Collapsible Steering Columns and Human Arm Strength

[BenB]

I am working on designing a collapsible steering column. I basically have a cylinder that the steering column can slide into, which is preloaded with a spring that you would need to basically get into a crash to match the preload limit and start compressing the spring.

I think that ideally you would want to reach the end of the stroke of your mechanism (ie full spring compression) at about the point where a typical person's arm would break. This sounds kind of violent, but I think it is the safest design you can have since as you use up your travel you basically do not have a collapsible mechanism anymore and if the arm is broken from bottoming out the mechanism it would have broke long before that if the mechanism wasn't there.

My question: I am trying to figure out what the break strength of a human arm is. The bones in the forearm seem like they are the limiting factors (Radius and Ulna), although it wouldn't surprise me if it was actually the wrist or hand, but I couldn't even figure out how to analyse that (before anybody suggests it... real world testing is a horrible idea in this casehttp://fsae.com/groupee_common/emoticons/icon_smile.gif. I think these bones are typically about 3/4" in diameter and in all my internet searching it looks like bones compression strength is around 22ksi. (this assumes around a 20-30year old male)

http://jap.physiology.org/cgi/content/abstract/10/3/493

I got a compression limit of 9000/bone or 18000lbs/arm or 36000lbs for 2 arms! This seams way too high to me. Does anybody have any experience trying to design a safety system like this? Maybe there is an industry standard that is way different than the way I am doing it.

[BenB]

I am working on designing a collapsible steering column. I basically have a cylinder that the steering column can slide into, which is preloaded with a spring that you would need to basically get into a crash to match the preload limit and start compressing the spring.

I think that ideally you would want to reach the end of the stroke of your mechanism (ie full spring compression) at about the point where a typical person's arm would break. This sounds kind of violent, but I think it is the safest design you can have since as you use up your travel you basically do not have a collapsible mechanism anymore and if the arm is broken from bottoming out the mechanism it would have broke long before that if the mechanism wasn't there.

My question: I am trying to figure out what the break strength of a human arm is. The bones in the forearm seem like they are the limiting factors (Radius and Ulna), although it wouldn't surprise me if it was actually the wrist or hand, but I couldn't even figure out how to analyse that (before anybody suggests it... real world testing is a horrible idea in this casehttp://fsae.com/groupee_common/emoticons/icon_smile.gif. I think these bones are typically about 3/4" in diameter and in all my internet searching it looks like bones compression strength is around 22ksi. (this assumes around a 20-30year old male)

http://jap.physiology.org/cgi/content/abstract/10/3/493

I got a compression limit of 9000/bone or 18000lbs/arm or 36000lbs for 2 arms! This seams way too high to me. Does anybody have any experience trying to design a safety system like this? Maybe there is an industry standard that is way different than the way I am doing it.

[Mikey Antonakakis]

Mythbusters tested the strength of human bones... I think they managed to get about 500 pounds to hang from the middle of an arm bone suspended at the ends... I'm not far enough in my mechanics classes to figure out how that kind of stress would relate to compressive stress though.

[rjwoods77]

Ben,

Dont try to recreate the wheel. NHRA has collapsible steering columns. Its a bolt in sheer. Look up steering components on Chassis Shop and you will see it. It you get in an accident bad enough to collapse the steering column that that car has no business to be continued to drive without tearing it apart to see what else is broken.

[rjwoods77]

Ben,

http://secure.chassisshop.com/partlist/5828/

http://sweetmfg.biz/products3.asp?edit_id=63

[Wesley]

Rob is right - most columns are telescoping tubes with a shear pin joining them - just calculate the shear force generated to find your pin size.

[Brian Evans]

mostly the body-parts that stress test steering columns are chests and heads. Having spring to create a reverse energy mode is about the most wrong thing it would be possible to do, from a surviving a crash point of view.

Brian

[BenB]

I think that saying its the most wrong thing to do is an overstatment. I would rather have a spring-loaded steering column than a solid, non-collapsible one, but thanks for the information on the shear pin-type system. Sounds like there is some good reasons for doing that. Thanks for the information.

[Steve O]

I am interpreting this functioning in such a way as to move your steering wheel out of the way of the driver in case of a crash. I'm not sure about you, but there is very limited space for our steering wheel to go to get out of the way of the driver, and this should not be the issue anyway. Breaking your arm would be a non-critical/non-life threatening injury and your head and chest should not be able to make it to the steering wheel if your harness is properly adjusted. What you should be concerned with is intrusion from the steering wheel; in other words if you crash in the front end and that force wants to drive the steering wheel into the driver. In this instance you don't need to know what the strength of your drivers bones are at all, you only need to know the strength of your mount keeping your steering wheel from going towards the driver. Put an anti-intrusion mount and calculate the shear pin to break before that does. That will cause your steering column to collapse and the only force is the force exerted on it by the driver. I think although he didn't back him self up well, Brian raises a good point in the fact that a spring setup will absorb the energy, only to have to dissipate it back in the opposite direction...in other words, back into your driver. Either way, most of these setups the steering wheel is stationary and the column moves;therefore, the force on your steering wheel from the driver's bones is irrelevant.

Steve

[ibanezplayer]

I agree, way easier to either buy/copy a current design than try and create your own.

Also, forgive me if I'm wrong, but the way I understand so far is the shaft collapses against the spring to absorb the impact, but then what happens? Does the spring push the wheel right back towards the driver?

Just grab one from the chassis shop

[BenB]

That was the idea. The spring absorbs the impact, but after that it would push back to its original position, but never push hard enough to injure the dirver unless you bottomed out the mechanism, but as Rob said if you are using your collapsable steering wheel feature you are probally AT BEST out of the race and won't need the steering to return to its original place anyways.

I am going to do the shear pin thing

[CappyUMD]

I don't see the point of this. In a frontal crash, the seatbelts will prevent the driver's torso and arms from moving forward much at all. Even if they do, the elbows are not locked in a fully extended position. Muscles (not bones) are the limiting factor.

If the car is crumpled to the point where the steering wheel hits your chest, you're toast anyway.

[Garlic]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by CappyUMD:
If the car is crumpled to the point where the steering wheel hits your chest, you're toast anyway. </div></BLOCKQUOTE>

That's ignorance. The entire point of a collapseable column is for exactly that occurance, and it's not that hard to imagine it could happen in these cars (especially with a top-mounted rack), with their lack of front structure.

If the wheel is pushed into the driver, the driver is not 'toast anyway', a collapsable column would probably save a life or at least serious injury.

[Brian Evans]

Interesting trivia tidbit from research into head and neck restraints, and crash analysis from Champ cars (indy cars). Prior to HANS being used, many crashes that had excessive G-force loads were routinely survived, and no one really knew why. Analysis of crash footage of such wrecks showed that what actually happened was that the driver surged forward dramatically, was guided straight forward by the high and strong cockpit sides - and rather than having a snapping of the whip effect on the head with subsequent broken neck - the head impacted the steering wheel at just the right time to absorb the energy and stop the neck injury from occurring. With full face helmet, they didn't even eat the steering wheel. They walked away.

Brian

[CappyUMD]

In these cars steering racks are mounted behind the driver's feet, so if the rack gets crushed back by more than a foot, the driver's legs are toast.

[Pete Marsh]

There is a big difference between broken legs and dead! If the addition of a shear pin can save someones life it seams like a great idea to me.
I'm assuming the steering gear has a fixed shaft from the rack, that roughly points at the driver? If it has double uni's on a low mount set up then then it probably wouldn't need anything else.

Pete

[CappyUMD]

I'll buy that a blow to the torso is far more likely to cause death than broken legs, but is it even possible to crush the frame that far? We're talking 200mm of impact attenuator plus 2 feet or so of frame (depending on steering wheel and rack locations).

You could also argue there's the possiblity that the rack could be yanked back by the steering tie rods if they remained intact but the rack mounts failed. It's a pretty remote possiblity if you ask me, but at least reinforcing rack mounts offers a performance related benefit.