Shifting the Posts - Goal Achieved

Shifting the Posts - Goal Achieved

Most bikes have two posts, both of them supporting major contact points, and both principal stress points on the bike.  Up front there is the fork and steering post unit, and most bikes have a seat post.  

Francesco Moser Time Trial Bike

I say most, because there have been some experimental designs that dispose of the seat post altogether and have the saddle mounted on the frame directly.  

Also, some top-end carbon time trial bikes have an aerodynamic seat post integrated into the frame itself.  The pillar is cut to suit the rider, and with all things carpentry, you can cut off but you cannot add on.  This typically involves a new frame.

Ridley TT bike with integrated seat pillar

The vast majority of us ride adjustable seat posts - a ring clamp around the top of the frame seat tube secures the seat post in place, and we can raise or lower the post at will.  Both directions.

The post that must be cut is the steering post, so if we are installing a new fork it is always a good idea to start with a high stem (and hence handlebar), and trim downwards until you find the handlebar height that suits you.  The only way to avoid this is to go retro, and use a quill stem that fits down the inside of the top of the steering tube and is held in place by a long expander bolt - this system is described in the post on stems, earlier on this site.

So, when installing the fork, the routine goes like this.  Fork crown race and bottom bearing cups for the headset.  Insert the steering tube up the inside of the head tube, and then drop in the top bearing cup and the top headset stack cover.

headset-stem assembly using external headset

You can  now, if you want low bars, slide the stem onto the steering tube, but it is wiser to slip on a couple of headset spacers before the stem, raising it a couple of centimetres (spacers are available separately in 2, 5, 10 and 20 mm thicknesses).

It is likely that there will be several centimetres of steering tube standing clear of the stem.  This is what you cut off, the cut being made about 3 mm below the top edge of the stem (how to do this will be described later).  With the shortened tube now assembled, the top cap expander bolt is inserted into the tube and tightened.

Because the top of the steering tube is below the edge of the stem, the top cap anchors against the spacer-stem stack, and with the anchor bolt inside the tube, the steering tube gets drawn up against the top cap, giving a nice, snug fit.

If you have an aluminium steerer you can use the star-fangled expansion bolt, but do not use one of these if you have a carbon tube, as the stress can damage the carbon tube.  Instead, use an expansion bolt that provides expansion stress evenly about the wall of the tube with no danger of localised stress fractures forming.  Considering that you lean forward onto the bars, you really do not want a tube failure.  This will result in your face meeting the road faster than you would really like.

So, what to look for in a fork?

How smooth a ride your bike gives depends on a few key factors.  Frame material and geometry is central to the overall quality of the ride, but all road feedback, bumps and all, come to the rider through the forks in the front, and the frame for the rear - seat and chain stays, both of which are connected to the seat tube and upwards through the seat post to the saddle.

sample pattern of carbon fibre lay-up

Up front, forks take the first hit of any bump in the road, and this is transmitted straight up the steering tube to the handlebars via the stem.  Metal forks absorb knocks and vibration through flexing, bending slightly on each impact.  However, the flexing, and the return of the fork material to its original shape, produces an irregular vibration that translates up to the bars.  Carbon composite forks, as with frames, are made of multiple layers of woven carbon fibre laid up and bonded with resin, similar in technique to fibreglass but significantly stronger and lighter.  The carbon components are much stiffer, with less flex, but more importantly the knocks and vibrations are dissipated through the layer interfaces, greatly minimising the vibration and chatter that makes it up to the handlebars, giving a much lighter feel to the front wheel.

Having ridden (and still ride) bikes with steel, aluminium and carbon forks, carbon has my vote.  There are many carbon forks out there, with many generic posts being produced in Asia, with a range of weights and geometries to match.  The forks that ended up on my table are Cannondale Synapse carbon forks, with carbon steering tube.  Weighing in at 350 gm, it is one of the lighter units out there, and cost a paltry NZ$141.05 from Bikewagon.  

As with many components, weight comes at a premium - cheaper forks are heavier.  Admittedly, Cannondale is an established brand, and I would trust it more than an unbranded, generic unit.  For such an important part of a bike, that is important, as failure is not an option.

SImilarly at the back end, a carbon seat post absorbs vibration the same as at the front end. It is perhaps of greater importance under the seat, as metal's ability to absorb bumps through flexing ceases to become relevant when the tube is not experiencing direct impacts.  Although the majority of tubes remain aluminium, carbon is becoming standard, and in order to increase compressive strength some posts are alloy wrapped in carbon.


Price is also relevant to weight, but is influenced by material, as new alloys can give a post of comparable weight to most carbon units.  The other consideration is the alloy mount for the saddle rails.  I prefer the adjustable offset.  The deal that I found was from Brinvo (who also sold me the bottom bracket).  Made by Felt, the 1.2SP3 seat post comes in at a relatively lightweight 205 gm and cost NZ$54.63.


So, front and back, the pillars that support the main contact points of hands and butt each have shared characteristics, function and material technology.  Not bad for woven tubes of carbon.