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Spacers above stems (engineering not cosmetic) question

by velochris
Mon, Apr 03, 2017 12:54
15

Bianchi are stating no more than 5mm of spacers should be above a stem.

http://www.bianchi.com/global/news/news_detail.aspx?newsidmaster=360100&...

In the past I always left 10-15mm, did a few rides to ensure I was fully happy with my position and then cut down to leave 5mm above.

Can anyone provide an engineering explanation why more than 5mm could be dangerous?

Ps I use a torque wrench and set just below recommend torque to prevent tube damage.

I try to make the expansion part of the expander bung sit in the middle of the stem. The one Bianchi provide is so small I doubt this makes any difference.

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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15 comments

Avatar
trohos | 7 years ago
1 like

Safety i thing. In many cases when you leave that space, the topcup comes above the handlebar height, sticking out.

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dottigirl | 7 years ago
1 like

I managed to bollocks up a carbon steerer tube by overdoing the expander nut thing. Was almost impossible to get spacers on and off. Stem tension was probably the only thing keeping it together. Luckily, the same fork was subject to a product recall a short time later (for a different matter), and the replacement fork had an alloy  steerer. 4

Avatar
CXR94Di2 replied to dottigirl | 7 years ago
2 likes

dottigirl wrote:

I managed to bollocks up a carbon steerer tube by overdoing the expander nut thing. Was almost impossible to get spacers on and off. Stem tension was probably the only thing keeping it together. Luckily, the same fork was subject to a product recall a short time later (for a different matter), and the replacement fork had an alloy  steerer. 4

 

You were lucky.  You only need a small clamping force for the top cap, then using the stem bolts to hold that position.  If, after a few rides the fork has a little play just reseat it again, literally a 2 minute job for 3 bolts

Avatar
Woldsman | 7 years ago
0 likes

Erm, hoping this a manufacturer-specific thing...

 

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beezus fufoon replied to Woldsman | 7 years ago
0 likes

Woldsman wrote:

Erm, hoping this a manufacturer-specific thing...

 

is there a reason you've done that - like having a bit of a backache occasionally and switching the spacers and stem for a more relaxed position?

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Woldsman replied to beezus fufoon | 7 years ago
1 like

beezus fufoon wrote:

Woldsman wrote:

Erm, hoping this a manufacturer-specific thing...

 

is there a reason you've done that - like having a bit of a backache occasionally and switching the spacers and stem for a more relaxed position?

Exactly.  I felt that my back was uncomfortably rounded and I was also sticking up in the air (my main bike is a 2010 Kinesis Racelight T2 with a 10.5cm head tube). 

So I fitted a longer stem and put the spacers on top for a similar position to that on my 'winter bike'.

Avatar
gonedownhill replied to beezus fufoon | 7 years ago
1 like

beezus fufoon wrote:

Woldsman wrote:

Erm, hoping this a manufacturer-specific thing...

 

is there a reason you've done that - like having a bit of a backache occasionally and switching the spacers and stem for a more relaxed position?

 

It is also a good idea to not cut the steerer short in case of resale. I'm about as flexible as a breadstick and would never buy a bike where the steerer was cut such that I had to ride slammed.

Avatar
davel replied to gonedownhill | 7 years ago
5 likes

gonedownhill wrote:

beezus fufoon wrote:

Woldsman wrote:

Erm, hoping this a manufacturer-specific thing...

 

is there a reason you've done that - like having a bit of a backache occasionally and switching the spacers and stem for a more relaxed position?

 

It is also a good idea to not cut the steerer short in case of resale. I'm about as flexible as a breadstick and would never buy a bike where the steerer was cut such that I had to ride slammed.

This is the excuse I use to explain why I've not cut my steerer and have got a longer and uncut 'woldsman' on 2 of my bikes.

But it's mainly because I just can't be arsed... 

Avatar
Rich_cb | 7 years ago
1 like

Could be a safety thing.

Geraint Thomas ruptured his spleen on his spacer stack!

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Nixster | 7 years ago
1 like

Those are the engineering reasons but why the 5mm limit I don't know, although I believe Cannondale say the same.  The expander plug on my Supersix acts over about 10-12mm length of steerer and the stem clamp length must be close to 40mm so it's entirely possible to position it within the clamp with more than 5mm of spacers over the stem.  So I file it under BS personally although there are obviously limits. 

Avatar
StraelGuy | 7 years ago
2 likes

Sorry Dave, I'm not as posh as you, my steerer tube is aluminium and has a SFN .

 

Beezus, thats a good pont, having the expander wedge thing within the stem area would prevent accidental crushing of the steerer tube.

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beezus fufoon | 7 years ago
1 like

if the steerer is carbon then it's preferable to have the expander bolt in line with the stem clamp

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DaveE128 | 7 years ago
2 likes

The only thing I can think of is that a steerer top cap retaining wedge (a star fangled nut in a carbon steerer probably isnt' a great idea) exerts a significant outward force on the inside of the steerer that could perhaps crack it if not counteracted by the stem clamp. In my experience steerer spacers don't sit tightly around the steerer (so you can slide them on and off easily and without causing scratches. Some even only make contact at three small points to save weight.) so they can't provide this reinforcement. If there are too many spacers above the stem, the steerer top cap retaining wedge will be partially or completely above the level of the stem clamp.

That's my best guess as an engineer.

Alternatively, they're just trying to enforce "the rules" or something! 3

Avatar
StraelGuy | 7 years ago
0 likes

Not really, no. Keeping the spacers to 5mm over the stem keeps the star fangled nut within the stem clamp area which may help prevent the steerer tube being crushed if the stem's over-tightened but that's grasping at straws. Once the headsets adjusted and the stem clamp is tightened, any steerer tube above the stem is largely redundant from an engineering point of view.

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wycombewheeler replied to StraelGuy | 7 years ago
0 likes
guyrwood wrote:

Not really, no. Keeping the spacers to 5mm over the stem keeps the star fangled nut within the stem clamp area which may help prevent the steerer tube being crushed if the stem's over-tightened but that's grasping at straws. Once the headsets adjusted and the stem clamp is tightened, any steerer tube above the stem is largely redundant from an engineering point of view.

star nut position within the sterner tube is the same whether the spacers are above or below the stem.

Bianchi are probably against it because it looks so bad and they like their bikes to be beautiful.

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