Details on how the GT Marathon Pro is made have been pretty hard to come by, but we do know that a ‘vendor’ (as they’re known in the trade) makes the frame and is responsible for the lay-up of the carbon monocoque. This is constructed from a blend of TR30 and TR50 unidirectional fibres — one being very stiff with low tensile strength the other being lighter with higher tensile strength. These two types of carbon are used in different places, depending on what strength, weight or stiffness characteristics GT wants in each of those areas. GT calls this process Force Optimised Construction. The carbon finish you can see on the bike is a 12k (think 12mm wide) cosmetic layer.
Although GT doesn’t manufacture the frame, it has a say in the way it looks. It has decided to form the forward shock mount into the down tube, but it’s not sleek and we wonder if it wouldn’t have been simpler, cleaner and lighter to go for a bonded mount like on the Giant and the Scott. The finishing isn’t that good either, with some flaking around the bushings.
If you want to show off your new frame, what better way than to route the derailleur and rear brake outers inside the monocoque? The exposed inner cable is protected, but GT needs to sort out the rear derailleur routing because it has rubbed a deep groove into the seat tube. Cables tucked away under the down tube look OK, but with this tube profiling that’s impossible, so too is fitting a Crud Catcher style mudguard or a battery pack for most lightsets.
GT has saved over 100g by switching to a carbon-fibre swingarm for 2008. It uses a similar construction technique to Ritchey, which we’re assuming is some sort of carbon wrapping. The dropouts, lower yoke and bottom bracket link are aluminium and there are alloy sleeves for each of the shock mounts, headset cups and seat tube. GT claims the frame weighs 6lb with shock; our test bike weighs 27.07lb.

Like all iDrive or Independent Drivetrain bikes, the Pro is a single pivot with a floating drivetrain. For the most part the bottom bracket (ID) is isolated from suspension movement, but there is a small amount of anti-squat or chain growth to improve pedalling efficiency. It’s a pretty active system and on this 4in platform it feels pretty similar stood up or seated.
The Fox float RP23 shock is a lot higher in the frame on this bike than either the Giant or Tomac, so the ProPedal lever is much easier to reach while riding. Flicking through the three ProPedal settings has a bigger effect on the GT than the others and seems to iron out a bit of the mushiness we could feel in the granny ring.

GT has increased fork travel this year to 120mm and apparently tweaked the geometry to accommodate it. The head angle comes out at 69.5 degrees, which is the slackest on test but the Pro is also the highest (the bottom bracket is 13.5in), so feels like a big bike. It’s the second heaviest bike here, but with our tyre upgrade carried speed very well and despite the low tread offers plenty of traction for those steep, slippery climbs. It scorches rocky sections and even with a long wheelbase, which would usually blunt a bike’s sharpness; we didn’t feel that much slower piloting the Pro through twisty singletrack. Increasing the fork travel on the Marathon for 2008 increases the bike’s potential for tackling rougher race courses, like the Roc d’Azur for example. On the downside, mud clearance is poor for the majority of UK riding, cable routing a mess, the stem is too long, you can’t really carry it on your shoulder and we think GT has made the wrong choice fitting Kenda Nevegal tyres on a marathon bike.

With such a big foot firmly planted in the short-travel trail bike category we’ve got to question whether the Marathon Pro is actually a good marathon bike. It lacks the instant snap of acceleration we could feel on the Scott and Giant and the weight and slacker geometry and, to some extent the component choices, are going to put off die-hard marathon racers. We’d say the GT Marathon Pro is a better cross-over bike for a rider looking for an aggressive, shorter-travel trail machine that they can trick out with lighter wheels and tyres for weekend competition.