From UR Baja SAE
Jump to: navigation, search

Absolute Truths

There are few absolutes in engineering, but at the Baja SAE competition these few truths have become evident:

  • Lighter is better. Reduce mass and rotating inertia for a faster car. The only event that doesn't penalize performance for mass is Load Pull (however, placement of the center of gravity is very important to Load Pull.
  • Test and tune or suffer. A tested and tuned B grade car does better than an untested A grade design.

Find guidance in the posted results

Patrick Lewis has a bunch of data from published scores and vehicle characteristics at the Alabama competitions

Focus on the Score

  • Cost Report: the guidance for the cost report in 2010 was that your target prototype cost should be about $8,000 for 1st place in Cost, and every $100 dollars above that costs a point. Participants on the Baja SAE Forum came up with this formula based on past results and the prototype costs some teams shared, but it is only an approximation because prototype costs are not published and the judge's cost correction amounts are unknown. With inflation and the increased cost of common materials like steel this equation will change. Perhaps the current target is $8,500 and the penalty is still around 1 point per $100 above that.
  • Design Judging vs. performance and cost

Design Trade-offs

Engineering is the application of science to real world problems. It is different from science because a good engineer evaluates trade-offs while optimizing his or her creative design for the optimum product.

  • Added features come at a cost. Monetary cost, weight penalty, complexity, or otherwise. Justify adding features by increasing your net score.
  • Weight the focus you place on design for each event. Preoccupation on designing for the constraints from one event may make others worse. For example, a rock-crawler design might be slow and do poorly in Land Maneuverability or Endurance.
  • Prioritize design work based on capturing real improvements to your score over last year.
  • Have realistic goals for the scope of each year's design. Avoid the high risk of a complete re-design from which you enter the learning curve from next to zero prior experience. Treat each year's car as a continuous improvement over the past year. Even with this attitude, cars 2-3 years apart chronologically can look and perform drastically different through incremental changes.

Preparing for Competition

Duty Roster for Competition

Nothing is more frustrating than having a broken car during a dynamic day and a bunch of people standing around it with their hands in their pockets. Another bad situation is if one person works on the car and refuses to accept help. For this reason you need to establish leadership in the pit, specific jobs for everybody to do, and good communication of everything that is going on during the day. Any team member that doesn't have a duty on the roster should be walking around the competition to check out other teams' cars and take notes. Create a worksheet for recording these competitive analysis notes.


Training the Drivers

Don't learn the characteristics of your car and how it handles at competition -- that's too late. You should have finished building the car in time to evaluate your design, fix design or fabrication mistakes, and practice driving. Two benefits of driver practice are that you teach your members to drive the car and you can more fairly select the best driver for each event. Even if you can't use a practice track, you should be able to learn most of the important driver skills on a flat, dirt or gravel lot. For specifics, see this page for Driver Training exercises.

Spare Parts

Get bolt/nut organizer boxes and fill them with the fasteners and small miscellaneous parts (e.g. cotter pins) that you need to service the car. ONLY the parts you need. Keep these separate from the myriad loose/random fasteners that you'll pack but probably don't need. It saves a lot of time, and time spent fixing the car equals points lost in the competition.


Similarly, pack at least two small red toolboxes with only the tools you need to service the car. Add zip ties, safety wire, duck tape, etc. so you can do field repairs as you are permitted. Some teams call this a "Go-box." If you make it into the top-10 cars you will need a Go-box to disassemble parts of the car as instructed for the post-endurance race inspection.

Personal Packing List

Don't forget any important gear you might need in your rush to pack your bags to go to competition. Here's a packing list Patrick Lewis made for the 2010 season: BajaSAE_Competition_Packing_List.pdf

BajaSAE Competition Packing List.JPG

Packing the Truck

Ratchet straps. Lots of ratchet straps. But seriously, pack smart and pack efficiently.

  • Tool chests and cabinets should be ratchet strapped to the wall of the truck. They are extremely heavy and you do not want them to shift during transit. The side with sliding drawers should face the wall so they cannot open and spill their contents.
  • Strap in the car to brace it for both forward and backward motion in the truck. Strap it against a tool cabinet or another solid object in the front of the truck if possible. Use padding to keep abrasion from the straps from damaging the car's paint.
  • Pack spare parts, fasteners, and other equipment in boxes, cabinets, or Rubbermaid totes to keep the clutter under control.
  • Pack the spare parts you need specifically for your car together and all surplus fasteners separately -- don't waste time searching a big box of random fasteners for special nuts and bolts you need for your car.
  • Load the truck last with luggage, food, and your club's personal effects that you want to get to quickly when you arrive at a hotel.

Race Fuel

Get fresh, ethanol-free gasoline to run in your Baja Car and in any Briggs & Stratton small engine you wish to keep running well. B&S engines do not run well with common pump gasoline (up to 10% ethanol). The ethanol has less energy, so the power output is lower. More importantly, many of the engine components are damaged by ethanol -- seals degrade and some metal parts rust. You might not care about your lawnmower, but you want to find ethanol-free gasoline for racing . You will usually find this gas in 89-91 octane and is specifically labeled ethanol-free.

Static Event and Technical Inspection Strategy

Design Report and Judging

Cost Report

  • Completeness
    • Include every system on the car and name every part of every system
    • Include all required receipts and invoices
    • Provide G-code and other CAD-CAM documentation for CNC processes to defend machine time
    • Provide notes on manual machining operations including a list of machining steps
  • Accurate and frugal cost
    • Make material and process decisions during the design stage with cost in mind.
    • A part that you would shear or punch should be costed that way -- cuts and drilled holes are more expensive.
    • Avoid massive penalties during an audit by correctly estimating prototype cost on the Cost Report and Cost Adjustment.
    • There is no benefit from adjusting prototype cost down with the Cost Adjustment, so do not overestimate by unreasonable amounts.

Business Presentation

Technical Inspection

Brake Test

  • Balance your brake system for the optimum front/rear distribution brake bias to the front due to weight transfer
  • Increase tire inflation for reduced traction on asphalt if possible
  • Use your lightest driver to reduce brake effort to lock tires, but strong legs might be more important if you have inadequate hydraulic and mechanical brake gain.
  • Make sure the brake pads and rotors are cleaned with brake cleaner and properly broken in, a process known as Bedding-in.
  • Use brand new brake fluid at competition because 'wet' fluid has a lower boiling temperature and can cause a phenomenon called brake fade. Brake fluid is hygroscopic and can absorb water in as quickly as one week.

Dynamic Event Strategy

Main Article: How to Drive Baja

Order of Events

Generally the suggested order of events is:

  1. Acceleration
  2. Maneuverability
  3. Load-Pull
  4. Suspension and Traction, (Design Presentation given before Suspension and Traction)
  5. Mud Bog

This order is based on the ranking of events least likely to break something, and thus being taken out of the day to repair something.

However, if the car is sufficiently tested and breaking things is not a concern, then the priority becomes the drivers knowledge of the track and tuning of the car. With this in mind the order becomes:

  1. First Acceleration Run
  2. Tune for next run
  3. Second Acceleration Run
  4. First Load Pull
  5. Tune for next run
  6. Second Load Pull
  7. First Run Maneuverability
  8. First Run S&T
  9. Tune for next run
  10. Second Run Maneuverability
  11. Tune for next run
  12. Second Run S&T

This order gives the maximum amount of time for drivers to examine and think about how to approach the Maneuverability and S&T course


  • Drive smart. To finish first, first finish. Nobody wins the endurance race in the 1st lap, but many nervous and inexperienced drivers will wreck their cars beyond repair because of driver error. This goes for the first few laps of the replacement driver at the halfway point too. Drive smart and stay away from the bad drivers. Don't pass on the inside of a corner in most situations because the other driver won't know you are there (they can't hear you over the sound of their engine, and they can't see you because almost no cars use side view mirrors).
  • Fast Pit Stops can advance your car by the equivalent of a lap or two, which is often the difference among the top teams in the 'fast pack' on the track. Practice quickly exiting the car, refueling, exchanging drivers, and buckling up the driver.
  • Fast repairs - organize your tools and spare parts for quick repairs. Communicate your failed components quickly back to the pit so the team can prepare everything needed to do the job before the car even gets towed back. With difficult jobs, sometimes working slowly and carefully is faster that rushing and making mistakes only to bring the car back into the pits from another incomplete lap. Design your car so that all maintenance and component swaps can be done as quickly as possible, including engine replacement.
  • Have spares - a spare upright, A-arm, CVT, gearbox, tire... anything you might be tempted to go without because of a lack of funds or time to fabricate could cost you up to 400 points if it fails during the endurance race and you are unable to continue without it. Heed Murphy's Law; anything that can go wrong will go wrong and in racing, the failed part that most likely ruins your event is the one you don't have a spare of.


A car that performs well in Acceleration will do well in almost every other event also. No top ten team scores poorly in acceleration because it is fundamental to a high overall score. [need link to a page that explains modeling acceleration times]

Areas of focus:

  • Reduce Mass. The Chief Engineer, Drivetrain Lead, and Suspension Lead should all scrutinize every part of the car to demand that they are as light as possible, within the constraint of reasonable cost. You can elect a committee member to the design board who can focus on mass reduction (nickname: "Weight Weenie").
  • Reduce rotating inertia. Accelerating rotating parts is kinetic energy that is stolen from the longitudinal acceleration of the vehicle. [need link to a page that discusses rotating inertia and calculation of effective mass at the tire -- also a page showing how to use the pendulum to experimentally measure parts' inertia]
  • Transmission ratio selection and efficiency
  • Efficient suspension alignment. Minimize sources of rolling resistance: limit camber and toe to reduce wheel scrub.

Consideration for other events

The compounding effect of quickly accelerating between obstacles and between turns makes Acceleration worth focusing on as a primary design objective. Likewise, slow vehicles will take compounding point losses in every event, especially Endurance.

On-Site Tuning Tips:

The key here is to maximize efficiency and immediacy of getting engine power to the ground.

  • As much as suspension allows, set all four wheels to 0 degrees toe and 0 degrees camber. This will minimize energy spent deforming the tires.
  • Fill tires to max PSI (usually 7) perhaps a bit more (8 or 9psi). Same principle here, less compliant tires deform less and thus less energy is put into doing so.
  • If wheel spin is occurring, consult the wheel spin section of Load Pull, the same principles apply here.

Load Pull

Design tips:

Load Pull is a low-speed event that you can design for using dynamic and steady-state free body diagrams. The main goal is to increase the tractive effort that the rear tires can apply to pull the load, which is usually a tractor-pull machine, heavy boat anchor chain, or a large object like a bus or truck. In scenarios like pulling a truck, initial launch performance and tractive effort combine for a score based solely on time (incomplete pulls usually earn very few points). However, tractor-pull machines and chains which progressively add load the further the vehicle travels are judged almost completely by tractive effort because teams are not expected to max out the load, what is called a 'full pull.' Each scenario requires a slightly different strategy.

Main Design Factors:

  • Tires - tire traction is obviously the most important factor. All other design factors are meant to allow the tires to apply the maximum force on the ground. Tire tread and compound interact with the ground surface at the tire contact patch. The capacity for traction increases proportionally with load on the rear tires and with the size of the tire contact patch. Tire pressure is a major factor for controlling the size of the contact patch, but lowering it too far can increase rolling resistance and reduce tractive effort.
  • Center of gravity - The height and fore/aft position is a factor for the weight transfer and tractive effort calculations
  • Hitch height - the load on the vehicle acts through the hitch point. If the hitch is too high the car will wheelie and reduce the force applied to the load. If it is too low the weight will not transfer to the rear tires for greater traction. Perhaps the best location is slightly below the height that would cause a wheelie.
  • Transmission - The final drive ratio, transmission ratio, and launch performance combine to determine how much tractive effort the tires will have during the Load Pull event. The ratios determine the torque the engine applies to the rear wheel, and the transmission tuning dictates how much torque the engine can supply as the vehicle launches from the start while slipping until fully engaging.

On site Tuning tips:

  • If the rear wheels spin:
    • Let more air out of tires - This is an old rock crawler trick, not only does letting air out of the tires increase the size of the tire contact patch but it also makes the tire more compliant and thus conform to the road surface better and lend more grip.
      • Downside: More compliant tires means higher rolling resistance and reduces speed for timed runs
    • Increase rear wheel loading - Simply put this mean increasing the torque about CG as well the vertical component of force on the tow hitch. This can be done by:
      • Increasing spring rate and/or preload of the rear shocks - This will raise the car and the force applied to the rear wheels
      • Increasing damping rate - This will only aid with getting started and may reduce traction on bumpy surfaces and thus should be used with caution, the additional stiffness will apply increased force
  • If the car wheelies:
    • lower the ride height of the car's rear so that the moment created by tractive forces is reduced
      • decrease the spring rate and/or preload of the rear shocks
      • reduce the length of extension limiting straps or other devices on the rear shocks
  • If you have a long time on a timed event
    • Brake stall the car on the starting line to increase the initial torque from the engine during launch (appropriate for CVTs, centrifugal clutches, and Torque Converters).

Land Maneuverability

Main Design Factors:

Track and Wheelbase

On site Tuning tips:

Suspension and Traction

Main Design Factors:

Ride Height and Travel

On site Tuning tips:

Mud Bog

On site Tuning tips:


(This event might be discontinued since 2012)

  • Use chevron pattern tires in the rear installed backwards as your propulsion. They act as paddles.
  • The vehicle's normal steering input to front tires can act as rudders
  • TTU's 2010 car used cutting brakes and an open differential on the rear wheels to lock an inside wheel and create 100% power flow to the outside wheel for sharp turns to maneuver around buoys
  • Adding fenders to split the rooster tail of water and redirect it rearward greatly improves the vehicle's speed in water
  • Design a floatation device that can keep the wheels submerged to approximately half their diameter. Any more and the tires are just churning water.

Consideration for other events

Trade-offs with Endurance:

  • Endurance courses at a competition including a water event will include at least a small section of water. Competition sites like RIT (Hogback Hill at Palmyra, NY) have approximately 100 seconds of water per 530 second endurance lap time. Major gains in lap time can be made by improving speed in water due to the large percentage of time in water.
  • The floatation device must remain attached and keep its buoyancy throughout the endurance race. Protect the bottom of the floatation from getting gouged by rocks and obstacles (several layers of fiberglass has proved to be good). Make sure to use robust fastening to the vehicle so that the floatation does not fall off (for example, robustly anchor webbing belts in the floatation and strap them to the vehicle; ratchet straps are perfect but slightly heavy).

Trade-offs with Dynamic events:

  • Although sand tires with their large paddle design appear perfect for propelling the car in water, they are untenable on dirt and grass for all other events.
  • Lighter cars are always better. A lighter vehicle requires less floatation, and less floatation means better ground clearance and clearance between the wheels.
  • Most teams will have poor performance on suspension elements because of floatation and event organizers will usually tame jumps, logs, and other obstacles on the endurance course. Lower your expectations for vehicle handling and suspension requirements compared to a Midwest or West style competition.