ACR FLC CHANGES ALT DUE TO TURB AFTER ATC HAD REFUSED THE ALT CHANGE.

Narrative

WHILE ENRTE MDW TO CLE AT FL250 WE ENTERED AN AREA OF LIGHT THEN MODERATE TURB. WE CHANGED OUR ALT TO FL190; HOWEVER; THE MODERATE TURB CONTINUED. WE REQUESTED A LOWER ALT; HOWEVER; ZOB SAID HE HAD TFC AT 17000 FT (A DC9) WHICH WE BOTH HAD IN SIGHT. SHORTLY WE ENTERED A BRIEF BAND OF SEVERE TURB AND INFORMED ZOB WE WERE DEPARTING FL190 VFR (WE WERE IN VFR CONDITIONS AT FL190) TO ESCAPE THIS SEVERE TURB. WHEN WE REACHED 17000 FT THE AIR BECAME SMOOTH. WE CONTINUED OUR FLT TO CLE WITH NO OTHER PROBS. THE ONLY PROB WE HAD WAS THE LACK OF ZOB SUPPORT TO WORK OUR REQUEST TO GO DOWN TO A LOWER ALT. I ASSUME THE 17000 FT TFC (WHICH WE HAD IN SIGHT) BECAME MORE OF A CONCERN THAN OUR REQUEST TO ESCAPE THE SEVERE TURB. I FELT THE ONLY CHOICE I HAD WAS TO FLY VFR TO A LOWER ALT ON MY OWN SEPARATION TO ENSURE OUR PAX AND CREW MEMBERS SAFETY. WE UNDERSTAND HIS CONCERN FOR TFC SEPARATION; HOWEVER; WHEN WE BOTH HAD EACH OTHER IN SIGHT; I DON'T UNDERSTAND WHY CTR COULD NOT AT LEAST ADVISE OF EACH OTHER'S POS 'ONE MORE TIME' AND CLR US TO MANEUVER AS REQUIRED TO ESCAPE THE SEVERE TURB.