This week: Chassis

According to SAE (Society of Automotive Engineers) rules, each Baja team may use a frame no more than two years.  This is mostly for safety reasons, as the main part of the frame is the roll cage, which protects the driver in a rollover.

By our team’s definition, chassis and frame are somewhat interchangeable.  Technically the chassis is a broader reference to the vehicle, including the frame, suspension, and wheels.  When designing the frame, you are also designing the chassis, since small changes in the frame hugely influence chassis.

This year in our chassis design, the main goals were stiffness, a 50/50 weigh distribution, and a shorter wheelbase.  I will go more in depth into each of these design goals.  Below was the final design of our frame.

Source: Author

Stiffness, not to be confused with strength – which defines how hard the frame is to actually break – is the ability of the frame to resist deformation, such as twisting or bending.   When your car hits a bump, you want your shocks to be taking the hit.  Because of this, the stiffness of the frame should be ten times the stiffness of the shocks, to make sure that the shocks are actually doing their job.

Weight distribution is a very import aspect of any vehicle, especially a small Baja racer.  Weight distribution simply is where the weight of the car sits on the tires.  Some of the car’s weight will be on the front tires and some on the rear.  A 50/50 weight distribution is an even distribution of 50% of the weight on the front tires and 50% of the weight on the rear tires.  Pushing more weigh towards the rear will help you get more traction in your drive wheels, which could mean better acceleration.  More weigh in the front will help you get more traction with your steering wheels, which could mean better turning performance.  Finding the balance between these is important, and in our testing at the beginning of the year, we found that we could go from our previous 40/60 front/rear ratio to a 50/50 without losing much traction in the drive wheels.

Wheelbase can play a factor in stability and turning radius.  A longer car will be more stable, but will sacrifice a smaller turning radius.  One of the goals for our entire car this year was to get a smaller turning radius to improve our low speed handling, so we shortened the wheelbase of our car by 9 inches.  Additionally we determined that we would not sacrifice stability by shortening it as long as we made no changes to the width of the car.

This week: Competition

This week in Baja, we went to our competition.  This year’s competition was in Bellingham, WA and was comprised of both static and dynamic events.

Thursday and Friday consisted of the static events, including a sales presentation, technical inspection, and design presentation.

The majority of the dynamic events were on Saturday.  These included a brakes test, then hill climb, acceleration, maneuverability, and rock crawl.  Sunday held the main event in store – a four hour endurance race with all the cars on the track at the same time.

Our sales presentation and design presentation both went quite well – we put a lot more focus into these than prior years.  Technical inspection led to a few issues: we had to replace a few frame members and modify our splash shield which keeps gas off of the engine.  After those were fixed, we were allowed to continue on into the dynamic day.

The first thing any car must do in order to be allowed to drive in any other dynamic event is to pass a brake test, in which it must get up to speed, then hit the brakes and lock up all four tires.  Dynamic day yields interesting strategic challenges, since weather can play such a massive role in the performance of our vehicle.  Early in the day it was muddy, and rain was forecast for the afternoon.  We decided to get our runs in at around noon, as it was getting dry, but the clouds were still very threatening.  Below is a picture of us preparing for a very muddy hill climb.

Source: Author

Unfortunately for us, the rain never came, so conditions continued to improve through the day, and teams that saved their runs for the end of the day benefited from much firmer ground, making our runs much less impressive.

Even with the worse conditions, we still had an excellent time through rock crawl, good enough to get us 5^th in the event (out of 80+).  Below is a picture of our car coming into the hardest corner of the very tough obstacle course.

Source: Author

As you can see some other teams did not fare quite as well as us during the rock crawl event.  This is one of several cars we saw flip at various sections of the rock crawl, which included climbing over 3 foot logs and up a foot and a half concrete wall.

Source: Author

The main event, the 4 hour endurance race worth 40% of the points for the entire competition is a challenge to teams’ ability to keep their car on the course.  Based on our past performance, we calculated that each 34 seconds we are off the track equates to 1 out of 1000 possible points for the competition.  This year, we had two minor fixes that took us off the track for a total of about 40 minutes.  Below is a picture of us (car 65) preparing for a pass during the endurance race.

Source: Author

At the end of the weekend of racing, we got 14^th place.  Our goal was to get into the top ten, but if a few more things had gone our way, we feel we would have made it.  If you are interested in seeing more in depth scoring or seeing what other schools we competed against, this link will lead you to a download of the excel file of all the final scores.
http://www.sae.org/servlets/pressRoom?OBJECT_TYPE=PressReleases&PAGE=showCDSNews&EVENT=BAJA&RELEASE_ID=2110

Source:  Author

This week: Steering

This week in Baja, it was crunch time.  With competition next week, everyone has been working hard to put the finishing touches on the car.  On Thursday, we got a few final parts in, and Friday night was spent on the final assembly of the car.  Today, we got our last day of testing in, and we feel confident in the abilities of our car.  Below is a picture of our competition ready car.  The only task that remains is putting on sponsorship stickers.

Source: Author

Rather than expounding on the difficulties of final assembly, I will go into a brief discussion of steering systems.  Most cars have fairly similar steering systems: the front wheels angle in the direction which you want to turn.  Steering is a bit more complicated, however, as there are several factors which can make the steering system better.  Ackerman steering is the practice of avoiding slip as can be seen below.

Source: http://www.beam-wiki.org/wiki/Steering_Techniques

In order for both front wheels to turn freely, without having to skid a bit sideways, is to have both tires travel in a circle about the center of the turning circle.  As you can see in the left picture, this means that the two front tires will no longer be parallel during turning for this to be the case.

An additional concern is called bump steer.  When you are driving along and you hit a bump, if the steering wheel tries to rip out of your hands, then you just experienced bump steer.  In the gif below, the suspension is articulating through its motion.  You can imagine that if bump steer is an issue, then the wheel would be turning slightly throughout the full range of motion of the suspension.

Source: http://en.wikipedia.org/wiki/Multi-link_suspension

An additional example of bump steer can be seen in this video.

Source: YouTube, Peter Basica

This week: Brakes

This week in Baja, we broke the car. Several gears in our reduction gearbox were damaged. This most likely occurred when one tooth broke, then bounced around in the gearbox, breaking other gears. Luckily, they were all smaller gears which can be replaced easily, and there was no additional damage to other gearbox components. The team is now waiting somewhat patiently for the inbound replacements.

Before the gearbox broke, I was preparing to modify the brakes to improve braking performance. To achieve the maximum possible deceleration due to braking, all the tires should be right at the verge of skidding. Anti-lock brakes keep all the wheels right at the verge of skidding. On a small vehicle without anti-lock brakes the ratio of braking between the front and rear tires depends on numerous factors.

The majority of braking is done by the front wheels since the deceleration of the car causes the weight to shift forward.  Anyone who has been in a car that slowed down somewhat quickly has felt these effects.  Due to this, more weight on the front tires means you can try to stop those tires more before you start them slipping.  The figure below illustrates how the rate of deceleration (directly related to the magnitude of friction with the ground) affects the ratio of braking forces required.  These are numbers based off the calculations I did this year to determine the size of brakes required.

Figure 1.  Contributions to stopping power from front and rear brakes
Source:    Author

If the ratio is not as this figure shows, then one set of tires will start skidding while the others are not sufficiently slowed. The video below shows what happens when your rear brakes start skidding first (also known as rear bias).

Source: Vimeo.com

As you can see, excessive rear balance can cause you to spin out.  On the other hand, excessive front balance will stop you, but only in a straight line.  Your skidding front tires will be unable to steer as they will just keep sliding, regardless of where they are pointing.

All of these descriptions should make you very happy to live in a day where anti-lock brakes keep most of this biasing out of the drivers control, as the anti-lock will keep the one set of wheels from skidding while still allowing the other set to continue increasing the braking force.

If you are interested in learning more about braking, this is an excellent presentation that I looked through a lot when I was initially learning about the surprisingly complicated braking system.
www.sae.org/students/presentations/brakes.ppt