Gas station without pumps

2014 July 14

Need new mesh seat for recumbent

Filed under: Uncategorized — gasstationwithoutpumps @ 21:28
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I need to replace the mesh seat on my recumbent bicycle, because one of the buckles snapped yesterday. The mesh itself is badly stretched and abraded, and a few of the webbing straps are badly worn, so it is not worth repairing the seat—it’s replacement time. I can still ride the bike, but it isn’t as comfortable with the front strap no longer functional.

Now I’m trying to figure out exactly what fabric and parts to get.  One person on the Ryan owner’s club mailing list conveniently provided a parts list recently, though the seat I have currently does not exactly match his list (for example, I have all 1″ webbing, no 3/4″ webbing).  Here are some things I’m trying to decide:

  • What type of mesh should I get?  He recommended black Leno Lock mesh from Outdoor Wilderness Fabrics MESHBLK at $14.03/yard, but I’m also considering Phifertex Vinyl Mesh at $12.95/yard, which is available in many colors, or Phifertex Plus at $17.95/yard, which would provide less stretch, but also less ventilation. The Phifertex Plus is sold as a sling mesh (capable of supporting a person’s weight), but the others are not.  I suspect that any fabric rated for seats will have too little ventilation for the recumbent. The leno weave fabrics are likely to provide more stability in an open mesh, because the warp threads twist around each other, rather than running straight, locking the weft threads in place. The bentrideronline forum posts generally recommend the Leno lock mesh from Outdoor Wilderness Fabrics, so I’ll probably go with that, even though it is a bit too stretchy.
  • What sort of webbing should I get?  The edges of the seat use 2″ webbing to stabilize the seat and attach the straps, plus a couple of diagonals from the center front to part way up the sides, to support the weight of the rider.  The rest of the straps are 1″ wide.  But should they be nylon, polypropylene, or polyester straps?  Nylon has high strength, but is rather stretchy. Polypropylene has less stretch, but poor abrasion resistance and UV resistance, and polyester has the best UV resistance and the least stretch (about half as much as nylon webbing of the same weight under the same stress).  It also doesn’t absorb water, and is more resistant to mildew and rot.
    I can get black polyester 1″ webbing for about 35¢/foot, and 2″ black polyester webbing for about 75¢/foot, but colors are a little more expensive: I can get 10 yards of red 1″ with reflective stripes for $18.90, or plain red for $1.48/yard. For a bicycle application, the reflective stripes may be a useful safety feature. Red 2″ seatbelt webbing would be about $10 for 5 yards.
  • I also need to get buckles for the 7 cross straps and the two straps that go over the top of the seat.  I’m undecided between simple side-release buckles (Fastex FSR1 59¢), and dual-pull side release buckles (generic GTSRD1 47¢) from Outdoor Wilderness Fabrics. Cam lock buckles (generic GCB1 46¢) are also a possibility. I’ll also want a a tri-glide for each loose strap end (generic GTG1 12¢).

So, unless I can get a new seat from the manufacturer of my bike (Longbikes in Colorado), even though they discontinued this model about 10 years ago, I’ll probably be making my own seat soon.  It’ll cost me about $50–60 for materials, but I suspect that an already sewn seat would cost more like $150, and I wouldn’t have the option of red straps with reflective stripes.


End of an era

My son has his last performances at West End Studio Theatre this summer—his last summer before college.  He has had theater classes with Terri Steinmann and various of her staff members since the Wizard of Oz class in July 2004, and he has been performing on the WEST stage since they opened in 2007.  Between Pisces Moon (where Terri taught before founding WEST) and West Performing Arts, he has done at least 42 classes with them (I’m not sure how to count the Dinosaur Prom Improv troupe, which he performed with for two years—I counted that as only one class, though it probably should count as more, as there were weekly practice sessions for the two years).  Adding up all the course tuition over the 10 years he’s worked with them, I think we’ve paid around $20,000, averaging $2k a year—well worth it for the pleasure and the learning he has gotten from it.

This past weekend he performed as Otho (the interior designer) in Betelgeuse. After seeing the movie, I did not know how they would pull it off as a stage play, but they did quite a good job of it—particularly since they did not have the complete script until a few days before they performed (a long-standing WEST tradition of writing the script after rehearsals have started).  There were two casts (the morning class and the afternoon class), but I only saw the afternoon cast’s production—I understand that the interpretations of essentially the same script and set were quite different for the two casts (costumes had to be different, because the actors were very different sizes).

He has one more class with them this summer—the summer teen conservatory with Santa Cruz Shakespeare, which I believe still has room for another student or two (the conservatory is limited to about 12 students).  He’s done their Shakespeare teen conservatory for the past four years—it is quite different each time. The conservatory is probably West Performing Art’s most advanced theater class.

After this summer, not only will he be finished with West Performing Arts, but the West End Studio Theatre, where about half his performances have been, will be closed. We joke that they can’t go on without him, but the truth is that they are losing their lease.  They’ve been renting on a year-to-year contract for eight years, and the landlord has found a tenant (a beer brewer) willing to lease the space on a longer term lease.  The parting is amicable, but everyone will miss the W.E.S.T. space, which has been much more flexible and functional than any of the other spaces children’s theater has used around the city.

West Performing Arts will continue classes at the Broadway Playhouse and at schools, but they’ll need more space for classes than Broadway Playhouse can provide, especially for their popular summer classes, so they are looking for a new home. If anyone knows of spaces that might meet their needs (ideally, two large adjacent spaces that can be used for classes, one of which can be a flexible performance space, totaling about 10,000 sq ft, with storage, office space, and nearby parking and not needing a lot of renovation).  They don’t have a lot of money (they’ve been keeping the classes affordable), so the typical $15–20/sq.ft./year leases locally are probably beyond their means.  If anyone has any leads for them, their contact information is on their web site.

2014 July 12

Impostor syndrome

Filed under: Uncategorized — gasstationwithoutpumps @ 10:56

I know that many students feel at times like they aren’t capable of doing what they need to do to ace their classes, to graduate, to move on into the “real world” or higher up in academia. Sometimes they feel like they are just “faking” being smart, and that someone will catch them at it.

This is known as “impostor syndrome” and is quite common—Wikipedia even has a page explaining it.  People from underprivileged backgrounds or who have been socialized to think of themselves as somehow inferior suffer from it more than those who have been taught to be confident in what they do.  For example, women in physical and computational sciences often doubt themselves, even when the objective evidence is that they are quite capable.

Even tenured professors, who have passed through many tests of their resolve and ability, often suffer from impostor syndrome.  I suggest the following reading (all from a single author) for those who are wrestling with this problem (the author selected these posts herself from her larger body of work):

Maria Klawe, president of Harvey Mudd College, has a good, short article on her own experiences with impostor syndrome in Slate.  For a somewhat younger perspective, Alicia Liu’s article Overcoming Imposter Syndrome is worth reading.

Incidentally, there is a flip side to the problem, of students (often, but not exclusively, male students from privileged backgrounds) having too much confidence and not being aware when they are out of their depth, failing to ask for help when they need it.

Both problems can be tackled with the same approach: seeking outside verification of your abilities and paying attention to the feedback. This is easiest while being a student, as there are many formal mechanisms in place for honest feedback—it gets harder when you have to rely on the more random mechanisms of journal paper reviews and grant proposals or pats on the back from co-workers.

As a community, we can all help with both problems by providing honest feedback (neither ego strokes nor unwarranted criticism) when asked for it, and by asking for feedback ourselves.

For my part, I tend to see the negative both in my own work and in others’ work, and I am working on trying to increase the amount of positive feedback I give people.

2014 July 7

Crowdfunding for UCSC iGEM project

Filed under: Uncategorized — gasstationwithoutpumps @ 19:34
Tags: , , , , ,

The UCSC undergraduate team for the iGEM synthetic biology competition have put up a crowd-funding web site to try to raise the money they need for their contest entry.

Their design project is to engineer a bacterial strain for cellulosic alcohol production—not ethanol, but butanol, whose energy density is more compatible with the existing gasoline infrastructure and that does not absorb so much water.  Conventional ways of creating butanol are too expensive, so recombinant bacteria are a promising approach.  Using cellulose as a feedstock avoids competing with food production, as waste paper and other non-food sources can be used.

They are not trying to do everything at once—they are working this summer on getting butanol production from glucose engineered into Haloferax volcanii, a halophile that their mentor has worked with a fair amount.  I’m not sure what their reasoning is for using a halophile—perhaps they just wanted to work in an archeon, and H. volcanii is one of the best-established model organisms for Archaea.

Their mentor for the project is donating his time, so all the costs are unavoidable reagent, equipment time, or registration fee costs.

The team description (including membership) is at, and the wiki where you can follow their progress is at (though they’ve nothing there yet but an introduction to the project).

I gave a token amount, and I urge others to do so also (or more if you are feeling generous).  They’re currently about halfway to their goal.

2014 July 6

Battery connectors

Filed under: Uncategorized — gasstationwithoutpumps @ 02:32
Tags: , , , , , , , ,

I spent a little time today working on my book, but I got side tracked into a different project for the day: designing a super-cheap coin-cell battery connector. I’ve used coin-cell battery holders before, like on the blinky EKG board, where I used a BH800S for 2 20mm CR2032 lithium cells. That battery holder is fairly large and costs over $1—even in 1000s it costs 70¢ a piece. So I was trying to come up with a way to make a dirt cheap coin-cell holder.

The inspiration came from the little LED lights that “glovers” use inside their gloves. They are powered by two CR1620 batteries (that means a 16mm diameter and 2.0mm thickness for the battery). Because the lights have to be made very cheaply, they don’t use an expensive holder, but put the negative side of the batteries directly against a large copper pad on the PC board. The batteries are held in place by the positive contact, which is a piece of springy metal pressing the battery against the board—and each manufacturer seems to have a slightly different variant on how the clip is made.

Unfortunately, I was unable to find any suppliers who sold the little clips—though I found several companies that make battery contacts, it seems that most are custom orders.

My first thought was to bend a little clip out of some stainless steel wire I have sitting around (not the 1/8″ welding rod, but 18-gauge 1.02362mm wire). That’s about the same thickness as a paperclip (which is made out of either 18-gauge or 19-gauge wire), but the stainless steel is stiffer and less fatigue-prone than paperclips. I was a little worried about whether stainless steel was solderable, so I looked it up on Wikipedia, which has an article of solderability. Sure enough, stainless steel is very hard to solder (the chromium oxides have to be removed, and that takes some really nasty fluxes that you don’t want near your electronics). So scratch that idea.

I spent some time looking around the web at what materials do get used for battery contacts—it seems there are three main ones: music wire, phosphor bronze, and beryllium copper, roughly in order of price. Music wire is steel wire, which gets nickel plated for making electrical connections. It is cheap, stiff, and easily formed, but its conductivity is not so great, though the nickel plating helps with that. The nickel oxides that form require a sliding contact to scrape off to make good electrical connection. Phosphor bronze is a better conductor, but may need plating to avoid galvanic corrosion with the nickel-plated battery surfaces. Most of the contacts I saw on the glover lights seemed to have been stamped out of phosphor bronze. Beryllium copper is a premium material (used in military and medical devices), as it has a really good ratio of yield strength to Young’s modulus, so it can be cycled many times without failing, but also has good conductivity.

Since I don’t have metal stamping machinery in my house, but I do have pliers and vise-grips, I decided to see if I could design a clip out of wire. It is possible to order small quantities of nickel-plated music wire on the web. For example, sells several different sizes, from 0.1524mm diameter to 0.6604mm diameter. I may even be able to get some locally at a music store.

My first design was entirely seat-of-the-pants guessing:

First clip design, using 19-guage wire, with two 1mm holes in PC board to accept the wire.

First clip design, using 19-gauge wire, with two 1mm holes in PC board to accept the wire. This design is intended for two CR1620 batteries.

The idea was to have a large sliding contact that made it fairly easy to slide the batteries in, but then held them snugly. Having a rounded contact on the clip avoids scratching the batteries but can (I hope) provide a fair amount of normal force to hold the batteries in place. But how much force is needed?

I had a very hard time finding specifications on how hard batteries should be held by their contacts. Eventually I found a data sheet for a coin battery holder that specified “Spring pressure: 50g min. initial contact force at positive and negative terminals”. Aside from referring to force as pressure and then using units of mass, this data sheet gave me a clear indication that I wanted at least 0.5N of force on my contacts.

I found another battery holder manufacturer that gave a tiny graph in one of their advertising blurbs that showed a range of 100g–250g (again using units of mass). This suggests 1N-2.5N of contact force.

Another way of getting at the force needed is to look at how much friction is needed to hold the batteries in place and what the coefficient of friction is for nickel-on-nickel sliding. The most violently I would shake something is how fast I can shake my fingertips with a loose wrist—about 4Hz with an peak-to-peak amplitude of 22cm, which would be a peak acceleration of about 70 m/s^2. Two CR1620 cells weigh about 2.5±0.1g (based on different estimates from the web), so the force they need to resist is only about 0.2N. Nickel-on-nickel friction can have a coefficient as low as 0.53 (from the Engineering Toolbox), so I’d want a normal force of at least 0.4N. That’s in the same ballpark as the information I got from the battery holder specs.

So how stiff does the wire have to be? I specified a 0.2mm deflection, so I’d need at least 2N/mm as the spring constant for the contact, and I might want as high as 10N/mm for a really firm hold on the batteries.

So how should I compute the stiffness of the contact? I’ve never done mechanical engineering, and never had a statics class, but I can Google formulas like any one else—I found a formula for the bending of a cantilever loaded at the end:
\frac{F}{d} = \frac{3 E I}{L^{3}}, where F is force, d is deflection, E is Young’s modulus, I is “area moment of inertia”, and L is the length of the beam. More Googling got me the area moment of inertia of a circular beam of radius r as \frac{\pi}{4} r^{4}. So if I use the 0.912mm wire with an 8mm beam I have
F/d = 200E-6 mm E.

More Googling got me some typical values of Young’s modulus:

material E [MPa = N/(mm)^2]
phosphor bronze 120E3
beryllium copper 135E3
music wire 207E3

If I used 19-gauge phosphor bronze, I’d have about 24N/mm, which is way more than my highest desired value of 10N/mm. Working backwards from 2–10N/mm what wire gauge would I need? I get a diameter of 0.403mm to 0.603mm, which would be #6 (0.4064mm), #7 (0.4572mm), #8 (0.5080mm), #9 (0.5588mm), or #10 (0.6096mm), on the site. I noticed that battery contact maker in Georgia claims to stock 0.5mm and 0.6mm music wire for making battery contacts, though they first give the sizes as 0.020″ and 0.024″, so I think that these are actually 0.5080mm and 0.6096mm (#8 and #10) music wire.

It seems that using #8 (0.020″, 0.5080mm) nickel-plated music wire would be an appropriate material for making the contacts. Note that the loop design actually results in two cantilevers, each with a stiffness of about 4N/mm, resulting in a retention force of about 1.6N. The design could be tweaked to get different contact forces, by changing how much deflection is needed to accommodate the batteries.

How much tweaking might be needed?  I found the official specs for battery sizes (with tolerances) in IEC standard 60086 part 2: The thickness for a 1620 is 1.8mm–2mm, the diameter is 15.7mm–16mm, and the negative contact must be at least 5mm in diameter.  The standard also calls for them to take an average of 675 hours to discharge down to 2v through a 30kΩ resistor (that’s about 56mAH, if the voltage drops linearly, 67mAH if the voltage drops suddenly at the end of the discharge time).  If the batteries can legally be as thin as 1.8mm, then to get a displacement of 0.2mm, I’d need the zero-point for the contacts to be only 3.4mm from the PC board, not 3.8mm, and full thickness batteries would provide a displacement of 0.6mm, and a retention force of about 4.8N.

If I were to do a clip for a single CR2032 battery, I’d need to have a zero-point 2.8mm from the board, to provide 0.2mm of displacement for the minimum 3.0mm battery thickness.

So now all I need to do is get some music wire and see if I can bend it by hand precisely enough to make prototype clips.  I’d probably change the spacing between the holes to be 0.3″ (7.62mm), so that I could test the clip on one of my existing PC boards.

Update 2014 July 6: I need to put an insulator on the verticals (heat shrink tubing?), or the top battery will be shorted out, since the side of the lower battery is exposed.


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