Mousetrap Vehicle C

[Alex-RCHS]

TL:DR there are too many factors for such and overarching statement to be made

Small wheels arent better for speed. They will generally lower the time score though. Consider the equation (I = mr^2 with the coefficient somewhere between 0.5 and 1). If we consult this it’ll be obvious that small wheels will accelerate the vehicle faster. Larger wheels, like you said, will have a higher top speed but given the track size will probably never be able to reach it. They also slow the reversal down a ton.

So now we know that for time score small wheels will generally win because quickly speeding up and reversing direction is more valuable than having a higher potential top speed. The problem is we’re assuming the lever arm length stays constant, which it won’t. To travel the full length of the track we will have to lengthen the lever arm as the wheels get smaller, which decreases the force applied to the axle, which increases the run time again. Not to mention axle thickness which varies the torque the axle receives also.

Fiddling with all these factors will take a lot of experimenting, unless there’s an easy way to calculate the optimal setup. I’d suggest buying a pair of mid sized wheels with reasonable mass first and optimizing your vehicle around those. Don’t consider the rotational inertia too much because you can compensate largely with lever arm length and axle thickness. In my experience picking the wheels out first has been easiest because changing wheel size implies buying an entire new set of wheels while varying the other two factors are much easier.

Actually, unless you’re using ridiculously large wheels the rotational energy invested in the wheels accounts for very little of the vehicle’s energy. The main issue is how the spring energy is distributed over the course of the run. Our situation of having a distinct (mostly) set distance means that the energy should be distributed mostly equally between the forward run and the reverse, which you can control via the lever arm length and wheel/axle ratio.

OK great. There were also a lot of people talking about making an axle thicker with tape and I was little confused on what that did or why you would do that.

That’s simply to change the wheel/axle ratio. There’s more info on that on the wiki I believe, and elsewhere online. And the guy explains how the wheel/axle ratio affects ideal mechanics advantage in the video

[Kenshi Takahashi]

That’s simply to change the wheel/axle ratio. There’s more info on that on the wiki I believe, and elsewhere online. And the guy explains how the wheel/axle ratio affects ideal mechanics advantage in the video

Great, thanks!

Edit: For the lever arm that causes the axle to rotate, should the string be pulled up directly upwards (perpendicularly to the axle), or angled backwards or forward? Also, if you use a short lever arm such that there isn't enough string for the car to reach the farthest distance do you just add a bunch of extra string and the car moves with the energy?

[4Head]

That’s simply to change the wheel/axle ratio. There’s more info on that on the wiki I believe, and elsewhere online. And the guy explains how the wheel/axle ratio affects ideal mechanics advantage in the video

Great, thanks!

Edit: For the lever arm that causes the axle to rotate, should the string be pulled up directly upwards (perpendicularly to the axle), or angled backwards or forward? Also, if you use a short lever arm such that there isn't enough string for the car to reach the farthest distance do you just add a bunch of extra string and the car moves with the energy?

For applying torque to the axle the direction the string is pulled up won’t make a difference but pulling it straight up (ie in ready to run the string between the arm and axle is perpendicular to the ground) or angled forward (lever arm shorter than distance to axle) is preferred. If the string is angled backward then the lever arm is too long and adds nothing to the vehicle except extra weight; it won’t pull any more string than the distance from the mousetrap spring to the axle.

Only use the amount of string you need. Excess string doesn’t contribute to propulsion and can get caught in the axle. The length of string should be the distance from axle to spring of mousetrap + lever arm length.

[Kenshi Takahashi]

That’s simply to change the wheel/axle ratio. There’s more info on that on the wiki I believe, and elsewhere online. And the guy explains how the wheel/axle ratio affects ideal mechanics advantage in the video

Great, thanks!

Edit: For the lever arm that causes the axle to rotate, should the string be pulled up directly upwards (perpendicularly to the axle), or angled backwards or forward? Also, if you use a short lever arm such that there isn't enough string for the car to reach the farthest distance do you just add a bunch of extra string and the car moves with the energy?

For applying torque to the axle the direction the string is pulled up won’t make a difference but pulling it straight up (ie in ready to run the string between the arm and axle is perpendicular to the ground) or angled forward (lever arm shorter than distance to axle) is preferred. If the string is angled backward then the lever arm is too long and adds nothing to the vehicle except extra weight; it won’t pull any more string than the distance from the mousetrap spring to the axle.

Only use the amount of string you need. Excess string doesn’t contribute to propulsion and can get caught in the axle. The length of string should be the distance from axle to spring of mousetrap + lever arm length.

Thanks! And would you recommend the lever arm rotate the front or back axle of the car?

Also, if you have small wheels, the lever arm would have to be longer, while larger wheels allow for a shorter lever arm. If you lose torque with a longer lever arm, wouldn't having smaller wheels be a problem, even if they don't need as much force to complete a rotation?

[PM2017]

That’s simply to change the wheel/axle ratio. There’s more info on that on the wiki I believe, and elsewhere online. And the guy explains how the wheel/axle ratio affects ideal mechanics advantage in the video

Great, thanks!

Edit: For the lever arm that causes the axle to rotate, should the string be pulled up directly upwards (perpendicularly to the axle), or angled backwards or forward? Also, if you use a short lever arm such that there isn't enough string for the car to reach the farthest distance do you just add a bunch of extra string and the car moves with the energy?

For applying torque to the axle the direction the string is pulled up won’t make a difference but pulling it straight up (ie in ready to run the string between the arm and axle is perpendicular to the ground) or angled forward (lever arm shorter than distance to axle) is preferred. If the string is angled backward then the lever arm is too long and adds nothing to the vehicle except extra weight; it won’t pull any more string than the distance from the mousetrap spring to the axle.

Only use the amount of string you need. Excess string doesn’t contribute to propulsion and can get caught in the axle. The length of string should be the distance from axle to spring of mousetrap + lever arm length.

Is it not twice the lever arm length?

[4Head]

Great, thanks!

Edit: For the lever arm that causes the axle to rotate, should the string be pulled up directly upwards (perpendicularly to the axle), or angled backwards or forward? Also, if you use a short lever arm such that there isn't enough string for the car to reach the farthest distance do you just add a bunch of extra string and the car moves with the energy?

For applying torque to the axle the direction the string is pulled up won’t make a difference but pulling it straight up (ie in ready to run the string between the arm and axle is perpendicular to the ground) or angled forward (lever arm shorter than distance to axle) is preferred. If the string is angled backward then the lever arm is too long and adds nothing to the vehicle except extra weight; it won’t pull any more string than the distance from the mousetrap spring to the axle.

Only use the amount of string you need. Excess string doesn’t contribute to propulsion and can get caught in the axle. The length of string should be the distance from axle to spring of mousetrap + lever arm length.

Is it not twice the lever arm length?

That’s only if the lever arm reaches the axle completely. If the lever arm is shorter than the distance to the axle then you need a little bit more string to make up that difference.

[biz11]

I have ball bearings for my drive axle, but I haven't been able to find any to fit the threaded breaking axle. I'm using an 8-32 threaded axle for the breaking system. If anyone knows where to find bearings to fit that it would be much appreciated.

[retired1]

An 8-32 thread will have a max diameter of 0.15870 to 0.1640 with 0.1631 as a nominal.
Measure your rods and then find the bearing that is about the same size ass your rod. If the rod is too small, you can tightly wrap 1 to 4 turns or teflon tape where the rod will just barely enter the bearing. If you rod is a few thousands to big, chuck it up in a drill motor and hold sand paper to the required area. Check frequently. Also remember that the rod will be quite warm, so that the diameter will be slightly larger than it will be when cold.

[Alex-RCHS]

What are most of you using to aim?

[MangoTTT]

Hey any prediction on rules for next year for mousetrap? I feel there's not a lot to add...