Monday, November 7, 2011

Car roof carrier - transformer (Part II, The Chair)

Finally, the test drive of the chair is completed with a great success in the stern conditions of Mojave desert (California). See the photo set of the construction details explained below.

The transformation time I'm getting is less than 20 minutes from opening the trunk door to getting in the chair. However, I have in mind several easy enhancements for particular elements used in the construction, which might halve that time easily, while also removing one of the hand tools out of two required for transformation completely.

The assembled weight of the chair is equal to the assembled weight of the cradle, minus 3 styrofoam inserts and equals to 17 pounds (~8 kg). There are some possibilities for improvement there too, but I felt comfortable enough with that weight while actively observing various objects for 6 hours straight, moving the chair around and adjusting its height often.

What's left to do is to apply some protective finish (more on that later) to all of the surfaces, to add a permanent soft sitting pad, and perhaps a soft backing. So far a folded several times yoga (gym) mat worked just fine for me.

Construction details

The chair consists of two primary pieces: the back board and the sliding seat. For those not following the ordeal: everything is made of 3/4 inch redwood plywood.

The most important element of my mechanical design is the seat's height adjustment mechanism. It has 2 stages - the bending stop block and the side clamps mechanism. More specifics on that below.

On the image to the left:
(for further reference)
A - the side clamp piece, one on each side. They are also serving perfectly as armrests and chair adjusting and carrying handles.
B - the front bending stop block threaded rod's terminal.
C - two screws holding the L-shape of the rear/top bending stop block.
D - the seat's pivoting screw.
E - the hand screw holding both front legs (cross rail screw).
F - adjustable legs at the lower side of the legs/rails.
G - foot rest (rather narrow and fixed, but often working well enough to assit with climbing up).
H - L - shapes at the front edge of the seat, which are used to reassemble the chair into the cradle (also the future foot rest attachment points).
I - legs/rails tops.
J - the back leg (repurposed rear side of the cradle).

The height adjustment mechanism exposed

This is the image with the left armrest (A) removed from the pivot point (D). The L-shape (C) visible on the back end of the armrest is the top pressure point of the bending stop block chair holding mechanism. It is pushed into the rear side of the back board by its side under the body weight. The bottom pressure points are formed by the 3/8 inch chair width's long threaded rod (B), which is driven into the front side of the back board under the body weight on the seat.

The bending stop block's functional diagram. Colored green - are the most crucial steel parts of the mechanism. red arrows represent crucial forces fixing the chair seat in place on the back board as soon as the weight is applied to the seat (leftmost arrow). In blue the armrest lever is contoured. Rotating it up (around the left green axis ) will disengage bender elements (two green pieces to the right from it) allowing the seat's free sliding up and down along the back board.

To prevent the seat from sliding down when there is no weight applied - I'm using a simple clamping mechanism based on the 3/8" thumbscrew, clamped in the slot of the tightening plywood handle (it's visible under the armrest (A) on the image above). The clamp effectively bites the side of the back board and the side of the seat's backrest holding the later in place by friction, however the primary force, holding the seat in place under the full body weight, is created by the bending stop mechanism. The most of the body weight is applied to the L-shapes (C), hence the 3/16" steel is used there.

To adjust the chair's height you need to undo the side clamp lever for one revolution, lift both armrests up a little to release the bending stop block, and slide the seat up or down along the back board holding it by these armrests (which prevents the engaging of the biting mechanism). When armrests are released their ends are falling down and engaging the biting automatically. It helps to push down on the seat's edge to fix the position better. Then apply the side clamp to prevent accidental seat slippage, as the biting will weaken in case the seat accidentally rocked upwards. It will simply slide down under its own weight in that case. All of that takes around 5 seconds without the need of a flashlight, including jumping off the seat and jumping back in.

The rails

This is how the chair looks from the back. The single screw with wide handle (cross rail screw (E)) holds both rails by the single point in the middle at the certain angle with the 4-prong T-nut in the back board. At the low edge, both rails have a wide notch going over the side of the cradle's side board. These notches are of a specific shape preventing the rails from sliding out of the grip. They are taking the actual body mass (not the cross rail screw E). However, for an ultimate security, I'm using a bungee cord to keep them in place (thinking of some improvements, though).

At the ground end of each rail, I'm using a 3/16" steel  L-shapes threaded for adjustable screw-legs, taken from some old furniture piece. These legs allow changing the slant of the back board a little to accommodate the lowest seat's height center of gravity. These legs are made of hard plastic with quite a smooth surface, what makes them safe to use the chair on a thin plastic ground cloth.

At the very top the rails' "X" is protruding quite high above the chair's top, providing handles helping to climb in higher. I'm also using the gap between them and the back board to tuck some stuff I may need at hands.

Still thinking about the perfect width of rails I want. Perhaps, to save some weight I should trim them in half or so. Or I can drill many 2" holes along them and cover their sliding surface with some plastic sheeting. I like to keep things looking reassuringly beefy, though.

Reverse transformation (to the OTA cradle)

1. Reattach the seat part to the top of the back board. Initially I've been using Allen wrench on 2 M8 hex bolts (visible on the left bottom side of the image with that ratchet key attached). Since then I have installed 1/4"x20 wood T-nuts into the bottom flange instead and using my universal wireless electric screwdriver on machine screws coming from the top. The initial solution led to the damage of the threads' starts in the soft steel of L-shapes way quickly. The 1/4" thread size seems sufficient with the hard steel nuts.

Closeup of the left joint point. Both M8 bolts are protruding too much, so some cutting is due. On the top, you see both rails already attached to the flange by small wood screws. (all of the bolts are cut flush to date and standardized on 1/4"x20 for all non-permanent connections for interchangeability; the flange screw holes for rails are reinforced with plastic inserts).

2. Detach both rails (the single screw (E) is the only screw to undo here from the 4-pronged T-nut in the middle of the back board) and mount them on the other side (on the front) of the back board.

Close up. The rails mounting mechanism is in development still. But the bottom (cradle rear's) end of the rail is using the screw leg for attachment to the side panel of the cradle as pictured. The leg's screw is simply going into the provided tight hole creating one attachment point. While towards the front, the rail is simply screwed down to the back board's top flange side (see above).

The position of rails is crucial for easy sliding into the car. Both rails' ends can bend a little to the sides accommodating. Since the initial version, two rubber wheels were installed on this end for better mobility. As the result rails had to be put closer to the back board there, so they are now resting directly on the edge of the side board of the cradle. The footrest above it got notches on the sides to accommodate the new position of rails as well, without compromising any of its functions (foot step, crossbar stop) much, and even adding a new one - protecting the business side of the chair's legs notches from possible damage due to sliding over a hard obstacle).

3. Almost done! Roll it over now. The cradle on rails is ready to accept the telescope's OTA (over three original packaging styrofoam inserts). Note the small screw end protruding from the center of the back board. That's the storage position of the cross rail cross screw (E), which is also serving as a holder for the middle styrofoam insert while loading the OTA in that position.

4. The telescope "mummy" is secured in the cradle with 2 heavy duty rubber bands going through four 2" round holes visible on the image above. The cradle is now ready to slide inside. Since the initial version the front (top on the image) styrofoam insert had been removed, as on that side, the soft seating cushions of the chair provide enough protection for the OTAs front end. Also, the sides are now tightened with the mandatory ratchet tie down strap going over the top of the cradle wall to wall. That strap makes sure the OTA cannot move along its axis, and also can be used as a top handle (for the single hand carry). The remaining two styrofoam inserts were covered with a thin layer of closed cell foam, preventing damage to the styrofoam and dampening horrible squeaking sounds in the car while riding (styrofoam against plywood for 5 hours? - brrrr).

5. Slide it in and use the holes in the seat / front wall to secure the cradle by the front car seat's headrest rods (a bungee cord works well there too). Done! ~20 minutes later.

Construction notes

  • The primary structural integrity of the construction is provided by 4 steel wall shelving holders (white on the images, marked by (K) on some of them). All of the chair's primary structural nodes of the construction are sticking to their bolted ends.
  • There are some wood screws used, but they are exclusively for closing the gaps between the pieces now (compared to the phase 1 when they were structural).
  • The rest of the mounting hardware are beefy bolts (1/4" - 3/4", M8) with wide washers and 3/16" steel plates with manually threaded holes on the opposite end instead of washers and nuts.
  • The chair looks a bit uplifted on the images. That's real. The body weight will bend the back board a little bit forward, so a small additional slant was introduced to compensate for that.
  • Still thinking about padding. There is an idea to replace the styrofoam insert for the OTA's front side (done, see later posts on the subject).
  • It is hard to find thick steel, which will hold the threading well enough. But I've found a perfect supply of L-shapes and steel plates by cutting some cheap property fence furniture (gate handles, hinges, etc), available from common hardware stores like Ace, with the Dremel tool, then drilling and tapping them.
  • The construction looks complicated compared to the famous Catsperch. However, in fact, mine is much easier to repeat because it is less sensitive to the materials selection, to the craftsmanship quality (cutting, driling, and assembling errors), even to an improper use of hand/electric tools. And it's way more versatile. It can be used to secure the OTA on the car's roof or to slide it easily into the back of a hatchback car. It can be used as a sturdy long table, and it has armrests, footrests, and the true backrest! Also when disassembled it takes a minimal space (two nesting one in another L shapes standing at the wall). 
Pending improvements
(notes to myself)

  1. (done) Make some permanent soft cushioning for the seat converting to the OTA padding.
  2. (done) Cut bolts flat to the surface.
  3. (done) Use some coating to improve durability.
  4. (done) Add small wheels.
  5. (done) Make a handle on the rear end.

The next stage is finishing.

(Back to Part1)


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