Gas station without pumps

2016 October 5

Broken bike seat

Filed under: Uncategorized — gasstationwithoutpumps @ 17:25
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Yesterday was not a good day for me.

First, I spent most of the day struggling with the homework for the control-theory class I’m sitting in on. The course is dual listed as an undergrad and grad course, with shared lectures but different homework and projects. The undergrad part of the homework was straight-forward, and I finished it Monday night, but the two additional problems for the grad students were tough.  One of them had a simple “engineering” solution that I got quickly by formal manipulation of the formulæ, but I could not justify some of the steps, since they involved a integral that was not finite.  The other problem was not difficult, but involved a rather tedious amount of algebra to linearize the system—the professor had done the linearization in lecture notes,  and we were just supposed to check it for the homework, but he’d made an error in algebra, so I had to redo the whole thing.

Late in the afternoon, I decided to take a break and replace the sump pump that had failed sometime in the past couple of weeks.  Originally I was going to disassemble the pump and see if the problem was repairable (I think that the switch for the float is not turning on reliably, possibly from corroded contacts), but I decided that I could do that later to have a spare pump, meanwhile getting a working sump pump.  (My house is built over a seep where an aquifer comes to the surface, and the water table is about 3 inches below the surface—during wet years, the water table is sometimes right at the surface.)

I put the old pump in my panniers and headed down to hardware store, when my bike seat suddenly failed.  I tried riding for a block with the failed seat and gave up and returned home.  The failure was right at edge of the block that holds the horizontal crossbar at the front of the seat:

Here is a view from the front showing the tubing displaced vertically from where it belongs.

Here is a view from the front showing the tubing displaced vertically from where it belongs.

A closer view shows a very clean break right at the surface of the block that clamps around the tube.

A closer view shows a very clean break right at the surface of the block that clamps around the tube.

I probably should have had some warning about the imminent failure, as the bike has been creaking a bit more than usual when I pedal for the past several months, but I was never able to track down the creaking. I’m not sure I could have seen the crack that was probably propagating, since it was flush with clamp block.

The seat on my Longbikes Vanguard is not a standard, off-the-shelf component, so I’m probably going to have to custom order a new seat from the manufacturer (who no longer make the Vanguard model, so probably has no spare seats built) and wait weeks or months for one to be built.

I got my old upright bike down from the garage wall, inflated the tires, adjusted one of my panniers to fit the different rack, and headed off to the hardware store, carrying the old pump in the pannier. At the hardware store, I could not find a sump pump with the same outlet size as the old one (they all had bigger outlets). I needed to match, in order to hook the sump pump up to the existing plumbing. Luckily, they did have a reducer that would adjust for the difference.

After buying the pump, I went out to my bike and realized that I couldn’t fit both pumps into one pannier—in fact the new boxed sump pump wouldn’t fit into the pannier even by itself. Normally I carry a bungee cord or two for strapping stuff onto my rear rack, but those were left on the other bike. So I had to go back into the hardware store to buy some new bungee cords—not a big deal, but an irritation.

The bike was a bit wobbly on the way home—I’d forgotten how much difference a high center of gravity makes on an upright bike—and the bike has much twitchier steering than my recumbent anyway—but I got home without incident.

On getting home, I immediately attached the pluming to the new sump pump and lowered it into the sump. Let me correct that—I tried to lower it into the sump, but it wouldn’t fit. The pump was a couple of inches wider than the old pump and though the hole at the top was more than wide enough, it narrowed significantly where the bottom of the foundation for the house spread out, and the remaining hole was simply too small for the new pump. This was particularly frustrating for me, as I was meeting my wife downtown for dinner in less than an hour, and I was going to have to walk rather than bike, so I only had about 10 minutes to come up with a fix.

I then remembered something that should have occurred to me much earlier—I had another one of the small sump pumps in a different sump in the back garden. Quickly pulling it out and attaching the plumbing got the main sump working again (though I still need to recheck the plumbing for leaks). And it turned out that the garden sump was wide enough to accept the new pump—problem solved!

I cleaned up, grabbed a backpack so I could do some shopping after dinner, and walked down to the library to meet my wife. After the stresses of the day, I felt the need for comfort food, so we went to Betty’s Noodles, a hole-in-the-wall Chinese restaurant in the bus station. This restaurant has taken over the niche that Little Shanghai used to fill of providing cheap, tasty Chinese fast food (noodles and rice bowls).  I had ma-po tofu over Chow Fun noodles, which went a long way to de-stress me.  Going to Mission Hill Creamery for a plum sorbet cone afterwards helped also.

On the walk home, a couple blocks before we got home, I realized that I had not done my shopping! I decided not to go back downtown, but to do without my chocolate soymilk for a couple of days, until I can go shopping again.

This morning I finished the homework and submitted it. I’m still a bit bothered about the inverse Laplace transform problem that  can be formally solved but that ends up with a function that doesn’t have a Laplace transform, but I’m pretty sure I did what was expected. After turning in the homework, I realized that there was a possible different interpretation of part of the linearization question than what I did, so I queried the professor about what he really meant.  (The homework isn’t due for a week, so if there is a clarification needed, he can get it to the grad students before the homework is due.)

The TA does not grade my homework, since I’m just auditing, but I’m doing the homework using Python instead of Matlab, so I’m sharing it with the TA and professor anyway, so they can see whether it would be worth switching to free tools.

Currently, the scipy.signal package and matplotlib seem as easy to use at Matlab, but there is no equivalent of SIMULINK, which the professor is relying on for students doing simulations.  I can do the simulations in Python, but setting them up is all text-based, and requires thinking explicitly about the state vector, rather than having a GUI that does all the setup for you.

I bicycled up to campus today on my old upright, after adjusting my other pannier to fit the rack.  I had forgotten how uncomfortable an upright bike is.  This evening my neck and shoulders are sore, and I have chafing on the inside of my thigh.  I really hope I can get the recumbent seat replaced quickly, so that I can go back to riding comfortably!  It might even be worth taking the seat to a local frame-builder and finding out whether they could replace the tube, even if only for a temporary fix. (Although most of the bike is chrome-moly steel, the seat appears to be all aluminum tubing.)


  1. John Caletti is an excellent frame-builder in Santa Cruz. I don’t know if he does this kind of work.
    Indomitable: switching bikes, buying bungees, swapping pumps, etc. Well done!

    Comment by miguelaznar — 2016 October 5 @ 17:52 | Reply

    • Thanks for the pointer. I see the John Caletti works in titanium and steel, not aluminum, so may not have the right tools or experience for welding aluminum tubes. His bikes do look nice for uprights, though.

      Comment by gasstationwithoutpumps — 2016 October 5 @ 20:07 | Reply

      • Two random things… one, John Caletti is Cory (Irimes) Caletti’s (of RTC) husband. He does good work… I can’t recall, but, I have a vague recollection he does tig welding. Other thing is, have you ever considered making your own soy milk? I have a soy milk machine that I’m not using much (I was really into it for a while) because I now am into making nut milks. If you want to try it for a while, let me know. It is more work, but, it’s a nice DIY project that saves money, avoids aseptic packaging, and you get to control things like chocolate level, sweetness/sweetener, etc.

        Comment by whatisron — 2016 October 5 @ 21:01 | Reply

        • I’m sure that John does good work with his chosen materials (Ti and steel), but if I go for a repair, I’d rather work with someone who has a lot of aluminum experience.

          I’ve thought about making soy milk, but it seems like a lot of trouble for the small amount I use (about 700ml a week). Also, it would take me a long time to find a recipe that I liked—most of the commercial soy milks are almost undrinkable for me. (I happen to like the organic soy milk from Trader Joe’s, though I’m sure it is full of emulsifiers, sugars, and other things that purists would disdain.)

          Comment by gasstationwithoutpumps — 2016 October 5 @ 21:08 | Reply

  2. The only thing I can help with is the bike seat. I was a bike mechanic for 14 years. A temporary fix would be to find an aluminum slug (solid not a tube) the size of the inside diameter of the broken seat tube and glue it in temporarily. I doubt you will be able to repair the break with a weld. That is a really bad stress location for a weld. The weld will be strong but both sides will not be. Welding aluminum does all sorts of bad things to the temper.

    Comment by gflint — 2016 October 10 @ 11:53 | Reply

    • I agree that any repair will need to add a new inner core to provide strength. I had not thought of gluing rather than welding. Either approach brings up the same problem for me—how do we get the new core into the tubing? It is possible to displace the tubing enough to insert a rod in one side, but how do we get it correctly positioned spanning the break? If it is going to provide sufficient strength, it can’t be so loose that it can slide around inside the tubing.

      Comment by gasstationwithoutpumps — 2016 October 10 @ 18:04 | Reply

      • The only way it would work is if you can spread the broken ends a couple of cm apart. Tap the slug into the clamped side with a rubber hammer (a regular hammer might mushroom the end) and let the glue set. Then flex the unclamped end enough to slip onto the slug and glue it in. Definitely not a long term fix but it might get you back on the road. I have done the same type of repair with wood doweling. The wood does not require glue because it can be a lot tighter tolerance but it does not look like you have a good way of tapping the ends of the break together once the dowel is inserted. I sure hope the bike builder has a replacement part. If they do not then you are going to have to get real clever. If you try the aluminum slug solution be sure to freeze the slug first. If the tolerance is really close the freezing will reduce the slug’s diameter and when it warms up it will be really tight. This is how you get steel bearing races to fit real tight in aluminum engine cases. Freeze one, heat the other.

        Comment by gflint — 2016 October 11 @ 07:16 | Reply

  3. I removed the seat from the bike today and measured the ID of the tubing. It seems to be 1.54cm ID, or 19.4/32″. I want a 15mm diameter rod about 10cm long, probably 6061 alloy with T6 temper.

    At first I couldn’t find any, but I did find 9/16″, which is 14.3mm, and I ordered a piece. The question now is whether having an extra 1.1mm clearance would provide sufficient strength, assuming that I used a thick layer of epoxy as the adhesive.

    Since then I found that McMaster Carr has 15mm tempered aluminum rods: 15±0.23mm 6061-T6 (part number 4634T39) for $4.03/ft and stronger 7075 alloy (with T6 or T651 temper) with tighter tolerance for $19.92/ft (part number 9403T57). Now I have to decide which to get. Is the extra strength needed here? Does the somewhat higher corrosion resistance of 6061 make it a better choice? (I think that being fully enclosed inside an aluminum tube should make the corrosion resistance fairly irrelevant.)

    Comment by gasstationwithoutpumps — 2016 October 11 @ 14:30 | Reply

    • I would go with the tighter tolerance. You can always sand it a little.

      Comment by gflint — 2016 October 12 @ 09:59 | Reply

      • I decided to use the 15mm 6061-T6 aluminum. It should arrive this weekend, I think.

        Now I need to decide what adhesive to use. I only really get one shot at this, and I’ve not previously done aluminum to aluminum gluing, so I don’t have experience to base it on.

        Comment by gasstationwithoutpumps — 2016 October 12 @ 10:04 | Reply

        • I would try something that stays flexible. The glue is just there to keep the slug from moving laterally. A hard glue like epoxy might just break up when the seat flexes. Depending on how tight the slug fits a silicone glue would be what I would try. An engine re-builder at a shop might know of some good stuff that will stay flexible over a long tome.

          Comment by gflint — 2016 October 13 @ 07:23 | Reply

          • Most of the easily available silicone adhesives are one-component adhesives that rely on reactions with water in the air. That doesn’t work well for a thin layer between two metal parts—the water would not penetrate more than a cm or so into the layer, so most would end up unglued. The two-part silicone adhesives don’t seem to be sold much to consumers—the two-part ones require special dispensing guns (like caulking guns but with two plungers).

            A flexible epoxy like 3M Scotch-Weld Epoxy Adhesive DP100 might be a good choice, but it also relies on a $50–$60 applicator gun.

            Flexible epoxies that don’t need applicators (like MarineTex FlexSet, G/flex, or TotalBoat FlexEpox) may be a better choice for me.

            Comment by gasstationwithoutpumps — 2016 October 13 @ 11:32

  4. […] started figuring out how to fix my bike seat (see Broken bike seat).  The idea is to insert at 15mm diameter aluminum rod (6061 alloy, tempered to T6) that spans the […]

    Pingback by Flexible adhesives | Gas station without pumps — 2016 October 13 @ 13:23 | Reply

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