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Old 04-09-2008, 02:01 PM   #1
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Default Need help building 3 wheel rc car

Hi everyone, similar to the previous thread, my group needs to build a rc car for an engineering class. We would appreciate any advice and sorry for the long post.

The requirements are:
3 wheels (2" diameter maximum)
Motor - Graupner speed 400 7.2V
Battery - 2500 maH 7.4V 20C LiPo
450 dollar budget for everything
Everything besides gears, wheels, tires, electronics, and suspension has to be fabricated
Track Surface - Is made of outdoor tiles with grout in between

3 Races - straight line, slalom, and towing (around 4-5)

The setup we have so far is 1 wheel rwd and 2 wheel steering in the front. Thus with the given conditions: would the advantage of having a rear suspension offset the added weight? Also, because its only 1 wheel drive, i'm worried about the traction for the towing. What tires would you recommend (something wide for the rear wheel)? And lastly, what would be the ideal weight distribution be for each case?

Thanks again!
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Old 04-09-2008, 05:30 PM   #2
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first off by no means am i a smart person im a second year mechanical engineer but trust me other people on here will blow me away, but i thought id help kick u off
for me the towing and slalom are the bigest problems for u the drag race will require the same qualities as the towing so u can kill two birds with one stone, personally id probably run a super soft rear suspension so that when u touch the throttle all the weight trans fers to the rear wheels which will br good for towing and drag race the flip side no on power steering so u have two option change springs between the events or just steer off power the later will be harder but depending on rules might be ur only option
also keep in mind with slalom u need quick off centre steering so keep your weight centralized this will help and maybe run a front sway bar.
so id run front independent suspension so a touring car set up or maybe 1/10th buggy, and rear pod running shocks instead of a t plate or link suspension kinda like running a solid rear diff in a race car the rear axle is solid but u still have suspension.
forgot to mention 1/8th scale on road will have the widest slick tire, there foam to so u can soak in tracktion compound for that holeshot advantage. so ide use them in the rear and run what ever the suspenion u use on the front the corresponding tire so 1/10th use 26 or 28mm foams
thats my 2 cents 90% sure some one else will have a better idea
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Old 04-09-2008, 06:02 PM   #3
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We did something very similar to this in high school physics, except we didn't have the slalom. We did the opposite of what you are currently purposing, we had two wheels in the back and one in the front. That third race adds an added complication. Ours were not RC either, but they used 540 motors and 7.2 volt batteries. For the drag race and the tow you really do want two drive tires in the rear, but at the same time you want two tires up from for steering. If you really want to get complicated you could build a "tank drive" of sorts. Drive the rear wheels with the motor, but have a diff and also a brake on both rear tires. Let the front wheel be a caster wheel or steer it as well. To turn left, slow down the left rear tire, to turn right slow down the right rear tire. If you also have a servo on the single front tire you can smooth out the abruptness of the rear steer method. The rear steer also helps you if you front end comes off the ground. Can you make any changes between events? If so you might want to consider an adjustable wheel base. Short for slalom and longer for the other two events.

How are the events weighted? Are they all equal? If they are, you should probably build a car that is best for the most events (drag and pull).

Keep us informed.
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Old 04-09-2008, 06:48 PM   #4
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man $450? thats going to be tough, or maybe not, since youre fabricating a chassis. I think you should make it two wheel RWD, and then a single damper wheel in the front, preferably an rc motorcycle wheel, which is round, and use a servo, to mover it side to side, with free right/left movement.
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Old 04-09-2008, 07:08 PM   #5
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If I had to build a three wheeled car I would always have two wheels upfront! Don't build it like a harley trike with the one wheel up front.

Second I would pick up a used 1:12 or 1:10 pan car.
You need to scavange from the car:
Front a-arms
Steering knuckles
Servo mounts.
Linkages and servo saver
Left Rear Axle Hub


You can use anything as your chassis.
Birtch Plywood for model airplanes,
PlexiGlass
G10 fiberglass sheet
Graphite.

You will need to buy
Four 1/4" setscrew collars (airplane)
Two Gear Hubs http://servocity.com/html/set_screw_hubs.html
1/4" round stock (K&S stock)
1/4" .020" shims


Next you need to build motor pod similar to what was on the pan car you bought. The only changes is that it needs to be longer to accomendate: Motor, Layshaft, and Wheel on the inside of the pod. The bulk heads of the motor pod do not need to be alumnium, they can be the same material as the chassis. You do not need to worry about milling out the rideheight spacers ovals, just drill a single hole for the bearing at the proper height since you already know your wheel diameter. Since the wheel is now inside the motor pod, a lay shaft is needed to add another gear into the system since a pinion will no longer reach the spur with the wheel in the way. You can use a 1:12 Left side hub to adapt a gear or you can buy a gear hub. For the inside of the motor pod you really need an 1:10 Left side hub that can be loosened without removing the wheel (pretty much not a pancake hub). This way the axle can slide though the bulkhead, then the wheel and then the bulkhead.

Before you start drilling on your bulk heads. Make sure the center distance between the spur and layshaft gear are proper and do a test on some scarp. When you get the center distance right drill a hole for a flanged bearing on both bulkheads at the same time. The motor mounting holes need to be on the LEFT side of the motor pod due to the layshaft.

I hope don't confuse you, but I am working on a three wheeled dragster so I already put thought and materials into it.
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Old 04-09-2008, 11:17 PM   #6
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Maybe you can get some ideas from this thing:

http://www.t-rexvehicles.com/

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Old 04-10-2008, 06:35 AM   #7
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Or here

http://spyder.brp.com/
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Old 04-10-2008, 07:41 AM   #8
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Why not make it all wheel drive?
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Old 04-10-2008, 07:48 AM   #9
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You get to do this for school? Sweet
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Old 04-10-2008, 10:48 AM   #10
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Quote:
Originally Posted by CCristo View Post
Why not make it all wheel drive?
He only gets one motor. The gearing to run all three wheels on one motor would be very inefficient.
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Old 04-10-2008, 10:53 AM   #11
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Another note: Make sure you hitch for the towing competition is higher or equal to the point on the load. If you hitch point is lower than the load/sled/chain, it will pick up the back end of your car.

Quote:
Track Surface - Is made of outdoor tiles with grout in between
Could you describe these a bit better? Are they ceramic? Rubber?
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Old 04-10-2008, 11:02 AM   #12
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Quote:
Originally Posted by kufman View Post
He only gets one motor. The gearing to run all three wheels on one motor would be very inefficient.
Touring cars only have 1 motor and they are all wheel drive.
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Old 04-10-2008, 11:09 AM   #13
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Quote:
Originally Posted by CCristo View Post
Touring cars only have 1 motor and they are all wheel drive.
But they are very inefficient and heavy compared to a pan car.
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Old 04-10-2008, 12:32 PM   #14
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I forgot to mention. If you can not use the the prefabed a-arms, you can just pinao wire folded into an V shape then insert a rubber gromet to act like a pivot ball. The radius bend in the piano wire should hold the grommet in place, if not just complete the radius more into a circle then form the V shape. This is now your "homemade" a-arms for top and bottom. Not sure of the size of grommet/washer but you will need one with an 1/8" hole for the king pin.



Also, for towing you will want a shorter wheel base. It will help with tighter turns. For drag racing you want a longer wheel base, but it is not as important as turning is for the towing course.
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Old 04-10-2008, 01:55 PM   #15
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If you are allowed to use two motors then just hook up a motor to each wheel & use steering like a tank. Not to mention you will have double the power. Then you can pick up a cheap 4 ch plane radio & use the 2 joysticks for your two throtle servos w/ reverse. I mean, two $50 escs, two $30 motors, a $50 radio, $40 in tires? That should easily land in your budget. You can build a chasis shaped like a T. Then go and mount the wheels w/ motors on each side & then mount the other wheel which is nothing but balance, you want it to slide around. If you are only allowed to use one motor then hook up the motor to a diff & use a braking system that will allow you to lock one wheel & deliver power to the other wheel.
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