Q&A: Broken frame: seatpost binder; best tire pressure; Auto Bike derailleur

Here are three interesting bicycle technical questions that arrived to kick-off June:

Q: Hi Jim,
I've long followed your work on RBR and on your website and really appreciate both (although I'll confess that the whirring freewheel on your website makes me reach for the volume knob!).

I have an unusual problem that perhaps you can give some advice on.

The bike is an aluminum frame Danish "city" bike, V-brakes, Shimano Nexus hub, etc. The problem is that the top of the frame's seat tube has broken and will no longer hold the seatpost in position (the design is not a separate seatpost clamp, rather the older style bolt through the "ears" at the top of the frame). We've considered and discarded the following ideas:

1. Weld the ears back onto the frame. As mentioned, it is aluminum and we don't have the technology for that.
2. File off the ears and get the biggest available seatpost clamp to fit over the top of the tube. A no-go since there isn't enough metal and seatpost clamps are not big enough to make this work.
3. Pour some corrosion-inducing agent into the seatpost to lock it in place. Unfortunately the seatpost could never be moved again...
4. Drill a hole through the seat tube and seatpost and insert a bolt. This one just seems like a bad idea.

My latest idea is to buy or fabricate a new seatpost that acts as an expanding wedge such as older handlebar stems. I can only find very short expanding wedge seatposts for BMX bikes on the Internet. Do you have any experience with such a thing?

Thanks!
Tim

A: Thanks for the kind words about my site, Tim, and sorry about my freewheel noise – there’s a long story behind that but I’ll spare you.

Without seeing it, it’s hard to know if this would work, but it’s pretty easy to work with steel. So you might be able to get a flat piece of steel, maybe an inch wide, about 1/8 inch thick. Home Depot has stuff like this and Ace Hardware, etc. You might find a piece almost anywhere too, some recycled metal or trash.

Next, you would make a cardboard or paper template, wrapping paper around your seat tube and shaping it into a ring with an open end with tabs jutting out 90-degrees on either end.

You would then lay your template on the steel and trace the shape. Drill holes in the ends (that will be the tabs that your new seat bolt goes through). Then you would cut the shape out of the steel with a hacksaw and file.

Then you’d shape the steel, probably bending the tabs in the ends first since that will be easy and then shaping the steel into a ring. For that you might use your frame as a bending jig, or your seatpost.

If you get good steel, this should work, as long as there’s enough room on the frame to fit it on there. If the tabs bend when you tighten the bolt, you could try better steel, or try heat treating your clamp by heating it cherry red with a propane torch and then dropping it into water to quench and harden it.

If the seatpost is a good fit, it shouldn’t take too much to tighten it. You can paint your seat collar to prevent rust and look good.

Your idea of a wedge type seatpost is a good fix too. Those are available but hard to find. I recently saw that the Swiss company BMC uses one in their Team road bike (photo), but it’s carbon and expensive. The way they made theirs was clever though and the concept might work great for you.

They have a rubber piece at the bottom. When you tighten the bolt at the top, it pulls up and mushrooms the rubber piece and that locks the seatpost in the frame. That might be an easy way to make something like this for you.

You just need a polyurethane donut the same diameter of your seatpost (or maybe just plain old rubber will work). Then you need any threaded rod that passes through the seatpost and through the elastomers/polyurethane donut. Put a washer and nut on the top and bottom. Locktite the bottom nut or crush the threads so it can’t vibrate off. You would only need to tighten the top nut to compress the elastomers and lock your seatpost in the frame.

After writing this, I think the elastomers idea would be easier to try than making your own seat collar, so maybe you should start with that.

I hope this helps you fix the seatpost. Let me know how it goes,
Jim
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Q: Jim, could you please settle the debate about tire pressure? I found only a little better ride at 83 psi, but less rolling resistance at 106. I run 700 x 28c Continental Gatorskin tires and my weight is 220 pounds. Is there a more ideal way to make a test run other than to try to just make the route the same at both psi and check the effort? I don’t have a watt meter. I normally run at 100 and am very happy to be getting about 2,200 miles on the tires but like everyone else looking to be better. What do you recommend?

Pam

A: Hi Pam,
I’m 160 pounds and when I ride on 28c tires, I run 90 psi front, 95 psi rear, and I do fine on the awful roads around here. On my standard Continental Grand Prix 4000 training tires, which are 25c, I ride 95 psi front, 100 psi rear since there’s less air in the tire.

Since you have 60 pounds on me, you need to ride at significantly higher pressures. Tire pressure and how it works for you is all about how much weight is on the tires and how much air is in the tires. Wider tires hold more air and more air supports more weight better.

So, I would recommend trying even more pressure, say 110 pounds in the front and 115 in the rear. This should feel better than 106 psi and provide an efficient ride. If you don’t feel a difference, bump it up slightly more to 115 front/120 rear. I can’t remember what the label on your tire says for max inflation, but my best guess is that it’s around 125 psi to 140 psi. At 220 pounds you should be toward the high end of the scale or else your tires will mushroom too much and add drag and not offer enough cushioning/support/grip.

Here’s a Bicycle Quarterly article (great magazine, by the way) with a more technical explanation and a way to actually measure your tires to determine the best pressure for your weight. This is based on research done by engineer Frank Berto who used to work at Bicycling Magazine with me. He’s a smart guy and author of the book The Dancing Chain. That's a history of bicycle derailleurs, which is a nice segue to the next Q & A...

Hope this helps you enjoy your tires more,
Jim
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Q: Greetings Jim,
I have a Auto Bike. You know, the one that shifts on its own, which is why it's called the "Auto" Bike. The rear derailleur shifts into the different gears according to the speed of the rear wheel. And it does it because it's a special rear derailleur that reacts to weights spinning on the rear wheel.

The trouble is that my Auto Bike is used and I got it without that special rear derailleur - unfortunately. Do you know where I can find one? I've asked the local bike shop but they don't even want to bother with it. I've looked online but can't seem to find anything, just discussions on how bad the bike is.

I like the bike (I like weird things) - even though it's only a one-speed right now. If I can get an Auto Bike derailleur and get the automatic shifting working again, I think it will be a good bike for my wife. Can you please help me find one?

Got my fingers crossed!
David

A: Thanks for the email, David. I don’t know if they will be able to provide the right derailleur to make your Auto Bike auto shift again, but why don’t you try contacting the company that appears to have taken over where Auto Bike left off. They're called LandRider. The bike is different but it does auto shift, so maybe, just maybe, their derailleur will work on your bike.

If they can’t help you, what I would try is to find one on eBay by searching on “Auto Bike” once a week for a while. You might find one if you’re lucky. You might also see an entire Auto Bike for sale and if it is not too expensive it might be worth it to buy the whole bike to get the derailleur as you’d also get all the other parts and would then have backups for everything.

Last, you could put a regular shifting system on that bike if you wanted, but then it wouldn’t auto shift so that may not be of interest to you. But, just in case, that’s an option.

Hope this helps – and I think the Auto Bike is kind of cool. You can’t argue with the ingenious way it shifts and it’s sure a no-brainer to ride it. I think some bike shops don’t like it because they know it was sold online for cheap and wasn't designed to be a quality bicycle that's easy to service. But, if it was me, I’d be happy to help you fix it because it’s really fun fixing unusual bicycles and a sign of a good mechanic who can fix things even if it’s not that familiar to him.

So, you might want to ask at some other shops. It isn’t out of the realm of possibility that some shop in your area has worked on Auto Bikes and might even have a used derailleur that they took off one that they could use to fix up your bike. You would have to get a little lucky, but I always saved everything in my shop and there are other shops that do this.

You could just get on the phone and keep calling shops. You might get lucky. You only have to find a shop that likes helping all cyclists and keeps a wide assortment of used/old parts.

Let me know how it goes and have fun with that Auto Bike,
Jim
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