THE CONVENIENT CYCLIST

Frame strength matters more with folding bikes

A few weeks ago, I was having a phone conversation with a caller in the folding bike market when he suddenly realized the bikes he was asking about weren’t Walmart-priced. He was polite, but couldn’t get off the phone fast enough, so he could find a cheaper bike.

As I hung up, my thoughts were about what that actually means, engineering-wise. A bicycle in use must respond well to a barrage of repetitive forces from many different directions. Structurally, what do you give up when you optimize for cheap?

Some people, like jamalik on the SimScale Forum, study this topic for fun. He simulated test loads of a typical bicycle under two routine stress conditions:

  1. Sitting and riding on an irregular road.
  2. Leaning forward while braking.

What parts of a bike get the most stress?

 

Finite element analysis (FEA) has a nice standard for displaying data. The hotter the color (red at the top), the more stress. The cooler the color (with blue at the bottom), the less stress. Here’s how a typical bike looks during standard riding.

When seated and riding, the most stressful area is the seat tube.

Basically, the stress here is like pushing down on a horizontal straw, except supported by a dual triangles that smoothly disperse the forces. Triangles are important!

But when you’re braking hard, and pushed forward by momentum, the forces on the frame change completely.

Now the top stress area shifts to the front of the frame.

Braking introduces the new variable of  displacement–how much your frame flexes under a heavy load. It’s not inches as depicted here, but it is enough for experienced riders to notice and appreciate the displacement differences among different frame materials (steel, aluminum, titanium, carbon).

Even more than last time, you can see the importance of that front frame triangle. Not only does it disperse loads better, but here it minimizes displacement that affects ride quality!

The front triangle of a CHANGE bike is, well… normal.

A standard front triangle in a CHANGE bike leads to less displacement, a better ride, and less chance of a broken frame (and the only ruggedness-certified folding bike in the world).

In the folding bike arena, the closest other bike to this design is Montague, which uses a smaller triangle connected to the top tube.

It’s not a big triangle, but it’s something.

Referring back to our two stress diagrams, this design may be just as good for sitting and pedaling as a regular bike. On one hand, it lessens the seat tube stress by providing a second way for vertical forces to dissipate down. But some of that force also gets transferred to a tube with a hole drilled in the middle! At best, it’s a wash.

Displacement is another issue. All forces causing displacement at the front of the bike must pass through the top tube, which is why it’s so big, beefy, and oval. Maybe that extra mass is enough to counter the extra flex during braking. If so, it still sends all the forces back toward the seat. Does that make a difference?

Maybe. It certainly doesn’t help.

What about “cheap” folding bikes?

 

When you eliminate that second tube in the front (called the downtube) and don’t provide ANY triangle at all to disperse the forces, anything can and does happen.

  • Weight becomes an issue because you’re pushing sideways on a single hinge joint–think of extended sideways pressure on your knee.
  • Flex (and frame weight) becomes an issue because you’re riding on one long, beefed-up bar.

As a result, this happens…

Tern bike: hinge joint shear

Or this…

Dahon bike: hinge pin failure

In all fairness, you can save money initially by buying a Dahon or Tern bike. They are among the most popular low-end bikes. But if you are really going to ride your bike, with the feel and utility of a full-size bike, then you’re ultimately looking for an entirely different experience.

The diamond frame is a time-tested, stress-optimized design for the best, most reliable bicycling experience. Don’t settle for less.

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Bob Forgrave
President, Flatbike.com

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8 thoughts on “Frame strength matters more with folding bikes

    1. I imagine it cost quite a bit! I see it’s electric.

      Unfortunately, that also means battery and motor weight pushing down on a single joint weld, that ultimately failed.

      This raises a good point, that even with an exceptionally strong hinge, the weld on either side can fail. So in a way, this article is wrong. A Dahon frame design does not have a single point of failure, but three of them together–any of which can break the bike in two. And two of the failure points are highly dependent upon welding quality issues (correct heat, not too much grinding, right filler, etc.). Ultimately, though, the forces are simply too great for this design.

      Why did it break? A rider puts significant torque (rotational force) on the frame, but the force is greater on the side of the hinge with the rider, motor, and battery. So that’s the weld side that broke. Expected physics, and no frame reliability certification.

  1. this blog made me make up my mind as I am planning to buy a “fairly priced” folding bike (space problem at home) but worrying about the joint concerns because I am living in a highly urbanized city with lots of bumps and humps on the road and thinking that the joint will eventually give up caused by continues stress. thanks so much for your input.. will just stick with a regular mountain bike 🙂

    1. Good research, Karlo. Ultimately, you’re looking for affordable, reliable transportation that fits anywhere. A cheap folder and a full-size mountain bike each fit two of these, but you don’t want to give up reliability/safety.

      BTW, you may be interested in how another MTB rider made his peace with a 500 sqft apartment–in his own words: https://flatbike.com/when-less-is-more/

      1. Indeed, good research. I used a Dahon Mu P8 for about 5 years and 18000 km of commuting, thus fast ride, not weekly leisure in the park. And the hinge broke, actually pieces of the frame that plug into each other inside the hinge. Some said I got a good service from this bike. But it still hurt to look at it broken, useless. I’m now using a mountain bike.

        1. Thanks, Cristi! 18,000 km on a hinge-folding bike is indeed a long time between failures–congrats! Not surprising at all to see the hinge as the weak point. It’s also a lot of distance on 20″ (pothole-sized) wheels. While that size of wheel is the most convenient for tiny packing, it also transfers the most jarring to frame and body. Ouch. Your MTB must be a lot more comfortable and reliable.

          One point in this article is that you don’t have to choose between folding and reliability. As a CHANGE dealer for four years, selling full-size folding bikes, we’ve heard of no frame failures ever and only two component failures. Now you can ride an MTB, or rugged hybrid or road bike, AND enjoy the convenience of a bike that folds!

  2. Hello, I live in Lima Peru and I’m looking for an electric folding bike. Lima is a terribly crowded city and the traffic gets worse every month. I wanted to change from driving to riding a bike. I’m close enough to work that I can do this but far enough that a normal bike will make me arrive at work wanting to sleep (I’m a teacher so I need all the energy I can).

    I see many brands mentioned but I was wondering if you have any good folding electric bike brands that you could recommend. I want a folding bike because there are places I need to go to that are very far for a bike but in that sector I need to move a lot. Hence wanting a folding bike I could take on the bus or taxi to that place and then move around freely. But I am concerned with how well a folding bike can manage the streets of Lima, which look like minefields in several parts. Any recommendations? Thanks a ton?

    1. Hi, Sofia. I’ve been researching this for a couple of days. It’s a tough dilemma, because there are three assumptions with folding electric bikes:
      1) A battery and a motor will add at least 10kg to your bike.
      2) A folding single top tube needs to be beefed up for strength, adding another few kg.
      3) You have a motor anyway, so the extra weight won’t matter, right?

      Put these together, and you have an electric folding bike that may be rugged enough for Lima roads, and may fold small enough for taxis and buses, but you are heaving 25kg on and off transit, which is a workout by itself. Looking at what’s in the market, the MOAR e-bike folds, and has passed EN-14766 ruggedness testing, which means something. But I can’t get their website to load, don’t know weight, and do know that the bikes are made in China, so I don’t know what tradeoffs they’ve made. It’s a fat-tire bike so that adds even more weight.

      One possibility is a CHANGE 611 with a Geoorbital wheel–an electric, removable front wheel with built-in battery. This gets the frame weight down, and means that you can lift the bike in an out of the taxi in two parts: an 11-kg bike and a 10-kg wheel. Putting them together again takes one minute. But with 26″ wheels, a custom disc brake is needed. After that initial effort, you’re on your way…

      Does that help?

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