At any speed your neck is taking all the stress and twists. At 90+ it's a lot of stress so your neck gets a few degrees out of wack and the back end follows... In the old days we would weld plates on the raked necks to help this but even the plates didn't help at 90+. Being you own a chopper with a large rake doesn't help. Also the down tubes twist and even with a brace it won't help over 90. These are cruising bikes not crotch rockets...