SAE Baja design

[Forspeedrs]

I am new to this site; I stumbled across it for the first time tonight.

I’m a junior in mechanical engineering, and partake in SAE’s intercollegiate design competition known as SAE Baja. Basically we design and build an off-road vehicle with a 10 hp Briggs and Stratton engine.

Although it sounds like a glorified go kart, we do try to engineer it to the best of our abilities. Since we are power limited, weight plays a huge roll in performance.

One problem last year’s vehicle exhibited was the severe tendency to understeer. This was an ongoing argument on how to fix the issue, where many believed it was a steering geometry problem. The car has 4 wheel independent suspension (SLA front and rear). I on the other hand felt that the condition was due mostly to the locked rear axle that provides severe yaw damping. Just to prove it, I removed a half shaft and drove the car… needless to say, it handled outstanding in comparison, with a bit of oversteer. Based on the CG, the car should want to oversteer, as the majority of weight is concentrated toward the back (approximately 40/60 weight distribution).

This season’s solution has been to switch to the use of a differential. I’m sure that this will help, but again, the issue here is weight. Next year I will head the suspension design as my senior design project. I would like to use the locked axle because of decreased weight, better packaging, and the traction limitations we encounter (the locked rear definitely outperforms the differential when it comes time for serious off-roading).

My question is, if I keep enough parameters leaning toward oversteer, will it be enough to counter the yaw damping of the axle?

I plan to keep the rear RCH up, (not too high though), to get the roll axis to be high in the back/ low in the front. Also, we don’t run anti roll bars because of some of the events such as rock crawling, yet I feel if we had added some more roll stiffness to the rear, this would help as well, leaving the option to disconnect the anti-roll bar for certain events.

I could go on all night, but I would like some initial feedback, ideas, criticism, etc. All are welcome.

Thanks

Lou

 

[patprimmer]

You will need a differential to overcome the initial under steer problem.

A steering brake also works wonders at creating turn in on very loose surfaces with most weight over the rear axle.

Regards

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[JHarshbarger]

Our SAE Baja (University of South Florida) uses a swing arm rear suspension. This system always gets ragged on by the judges for using a primative design that has many disadvantages from an independent rear. However, our team consistently finishes first in agility, endurance, and even maneuverability. The swing arm is very light (made from welded laser cut plate steel) and gives the car a very strong oversteer behavior.

I have seen many teams that try to "kill" their rear traction in order to get their car to oversteer. Although I agree with your decision to not run ARB's in order to increase articulation, the SAE Baja series does not require a lot of articulation and instead favors speed and agility. I believe that cornering speeds that our car sees is mainly due to the very high rear end roll stiffness and the 5-speed transmission. Although our Baja gets rocked from time to time going over logs and other one-wheel-bump obstacles, this is only a small part of the competition. A removeable/adjustable ARB would give you an advantage in all events, but it would just add more weight.

If you look at some of the racing ATV's on the market, you'll notice that many of them use a swing arm setup (ie. Polaris Predator, Yamaha Raptor). In fact, those ATV's that have gone with the independent rear have not had the same success in the racing industry.

I'm not saying that you can't get good performance out of a independent rear suspension. This is just would I've heard and what has worked for our team. The majority of the teams use independent rear suspension and many have been successful with it. I just thought I'd give you the view from the other end of the spectrum.


-Joel

 

[Forspeedrs]

Thanks Joel,

I saw you guys flying around the track in Wisconsin last year, No matter what the judges say, your team gets noticed at these events, I think you guys definitely have something good going on. I agree with the idea on the swing arm, we have an old car with tons of issues, but the swing arm does result in a decent handling car with a good bit of oversteer (obviously also a high rear roll stiffness. This year we toyed with the idea of resorting back to a swing arm, but again resulted back toward independent rear, w/ the differential. We'll just have to wait and see how it works out.

 

[Fabrico]

Sounds just like the big off-road cars. Rear, and most mid-engine cars have open differentials. Front engine are all locked. With that much weight on the rear and IRS, you should not need a locked rear. An open diff would allow you to soften the suspension, improve handling, and possibly achieve even more traction than you have now. I know of at least one desert race Trophy Truck that has an on-off lever in the cab for the sway bar.