Rollcage in road car - not adding risk

[Drmike]

Ive read postings here and other places saying that a rollcage in a dual purpose road/track car will add risk on the road due to potential for non protected head collision (not wearing a helmet on road or using a harness)

I get the fundamental idea that a 3 point seatbelt will allow more body movement which could make the likelihood of head to rollcage collison higher than if a harness is used.

Im an engineer and so tend to over think things over/through - so heres my thoughts on this - Im posting here to see what people think in case ive missed something or just to get general feedback/opinion.

Some background:

The rollcage Im looking at is a JAF approved cage that doesnt have a diagonal roof bar. See attached (roof not fitted).


So here we go:

The side bar (running across the door top) is running flush to the roof line so effectively in the event of a crash the head is now a few inches closer to an imovable object. My thinking on this design rollcage is:

Head impacting to side bar:

1) Im wondering if you will actually sustain less head injury from contacting with a foam covered bar than you will a fiberglass hardtop. (try hitting your shin with a piece of 4x4 with or without a thin foam cover and see what a big difference even a thin foam covering it makes) Theres only a few inches of clearance between the head and roof anyway so your heads going to hit the roof anyway. Im wondering if having less clearance and having a foam to cushion the impact could actually be benficial.

2) The closer proximity to the head will mean less momentum is built up (by the mass of the head as it moves) before it contacts - which I suspect would reduce injury to the head and the reduction in torque may help reduce extent of neck injury.

Head impacting to front cross bar (across windscreen top):

1) With no rollcage your going to headbut the fiberglass roof/screen top trim. With one, your going to headbut a foam covered bar. I guess the roof trim (if fitted) is going to provide some cushioning (as it is foam) (As mentioned the JAF rollcage has no diaganol across the top.)

So basically Im assuming some benefit in case of side movement of head and regarding forward movement Im assuming if the cushioning (padding) on the rollcage provides as much or more cushioning than the (foam) screen/roof trim panel then no additional risk. If a harness is used (on the road) then risk of impact to front bar is zero anyway.




So basically Im considering getting the JAF cage fitted and either using a harness if I cant suitably pad out the bar to match the cushioning of the roof trim or a 3 point if I can.

One other consideration is average road speed is 10miles per hour around Tokyo I would benefit from rollover safety on (and off) the track. Having just sat through more footage of rolling cars (with drivers walking away) Im seriously interested in weighing up the pros and cons.

Id be interested in anyones thoughts, feedback and/or corrections to my logic.

[xtn]

Originally Posted by Drmike

Im wondering if having less clearance and having a foam to cushion the impact could actually be benficial. The closer proximity to the head will mean less momentum is built up (by the mass of the head as it moves) before it contacts - which I suspect would reduce injury to the head and the reduction in torque may help reduce extent of neck injury.

I am not qualified to address your entire post, but I would like to comment on the above misunderstanding of physics.

Imagine you're driving 40mph and you impact a fixed object. Your head does not "build up" momentum as it slings forward. Your head already HAS 40mph worth of momentum, and it's hurtling towards that bar. The bar could be 1 inch away or 10 inches away and it doesn't matter. The mass of your head has the same momentum.

Actually it has less momentum the farther it travels, what with some energy being absorbed by your torso being restrained. So at the moment of impact your head has the highest kinetic energy and begins to loose momentum as it travels forward and slows. The further away something is when your skull hits it, the less damage it is likely to get, assuming everything else is equal.

xtn

[fishguy]

drmike,
looking at your harness set-up, harness going over the seat rather than through a proper slotted seat.
start over, that seat harness set-up is all wrong
how about rethinking this fisrt critical part of your safety seat up from an engineering point of view.
better yet, refer to this, since the a few in the LT peanut gallery here will ask for my degree in engineering to be able to make any comments.

Installation- and Operating Instructions
see page 31, far right picture with the big red x through it.

i sat in the same set-up you have now when i was at the dealership one day with my car.
by loking at it, i could tel it sucked, when i sat in it, i could see how enourmous trauma to ones body would be very easy to have happen when the belts "attempt" to restrain you. i say attemp, because it felt to me like i would get ripped out of the seat, and have both shoulders destroyed in the process.

[Drmike]

Originally Posted by fishguy

drmike,
looking at your harness set-up, harness going over the seat rather than through a proper slotted seat.
start over, that seat harness set-up is all wrong
how about rethinking this fisrt critical part of your safety seat up from an engineering point of view.
better yet, refer to this, since the a few in the LT peanut gallery here will ask for my degree in engineering to be able to make any comments.

Installation- and Operating Instructions
see page 31, far right picture with the big red x through it.

i sat in the same set-up you have now when i was at the dealership one day with my car.
by loking at it, i could tel it sucked, when i sat in it, i could see how enourmous trauma to ones body would be very easy to have happen when the belts "attempt" to restrain you. i say attemp, because it felt to me like i would get ripped out of the seat, and have both shoulders destroyed in the process.

Thanks for your feedback but

That pic is not my car.

My car has a Lotus Sports seat and 6 point in it

BTW, Here in Japan theres a very popular JAF Lotus Cup. The car in the pic is a stock JAF Lotus cup car. They all use this seat setup.....I dont know why but guess they dont consider it important. Pics Ive seen of drivers racing you can see the harness not sitting right on the shoulders....

[Drmike]

Originally Posted by xtn

I am not qualified to address your entire post, but I would like to comment on the above misunderstanding of physics.

Imagine you're driving 40mph and you impact a fixed object. Your head does not "build up" momentum as it slings forward. Your head already HAS 40mph worth of momentum, and it's hurtling towards that bar. The bar could be 1 inch away or 10 inches away and it doesn't matter. The mass of your head has the same momentum.

um,

I would assume that both head, body and roof are travelling at the same speed driving down the road (prior to crash) . After the crash Its the 'relative' difference thats important.


I would have thought the head starts off with zero relative velocity (to to the roof) then starts to accelerate as the car slows. Depending on how far the head is 'allowed' to travel/accelerate will effect the final speed at impact. Hence the Closer the restriction is the lower the impact force would be.

Momentum is just = Mass X velocity

we are concerned about the relative velocity difference between the two objects (head and rollcage) after the crash.
Of course the head has momentum before the crash, its traveling at 40mph but so is the roof. The question is what does the head do with that stored energy when the car stops? It accelerates the head forward. As it does so the head will accelerate untill it hits an object or the kneck absorbs the forces. The closer the object is the lower the relative velocity at time of impact so the lower the impact force (all other things being equal)


...I think

Actually it has less momentum the farther it travels, what with some energy being absorbed by your torso being restrained. So at the moment of impact your head has the highest kinetic energy and begins to loose momentum as it travels forward and slows. The further away something is when your skull hits it, the less damage it is likely to get, assuming everything else is equal.

xtn

hmm I have to say that doesnt sound right to me at all. I agree that head/body etc will accelerate forward then decelerate as the restraints etc take over- but in the case of a side bar head impact I doubt theres going to be any deceleration as its inches away, you only have the thin kneck transmitting restraining forces and a 3 point does not offer much side ways restraint. In the case of the front head impact I think again the head would impact before a 3point restrained much.



Force = Mass x acceleration? Mass is fixed, how far it accelerates will depend on distance head is free to travel. (eg think about race seats with side neck restraints, or think about a Hans device. These work by stopping the head before it builds up too much kinetic energy.) Sure seatbelts will help eventually stop forward acceleration in a front on movement but I doubt that will happen before your head hits the front roof and certainly the head will impact the side without restriction from a 3 point.


kinetic energy = 1/2mv2, again mass is fixed so we worry about the velocity - which for us is relative velocity (between head and roof) so the velocity at head impact will depend on how far the head is allowed to accelerate (move) before it hist the restriction...I think...

[Simba]

Originally Posted by fishguy

i sat in the same set-up you have now when i was at the dealership one day with my car.
by loking at it, i could tel it sucked, when i sat in it, i could see how enourmous trauma to ones body would be very easy to have happen when the belts "attempt" to restrain you. i say attemp, because it felt to me like i would get ripped out of the seat, and have both shoulders destroyed in the process.

One caveat to this is in a situation where one is too tall for a given seat with harness holes. I'd personally rather end up with some potential shoulder trauma than a compressed spine, as would be the case given a seat with harness holes below the tops of the driver's shoulders.

That's generally not an issue with a properly sized seat in one's own car, but I've seen more than a few taller people take someone else's car for a run, with the harness belts more or less running vertically down the back of their shoulders. That situation is much worse than looping the belts over the seat back.

[rnr]

We should also remember that the fiberglass roof and the windshield surround are NOT immovable objects. In an accident the roof usually pops off and the windshield deforms which gives you more cushioning than a naked roll cage which is an immovable object.

In the end the car is currently designed to survive a crash - by adding a roll cage you are changing the parameters of the crash which can lead to different problems.

[Chris Mackey]

The general concerns about safety led me to the conclusion that road cars and track cars are different beasts. I do autocross and low speed track events now and then in my Lotus, but I am saving to buy a car built specifically for the track.

[Drmike]

Originally Posted by rnr

We should also remember that the fiberglass roof and the windshield surround are NOT immovable objects. In an accident the roof usually pops off and the windshield deforms

Exactly - which is why I wouldnt want to roll one without a cage

In the end the car is currently designed to survive a crash - by adding a roll cage you are changing the parameters of the crash which can lead to different problems.

Can you elaborate on these problems? I assume a cage wont effect front or rear crash situation. The chassis doesnt to me look like its designed with a safety zone- Instead relying on seperate crash structures (which would be uneffected by cage fitment)

I suspect this is one of those often quoted urban myths - Cup cars are road legal here with lotus sports cages - I doubt they (JAF or Lotus) would allow road cars to be sold like that if there was a negative effect to cars crash strength (for want of a better word)

[rnr]

Originally Posted by Drmike

Exactly - which is why I wouldnt want to roll one without a cage

Except its almost impossible to roll an Elise on the street (or even the track) - its waaaaay more likely that you will be in a head-on or rear impact

Originally Posted by Drmike

Can you elaborate on these problems? I assume a cage wont effect front or rear crash situation. The chassis doesnt to me look like its designed with a safety zone- Instead relying on seperate crash structures (which would be uneffected by cage fitment)

The simplest thing is that the designers would have (hopefully?)left enough room between the seat and the windshield so that in a "typical" accident the occupants are restrained enough that they will not hit anything. By adding a cage you are taking away around 4 inches of the roughly 2 feet of space which is pretty significant. Its even worse in the case of a side impact where your head is just inches from the side bar. If you add padding to the bar you are reducing the distance even further which practically guarantees that your head will hit the bar.

Originally Posted by Drmike

I suspect this is one of those often quoted urban myths - Cup cars are road legal here with lotus sports cages - I doubt they (JAF or Lotus) would allow road cars to be sold like that if there was a negative effect to cars crash strength (for want of a better word)

Just because they are road legal doesn't mean I would want to be in an accident with one. You could drive a Chinese BS6 sedan but would you really want to be in an accident in a car with zero stars in Euro NCAAP? In the end its your decision - personally I would not drive a caged car without a helmet but thats a personal decision and not one that I can make for you.

[Drmike]

Still thinking out loud.

Heres an illustration of what im thinking


The green line is a plot of the velocity of a decelerating car (crash or whatever)

The blue line is the velocity of the drivers head.

As the car starts to decelerate you can see that iniially the head is still moving at the same original speed.

If the head contacts with a fixed part of the car quickly ( point 1) then car will not have decelerated much and the difference (between head velocity and car velocity) will be less which means less impact force.

The pink line is what happens when it takes longer for the head to make contact with the car.

If you look at the difference between V(1) and the green line and V(2) and the green line at that time you can see that the impact force is a function of how close the head is to the fixed part of the car (all other things being equal)


....I think

[Drmike]

Originally Posted by rnr

The simplest thing is that the designers would have (hopefully?)left enough room between the seat and the windshield so that in a "typical" accident the occupants are restrained enough that they will not hit anything. By adding a cage you are taking away around 4 inches of the roughly 2 feet of space which is pretty significant.

Yes about 3 inches I guess. BTW where does the 2 feet come from?

Its even worse in the case of a side impact where your head is just inches from the side bar. If you add padding to the bar you are reducing the distance even further which practically guarantees that your head will hit the bar.

hmm I suspect if im getting hit from the side by a truck that the air between my head and the door top isnt going to offer me much protection from the truck. A rollcage tube will do more I suspect

If a truck is going to hit my head the force of impact will not be higher if it hits a rollcage bar first. Laws of physics say it must less. I guess there is an argument that if the truck impact deformed the car but the deformity wasnt large enough to contact me that adding a rollcage could narrow the gap causing force of impact to be transfered to me rather than absorbed by the car. I considered this but given the proximity to the edge of the car anyway and lack of structural strength there, I cant see any 'absorbtion' or deformity potential there.

Just because they are road legal doesn't mean I would want to be in an accident with one. You could drive a Chinese BS6 sedan but would you really want to be in an accident in a car with zero stars in Euro NCAAP? In the end its your decision - personally I would not drive a caged car without a helmet but thats a personal decision and not one that I can make for you.

I hear yah and appreciate that, and this is why Ive been milling about trying to look at the reality of it (rather than just accept blindy, trying to think for myself)
The decison isnt made yet by anymeans - i just need to convince myself one way or the other the facts.


cheers
m

[rnr]

Few things

1) you are not including the seatbelts which stretch and lock to slow your body down in the case of an impact. Therefore the the further away the point of impact the less force you will take

2) The head still needs to get from 40 mph to zero mph regardless. If you looks at this video of the Tesla Crash test (Video: Tesla Roadster crashes into a wall - MotorAuthority - Car news, reviews, spy shots) you will see that the windshield support is already at zero when the dummys head gets close to it. The less space it has to get to zero the more damage you will sustain.

3) You will see that with the airbags and the seatbelts the drivers head did not hit the windshield and I think we can all agree that its much better to not hit anything at all.

[rnr]

Originally Posted by Drmike

hmm I suspect if im getting hit from the side by a truck that the air between my head and the door top isnt going to offer me much protection from the truck. A rollcage tube will do more I suspect

If a truck is going to hit my head the force of impact will not be higher if it hits a rollcage bar first. Laws of physics say it must less.

A rollcage tube could do more damage than the truck:

1) You be more likely to hit it since it will be very close to your head. You will hit your head in even minor accidents which would previously have been safe

2) Your neck again helps deccelerate/accelerate your head. In the case of a sideways impact your body goes from zero sideways motion to say 20mph and then back to zero. In the stock design your body will start puling your neck and head sideways so that by the time the truck intrudes into the cabin you are already doing 10 mph which leads to an effective impact of 10 mph. By adding the bar your head will possibly hit the bar directly at 20 mph because there will be no space for it to move sideways under acceleration

3) And finally you may be underestimating the side protection of the Elise. We are sitting between two massive chassis beams which will provide some decent protection already (for the street).

[Drmike]

Originally Posted by rnr

Few things

1) you are not including the seatbelts which stretch and lock to slow your body down in the case of an impact. Therefore the the further away the point of impact the less force you will take

Im not including the seat belts because they wont have any effect when it comes to the side bar of the rollcage (across the door top). They would only help with the front bar and that one is less likely to hit anyway as its further away and Id probably wear a harness anyway which would prevent any contact with that (but not the side bar)

I agree that with forward restraints you will reduce relative velocities (between head and car) but not right off. At first (as the belts stretch) you will see an increase in relative velocities. So you want to have any restriction as close as - or as far as away possible. The danger point would be at the point where the relative velocity peaks.

2) The head still needs to get from 40 mph to zero mph regardless.

not really. Take a look at my graph, Your head has to go from 40mph to the same spead as the car. As soon as it makes contact with the car the 'impact' is over. You can hit your head on the roof of a car travelling at say 200mph and get less injury than if you hit it at 50mph if your head is only 1/4" from the car in the former and say 6" in the latter. Its the relative velocities thats important

If you looks at this video of the Tesla Crash test (Video: Tesla Roadster crashes into a wall - MotorAuthority - Car news, reviews, spy shots) you will see that the windshield support is already at zero when the dummys head gets close to it. The less space it has to get to zero the more damage you will sustain.

good stuff.

3) You will see that with the airbags and the seatbelts the drivers head did not hit the windshield and I think we can all agree that its much better to not hit anything at all.

no argument there, but I think we can all agree that you are going to hit your head on the side with or without a rollcage as its just inches away

[Drmike]

Originally Posted by rnr

2) Your neck again helps deccelerate/accelerate your head. In the case of a sideways impact your body goes from zero sideways motion to say 20mph and then back to zero. In the stock design your body will start puling your neck and head sideways so that by the time the truck intrudes into the cabin you are already doing 10 mph which leads to an effective impact of 10 mph. By adding the bar your head will possibly hit the bar directly at 20 mph because there will be no space for it to move sideways under acceleration

If the bar is deformed by the truck so that it touches the head then surely that would suggest that if there was no bar there the truck would hit the head anyway?

Like I said before 4 " of air isnt going to decelerate a truck.

I would agree IF there was a crash structure that would take up impact forces and IF a rollcage would nulify the benfit of that.

3) And finally you may be underestimating the side protection of the Elise. We are sitting between two massive chassis beams which will provide some decent protection already (for the street).

So sitting behind two massive chassis beams isnt a bad thing (truck collison force transfer wise) but a roll cage bar is?


Seriously thought, thanks guys for all your input! It helps a lot

[rnr]

Originally Posted by Drmike

If the bar is deformed by the truck so that it touches the head then surely that would suggest that if there was no bar there the truck would hit the head anyway?

Like I said before 4 " of air isnt going to decelerate a truck.

I'm not saying that 4" will decelerate the truck. I'm saying that the 4" of space will allow your head to accelerate/decelerate slowly which could be crucial in an accident.

Originally Posted by Drmike

I would agree IF there was a crash structure that would take up impact forces and IF a rollcage would nulify the benfit of that.

So sitting behind two massive chassis beams isnt a bad thing (truck collison force transfer wise) but a roll cage bar is?

The chassis beams and the door beams (in the doors) are a crash structure and are designed to both take a hit and give the occupant room in the cabin. It is is impossible to hit your head on the chassis and if you get hit with enough force to hit your head on the door bar you will likely already be dead.

The roll cage bar on the other hand intrudes quite significantly into the cabin and is an immovable object that has the potential to make what would have been a minor side impact into a serious head injury (without a helmet).

[cemaykan]

just wear a helmet at all times...?

[Hello Kitty]

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[rnr]

Both of these clips are from competition hill climbs. I still stand by my assertions that on the street you are much more likely to be in a front/rear impact compared to a flip.

Originally Posted by Hello Kitty

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