The Geometry of your Motorcycle
Whether or not you understood it at school, geometry plays an important role in determining how your motorcycle feels and handles. So it can't be ignored. But you don't need to be a geek to tell the difference between rake and trail.Walk around any motorcycle showroom, flick through magazines or read articles on Internet, and you're guaranteed to come across a motorcycle specification panel at some point. But apart from glancing at the peak power and torque figures, which other information do you retain? No, not a lot; we're the same. Which is hardly surprising, given the density of information packed into a specification panel. And if the numbers don't mean much to you, they're a tad boring too.
But there's often an important story hidden away in these information cluster-bombs – and it's nothing to do with how much power the engine makes. These figures, if you're able to interpret them, tell you what to expect when you try and ride down a twisty back road, and how it'll behave while using countless tanks of fuel heading up to the North of the country. The language in which these insights are written is geometry.
Don't panic – there's not risk of after-school detention. The measurements we're talking about are grouped under the 'geometry' banner, you don't need a white lab-coat to understand, use or even calculate the basics for yourself.
Think of a motorcycle with a long wheelbase and another with a short wheelbase. Chances are, you thought of a tourer or cruiser first, and perhaps a 600cc sportsbike second. Why is that, and are you right?
Even if you don't feel comfortable defining exactly what wheelbase is and how it affects things, it's patently obvious that most tourers are relatively big, long and heavy, whereas most sportsbikes are small, short and light.
Which begs the question – why? But before we get too carried away, we need to look at the small matter of what wheelbase actually is. For the sake of this article, we're going to define it as the distance between the contact-patches of a motorcycle's front and rear tires. Of course, it's impossible to measure to the center of a contact-patch unless sitting the motorcycle on a sheet of glass and measuring from underneath. So, it's simpler to measure from the center of both wheel spindles, but you must be careful when doing this because it may not be accurate unless you hold the tape-measure parallel to the floor.
The reason the tape must be parallel to the floor is simple. Think of a motorcycle with odd wheel sizes – something like Yamaha's XT1200Z Super Tenere, which has a 19-inch front wheel and 17-inch rear. If you use a tape-measure to measure directly from the center of one wheel spindle to the center of the other, it will be angled very slightly uphill, which would mean your measurement is slightly longer than the actual distance between contact-patches.
Interestingly, if you go outside and measure the wheelbase of your own motorcycle right now, it's likely not to agree with the specification for your motorcycle. The reason for this is explained more full in the 'Static versus Dynamic Measurement' explained later on, but for now just understand that hings like chain-stretch, suspension settings and even how much petrol is in the fuel tank all change the measurement you obtain. So what can we actually learn from such a hard-to-measure and variable value?The answer is we can use it relatively. In other words, if you have two otherwise-identical motorcycles, the fact they have different wheelbase measurements tells you they will handle differently. If you've ever fitted new chain and sprockets to a motorcycle, you may have noticed that, afterwards, the motorcycle seemed keener to lift the front wheel, had more grip and cornered more tightly. You weren't imagining this.
A new chain is shorter than an old one because there's no wear in the links, meaning that, when it's fitted, the rear wheel is adjusted forwards. Moving the rear wheel forwards, on any motorcycle, shortens the wheelbase by up to 70mm. In doing so, you place more weight on the rear wheel and move the rear axle closer to the motorcycle's center of gravity. You also bring the rear wheel closer to the front. Imagine both front and rear wheels are painting lines on the road: now that they're closer together, the rear wheel's painted line will follow – and cross – the front wheel's line in a shorter distance. Hence, you turn tighter.
Trail is what keeps your steering straight even when you take your hands off the handlebars. It is a fairly simple measurement – although tricky to describe.
In terms of the measurement itself, it reveals how far behind the steering axis the center of the contract-patch sits. Hmmm, still not clear? Well, try this. Imagine an articulated lorry. The bit where the trailer attaches the cab is just like your headstock. The length of the trailer is the same as our trail measurement – more trail equals a longer trailer. The trailer's wheels are like your wheel's contact-patch. When the lorry drives forwards, the trailer swings around behind to follow the lorry cab because the tires want to stay behind the steering axis. The longer the trailer, the slower it swings around, but the more stable it will be. When your motorcycle moves forwards, your contact-patch also swings behind the steering axis – meaning your trailer also swing into line.
Using the same analogy – this explains why cars towing trailers seem to tie themselves in knots at a lower speed than lorries and their trailers. The same logic applies to motorcycle steering. The more trail a motorcycle has, the more inclined the steering will be to follow the steering axis and, in other words, the less flighty the handlebars will feel. But there is a downside to this. While more trail means more stability under braking, cruise and acceleration), it means you'll need to work hard to deflect the contact-patch from following the steering axis when you want to turn. The upshot is the motorcycle feels less responsive to the rider. And following that logic, it's clear there's a compromise between stability and steering effort/responsiveness.
However, there's another compromise to be considered too. Once a motorcycle is actually leant over, the more trail it has, the more the steering will want to turn in to the corner and the better line it will want to hold. So making a motorcycle handle isn't just about adding more trail – otherwise it becomes equally unridable.
While you would never buy a motorcycle on the geometry specs alone, they're worth paying attention to. For instance, if you want a sportsbike but expect ride it mostly on bumpy roads, more trail is a bonus – it indicates the motorcycle will be less easily upset. Equally, if you're lucky to live where the roads are full with hairpin bends, then perhaps a shorter wheelbase will be the deciding factor – it signals that the motorcycle will turn faster and tighter.
