School Project. Brainstorming Help.

[BP-Revo]

Hey everyone,

So this quarter I'm taking a class at my school called Intro to Mechanical Design. Besides the basic homework and quizzes/tests, there are projects. Basically, we are given requirements and have to design something to fit those requirements, and turn in a design report. We also have to make an oral presentation to the class on how it works.

It's groups of 3, and you all get the same grade on the project but oral grades are individual.

The first project we did pretty well. 98% on the project and I got 90% on the oral report while my other two partners got 88%. Second project we just presented and haven't gotten back, but he's already given us the third project, and this is why I've come to you guys for some extra ideas.

Basically we have to design a model car (an "RC" - except that it's not remote controlled) to meet some requirements. The requirements are...

It has to be at least rear wheel drive (4WD is optional).
It will run off a radio shack DC motor on two AA batteries.
Maximum wheel size is 7 inches wide, 10 inches long.
Wheels are a maximum 1.5 inch diameter.
Manually steerable (basically aim the wheels and lock them in position)
Not be a commercially sold kit (so I can't just get an RC18T and drop the motor in lol)

The car has to be capable of doing a series of "challenges."

The challenges consist of:

25 foot long track with .5inch tall bumps. Whoever can get to the end quickest without hitting the walls of the course the quickest. Basically an off-road drag race of sorts. I've seen the course and it's more or less just wooden slats. (Quickest one get's extra credit.)

Hill Climb. Basically there is a board with an adjustable angle. Whoever can keep climbing at the steepest angle wins. (Highest angle gets extra credit.)

Tractor Pull. Minimum weight that can be pulled is 30lbs. However, the more we pull the more points we get. 90lbs is the max, if you can pull 90 you can make an attempt to pull a Chevy 357 V8 that's on an engine stand. (If you can pull that you get extra credit.)

Hill/Valley style "off road" track. It's just dips and hills and you have to not get stuck/high centered. Quickest to make it through wins. (Extra credit for being fastest).

Circle a 6 foot diameter circle without deviating more than 1 foot to either side (this is where the steering comes in). (Fastest time gets extra credit)

We will allowed to change things like gear ratio in between each event.

So the goal is to win as many as possible and spend the least amount of money. Apparently some groups in the past have spent over 600 dollars building it with a custom machined carbon fiber chassis and still didn't win.

My group is looking at me since I'm the RC Guru. My RC experience sure helped with the first project.

I've got a few ideas down already.

I've decided that I want it to be 4wd (ideally), though it might change if it gets really complicated. I was thinking to allow easy change of gear ratio's, I would have two gear reduction transmissions. I would make it so that I could swap out one of them for a simple shaft when I want a higher ratio, or keep both in for a super low ratio.

I've also decided that I'm going to use a friction drive with a wheel on a wheel to get more ground clearance, since the maximum tire size is only 1.5" and the obstacles are .5". (Basically, two wheels, one on the other - the upper one is the one held by the suspension arm and driven, and the lower one is driven by the upper one). Though I was thinking I want it to be quickly convertible back to using one tire to lower it. I figure it would make it better for the hill climb.

What I'm trying to work out is the suspension while keeping drive to all the wheels. I doubt the professor wants us to just slap on an RC suspension assembly (though I could very well do that)...I have a Mini E-Revo.

Right now I'm thinking a solid axle setup? It would work very well through the rough sections and it wouldn't have a negative effect on the circle course since that course is flat. I can only see a possible issue on the tractor pull.

Any ideas? We need to get started quick, it's due March 11 (just before finals). I don't want to have this taking all my time last moment and screwing my other classes over.

For materials I have quite a bit of 2.3mm 6061 aluminum sheet that I figured we could use extensively for the chassis and other main plates.

PS. I'm going to ask the professor if I can bring in my 4S Neu 1112 Powered Mini E-Revo to do the challenges just for kicks.

[JThiessen]

Why bother with the complexity of a suspension? Just a thought.

Also, you might consider the benefits and drawback of using spring loaded self centering steering. Might help on the obstacle course.

[BP-Revo]

From my understanding there has to be some sort of suspension system.

[lincpimp]

Front wheel drive will help with most of the challenges as you will be driving the steered wheels. Since you have to set the car up and let it go you want any bumps to have the least effect possible. I would go with solid axle design, with a diff loaded with heavy fluid. 3v max to a small motor will not be much. You will have to gear it uber low to pull 30lbs...

[What's_nitro?]

Could you use this? I think it might use the same motor, just a hunch. Just need to find a way to mate the output to a diff. "8 different gear ratios (4:1, 5:1, 16:1, 20:1, 25:1, 80:1, 100:1, 400:1."

[rabosi]

To me it seems like a crawler type (low geared) vehicle might be more effective in most of the challenges. With preset steering I think you wouldn't want it going very fast because bumps will probably throw it off course the faster it goes.

[BP-Revo]

That tranny is an excellent find What's_nitro? Thanks!

If I can buy two and then hook a bunch of them up in a row, I could get a reduction of 160,000:1, which I think would be more than enough needed.

Linc, since for the towing competition there is no speed requirement, I could have it geared down so low it takes 10 minutes to move 1 inch and it'll be fine. As long as it tows it.

I also agree that solid axle seems like the best bet at this point.

rabosi,

Very good point. I think it will be more ideal to accelerate quickly out of the hole and maintain a decent average speed rather than gear it too tall so acceleration is hampered and then have the vehicle unstable as the speed increases.

[What's_nitro?]

No problem dude! I see those Tamiya gearboxes every time I browse the Tower site... Until now I had no idea what to use them for!

[BP-Revo]

Hahaha. Yeah, they're prefect if you want a gear ratio reduction of 160K to 1!

[What's_nitro?]

You know, if that motor puts out 10K rpm you'd be looking at 1/16 rpm at the wheel...

[BP-Revo]

Lol. The motor probably puts out less to be honest. I don't think I'd realistically use the full 160K to 1. Probably more like 40,000:1. But if I end up needing the extra torque, it'd nice to know I'd have it!

[What's_nitro?]

Just remember- those gears are plastic!!!

[shaunjohnson]

hehe...see if you cant get a heading hold gyro to work so it wont hit the walls then you can make it go as fast as the AA's will allow

[Finnster]

Another option is the worm gear axles from a losi mini crawler. Won't have the native gear reduction the tamiyas would, but they are v strong and wont break. Perhaps you could even hook them together.. IDK.

Tamiya also makes a worm gear box like the one posted above.

I think Linc is right on making it FWD. May help with the pulling as well. RWD would likely just backflip. A crawler type veh seems best suited for this challenge

[simplechamp]

Originally Posted by lincpimp

Front wheel drive will help with most of the challenges as you will be driving the steered wheels.

Originally Posted by Finnster

I think Linc is right on making it FWD. May help with the pulling as well. RWD would likely just backflip.

The rules he posted says it has to be either RWD or 4WD.

Are the batteries going to be alkaline or NiMH?