Gravity Vehicle C

[fishman100]

Does anyone have anyone have any ideas for making sure your ramp is lined up with the dot. Because when we run our car it is always 2-5 cm to the right or left of the dot. So any ideas?

According to rule 4b, the center of the Start Line is marked and that mark is perpendicular to the Target point, so if you align your ramp with the dot on the start line and it ends up 2-5 cm to the side of Target Point, then your vehicle must be curving. I would check if the axles are parallel and your ramp is level (one side isn't higher than the other, etc) to ensure that the vehicle isn't curving.

If it does curve and the curve is consistent, then you could always move your ramp a few centimeters over so that the vehicle lands on the target point.

[mrsteven]

I'm running my axles on 1/4'' steel rods (one threaded for a break) and the wheels I ordered have a larger inner bore than a 1/4'' to insert. Any ideas on how to mount my wheels onto the axles so that they're in the center and fit? I'm a bit stumped.

For the threaded rod I'm thinking sandwiching between 2 bolts which I'm not sure I'd ever be able to keep center.
For the non-threaded rod, I have no idea

[Balsa Man]

I'm running my axles on 1/4'' steel rods (one threaded for a break) and the wheels I ordered have a larger inner bore than a 1/4'' to insert. Any ideas on how to mount my wheels onto the axles so that they're in the center and fit? I'm a bit stumped.

For the threaded rod I'm thinking sandwiching between 2 bolts which I'm not sure I'd ever be able to keep center.
For the non-threaded rod, I have no idea

Interesting challenge.
The threaded side is easier. You'll need a bushing, with an i.d. of 1/4", and an o.d. that matches the size of the hole in the wheel. Try your local Ace Hardware- they have (or at least the one's I've seen have) a very complete assortment of hardware bits that includes bushings of varouus sizes- metal and nylon; good chance they'll have something that will fit. The length of the bushing needs to be = or a bit less than the thickness of the wheel. An option, if not, is a bushing with the right o.d. to fit the wheel hole, bit w/ a smaller i.d. Easy to drill out the center hole w/ a 1/4" drill. Then use nuts and washers on both sides. If you run into wheel slipping/rotating on the axle, options are larger washers with a rubber washer against the wheel, or glueing (a problem if you need to take it apart), or drilling the axle (like a 1/32nd hole) putting little steel pin through that sticks out just a bit beyond the axle, like a 32nd or a 16th each side, then filing/Dremeling in little slots in the wheel next to the axle that the pin will engage in. Music wire (Hobbytown carries it) at 1/32nd" diameter works for pin making. Brass might work, and it's easier to work with.
On the non-threaded side, you'll probably need to work out some king of pinning, one to lock bushing to wheel, and then to lock bushing to the axle.

[mrsteven]

I'm thinking on bushing the threaded and gluing in place, similar with the nonthreaded. It is a bit more permanant, but I suspect I won't need to change them ever as long as they're initially aligned correctly.

With low ramps and such, I'm evaluating my designs for the car again for the best combination. Last year we saw lots of cars (a foot or longer) by 5-7 inches. With a shorter ramp and lower speeds, I'm thinking of narrowing my car and shrinking it. Maybe low end of the spectrum at 5 in wide and 11 inches between the axles. Doing so I can see that it has a much smaller profile than what I've seen thus far, however I can't see a large advantage to making it larger. If you have the same ratio of width to length for stability to go straight (Not perhaps an exact 'value' but a reasonable range), it shouldn't matter actually how long or wide you start, if my logic is correct, as long as both are scaled back similarly.

This method leaves more weight on the table for you to distribute and manipulate as you like.

[Balsa Man]

Your analysis sounds good on all points.
One thing to consider before you glue it up- the possibility that when the brakes go on you have enough torque to shear the glue- easier to maximize strength when you're initially doing it, than trying to come in after it fails and beef it up. If you go w/a metal bushing and roughen inside and outside of the bushing w/ like 100 grit sandpaper, good chance it'll work. If you go w/ a nylon bushing, glue adhesion/shear strength will be a lot less. You can help that with cutting/filing little groves- on the inside and outside of the bushing, and the inside of the wheel hole- creates little locking keys... best bet on glue, either a good epoxy, or rubber-enhanced CA

[Flavorflav]

Shearing the glue between the threaded rod and the inner race of the bearing is something that we've always been afraid of, but never actually had a problem with. You should probably be okay unless you get a very low-friction bushing. Scoring it as Balsa Man suggests couldn't hurt, though.

[mrsteven]

Shearing the glue between the threaded rod and the inner race of the bearing is something that we've always been afraid of, but never actually had a problem with. You should probably be okay unless you get a very low-friction bushing. Scoring it as Balsa Man suggests couldn't hurt, though.

What did you use for adhesive?

[Flavorflav]

CA. You want to be very careful with it - you don't want to lock up the bearing. I must tell you, though, we've never tried it with an insert - you'll have an extra potential weak spot between the race and the bushing. If it's a smooth plastic I would definitely score it or sand it with really coarse paper before you glue.

[Balsa Man]

CA. You want to be very careful with it - you don't want to lock up the bearing. I must tell you, though, we've never tried it with an insert - you'll have an extra potential weak spot between the race and the bushing. If it's a smooth plastic I would definitely score it or sand it with really coarse paper before you glue.

Just for consideration - data on shear strength, for metal to metal joints; "regular" CA compared to rubber-enhanced CA ~3,000 psi vs. ~4,000 psi; the rubber enhanced CA is ~33% stronger in shear, and given the slight elasticity, less subject to failure under shock loading...

[catfoot]

How does one tell the quality (well, friction, or the equivalent experienced by bearings) of bearings if ordering online? Specifically, I've got my eyes set on a set of bearings from Amazon with high RPM, but I'm sure that doesn't mean much in the resistance department. Is there any advice you guys could offer on selecting bearings?