Right from the start, the new Shimano XTR groupset was designed  around disc brakes rather than V brakes, but they knew that the main obstacle for acceptance by the Pro’s would be weight. Their design brief was therefore to keep the same big stopping power of their proven XT 4 pot discs but get the system weight down near that of XTR V’s. So did they suceed?

As we’ve already revealed the lever itself is now responsible for shifting too, which means a cunning linkage to join the brake action to the mineral oil reservoir mounted inboard of the shifter housing.

It worked fine over our test weekend and despite the rather overcrowded looks the whole STI unit comes in at 423g without the thumb lever – only 9g heavier than current XTR V brake.

Pipes are resin rather than braided which makes cutting and fitting a lot easier and the reservoir uses the same rectangular tank as XT so the same cunning bleeding clamp tool can be used.

The real cleverness is at the far end of the pipe though. Rather than the two piece calipers most other brakes use Shimano’s advanced casting technology has let them make the whole caliper in one smoothly moulded piece. This means no flex between the two bolted halves and no need for heavy bolts either. As there’s less flex they’ve also moved from a four to a two pot system with caliper weight (with pads) dropping to 98g for the front and 100g for the rear.

The rotor is also a cunning piece of design, using the same 160mm diameter as standard XT, but with a steel braking surface mounted on an aluminium spider, which then slides onto splines on the hub before bolting into place with a lockring. This is exactly like early Hope brakes and Shimano claim lightness, convenience and safety (they reckon 6 bolt systems can work loose) as the reason. It also means that the XTR discs can only fit on XTR hubs, although as Deore and XT discs are compatible you can use them and a conventional 6 bolt hub at a slight weight penalty.

The disc is only planned in a 160mm XC version so far but you can choose between smooth, soft resin pads, more hard wearing but noisier metallic pads or the Shimano recommendation of one of each.

We were the first on the brakes after only a quick bedding in spin round the car park, but their bite sharpened up within an hour or so of riding. Obviously 160mm discs aren’t going to match the power of bigger DH discs, but the stiff one piece caliper gave easily enough bite to slide either tyre in wet or dry conditions. With the soft pads modulation and smoothness of power increase was absolutely excellent making for true fingertip control. Even in torrential rain the new brakes shook off water, stopping well. And, as you’d expect for an open system, there was no sign of boiling after a long, deliberate drag of the rear brake downhill.

The really impressive thing about the brakes is the control though, letting us feather braking superbly in even the sketchiest conditions, which is something the XT 4 pots were always a bit too violent to manage.

After the rainy ride the day before the resin pads hooted and squealed but this went as they dried out – a clear advantage over Hope’s Mini then! Shimano pads generally have a good reputation for lifespan too, so we doubt these will be much different.

Mixture of fleeting sunlight then alternating torrential Alpine thunderstorms and light mountain drizzle served on a mix of long fire road climbs followed by very steep woody, rooty and generally treacherous singletrack descents.

Pretty much ideal ‘mock British’ conditions for testing disc brakes and shifter systems then.

At first the new brake looks a very uncompatible beast, what with it’s new splined disc etc. However the levers work with any Shimano disc brake (and vice versa) and the calipers work with existing 160mm Shimano discs which means you can still your old 6 bolt hubs. Although mineral oil compatibilty means they may work with Magura, we haven’t tried it yet, but they ceratinly won’t work wth DOT fluid systems like Hayes and Hope

The superbly precise control and modulation of the power – probably the best yet from any system, even 4 pots.

The light weight. We didn’t have any scales to verify it but Shimano claim the entire system to be only 80g heavier than a V brake set up (if you count the weight of brake bosses). We haven’t got any comparable figures to hand (the combined shift and brake unit stops direct comparison) but that’s got to be pretty close to the previous featherweight title holder Formula’s B4.

All weather performance seems good although they do squeal when damp.

No pump up or fade. As it’s an open system there’s no dials to twiddle to compensate for heating up.

Mineral oil. Harder to find but kinder to paint, skin and fluffy animals everywhere.

• The new spline fitment is a ‘only our hubs’ pain but there are ways round it.
• We look forward to seeing more disc size choices for freeriders or weight freaks.

The rest of the groupset bits.