The Tharsis from Shimano-owned PRO is a light carbon fibre post – our 375x27.2mm sample came in a couple of grams over the claimed 175g, but that still puts it in the premier league. The shaft has thicker material on the more heavily-loaded front and rear walls, and the finish is excellent, with no excess carbon strands or gobs of resin inside.
A smear of the supplied assembly paste ensures no danger of slippage. The moulded wraparound head forms a single piece with the shaft. There’s 10mm of offset, putting the head usefully in between a pure inline and traditional offset position. The single-bolt clamp is well thought out – the jaws open wide enough to get the seat in without any dismantling, and there’s a spring inside that pushes them out and keeps everything aligned.
Tighten the bolt just enough to hold the rails fore and aft and you'll still be able to adjust the angle before cinching it down to lock everything in place. Alternative clamps for ovalised carbon rails are included. In use it has proven to be secure, with a hint of comfort-boosting flex in the smaller size.