Apalrd's Motorcycle/Tricycle

Since I’m home for Christmas break, I’ve spent the last week or so building things out of Vex IQ.

This is a ‘motorcycle’ (Actually it’s a tricycle). I planned on building a car to use the differentials (since we have a lot of them and have never used them, and to demonstrate how they work). But, proper Ackerman steering was too difficult for a quick prototype, so I ended up with this, and later built the car. It drives okay, and the Modkit code is attached.

When driving, both motors drive the center of the differential, with the two side outputs connected to the two rear wheels. It works as expected, with one wheel spinning faster when turning, but since the differential is entirely open and the friction is very low, if either wheel is lifted the other will not receive any torque.

Steering is done using a single motor. The joystick position is multiplied by a gain and used as the steering angle directly (so a centered joystick should result in centered front wheel). However, the Modkit set angle function appeared to block until it got there, which was very slow and unresponsive, so I wrote a bang-bang controller in a forever loop.

A link to an album with many images:

For the Modkit code, rename it to an mk4v file, since those can’t be attached to this forum for some reason.

Motorcycle-0.mk4v.txt (6.84 KB)

1 Like

Just wondering a few things.

  1. Is the SR-71 Blackbird yours? If so, can I borrow it this weekend? I have a few errands to run and not enough time :wink:

  2. Since both your posted designs use differentials, I am interested in your opinon (as well everyone else’s who reads this.):

I suggested to a contact at vex a while ago that they add a little extra weight to the differential mold so that the side opposite the central bevel gear is heavier (i.e. balanced) when the differential spins. So far, no response, but have you noticed the unbalanced weighting affecting your designs - either positively or negatively?

Hope you can find time to keep building / posting when term resumes!

  1. Nope, but the photo is, I took it at the Seattle Museum of Flight. It’s actually an M-21 blackbird and carries the D-21 drone (which is only a few pixels tall in this picture).

  2. I have not noticed any difference to the unbalanced weight. Since both side gears weigh the same, their inertia should be the same (in both of my designs which I have posted). The carrier inertia is separate and doesn’t affect either side. I did build other non-symmetrical designs with the differentials (including a CVT which I might post soon) and did not see any issues due to imbalanced weight, in fact the total weight and friction is very low. Having both a 24t and 36t gear available on the differential is quite useful, it gives more flexibility in gear ratios. With the car in the other thread, I initially built it with all 3 differentials and, as expected, lifting any wheel would cause it to spin at 4x the input speed, and very little friction was required on the other wheels to make this happen.

2b. Another side use of the differential is as a gear with a round hole in it. In some mechanisms it’s nice to transfer shaft power into the mechanism through the same axis that it rotates by, so both of the motors can be mounted remotely, and using a differential, turntable bearing, and pegging through the diff, bearing, and structure of the mechanism allows you to put a shaft through the differential housing to transfer power into the mechanism and rotate it by the 36t gear on the differential. So just the gear with round hole might be a good product idea for VEX <goes to post on product ideas thread>

Thanks for the reply. Yes, I agree on the free spinning gear and multiple gear ratios. I also had the same product idea for a free spinning gear. Vex makes one, just not for IQ.

I haven’t posted all my product ideas. Still tinkering. Meanwhile, I have a bot with four indepently movable arms, each on the same shafts as their drive shafts. It uses 6 motors and tank treads. It can climb over almost anything - including an overturned vex IQ super kit bin!

Might post this one in the showcase…


I can’t reply to your PM to you for some reason. Could you check your settings?