The chainstays connect the rear axle to the bottom bracket. At some point the need to pass between the tyre and the chainrings.
There has to be a compromise to do this.
- Less tyre clearance
- Longer chainstay
- Smaller cross section of chainstay (less stiff all else being equal)
- Indirect path of chainstay (dropped chainstay)
- Wider chainline
Every frame design must solve this problem in some way.
Cannondale has chosen to solve this problem by widening the chainline. That is essentially what Ai is, a wider chainline.
MTB 55mm vs 49mm standard or 52mm boost
Road 51mm vs 45mm standard or 48mm boost
It is a good solution in that, all else being equal, allows
- bigger tyres
- shorter chainstays
- more direct chainstay path (weight/stiffness?)
- maintain cross section (stiffness)
It has the downsides of
- proprietary cranksets/chainrings to achieve different chainline
- wider q-factor in some instances
- assymetrical dropouts relative to midline of bike resulting in wheels that can't be a direct swap for symmetrical dropout bikes (rear everything else)
Note that the assymetrical dropouts have an added benefit of a dishless rear wheel which, all else being equal, will be more durable and possibly stiffer.
Like everything. The buyer must decide what is important to them. For those who like to customise, like users of this forum, it can be a turn off, or a challenge to solve!
. But for the majority of users who ride the bike as it comes out of the box these design features are a selling point.