This is like my experience trying to explain to a 6 year old why the 4 wheeler that has 4 wheel drive won’t go twice as fast as the one that is two wheel drive.
A SUV doesn’t take significantly longer to stop than a sub compact car.
If stronger brakes were put on a tractor trailer it wouldn’t take longer to stop either. A problem could be that the brakes are deliberately weak to prevent a jackknife from having the rear wheels of the tractor skid. The weight distribution in the trailer isn’t always the same. ABS would be important. The stopping distance standard existed before ABS was common place.
Then we are into increased friction on the tires, whatever formula that is. But it is a non factor, given real life experiments of watching a loaded truck try to stop.
No, wrong. The tires braking capability with load is non linear. If you can create 5000 lbs of stopping force atthe road unloaded you will not be able to create 10,000 lbs at 10,000 lbs load. There is not a constant coefficient of friction.
This, too, is completely wrong. For the same reason. A high center of gravity shifts more load to the front tires than a subcompact. The non linear nature of the tires means the it cannot generate the front braking force as a lower, lighter car.
The 911 and Corvettes stop better because they have low cg, carry more weight on the rear axle so any weight transfer evens out the load on the tires making braking performance much better than the heavier, front engine Chrysler product.
The reason I happened to know the above is partly b/c of my esteemed colleague @Bing. A few months ago Bing pointed me to a learning resource called “great courses”, something like that. I happen to be listening to another title in that series, “Classical Physics”. It’s sort of an abbreviated, watered down version of a basic college physics 101 class. But still pretty interesting. The professor giving the lectures hails from from a university or college in Boulder Colorado as I recall.
This came from the Bosch Automotive Handbook. Specifically addresses commercial tire load and braking traction. Note when the load triples from 10kN to 30kN, the braking force is about 9kN while the force at 30 kN is only about 23kN
Also keep in mind a loaded semi’s center of gravity jumps tremendously. That reduces braking loads from the trailer axles and shifts it to the tractor. Both of which have been required to have ABS since the late 90s.
Yeah I’ve got a whole library now after the lockdown. They are excellent mostly in the areas I’m interested in. I’ve only run across two that I would not recommend. Indian history and great presidents. I didn’t send them back though just skipped through them. Very one-sided and only half the story. I’m going through the civil war again with prof Gary Gallagher. Just excellent. He has a number of YouTube’s too including the Darden mba extra credit. I start one of his lectures and have to finish it. He repeats you can’t understand our country today without an understanding of the civil war. Great time for reading too. I like people that debunk common misconceptions and always have.
Are you sure it’s 9kN? It looks more like 8. 8 * 3 = 24, which is close to the 23kN figure. It does show that lower tire pressure helps improve braking performance significantly.
But yes I see that the breaking force doesn’t increase as much as the weight does. I wonder if the reason is that the rubber on the tires at the pavement contact area gets torn off more easily. If the weight is 3 times as much, then the rubber that the tire is made out of would need to be 3 times as strong to maintain braking performance.
Tires are string and rubber supported by air. There is a non linear “spring-y” connection between the rubber on the ground. Then the tread itself is flexible.
The chart I posted is representative but every tire design is different so the specific performance depends on that design. So…
The raised tread blocks deform to the roughness of the road. A bit, to a lot, of rubber is left on the road when braking. If the rubber is softer, traction improves so braking improves until it is too soft. But the tires wear faster. If the grooves are smaller or eliminated, traction in the dry improves but wet performance suffers. Lower pressures improve grip, to a point, and they ride softer, but handling and life decrease.
The string, nylon, steel, kevlar, ect, that builds the tire provides the support for all the loads. Tires are designed for the maximum load so they must compromise the unloaded performance. Tire companies would like max braking at max load so low load is compromised.
Tires are amazing inventions made with huge compromises
After listening to much of the audio book, I have to say this physics professor is very good at explaining the important principles of physics in understandable, commonsense terms. But explaining how a car works? Applied Physics, not so much … lol … He proferred (no pun intended) the theory that the reason a car’s ignition key is made of metal is b/c the metal in the key is what conducts the electricity to power-up the starter motor … lol …
Probably because that field of study takes up all his time. There’s a reason that college physics classes are 5 credits and math classes are 4 credits and Humanities are 3 credits. Most semesters in college I’d take 18 credit hours…EXCEPT when I had a physics class scheduled. Then I’d drop to 12 or 15. Physics class took way more of my time than any other subject.
I worked with a brilliant engineer who had a PHD in Physics (from MIT) and another PHD in Electrical engineering (from Syracuse University). I watched him walk into the cafeteria and struggle with the operation of the new vending machines that accepted credit cards. A 70 hour work week was a very light week for him.
I’m gonna get banned today, but I generally have a pretty good mechanical aptitude but after a long flight to Israel, encountered a coke machine and desperately wanted a coke. I tried credit, debit, us cash, and euros and never could get a coke out of the dang thing. So don’t judge a guy by his ability to use a vending machine designed by engineers.
Similarly, the first time that I was in Rome, I couldn’t seem to get the ticket vending machine for the Metropolitana (their subway) to work. It kept spitting my Lira bills back at me. It was later in the day that I discovered that my local bank had given me “old” Lira, and the machine only accepted “new” Lira. To the casual observer, I probably seemed clueless.
I had a very… spirited… discussion with my local bank manager when I returned home.
Since purchasing coca-cola in a foreign country topic has already been invoked, I’ll tell my story. Tourist walking the streets of Paris, I noted the price of a can of coca cola varies widely, from 50 cents to over $3 per can, depends mostly on how close the shop is to a subway stop. Being the frugal type I purchased a can for 50 cents 10 blocks from a subway stop, then walked toward the stop. Near the subway stop I decided to browse another shop , still holding my 50 cent coca cola. I showed the can to the clerk as I walked in, who nodded. Didn’t work. The clerk wouldn’t allow me to leave until I paid $3 for the same can I had already purchased at the other shop ! … lol …
Heh heh. If it was during their months long holiday, all the actual French were gone and foreigners were manning the shops. I ordered a coffee Americana at a restaurant and then watched them snicker in the back ground when they delivered that little tea cup of foul stuff. And if you order a hamburger keep sending it back until all the red is gone. Yeah those ugly Americans.
