The Allston is the first Montague folding bike to offer a carbon belt drive in place of a traditional chain. The Gates belt combined with a Shimano Alfine 11 speed internal gear hub provides a quiet, clean, and nearly maintenance free drivetrain.
Proper tension on the belt is very important, as it ensures the system is as efficient as possible and has the longevity belts are famous for. A new Montague Allston will arrive with the belt pre-tensioned, and since the bike uses vertical dropouts the rear wheel can be removed and replaced for tire changes without loosening belt tension. However, it’s good to know how to make these adjustments if necessary.
Belt tension is achieved through the use of an eccentric bottom bracket. The bearing cups are fitted into a larger cylindrical plug, which can rotate in the frame’s bottom bracket shell. The bearing cups are off center, so rotating the whole units moves the crankset forward and backward relative to the rear wheel.
To make an adjustment, you will need a 5mm allen key to loosen the eccentric BB.
From the non-drive side of the bicycle, insert the allen key into the center hole on the side of the eccentric BB. Make sure it’s pushed all the way in and seated in the recessed bolt. When attempting to turn it, you should feel resistance.
Loosen the bolt securing the BB by turning it counter clockwise.
With that bolt loose, It’s now possible to rotate the entire unit. You can get leverage by engaging two of the holes on either side of that bolt. This can be easily done with a freewheel pin spanner, in this case a Park Tool SPA-1.
The end of each prong can fit into the two outermost holes and allow you to rotate the BB. With it in this position, rotating counterclockwise will move the crank toward the back of the bike and loosen the belt tension, while rotating clockwise will move the crank toward the front of the bike and tighten the belt.
Gates provides two ways to check belt tension; a special tool you can purchase or a free app you can download to any smartphone. The app uses your phone’s microphone to measure the frequency of the sound made from plucking the belt. Just like a guitar string, the tighter the belt the higher the frequency.
With the app open, position your phone near the belt and pluck the center. You should hear the vibration which will be recognized and measured by the app. For internal gear hubs Gates recommends 35 – 50 Hz, which translates to 28 – 40 lbs of tension.
When you have the desired belt tension, be sure to re-tighten the bolt recessed in that center hole on the non-drive side. Insert the 5mm allen key back in the whole, and turn clockwise. Now you’re ready to ride!