Skid Marks, How fast

Hi,

My 89 Testarossa was recently smashed by a third party. While I work out what to do I want to work out the approx speed before braking started.
Facts are:

Skid Marks: 109 metres
Estimated speed at impact 45 km's p/h
Road surface: Worn Tar
Tyres: Brand New Pirrelli P"0"'s 18" 245's front 295"s back on TR rims.
Road speed limit 50 km/h

Is there a web site with a formula to work it out. I have attached some photos for research.

Kind regards
PV908
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http://www.highwaycode.gov.uk/09.htm
20 MPH Stopping Distance table
Think ---- Brakes
6 metres + 6 metres = 12 metres (40 feet) or 3 car lengths

30 MPH Stopping Distance table
Think ---- Brakes
9 metres + 14 metres = 23 metres (75 feet) or 6 car lengths

40 MPH Stopping Distance table
12 metres + 24 metres = 36 metres (120 feet) or 9 car lengths

50 MPH Stopping Distance table
15 metres + 38 metres = 53 metres (175 feet) or 13 car lengths

60 MPH Stopping Distance table
18 metres + 55 metres = 73 metres (240 feet) or 18 car lengths

70 MPH Stopping Distance table
21 metres + 75 metres = 96 metres (315 feet) or 24 car lengths

So...

109 Metres of brakes, huh...
20 / 6 / Increasing distance for every 10 MPH (Integer Sequencing)
30 / 14 / 8
40 / 24 / 10
50 / 38 / 14
60 / 55 / 17
70 / 75 / 20

The closest match for 8, 10, 14 Integer Sequencing is here:
http://www.research.att.com/~njas/sequences/?q=8%2C10%2C14&language=english&go=Search
0, 1, 3, 5, 8, 10, 14, 16, 20, 23, 27, 29, 35, 37, 41, 45, 50, 52, 58, 60, - so,
Guesswork:
80 / 98 / 23
90 / 125 / 27

98's too low, 125's too high

Probably doing (at least) 85 MPH or 136.79 - which is 86.79 kmh over speed limit.
http://www.mr2ownersclub.com/converter.htm

 

Below you will find some web sites that may give you a start. One of the keys is finding the coefficient of friction for the particular roadway. The chart found here http://www.collisionrecon.com/services/Speed%20Chart%20A.pdf may give you some idea of the speed if you know the coefficient of friction of the road. The chart assumes 100 percent braking efficiency on a level surface. I can’t vouch for any of these sites as I just did a quick search. The easiest and most expensive way to figure this out correctly is to hire a qualified accident reconstruction expert.

http://www.pimall.com/nais/n.skidm.html

http://www.neptuneeng.com/SAEPapers/950354.pdf

http://wps.prenhall.com/esm_giancoli_physicsppa_6/0,8713,1114162-content,00.utf8.html

http://www.waltersforensic.com/articles/accident_reconstruction/vol1-no8.htm

http://www.bsharp.org/physics/stuff/skidmarks.html

http://www.collisionrecon.com/services/momentum-analysis.html

 

Holy crap! that first pic is very telling. they relized that they needed to stop before they probably could make out your car. that alone says they were going way way too fast.

any details? what happened?

 

Where was this? Looks like Inner west, but the Council area will help you work out what the bitumen compound is and hence a co-efficient of friction.

Looks like a very straight line for a 100+ metre skid and a pretty short street. Is there any chance it was a small front wheel drive car that was doing a burnout with the handbrake on?

The skid marks were not made by your car (were you stationary all that time before they hit you?) so the details of your tires are not relevant. What sort of car was it that hit you? Do you know the tire sizes and make etc? I suspect they have/had less grip than yours! I also assume it is small (based on the tire marks) and old (no ABS)

Either way, since the Police have marked the road, they will establish all the facts in court when they prosecute the driver. Whatever speed comes out in their evidence will be your best bet with determining what to do. Remember of course that the driver HAD to be doing more than 95k, which means they were doing more than 45 over the limit, so therefore a mandatory 6 month disqualification and 6 points, plus a court date. Add on the negligent driving charge due to the accident (possibly even a dangerous driving charge, although if you were not injured that may not happen).

And... Pics of both cars please! The pathetic side of me wants to see the damage...

Just my .02

 

Hi,

I am not sure I understand well, but you are saying that you are trying to estiamte the initial speed of a TR which left 109 meters of skidmarks before hitting something at 45 km/h ?
In that case:
I do not think the normal formulas work because the car had its wheels locked up and was just sliding. hence very limited brake efficiency.
Shame the driver did not know how to brake: it would probably avoided the crash altogether. I guess no ABS nanny on 89' TR.
What is clear is that driver was over speed limit though

 

thats in Sydney,where abouts did it happen,and how did it happen

 

My car hit a stationary trailer double parked. The flat spots on my tyres tell the story. One big flat spot on each front wheel. It is a long street prior to braking.

 

109 metres is a VERY long skid mark in a residential area. You also say the front tires are flat-spotted, but say nothing about the rear.

I would look at the possibilty that the third party driving your car had their foot on the gas and brake pedals at the same time while trying to panic stop. This would greatly extend the stopping distance and result in skid marks from the only the front wheels very similar to what you are looking at.

They were likely well over the speed limit, but they might not have been going quite as fast as you would think.

 

This reminds me of the following pictures...
Ciao
Eugenio
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Uh....................why were you driving so fast in a residential neighborhood?

 

It was his mate driving the Testarossa ernie. I mean his mate was thrashing his Testarossa.

 

On race track with Hoosier R compound tires without locking the brakes, a 360 Challenge race car can easily decelerate at about 115kph per 100 meters.

On street tires and locking the brakes, deceleration will be less than that. So I think you can safely assume the 115kph per 100meters is the upper brake performance limit. Extrapolating that out to 109meters of braking distance and the final impact velocity is 45 kph, then car would have been going at most 115kph/100m * 109m + 45kph = 170kph.

My guess is the car was going at about 120kph at the time driver locked the brakes.

 

The skid marks are straight, ABS would not have helped. ABS doesn`t make you stop faster it only helps you remain in control while stoping.

 

I can't imagine the speed required to have a TR leave skid marks of around 109 meters (350 feet++)!!! The driver must have been going well over 120 mph (in the range of 200 kph! See if anyone here in the TR forum has been able to leave marks anywhere near that length and at what speed. Foolishness, and obviously dangerous.

BT

 

Not true. The coefficent of friction for good tires is about .95-1.0 when they are rolling, but drops to something like .80 when they are sliding. ABS keeps the wheels turning and keep the friction up, reducing the stopping difstance for the average driver.

 

True, kinetic friction coefficient is a bit lower than static, but most ABS systems will not make you stop in a shorter distance at least in wet or slipery conditions. Most ABS systems won't keep you at the absolute top of the mu curb, where your friction coefficient would be higher.

 

Those pics are unreal! I can't believe how straight the marks are.

Obviously another reason to never let someone else drive your car. Sorry for your loss/damage.

What was your TR's 60-0 braking spec??? I might be in a road test of some sort.

Either way I think the police do those calc's all the time when there is an accident...start with them.

 

Good call. Note that it looks like only one set of wheels left skid marks, not two. Surely the rears would lock first under hard braking with the weight transfer and no ABS. If only the front wheels locked either the rear brakes are terribly inefficient or something (horsepower, for instance) was keeping them from locking.

 

I don't see that anybody's got to it so....

From physics 1

F=ma and F= NCf where N=ma(gravity), agravity =9.81m/s^2

so, ma=magCf, dividing by m, a=agCf

assuming the Cf for a locked and sliding tire is 0.8, maybe somebody can find a better number for this

a=9.81*0.8=7.848

Then
V^2=Vo^2+2a(X-Xo), rearranging,
Vo^2=Vf^2-2a(X-Xo)

Vf=45, X-Xo=109, a=-7.848 (minus because you're slowing down)

plugging in,

Vo^2=45^2-2(-7.848)(109)
Vo^2=1867
Vo=43.2m/s
Vo=155 km/h

If you find a better number for Cf, just plug it in to get the new speed.

 

Motorweek ran the braking tests on the TR years back, 130-ish sounds about right.



The 512 60-0 braking numbers;

512tr - 130 Feet.
512m - 127 Feet

 

I am glad the rear brakes did not lock up as the car could have lost total control.

I was always under the impression that the rear brakes are designed to not lock up when the road is in good condition for the Ferraris' designed with disc brakes.

 

wow, just wow.

from nearly 2 months ago: http://www.ferrarichat.com/forum/showthread.php?t=104326

 

I forgot to post a step....sorry
45km/h needs to be converted to m/s, so 45*1000/3600=12.5m/s. The answer's right, I just forgot to type in a line.

 

even with locked tyres, would it really take 110 metres to slow from 155 to 45kph? i'm thinking closer to 200kph...