Well I don't wanna get too deep into this and off track, but as you bring it up again - As mentioned, I'm a bike mechanic. Not like I tinker at home, but like I was a head mechanic in a reputable shop in a major cycling town before changing careers. I also am into all sorts of biking, including MTBing, and am fairly familiar with the capabilities of the bike. I've never had my hands on a Banshee, but if you're having stability issues at 40mph, I'd look to things like your wheels or the tires you're using as a first step. Check all of your suspension bushings and bearings to make sure they're tight. I assume this is a through axle bike, so if your axles are tight, you shouldn't have issues with flex there. You'll want something like a Fox 36 or beefier for no flex at those speeds and weight. Your frame shouldn't have a lot of flex to it. I'm not saying it'll be like it's made of solid marble type stiff, but it shouldn't have enough flex to give concern. I'd really start with looking at wheels/tires, though. Are your rims wide enough for those speeds? If you have a "light bulb" shape to your tires, they aren't. If you take a wheel off and put it on a work bench, axle on the bench, and push hard on either side of the rim, with your hands and 3 and 9 oclock, does it flex at all? It should be stiff with zero flex. Are you using quality street tires with a low, stable tread pattern? I've bombed a long mountain road on a loaded touring bike at 40mph and it felt solid. Just gotta make sure the bike is stiff, set up well, and everything is tight and in good condition.