Introduction: Engineering a 3 Wheel Vehicle Chassis
First Prize in the
Car and Motorcycle Contest
My university senior project was extremely ambitious. Over the course of a year (2011), myself and three other mechanical engineering students designed and fabricated a complete solar race car chassis including the frame, suspension, hubs, spindles, brakes, & steering.
Its been almost 5 years now but vehicle design is just as fascinating to me today. In retrospect any one of the above systems would have been an adequate senior design project. My decision to attempt all of them together reveals the ignorance/arrogance/ambition of my younger self. In the end we were successful though and I learned a heck of a lot about chassis design. I'm sure there are a few more ambitious university students out there who could benefit from my experiences.
This instructable will discuss the engineering process of designing a custom 3 wheel vehicle chassis. The project consisted of research on vehicle dynamics theory and calculations, 3D modelling, structural stress analysis, designing around human factors, component sourcing, and lots of time spent fabricating.
Note: My senior project group's involvement in the car was limited to the mechanical chassis portion only. All the solar panels, battery design, bodywork, and system integration were later done by other students.
Step 1: DESIGN GOALS & CONSTRAINTS
At the start of any design project you have define your goal and all the constraints that will shape your project. You don't want to miss anything and later have to change your design because of it.
Timeline: We had to actually finish the new chassis by the time we graduated, otherwise it would be unlikely that someone else could jump in and finish our work. See Gantt chart above. The real timeline turned out to be everything shifted and squished to the right because we were all busy full time students.
Meet Regulations: We designed the car to meet the American Solar Challenge Race regulations.A 27 page document listing things like: Driver must be sitting in upright position at no less than 27 degrees. Driver must have no less than 10" of space between any part of his body and the frame. The overall dimensions of the car must not exceed X'. There must be adequate crush space, visibility, and so on.
Improvement vs Old Solar Car: Our goal was to make quantitative and qualitative improvements when compared against the schools existing old car. We ran the old vehicle through a few tests as specified by the regulation including a slalom, figure 8, braking distance, and turning radius test.
Practical Constraints: The car had to be able to fit through a standard double door, so we could get it in about of buildings. The wheels were heavily regulated by the race organizations and they were so expensive to buy new that we had no choice but to steal the existing wheels of the old solar car. The vehicle had to be powered by a single very special high efficiency electric motor that was designed to mount to one of those wheels.
Another issue was that some of the schools tools were shared by multiple engineering clubs. This was a problem because the clubs saw themselves as competitors for the same scarce resources and so they hoarded them. We ended up using a lot of our own personal tools and making the school buy us some new tooling. Without getting too deep into our financial setup it should suffice to say that money was also a significant practical constraint.
We purchased most of our parts online from Jegs.com, SummitRacing.com, Ebay, McmasterCarr, and Amazon. The fasteners and raw materials came from local stores.
Step 2: DESIGN CONCEPT
SIUE has a really weird design as far as solar cars go. This section is important to understanding where that came from.
We chose to base our design off an Australian solar car built in 1987. The idea was that the car body would completely separate from the panel module, kind of like a truck and its bed, so that future student teams could just focus on designing the panel. All electrical sections would be completely segregated from the cockpit. It was meant to make everything cheap, reconfigurable, and easy.
Keep in mind that when we started this project it felt important to simplify things for future students because there was very low participation rates in the club at that time. The team had almost no money budgeted for it prior to our pursuing this project, so this was a big deal for the school. Flash forward and the team ended up growing substantially later on and, as you will see, future solar car teams decided it best to integrate the panels and cockpit anyway. Three cheers for continuity, Hip Hip...Booo!
The new chassis design was also special because we decided to make it a three wheeled vehicle. Three wheels is pretty standard for competitive university teams. Dropping the extra wheel significantly reduced the complexity, weight, and rolling resistance. It also reduced the car's polar moment of inertia giving it a better steering response (so it handles more like a Mazda Miata and less like a Buick). Unfortunately losing that wheel also increased the chances of the vehicle rolling over! (More on this later).
Now take a second to compare our design against one of the best solar car teams out there, the University of Michigan. (See pics of yellow car). Their car costs around a million dollars due to the high performance materials they use. Their team of 70+ well organized engineers build a new one every few years. SIUE's solar car team is the underdog's underdog in this race.
Also, looking back now I think we prioritized performance handling a bit too highly in our design. Our four man group was really interested in formula car racing. Whoops.
Step 3: 3 WHEEL VEHICLE DYNAMICS
As mentioned we were driving our car from a single very expensive electric motor. A one wheel drive on a four wheel car is not dynamically stable and is a great way get stranded in the mud too! Propelling a 4 wheel vehicle from a single wheel results in a single force acting off-center to the center of mass of the vehicle. The result is a torque that makes the car turn when you accelerate. A three wheel vehicle solves this problem by making the center wheel drive.
OK but there still a big question: What are the advantages to having two wheels at the front instead of the back on a three wheeled vehicle?
To answer this, we should first define the different three-wheeled vehicle options and compare them for a few different desirable vehicle performance characteristics: (See pictures)
Delta: One wheel in front, two in back.
Tadpole: Two wheels in front, one in back.
Dynamic Stability: When a vehicle is said to be dynamically stable it is meant that it reacts safely and predictably under various driving conditions.
When designing a chassis, we can choose how the car will react when turning too fast. One of two things will always happen: either the car wheels will slip relative to the ground, or the vehicle will tip over. Obviously, slipping is the desired outcome. Keep this in mind for the moment.
When the car does slip out of control on a fast turn, we can design it in such a way that we know whether the front or rear wheels will slip first. This is important because if the rear wheels slip first, the vehicle runs the risk of spinning out of control (oversteer). If the front wheels slip first (understeer), you won't spin out and it is easier to regain control. Understeer is considered a safe dynamic response to slipping in a turn and is designed into all commercial cars. Which wheels will slip first is a function of weight distribution and weight transfer turning turns.
The problem for delta vehicles is how to distribute their weight and control their weight transfer during a turn to avoid undesirable outcomes. If you design the weight distribution for a heavy front bias to achieve understeer, you increase the risk of tipping over. Alternatively, if you increase the weight distribution on the rear tires, the vehicle will oversteer in hard turns, also bad.
We also need to consider nose diving, which is exactly what it sounds like. When you slam on the brakes as hard as possible, the vehicle will either skid to a halt or the rear wheels will lift off the ground. This is also a function of weight distribution and weight transfer. It would seem that the delta design has an advantage here because it naturally lends itself to having a rear biased weight distribution. But in the real world, a hard stop doesn't always occur when traveling in a straight line. If you stop hard enough while turning with a delta vehicle, the weight will transfer to the front wheel enough to cause the vehicle to flip over at an angle. See the first ten seconds of this Reliant Robin car video. Category Winner: Tadpole
Braking: Because your weight transfers to the front when you decelerate, the front wheels on any vehicle provide the majority of your stopping power (something like 60-70%). The delta is at a disadvantage considering the weight distribution issues discussed above and the fact that it has one less front tire to brake on. Category Winner: Tadpole
Simplicity of Design: Steering is definitely easier to design for the delta. No special considerations need to be taken into account to avoid lateral wheel slipping on turns. The tadpole design has to incorporate extra linkages to approximate Ackermann steering geometry to prevent wheel slippage. Front suspension design is definitely easier on the delta. The best choice is the telescopic fork positioned with some degree of castor angle to ensure that the car drive straight when you let go of the wheel. The rear suspension design can be any number of options.
The reverse is true of the tadpole. Multiple choices are available for the front suspension, while the rear suspension is much easier to design (the swing arm being the obvious choice). Imagine designing a toy tricycle for a child. Have you ever even seen a tadpole design like that? Category Winner: Delta
Aerodynamics: The tadpole design lends itself better to the aerodynamic tear drop with the correct length/width ratio more easily than the delta. (The ideal teardrop width/length ratio being 0.255) The image above shows how poorly the correct shape fits the delta design. Plus, you would have to encase more empty space with the delta.
Category Winner: Tadpole
Powertrain: The delta design has more disadvantages when selecting your drive wheel. If you go with front wheel drive, you risk putting too much weight at the front of the vehicle and steering that wheel might get in the way of powering it. If the you choose the back, you need to add a differential gear to the rear wheels.
Even an electric motor is troublesome. If the motor is on the front, you don't want the heavy batteries up there for weight reasons. But if the batteries are in the back you have to run the main power cables to the drive motor a long distance and through to the passenger cabin. Either way you have a disincentive to have too much weight on the front driving wheel, so you miss out on potential traction.
A tadpole with rear wheel drive gets the best of both worlds. No differential is necessary and you can keep a good 30% of the vehicle weight on the drive wheel to maintain traction. Category Winner: Tadpole
Conclusion: For most vehicles the tadpole is clearly the best option on three wheels. However, the delta will always have value for niche applications.
Step 4: 3 WHEEL VEHICLE DYNAMICS CALCULATIONS
All of those factors discussed in the previous step can be quantitatively predicted with reasonable accuracy using a spreadsheet I made for this purpose (attached at the end of this step).
I started by referring to the Microsoft Paint concept art from the "design concept" step. That allowed me to estimate some reasonable numbers for the overall car dimensions and the locations of everything in it. This enabled me to do a static analysis of the car (all on paper) to determine its weight distribution on each tire as well as its center of mass.
In the following sections I calculated safety factors for each of the dynamic stability issues previously discussed.
In the slipping vs tipping case I pretended the car was going through a figure 8 turn and calculated the applied lateral force that would act on the car's "neutral steering point". (This point is different than the center of gravity because the friction from the tires is considered. This is unique to a three wheel vehicle!)
From there I found the maximum velocity the car could possibly take the turn without either slipping or tipping. Then I made sure that the safety factor for a slip condition was less than that for a tip condition, ensuring that the car would never tip (unless you hit a pothole).
I did a similar thing for the nose diving analysis to make sure that if the front wheels were locked up that the car would skid to a halt instead of flipping over the front tires. This turned out to be an insignificant issue because our vehicle was so long.
One of the really tough things was figuring out how exactly the weight would be transferred through the tires while turning. If the car had no suspension system and was literally just a box on wheels then we could easily calculate the weight transfer as a function of the location of the center of vehicle mass relative to the wheels.
But things get complicated when you put the car on springs. The point of suspension is to smooth the ride and keep the tires in contact with the ground at all times. Good suspension designs will also keep the tires positioned upright and pointing in the desired direction at all times.
During a turn on a three wheel vehicle the weight of the vehicle will transfer both laterally AND front/back. Our rear suspension design was simplicity itself so that was easy to calculate around. The front, however, gave us the option of choosing how the car would transfer its weight during a turn. I ended up designing the suspension so that the 'roll axis' pointed downward just a little causing the weight to transfer forward on a turn and reduce any possible oversteer effects, just in case the finished car was not assembled as we intended. (And it wasn't!) This back to front weight transfer also had to be considered for the tipping vs slipping analysis.
Step 5: DESIGN STRATEGY
The excel document from the last step was a great tool for picking the basic dimensions of the vehicle on a large scale. But the process of actually designing parts on such a massive interconnected project is still difficult. Our original intentions were to use as many off the shelf parts as possible to save us the fabrication work. But there were so many systems that rely on other systems that we felt like we couldn't nail down enough to even get started!
The Outside-In Approach: Eventually we realized that by starting with the tire placement we could design our way inward in the order of tires>suspension>frame>frame sub-components. The frame's minimum dimensions were constrained by the drivers compartment, so we used that to estimate how far in we could go.
The best way to do it was to start laying everything out in 3D using AutoDesk Inventor and iterate from there. The rest of this instructable is laid out in the order each main sub assembly was designed.
We also realized that the rabbit hole of design perfect would go as deep as we were willing to dig. What we needed was a technique for verifying that our designs were adequate without having to delve too deeply into individual component design.
The Comparative Approach: By using another pre-existing commercial vehicle as the basis for comparison we saved ourselves from having to 'reinvent the wheel' for a few standard components. For example: we found an all terrain vehicle design that had a similar weight distribution to our car, so we took the brake calibers and rotors from that machine and used it on ours.
This is a valid design technique and is similar to one used by the FDA to review new medical devices, called 'substantial equivalence'. This means that if you can prove that your medical device performs at least as well as an existing medical device that's already approved for public use, then yours will be approved also.
Step 6: WHEELS
The rims & tires were already provided and met the regulation specs so that part was easy. They are tall and skinny, designed for minimizing rolling resistance and weight at the cost of cornering grip and braking traction. Because the tires were already specified there wasn't a lot of work for us here but I do want to mention an important often overlooked issue regarding race car wheels.
Wheel Aerodynamics: Aerodynamically, tires exposed to open air are a big no-no. While they are easier to access this way, exposed tires disturb the airflow around the car resulting in a greatly increased air drag. This is because wind resistance is an exponential function of velocity. The center of the tire travels at the same speed as the car, but the top of the tire travels twice as fast as the car speed, resulting in greater air drag forces. Exposed tires on a solar car can greatly reduce your range so professional teams cover them up with wheel fairings.
Cool Tire Science: Tires are much more flexible than you might think. I thought it was interesting that drag race car tires deform so rapidly upon take off that they can vibrate enough to knock the driver unconscious!
The moment the driver accelerates from zero, the rim starts spinning faster than the rubber tire. The tire reacts by deforming and wadding up in the front of the tire patch and pulling tight against the rim on the back of the tire patch. As the tire spins, the wads flop down on the ground in the front causing vibrations which can be be severe enough to knock the driver out . Tire stiffness/geometry/grip/rotational speed have to be taken into account when designing a race car to avoid tire shake. This was never a concern on a solar car though.
Step 7: HUBS & SPINDLES
We soon realized that our rims were so custom that we couldn't adapt prefabricated parts from other vehicles to mount to it. The rims had a six hole mounting pattern and for this reason we reluctantly had to design custom hubs to mount to it.
The hubs job is to provide a spinning mounting point for the rims. The hubs house bearings pressed inside them and they also have the brake rotor mounted to them.
Even more unfortunate was that the only way to get the suspension characteristics we needed out of something that could bolt to our custom hubs was to design a custom spindle as well. The spindle's job is to hold the entire suspension together. A spindle shaft runs is pressed into it and it comes out the far side to provide a surface for the hubs to spin on. The spindle also has mounting points for the brake calipers, the steering knuckle, and the rod ends on the control arms.
Step 8: FRONT SUSPENSION
The only way we were going to get the performance we wanted out of our suspension was to go with an unequal length double a-arm design. (In retrospect this is a high performance suspension design and was overkill for a solar car.)
We had a lot of goals for our suspension design: (Check out the video file at the end)
- Keep the tires upright at all times (minimize camber).
- Keep the in/out distance the tires move during articulation at a minimum (minimize tire scrub).
- Not let bumps change the direction the car travels (zero bump steer).
- Provide enough adjustability to keep the tow angle at zero (keep tires parallel).
- Provide enough travel and an appropriate spring rate to keep the tires in contact with the ground during turns.
- Minimize the overall weight of the suspension system relative to the rest of the car (keep a low sprung/unsprung weight ratio)
The angles that the control arms sit at have a huge effect on the behavior of the finished vehicle. To get it right we used a couple different methods:
Geometric Modeling: Use 2D modelling to check suspension in various positions to make sure there is room for everything and that it all articulates as it should.
Kinematic Modelling: Use free body diagrams and calculations to make sure the spring rate is appropriate.
In the end we used single-adjustable coil-over shocks (oiled filled dampers) that fit up with 200 lb/in springs. It was setup to be compressed 50% of its 4" travel when the car was fully loaded. Note that all of this was done based on our estimate of the finished cars weight. If we were wrong then the suspension wouldn't perform as desired! Here's a quick demo of the finished product:
Step 9: REAR SUSPENSION
The rear suspension design we used is called a swing arm. It was made entirely from 1.5" OD steel (we called it the beef rod) so that it could resist the large torsional force of having an offset load. Because the weight distribution of the car was almost equal on all three wheels, we could use the same suspension geometry here that we used in the front.
One drawback with this type of rear suspension on a three wheel vehicle is that there is potential for 'roll steer.' That is, when taking a turn causes the weight to shift in the car and the frame itself rotates causing the rear tire to also rotate and travel laterally. (Just like you can turn on a motorcycle just by leaning.) Fortunately, the effect in practice was minimal.
Step 10: FRAME DESIGN
The whole frame is made of 6061 grade aluminum because it provides a good strength to weight ratio at a reasonable cost, especially when compared with steel. All tubing used is 1" OD 1/8" wall to simplify design, purchasing, and fabrication.
The big problem with aluminum though is that it is difficult to weld. We were very lucky to have access to a TIG machine and one of our group was a skilled welder.
This type of tubular shell frame design is called a 'space frame'. A popular alternative used on solar cars is called the 'monocoque frame' which integrates the outer body with the frame components.
The frame itself required an enormous amount of physical labor. We spent hours measuring and cutting all the tubes to length, hoping we were keeping the waste to a minimum. A free online tool I found extremely helpful for this task is called the cut list calculator, which automatically calculates the most efficient way to cut a set of tubes or boards or anything.
Another lesson we learned the hard way was that tube bending is not of those things you do without a plan of action. The trouble is that normal drawing details call out the center points of the bends but not the bend start and end points, which vary with the bending radius in non-obvious ways. We screwed it up twice before taking the time to enlist a free online program to help us know where to bend.
(If you like finding resources like these then check out my whole list here!)
Step 11: STEERING DESIGN
The hidden difficulty behind steering a car is that when you turn the car the inside tire travels along a smaller radius than the outside tire. The solution is to design a special linkage in your steering system to satisfy 'Ackermann steering' conditions.
In practice this was pretty difficult to achieve. We used 2d modelling to get as close as we could but there seems to have been some discrepancy between the model and real life. The final steering system wasn't perfect, but it was good enough. We were proud that the steering column had zero play in it and that the steering wheel was made to be removable.
Step 12: BRAKE DESIGN
In retrospect it was overkill to put brakes on all three wheels! (Again, we focused a bit too much on handling performance). Having brakes on just the front tires would have been adequate because the majority (60%-70%) of your braking power comes from the front due to the weight transfer from decelerating.
Adding that rear brake was a major hassle too, because the motor drive wheel was never intended to have one. We ended up running a rod through the center of the rear wheel to provide a place to mount the caliper. Then we mounted up a weird series of disks to enable us to mount the calipers directly to the rim.
We also gave the brake system adjustable length pedals to suit the driver's height.
Years after we finished the chassis, SIUE competed in Formula Sun Grand Prix 2014 and took home 1st prize in the stopping distance portion of the competition. This was a great honor for the school because the world of solar car racing is prestigious. Just meeting the qualifications and being allowed to compete in a large competition is a big success. ...Dang!
Step 13: FAB & ASSEMBLY NOTES
We wasted a LOT of time trying to get the settings on the welder set correctly for the aluminum. At one point we were able to make nice looking welds that had no actual strength! (Testing is important!)
We used a really simple system to keep track of building the frame parts. We published all detail drawings into pdfs and printed them ONCE. Then we'd write 'DONE' on the paper in sharpie when the part was done. No need for frills, just a method to keep from making the same things twice.
For certain parts, printing up drawings on a 1:1 scale and tracing them onto the metal is the easiest way to build it.
For all the tube notches we used an industrial Tube Notcher Jig with a 1" mill bit.
The tabs that all the rod ends mount up to were custom made using sheet aluminum and a Rotozip.
Step 14: COMPLETELY FINISHED CAR
Here is a video of the completely finished car doing some racing in 2015 (Remember that my involvement ended with the chassis in 2011). 100% of the credit for the hard work on the body, electronics, and getting out to the race belongs to the current team!
Step 15: RECOMMENDED READING
This project was an immense amount of work but I learned so much and am demonstrably better off for having done it. For our efforts this project was awarded as SIUE’s ‘2011 Mechanical Engineering Senior Showcase Project’. My personal experience even helped me get a job after graduation.
For any current students reading this I highly recommend that you participate in some sort of engineering club activity to help you get real hands-on work experience. For any those interested in solar cars or race car design I've put together a list of the books I used to figure all this stuff out. None of this would have been possible without these resources.
- The Winning Solar Car: A Design Guide for Solar Race Car Teams
- How to Make Your Car Handle
- Tune to Win: The art and science of race car development and tuning (or any other book by Carroll Smith)
- PDFs of 3 wheel vehicle dynamics technical references are attached.
And as always, if you enjoyed reading about this project then let me know and please take a second to vote for this project in the "Instructables Car and Motorcycle Contest!" (Sept 2015)
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Please be positive and constructive.