Note: the photos of the ASS on the red frame is the final production version with the silver brazed alignment cam and bronze brazed clamp boss. On the Turkish green frame you see the prototype ASS.
Eerder metaal #ASS (Anti Seatpost Slippage device) Mk3 is designed to act as additional clamp on top of the Brompton seatpost clamp to minimize the seatpost sliding down. The Mk3 version has the lowest stack height of all three versions (making your Brompton more compact than before), the most effective (actually very effective!) and the lighest. Also it's the first version to feature the alignment cam to prevent the seatpost from rotating when the original Brompton quick release can't keep it from rotating.
The #ASS is very useful for heavy riders to not have to overtighten the seatpost QR clamp (to prevent destroying the plastic seat sleeve and eventually causing the seat tube to develop cracks near the end of the slot at the back side.
The #ASS is also very effective on titanium seatposts which are due to the lower stiffness harder to firmly clamp and thus more likely to slide down during the ride. (the Young's Modulus of titanium, which defines the materials stiffness, is about half of that of steel)
The #ASS can be used in combination with a #seatposthugger or big saddle bag to prevent the seatpost from sliding down as a result of all the extra weight.
It features an adjustable (pull and turn) Elesa thumb lever. When opened 1 1/2 to 2 turns counter clockwise the ASS can slide up or down the seatpost and a when tightened it's pretty tight and locks the seatpost into the set height.
- The original seatpost clamp, the sleeve and eventually the seattube, or frame if you like, hold the load of your body weight. Which basically is a torque or 'moment' [Nm] (force on saddle multiplied by seatpost length) This 'moment' can be split/converted into a downward force inline with the axis of the seatpost (causing it to slide down if not clamped well) and a force perpendicular of the seatpost acting up on the seatpost sleeve inside the seattube (causing the seatpost and the sleeve to deform slightly)
- This downward force is obviously what makes the seatpost slip and slide down. This is made worse by your inner thighs pushing against the nose of the saddle when making turns and keeping balance. Pushing the seatpost down and rotating it at the same time is an effective way to slide your seatpost down as we all know. The force of your weight is not a constant factor but you're bouncing, to some degree, up and down caused by your pedal strokes and the bumpyness of the road you're on. This alternating 'moment' on the seatpostclamp converted to a resulting downward force and some material deformation causes the seatpost to work it's way down, slowly but surely. The heavier and or longer you are, the worse it gets. No suprise, I guess.
- While the ASS is just the stop for sliding down, it takes no perpendicular force, so there's no deformation of the seatpost or seat sleeve in this area outside of the frame. It's just a perfectly round surface and there's no wiggling down as a result. It just takes the force inline with the seatpost and it does it well.
- summery: The seperate ASS clamp is more effective for clamping against slippage because it doesn't have to take the moment caused by your load at the end of the long seatpost with deformation of the post at the bottom as a result. This deformation makes it hard to get a good and permanent grip on the seatpost to prevent it from sliding down.
Specs:
- RAW lacquer, clear gloss powdercoated with an industrial look, some discoloration from brazing might be visible
- Bronze and silver fillet brazed, (RAW, unfiled fillets)
- 0.9mm wall thickness Chromoly tubing
- including the original, but modified Brompton seat post sleeve. The sleeve is not glued for easy replacement if ever needed. It's hold by the cam, so won't come out by itself.
- adjustable Elesa thumb lever
- weight complete: 58 gram