Assisting with a CSY crash. This is the best well defined CSY that I have ever seen. Measured a chord on the front right tire mark outside edge about two feet after right rear crosses over. Pulled the chord 88 feet and took a middle ordinate of 20 inches. Took second chord on the same tire mark starting where the first chord finished, so after the 88 feet mark. the second chord was 103 feet with a 27 inch middle ordinate. Roadway friction tested around a .76 with drag sled. This is a level road way with a 1 to 2 degree super elevation so thats not a factor. Vehicle has a track width of 55 inches. In putting all of this into the critical speed equation I have a resulting speed of 81.2 mph on the first chord measurement and an 81.8 for the second chord measurement. So my question is why is the speed so close and shouldn't it be slowing down? I will also say that this is a large yaw. My books tell me to take a chord of at least 30 feet with a middle ordinate of at least 6 inches. My only explanation is that for the first half of the yaw it seems that the vehicle is slowly starting to get sideways. But toward the end is where the vehicle gets totally sideways and then begins to roll. Any ideas ar greatly appreciated. Ken
It sounds like this vehicle might have had the accelerator applied as it left the marks. What direction were the striations heading? They should have been parallel to the axle on which the tire was mounted. The striations would "lean" forward if the wheel was under braking, or rearward if under positive acceleration.
-- Edited by DDavis on Thursday 30th of April 2009 06:17:44 PM
Apart from interest factor, why do the second measurement? We measure the start of the first chord as soon as there evidence of tyre seperation - tend to use about 12-15 metres chord, tyre separation not being more than 50% of track width at the end of the chord and mid-ord preferably more than about 300mm. So broadly similar to the spec you mention. The second chord you have measured over such a long distance could, and I put it no more strongly than that be a bit of measurement error. Did you make any secondary measurements to see whether the arc of the curve varied along its length and you just happend to measure the mid ord where the arc came up with your result. Likewise there could be argument that whilst it is a striated mark it may be that the vehicle is under acceleration at the 'edge' of loosing complete control. You don't mention what happens later in the mark - does the vehicle hit something or does the mark cease and the vehicle is no longer under CSY?
The vehicle continues in a yaw until it is completely sideways and it bottoms out in a median and rolls three times. The stirations were parallel to the axle. I was wondering how the long second chord would affect my outcome.
The vehicle continues in a yaw until it is completely sideways and it bottoms out in a median and rolls three times. The stirations were parallel to the axle. I was wondering how the long second chord would affect my outcome.
I'm curious as to why you'd use an 88 foot chord in one instance, and 103 foot chord in the second? I don't know that it would necessarily change the outcome, but it seems a little odd to not be consistent.
Was there something about this particular yaw which implied you should do that, or what? What kind of measuring system were you using because if you're using a measuring tape of some variety, the longer you stretch it the less precise it will be.
Also, you might consider using a tangent equation to run the whole thing through.
-- Edited by ashman165 on Friday 1st of May 2009 09:02:24 AM
__________________
Regards, Johnathan
"Ending a sentence with a preposition is a situation up with which I shall not put." - Sir Winston Churchill
The first chord had to be at least 60 feet in order for me to have at least a 6 inch MO. As for the second chord, I just pulled the tape a certain distance and stopped. There wasnt any reasoning to the distance. SO should I have only went say another 60 feet. I also mapped this scene with a total station and I have been using my visual statement program to measure the CSY and it seems to be coming up with the same speeds as I have mentioned. So Im sure that its right on target. My newest book from john daily says to pull two chords and they must be at least 30 feet. SO that where Im getting at is if one is 30 should the second be 30 or 50 or 80 or does it even matter. What I will do on monday is measure the yaw on the program and use a few different lengths just to see what the outcome will be. Thanks Ken
The first chord had to be at least 60 feet in order for me to have at least a 6 inch MO. As for the second chord, I just pulled the tape a certain distance and stopped. There wasnt any reasoning to the distance. SO should I have only went say another 60 feet. I also mapped this scene with a total station and I have been using my visual statement program to measure the CSY and it seems to be coming up with the same speeds as I have mentioned. So Im sure that its right on target. My newest book from john daily says to pull two chords and they must be at least 30 feet. SO that where Im getting at is if one is 30 should the second be 30 or 50 or 80 or does it even matter. What I will do on monday is measure the yaw on the program and use a few different lengths just to see what the outcome will be. Thanks Ken
Well, like I said, I'm not sure that it'll change the outcome because I haven't seen the scene or anything. And I understand that mathematically, you should wind up with certain similar results, but I can't think any particular reason other than "what if" to use different lengths.
While we would expect a vehicle to gradually reduce speed as it is rotating, there may well be many reasons for which it could gain a slight degree of speed, or even appear to remain at the same speed. And to be frank here we're dealing with 6 tenths of a mile per hour in a formula which operates under a rational expression and a square. Goofy things can happen under there. I guess at the end of the day, I'd have to take a look at it myself to figure out precisely what's going on.
__________________
Regards, Johnathan
"Ending a sentence with a preposition is a situation up with which I shall not put." - Sir Winston Churchill
Why do two chords of the same mark? What is the reasoning when the aimof the csy calcs is to ascertain the speed of the vehicle at the earliest opportunity after tyre mark separation. A 30 foot chord is really very short and any minor eror in the mid ord can result in significant errors to the final answer. Perhaps you have answered the matter with the comment that you had to have such a lng 1st chord to get any noticeable separation ( 6") which would suggest that the vehicle may not have been in rotation sufficient to fulfill the calcs that are used. Again when the midord is so small, any small change can have result in high variations later. When youdid the survey, how many plots did you do along the chord- assuming youusd a short height prism to minimise error of pole tilt. One option using the software is to check the radius of the curve using each end of the chord and each plot point separately and see what you get. It is wrth considering that just becase you have good looking striated marks, does not mean that the csy WILL be valid, you stillneed to show the other factors.
at the WREX conference in College Station TX in 2000, 32 experienced accident reconstructionists measured the same yaw mark. The range of radii measured by those individuals ranged from 155 to 215 feet. The simple answer to your question is that yaw marks are difficult to measure with absolute precision. In general, speeds calculated from yaw will correspond with actual speed plus or minus approximately 10%. The small difference you are finding is probably not significant.
