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Its custom from its frame geometry up. Specifically if you've ever tried to ride a velodrome optimized track bike with steep angles, short wheelbase and a high centre bracket on a flat dirt track you'd understand why. Stability is king racing on flat dirt tracks. It was 50yrs ago. All of the attributes assigned to a good velodrome frame serve to destabilize you racing on dirt.
If you'd actually read the text on the link you'd already know this.
Yeah, but is it bespoke?