Yep that's the issue, and also the intended fix. The main boom will have to actually be offset to the right a bit, because it's the left wheel that farther over since it's the wider hub. But yes, I'm going to measure wheel center and work based on thatTaking a good look at the picture of the rear wheel enclosure I'm gonna assume you have used on the left a rear wheel with the gears and on the right a front wheel. The different width of the hubs being due to the gears. One possible solution is move the main boom of your frame slightly to the left. Check it by measuring from the center of each tire, marking the middle and welding the main boom at the spot where center is.