Softail mountain bikes last excited this magazine some two decades ago with their smooth rides, but now the flexible hardtail is making something of a comeback.

In May, BMC launched its Teamelite with a seatstay elastomer damper, and now Trek has revealed its new Procaliber carbon-fibre hardtail with a remarkable flexy seat tube for added comfort and pedalling performance when seated.

Big in the 90s, the softtail offers a bit of give at the rear end without adding too much weight and complexity. Today it’s all about carbon-fibre, which can be engineered to be flexible, strong and light.

Trek and BMC are promising all the pedalling efficiency of a lightweight hardtail with some of the comfort and grip of a full-susser.

Trek Procaliber

First there was the Stache 29+ bike from Trek, which has shock-absorbing supersized tyres. Now there’s the Procaliber, the hardtail that thinks it’s a full-suspension bike, squishing out up to 11mm of vertical flex through the seat tube.

The system is called IsoSpeed Decoupler because the seat tube and down tube are detached from one another. They’re still connected by two bearings and a bushing, but you can see the seat tube flexing under your weight if you push down on the saddle.

Trek Procaliber: 71 pre cent more compliance than a carbon hardtail (Keizer)

Pedal efficiency remains undiminished, but it doesn’t give any extra compliance when you’re standing up (unlike the BMC). The additional comfort is only perceptible when you’re sitting down.

So does it work? Trek says it has 71 per cent more vertical compliance than a regular carbon hardtail, and it’s definitely noticeable from the saddle — we found it helped when trying to lay the power down over washboard trails.

However, with the three-bike range starting at £2,250, it’s hardly technology for the masses…yet. Isospeed has now trickled down to the aluminium frames in Trek’s road bike range, drastically reducing the price, and this would be an obvious direction for its mountain bikes too.

BMC Teamelite

Micro Travel: elastomer enabled back-end flex (Burgess)

BMC’s softtail system is called Micro Travel Technology and works slightly differently to Trek’s: the seatstays have an elastomer damper built into them that lets the back of the bike flex vertically by 15mm.

This means the bike will provide some damping when you’re sat down pedalling, and also when you’re stood up and descending. There are three different damping ‘tunes’ available, for heavier or lighter riders.