Accelerations experienced from driving over road debris 1

[smurf40]

I ’m looking into the situation where a vehicle has run over at speed (approx 60kmh) debris lying on the road with respect to accelerations the driver may experience.

The worst scenario as I see it could be the driver has braked as hard as possible causing the vehicle to compress fully the front suspension before impact.

The debri in question is a tire from another vehicle involved in a previous crash.

I don’t believe most of the vehicle crash simulation programs are able to simulate this situation accurately as firstly they use a single stiffness/damping value (assumed linear) for each wheels suspension.

I have read other threads regarding potholes/curb impacts and can see this is not a trivial exercise to calculate.

What I am looking for is perhaps some general statistics from actual physical pothole/curb tests indicating the range of accelerations a passenger could expect to experience in such a case.

Could I perhaps estimate (assuming full compression of the suspension) the displacement path of the front axle on the wheel that impacts the tire on the road and from this calculate accelerations at the position where the driver is seated. This will not however take into consideration any deformation occurring in the main structure of the vehicle which may dampen the final acceleration the driver may experience.

 

[GregLocock]

"What I am looking for is perhaps some general statistics from actual physical pothole/curb tests indicating the range of accelerations a passenger could expect to experience in such a case."

Hand waving answer - not much at all. The airbag wouldn't trigger. I'm sure many people you know have hit walls, kerbs or potholes, or tires.

Put it another way. If you hit a kangaroo, which weighs about 35 kg, at 100 kph, then expect a trip to hospital. But that is much faster, and directly into the front of the car or windscreen, not running over the obstacle, which is, when all is said and done, exactly what a wheel is designed ot do.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[smurf40]

Thanks for you reply Greg.

The question is perhaps too general (thought I was going to be red flagged there for a while)

Like you say the wheel will roll over this obstacle with little change to the forward velocity or lateral movement. The wheel riding up over the debri will experience a catapult effect, that is to say the corner of the vehicle (where the wheel is attached)will be thrown up in the air.

It is this movement I'm interested in.

People injured in vehicle collisions/accidents are subjected to these forces.

I am looking to quantify these forces to basically indicate whether people may or may not have been injured due to the intensity.

Regards,
Chris

 

[GregLocock]

The only reason I can see for asking this question is legalistic. As such, I'd rather be paid to provide accurate answers than unpaid to provide guesses that will be shot down by measurements.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeHalloran]

A 1970 M-B 280S (sedan) traveling at ~90mph and impacting a truck tire and rim lying in the road aligned with the right front wheel, experiences the following:

- RF car wheel impacts inner fender and firewall, after suspension arm pivots fracture. Unibody fwd of firewall is bent a little, rt outer fender develops a nice bulge behind the wheel opening.
- RF and RR car wheels (steel disc) deform to heart shape, like a spoked motorcycle wheel that's been impacted.
- Trailing link locator for front suspension subframe fractures, misaligning isolators carrying frame. Front suspension subframe (floating crossmember) travels back and up and impacts engine oil pan.
- M6 Hex nut dropped by idiot owner during prior exhaust maintenance, lodged in goo atop front suspension subframe, penetrates expensive die cast engine oil pan, leaving nice clean hexagonal hole.
- Excursion of RF wheel ruptures brake line, so brakes are lost, as idiot owner finds out later in sequence.
- Rt side of car becomes elevated to a noticeable degree. Truck driver chatter on CB suggests car was on two wheels for some distance. Idiot owner just hangs on, tries brake once, tries not to oversteer. Car does not roll over, comes back to earth with shiny side up, rolls to a stop, eventually.
- Idiot owner, on side that was not directly impacted, complained of muscle pain in vicinity of seat belt for two days, accompanied by localized redness. No bruising visible. No injuries detected.
- Idiot owner violated FCC rules against profanity on the Citizens Band radio, at length, with a lecture about truck drivers chaining their spare to the truck, etc.

The doorframes were not distorted, at all. [The car was repaired, driven for another five years, stored for 25 years, then donated to charity. Idiot owner loved the car, hated POS engine.]

;---
Which is a way of saying that it's not a linear problem, because so many things deform plastically and/or fracture in between the path taken by the wheel's theoretical centerline and the path taken by the occupant.
... and so much of that is dependent on the particulars of the car.

As my friend Gerhardt said,
"Vell, dot's how de Mercedes ... bends."




Mike Halloran
Pembroke Pines, FL, USA

 

[smurf40]

Fair enough.

Yes, your right it is for legal purposes that I have been envolved in this. I am however a mechanical engineer not automotive and therefore asked for references to physical pothole or curb tests in my first question. A reference to such a test is basically all I need.

I have found such a paper..."SAE Technical Paper 2002-01-0557 Curb Impacts - A Continuing Study In Energy Loss and Occupant Kinematics" which looks interesting.

Thanks,
Chris