Panniers - Racks - Bags - Bike Luggage

So, after rambling in the bikepacking thread, I'm seriously considering adding a rear rack to my audax bike. Something like a Tubus Fly, mounted on standard eyelets. I'll be using micro-panniers (I'll probably have to make them myself), and I don't think the load will exceed 5 kg (and will probably bit a fair bit less).

Will this change the supposedly magical ride quality of my high-end steel frame? Because of either the weight itself, or the fact that the rack itself might stiffen the rear triangle (not sure if that can actually play a role!).

Here are the tubing specs, for those with a lot of framebuilding experience :

• Top tube 28,6mm - Velospec 75 - 0.7 x 0.4 x 0.7 mm
  
• Seat Tube 28,6mm - Velospec 75 - 0.85 x 0.6 mm
  
• Down tube 38,1mm - Velospec 75 - 0.7 x 0.4 x 0.7mm
  
• Head tube 44 mm - CrMo CNC
  
• Chain Stays - CrMo - 425 mm length
  
• Seat Stays - CrMo - 10 x 0.9 mm
  
  

Here's what it looks like, except I've had standard rear rack eyelets brazed on the dropouts and at the top of the seatstays (and I have a generic carbon fork). As you can see, the seatstays are very thin.

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