A custom, tilting trike, solo seat vehicle based on moto components
This just popped up in my RSS feed. It’s an inexpensive, lightweight rearview camera system designed for use on bicycles. I could see this system finding its way onto motorcycles or even ATVs. At just $180, that’s pretty accessible for the Streetliner. I’d like to run at least a rearview camera, if not side view cameras as well to replace traditional rear-view mirrors. A system like this could remove all need for a rear window and help keep the outer aerodynamics nice and slippery.
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.
Pros:
Cons:
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.
Pros:
Cons:
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.
Pros:
Cons:
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?
There’s been a lot of activity on the Streetliner drawing board this week. If you’ve been following along, you’re familiar with the shape above. This ’30s era race car inspired shape is what I’ve been showing people when they ask about what I’ve got in mind for this project. I love it, but I knew all along that this shape would inevitably change and evolve.
Likewise, one major aspect of the Streetliner’s design had yet to be worked out in concept: entry/exit. How the hell do I get in and out of the thing? It’s not that entry and exit is terribly complex, but in figuring out a good way to get in and out, it meant big adjustments to the exterior design of the car. So like any design undertaking, this needed criteria.
So starting with the current exterior design, a handful of things have been adjusted since I first penned Design Concept Alpha. The overall length increased, the front wheels got larger, the wheel pants got longer and no longer turn with the front wheels. I really like this shape overall. The snout shape of the front end is very pleasing, the rear has a lovely duck tail quality, and I especially like this design because the vehicle looks even better with its canopy on. All of that shape, however, is mated to an underlying safety cage and chassis:
With arches as the vertical pieces, the safety cage design is pretty straightforward. Heavy duty curved bulkheads are connected by heavy duty rails, then everything is cross-braced in a truss of smaller diameter steel. The angled front and rear main plates actually create rudimentary “crumple zones” where impact damage would send the motor assembly and or front suspension components under the vehicle in the event of an impact. This design is structurally sound, but it has one major flaw: how the hell do you get in and out of the thing? The height of the cockpit opening lip is right at 36″ in this design. I’m pretty tall, but that’s still quite a height to throw a leg over while getting in and out. I also realized that I’d only given myself a 24″ deep opening front to back. I’m not a whole lot narrower than that myself (it’s winter weight, I swear!). So the practical concerns of getting in and out of a high, narrow opening are pretty significant. But even beyond entry acrobatics, with such a narrow opening, I wouldn’t be able to see my hands or any cockpit gauges. That top opening needs to grow and dammit, I need a door. The tricky bit is how do I add a door without compromising the safety cage?
One thing at a time. I added an approximation of the front suspension “box” and see where and how that should tie into the frame. I lowered the bottom rails to tie in to the front and also simplify the rear subframe where the mono-shock would attach. I also moved the main roll hoop back just slightly where previously it was implied that it would overlap my shoulders. Upon further reflection, I realized that this being a single seater, I only need one door. Even though my diagrams here show the opening on the left side, I think I’m going to opt for the right side having the door. Since the majority of motorcycle accidents involve people violating your right of way from the left, it makes sense to me to leave the left side solid. That written, the underpinnings of the door as I’ve envisioned it are as substantial as the major parts of the cage. That way when it’s closed and latched, the door becomes part of the safety cage. I like to think of it like the harness that comes down and locks in when you get on a roller coaster. Solid. Also, thinking about an impact scenario, that’s a force into the cage, so if the door is structurally captive against being pushed through the opening, it ought to be as good as solid. With the door opening on one side of the vehicle, the canopy (when attached) could hinge along the opposite edge — making for a very easy time getting in and out.
With that adjustment made to the frame, the body shape needed adjustment, as you can see below. The opening needs to be increased to meet my design criteria for being able to exit the vehicle in a pinch through the top without using the door. The bottom profile of the body shape also needed to be adjusted to account for the front suspension box.
Now with the shape updated, I like it even more. The larger top opening not only gives the whole vehicle a better proportion. It looks smaller and more trim overall. Before now, with the length being about that of my MINI (which I know, isn’t exactly big), the Streetliner has looked strangely large. Now it looks much more like the race car cabin scooter it was always meant to be. I also reshaped and shortened the wheel pant to account for the door opening.
I really felt like the new shape came into its own with the canopy in place. Not only was it less bubble-shaped, but it completes the curve created by the tail. I can also imagine much better visibility and comfort within the cockpit. That led me to consider some alternate front end shapes. One of which was the sloped, Ferrari GT-style nose. It would borrow its hood scoop aesthetics from a different era than I’d previously been thinking, but I really like it. Forward visibility would be better and overall aerodynamics might be a tad stronger with this sloping approach.
All that remained at this point was to add some visual interest to these basic shapes. This included sculpting the rear and adding a sort of LeMons-style front lip to the wheel pants. I really don’t want to overlook subtle details throughout the shape. Sure, a perfectly smooth Velomobile kind of shape is terrifically aerodynamic, but without at least some minimal sculpting, I think the shape would look like it were stuck in the ’70s and just generally unfinished. These details will surely evolve as the project progresses, but I’m loving it so far.
As I look back on the progression, it’s amazing how much influence a little thing like a door can have, but all for the better! This shape still has a lot of classic Italian sensibility, a lot of race car mojo, and plenty of salt flats shape credibility. As much as I love the ’30s sensibility of Concept Alpha, I like this even better. But more than that, I’m glad to have another big piece of the conceptual design puzzle in place.
I’ve refined my safety cage a bit and done some more research into safety cages used in racing. I found a kit that featured 1.75″ tubing with .135″ thick walls. Running that through the online weight calculator I’d found, that comes out to 2.133 lb/ft. I also did a calculation on 3/4″ tubing of the same wall thickness for use in cross ties and such. Figuring out the length of the tubing in this design, I was able to do some weight calculations:
This is all obviously napkin math, but the result is pretty surprising. If I use the 1-3/4″ tubing for the main structural hoops, then tie it all together with 3/4″ tubing, my entire safety cage and main chassis would only weigh about 100 lbs 130 lbs. This doesn’t include some sub-frame pieces and none of the front suspension components, but still, that puts me well on my way, I think, toward making my < 600 lb design criteria.
I love LEGO! The next best thing to working this out by hand with a big pile of LEGOs is using their software to render a mechanical prototype of Project Streetliner. Obviously, the proportions aren’t any kind of exact, but seeing the basic shape and structure in 3D has me pretty stoked. Also, having to work through the basic mechanical details (what stays solid, what’s allowed to pivot, etc.) has given me a much better understanding of the mechanics involved. All in all, it’s not that complicated. It’s also really interesting to start thinking about the structural considerations of the safety cage / chassis.
Now that I have this so much more planted in my mind, I’m actually tempted to skip the functional model all together and go straight for the Pathliner prototype. That’s probably not a great idea, but it’s a temptation all the same. I just want to keep moving — keep making progress.
Some may wonder why I’m insisting on a tilting 3-wheel design for Project Streetliner. Watch about :30 of this video and it’s pretty obvious. The trouble with non-tilting trike is well explained by Wikipedia:
The disadvantage of a rear drive, non-tilting three wheel configuration is instability – the car will tip over in a turn before it will slide, unless the centre of mass is much closer to the ground or the wheelbase is much wider than a similar four wheel vehicle.
So unless the wheels tilt, you’ve got to essentially crawl around corners. I’m not planning to race this vehicle, but I do want to be able to take evasive action in traffic. So tilting it is! A vehicle like the Morgan was low enough and wide enough that it doesn’t really have this issue. But if I want to really benefit from a smaller frontal aerodynamic cross section, I’m going to need to keep things narrow. The advantages of a Piaggio MP3-style front end keep piling higher and higher.
This forum post has lots of great info on the basics of automotive safety/roll cages.
