The biggest advantage usually only becomes clear to belt cyclists later, for example in a quiet minute at a mileage of 3000 plus: Hey, I haven't had to clean, oil or re-tension the drive or even, as in the past, replace my chain - advantage belt drive!
If you compare the chain and belt drive on a hub gear system, the slightly softer start of the belt is noticeable: The stiff toothed belt is tightened a little by the application of force and fits into the recesses of its sprockets, the drive pulleys. There is definitely no elongation of the belt: Tension cords made of carbon fibres (Gates) or twisted aramid (Conti) run inside, which definitely do not give way in length. This is why the belts of both types are sensitive to lateral impacts or shocks, as these tension cords can be damaged. Skipping, i.e. sliding the toothed belt over the recesses of the sprockets with brief overstretching, would also destroy the fibre lay-up: Tearing of the pre-damaged belt under heavy load can then no longer be ruled out.
The key to perfect running therefore lies in a defined belt pre-tension during installation. Here, the Gates system requires a relatively high tension, which should correspond approximately to that of a chain drive. Gates provides a measuring tool with clear readability for this purpose. Conti, on the other hand, requires significantly lower pretension values, with larger and differently shaped belt teeth providing safety. But even here, the pre-tension is not simply arbitrary: it is only so low that it can even be easily adjusted by hand. However, parallelism errors between the front and rear pulleys or a rear end that flexes sideways under load are not good for either system: the belt then no longer lies completely flat on the tooth webs of the drive pulleys under load and could run off sideways or wear unevenly. Gates counters this with its centertrack system: A longitudinal groove in the belt runs on a corresponding rib of each pulley and thus always guides the rotating belt in the centre. With Conti, the longitudinally parallel aramid tension cords are twisted in opposite directions so that the belt tends to centre itself: even if you try to move the rotating Conti belt sideways with your finger, it runs back on track by itself within a few centimetres. In addition, so-called flanged pulleys on the drive pulleys prevent the belt from running sideways, outwards at the front and inwards at the rear.
Bicycle and frame manufacturers have now also done their homework: the belt frames are technically mature and the tighter tolerances have been accepted and implemented. About five years ago, some manufacturers still had to subject all belt frames delivered from the Far East to a 100% inspection and often send back the rejects.
In order to insert the closed belt into the frame, an openable rear triangle must be constructed: In most cases, two overlapping aluminium surfaces are bolted together on or in the seat stay. However, these solid parts should preferably not be located in the power path of the frame tubes, but are more intelligently accommodated closer to or even in the dropout. The belt tension itself is always achieved by changing the length of the chainstay: Either you turn an eccentric in the bottom bracket, which shifts the crank axle. However, this inevitably changes the bottom bracket height, which not everyone likes. Alternatively, the rear wheel axle can be mounted in movable axle pieces, with or without adjusting screws, which make it easier to align the axle precisely. There is a certain risk of skewing or tilting the axle. Careful work is therefore definitely required here!
In practice, the two belt drives are on a par. Blindfolded, we couldn't detect any difference in driving behaviour. The Conti CDS is a little easier to handle, but the effort involved with the Gates is also always manageable. The advantage of easier installation of the Conti system is more an issue for bicycle manufacturers than for the cyclist himself: They can simply forget about their drivetrain for the next few thousand kilometres. No matter which one.
