Gravity Vehicle C

[mrsteven]

What are we thinking about with breaking systems.
There is the usual sting and threaded axel approaches but I feel like there is a better way. String recoils, and I know last year we discussed a way to stop the recoil but I'd think there is a better solution than to jerry-rig it. And threads you get friction and serious skidding from going Max V to 0 m/s in almost an instant.
Of the two, I'm leaning towards the string method simply because the recoil can be more easily controlled and since the skid seen in threads will change in magnitude depending on the surface/wear/dust/oil. Unless someone has found some fairly significantly gripping (stick almost?) wheels that stop skidding in its tracks.
Thoughts?

[olympiaddict]

What are people using for bearings?
In the past I've used skateboard bearings, with threaded rod and nuts tightened against the flanges on either side of the bore of the bearings, with great success, but for Robot Arm, not Gravity Vehicle. In that case, I'm using ball bearings more for strength and reliability than for the low-friction needs of this event.
While skateboard bearings could work for Gravity Vehicle, they have a fairly wide inner diameter, making it difficult to use thinner threaded rod to reduce rotational inertia and still get it straight.
I've looked online, but while skateboard bearings seem to always have an ABEC rating, a lot of other bearings I've seen are difficult to evaluate in terms of how much friction they cause, and I'm also not sure to what degree small differences matter since this is my first year with this event. However what with the high forces at play in this event I'm guessing a lot more than other applications.
so can anyone give me some guidance as to where to look for appropriate bearings?

[iwonder]

There aren't really any high forces at play in this event... And ABEC ratings don't always tell you if a bearing will have a high or low friction, they just tell you the tolerances that the bearing was made to(which does effect it, but the type of lube in the bearing and other stuff can too). I get bearings from McMaster-Carr or Stock Drive Products, but I'm sure there're other sources too.

[olympiaddict]

what I meant was that, compared with other events, there is more of a need for very low-friction bearings. In Mousetrap Car, which I did last year, using ball bearings didn't really seem practical because bushings seemed to be doing the job fine, and we didn't want to add too much extra mass. In addition, the bearings I'm using in Robot Arm are definitely necessary, but the waist joint isn't rotating at very high speed like these axles and wheels will be.
Forgive me, because I haven't taken Physics yet, but won't the negative effect of friction be greater when you have a heavier car traveling at a higher speed? For example, our mousetrap car was in the ~300g range if I remember correctly, and it got a time score around ~8 seconds for 10m, whereas these vehicles are 1.5kg and some teams get scores of around 3 seconds if I remember correctly.

Yeah, in the past I've replaced the lube in the bearings with lower viscosity stuff to reduce the resistance, which has been fairly effective, but they need to be re-lubricated fairly often.

Thanks, I'll check them out

[iwonder]

Ahh... Well, the friction assumption you made is true for kinetic friction(one object sliding past another) but in this event you've got to worry about the rolling friction(basically how much the wheel deforms due to the vehicle weight, think of rolling a ball down a solid ramp vs a ramp made of foam) and the losses in the bearings. In fact, the heavier your vehicle(assuming the bearing loss doesn't increase, wich it proably won't) the better off you are for distance, becuase of the increased momentum. The small differences between bearings(assuming you get reasonable bearings) don't really matter as much at the lower levels of competition, in fact, I used bushing in my vehicle and made 2nd at states last year.

[_HenryHscioly_]

does the width of the wheel make a difference? if its the same material and same vehicle weight
not like, super soft, or super hard material(I'm using rollerblade wheels)
I would like to get some more friction when my vehicle brakes.
its not that fast at the end, but it still skids,.. actually, it is "jumping", or hopping...

[iwonder]

Friction is independent of surface area, so the width of the wheel wouldn't stop the skidding(this is counter-intuitive, but you can google it for a better explanation). Skateboard wheels are generally slick, so the skidding can be expected, but I see two possibilities... One, you can test a lot of data points for turns vs distance and find a quadratic fit for the data(brute force method) or you could wrap your wheels in something like the latex from a balloon(this is fairly common practice, and probably much simpler) to increase the friction when your wheels stop. I'm not quite sure how it could jump or hop at the end however...

[Flavorflav]

While it is theoretically true that friction is independent of surface area, it is not generally true in practice. The hop, though, suggests to me that friction is not the problem. The vehicle is hopping because the wheels are supplying plenty of frictional force, but the momentum of the vehicle is sufficient that tiny imperfections in the floor are generating enough upward force that the front wheels are breaking contact with the ground. I would suggest adding a little weight above the front wheels.

[Balsa Man]

While it is theoretically true that friction is independent of surface area, it is not generally true in practice. The hop, though, suggests to me that friction is not the problem. The vehicle is hopping because the wheels are supplying plenty of frictional force, but the momentum of the vehicle is sufficient that tiny imperfections in the floor are generating enough upward force that the front wheels are breaking contact with the ground. I would suggest adding a little weight above the front wheels.

That's certainly a possibility - it's impossible to say for sure what's going on without being able to see. There are other possibilities for what's causing this. One, which I suspect is most likely, would be 3 vs 4 wheel contact interacting with weight transfer. 3 points define a plane. When you add a 4th (as in 4 wheels), unless you have incredible precision, one wheel is going to be a bit out-of-plane; as the car rolls along, it's going to be on 3 wheels; maybe only by a few thousandths. If you put the car on a really flat surface (like steel plate, or a sheet of glass), you may be able to feel it- it will "teeter-totter" on one diagonal axis, e'g., left front and right rear can move up and down (slightly), right front & left rear stay in contact. Which 3 depend on weight distribution (fore and aft and left/right). If there's more on the rear, one front wheel willl be running "off the ground" (either actually not in contact, or with less load than the other). When the brakes go on, there is a weight transfer to the front. That brings the "off the ground"/lightly loaded wheel into sudden contact. The opposite wheel on the front axle will be sliding; coefficient of friction (in an elastic material- the tire- is a bit higher just as it starts to slide, decreases as sliding starts, so when the "off the ground wheel" is driven onto the floor, it briefly has more friction, and that can cause hopping. The amount of weight transfer, btw, depends (among other things) on where the center of gravity (CG) is relative to the plane of the axles- if the CG is above this plane, the moment, rotating the front wheels down is stronger...

[_HenryHscioly_]

have 3 wheels, front wheel has no breaking mechanism
my back two wheels wobble when they spin(~1mm off center) when spinnng and the front wheel is flexibly connected to the rest of the vehicle cuz I just used some bass wood for my thin chassis
I'm wonderng if it's one of those two problems, but not sure how I should address them.
I used 1/4" all-thread axle, accidentally bought stainless steel....thought it was zinc-plated, until i had super hard time cuttng it.....
i looked down the rod, and it seemed pretty straight though