1986 VT500c rider. Noticed this since I got it a few months ago but it doesn't normally effect me too much. Bike rides fine up to about 75 mph and I try not to speed very much but I've noticed once I go past 75 and the bike becomes very shaky. Especially around 85 or so there's a very noticeable shake. It isn't a tire pressure issue and maybe it's normal but it doesn't feel like it should be. My dad who's been riding for a long time says my tires look good so I haven't worried about changing them yet.