Roll cage standards, weight savings and a major oversight

Earlier this week I scrounged around online for SCCA safety cage standards and found this PDF. The information was not only interesting, it was intensely helpful. But before I get into all of that, though, let me say this. It is not my intention to follow the every letter of this standard. I’m simply using it as a guide.

It’s a lot of spec to pour through for the casual reader, so here are the highlights as they pertain to this project.

  • Must be of one of two designs: A high roll hoop (over the windshield), or low roll hoop (over the steering wheel).
  • Must be protect from roll over, impact with an obstacle, and impact from another car.
  • Must be able to support the weight of the vehicle if overturned
  • Must be able to withstand the forces withstood should the vehicle go sliding on the cage
  • Head restraint is mandatory and must prevent whiplash as well as prevent the driver’s head from striking the main roll hoop. Depending on what seat you use, the head restraint may or may not be required to be physically tied into main hoop.
  • Any forward bracing from the main hoop must be padded if the driver’s head is able to contact it.
  • Neither the cage nor the chassis can have an aerodynamic effect that creates lift.
  • The cage must prevent engine intrusion into the driver compartment.
  • Minimum tubing sizes for mild steel for vehicles under 1500 lbs: 1.375″ x .095″
  • The main hoop must be one continuous piece of steel, can have no more than four bends, those bends cannot total more than 180º, and the radius of those bends must be more than 3x the diameter of the tubing
  • The front hoop must also have no more than four bends
  • Rear hoop supports (cross bracing) must be straight
  • The rear hoop, if possible, should go all the way to the floor and all joints should be reinforced by gussets or sheet metal webbing
  • The main hoop must have at least two braces extending to the rear, and two to the front, attached no more than 6″ from the top of the hoop, with an angle of at least 30º
  • The front hoop requires two braces to protect the driver’s legs
  • Side protection should include at least two tubes connecting the front and rear hoops across all door openings
  • The roll bar must be able to withstand stress loading of 1.5x laterally, 5.5x longitudinally, and 7.5x vertically where x is the minimum weight of the car.
  • The race seat must be firmly mounted to the frame, and the back of the seat must be tied into the main hoop or its cross bracing unless the seat is specifically designed to be unbraced.
  • The main hoop cannot be less that two inches above the driver’s head
  • A straight line drawn from the top of the main hoop to the top of the front must pass over the driver’s head.
  • The main hoop must be at least 15 inches wide where attached

There’s a lot there, but there are a couple of things that had a really profound effect on my design. The first is the minimum tubing size of 1.375″. I’d been doing all my weight calculations on 1.675″ tubing and by scaling down to this standard, the estimated weight of my safety cage goes from about 130 lbs to about 80 lbs. That’s a huge difference, especially for a vehicle this light and with the efficiency ambitions that I have. So that’s a win.

The second significant standard wasn’t so kind. It’s this one:

A straight line drawn from the top of the main hoop to the top of the front must pass over the driver’s head.

Here’s what that looks like in my current design:

The problem here is obvious. I’m less concerned about the standard itself than what the standard is meant to prevent. Namely, the vehicle landing on my head. There are really only two solutions to this, at least that I can think of. Either both the front and rear hoops get taller (which kills my streamlining and visibility), or I’m going to have to brace across that gap in a way that clears my head. So I took a page out of the off road buggy book an designed a pair of curved A-pillars to connect the front and rear hoops.

What I’ve rendered here is incomplete, in the sense that there will be a lot more cross bracing than is currently showing. For simplicity’s sake, I’ve kept this rendering to just the main hoops and cross members (plus the door). This change doesn’t really have any impact on the body shape or my aerodynamics, thankfully.

What these new A-pillars do effect, however, is the canopy design and entry/exit. While the struts themselves may indeed bolt in and out in the end, they won’t swing over with the canopy as it previously worked. In fact the whole removable canopy idea is kind of out the window at this point, I think. Instead, there will likely be more of a t-top arrangement where the side windows and perhaps part of the roof are removed and stowed. That, or perhaps just go Jeep-style and do zip-out covers that are heavy canvas or nylon with polycarbonate windows suspended in the fabric. This does afford an opportunity for an elegant permanent windshield, however. By curving the A-pillars in as well as up, that compound curve (which will be a bitch to fabricate, but worth it), should make getting in and out (via the door) easy as any normal car, if not easier. And at just over an inch in diameter, they shouldn’t impede my visibility. While perhaps not as mean looking as the full canopy was, this new design is mean looking in a different way, and a whole lot safer. I’ll take that.

Happy Birthday Project Streetliner!

My website domain just renewed today, which means that this website (and by extension, this project) is essentially one year old today. I’ve never had a personal project of this kind of grand scope before, and certainly not one where a year of planning and concepting still isn’t quite enough. Yet here we are, a year in and a thousand ideas later. The idea has evolved a great deal in one year. The finish line got bumped to 2012. The scooter engine and CVT gearbox is now an ATV engine with a 5-speed and reverse. The aesthetic has come a long way too. Just look at where this started and where it is now in the renderings.

From this:


To this:

More than anything it’s been a great year of collaboration. This would not be the design it is today without YOUR input. So a big thanks to everybody who has gotten involved with ideas, observations and enthusiasm.

3D accurate renderings

So as things keep moving forward on the proportions and details of this design, I’ve noticed that my three-quarter renderings aren’t as accurate to the profile views I’ve been working from. I am not a CAD pilot. I don’t have the right software or the right experience to model this in 3D for real yet. That’ll probably happen before I’m done, but what I did want to see is what this design might look like from different angles. So what I did was take some advice from Twitter and pull my profile views into Google SketchUp. Now while Greg (on twitter) was suggesting I model the whole thing in SketchUp, it won’t give me the kind of results I want for the sheer amount of time I have to put into it. All I wanted was better perspective renderings I could use in Illustrator. This was this was the result:

What I think is particularly interesting is just how much the forced perspective changes the visual relationship between the wheelbase and the width of the body. In the front view, you can see just how far apart the wheels are, but in the other views, they look like they’re right on the body. It’s also interesting how it looks really mean from some angles, and just a tad messerschmitt from others.

Entry/exit cutlines and mechanism

People have been asking how getting in/out of the Streetliner is going to work, so I’ve rendered what that might look like. In concept, the canopy (when attached) will hinge along the right-hand side of the vehicle, and a car-style door will open on the left-hand side of the vehicle. (I’ve also added a lip to the wheel pants to kick the wind off the surface of the wheel just a bit. I really like the look of this detail as well.)


Thoughts on the canopy

While discussing the Streetliner with my collaborators at work, the subject of the canopy came up. That sparked an idea for me. Rather than having to form (or pay to have formed) a singular canopy, it’d be a lot simpler to build the canopy in sections. Each of those sections would be much smaller and much easier to make. I think I could pretty easily create forms and heat-bend polycarbonate panels for the windows and windshield. In fact, in this configuration, a glass windshield might even be viable if it weren’t too expensive to get fabricated. But beyond ease, having a roof on the canopy would mean the cabin would be cooler. It’s also that much meaner looking. I like that too.

Concept finalization


I’m gaining more and more momentum when it comes to finalizing the Streetliner concept. I’ll be putting together a more comprehensive full spec post later, but for now, here’s an update on the exterior design. I feel like I’ve finally found the art deco mojo I’ve been looking for all along. While the updates are subtle from concept Echo, a lot has gone on under the skin to help me arrive at this, the most finalized shape to date.

Most significantly, I realized that my seat and human analog were actually too large, as was my representation of the ATV drivetrain — each by about 20%. This meant that I could shrink the wheelbase as well as the cabin size by significant amounts. I had previously been concerned about the front to back balance of a rear-engine design, but these new proportions make it seem like it won’t really be an issue.

I accounted for a “jack shaft” that will inevitably be required to get proper rear wheel sprocket alignment. It also allows me to more easily monkey with the final gear ratio to the rear wheel. Additionally, it allows for easy fitment of a belt-drive, which will require much less maintenance than a conventional chain. I reworked the rear suspension in such a way that I can still get a lot of travel, but I don’t have to build a whole subframe back just to intersect the shocks. I’ve essentially just extended the rear swing arm. What may not be obvious from the side rendering is that there are arcs involved that would allow the shock(s) to be centerline instead of at either side of the swing arm. In the rendering there’s a ghosted wheel showing the full 6″ suspension travel. The jack shaft is inline with the pivot for the rear swing arm, which should keep things nice and smooth.

Front and rear, I’ve simplified a few things, and added others. Most noteworthy is the addition of a rear window, which is something I hadn’t had in the concept previously. The view won’t be great, but it’ll be much better than nothing. I’m thinking that rather than a rear view mirror, I’ll utilize a rear-facing camera built into the high brake light assembly. That will have a better view than any mirror system that would conceivably work (although I did have a hilarious rear-view periscope idea). I also abandoned previous modern-style brake lights and turn signals in favor of more conventional round lights. This is actually more in line with the classic design language from cars in the eras I’m trying to emulate. In the end, I’ll be somewhat limited in what I can find off the shelf, but standard round light will actually look better, in my opinion. I was designing those other signals and lights almost in a vacuum. I think they were cool looking, but in the end, such modern details would look out of place in this shape, I think.

Lastly, I finally paid some real attention to the front cross-section of the vehicle. I was able to resolve the shape such that I now have a completely flat front suspension parallelogram, but still enough body and wheel clearance to get about 40º of tilt out of the vehicle before the wheels bind or the body touches the pavement. I know for a fact that I’ve never put 40º of lean on any two wheeler I’ve ever owned, so this ought to be plenty. What’s more the cabin ought to be pretty comfortable, with little compromise toward comfort. It should just fit me, but with some reasonable room for comfort.

So let’s hear it. What does everybody think? If you’ve got other ideas, now is the time for them. As far as I’m concerned this is the design I’m moving forward with. I’ll hopefully be starting a small scale foam shape prototype today and I’ll share progress as soon as there is any. I’ll also be doing some small-scale safety cage models, likely in brass tubing, for structural testing and evaluation by people who know more about this crap than I do.

Safety cell construction: looking for your thoughts

I’m getting to a point of critical mass with this project. I’m feeling like it’s truly time to “shit, or get off the pot” as they say. While I have no doubts as to my desire to truly see this project happen, I’ve been distracted of late and have a few lingering things I need to figure out. So I’m throwing those topics out for discussion to keep them moving forward.

Firstly, I need to finalize the broad chassis design when it comes to the safety cell. One of my major design criteria for this vehicle is that the driver be completely protected, much like a race car. Obviously weight is a consideration, but every bit of stiffness gained from driver compartment reinforcement is also stiffness gained for better handling. I feel very finalized in my plans to have front and rear sub frame assemblies that hold the front suspension and engine respectively. These would be tube steel and would bolt onto the safety cell in some manner of catastrophic break-away fashion, regardless of the safety cell’s construction. The question then remains of simply which method of safety cell construction to adopt. Here’s what I’m considering:

Steel tube roll cage — This would be similar to a rock crawler, drag racer, or any number of purpose-built oval track race cars like dirt track or even NASCAR vehicles.


  • Relatively lightweight at approximately 100-150 lbs.
  • Very sturdy.
  • Lots of known quantities and standards from organizations like the NHRA.
  • Relatively easy to construct and customize.
  • Reasonably inexpensive.
  • Provides structure for the chassis as well as protection for the driver.
  • Very easy to just weld things onto it or cut things off of it.
  • The thickness of the safety cell could be less than 2″ , which would help keep everything low-profile.


  • Very rigid geometry in terms of shape. Complex curves are really hard.
  • Requires some extra equipment (welding rig and pipe bender).
  • Not as lightweight as some alternatives.
  • Provides only stiffness — no energy dispersion.
  • Would need to be jig built in order to prevent heat warping.
  • Would have to create separate inner cockpit lining and separate outer body shell that would attach inside and outside of the cage.

Composite safety “tub” — This is the kind of safety cell used by may alternative vehicles like the Aptera and is based on F1, Indy, and even high speed race boat designs. It’s also comparable to many of the structural construction techniques used in making high performance small aircraft.


  • Very lightweight and rigid.
  • Can be made in any shape.
  • Inner surface can be the cockpit lining and even be contoured to make up the seat.
  • Outer surface can literally be the body shell, so no body attachment necessary and weight/complexity savings in unifying the body with the structure.
  • Can be constructed with simple tools for shaping foam and then laying the FB or CF cloth.
  • In an impact situation, the layers of composites and foam would not only provide protective rigidity but would actually absorb a lot of energy.


  • Fewer known quantities in terms of rigidity and real-world safety (although that information is probably available through F1 and other sanctioning bodies).
  • Could be more difficult to adjust it on the fly if a big change was needed after it was made.
  • Need to do a cost analysis, but it might be slightly more expensive (only because steel is actually pretty cheap). Not sure how much that much foam and FG and resin will cost.
  • Will be thicker (possibly 4″-6″ inches thick), which could create clearance issues during lean and general bulk

Aluminum and fiberglass monocoque — This is an idea I had yesterday after a brief visit to the EAA Airventure Museum. The idea would be to utilize flat, machined aluminum bulkheads and ribs that interlock at right angles to each other. Then I’d fill the gaps with foam and fiberglass a skin inside and out. Put another way, it’s the F1-style safety cell, but instead of just being solid foam, it’s got aluminum bulkheads and bracing throughout.


  • Stiffer than the all-composite cell, but likely still lighter than the steel tubing cage.
  • Inner structure would add a lot of stiffness, probably surpassing either of the other two designs
  • Flat bulkhead designs mean that all the truly structural components could just be water-jet cut and then put together like a kit. It’d be self-aligning and could just be MIG welded at the joints with an aluminum-core wire.
  • It would provide solid metal anchor points for things like the seat, harness, and the subframes front and rear.


  • Heavier than the all-composite cell
  • CNC work required to do it right. It wouldn’t be something I could make correctly myself. That means expense both in programming and machine time.
  • Might simply be overkill, although over-engineering the safety cell isn’t really a bad thing, now is it?

My favorite option at the moment is the third concept. Perhaps just because it’s the newest idea, but there does seem to be a lot of elegance to the interlocking aluminum bulkheads. I’m looking for input. Give me your thoughts. What have I overlooked? Is there a structural engineer in the house?


Design concept Echo

It’s little more than doodles at this point, but this is Concept Echo for the Streetliner. Of all the concepts I’ve rendered ’till now, this one is easily the closest to what I’ve seen in my head in terms of its sensibility. It’s got some wonderful ’30s and ’40s era automotive cues, but also some modern flair. Most of all, the rear end is really feeling the streamliner locomotive, art deco vibe I’ve wanted to always have present in this vehicle. The next step, I think, will be to render it in floral foam, then shell and paint a mockup. I’ll be doing this at a much larger scale than before, set off of 4″ wheels and built onto a sort of mockup under-chassis that should let me fit a number of test body shapes moving forward. Think of it as the small scale prototype for shape and aesthetics, rather than mechanicals (that’s a separate, ongoing thing all its own). 

Introducing contributor Nate Erickson

A few weeks back, Nate Erickson — colleague and crazy talented industrial designer at the product development firm where I work — kindly volunteered to do some Streetliner sketches for me. His love for cars and his professional abilities mean that Nate has an ability to think in shape that I can’t even approach. What I was hoping for, and what he’s constantly delivering, was that he’d bring some fresh aesthetic thinking to the Streetliner project. This sketch is one of many groovy renderings he’s done lately.