Collision velocity and load absorption profile as a function of time

[rani]

Hey everybody.
I want to calculate the forces that works on the driver's seat. In the FSAE Rules 2011 the rules of the energy absorber are to take 7350 j in a 7 m/s velocity.
But I don't understand if the 7 m/s velocity is the one that I have to use when I calculate forces on the driver and his seat, or I have to take another velocity(faster).
Our advisor asked us abuot load absorption profile as a function of time.
How you recommend us to get it?

Thank you, and good luck

Rani

[rani]

Hey everybody.
I want to calculate the forces that works on the driver's seat. In the FSAE Rules 2011 the rules of the energy absorber are to take 7350 j in a 7 m/s velocity.
But I don't understand if the 7 m/s velocity is the one that I have to use when I calculate forces on the driver and his seat, or I have to take another velocity(faster).
Our advisor asked us abuot load absorption profile as a function of time.
How you recommend us to get it?

Thank you, and good luck

Rani

[Tech Guy]

Rani,
Unless the driver stuffs it going backwards, in a crash, if you neglect any minor component from the driver's butt sliding forward against the front lip of the seat, the only load on the driver's seat is that from the mass of the driver, i.e. vertical! In a frontal crash, the seat belts take all the load. (Laterally, the seat has to withstand the lateral "g" loads from cornering.) From a dynamics point of view, velocity has nothing to do with force, but acceleration (or deceleration) does. The 7350 J and 7 m/s you quote are part of the test conditions for the impact attenuator. They may, or may not have correlation to what could happen in an incident with an FSAE car. In this test, the Rules say the average decel cannot exceed 20g and the maximum decel cannot exceed 40g. The load profile relates directly to the decel profile, which is not given in the Rules. It's up to you to define an appropriate profile.

[Tom W]

I'm not 100% certain what you are actually trying to find here. As Tech Guy says the loads on the seat are pretty minimal in a frontal impact. In a side impact then you want some form of load path through your drivers shoulder because ribs just break and puncture lungs. This is probably easier to organise through your chassis than your seat.

If you want to calculate vehicle velocity during a crash from 7m/s then you can use the data that you use to calculate the energy absorption capabilities of your impact attenuator. To check that your impact attenuator absorbs 7.35kJ you will almost certainly make up a load v displacement graph and use dw=F.ds to calculate the energy absorbed. You then have several options to calculate the vehicle velocity throughout the crash. Probably the easiest is just to start your work function at 7.35kJ and then take energy out of it according to your dw=F.ds calculation, then you can use the remaining kinetic energy to calculate velocity according to v=sqrt(2*E/m).

This is conservative because you are not accounting for seat belt flex, crushing of the chassis etc. etc.

The impact attenuator is designed around a 300kg car (including driver and fuel), you then have to decide if you want to use this value or some other value that better represents your car.

[rani]

OK guys.
Thank you for your help... I will try to use them...

Ran Ifrah
Composite Materials Teem(seat and covers)
www.BGRacing.com (http://www.BGRacing.com)