I really don't think that Ortlieb Back-Rollers are huge. They are in fact slightly smaller than many other touring panniers from Vaude or Arkel. Besides that, they are tapered at the bottom instead of being just square which helps against heel strike. So does the excellent mounting system that means you can move the pannier quite a way back.

Since the Tubus Vega is the same as the Tubus Logo just without the double rails, I can give a good estimate on heel strike. On a bike with 42 cm chain stays (vs 40,5 cm for the Allez) there are ample space between the Back-Roller and the heel at any realistic angle. The shoes are Shimano winter boots size 13 (US), crank arms are std. 172,5 mm, the cleat is placed in the middle position.

All in all there should be a good chance that the OP won't have heel strike with the Tubus Vega rack, even with short chain stays.

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Regards