Every motorcycle is made up of a design instilled by its builder. This design echoes through the lines that make up the silhouette of the motorcycle. These lines may be implied rather than overt and they make up the underlying look and feel of your motorcycle.
Beauty is, of course, in the eye of the beholder and personal preference trumps all when it comes to design choices on your one-of-a-kind bike. However, there are tried and true guidelines that may help you understand why one bike looks appealing and others seem ‘off’. They might just arm you with the vocabulary to refine your own design language so it doesn’t look like you’re riding a can of alphabet soup.
Following these guidelines won’t make your bike blend in with the crowd, either. For example, smart, individualistic color choice like blacking out certain panels can do a lot to accentuate the lines you like best. And when looking at inspiration bikes, keep these lines in mind to see how they influence the type of design you want.
Generally, the most dominant line on a bike, the *foundation line* runs from front to back. On many bikes, it travels from the steering neck, along the underside of the tank and follows the frame or seat out towards the rear of the bike through the fender. And it doesn’t have to be a straight line. On cafe racers it often is rigid and horizontal, yet for a bobber or a chopper it is often a reclined line from the steering neck down to the back of the bike.
If you were to look at the bike as a stick figure or as a crudely drawn sketch, it would be the dominating shape that you see that connects the front to the back. The foundation line also does a lot to impart the feeling of speed. It's no accident that most fast-looking bikes like flat trackers and café racers often have a flat foundation line parallel with the road. On many sports bikes the foundation line tips forward, giving a very aggressive nose down appearance to the bike.
After the foundation line, these are the other dominant lines of the bike called *primary lines*. These are generally lines that are more vertical than horizontal. Often these are the rear shocks and major frame tube fore and aft of the engine.
A clean bike keeps these parallel or at least moving in the same direction. Too much difference in the direction of these lines can make a bike feel messy, which should be avoided unless this is the look you want.
If you do want differences in the primary lines, make the difference deliberate and noticeable so it looks like you intended to do it, rather than poor workmanship that resulted in one primary line being a few degrees off any other two primary lines.
Every motorcycle has a point in the front and rear where the bulk of the bike ends. These are the *cut-off points*. Often this happens at the outermost edges of the front or rear fenders, but other notable cut-off points are the headlight and back of the seat or taillight, or wheel hubs if not running fenders.
For many builders wanting to build aggressive and peppy bikes, the cut-off points are kept inside the front and rear hubs. Going out past the hubs with fenders will lengthen the look of a bike. If you have a large rear fender that extends out past the rear hub but no corresponding extension past the front hub with a fender, your bike may end up looking lopsided and unintentional. So, aim for harmony between the front and rear cut-off points.
Two heights are considered: the seat height and the overall height of the bike. While both are important individually, where they intersect can affect the look of the bike. On a chopper with ape hangar bars, there is a vast difference between the two, giving a very tall look at the front of the bike, becoming a small wedge towards the rear where the rider sits. On a cafe racer, the intent is often to minimize the distance between the seat height and the overall height of the bike.
Your height obviously impacts how well you can handle your bike, so you can customize seat height for yourself by changing suspension or wheel size until you find what is comfortable. If you plan to sell the bike it might be a good idea to not go too far outside the normal range of seat heights.
Often a customizer might want to increase the suspension height to give a bike a ‘lifted’ and tall feel. This can end up with a very tall seat height, so a way to combat this is to fit smaller diameter wheels, which lowers the seat and accentuates the height of the bike. Sometimes the opposite is true, where a stripped back bike can start to feel tiny once a 6’2” rider throws a leg over it. So, fitting large wheels can add some size back to the feel of the bike as well as lifting the seat height.
This is a measure of how visual density is distributed, often centered around the engine of the bike. It varies a lot from bike to bike and where - within the bike's silhouette - the visual weight is evident. A bagger or cruiser has a lot of weight overall, feeling bulky all the way from front to rear because of a big engine, big tank, big fenders, and a long frame. The weight is quite consistent throughout the bike.
A streetfighter however, with no or minimal fenders, a short stubby seat, and exhaust kept short and low down, would have the bulk of its visual weight only around the engine and tank. Other styles like flat trackers and cafe racers take a similar approach, trying to keep the visual weight around the engine and tank and giving the rest of the bike an open and light look with minimal fenders and open sections of frame.
The bone line can be hard to grasp at first. It's not a hard line like the foundation line or the primary lines that are forged by the frame, seat, or tank. Rather it is the line where light reflects off the bodywork, generally occurring at the widest part. On a car for example, this is often about one third down from the door glass, often running through the door handles. It's a line your brain picks up when looking at the culminative effect of all the parts (tank, headlight, fairings) where the sense of shading on the bike goes from light to dark. On a bike this roughly horizontal line often occurs about mid height of the tank and runs through the controls and the midpoint of the headlight cowl or front fairing.
These lines made are made up of the forks, exhaust, and smaller frame members. They won’t be moving in the same direction as the primary lines and are often aligned in a contrasting direction. They may be complimentary to the primary lines and softer, or there may be one strong secondary line as a visual contrast to the primary line, such as a strong upward-trending exhaust on a chopper that contrasts frame lines.
Secondary lines also tend not to be parallel to each other and may not always be moving in the same direction, but a good bike design tends to not have many strong competing secondary lines, as this can give an impression of poor workmanship and messiness. When in doubt, keep it simple.
This is the angle of the front forks compared to an imaginary vertical line through the center of the head stem. Increased rake pushes the front wheel further away from the front of the bike frame. Choppers tend to have a relaxed rake, sports bikes a very small or steep rake. Not only is there a visual importance, but there are also important handling considerations with rake (and an associated parameter, trail). Bikes with a steeper rake tend to be able to steer and change direction quickly, but too steep a rake and a bike can become unstable.