Fail Of The Week: How Not To Build Your Own Motorcycle

There’s a saying among writers that goes something like “Everyone has a novel in them, but in most cases that’s where it should stay”. Its source is the subject of some dispute, but it remains sage advice that wannabe authors should remember on dark and stormy nights.

It is possible that a similar saying could be constructed among hackers and makers: that every one of us has at least one motor vehicle within, held back only by the lack of available time, budget, and workshop space. And like the writers, within is probably where most of them should stay.

[TheFrostyman] might have had cause to heed such advice. For blessed with a workshop, a hundred dollars, and the free time of a 15-year-old, he’s built his first motorcycle. It’s a machine of which he seems inordinately proud, a hardtail with a stance somewhere closer to a café racer and powered by what looks like a clone of the ubiquitous Honda 50 engine.

Unfortunately for him, though the machine looks about as cool a ride as any 15-year-old could hope to own it could also serve as a textbook example of how not to build a safe motorcycle. In fact, we’d go further than that, it’s a deathtrap that we hope he takes a second look at and never ever rides. It’s worth running through some of its deficiencies not for a laugh at his expense but to gain some understanding of motorcycle design.

This bike has a classic tubular frame in which the engine sits and is not part of the bike’s structure. Modern sports bikes will usually adopt a different approach in which the engine, as the largest and most rigid component, becomes part of the structure, but the Honda engine hails from a different era.

Frame Design and Kinky Pipe

How the headstock on a tubular motorcycle frame should be attached, a reproduction of a Norton "Featherbed" frame. Flattrackers and Caferacers, (CC BY-SA 2.0) via Flickr.
How the headstock on a tubular motorcycle frame should be attached, a reproduction of a Norton “Featherbed” frame. Flattrackers and Caferacers, (CC BY-SA 2.0) via Flickr.

At the front of a well-designed frame, all the tubes should meet at the headstock. The pivot on which the forks sit, a vertical section of tube which forms the axis of motion for the steering. This section of the frame is crucial to the integrity and safety of the bike as it experiences some of the largest forces while on the road, so if you were to examine this section of a commercial frame you’d find some of the heftiest welding and bracing.

In [TheFrostyman]’s frame the headstock hangs out in front of the rest of the bike almost as an afterthought, held only by a short section of two parallel tubes. Inexplicably he brings the vertical members of the frame to its spine a short distance behind the headstock rather than to the headstock itself. At best this headstock will be flexible and give the bike poor handling, at worst it will bend or fatigue and break, leading to the bike disintegrating at speed.

How not to bend a piece of tube. Compare with the curves of the Norton frame above.
How not to bend a piece of tube. Compare with the curves of the Norton frame above.

Moving through the frame, he has its bottom members curve round below the engine and he’s incorporated some diagonal bracing. Unfortunately though rather than make his angles either by careful use of a tube bender or by welding together individual sections that meet at an angle, he seems to have formed them in a vice  and left his bends with kinks. This will have introduced a significant weakness to the structure, when exposed to the forces it will have to cope with on the road it is likely to collapse. There is also a question of the frame bracing around the seat, he’s taken the frame out to the rear to form a mudguard without taking it up to support the seat. This creates a huge weak point right beneath the rider, it would take a brave or foolhardy rider to sit on a motorcycle seat likely to collapse at the first bump.

Resting on a Chain, a Wrench, and a Prayer

The rear wheel arrangement is probably the most visibly scary piece of this build though. It’s a hardtail design – no rear suspension – which you’d usually expect to see using some kind of double-triangle frame geometry similar to that you will probably be familiar with from a bicycle. [TheFrostyman]’s machine eschews that tried-and-tested design in that the front half of the two-triangle model is missing. The bracing upwards towards the rear mudguard will contribute next-to-nothing to its integrity, so all the strength of the bike lies with his bottom rails and a bent tube touching the top of his gearbox. This is likely to create a weak point in the bottom rails just behind the engine, this frame will not stand up to much punishment.

The lack of a rear brake caliper will be the least of the rider's worries.
The lack of brake calipers will be the least of the rider’s worries.

We’ve saved the best for last with this machine. The rear triangle in a hardtail frame supports the rear wheel, it is crucial to the safety of the finished motorcycle. If you design a hardtail frame, this part of it will be made from hefty tube with careful reinforcement and welding. A section of chain welded solid for the top member and a wrench used as the bottom member should not be your first choice if your life is going to depend on it, you are not going to be able to save yourself with an awesome stoppie if the rear wheel drops off while you are riding the bike.

It’s tempting to sit back and laugh at the builder of this motorcycle for getting it so wrong. And yes, this is a Fail Of The Week post because he has failed miserably to make a safe and rideable motorcycle. But to do so would be unfair, for even with its fails it is an achievement to have built it at all, and particularly so for a 15-year-old. If we could leave [TheFrostyman] with anything it would be the exhortation to never give up, and to return to this project and have another go at his frame. Study a few commercial frames of similar design, and pick  carefully the tube sizes for a MkII frame that addresses the deficiencies of the original. Above all, keep at it!

If you have an interest in motorcycle frame design, we’d wholeheartedly recommend reading every page of [Tony Foale]’s site, meanwhile [TheFrostyman]’s Imgur gallery of the bike is below the break. Those who have their own stories of inadequate motorcycle design, please share your story in the comments. We’d love to hear about who helped you learn a better way of doing things.

2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which celebrates failure as a learning tool. Help keep the fun rolling by writing about your own failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

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