Trek's latest Procaliber hardtail mountain bike has a new frame compliance system based around a large hole in the top tube.Called IsoBow, the compliance system replaces Treks IsoSpeed design and it's said to be lighter and simpler than its predecessor.Elsewhere, the new Procaliber keeps up with cross-country mountain bike trends, with fork travel increasing from 100mm to 120mm and a more aggressive frame geometry. Trek says the bike has been designed with anyone in mind from rugged gravel riders to World Cup XC racers.The Trek Procaliber starts at 1,200 for the base aluminium Procaliber 8 with the high-end, carbon-framed Procaliber 9.7 AXS topping the range at 4,250. A frameset is also available for 1,400.International pricing is to be confirmed.IsoBow frame complianceThe IsoBow does look simpler than the IsoSpeed it replaces. - TrekTrek says the new IsoBow frame compliance design is lighter and simpler than the IsoSpeed system it replaces, and that its athletes cant feel a difference in compliance between the two systems in a blind test.The new design utilises flex in the rear triangle to smooth out trail imperfections. The seat stays continue past the seat post allowing them to flex and dissipate vibrations.The new system leaves a large hole in the crossbar. - TrekThe Lapierre Prorace CF 9.9 has a similar system, which is a bit like GT's Triple Triangle Design.Only carbon Procaliber models feature IsoBow. Trek says reduced vibrations lead to less fatigue over time. - TrekThe new frame has a floating brake rotor mount similar to Treks full-suspenionSupercaliber XC bike. The mount is said to isolate braking forces from the seat stay flex to increase comfort and provide more consistent braking.Trek says the new Procaliber frame is on average 150g lighter than the previous generation across the sizes.Increased travel and clearanceTrek has designed the Procaliber for elite competition. - TrekThe new Procaliber now uses a 120mm suspension fork. This is in keeping with other cross-country hardtails which have increased suspension travel in recent years.Also following a trend in XC mountain bikes, the new Procaliber can accommodate wider mountain bike tyres, measuring up to 2.4in.Inside the front triangle is room for two water bottles, which should be enough to keep you hydrated on longer riders or in marathon races.Trek hasnt given the bike any frame storage, though there is a mount on the underside of the crossbar where you could mount a small pack.Modern XC geometryThe Procaliber has grown longer and has a slacker head angle. - TrekTrek has also modernised the Procalibers geometry. It now features a slacker 67-degree head angle compared to the 68.8-degree angle of the previous Procaliber.The effective seat tube angle has also steepened to 74.3 degrees from 73.8 degrees to maintain an efficient pedalling position.Reach has increased, too, growing 10mm to 460mm in a size large.Trek says all this has resulted in a bike with calmer handling and increased confidence which should lead to increased speed.Trek Procaliber spec and priceProcaliber 6The Procaliber 6 is the entry-level aluminium frame. - TrekFork: RockShox Judy Silver, 120mmDrivetrain: Shimano XTBrakes: Shimano MT200Wheels: Bontrager KoveeTyres: Bontrager Sainte-Anne Pro XR 2.4inPrice: 1,200Procaliber 8The Procaliber 8 features an SLX shifter and better brakes than the Procaliber 6. - TrekFork: RockShox Recon Gold RL, 120mmDrivetrain: Shimano XTBrakes: Shimano MT410Wheels: Bontrager KoveeTyres: Bontrager Sainte-Anne Pro XR 2.4inPrice: 1,400Procaliber 9.5 Gen 3The Procaliber 9.5 is the entry-level carbon bike - TrekFork: RockShox Judy Gold, 120mmDrivetrain: Shimano Deore 12-speedBrakes: Shimano MT200Wheels: Bontrager KoveeTyres: Bontrager Sainte-Anne Pro XR 2.4inPrice:1,950Procaliber 9.6 Gen 3All Procalibers come with Bontrager Sainte-Anne Pro XR 2.4in tyres. - TrekFork: RockShox Reba RL, 120mmDrivetrain: Shimano SLX 12-speedBrakes: Shimano MT500Wheels: Bontrager Kovee Comp 25Tyres: Bontrager Sainte-Anne Pro XR 2.4inPrice:2,350Procaliber 9.7 AXS Gen 3The Procaliber 9.7 AXS features SRAM's GX Eagle AXS Transmission - TrekFork: RockShox SID, 120mmDrivetrain: SRAM GX Eagle AXS TransmissionBrakes: SRAM Level Bronze 4-pistonWheels: Bontrager Kovee Elite 30Tyres: Bontrager Sainte-Anne Pro XR 2.4inPrice: 4,250