Intro to car construction methods and planning for side impact

One of the weak points of many kit cars is side impact protection. This is something that’s hard for kit cars to replicate like it exists in production cars due to the vastly different construction method. This requires a brief introduction into basic car construction methods.

Most modern cars are built as a unibody where large portions of the body are actual integral and stressed parts of the chassis. This is in contrast to body-on-frame construction, where as the name suggests, there’s a frame which holds the load and the body just rest on top, unstressed. Body-on-frame is still a common construction method for trucks and buses. To compare, here’s a picture of the body of a Lamborghini Aventador and the chassis of a Toyota Tundra:

Lamborghini Aventador chassis (unibody construction)
Toyota Tundra chassis (body-on-frame construction)

Unibody is popular for many reasons, including cost for large-production vehicles. If you don’t need to build something extremely rugged like a truck and cost is a major factor, every piece of metal you put on the car should have a purpose. Thus you make it a stressed member which means unibody construction. Moreover, if you look at where the doors would be on the Lamborghini above, if you can guarantee that the pieces in front and behind the door will hold up in a crash, you just need to make sure the door does too and you will have completed the crash cell at the same time.

Beside impact protection and in many was much more important are chassis dynamics considerations. The way in which a unibody is loaded is very different. In a body-on-frame vehicle, all the stress for a vehicle is placed on the lower chassis. Trucks tend not to need a lot of torsional rigidity (think twisting the front with the rear like breaking an ice cube tray) and they’re build to withstand lots of other loads anyway, so the chassis is pretty solid. In contrast, a supercar where weight matters like the Lamborghini, you want to be able to build a thinner lower section because you can run many loads via the A and B/C pillars (connections between a car’s roof and subframe) as well. That’s a unibody – loads run throughout much of the car, including many “body” panels.

There is also a third major construction method, common really only in kit cars nowadays, which is called the spaceframe. As the name somewhat suggests, there’s a frame on the inside which holds up an exterior structure. In kit car terms that means a bunch of tubing on the inside with a fiberglass body on top. No stressed flat sections, just tubes, round or square. The highly popular Shelby Cobra replica from Factory Five, which I built a few years ago, is built this way. I’d argue it’s a lesser construction method from a technology purist’s perspective because it’s typically done in steel and is heavier, but it’s also a fairly cheap production method (design and construction) which is great when you think just about enabling more people to buy the car and getting their butts in the seat.

So with that out of the way, what’s my kit? Well, it’s a “monocoque”. For those of you who clicked the above link to unibody, you’ll notice that you actually landed on the Wikipedia page for monocoque. They’re very similar, basically the same depending on who you talk to. Monocoque also is not tubular (spaceframe) on the inside nor is it body-on-frame. What it comes down to is that race cars are monocoque because it was developed as a weight savings method (and the only way to build cars out of carbon fiber), whereas unibody is used for mass-production vehicles where the major factor is cost of construction/materials and ease of building a safe car. Formula 1 cars are monocoque for example, a Honda Civic is unibody…

My car is a monocoque (unibody) due to the aluminum tub which is the center section. This is reinforced by a steel structure which is the black part in the picture below.

Monocoque chassis with steel roll bar

I will admit that calling this car a true monocoque is tenuous, but that’s not the point. It’s probably the most monocoque (and thus lightweight) kit car out there, even if that requires tubular reinforcement, which brings up the second part of this post: reinforcement for side impact protection. I wanted to be sure that I have an even greater chance of surviving side impacts from soccer moms in SUVs and any obstacles in my way on a track day (sideways in my way…). You sit really far in the center of the car already, but why not have a little extra insurance short of the full race cage.

That meant installing door side bars which complete the loop of the front and rear hoops. Going with the white exterior theme, I decided to paint these white as well upon fitment. The passenger side went in really well and is easy to remove. The driver side was a b&%$# to get in, so I had to actually paint it when it was in the car, because it ain’t comin’ back out – ever. I’m pretty happy with the result. I think they will truly actually make the car quite safe and best of all, they look real racey. Guaranteed 15 horsepower gain.

Door side bar, passenger side

2 thoughts on “Intro to car construction methods and planning for side impact

  1. Howdy! I know this is kinda off topic however I’d figured I’d ask.
    Would you be interested in exchanging links or maybe guest writing a blog post or vice-versa?
    My blog addresses a lot of the same topics
    as yours and I think we could greatly benefit from each other.

    If you are interested feel free to shoot me an e-mail.
    I look forward to hearing from you! Superb blog
    by the way!

  2. I don’t see anywhere how to purchase which you are building are you selling these bodies for building kit cars and how to find it

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