying me a new car.

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thatgymdude@reddit (OP)

The other driver was at fault not my friend.

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AnonymousEngineer_@reddit

No. The force on occupants inside is a result of the rate of acceleration/deceleration. You could be on a sled travelling at 60km/h and have the same forces put on yourself with nothing more than a bungee cord.

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woodsides@reddit

Yeah essentially. Your body doesn't feel speed, it only feels acceleration. F=ma, so the heavier you are and the harder you brake, the more force you're going to feel. Plus, all that force is mostly concentrated over a small piece of area around your where your seatbelt sits on your chest, so you feel it a lot more.

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RetiredBSN@reddit

The seat belts may give a little, but then they will lock up to prevent you from hitting the dash, so it is going to be on the violent side, but better than colliding with the dash or getting thrown through the windshield.

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R_V_Z@reddit

> Your body doesn't feel speed Disclaimer: Until you get enough speed, and then your body *really* feels it. Don't approach C, kids.

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Pixelplanet5@reddit

and the same thing applies to many other things, for example we dont feel temperature, we feel the change of temperature compared to our body temperature.

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accountforrealppl@reddit

I would also add that a rivian would have a relatively soft suspension with a lot of travel, and the heavier weight would probably load it up more. I believe the dipping sensation from the suspension coupled with it bouncing back more can contribute to the feeling of harsher braking. Anecdotally, I can come to a stop pretty quickly in my miata without it feeling like I'm slamming on the brakes since it has a stiffer suspension and does very little pitching down or bouncing when I come to a hard stop. Coming to a stop at the same speed in my e class (luxury trim with the softer suspension) can *feel* a lot more violent because the entire front of the car dips down substantially and bounces back up once I fully stop, even if the acceleration I'm actually experiencing is the same.

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Bassracerx@reddit

This is a good point so the passengers bodies are not only accelerating forward but also down with that soft of a suspension and increased travel i can see you getting the tummy flips. Likely its just a quirck of an electric car. Using tbe regen motor plus friction brakes in a panic stop probably felt like your eyes were coming out of their sockets!

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StinkySoggyUnderwear@reddit

Cars have weight transfer

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AnonymousEngineer_@reddit

That is not how weight transfer works. The mass of the car's chassis does not get transferred to the occupants of the car when it accelerates or decelerates. If it did, nobody would be able to survive a trip on a train.

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StinkySoggyUnderwear@reddit

But the passengers will feel the momentum, which creates weight transfer.

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LevoiHook@reddit

You will feel the nose dive, but light cars do the same. It is only a matter of centre of gravity and suspension. The weight of the car itself is not an issue.

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StinkySoggyUnderwear@reddit

Suspension.

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StinkySoggyUnderwear@reddit

But it will throw you around

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itsaPHound@reddit

This guy physics

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sendme_your_cats@reddit

This guy this guy's

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B00marangTrotter@reddit

He's not you buddy guy.

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DudeWhereIsMyDuduk@reddit

> You could be belted into a sled travelling at 60km/h Goddamn John Stapp would've had the best Reddit AMA (I realize his speeds were like four times that lol)

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TheDistantEnd@reddit

The weight of the car is a factor insofar that the brakes on the car will be much more powerful to stop its mass. The Rivian's braking was so violent mostly because that R1S has powerful brakes meant to stop its mass. The uber brakes, coupled with you guys not expecting the sudden stop, is why it was so rigorous. I'd be curious to see the traffic conditions that led to the emergency braking. I'm sure driver skill and reading the situation is also a factor to why the car had to code red stop flat from 40.

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thatgymdude@reddit (OP)

The traffic conditions were normal and it was just an idiot in a hatchback who took a left turn and pulled put in front of us without waiting for us to go by. My friend is a decent driver but no one could react fast enough to that honestly. Based on the footage the car pulled out about 4 car length from us while we were doing around 40mph. If his Rivian hit that tiny little car I am sure we would crushed it like a tin can, the otjer driver was 100% at fault.

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Razathorn@reddit

The weight of the car and its orientation affect the rate of deceleration, which is what you felt. It's not that being heavier or lighter affects it independently. In general, the heavier a car is, the slower it slows down, and the less deceleration you as an occupant will feel, but this is only part of the story. The lighter the car is, the less friction it can create as weight is transferred to the front tires as the front slams on the brakes, but also the less mass it has to slow down. Your position in the car has an effect too, and all of these things are somewhat elementary. The thing that REALLY matters is that the available weight is optimal for transfer of force to the front tires and the brakes and tires being optimal. Weight of the car is just one piece.

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FriedEldenRings@reddit

i am wondering if the body roll forward can affect the g forces you experience during a hard stop. because in an suv you'll be pitched out of your seat due to the longer suspension travel compared to something like a sedan. and we know from things like an atlatl that the acceleration is additive when there are multiple sources of acceleration.

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BluesyMoo@reddit

Weight of the car is not a factor. Just how sudden the stop is. It means that the Rivian has pretty good braking capability then.

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MercSLSAMG@reddit

It also brings up a great point about autonomous/semi-autonomous driving - the vehicle is stopping before the occupants are bracing for a sudden stop. When doing a sudden stop manually you are pushing hard not only against the brake pedal but you will also push against the steering wheel out of reflex. That doesn't happen when a vehicle does it on it's own which leads to the seatbelt taking all of the force of the body trying to go forward.

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bpnj@reddit

So fewer broken wrists and ankles then?

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MercSLSAMG@reddit

But more broken ribs, collar bones, and possibly pelvises, and like in this case there doesn't even need to be an impact while for wrists/ankles a crash does need to happen. Imagine if you work in a job that requires you to be physically fit and you have to go off work for 4-6 weeks from your car stopping too fast with no impact...

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bpnj@reddit

So you’re saying braking is worse than the crash that the braking prevented? The deceleration rate would be much higher in an actual crash than an automatic braking event.

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MercSLSAMG@reddit

The hope would be there would be much less collisions so the collisions+sudden braking injuries would still be less than it currently is. Whether the injuries are worse depends on what happens - a simple wrist/ankle break is a 6 week process that while annoying is relatively easy to deal with. Broken ribs sucks, and does have the possibility of puncturing a lung, but typically something that is a simple recovery. A broken pelvis though is a big issue, typically you're bed ridden for a bit and very light walking for a bit after. The bigger issue is dealing with your workplace - getting into a car accident and being injured is going to be no issue to get time off for 99% of people. Being injured because your car broke so suddenly - you're going to be the butt of jokes for a while and some places are not going to be accommodating. It's one of those unintended consequences that most would not have thought of, I'm not saying to abandon the technology because no collisions is better than having them. But as we're seeing with this post you could stop all collisions but there would still be instances of hard braking that can cause injuries.

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thatgymdude@reddit (OP)

Modern cars are already at this point with how strong their automatic braking systems are. My friend ended up with a collarbone injury and my shoulders are injured plus it knocked the wind out of me instantly and we both are physically fit and strong dudes. Imagine that happening to kids or elderly people and how they would fare. Sure it is better than a crash but these automated systems if they make a mistake you will be injured regardless.

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thatgymdude@reddit (OP)

You are right, it reacted before he could press on the brake pedal and before I could say something or grab anything. My Lyriq did this once but I already was covering the brake pedal and was expecting something bad. This was immediate and violent, it saved us from the accident sure but you will be injured regardless. If this triggered by mistake in a car with children for example like how phantom braking happens in some EVs that would be bad.

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zxrax@reddit

for clarification, braking ability in a single emergency stop situation is most strongly affected by the tires rather than the actual brakes. That said this is slightly different in EVs because they can modulate deceleration using the e-motors (i.e. via regen braking) with substantially more fidelity than a traditional ABS system, so where the brakes would lock up and trigger ABS in an ICE car, an EV can use its e-motors to apply deceleration force more judiciously and brake exactly at the threshold the tires can withstand instead.

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Captain_Alaska@reddit

Not how it works. For starters the car can’t know what the limit of traction is without exceeding it. Secondly the vast, vast majority of EVs use a single motor across two wheels with a differential and have literally no ability to modulate power (regen or acceleration) independently to a wheel.

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markeydarkey2@reddit

The relatively limited regen in most is also not going to do much in emergency braking conditions. That precise motor control can be great for low traction situations but when you need 1.0g of decel and regen can only do 0.4g most braking force will come from the physical brakes in an emergency stop.

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StanknBeans@reddit

Someone failed physics. It matters less as brakes size up proportionately, but in hard ABS braking, weight absolutely has a significant impact because contact patch can only be so big before its no longer functional.

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BluesyMoo@reddit

You're wrong. All of the mass, ABS, contact patch... all those factors, they boil down to one number which is the deceleration Gs. Or "how sudden the stop is" in plain words.

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StanknBeans@reddit

And what factor do you think determines the maximum amount of deceleration Gs achievable by a car? Mass, contact patch and size of the braking contact. Some /r/confidentlyincorrect shit you got going on.

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AdamN@reddit

You could imagine however that a lighter car (and fatter tires/bigger discs/more pistons) could allow the auto braking to further minimize the max G during the stop. I’m not sure if these systems are that elegant though or it’s just that they are activated and then stop as quickly as possible.

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aroc91@reddit

They mean in the context of OP's question, smartass. The alleged effect on the occupants. They're not talking about the effect of the weight on the braking system.

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hotakaPAD@reddit

Yea EVs have low center of gravity. That helps them stop

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FlintHillsSky@reddit

Because having the weight low and in the middle reduces weight transfer to the leading tires. That means that the front tires don’t have to do extra work to absorb more inertia and the back tires are not unloaded and lose traction.

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thatgymdude@reddit (OP)

I can agree, you WILL stop, after it was over I was impressed by how fast it stops.

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Lonely_Fisherman_233@reddit

So from a physical standpoint, the weight of the car doesn’t directly affect your experience; only the rate of deceleration does. Because Rivian is a premium electric car, and the R1 is large, I wouldn’t be surprised if it has very large and powerful brakes, allowing you to come to a stop more quickly. That’s great for avoiding collisions, but as you pointed out it can itself have similar effects to deceleration due to a crash.

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ReV46@reddit

Most people don’t realize how fast a car can stop. I have a track car and when I take friends on track, the universal comment is how shocking and violent the braking is.

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thatgymdude@reddit (OP)

I have stood on the brakes in a Huracan Performante and as face melting at that was with a harness honestly the Rivian was much worse. Sure the Gs are more sustained but a violent stop from 40 almost instantly does alot more damage. The Shelby GT500 was a close second behind the Huracan and after about 12 laps of that brute of a car I had enough. The worst part about tracking imo is the side Gs, even if you are in shape like I am it's very tiring and the Huracan added the mental exhaustion to it too. F1 drivers must be inhuman to tolerate that.

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AnonymousEngineer_@reddit

> I have stood on the brakes in a Huracan Performante and as face melting at that was with a harness honestly the Rivian was much worse That's because you were a passenger in the Rivian and you were driving the Huracan. When you're driving, you're in control of when you hit the brakes, and you will be bracing yourself against the steering wheel and pedals prior to doing so. When you're a passenger, you don't know the exact moment of braking , especially if it's due to AEB as your example was. So the element of surprise works against you.

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thatgymdude@reddit (OP)

I rode shotgun in the Shelby GT500 before I tracked it myself and I see what you mean. Even though that had a harness too anytime the owner touched the brakes you it squeezed you like a vice. I was shocked a simple Mustang could do that and the Huracan was on another level. I wouldnt want to experience that car in a track setting as a passenger, standing on the brakes in it makes your vision blur its so harsh.

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GimmeChickenBlasters@reddit

> I rode shotgun in the Shelby GT500 > I was shocked a simple Mustang could do that and the Huracan was on another level. LMAO that "simple mustang" will obliterate 90% of high end sports cars including most Huracan trims https://www.caranddriver.com/features/a23319884/lightning-lap-times-historical-data/

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thatgymdude@reddit (OP)

No kidding, I drove a v6 mustang as a teen and thought they were are all pieces of garbage and the GT500 changed my perception of them incredibly fast. I drove it both on the track and on the street and I cant fathom how someone can have it as a daily, floor it and you will be at triple digits begore you can look down at the speedometer.

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GimmeChickenBlasters@reddit

> I drove a v6 mustang as a teen and thought they were are all pieces of garbage and the GT500 changed my perception It depends what generation it was. Prior to 2015, yea, they mostly were. The S550 generation brought IRS and a much nicer interior which transformed the Mustang from the but off a "can't turn" joke into an actual decent sports car.

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UncleBensRacistRice@reddit

It's because you weren't ready for it. That huracan will stop in a shorter distance every time compared to tje rivian, but when you hit the brakes then you knew what was going to happen and reflexively clenched before it happened. You never got that chance with the rivian

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Averageinternetdoge@reddit

Oh yeah. I have a 20 year old car with good tires and abs and even that thing stops on a dime if you really step on it. I've had to do emergency stops a couple of time with it and holy shit, it's so much more powerful than the cars I learned to drive with. And remember, that's 20 year old cheap tech! Op's rivian will be *much* more powerful.

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Shomegrown@reddit

A Rivian will have much more brake dive than a sports car (softer suspension, more tire aspect ratio, higher COG) which adds to the sensation of something violent happening. A GT3RS brake harder but may feel less dramatic without body pitch changes and supportive seats/belts that keep you restrained.

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DetectiveNarrow@reddit

The bigger the car that more force it needs to stop Rivian is already a big ass vehicle combined the fact it’s an EV could be dual motor etc, so it needs a powerful breaking system and you guys weren’t expecting it.

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strongmanass@reddit

F = ma m_R1S = 3206 kg a = (v2 - v1)/t v1 = 11.1 m/s v2 = 0 (you came to a stop) Let's say t = 3 seconds a = 11.1/3 = 3.7 m/s^2 F = 3206 kg * 3.7 m/s^2 = 11,862.2 N = 11.862 kN ~ the weight of a small hippo That's not necessarily the problem. What hurt you both is probably the jerk. * Velocity is the rate of change of position * Acceleration is the rate of change of velocity * Jerk is the rate of change of acceleration In an emergency stop the jerk will be high and impart large forces over a short period of time.

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rosen380@reddit

"What hurt you both is probably the jerk. " And the jerk tgat cut you off :)

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thatgymdude@reddit (OP)

Thank you, it was how sudden it was and since that car has such strong brakes it really amplified it.

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srcorvettez06@reddit

The seatbelt pretensioners may be what knocked the wind out of you more than the actual panic stop.

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StinkySoggyUnderwear@reddit

The weight of the car matters a lot here. But it’s not necessarily the weight of the car itself, but the way weight transfers in the chassis and suspension. Ride height matters too. Most cars are built with anti-dive characteristics, some aren’t done as well others. Front heavy vehicles will dive more because there is more weight available to transfer forward where it will make the car dive.

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woodsides@reddit

Your body doesn't feel speed, it only feels acceleration. F=ma, so the heavier you are and the harder you brake, the more force you're going to feel. Plus, all that force is mostly concentrated over a small piece of area around your where your seatbelt sits on your chest, so you feel it a lot more.

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Zombie256@reddit

Opposite the weight of the people in the car affects the car’s dynamics

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ChapekElders@reddit

It was so violent because you weren’t expecting it in the slightest.

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Sharp-Hotel-2117@reddit

This right here, no prep so you got tossed into your belt. F1 cars corner with upwards of 5gs and brake at the same rate, the drivers are cinched in and expecting the forces though. Imagine the neck injuries if those cars did anything (velocity) related automatically, the drivers are in tip-top shape but when your head suddenly acts like it weighs 5 times more than normal and wants to be elsewhere its gonna hurt. I'd say with good/great tires and a sophisticated ABS system a Rivian can pull near a full g in braking. That's shifting from your regular 1g "Im just sitting here" to 1g of "my body wants to be moving at 40 mph forward" in the blink of an eye. Out of the blue that's a serious slap.

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edgarecayce@reddit

One thing is a higher car will “lurch” more in a panic stop - continuing to lean forward after the tires stop, and then bouncing back. A car with a lower center of gravity does that less.

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2Drogdar2Furious@reddit

Yo... you shit your pants?

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thatgymdude@reddit (OP)

No sir

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2Drogdar2Furious@reddit

Oh, I read "pants-shiting experience" too literally I guess 😅

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DudeWhereIsMyDuduk@reddit

As someone who was hit head-on by a Toyota Highlander in a body on frame 4x4 with a winch bumper at 50mph, the lasting memory I have of it (beyond the view of two headlights directly in front) is the speed. Humans do a pretty shit job at perceiving quick events, and I get why. What really affects people inside are crumple zones. The 21 year old who hit me walked out, my tibia telescoped in on itself and the bones in my shifting hand were rearranged somewhat.

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GGCRX@reddit

It's not the weight of the vehicle but the braking distance of the car that impacts you. Some big vehicles like the Rivian have a relatively short braking distance, and some small vehicles like the Yugo seem to take forever to stop because their brakes and/or tires suck. It's YOUR weight that impacts the injury you get when the car stops quickly. The seat belt has a mechanism that makes it get very tight almost instantly when a rapid deceleration is detected. It then resists your momentum which is a function of your speed and your weight. The seat belt is trying to slow you down, and you (and your weight) are trying to overwhelm the seat belt. The heavier you are, the more force will be applied by your body to the seat belt and vice versa, and it's that vice versa that causes injuries. All that said, it doesn't mean only fat people get hurt by seat belts. Even a 100-pound person will commonly get seat belt rash in a fast stop.

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EddieMcClintock@reddit

Heavier cars require more force to stop in the same distance as lighter cars. Most modern cars should be able to fully stop from 40mph in a very short distance.

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dietchaos@reddit

The bigger it is the more force is needed to stop it. Not all braking systems are equal and some are much more effective than others. The more effective it is the more a sudden change in momentum is applied to you.