Longer steerer or stem raiser?

[MartinC]

The load on the bearings wouldn't be my main concern. That would be the bending stress on the steerer tube where it exits the top bearing.

On a steel steerer I'd be relatively relaxed about a big stack of spacers but on a material that handles fatigue differently like carbon or aluminium I get a bit nervous beyond about 30mm of spacers. Being a bit risk averse with steerer tubes is a good idea in my book. An extension will create the same stress as a big stack of spacers.

I'm sure someone will be along shortly to say that they've had 150mm of spacers on a 1" carbon steerer for 20 years and it's not been a problem (yet) - there's no accounting for taste!

 

[mickle]

The 'bending forces at the top of the headset' of which you speak - how might they differ between 500mm of handlebar/stem rise and 500mm of 'spacered' steerer?

 

[jack the lad]

The load on the bearings wouldn't be my main concern. That would be the bending stress on the steerer tube where it exits the top bearing.

On a steel steerer I'd be relatively relaxed about a big stack of spacers but on a material that handles fatigue differently like carbon or aluminium I get a bit nervous beyond about 30mm of spacers. Being a bit risk averse with steerer tubes is a good idea in my book. An extension will create the same stress as a big stack of spacers.

I'm sure someone will be along shortly to say that they've had 150mm of spacers on a 1" carbon steerer for 20 years and it's not been a problem (yet) - there's no accounting for taste!

Isn't this just a bit irrational. The forks themselves are made of the same materials, but are thinner, weaker and unsupported for around 400mm, with a lot more forces acting on them.

My Brompton has at least a couple of feet of steerer/stem above the top bearing. It might need some slight adjustment in 20 years rather than 30! If you are not downhill mtb'ing, jumping or trials riding the forces aren't enough to worry about.

Just worry about the aesthetics. Mrs J has a mid-length head tube, a middling long fork steerer, a mid-riser stem and a mid-riser bar. I had to keep going up and up until she found the position she likes - something ridiculous with her hands roughly in line with her shoulders. I could have achieved the same position with any one of a long head tube, a long steerer with a big stack of spacers, a high rise stem or ape-hanger bars, but splitting the height between the 4 components looks more balanced and neater. And that's what matters!

 

[GrumpyGregry]

Care to elaborate?

Head tube length will make a difference - the further the headset bearing races are from each other the lower the load on them. But there's bugger all in it.

Two feet of steerer with spacers or two feet of hi-rise chopper bar - if the bars are in the same place (and all other things being equal) the loading on the headset bearings will be exactly the same.

With chopper bars, and brompton bars ime, the leverage is usually enough for the bar to rotate in the bar clamp, ocause the bar to collapse, well before the forces involved can damage the headset or steerer tube. But I've seen more than a few mtb's at trail centres with long steerers* and, even, horror of horrors, risers mounted on top of them with snapped steerers and/or farked headsets. If you are light and don't make big hits all will be good. If you are a clydesdale and clumsy or like big air the outcome over time will be different. The steerer is acting as a lever. There was a bit of an engineering thing, complete with equations and everything published in one of the US mags some while ago. I'll see if I can find it.

*lots of mtb-ers leave their steerers at 200+mm on their sus forks to make it easier to resell the forks, if they've been cut down to say 170mm as per the specs for the frame/headset/fork you really limit your market for resale. But even with an extedned steerer most people would be running the stem low on the steerer tube.

 

[GrumpyGregry]

My Brompton has at least a couple of feet of steerer/stem above the top bearing. It might need some slight adjustment in 20 years rather than 30! If you are not downhill mtb'ing, jumping or trials riding the forces aren't enough to worry about.

I stand to be corrected but my Brompton was the same, except the steerer was considerably heavier gauge/duty tube than a conventional steerer tube.

To better resist the leverage forces perhaps?

 

[Rob3rt]

I cant be arsed to read this whole thread. But may I ask, how is the force on the top of the headtube due to a few extra spacers or a long stem going to cause any more leverage than that at the bottom of the head tube, as caused by the forks that are about 40-50cm long?

 

[mickle]

 

[TheDoctor]

Well done, Mickle!!
You've managed to find the one bike that's even uglier than a Thorn. IMHO, obviously.

That's a Deawoo Shuttle, is it not? I have one.

 

[MartinC]

The 'bending forces at the top of the headset' of which you speak - how might they differ between 500mm of handlebar/stem rise and 500mm of 'spacered' steerer?

Very little in my view. But that's what I said originally.

 

[mickle]

Well done, Mickle!!
You've managed to find the one bike that's even uglier than a Thorn. IMHO, obviously.

That's a Deawoo Shuttle, is it not? I have one.

OT - Quite the worst handling bike I've ever ridden. My mum bought me one for xmas one year because I wooed at one in a shop window! Horrid. I turned mine into a kids bike eventually.

 

[MartinC]

I cant be arsed to read this whole thread. But may I ask, how is the force on the top of the headtube due to a few extra spacers or a long stem going to cause any more leverage than that at the bottom of the head tube, as caused by the forks that are about 40-50cm long?

Yes you can, because I can be arsed to be polite. The construction at a fork crown is normally very different to that at a point towards the top of the steerer tube. The leverage comes from from the distance from the top race to the handlebar clamp not from the length of the steerer stack. The more important effect of the big steerer stack is to allow flex that creates a potential fatigue problem.

Brompton steerer tubes are very short - it's a threaded headset, it doesn't extend much above the head tube. The external 'steerer' tube fits in like a quill stem and is a much bigger diameter and guage than a traditional steel steerer tube. The posted picture of a folder looks very similar.

 

[snailracer]

...I've seen more than a few mtb's at trail centres with long steerers* and, even, horror of horrors, risers mounted on top of them with snapped steerers and/or farked headsets. If you are light and don't make big hits all will be good...

Now, surely, at a MTB trail centre you will also see snapped frames, wonky forks, pretzeled wheels, missing cranks, snapped pedal spindles, torn-off saddles and broken riders, no? Crashing can destroy anything.

 

[g00se]

What you need on that is a BMX layback seat post - just to complete the look:

As for the stack of spacers - my cousin in Australia has a road bike with a huge steerer and flat bars (she's getting on a bit). Had built by a reputable builder so they must be quite sure of the stress. There's at least 8 inches of spacers.

It looks ridiculous by the way.

 

[GrumpyGregry]

Now, surely, at a MTB trail centre you will also see snapped frames, wonky forks, pretzeled wheels, missing cranks, snapped pedal spindles, torn-off saddles and broken riders, no? Crashing can destroy anything.

yet oddly, having seen all of those and more, I've never seen a steerer/upper headset gone on a bike where the bars were in the specified position...

 

[snailracer]

yet oddly, having seen all of those and more, I've never seen a steerer/upper headset gone on a bike where the bars were in the specified position...

Not heard about the recent rash of broken steerers on Trek Madones, then? Trek blamed this on NOT fitting a spacer.