Dave W wrote:Who fiddles with their handlebars from time to time? ..... Once set my handlebars haven't been adjusted in years. ...
I think you have inadvertently answered your own question there
; the answer is 'nearly everyone with a new bike does...'.
BTW quite a lot of riders move the handlebars up and down with the seasons, to allow for winter clothing etc.
jb wrote: ...Can't say as I've ever noticed the bearings become rough, not like your average threaded loose ball headset does when over tightened.
Any headset with imperfectly smooth bearings will become rough when there is a lot of preload on it. My point is that
a) most A-head headsets NEED a lot of preload to work properly (unlike most threaded headsets) and
b) that well-intentioned folk apply more than enough preload quite unintentionally using an allen key with an A-Head and
c) I've only seen one or two headsets where the bearings won't quickly be irrecoverably damaged by such overtightening.
The minimum preload might be specified as 'that which is enough to prevent unwanted lateral movement in normal use'. With a headset that has 45 degree angular contacts and drop-in races, the minimum preload is approximately the same as the expected lateral service load. For an average bike/rider I estimate this is about 300Kg, although it could be less for a bike with a long headtube and more (a lot more) for one with a short head tube. A headset of this type with a lower preload will rattle in use and/or fret/wear on the abutting faces. Note that some A-Head installations have slightly squashy (rubber, plastic, badly fitting etc) parts in them; these can require more preload than expected in order not to allow movement in use.
The upper limit to the headset preload is determined by the maximum static load capacity of the bearing.
In service the lateral loading generates axial bearing loads. These add to the dead weight and the preload to generate the service loading on the bearing. So a bearing with (say) 300kg preload, 100kg dead weight load and 300kg lateral load will see ~600kg axial service load on the upper race, and about 700kg axial service load on the lower race. Shock loads will increase these values considerably.
The same bearing could easily be preloaded to 500kg (it doesn't take much torque on an allen key to do this; an M6 bolt will go to over two tonnes quite easily and even an M5 will usually do over a tonne). A 500Kg preload would generate service loads of ~800/900kg on upper and lower races respectively.
You can see that with a typical A-head headset (with drop-in bearing races) the service loads are determined (and indeed easily dominated) by the bearing preload.
By contrast a loose ball headset with fixed races and a 45 degree contact angle can be run with minimal preload; at (say) 100kg preload (which might generate a small rattle over bumps, but won't knacker the headset anytime soon, because it is a rolling not sliding contact) this would generate service loadings of 400/500kg on the bearings.
When trying to ride 'no hands' the service loads are lower and comprise almost entirely the bearing preload.
At any given condition, a rough bearing surface will feel less 'lumpy' etc in proportion to the service loading; so a 300kg preload headset will feel x3 as lumpy as a 100kg preload headset and a 500kg preload will feel x5 lumpier etc.
So I am somewhat critcal of headsets with drop-in races etc. As it happens, this means I am critical of A-Head headsets in practice, because they are the sort most often built this way, and where it is easiest to set the preload too high; IME a fraction of a turn more adjustment does generate a uniform axial displacement with a threaded headset (thus allowing elimination of play and minimal preload) but with an A-Head system this is not guaranteed, because there are more (springy) things to move and one of them is usually a sticky wedge. The second adjustment rarely behaves like the first, and if using torque to set a top cap, I very often find I have to release the wedge when making a second adjustment.
cheers
