Ride UK Tech Columns
#73 What the
hell is a Hidden Headset?
(This article first appeared in Ride UK issue No. 73 and is reproduced
here by kind permission of Ride UK.)
What the hell is a Hidden Headset?
Good question. And surprisingly hard to answer.
Fifteen years go when I became a born-again BMXer, headsets were utter utter
shite. You had two choices, a Tioga Beartrap in a size that fitted, or a
Tioga beartrap in a size that didn’t fit. These were all one inch threaded
headsets but for some reason there were several sizes to choose from, getting
the right one was always a pain. Once you had your headset fitted you then
spent most of your time tightening it. After every ride it was loose, shit,
it was probably loose five minutes after you started riding. This was only
1988 but headsets were working on technology from 1888, or rather NOT working
on technology from 1888.
Then came the Aheadset, no threads, YAY! One size, YAY! Stronger forks, YAY!
Stems that didn’t slip all the time, YAY! At last we had a standard to embrace,
OK so it still used a rough-arse unsealed bearing and it was still a bit
weak and flimsy, but it was undoubtedly forward progression. Over the last
ten years we have grown complacent and forgotten all about the nightmare
that was small threaded headsets; cups have got stronger and most people
can go months without touching their headset.
But now things are changing again, lots of frames are boasting a HIDDEN headset
and loudly proclaiming it to be a huge leap forward. Hooray! But hang on
a minute… What is so aesthetically displeasing about the aheadset that we
now need to “hide” it? Is this a real step forward or change for change’s
We are all familiar with the terms “ball bearing”; “sealed bearing”; “un-sealed
bearing” etc but they are often misused so lets clear up the definition first.
Sealed bearing; This is what most of us think of when we hear the words “sealed
A fantastically well made rolling element cartridge bearing, precision ground
to a very high tolerance so that it needs no adjustment and just spins beautifully.
Two little rubber shields keep some of the crap out and they fit into a housing
that is usually a slightly tight fit. They aren’t actually “sealed” but they
are at least bearings.
Sadly most headsets don’t use these. Instead we have an array of balls loosely
tied together by a soft metal “cage” to make a ball necklace that we drop
into a cheap steel cup. A “cone” then sits on top and we adjust the assembly
to take the slack out.
So why don’t we use “sealed bearings” in headsets? Well the type shown above
just wouldn’t be up to it, although they are big and beefy they are designed
to take mainly “radial loads”, ie. loads at right angles to the axis of rotation.
Like wheel or pedal bearings. They can take some small axial loads more than
enough for a wheel say which only gets an axial load when you bang the wheel
sideways into the ground, for example fluffing a tailwhip. If you cut one
in half it would look like this:-
As you can see the contact points are at the inside and outside of each ball
so this is the direction it can best carry loads.
But there are other cartridge bearings which ARE designed to take big axial
loads. These are called “Thrust” bearings. A pure thrust bearing can take
huge axial loads so it’s the sort of bearing you put at the end of a ships
propeller shaft to take the thrust from the propeller and use it to push
the ship forwards.
BUT a pure thrust bearing can take almost NO radial load. Cut in half one
would look like this:-
As you can see this sort of bearing can just fall to bits and needs to be
loaded to hold it together, and the contact points are top and bottom.
So for a headset what can we use? Forks take a lot of force straight along
their length but they also take sideways and head on impacts that cause big
radial forces too…
Well there is another type of cartridge bearing called an “Angular contact
bearing”, these look more like this:-
With angled contact surfaces they cant take radial loads as big as a pure
radial bearing or thrusts as big as a dedicated thrust bearing BUT they can
take pretty big loads in BOTH directions unlike either of the others.
This is the basic bearing arrangement used in nearly ALL headsets. In our
current cheap Aheadsets the outer race is built into the cup and the inner
is the cone. But you can also buy fancy ones where a cartridge unit, that
looks a lot like the one above, is employed and sits inside a separate cup
and cone which act simply as seats.
So since these newer headsets have a separate bearing-race and cup, somebody
realised that the cup was now only acting as an adapter to let the headtube
hold the bearing unit. And that if the headtube was made bigger and had the
precise bearing seat built in the cup would no longer be necessary.
This would result in two less components which is obviously simpler and lighter,
it also means that the headtube can be a little longer to reach the normal
position where the bearing would sit in its cup. This means that the toptube
and down tube are welded a little further from the end which can help
If this was the whole story then there wouldn’t be anything to worry about,
but there is more.
It is an unfortunate fact that when you weld things they tend to distort.
SO when a headtube is welded on it tends to become slightly oval. The traditional
solutions to this are either doing nothing and hoping the tight fit of the
cup sorts it out; OR to ream and face the headtube.
Reaming and facing is fine for high end frames but it needs to be carefully
allowed for, plus it adds cost.
Reaming and facing an actual bearing seat is much more difficult so the manufacturers
of Hidden headsets have tried to design the problem out.
This is a representation of what a hidden headset would look like sawn in
As you can see there is a very slight gap between the bearing unit and the
headtube sides. This means that you will never have to hammer your bearings
in or out; Woo-Hoo!
Instead the bearing unit sits on a forty five degree chamfer. As the top
bolt is tightened the bearing unit settles down into this conical seat which
matches a conical chamfer on the outside of the bearing.
This ridge on the inside of the headtube isn’t the easiest thing to make,
hence the increased cost of a frame with a Hiddenset BUT it does add considerably
to the rigidity of the headtube, Flareing resistance is increased and the
headtube is less likely to deform during welding.
The downside is hard to explain but I will give it a go.
The first biggie is that the frame needs to be well made. As long as the
hiddenset is mainly on the top end bikes it will probably work very well,
but on cheaper bikes with a higher tendancy to distort under welding, things
could start going wrong. IF the headtube does distort from round to a slight
oval then the bearing will not sit on its forty five degree ledge properly.
It may touch only at the sides while the front and back sit a little away
from the seat. This would manifest itself as a very slight wobble under big
loads. Every time you hit a ramp it might click or groan. As time went by
this movement would reduce the bearing unit life drastically. And if it was
run with broken bearings all the usual problems of flared headtubes would
be likely, only mending a hiddenset headtube is likely to be even harder
then an ordinary frame.
Another consideration is also related to wobble. If the headset is even slightly
loose then the bearings can wobble around in their seats; in the same way
a funnel can rattle about in the neck of a bottle. If this isnt corrected
then it can wear the seats or even flare the headtube.
Lastly we should consider that the Hiddenset wasn’t developed for BMX, it
wasn’t even designed for mountainbikers. This is road-bike technology! Those
skinny but fit guys who hammer round the Tour de France are undoubtedly hard
on bikes in their own way but it is very different to us scum-bags launching
off ramps, steps and dirt jumps into oblivion.
Campagnolo who make the Hiddenset used to make BMX racing parts many many
years ago but they dumped us when things got tough in the Eighties. If the
hiddenset isn’t a success with road racers they wont keep it alive for us
So, if you are thinking of going for a frame with a hiddenset bear this in
mind, if it doesn’t work out there is no way to go back to a normal headset.
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