A great primer in what happens with each change.
Loads of people are getting into custom motorcycles these days, and some of the stuff coming out of various independent one one-person shops is just mind-blowing. There are lots of beautiful custom machines, but some of those beauties inevitably turn out unrideable. Why is that? It’s all about the bike’s geometry.
When you plan changes to a stock motorcycle, whether fully customizing or just making a few little changes, it’s good to have at least some awareness of how those changes will affect the bike’s handling. Unlike our four-wheel counterparts on the road, little aesthetic changes can add up to big handling changes.
As noted in this great video overview, there are aspects of a motorcycle that affect how it rides, like the angle of the forks in the triple tree, the distance from the steering head to the ground under the front tire, the size of the front wheel, the distance between the wheels, and the angle of the rear swingarm. We are not mathematicians over here, but the way the video explains it all makes a ton of sense.
You can imagine, too, a fully custom-built motorcycle might have many or most of these aspects changed from stock. It’s not unusual to swap out wheels or entire front ends. Custom builds commonly borrow parts like swingarms from other motorcycles. Choppers often have a wildly-increased steering head angle. It’s good to know how all of these changes can independently affect how a bike handles, and what the effects are.
A good builder will keep all of these changes and their potential results in mind, but must also know about the suspension as well. All of these modifications don’t just apply to custom motorcycles—they can also apply to any motorcycle you have attempted to make lower, for a shorter rider. Not only can lowering a bike change its handling, but it can remove suspension travel and change the stability of the bike.
Overall, changes to motorcycle geometry can completely alter how a bike handles on the road, and you may not love those changes. On the other hand, if you have a bike whose handling characteristics you do not love, this tutorial can give you some ideas about what to change, and how to change it, to make the bike handle the way you prefer.