Gas station without pumps

2014 July 28

PWM controller

Filed under: Uncategorized — gasstationwithoutpumps @ 21:37
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For the last couple of days I’ve been working on designing and building a PWM controller capable of handling moderately large currents at modest voltages (5A @ 9V). I don’t need high PWM frequency (4kHz should be plenty), but some of the loads I’m thinking of are rather nonlinear (very little current at low voltages, large but constant current at higher voltages).

I decided to try using an ATtiny chip as the PWM controller, because they are very cheap and can be programmed from the Arduino IDE (using an Arduino board as the ISP programmer).  I first prototyped the design using a Arduino ProMini (so I could add debugging statements and not have to worry about the 1kByte flash limitation of the ATtiny13A).  Once that was working, I rewrote the code for the ATtiny, doing some hand optimization to reduce the code size. I got it down to 880 bytes, but I can see why C is more popular than C++ for the really tiny processors—the C++ and Arduino overhead accounts for about 150 of the 1k bytes of flash!  I could make my code much smaller if I made it more special purpose and did everything in C rather than C++, but once it fit in the 1024 bytes, I stopped worrying about the size.

PWM controller using ATtiny 13A

PWM controller using ATtiny 13A

The design I came up with uses a low-dropout regulator to provide 5V for the ATtiny13A. A potentiometer provides analog input and a nFET provides a low-resistance switch for the output. I used the AOI518, because I have some on hand, but they are reaching the “end of life” so some other nFET may need to be used in a future design (perhaps one of the others in the series, such as the AOI516 which has a slightly lower on-resistance but a slightly higher gate capacitance).

The capacitors C1 and C2 are to keep the switching of the large load from propagating spikes into power supply.  The 470µF capacitor with a 5A current spike should limit the voltage slew to 10.6V/ms—with a 4.7kHz current square wave, this would be about a 1.1V triangle wave.  A polymer electrolytic is used (despite the price) to keep the effective series resistance small.  The one I’m using has an ESR of 9mΩ (@100kHz) and a ripple current up to 6.1A.  The leakage current could be as high as 1.5mA though.  C1 is a small ceramic capacitor to remove higher-frequency noise.  The big Schottky diode on the input is just to provide protection against accidentally hooking the circuit up backwards. I should probably add a 60¢ resettable fuse as well, to keep the current from getting excessive if there is a short.

When there is no load, the maximum current draw is 1.9mA for the pullup resistor R2, 0.9mA for the pot, 4–6mA for the ATtiny13A, and up to 1.5mA for C2, for a total of 8.3–10.3mA.  With a typically 80% efficient 9v switching power supply, this is a waste of about 100mW, or about 0.9kWh/year.

The pullup resistor R2 is not strictly necessary, but was added to keep the PWM pulses clean, turning all the way off even when the load had only tiny current draw—otherwise I could not get very low duty-cycle pulses to work well—the nFET acted as if it were a 1.3nF capacitor and kept a low voltage even when the nFET was off.  With the 4.7kΩ pullup, the voltage goes up appropriately, with a time constant of about 6µs.

So far, I’ve not tried the circuit with a large load—just with a small load that takes 13mA.  With that tiny a load, I don’t see any ripple in the power supply from the PWM.  With a 210mA load, the ripple on the power supply is noticeable: on top of the 20mV of ripple at about 20MHz with no load, I see 30mV of ripple synchronized with the switching of the PWM, when switching 210mA with a duty cycle of 1/2.  Without the 470µF polymer capacitor, the synchronized ripple is more like 200mV, indicating that the capacitor is indeed smoothing out the ripple.

The 30mV swing on 210mA implies that I should see about a 710mV swing if switching 5A.   That shouldn’t cause a problem in my design, but I do need to allow a little more voltage so that the swing isn’t a problem.  I could also use a bigger capacitor, but the polymer electrolytics are a bit expensive, and the regular wet electrolytics can’t handle a big ripple current—one with the same 6A ripple current rating would be 20 times the volume and 3 times the price of the polymer capacitor.

The high-frequency noise is almost certainly coming from the ATtiny, as it is synchronized with the PWM signal. The 5V power supply at the ATtiny is showing a 40mV ripple, which is large considering that there is a bypass capacitor on the Vdd pin.  There is a lot of inductance from the wiring on the board though, which could be increasing the propagation of the high-frequency noise.

New mesh seat finished and tested

Filed under: Uncategorized — gasstationwithoutpumps @ 17:50
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In Need new mesh seat for recumbent, I mentioned that I needed a new seat for my recumbent, as the one I’ve been using since 1999 (15 years!) is completely worn out.  I repaired the seat once before (replacing part of the front strap), but the seat was not worth repairing again, as the warp has worn away in several spots across the middle of the seat (apparently abrasion with the central strap behind the seat).

I bought new hardware and straps from Country Brook Design (through Amazon, since the prices were the same and the shipping seemed cheaper than a direct order from Country Brook Design.
I got

There was $7.08 shipping and handling on the order from Country Brook Design.

I also bought a yard of 840×1680 denier leno-woven mesh fabric from ahh.biz, but when I went to check the price again, it is now a “discontinued product”—I must have bought their last yard—including shipping the total bill was $20.20.

Since I already had sewing thread, the total cost for making the new seat was $76.11, but I have a lot of materials left over (the 25 triglide slides, 16 buckles, a little bit of seat-belt webbing and reflective webbing, and most of the leno-lock mesh).  I could have bought a seat from Greg Peek at Longbikes for $149.00 plus $15 shipping and handling, so making my own was less than half the price, despite having a lot of excess parts.  Also, my new seat has fancy red straps that are reflective at night, and the new straps are polyester rather than nylon, so should perform a bit better (less stretching, more UV resistant, faster drying).

I decided to use dual-adjustable heavy-duty buckles, so that they would be less likely to break and easier to replace if they did break (no sewing of the buckles). I also ran the 1″ straps all the way across the seat at the top, bottom, and middle (3 of the 7 horizontal straps), as these seem to be the high-stress parts of the design (based on the stretching in the old seat).  I ran the warp horizontally, as I thought that would result in less stretching than having the warp vertical (as on my old seat)—I’ll let you know in 15 years how that works out (if I remember).

The new seat came out a bit heavier than the old one (545g rather than 470g), probably because of the straps across and heavier plastic hardware. The extra 75g is completely irrelevant give how much my bike and panniers weighs.

I used my wife’s sewing machine (my treadle machine needs a new leather belt, or tightening of the old belt).  My wife’s machine had no trouble with the mesh or the reflective webbing, but going through the seat-belt webbing sometimes caused it to have problems, particularly where it had to go through two layers of seat-belt webbing, a layer of reflective webbing, and the mesh.  I think that the treadle machine would have had less trouble, but I didn’t want to take the time to fix the belt on the treadle machine.

The seat came out looking ok, and I took a short ride on it today to buy groceries—it is as comfortable as the old seat and much nicer looking:

In this side view, you can't see much of the mesh seat—just the straps on the sides.

In this side view, you can’t see much of the mesh seat—just the straps on the sides.

From the front, the red straps match the red paint fairly well.

From the front, the red straps match the red paint fairly well.

By using a flash on my camera, I could light up the reflective straps even in the day time.

By using a flash on my camera, I could light up the reflective straps even in the day time.

From the rear, the reflective straps should add a fair amount of visibility at night.

From the rear, the reflective straps should add a fair amount of visibility at night.

The heavy-duty dual-adjustable buckles seem to work well.  By keeping the straps fairly short, I did not need to use the triglides or keepers to hold the ends of the straps in place—they aren't long enough to be a nuisance.

The heavy-duty dual-adjustable buckles seem to work well. By keeping the straps fairly short, I did not need to use the triglides or keepers to hold the ends of the straps in place—they aren’t long enough to be a nuisance.

UCSB orientation

Filed under: Uncategorized — gasstationwithoutpumps @ 11:37
Tags: , , , , ,

Last week I finally had an opportunity to visit UCSB, where my son will be going to college in the Fall.  This college visit was a bit different from other ones we did together, as he had already filed his statement of intent to register (in UC jargon, the “SIR”).  So we were not deciding whether UCSB was a good place to apply, or whether to accept an admissions offer—this was an orientation session for new freshmen and their parents.

Because we don’t drive, and the Santa Barbara airport is very expensive to fly to from anywhere but LA (and somewhat expensive even from LA), we took Amtrak to Santa Barbara.  There are multiple ways to get from Santa Cruz to UCSB by train, but we took the simplest and most familiar: Highway 17 express to San Jose ($5), Coast Starlight to Santa Barbara ($51, but the price is higher if you don’t buy the tickets far enough ahead of time), MTD bus 11 to UCSB ($1.75).  (Note: the number 11 bus is not as fast as the 24X or 15X express bus, but those don’t run on Sundays, which is when we were going to Santa Barbara.  The Coast Starlight is a rather slow train, and there are bus+train combinations that are faster, but both my son and I tend to get motion sick on buses, so we preferred the train.  One could take the Greyhound to Santa Barbara for only $37, but that is about 6 hours on the bus.  One could also take Greyhound to Salinas ($12), then Amtrak 4740 bus to San Luis Obispo and the Pacific Surfliner to Goleta ($42), and MTD bus 15X  (or walk a mile and take the 11 on weekends) to UCSB ($1.75).  One could also take the Coast Starlight from Salinas to Santa Barbara ($12+$39+$1.75), reducing the bus time to the same as taking the Highway 17 express, and the bus to Salinas is probably less of a roller-coaster ride than the bus to San Jose.  The connections are not tight, so no time is saved by catching the train in Salinas—the extra time is spent waiting at the Amtrak station in Salinas.

Because it was a two-day orientation, they put us up (separately) in dorms for the intervening night.  Because we were using Amtrak to get to Santa Barbara, we needed an extra night before and after, which we also spent in a (different) dorm, managed as the UCSB Summer Inn.  All the dorms were in Manzanita Village, which is the dorm complex my son has requested.  The dorms were spacious with lots of closet space, but a bit too warm—a fan would be a useful addition to the fall dorm supplies.  The mattresses were also a bit too firm for my taste—my son may want to get a softer foam pad to put on top of the mattress.

Bus service for UCSB is not bad on weekdays, but is a bit skimpy on weekends.  Even at its best, it is not as frequent as UCSC bus service. Of course, the UCSB campus is more compact than the UCSC campus, and UCSB students mostly live on campus or a short walk away from campus in Isla Vista, so bus service is not as necessary.  Also, the UCSB campus and surrounding area is flat, so bicycling is very easy (even with the low-efficiency “beach cruisers” that southern California finds fashionable).

UCSB seems to take bicycling fairly seriously in terms of infrastructure.  There are no roads through campus, but there is a major bikeway and lots of bike parking:

UCSB has bike paths on which bicyclists have priority over pedestrians, and traffic is heavy enough during the school year that they found it useful to put roundabouts at a couple of the major intersections of bike paths.

UCSB has bike paths on which bicyclists have priority over pedestrians, and traffic is heavy enough during the school year that they found it useful to put roundabouts at a couple of the major intersections of bike paths.

Bikes have the right of way on the paths, and (unlike roads) the pedestrian crosswalks do not give the pedestrians right of way.  There are warnings and textured strips to caution the pedestrians.  Bicyclists are expected to walk their bikes when on the pedestrian paths (with frequent warnings about heavy fines), but this rule seems to be routinely ignored.

Bikes have the right of way on the paths, and (unlike roads) the pedestrian crosswalks do not give the pedestrians right of way. There are warnings and textured strips to caution the pedestrians. Bicyclists are expected to walk their bikes when on the pedestrian paths (with frequent warnings about heavy fines), but this rule seems to be routinely ignored.

There is even a separate skateboard lane on one of the main campus paths (taking up part of a wide walkway and paralleling a divided bike path).

There is even a separate skateboard lane on one of the main campus paths (taking up part of a wide walkway and paralleling a divided bike path).

Bike parking is copious, often with seas of bike parking near classroom buildings.

Bike parking is copious, often with seas of bike parking near classroom buildings.

Most of the bike parking is of a style that alternates high and low, intended for allowing tight packing of the bikes without handlebars interfering.  There is an adequate locking point for the frame, but not for the rear wheel.

Most of the bike parking is of a style that alternates high and low, intended for allowing tight packing of the bikes without handlebars interfering. There is an adequate locking point for the frame, but not for the rear wheel.

In a couple of places, the "low" version of the bike parking is installed diagonally, where there is not sufficient space for the bikes to be perpendicular.  This view shows the locking loop beside the wheel-holder clearly.

In a couple of places, the “low” version of the bike parking is installed diagonally, where there is not sufficient space for the bikes to be perpendicular. This view shows the locking loop beside the wheel-holder clearly.

In a few places, UCSB has wheel-bender racks that provide neither support for the bikes nor adequate locking points—these were clearly selected by someone who did not park a bicycle.

In a few places, UCSB has wheel-bender racks that provide neither support for the bikes nor adequate locking points—these were clearly selected by someone who did not park a bicycle.

Although the campus is compact and easy to navigate in, it is not small. A walk from the dorms my son hopes to stay in (Manzanita village) to the College of Engineering (where many of the faculty he might do research with have offices) is about a mile. Given the distances, the flat terrain, and the mild weather, bicycling is probably the best way to get around campus.

I saw a number of cyclists at UCSB, but very few wearing helmets. We were warned that even experienced bicyclists should probably avoid cycling on campus for the first two weeks of Fall quarter, as there were a lot of bike crashes during that period, often caused by new cyclists who did not know what they were doing. It seems that UCSB’s infrastructure efforts are not matched by bike safety education efforts. Bike theft is also a major problem on campus—the suggestion is to get an ugly old bike and use a good lock. There is a sale of abandoned bikes during Welcome Week in the fall, and my son will probably get a bike then, rather than lugging his from home. I think that a 3-speed with front and rear brakes is probably the ideal bike for UCSB conditions—easier to maintain than derailleurs, but more efficient than a one-speed beach cruiser.

UCSB has a few more students than UCSC (22,225 students in Fall 2013 vs. 17,203 at UCSC), and a higher proportion of grad students (12.9% vs. 8.8% at UCSC).  At the orientation, UCSB claimed to have the highest proportion of undergrads of any R1 research university, but they achieved this status only by using a non-standard definition of an R1 university, using the 62 invitation-only members of the Association of American Universities, rather than the 108 “RU/VH: Research Universities (very high research activity)” in the Carnegie Classification of Institutions of Higher Education, which is the more commonly used definition of “R1 university”.  UCSC is on that bigger list, and probably does deserve the status of the R1 university with the highest ratio of undergrads, which UCSB was improperly claiming.

Although UCSB has only 23.4% more undergrads than UCSC, they have a few much larger class sizes.  They have several lecture halls seating 300 or more students (Campbell Hall @ 860, Isla Vista Theater 1 @529, Lotte Lehman Concert Hall @468, Chem 1179 @354, Buchanan 1910 @306).  UCSC has 3 classrooms with 300 or more and none over 500 (Classroom Unit 2 @472, Media Theater @382, and Humanities 3 Rm 206 @301).  Unless he changes his schedule, my son will have a class in Campbell Hall in the fall: Linear Algebra.  His other classes will be tiny, all being College of Creative Studies computer science classes for freshmen, so having 10–20 students.  I’ve been suggesting to him that he delay linear algebra by a quarter or two, so that he can take the CCS physics series with the CCS physics majors.  It isn’t clear that they’ll allow him to do that, but he neglected to tell his computer science adviser that he was interested, and the CCS physics adviser was not around to talk to, so he’ll have to ask about that by e-mail, if he decides to try it.

The orientation was carefully designed to separate students from the parents, with the students talking with advisers and with other students, and the faculty hearing from administrators (and a few students).  There were a few combined sessions for students and parents, but not many (campus tour, welcome assembly, half the College of Creative Studies meeting, an Education Abroad Program presentation).  Supposedly we could eat meals together, but my son managed to make friends with a few of the other CCS students, so he had both lunches and the Monday dinner with them—I only ate with him for the Tuesday breakfast.  Incidentally, the dining hall had fairly good food—better than any of the other campus dining halls we’ve eaten at—and we were told by students that this was not a special “for the parents” thing, but that the dining hall food was routinely that varied and that good.

Most of the presentations had very little new content for me, as they were aimed mainly at parents who had not been to college (UCSB, like all the UCs, takes pride in what a large proportion of their students are the first in their families to attend college).  I did pick up a few tidbits of useful information, like getting my son to add me to the e-mails about the bills from UCSB, so that I can transfer the funds from the Scholarshare 529 plan without the delay of waiting for him to forward the bills to me.

The meeting with the CCS students and a couple of the CCS faculty was worthwhile, but sending us to the Letters and Sciences panel discussion afterwards was a waste of time—I would rather have had a chance to hear from the engineering faculty or students.

The only really good presentation was the “Packing, Prepping, and Parting” presentation for parents Monday night.  It was very entertaining, but made the strong assumption that all students would come by car.  Since packing and prepping are even more challenging for those coming by plane, bus, or train, it would have been useful to spend a little time on that. There was a brief mention of the Amtrak station in Goleta (which serves the Pacific Surfliner only, not the Coast Starlight) and no mention that MTD bus service to the train stations is limited on weekends (the move-in days), since the 15X doesn’t run on weekends.

The presentation “your student’s first year” was so generic as to be useless.  I was also rather surprised to see some copyrighted cartoons copied off the web without permission (the watermarks to show that these were unlicensed were still clearly visible).  UCSB is not so poor that they need to steal intellectual property from cartoonists, and it sends a very bad message to students about plagiarism and intellectual property to be so cavalier about copyright in an official university presentation.

The EAP (education abroad program) was one of 7 “co-curricular workshops” that we could choose among to attend together, none of which sounded very interesting.  My son fell asleep during it and I nearly did—they could have provided a lot more content in a much less boring presentation.

The Tuesday afternoon was dedicated to registering for classes, but the CCS students got that done early, so we had the afternoon to wander around campus, taking our own tour of the science and engineering buildings, which had only been pointed out as being “over there” in the general campus tour.  We did not go down to the beach, though that might have been a pleasant option to cool off.  We rarely get to the beach in Santa Cruz either—it is not big on our list of fun things to do.

On Sunday night and Tuesday night we ate in Isla Vista.  We had Indian food at Naan Stop Sunday night and Vietnamese food at Pho Bistro Tuesday night.  Both were adequate of their kind and clearly local rather than chains.  Isla Vista has about 30 eateries, all of “college student” styles, so there are places to go if the dorm food gets too repetitive, though the variety is somewhat limited.  Isla Vista does not have much else in the way of retail—students are expected to go 2 ½ miles to Kmart, Costco, Home Depot, and other shopping-center stores in Goleta for anything other than convenience store or bike store stuff.

Overall, my impression of UCSB is that it will be a good place for my son to go to school.  The CCS program gives him some small classes, good access to research opportunities, very flexible general-ed requirements, and an easy way to meet fellow geeks. He’ll have to put up with a few large classes (like linear algebra), but he’ll be past them fairly quickly if he makes good choices. The campus is well set up for bicycling, and it is not overly influenced by fraternity or sports culture (though both are more accessible than at UCSC).  The culture is a little more “southern California” than he is used to, but I don’t know if he’ll even notice the difference, other than the greater exposure of skin, which is driven more by the climate than by the culture.

 

2014 July 18

How to sell a widget

SparkFun has a tutorial on how to sell “widgets” through them at How to Sell Your Widget on SparkFun – Learn.SFE:

Have an awesome electronic widget that you want to get to market? Great! We are always listening for new ideas from our customers and the community. We get many inquires on this topic, so read this tutorial carefully to keep your product pitch from getting lost in the shuffle.

We’re lucky, here at SparkFun, to have an amazingly creative and talented group of customers. Not only can they identify gaps in the catalogs of electronics suppliers, they can create a gizmo that fills that hole. But, going beyond a prototype or even a limited-quantity production run, often the hardest task in getting your world-altering product out there is producing, marketing, and/or selling it to the masses. That’s where we come into the picture.

The advice there is seems fairly reasonable.  They offer a choice of two models: make it yourself and have them sell for you, or have them make it and pay you royalties.  They tell you how to pitch products to them and how to design for them to be able to manufacture stuff.  Most of the stuff they sell is PCB boards, so they give quite a bit of advice about PCB design to fit their pipeline—they use Eagle, but ‘All parts are placed on a 0.005″ grid. If possible, use a .05″ grid.’ That must get irritating with modern parts that are convenient round metric numbers, not mils.   They also want version numbers in the bottom copper, which is reasonable for some designs, but not all.  They also encourage people to use their Eagle libraries, but my son and I have found their pad layouts to be very sloppy (putting silkscreen over SMD layers, getting the keep-out areas off by a little bit, not fixing the fonts on the “>NAME” and “>VALUE” labels to meet their own requirements, and so forth).

Still, it is good that they put out their design rules and provide clear guidelines to new designers.

I’ve thought a few times about putting out some of my designs through SparkFun or Adafruit Industries—perhaps an improved version of the blinky EKG as a kit.  SparkFun now sells EKG electrodes and snap leads for them, and even have a heart-rate monitor board (based on the AD8232 chip) and the “BITalino” biomedical board, so I suspect that they are interested in the market.

The BITalino is outrageously expensive and their EKG electrodes are about 3 times the price of buying them at Amazon, but the AD8232 chip actually looks like a nice one for building an EKG front-end and reasonably priced, so I’m not sure they’d have much interest in a through-hole part kit for do-it-yourself EKG that isn’t quite as good, unless it could be sold very cheaply or as an educational product (which is what the blinky EKG is aimed at, anyway).

I have some other ideas for products that I might be marketable, but I don’t know whether I have the time to refine them to the point of pitching them to SparkFun.  I can justify some time spent on doing electronics as a hobby, some as necessary learning for teaching my applied electronics course for bioengineers, and some as engineering-for-manufacture experience (something I never had any instruction in, despite my years as an engineering professor). But when the electronics work starts cutting into the time I need to spend on writing my book, teaching my classes, or doing collaborative research with other faculty, then I have to draw the line.  I’ve also got a lot of administrative responsibilities now (undergrad director and faculty adviser for two BS degrees, Program Chair for bioengineering, and Vice Chair for the Biomolecular Engineering Department), so writing time and research time have gotten doubly precious.

I do have one project this summer that I’m going to try to get fabricated for me—it is all SMD parts, including some that are hard to solder by hand (pads under the chips), so I don’t want to do it myself.  The project also calls for a lot of identical boards (20 to 50 of them), so a prototyping house seems like the way to go.

I’m looking currently at Smart Prototyping to do the PC board fabrication and assembly—they may not be the cheapest, but they have a comprehensible pricing scheme on their website, and they replied within 12 hours of my request for a quote. They also have a nearly turnkey system—I send them the Gerber files and the Bill of Materials (BOM), and they’ll make the boards, buy the parts, and assemble the boards.  They’ll even test them for an additional charge, though these boards are simple enough that I can test them myself at about 5 seconds a board, so their testing would not be worthwhile unless they guaranteed their assembly (which none of the prototype houses can afford to do with untested designs).

I also considered Elecrow, which has a similar service, but their pricing information on the web page is rather vague: “For BGA or IC with pads under IC, The quotation will be a little higher.” and “We will give a discount for the PCB assembly service according the some factors (assembly time,Hard or Easy to assemble or requirements etc.).” I prefer sites that have clear pricing even if it is slightly higher, so that there are no surprises. I suppose I could ask Elecrow for a quote and see if they respond as promptly as Smart Prototyping did.

Incidentally, my design does not follow all SparkFun’s guidelines—for one thing, I placed parts on a 0.5mm grid, not a 0.005″ gird, and the board is not rectangular.  Still, if the design I’m working on turns out well, I might pitch it to them, as I see some potential for it appealing to the open-source hardware market, and the violations of their design guidelines made good sense for this application.  Note: I’m deliberately not saying what the design is—I’ll reveal it once I’ve gotten a working prototype, when I’ve decided whether I want to commercialize it or not.

2014 July 14

Need new mesh seat for recumbent

Filed under: Uncategorized — gasstationwithoutpumps @ 21:28
Tags: , , ,

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.

 

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