Gravity Vehicle C

[Balsa Man]

Just a question: Are you guys using wood or metal chassis for the vehicle? And what type of wood/metal?

The reason I'm asking this is because I didn't compete my vehicle last competition because our wood chassis warped, and so it bounced when it rolled. Now that we're going to rebuild it, we're wondering how we should build now to ensure structure strength.

One of the many interesting challenges in this event. Especially if you're going to run near max weight (which is advantageous), the forces transitioning off the ramp are significant- which means, let's call it the chassis plate has to be quite strong and stiff (so it doesn't flex, allowing critically located bits-like wheels) to move around. Once we got an appreciation of the forces involved, we actually went to a balsa core/carbon fiber sandwich- very light, and very stiff. A cheaper way to get some of the advantage this approach offers would be wood lamination- balsa core (we're using 1/2 inch thick- the lighter the better), with balsa sheet lamination. Run the grain in opposite directions in core and the outer sandwich sheets; 1/32nd for sandwich sheets (look for good hi-density ones). Stiff, and it pretty much won't warp

Once you have a nice stiff chassis plate, then you have a workable base to get the wheels on so all 4 are in even ground contact - a delicate, but important step - and they won't move around, as in up and down/bouncing under the loading of runs.

[sj]

We are using a custom cut lexan chassis with an aluminum frame for stiffness... all in all the car weighs in at 655 grams and can hit 10 meters in about 4.5 seconds depending on the floor.

[twototwenty]

Being new to this topic, I was having trouble understanding the use of ball bearings in the vehicle? Would someone mind explaining how they would be used? I searched this thread, but I couldn't find a clear explanation.
Also, what are people doing to attach axels of the vehicle to the car? I am drawing a blank on how to attach them without causing a lot of friction.

[Balsa Man]

Being new to this topic, I was having trouble understanding the use of ball bearings in the vehicle? Would someone mind explaining how they would be used? I searched this thread, but I couldn't find a clear explanation.
Also, what are people doing to attach axels of the vehicle to the car? I am drawing a blank on how to attach them without causing a lot of friction.

You’ll have to work out the….little building details to implement it, but here’s a quick overview.
Bearings have an inner diameter, outer diameter, and width. You need to match these up with your axles and wheels. There are two….cases- “dead axle” and “live axle.” With a dead axle, the axle is attached to the chassis plate, with the ends sticking out beyond the edges. The axle doesn’t rotate- the bearings are in the wheels, and the wheels rotate on the axle. The outside diameter of the bearings needs to fit snugly in your wheels, and the inside diameter needs to fit snugly on the axle. You need to build a way that the bearings can’t slide along the axle, once you have them located/spaced properly. The tricky part is getting the bearings solidly in the wheels, and the wheels aligned so they don’t wobble too much. With the live axle (which is what you’ll have/need for a wingnut or string winder braking system), the wheels are fixed on the axle- the bearings are positioned either inside or outside the wheels, and the axle rotates inside the bearings. You set up bearing holders so that that the outside diameter of the bearings fits snugly into them, and the inside diameter of the bearings fits snugly on the axle. A way to do bearing carriers is use a piece of some kind of bar- aluminum, plexi, or lexan. For instance, we are using 1/8th axles, with ¼” o.d. bearings, 1/8th” wide. Bearing holders are ½” x 1/8th lexan bar, with slots filed in ¼” wide x ¼” deep. The tricky part with a live axle is getting/making sure the axle is straight; if its not, you will get wobble, which will cause vibration at speed, and movement of the bearings (or if the bearings are tightly held, binding).

Hope this helps

[illusionist]

For instance, we are using 1/8th axles, with ¼” o.d. bearings, 1/8th” wide. Bearing holders are ½” x 1/8th lexan bar, with slots filed in ¼” wide x ¼” deep.

1/8 inch axles? Is that really strong enough to support the weight of the vehicle?

[Balsa Man]

For instance, we are using 1/8th axles, with ¼” o.d. bearings, 1/8th” wide. Bearing holders are ½” x 1/8th lexan bar, with slots filed in ¼” wide x ¼” deep.

1/8 inch axles? Is that really strong enough to support the weight of the vehicle?

Sure is, and that's at a full 2.5kg.
It's just a matter of the right material.....

[NinjaChicken]

How long does it take to receive a response to a rules clarification? I submitted one pertaining to this event about a week and a half ago.

[chalker]

How long does it take to receive a response to a rules clarification? I submitted one pertaining to this event about a week and a half ago.

It can take several weeks. Multiple people have to review the questions / responses, and we all are volunteering our time to do that outside our 'real' jobs. In addition, many of us are helping run tournaments this time of year, so we are overloaded with things to do.

[NinjaChicken]

It can take several weeks. Multiple people have to review the questions / responses, and we all are volunteering our time to do that outside our 'real' jobs. In addition, many of us are helping run tournaments this time of year, so we are overloaded with things to do.

Yes, of course. I was just curious. I'm very appreciative (as I'm sure many others are) of how much you all care for the program, and I hope to join your ranks one day

Anyway, back to gravity vehicle. I have rubber wheels, and they can be 'squished' for lack of a better term at the moment. My vehicle weighs about .225 kg, and adding even 100 g to it will either not affect it at all or make it run slower (longer time). Is this because the extra weight is adding pressure to the wheels, 'squishing' them, and increasing friction?

[Balsa Man]

Anyway, back to gravity vehicle. I have rubber wheels, and they can be 'squished' for lack of a better term at the moment. My vehicle weighs about .225 kg, and adding even 100 g to it will either not affect it at all or make it run slower (longer time). Is this because the extra weight is adding pressure to the wheels, 'squishing' them, and increasing friction?

.
Can't be sure without seeing it, of course, but I suspect that may be what's going on; one form, or element of "rolling friction." It's not friction in the sense of something sliding, but it is energy being absorbed. You have a certain amount of momentum (mass x velocity) coming off the ramp; it gets absorbed by the various forms/sources of friction, and anything else going on that takes energy. The rate at which momentum gets "eaten up" is a function of velocity; a higher rate at higher speed. lower rate at lower speed. When your wheels "squish" (deform), that takes energy (it goes into producing heat); that energy suck/absorption goes on constantly as the wheels roll each little increment. At really light weight, the deformation is negligible. Depending on the rubber, there will be a range of load (vehicle weight) where the deformation will be minimal; then as load increases, deformation (energy appetite) increases, and its not linear (i.e., doubling the load more than doubles the the energy going into deformation). Sounds like the "squisiness factor" of your wheels is in a range where the ~50% increase in mass (fm 225 to 325gr) is producing an increase in deformation energy that's greater than the increased momentum you get from the mass increase.