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Old 01-25-2012, 02:05 PM   #11 (permalink)
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You have a good point about semi's not being able to stop as fast as cars, but NHTSA is trying to change that. I think they're trying to cut the stopping distance in half, from 600 feet to 300 feet at 65 MPH.

It's very doable, a fully loaded semi has the traction to stop faster, but not enough braking power. The current favorite is switching the drums that 99.9% of semis have for disk brakes.

All i'm saying is don't assume that the trucks will always take as long to slow down as they do now.

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Old 01-25-2012, 03:37 PM   #12 (permalink)
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Quote:
Originally Posted by p38fln View Post
You have a good point about semi's not being able to stop as fast as cars, but NHTSA is trying to change that. I think they're trying to cut the stopping distance in half, from 600 feet to 300 feet at 65 MPH.

It's very doable, a fully loaded semi has the traction to stop faster, but not enough braking power. The current favorite is switching the drums that 99.9% of semis have for disk brakes.

All i'm saying is don't assume that the trucks will always take as long to slow down as they do now.
Trucks brakes can already lock up now. Unless you get into a runaway heat build up problem as when descending a mountain. The limiting factor of an emergency stop is mass vs contact patch and friction parameters of the tires. Unless they add a bunch more tires and sacrifice high tread life for traction with softer rubber, the stopping distances of big trucks will stay as it is.

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Old 01-25-2012, 03:51 PM   #13 (permalink)
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and, the idea that disc brakes somehow have more stopping power than drums is incorrect.
disc brakes shed heat better, but do not exert more braking force.
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Old 01-25-2012, 04:01 PM   #14 (permalink)
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OK, I searched NHTSA and found their research - they are saying discs stop faster in their testing. They used real trucks and real drivers to do the testing, no computer simulations. A total of 108 drivers were used. I got tired of reading and just copied the important details -
*Edit - read further into it - The stopping distance from full panic stop numbers are from a real truck, the 108 drivers were to test reaction time using a simulator

Anyway, i was completely wrong about the numbers- they aren't cutting in half from 600 to 300 feet, more like 30%.

Full panic stop from 60 MPH -
Regular S-Cam and drum - 317 feet
Enhanced S-Cam and drum - 252 feet
Air disc - 222 feet

http://www.nhtsa.gov/DOT/NHTSA/NVS/V...ons/811367.pdf

This document has the research on stopping distances - the previous one was just research on how drivers responded to better brakes
http://www.nhtsa.gov/DOT/NHTSA/NRD/M...OTHS809700.pdf

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Old 01-25-2012, 05:10 PM   #15 (permalink)
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Quote:
Originally Posted by p38fln View Post
OK, I searched NHTSA and found their research - they are saying discs stop faster in their testing. They used real trucks and real drivers to do the testing, no computer simulations. A total of 108 drivers were used. I got tired of reading and just copied the important details -
*Edit - read further into it - The stopping distance from full panic stop numbers are from a real truck, the 108 drivers were to test reaction time using a simulator

Anyway, i was completely wrong about the numbers- they aren't cutting in half from 600 to 300 feet, more like 30%.

Full panic stop from 60 MPH -
Regular S-Cam and drum - 317 feet
Enhanced S-Cam and drum - 252 feet
Air disc - 222 feet

http://www.nhtsa.gov/DOT/NHTSA/NVS/V...ons/811367.pdf

This document has the research on stopping distances - the previous one was just research on how drivers responded to better brakes
http://www.nhtsa.gov/DOT/NHTSA/NRD/M...OTHS809700.pdf

all things being equal, disc brakes do not provide more stopping force than drum brakes.
Now, in rain, they may...and if doing several continuous stops and building up heat, then they are also at an advantage there.

but in the dry, they do not provide more friction than drums....
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Old 01-25-2012, 06:58 PM   #16 (permalink)
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Quote:
Originally Posted by p38fln View Post
OK, I searched NHTSA and found their research - they are saying discs stop faster in their testing. They used real trucks and real drivers to do the testing, no computer simulations. A total of 108 drivers were used. I got tired of reading and just copied the important details -
*Edit - read further into it - The stopping distance from full panic stop numbers are from a real truck, the 108 drivers were to test reaction time using a simulator

Anyway, i was completely wrong about the numbers- they aren't cutting in half from 600 to 300 feet, more like 30%.

Full panic stop from 60 MPH -
Regular S-Cam and drum - 317 feet
Enhanced S-Cam and drum - 252 feet
Air disc - 222 feet

http://www.nhtsa.gov/DOT/NHTSA/NVS/V...ons/811367.pdf

This document has the research on stopping distances - the previous one was just research on how drivers responded to better brakes
http://www.nhtsa.gov/DOT/NHTSA/NRD/M...OTHS809700.pdf
This is interesting, and compels me to make a correction to my previous statement (last post on previous page):

I assumed that a truck took "3 or 4 times" as long to stop from highway speeds as a car.

With an average car, in good repair, taking 120-170 feet to stop from 60, and trucks taking 317, the stopping distance for a truck is notably less than "3 or 4 times" the distance for a car.

While I suspect a majority of trucks actually on the road would struggle to equal the NHTSA numbers, it does mean I've been overestimating the reaction time I have behind a truck in a panic-stop situation.
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Old 01-27-2012, 02:26 AM   #17 (permalink)
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Yeah, let's call it 'following'.

In my experience, when a truck does a crash stop (I had this happening to me while following about a second behind), you have the time to slam the brakes. Once that is done, you decelerate much quicker than the truck, so the one thing you HAVE to get done is move your right foot... so no fiddling with the mobile phone or nav unit etc. pp. when following. But this is no surprise.

In 'normal' situations, when the truck has to brake because of traffic buildup, lane swervers etc. pp., you even have to be careful not to over-brake and lose lots of fuel while re-accelerating :-)

Good point about nobody really caring for the 3-second-rule in heavy traffic. When I drive home about 3pm, traffic is mostly heavy in this part of germany. I usually look for a truck and follow. At this point, you have the guys on the left lane going by (about 10 km/h faster, if that much) almost bumper-to-bumper... even the stated 30 m are very, very rare to see. I'd say it is more like 10 m (30 feet)...

Considering this, I have to agree: you are much safer behind a truck at 30m distance than on the fast lane with half that or even less.

so long,

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Old 01-27-2012, 10:26 AM   #18 (permalink)
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The difference between the drums and discs in this case is anti-lock. Trucks have had antilock brake systems for several years now. A disc brake can unlock and re-apply faster that a drum/s-cam/shoe. Ask me how I know !
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Old 01-27-2012, 02:39 PM   #19 (permalink)
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I will occasionally follow a truck at a safe distance on the interstate. I am no tailgater by any stretch of the imagination. But one thing puzzles me. For some reason, the truck drivers seem to loathe cars following behind them. They will slow down, change lanes, and cut in front of other trucks in order to get the car out from behind them. At the same they are on the CB radio griping and complaining about it. If they paid as much attention to the road ahead of them and not driving through their rear view mirror, they would be a lot safer and the following car would be safer. There is this myth that being in a blind spot behind a truck is dangerous, when it is only really dangerous to be in the side blind spots of a truck. What is behind a truck is not going to hurt the truck or the driver. I wish they would realize this.
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Old 01-27-2012, 03:08 PM   #20 (permalink)
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Quote:
Originally Posted by jarawhead View Post
I will occasionally follow a truck at a safe distance on the interstate. I am no tailgater by any stretch of the imagination. But one thing puzzles me. For some reason, the truck drivers seem to loathe cars following behind them. They will slow down, change lanes, and cut in front of other trucks in order to get the car out from behind them. At the same they are on the CB radio griping and complaining about it. If they paid as much attention to the road ahead of them and not driving through their rear view mirror, they would be a lot safer and the following car would be safer. There is this myth that being in a blind spot behind a truck is dangerous, when it is only really dangerous to be in the side blind spots of a truck. What is behind a truck is not going to hurt the truck or the driver. I wish they would realize this.
If you're pissing them off, you're following too close.

Why do they not want you invisible? Because they CANNOT trust you to stop in time if they have to brake. Keep in mind they have no idea HOW close you are if you're too close for them to see. Are you wedged under their bumper or a full second back? They don't know. Imagine for a second you have something very low behind you... say a very fast recumbent bike on a slower road. Would you be comfortable with it behind you if you had zero visual reference on them? No. If the drive cannot see you he has to operate on the assumptions that you are right on his bumper and not necessarily sane/attentive.

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