Checking the wheel alignment on a bike is a really useful skill, especially when you’re out there looking to buy a new machine and you want to make sure the frame isn’t bent and the forks are still straight.
The first thing you need is a straight edge and an eight foot fluorescent tube will give you that.
Next, clamp your straight edge against the front tyre, then very gently move the steering until the straight edge is touching the front edge of the rear tyre and measure the distance between the straight edge and the rear of the tyre, it’ll probably be about 10mm.
Now repeat the process on the other side and use the chain tensioners to set the gap so it’s equal.
Don’t use the chain adjustor marks to align the wheels. Instead, use this method, then turn the adjustment bolts equal turns, maybe a flat at a time to get the chain tension right.
Once the wheels are aligned, crouch down at the back of the bike and look down the chain run, if it’s out, then there’s something wrong with the chassis.