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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: email@example.com. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
IMPACT PROTECTION IN GENERAL, AND PROTECTING AGAINST SPINAL COMPRESSION INJURY IN PARTICULAR
A fellow Locost driver had a relatively minor incident (caused by chassis bracket failure) on a road course where he ended up with a severe back injury. His thread (with good info) is here: http://www.locostusa.com/forums/viewtopic.php?f=38&t=17408
I am hoping you would be able to shed some light into a safer way to mount seats, or to cushion the impact to the driver in an incident where the chassis impacts the ground first. Foam? Collapsible brackets? I've looked into a lot of info, and really only come up with preventing compression injuries in front end impacts.
The link above leads to another: http://www.usa7s.net/vb/showthread.php?10503-My-Accident-at-Texas-World-Speedway. This is the driverís own account of the crash. Due to a suspension failure caused by hitting a pothole, the car left the track and went airborne due to an abrupt 3-4 foot dropoff. It landed approximately flat, and bottomed against the ground. The resulting upward acceleration and jerk caused compression fractures to several of the driverís vertebrae. He writes: Önot sure what Iím going to do here in the future, but I can tell you that I will not be sitting in an aluminum bucket thatís bolted directly to the frame thatís using a thin section of foam and my ass for padding. Seat mounts need to be able to absorb the initial shock of a vertical impact . . . . And Iím open to suggestions on this topic.
I should begin by mentioning that I do not claim to be a real expert on seat design, restraint design, impact attenuator design, or safety equipment design in general. Others have made careers specializing in these areas. However, I do know enough physics to have reasonably well informed opinions.
More or less coincidentally, I have encountered the same issue of spinal compression injury from bottoming impact in the course of mentoring a senior design project at UNC Charlotte. The students have been tasked with designing an improved frame/cage/cockpit for a midget (midget car, not
driver). Concepts developed may then also be applied to sprint cars and Silver Crown cars, which are similar in construction. The idea is to do for sprint cars and related classes what NASCAR did
for Cup car safety with the COT: create a driver cell with more room between the driver and things that could injure the driver, and then hopefully get that design adopted as a required standard.
One issue identified by industry partner Brown and Miller Racing Solutions was that of spinal compression injury. This has been brought to the attention of the sprint car racing community by the recent paralyzing injury to Kevin Swindell. His car got launched into the air, spun around, and landed flat while traveling backward. Not only did he land hard, but the brake rotor on the rear end center section reportedly came up into the seat and hit his pelvis.
A video of the crash is at http://www.802solutions.com/kevin-swindell-crash-analysis/. The rest of the site is interesting too. This is on the website of a company, 802 Solutions, that is marketing a product they call the Crash Pad. This is a pad intended primarily for sprint car seats, but also applicable to others, that is specifically intended to protect against spinal compression injury. According to the company, the material they use is the choice of the US military for this purpose, adopted after exhaustive testing.
This product is a new discovery to me, and I am not in a position to comment on it from personal experience or client experience, but it appears to be a good choice as a way to address the specific problem of spinal compression injury without major alterations to the rest of the car.
However, the whole question of whether to provide compliance of any kind within the seat structure, either with padding or by making the seat itself compliant, is highly controversial even among experts and is by no means a simple matter.
If the structure is compliant but resilient (springs back), the resilience can actually intensify accelerations imposed upon the driver. If the structure is non-resilient (does not spring back), it absorbs impact well, but because it stays deformed, after impact the driver is a looser fit in the seat and harness. This is then a problem if there is a second impact Ė and racing crashes often involve multiple impacts. The more deformation we allow, or the thicker we make any non-resilient padding, the looser the fit of the seat and harness becomes after deformation.
One thing that helps in a bottoming impact is reclining the driver. As little as 15 degrees can be a lot better than straight up, for a pure bottoming impact. This makes the cockpit area longer in a sprint or midget. This means the engine needs to move forward, or the front of the fuel tank has to move rearward. However, there is no guarantee that a bottoming impact will be purely that. Cars can take hits from any angle. There can be some impact that produces accelerations and jerks whose vector sum is aligned with the driverís spine, no matter how we seat the driver.
Another controversial issue is whether to mount the belts to the seat or to the frame. Probably the best answer to this question is that ideally it shouldnít even arise; the seat and frame shouldnít be
separate parts. However, if we are not racing a monocoque car, the seat will inevitably be a separate part. Production cars are always made with separate seats because for street use the seats have to be
readily adjustable. In these, the belts anchor to the unibody, and have inertia reels to accommodate different size occupants.
Heavy trucks often have suspension systems for the driverís seat, not so much for bottoming impact protection but rather for comfort in normal operation. In these, the belts (usually lap belts only) have to anchor to the seat. Sometimes the belts have a secondary anchorage to the floor, but then there has to be slack between that and the seat anchor, so the seat suspension can work. The belts also would have to anchor to the seat if we use a seat suspension system or compliant mounting of some kind to absorb bottoming impacts.
If the seat cannot move with respect to the frame, theoretically it doesnít matter much whether the belts mount to the seat or the frame. We do want the belts to all be as short as possible, provided we can get enough length adjustment. That argues for mounting them to the seat. If we accept that there is probably going to be some situation in which the seat moves with respect to the frame, itís better to have the belt anchorages move with the seat than to have them stay with the frame while the seat moves. On the other hand, anchoring the belts to the seat causes the belt forces to go through the seat mounts. The belts are trying to tear the driver and seat loose rather than hold them in place with respect to the frame. This requires the seat and its mounts to be stronger, and/or increases the likelihood that the seat will fail structurally or get torn loose.
Ideally, we want to place the driver inside a structure that guards against intrusion and holds the driver in place without injuring the driver. This driver envelope or capsule should have continuous smooth surfaces, preferably with a bit of padding. Padded tubes arenít awful, but continuous surfaces are better. The driver should be supported and restrained so that impact loads are fed into the driverís body in accordance with the bodyís ability to withstand them. Side loads, for example, should be borne primarily at the hip and shoulder, not the ribs. However, the ribs can absorb a bit of force, and that can allow the hip and shoulder to withstand a somewhat harder hit. Arguably, if the forces are perfectly distributed, you either donít break any bones, or you break a lot of them at once.
We then should add impact attenuation devices, but we want these to be mainly outside the driver protection envelope, not inside it or in the seat. Impact attenuators can be of various materials: foam, aluminum honeycomb, sheet steel or aluminum, even tubing. New materials for this are probably being developed as I write this; impact absorption technology is still not really mature.
We can lay out some performance requirements for impact attenuating structures, no matter how they are made:
1. They have to crumple, in something resembling a controlled manner.
2. They should crumple more easily in their outer regions and with greater resistance closer to the structure they are protecting. This is sometimes called graduated rigidity construction. This causes accelerations to build more gradually, i.e. it reduces jerk values.
3. They should be as thick as possible. The more thickness they have, the greater a distance they have over which to absorb an impact, and the better they can cushion the blow. This, of course, conflicts with packaging constraints.
4. While they have to be weak enough to crumple, they have to stay attached in a variety of crash scenarios. If they dislodge, not only do they not protect what they were attached to, but they become hazards in their own right.
In the case of an impact attenuation structure under the driver, this last criterion is particularly crucial. In many bottoming impacts, the car is still traveling horizontally at a good clip. An under-car impact attenuator has to withstand this without getting torn off. The vertical space constraints are also pretty severe in most cases. Generally weíre trying to get the driver and the whole car as low as possible.
Itís particularly nasty if the driveline is under the driver, as in a sprint or midget. These components are unyielding, and they can be driven upward with respect to the frame even if the frame itself never hits the ground. To some degree this can be dealt with by raising the driver and providing shielding and padding under the seat, but to achieve big gains, it would be better to re-think the whole layout of the car. The driveline really should go alongside the driver, not underneath. The driver can then be in a protective envelope with legs alongside the engine, an impact attenuator underneath the protective envelope, and a somewhat reclined seat back.
For best results, this structure should be a composite monocoque with the belts attaching directly to it, and a thin poured foam insert filling most of the space around the driverís sides and upper back, and fairly thin 802 padding under the pelvis and lumbar region.