Stopping on a dime?

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With a properly setup and cornerweighted vehicle, with sticky yokohama and tarox 6 pots on the front, 4pots on the rear,
my previous car (track toy) would stop from 70mph in 51 meters.

The official line from the uk govt. is 21meters thinking time andd 75meters braking time, giving a total of 96.

Good tyres, good brakes and optimised suspension geometry make the biggest difference to stopping distance.

That car died when a motorbike pulled out of a sideroad, I slammed on the brakes to avoid hitting him (which I managed) but the Mercedes C class behind me couldn't brake in time, rear ende me and bent the chassis beyon repair.

So sometimes the biggest brakes you can use aren't actually the best..
 
The 21 meter reaction time translates to roughly 0.6 seconds...and I think that is wildly optimistic. It allows for a slightly below average reaction speed, but allows zero time for the decision making process. I can press a button in 0.25 seconds if I am expecting to react, but the ability to process what is going on and make a decision on what to do (brake, how hard, steer, which direction) will take much longer...perhaps 1 second in an alert,focused driver...several seconds in the case of someone talking on a cell phone or otherwise pre-occupied...

During that time, the car is traveling 100 feet per second...and I submit that the average, slightly distracted driver will take the entire football field to recognize the situation, make a decision, and then react...the car's performance after that length of time is interesting, but it's the driver that is the critical factor in determining safe stopping/reaction.
 
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I dont plan on stopping, I plan to evade by steering. As said, stopping doesn't do you much good if the car behind you cant stop.
 
I exercise my reaction time every day in Boston.
lol.gif



Drink lots O' coffee
 
Originally Posted By: Olas

That car died when a motorbike pulled out of a sideroad, I slammed on the brakes to avoid hitting him (which I managed) but the Mercedes C class behind me couldn't brake in time, rear ende me and bent the chassis beyon repair.


So was it the fault of the person driving the C class? I would think they were following you too close.
 
I have Michelin Defenders on my DD. Its supposed to stop faster/shorter distance. I can't tell. Whenever I stop I have about the same distance from the vehicle in front of me, or up to the stop/cross-walk line. When I pull onto my driveway, I'm still the same distance from the garage door. When I park elsewhere, I'm still in the slot as usual (back end not sticking out).
 
To andyd: I wish I could alter space and time. I'd make it so your line, "I don't plan on stopping, I plan to evade by steering." was said in front of my driver's ed instructor from high school.

I say this not because he'd hit you with a baseball bat or anything like that.

He'd give you a colorful lecture on logic, presumption, egocentrism and 'sounding good'. I suppose it's good to be young. Kira
 
Depends on the vehicle. A good German car with real brakes, real tires, at full lock? You better have your seat belt on or your going into the windshield!

The downside of having good brakes is most vehicles well don't, so getting rear ended becomes a very big concern. I have had several instances where I had to make emergency stops on the highway only to have the guy behind me (thankfully) swerve into the brake down lane and stop 2-3 car lengths in front of me!

Several years ago their was a truck that lost control in front of me on I95 and I along with the three cars following me around 75mph had to stop, fast. I went to full ABS lock, naturally a car built for much higher speeds on the autobahn just dug in and well stopped, zero drama, no corrections needed with the wheel. The Saab right behind me did fine, they also have great brakes. The late model Accord behind him ended up in front of me! Afterwards the Accord driver said he was at full ABS but couldn't stop. That's what narrow all seasons plus small discs, and single piston sliding calipers get you.

YMMV.


Lastly this is why people should leave reasonable following distances at higher speeds. Naturally almost no one does this.
 
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Originally Posted By: hattaresguy


Lastly this is why people should leave reasonable following distances at higher speeds. Naturally almost no one does this.


This ^

While I realize you cannot predict every circumstance, you can improve your odds of not needing to stop on a dime by increasing your following distance.

The other time to do that, as I was taught in drivers ed, is when the vehicle behind you doesn't have sufficient following distance.

If the car behind you is tailgating, you can leave more room between you and the car before you, reducing the odds that you'll need to slam on your brakes.
 
Yeah, generally I leave even more stopping distance so the idiot behind me 15ft off my rear bumper doesn't nail it if I have to stop quickly.
 
I absolutely hate it when people tailgate me. I will pull over and let them go by as they are obviously way more important than I am and in a huge hurry. If I'm on the highway and someone tailgates me in the travel lane, I just take my foot off the gas until they realise what's going on and go around me.
 
Originally Posted By: Kira
To andyd: I wish I could alter space and time. I'd make it so your line, "I don't plan on stopping, I plan to evade by steering." was said in front of my driver's ed instructor from high school.

I say this not because he'd hit you with a baseball bat or anything like that.

He'd give you a colorful lecture on logic, presumption, egocentrism and 'sounding good'. I suppose it's good to be young. Kira
Heh Heh I'm 62. He could hit me with a baseball bat if I didn't take it away from him first.
smile.gif
The whole thing about evasion is you have to make allowances. For speed , it is distance. For weather it is speed. You never drive where you cant see or escape from. A trailer full of 35 yards of gravel going 70 mph doesnt stop for sour apples. You must make allowances. you want to avoid rear ending the car in front of you, also watch the tail lights 10 cars up, and so on. If you are paying attention you hardly need brakes.
 
No disrespect to real semi-truck drivers. I went to school got my MA class one license back about 1970. I used it just enough to realize how unsuited I was to driving trailer trucks. But I did learn a few things from other drivers, that I apply to driving a car.
 
If you can brake at 1g, stopping distance from 100km/hr (27.8m/s) is governed by
s=V^2/2g...just under 40m.

The bit before full brake application is completely up to the Driver's state of mind at the point that he needs the brakes, and at 100km/hr, there's 1-1/2 cricket pitchess passing a second.

Both my Caprice and Nissan can run 0.95g
 
Originally Posted By: Merkava_4
Originally Posted By: Olas

That car died when a motorbike pulled out of a sideroad, I slammed on the brakes to avoid hitting him (which I managed) but the Mercedes C class behind me couldn't brake in time, rear ende me and bent the chassis beyon repair.


So was it the fault of the person driving the C class? I would think they were following you too close.


So there's two points ffor me to adddress there..

It was the combined fault of the Biker AND the C-Class..
The biker should have checked to see if it was safe to pull out and I wouldn't have had to brake.
AND
The C-Class driver should have stopped/swerved to avoid me

So they both contributed to the accident, and neither one can be 100% accountable

You said you think the C-Class was following too close? Well yes, it was too close, but thats just another way of saying the C-Class brakes aren't good enough.

Like algebra, you can switch the equasion round...so...
He was following too close for his braking performance envelope
He wasn't following too close, it's just that his braking performance envelope was insufficient.
 
Originally Posted By: Olas
The official line from the uk govt. is 21meters thinking time andd 75meters braking time, giving a total of 96.

The site in question will use a very specific set of equations that use rounding and conversion factors to give the benefit of the doubt to a North American accused at trial (or under investigation). When a collision analyst determines speed based on skid marks, the speed given is quoted as a "minimum speed." They very well might have been going faster, as in the case of a vehicle with very good brakes and tires - i.e. compare an F1 car to an E-350.

If Shannow or I take our properly formulated Newtonian equations (rather than what's in the collision analyst textbooks) and get a good reading on the coefficient of friction for the road surface in question for the vehicle in question and use proper error management (i.e. at the very least, use of significant figures), of course one could get a much better prediction. However, that's not going to be what you see in an expert investigation of a crash scene.

The procedures in use have been there for an extended period and are standardized for court purposes and well accepted by case law.
 
I'm by no means a perfect driver. I probably drive a little too fast and shave traffic signals a little too much. I also will stop at intersections where my line of sight is obstructed by an illegally parked car, even though the light or the sign is in my favor. When I go down a busy street, my foot hovers over the brake and the radio is off. I try to maintain a safe following distance. I also have already lifted my foot off the gas in response to a brake light 10 cars ahead. The car in front of me has yet to react, by braking, to brake lights. The physics involved for me are as simple as not occupying the same space/time of another object. Mostly, it is paying attention and using what you got to keep going.
 
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