Weight limits...what happens if exceeded?

True, and of course braking force isn't constant unless the driver is too dumb or too weak to press harder on the brake pedal so the increase in braking distance is even less than that indicated. Modern car brakes have enough power to keep the wheels on the verge of locking up in a seriously overloaded car.

 

In classical physics, stopping distance is independent of mass. It depends only on the velocity (quadratic, as you have), the coefficient of friction, and the acceleration of gravity.

But classical physics assumes such things as providing a stopping force that increases linearly with mass without being affected by the amount of heat generated. Tires heat up quicker and become more likely to skid, brake pads/rotors are limited in rate of heat that they can dissipate, regen is limited to 100 amps @ 250V, etc. Another example, a car with drum brakes will have a longer stopping distance than the same car with disc brakes -- yes, cars used to be made with 4 drum brakes -- because the drums dissipate heat at a lower rate. If you want to improve your car's stopping power, then you need wider tires, larger calipers, and larger+drilled rotors.

Bottom line, all else being equal, stopping distance will increase with mass, but not linearly, more like second order.

(sometimes I miss talking about physics)

 

I appreciate your comments; it's refreshing to have a technical discussion. And I concede that the braking distance is not truly linear, but I don't think it's that far off.

If you make the assumption that braking distance is only really important in a panic stop then only the force of the calipers matter and fade should not be a factor -- in any single stop. The ABS hopefully operates the caliper brakes at the tire skid threshold so adding regen torque would not help. If you're descending the St. Bernards pass in Switzerland then it's a different story, but the question that then arises is "how did you get up there"?

You make a good point about upgrading tires. S33D's are really poor performers; Yokohama even states this on their sidewallls (320 "B B"). I replaced mine with Conti Eco Plus (600 "A B") and even these seem so so compared the the Conti Pros on my other vehicle (600 "AA A"). And these are not expensive tires.

Here's a physics question to ponder. The S33D's have a really low rolling resistance, and this implies that the rubber compound has fairly low hysteresis so less heat is generated as the tire is flexed. Given this, why is their heat resistance so poor? Also, any thoughts about the change in elevation of the vehicle CG with the increased passenger load?

 

Yes, the difference from a few passengers is small.

On a modern car, brake heating doesn't normally become an issue until the 2nd or more stop from some speed over 70 mph, sometimes well over 70 mph.

Heating of the tires is much less of an issue than it used to be because of ABS preventing locked up wheels.

You would be hard put to measure the difference in max braking stopping distance for the max effort first stop from the weight of a 3 or 4 added passengers.

 

And those four added passengers most certainly won't double the distance of the first stop, as was claimed earlier in this thread.

 

LRR tires are very stiff. If you flex something with less elasticity, then it will generate more heat.

I've attached a picture of the length/width center of mass for hoisting purpose. Having passengers will raise the vertical CG slightly and moving it slightly rearward, resulting in increased body roll, but in a way that would be barely perceptible.

The only time I noticed increased passenger load, when hauling the whole fan-damily, about 750 lbs of load, is that the powertrain struggles a bit more going uphill, and FE goes down.

 

Another issue is how the weight is distributed in the car. If the balance shifts too far to the rear, the steering becomes 'lighter', and the headlights point at the trees instead of the road. This is especially risky at night on a dark windy road. Don't ask me how I know.

Significant overloading can seriously compromise your car's handling, to the point of being dangerous. For slight deviances over short distances under good conditions, you might get away with it.

 

The changes they made to the rubber compounding and other tire design characteristics to reduce rolling resistant probably included changes that reduced the tires ability to handle high temperatures. Rubber compounding alone could account for that.

A B rated tire could actually run cooler than an A rated tire. The tires ability to withstand the heat without failure is an important part of the rating.

In general, if you optimize a part or a system to optimize one characteristic, other characteristics tend to take a hit.

 

With modern brakes the limiting factor in a panic stop will be the tires and the brake control system. Obviously all retarding force is generated by the tire contact with the ground. This is a function of tire size, composition, geometry, and weight, and the characteristics of the surface. There are too many factors to make any simple claims about the relationship between mass and braking distance without carefully spelling out the exact scenario under discussion.

In addition, the brake control system plays a big part. ABS brakes will prevent lock-up in most situations, but will not necessarily reduce the braking distance. ABS often increases the braking distance, but is considered a good trade off because directional control is maintained.

Tom

 

Short term? Day trip...I would think nothing would happen. If you were routinely exceeding weight limit, or exceeding weight limited coupled with demanding driving such as over mountain passes?

But taking a group of friends out for a day? I wouldn't expect that to be a problem.

 

Selective rule following at its finest.

On this same site we have a number of people who get upset when folks drive faster than the posted speed limit, some vehemently so.

But exceed the rated load carrying capacity of the car? Over-inflate the tires to at or beyond the manufacturer's rating?

No problem, and here's the logic behind why it's okay...

[sigh]

 

I was thinking the same thing.

 

I had 1500lbs worth of concrete plus myself (200) and the dog (75) in the Prius today. Almost Double the limit. The trip consisted of 10 miles on highway, 5 miles city streets, 2 miles windy country roads, and 1 mile down my private dirt road. Other than being even more gutless than it’s normal self, the only issue was the rly low ground clearance. Scraped coming out of the parking lot at home depot, but that’s it. These cars can take a beating and just keep on going. At least mine has.