Thread: SAE Baja Car

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    SAE Baja Car
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    Although we are only able to run an engine with about 7% of the power most of you guys are running, SAE Baja is a pretty impressive collegiate competition. *This past year, myself and roughly 4 other guys (technically 8 in the group) started the first team our school (WPI in Worcester, Mass) has campaigned since SAE’s first competition in the 70s. *We went at it with every minute of time we could and did some pretty impressive fab work and design. *I’ve been lurking on the site for a few months now and have seen reference to mini baja in other posts, so I assume most of you are aware of the basic rules (10hp Briggs and Stratton engine and safety stuff) so I wont go into any detail there. *We were only able to go to one competition this year ( the world challenge in Rochester, NY) as our budget and time constraints ran out. *But, we did manage to get 30th overall out of 140 teams even with a huge disadvantage in engine/ cvt tuning. *We were making about 5.5 hp instead of 9 possible. *We neglected to dyno the engine, and with the progressive governor, our CVT was set at a very low power rpm. *We also won rookie team of the year. *

    Anyways, here is some detail on the car. *Hope you all enjoy. *

    Chassis
    -Full chassis finite element analysis using ProEngineer and a large number of loading cases
    -peak stresses based on fatigue analysis of 5000 6 foot landings
    -4130 tubing in 12 different o.d. and thickness geometries (1.25”o.d to .5” o.d)
    -fully tig welded using 4130 rod
    -entire chassis annealed post welding
    -entire chassis then sent for full heat treatment to a hardness of 29 on the C scale
    -raised tensile strength by 25% and increased the endurance limit significantly allowing us to design a lighter chassis with a higher safety factor
    -no brittle areas around heat affected weld areas as is common with 4130 fabrication
    -all tubes were filled with an inert gas (which flowed) during welding to minimize hydrogen embrittlement
    -gusset effectiveness was analyzed using ansys to provide the best gusset geometry for a number of different joint cases ( gussets are actually most effective on the outside of tubes, not the center)
    -some of the chassis uses sheet construction. *Here dimpled holes were implemented to increase torsional rigidity by 216%, again finite element analysis was performed with a variety of geometries and loading cases to validate this

    Suspension
    -full kinematic modeling
    -double A arm front, non parallel, unequal length arms, fully adjustable coilovers, roughly 12” of travel without limit straps
    7 degrees negative camber gain
    raked mounts 20 degrees
    10 degrees of castor
    next to no toe change
    -semi trailing arm rear
    10” of travel fully adjustable remote reservoir coilover
    6 degrees of negative camber gain
    anti roll bar (tunable)
    minimal toe change

    Steering
    -rack and pinion
    4 to 6 degrees of ackerman angle depending on suspension travel
    extremely minimal bump steer

    Drivetrain
    Polaris P90 CVT
    2speed with reverse trans (never used low, only used reverse once when someone landed on me)
    final chain reduction
    it only went 30mph top speed

    there is so much more tech information if you are interested

    Website….

    http://users.wpi.edu/~minibaja/

    Here are some pictures of fab and testing….
























    We are going back next year. *Everyone in the picture above (except for the advisor) is doing graduate research on the car including data acquisition on damper tuning, heat treatment analysis, and CVT/ driveline efficiencies. *The rearend of the chassis is geting re-worked. *The front suspension might see a some serious modification as well. *We will be going back with full power, single speed, and a much more efficient driveline, much better damper tuning, and a lighter overall vehicle. *It should yield some serious improvements in results.
    WPI Baja SAE and Formula SAE
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    Re: SAE Baja Car
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    Protodie Master and Vendor ProtoDie's Avatar
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    Looks like you guys did some real nice work
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    Re: SAE Baja Car
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    The Wizard bdkw1's Avatar
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    That looks like to much fun!

    If your redoing the back half, try to get the motor down a little for a better COG.
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    Re: SAE Baja Car
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    I'm currently working on the cad model for the changes to the rear of the car. *The engine is much lower by 8 inches or so, limited by the tight fit between the semi trailing arms and the drive clutch. *It is also pushed forward towards the overall center of mass. *This will necessitate a new cvt center distance and single reduction transaxle. *I'm in the basic dimensions and packaging phase right now to get an idea of the size envelope we have to work with. *Thinking about a triple row 35 series chain to get the final sprocket size down a bit for better cv angles with the same ground clearance. *Also, the entire drivetrain will likely be mounted on an aluminum subframe mounted to the rest of the car for ease of maintenance and facilitating future design changes.
    WPI Baja SAE and Formula SAE
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    Re: SAE Baja Car
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    Super Moderator minibajaman's Avatar
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    You guys did an awesome job for a first year team. *This season was my last and I miss it already. *I was on the SUNY-Buffalo team.

    A couple things on your car:

    Frame:
    The use of frame FEA is great and the judges love that, which is a good start for a first year team.
    Analyzing gusset locations and sizing them appropriately is also very cool.
    However using 12 different tube sizes means higher manufacturing costs and lower mass producibility. *Sometimes using a few larger tubes instead of a bunch of small ones will yield similar stiffness and better manufacturability.
    The post welding normalizing is a very good idea; one of the first frames I built cracked at a HAZ a few years later when a teammate ran it into a tree.
    However full frame heat treating, while pretty cool, does really not seem necessary to me. *A frame built to the rules and properly triangulated will never catastrophically fail, and long term durability is not really a concern for our program.

    Suspension kinematic modeling probably helped you guys quite a bit since you had no previous vehicle to base your designs on, so that is a good use of that technology.
    The trailing arms look nice and stout.
    10 degrees of caster is quite a bit, you might want to try other settings if you can adjust it and see how it feels.

    As you have found that Polaris gearbox is not the best or most efficient; *my team used that before I joined about 7 years ago and after changing we never looked back, but another member is working on his own two speed gearbox design.
    Also as you found driveline efficiency is very important for low horsepower applications. *
    Make sure your CV joints move as freely as possible. *Every year I go thru mine and use a sanding drum on a Dremel to touch up the cages and races so that the balls can freely move around with no binding whatsoever. *It really seemed to help this year, especially since I rebuilt the gearbox too, as our car was one of the fastest on the track which contributed to our 2nd place Enduro finish. *Well that and my amazing driving skills .
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    Re: SAE Baja Car
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    You are absolutely correct about the mass producability of this chassis. *It is so far from producable its rediculous. *Simply in terms of welding, I believe there are between 120 to 140 hours of welding done to the car based on our consumption of gas. *The notching had to be extremely tight fitting. *There are crazy nodes everywhere. *It is definately not designed with production in mind. *It took us 73 days (maybe 74) to produce the chassis and suspension arms.

    We chose to use the project as our major qualifying project and kind of went crazy on the materials and structural engineering behind it. *We were also blessed with a team of very competent fabricators and we were all willing to spend way to much time in the shop producing our design. *Without an idea of the true loading cases, I think we over estimated loads in our fea. *As such, with the heat treatment in mind, the chassis is designed to handle more, and for a longer period of time, than what it really sees in competition. *Strain gauges and data aquisition will allow us to figure out how overbuilt it really is. *And, with that, the 2009 team can build a new, lighter, chassis. *Even so, it is fairly light, even with much more length in tubing than other teams. *This chassis will last a long time as a good test pig for years to come. *

    The trailing arms actually cracked. *We fixed them before the cracks propogated by boxing in much of the arm with dimpled sheet. *There should be some pictures somewhere. *After some testing with sensors on the car, we have found that the rear coilovers are bottoming out on hard woops and landings. *An increase in spring rate here should significantly decrease the stresses on the trailing arms. *Next year they will be made out of sheet and much lighter.

    I agree on the castor, its being considered with the redisign. *Although, the biggest issue we've found is the increase in steering effort as the rack gets closer to its maximum travel. *Changing the geometry and location of the rack should fix this change in steering velocity issue. *

    I'm sitting in the lab today working on the driveline design changes. *I graduated this past year as well and came back to grad school here because of this project. *My sophomore year I lost my way and took on civil engineering as a double major only to have an epiphany with ME through this project. *There are a bunch of tours coming through today and I cant shutup about how great the project is, haha.
    WPI Baja SAE and Formula SAE
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    Re: SAE Baja Car
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    Minibajaman- have you all ever mounted the engine at an angle in the chassis?

    I'm playing with the assembly model and tilting the engine forward 10 degrees will help with packaging quite a bit. *This seems within the 15 degree range I've heard of, and I cant imagine a 10 degree tilt will effect efficiency much, especially with the tiny amount of oil that goes into these things.
    WPI Baja SAE and Formula SAE
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    Re: SAE Baja Car
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    Super Moderator minibajaman's Avatar
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    Quote Originally Posted by bmies\";p=\"41199
    Minibajaman- have you all ever mounted the engine at an angle in the chassis?
    Yes, the last two cars built have the engines tilted at approx. 7 degrees. *No problems whatsoever. *Since the car is constantly bouncing around and driving on hills and such anyway I can't see why it would make much of a difference, but sticking to the Briggs number of max. 15 degress is probably a good idea.

    I'll post some pics of the latest car when I get home from work.
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    Re: SAE Baja Car
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    The Bob Ross of MBN Bullnerd's Avatar
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    Very cool .

    What did you use,where did you get, the front upright/spindles/balljoints from?Polaris?

    Hope to see more in the future.
    "Speed is time-time is speed"-Dennis Hopper

    Quote Originally Posted by TALON View Post
    did you use a special bigfoot camera or something ,you know all blurry could be a tree stump kinda thing .
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    Re: SAE Baja Car
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    Admin Gene's Avatar
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    Great thread and details. I feel smarter already!
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