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 case. 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 case. 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