Car Sluggish? Maybe It's Your Weight

In an eye-opening piece, USA Today reports that the average American is too heavy to fit in his own car — at least with passengers. The average weight for men in 2004 was 190 pounds — we’re guessing that’s a number that has not regressed in the past two years — while most cars can barely tackle a load of 200-pound passengers, according to their maximum load capacity, printed on a decal inside the front door of most vehicles.

The Mazda MX-5 Miata, Chevrolet Corvette and Honda S2000 sports cars don't rate higher than 400 pounds maximum. Not only does that include passenger weight, but also any cargo, like luggage. Even more surprising are numbers for large vehicles like the Dodge Magnum and Mazda CX-7, which come in nearly identical to much smaller ones like the Honda Civic and Scion xB at around 850 pounds.

The ratings stem from a government formula that isn’t divulged, but one automaker tries to avert fears by saying there are unpublished safety margins built in. Regardless, you should know what your vehicle’s weight limit is and drive the vehicle accordingly, including monitoring the tire pressure for different-sized loads. We'd like to see more realistic numbers put in place, otherwise it seems like the automakers are just covering their own — hopefully not too heavy — backsides with these low ratings. 

When researching a car, you can find this weight limit in the specification section either on manufacturer websites or on Cars.com.

Car Weight Limits Are a Big, Fat Problem (USA Today)

Comments

A few problems that add to this max load problem:
(1) safety equipment - more = heavier = taking away from passenger/lugguage load;
(2) luxury components (we now want more than a cassett deck) - mutli speakers, luxury seats, bluetooth...all those equipments and wires - each may not weight much, but when you add them together, that's a different story;
(3) safety zons/body materials - to sustain better crash reinforcement, the materials are heavier, with more structural re-inforcement;

These are just some factors.

I remember in physics when we were doing experiment on how a person's weight can affect fuel efficieny...I was totally surprised by how big of a difference 20lbs can make!

“Car Sluggish? Maybe it’s your weight” No, maybe it’s the small engine in the vehicle. Would the manufacturer’s GVW still apply if the vehicle came with a larger engine and after market upgraded brakes and suspension? And how could a Corvette with all that horsepower and torque be sluggish even if over 400 pounds were loaded into the vehicle?

It would seem that as long as the engine rpm stays the same, it shouldnt matter how much weight the vehicle is carryin, sure you'll burn more gas on take-off but once you get goin it shouldnt really matter. unless im way wrong then i dont know.

Yes, it sure does matter. Here's a very brief lecture:

You have vertical downward force from weight (total of vehicle, luggages, passengers, etc.).

Engine provides horizontal force (either left or right).

When engine pulls horizontal, say, left (arbitrary direction), so does the weight. Resulting force is a skewed force that points both downward (vertical) and horizontal. It's like the horizontal force is pointing West, Vertical force point South, RESULTING force is SW (SouthWest).

The more force to either Horizontal or Vertical, the closer the resulting force is to the increased direction. If you want the car go faster, you step on the gas pedal and that will result in the resulting force pointing to the "West" (horizontal).

Thus, if you put more weight (heavy people, luggages, etc.) the resulting force is pointing more toward the "SOUTH" direction, and the vehicle will barely move if you keep the engine at the same force (i.e. same rpm). You vehicle will slow down to almost stopping itself. What would you do? Step on gas pedal = more gas needed -> you can then move the car and have the resulting force pointing toward the horizontal direction (West) again. The more weight you put vertically, the more force you'll need horizontally to move the vehicle. Unfortunately, this really isn't a 1:1 ration because the engine itself has weight too, and fiction, and efficiency (our engine's efficiency is at most 30%), so to Ken L., if you use a large displacement engine, the weight of the engine, hence total weight, will increase significantly. Moreover, for a sport car, the engine is tuned for performance, not towing (towing in the sense that it's towing the vehicle weight), and so if you put more than 400lb on a Corvette...it will PROBABLY go...but by go I mean "WALK" :) (The weight of vehicle + passengers + luggages are more than what the engine has tuned to handle.)

I vaguely remember that for every 20N (Newton) of "weight" (approx 2 kg or 4.4 lb), the force exerted by engine needs to be more than 20N (exact figure I don't recall. So if you are a fatty of over 200 lb (85 kg), you'll have a downward force of 850N...I'm thinking of probably 2000N horizontally to compensate the weight - the equivalent of pulling someone with a weight of over 400 lb!

AR,
Your response is rather incoherent. Yes there's a downward force from gravity proportional to the weight of the vehicle and its cargo, but the road responds by pushing up on the car's wheel with an equal force (the normal force) canceling the gravitational force out. If these forces were not perfectly equal the car would accelerate upward or downward until the forces become equal (Newton's second law: f=ma). Similarly, torque from the engine is turned into torque on the wheels which creates a frictional force between the tires and the road that pushes the car forward; as the car moves a frictional force from the air (drag) pushes back on the car. Once again these forces must be perfectly equal if the car is moving at a constant speed. The weight of the car has no *direct* effect on the required force to maintain a certain speed.

However, weight is still an issue here because the increase in weight increases the frictional forces in the wheel bearings that oppose the torque applied to the wheels (dynamic frictional force = coefficient of dynamic friction*normal force). Since the wheel bearings have a low COF not too much extra torque is needed.

This leads me to think of a question long ago spinning in my head.

If I am mostly the only one riding my car, how does my weight on one side of the vehicle matter to the whole car's suspension, handling, etc?

Lil'Tom,

I'm no Physics major, but if i remember correctly, when we deal with the motion of a car, it's a 1-body system, hence not including the "Earth" itself. However, it's incorrect to say that gra. forces actually got canceled; otherwise the car would have weighted "0," wouldn't it? The mass & weight are still there, and if the weight was balanced, it would've been easy "work" for the engine :) Again, I'm no Physics major, but it was an experiment that we did a long long long time ago, and that's the explanation that was given to us from someone who has a Master Degree in Physics, and he could have pursue a Doctorate if he selected to.

J, hate to hear that u call me an idiot in another post, but that's ok, 'coz I was an idiot arguing with another idiot :( Anyhow, your weight does matter! While studying a case in mechanical engineering, I've learned that in some sports cars, the placement of battery and heavy "items" such as muffler and whatnot (except for dual exhaust) are actually at the opposite position of the vehicle. If you actually check out some of the ULTRA performance + light-weight vehicle, they put the driver position in the center (kinda like F1), rather than putting it in either side. However, in our day to day comute vehicle, the weight of the vehicle is >>> our weight, so our weight is like nothing that adds on to the suspension of the vehicle, hence really doesn't matter whther your seating position. Funny thing is that, if you observe someone driving a CAR but this person is overweight, you'll actually see that the car will tip to one side, and the suspension of the car is actually wearing out at an uneven rate.

Are vehicles really heavier than the behemoths of the 60's and 70's? What was there weight carrying capacity?

AR,
For the record I'm no physics major either but I think it's easy to see that if a car is moving in a straight line at a constant speed the acceleration is 0. Since the net external force on an object is equal to its mass times its acceleration (f=ma) the net external force (f) is also 0. Since the weight is a downward force there must be some counteracting upward force, otherwise the car would accelerate downward. This upward force is the normal force from the road. The weight doesn't directly effect the forward force because two forces are orthogonal to each other.

If you're looking at an airplane trying to fly east while being pushed sideways by a southern wind the plane would have to produce a northern force to counteract the wind (or be blown off course). That would require more fuel, but in a car the road keeps the car from moving downward.

Bah! You're ALL over thinking this!

It's not the acceleration forces that are the primary problem, IT'S THE CORNERING FORCES!!!

It's not the weight, it's the MASS.

This affects all aspects of performance, acceleration, braking, cornering and handling.

I'm no physics guy either but I can tell you why the automakers are covering their butts and it's because of what infosaur is talking about, rollovers.

Shoot! Missed the physics discussion. Oh well. Next time.