# sortaOT: skid marks (cars)

• posted
not much metalworking, my apologies.. but i know there
are quite a few (car) knowledgeable people here.
just curious, how fast would a small car (2500lbs?)
have to be going to leave 60' long skid marks on dry asphault?
wife at stake,
-tony
• posted
Note that some of the below instructions involve measurements of coefficient of friction and note the method of measuring skid marks.
(alter the CHXX on the above for different chapters)
Good luck.
Best regards, Spehro Pefhany
• posted
wow. thanks for the (great) quick reply. very informative. i am surprised to find that vehicle weight doesnt seem to enter into this.
thanks again -tony
• posted
No way to compute it. If you brake hard enough to lock the wheels, the cops have tables to determine speed, depending on road surface conditions. If only one wheel is locked or braking hard enough to leave a mark, it is harder to do. If it was spinning wheels from engine power, it is totally unknown unless you repeat it with a radar speed detector. Some cars just barely leave a mark, and almost all energy goes into accelerating the car - they get up to speed fast. And, some cars with light weight on the driving wheels just burn rubber, transmitting very little energy to the car at all.
If you were braking hard, 60 feet sounds like 20-30 MPH, but others can probably answer more accurately.
Jon
• posted
On Mon, 08 Dec 2003 21:28:52 GMT, "tony" brought forth from the murky depths:
0-30mph or so, depending upon tranny, tire width, torque, and HP.
Oh, you meant STOPPING speed, didn't you?
2-wheel disc or 4-wheel drum brakes? Bald P155x13" import rubber or on new hi-performance 16" Pirellis? Smooth or coarse asphalt? Hot or cold weather? Warm or cold tires?

Untie her. That's illegal in most states.
P.S: The disc/drum question is a moot point in a silly skid. If one can how to use the brakes _properly_, discs can help stop the vehicle a whole lot sooner under a whole lot of different conditions.
-- Sex is Evil, Evil is Sin, Sin is Forgiven. Gee, ain't religion GREAT? ---------------------------------------------
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• posted
I get about 38 mph assuming a coef of friction of .8
physics are phun!
Larry
• posted
On Tue, 09 Dec 2003 00:57:55 GMT, "Lawrence L'Hote" brought forth from the murky depths:
I used the google-provided formula of double the square root of 5 times the length of skid which gave me 34.64 rounded to 35.
Was that cof figure for worn imports or new Pirellis? Laden or unladen?
-- Sex is Evil, Evil is Sin, Sin is Forgiven. Gee, ain't religion GREAT? ---------------------------------------------