Saturday, July 30, 2011

tinyKart: Round 3

The quest for an ultralight go-kart continues...

In the last update, we had assembled the two critical front wheel modules, which allowed us to complete the aluminum plate and 80/20 sandwich that is the front of the chassis. With that, the basic structure of the kart was complete and we could get a look at the full size and shape of tinyKart:

So tempted to throw this on the railroad tracks...
The rear drive modules attach to the rear 80/20 frame using standard t-nuts, though we did have to drill an access hole on the inside of the side rails for the single 1/4-20 screw which holds the rear wheel shafts in place. Each drive module has a single Turnigy SK6374-170 motor driving an HTD 5M-15 belt with a gear ratio of 4.5:1. This will give a top speed of....I have no idea. It depends on the motor's performance under sensored control. More on that to come.

The standoffs and outboard plates for the drive modules have been fabricated, so all that remains are the belt tensioners. We may also modify the motor pulley to have a set screw hub on both sides to leave open the possibility of attaching a second motor to the outboard plate, with the two shafts coupled by the pulley itself. This would not be for the ultralight version, but rather for a high-performance follow-up. With just one motor per side, the peak force at the ground should still be close to 100lbf.

Thus far, the build had been fairly 2D: plates, extrusions, and right angles. The steering linkages and seat mount are where the interesting angles start to come in. Here's one of the tie rods:

Normally, a tie rod would just have one ball joint on each end, the 3D equivalent to a pin joint in a planar mechanism. But here I wanted the steering linkage to go in the 1" gap between the front chassis plates. So, I came up with this Non-Collocated Ball Joint which is really just two pin joints, one in the horizontal plane and one in a vertical plane. Together, they remove the same degrees of freedom as a single pin joint, but they allow part of the tie rod to be completely planar and thus fit in the tight space between chassis plates:

Vertical pin joint.
The horizontal pin joint fits, just barely.
The other end of the tie rod is a straightforward ball joint going to the driving link on the steering column:

The rod itself is 1/4-28 threaded steel rod with a 3/8" aluminum sleeve over it. Bling nuts on either side of the sleeve put the sleeve in compression and the rod in tension. This bit of preload makes the combined rod much stiffer and ensures it won't buckle on the "pushing" side of the steering linkage. Here's the whole linkage:

There are several ways to adjust the linkage: The easiest seems to be to change the length of threaded rod passing through the vertical pin joint block. This can be used to adjust the toe angle and alignment of the front wheels. These dimensions also depend on the angle of the steering column, which is adjustable (for now) by pivoting a block that acts as the bottom bearing for the column itself:

That potentiometer might come in handy...
Still to be fabricated is the second support for the steering column. Once that is in place, an overall fine-tuning and stiffness assessment will be necessary. Some of the pin joints could use a bit of tweaking to take out as much play as possible, but overall it seems like it will do the job. If there is one thing I am still worried about, though, it's the performance of the front wheel systems under load. It all went together too easily...

The seat mount was the last bit of critical structure to add. The bottom of the seat is just bolted to the 80/20 rail that comes across the front of the rear chassis assembly. The back, though, needs to be supported in 3D by a set of braces:

One brace, constructed in a manner similar to the steering linkage, connects the side of the seat to the side rail of the rear chassis. Another brace, a strip of 1/4" aluminum, links the side of the seat to the rear rail.  This provides stiffness in all directions and actually makes the frame itself stiffer as well, since it now has structure outside the plane of the main chassis rails. Okay, so it's not a space frame, but for a group that makes everything out of 80/20 and plate stock, I think we did pretty well in 3D.

The moment of truth: a weigh-in with a makeshift steering wheel (vise grip), the controllers, and the batteries dangling from various frame rails:


Noticeably absent are the tires and tubes, which should weigh about 3lbs total, the tensioners, the second steering column support, and the wiring. Not noticeably absent but probably the weight killer is minor hardware like controller mounts, switches, etc. I think we will need to take about 2-3lbs off of our current configuration to meet a 50lb target. Otherwise, 53lbs is the weight of one of Cap Kart's old lead-acid batteries, which was also a potential target. I would not be disappointed either way.


No comments:

Post a Comment