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Unsafe quill stems (threaded forks)

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BikeGremlin

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I wrote an article on this topic:
Unsafe and bad cycling industry patents

I've also made a video where I discuss the quill-stem problem among other things (the link should start at 3:04 where I show and discuss that):
Robust and cheap to run bicycle?

In that video, I received this (logical and commonsense) comment, to which the reply is worth posting here so that more people can read (and discuss) it - since YouTube comments get lost in the endless scroll (the bold emphasis made by me):

Nice bikes and well built!However, I cannot fully agree with you on some points. Quill stems are perfectly fine and reliable. There are quill stem adapters that allow you to mount modern stems and handlebars, so you can still get a 1" threaded fork and combine it with a modern front end. The limitation with older road bike frames is rather the 126mm rear hub width, which is not compatible with the modern 130mm standard. (yes, you can widen the rear triangle to 130mm, but that's not for everyone)I have built a number of bikes based on old frames (road and gravel) and upgraded them with more modern components; my cost breakdown usually is around EUR 500-700 for a really nice bike.Last but not least I think that modern brifters or STI's are superior to downtube shifters. The sweet spot for me is 2/3 x 9 gears, covering a wide range, is reliable and bomb proof. Look for a used Ultegra (6500) groupset, they are still around in good quality, or get new Microshift brifters, perfect!

There are two things worth discussing here:
  1. Adapters for ahead stems on threaded forks.
  2. The mounting system with a quill (for threaded forks).

Qull-stem to ahead stem adapters​

These adapters allow for mounting an ahead stem (designed for threadless forks) onto a threaded fork (designed for a quill stem). Here's what such adapters look like:

Threaded fork (quill stem) to threadless stem adapter
Amazon affiliate link (click on the image too)

With this kind of adapter, we're in the same position safety-wise. Yes, an ahead stem will mount nicely onto the adapter with two bolts (as long as you don't buy an ahead stem with just one bolt, which I definitely don't recommend), but the adapter itself is secured to the fork using the same quill system, just like a quill stem.

That leads us to the following problem:

Quill stem system safety risk​

My quill stem broke while riding, several decades ago. Since then, I've spent a lot of time and energy trying to determine the cause (succesfully) and find a way to prevent that problem through some form of preventive maintenance (unsuccessfully). Why is the latter practically impossible?

The entire system is secured by just one bolt and quill. Whether the bolt or the quill fails, you lose control of the bike (the handlebars can come right out).

The system isn't solidly fixed. It's held at the bottom, while everything above the quill (as shown at the bottom of the picture above, marked with "31 mm / 1.2 in") is of slightly smaller diameter than the fork steerer’s inner diameter, so it moves freely (there's no need to press it in). This means there's no preload, which causes it to wobble (often visible to the naked eye).

This makes it impossible to seal the system against water ingress, so aluminium stems tend to seize up (which, to some extent, solves the bolt-breaking problem :) ). However, this constant movement of the quill also increases the risk of it breaking due to material fatigue.

The bolt itself, if there are no manufacturing defects and it's properly tightened (enough preload), shouldn't break from fatigue due to this movement. However, if there is a manufacturing defect, it can break at any time - even after a year or two of riding.

The nature of the design is such that even weekly preventive maintenance doesn't eliminate the risk of breaking due to material fatigue (unless you regularly scan and separately test the bolt, which is, well - ridiculous and impractical).

Excluding letting an aluminium stem seize (which prevents headset servicing), the only real solution is a threadless fork and a matching ahead stem with two bolts on the fork steerer and four bolts to secure the handlebars (as shown in the video). With such a stem, the failure of any bolt won't result in loss of control of the bike, and any cracks in the stem itself are easy to spot from the outside.

Relja BrevityIsNotMyVirtue Novović
 
In the same video, I got a good quesion about the minimum stem insertion - so I'll answer it here.

Great content, keeping it simple and real! Do you happen to have any thoughts or experience on minimum insertion measurement for a quill stem? ie the minimum amount of quill stem you should have inserted into the headtube to keep things solid. Thanks

It is usually marked on the stem. I'd say it also depends on the stem's length - and the material it's made of. If I had to state a minimum (without seeing the particular stem and fork and headset combination, as well as the bike's intended use), I'd say:

- At least 5 cm (~2 inches) for the shorter stems (up to 10 cm high - or up to 15 cm in total stem height, if we include the inserted part in the measurement).
- At least 7 cm (over 2.5 inches) for the longer stems.

Please take this as a rule of thumb but not as a gospel - and use common sense and reasonable caution.

Relja
 

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