Welcome to our Home Schooling series. The idea is to present motorcycling technology in a simple form everyone can understand, even children. If you’re a British Superbike technician or a professional rider, stop reading and go make a brew. For the rest of us, pin your ears back, get a note pad and pen and read on. Unlike school, there won’t be any homework, there isn’t a test at the end and because you’re at home nobody knows if you’re picking your nose or eating your lunch early. Now concentrate on today’s lesson...
Enormously weighty and in-depth books have been written on motorcycle geometry, a subject that scares and confuses even some professionals, but we are going to bring it down to a very basic level. We are going to simplify it by looking at basic rake and trail, plus wheelbase. We aren’t going to look at offset, swingarm pivot, and other complicated terminology. Again, don’t worry, this is going to be easy, and after reading this article you’ll wonder why people get so confused and messed up. Geometry is simple in its basic form.
Basics: Using the above diagram: First we are going to say the forks go through the headstock, like a pushbike. We then draw an imaginary line down the fork leg, through the front wheel spindle until we reach the road. Then we must draw another imaginary line this one is vertical through the front wheel spindle to the ground once more. So, we have one diagonal line, following the forks and one vertical through the front wheel spindle… good.
Rake: The distance, from our imaginary vertical line (above the front wheel) to our diagonal fork line is rake, again see diagram, this is measured in degrees.
Trail: Below the front wheel spindle, you’ll notice the lines have carried onto the ground. The distance from our continued fork line to the imaginary vertical line is trail. See, simple enough and not too hard.
Forks out: There are many different terminologies, but essentially if we push the forks away from the bike, push them out or extend them like you’d find on a cruiser, then you’re increasing rake and trail, as the diagonal line is further away from the vertical line. In general, the more we move away from the vertical line, the more we push out the forks, the slower the steering, and the more stable the bike becomes. More trail makes the bike want to go straight.
Forks in: Again, there are many different terminologies, but bringing the forks in towards the bike, so they are closer to a vertical line, reduces rake and trail. This quickens the steering but increases the chance of instability.
Most bikes: Generally speaking, most bikes will have a relaxed rake and trail, neither too aggressive nor near vertical either, as they must be safe and be able to cope in all scenarios. Rake and trail will depend on the bike you ride – adventure, street or sports – and the size of the wheels.
Should we change rake and trail? You might be doing this already without realising as changing wheel sizes will change your rake and trial, so too fitting in the incorrect tyre size. Dropping or raising the forks through the yokes also changes rake and trail. Generally speaking, you don’t need to change your rake and trail, but some riders do it to allow the bike to steer faster on track. But again, while the steering might be quicker and stability perfect up to 100mph, after that it may cause instability or even a tank-slapper. You don’t want a 150mph tank-slapper, trust me.
Controlling instability: If we want to a bike to steer fast with aggressive geometry, we may have to add a steering damper to control the speed of the bars. This can be side mounted, which you’ll find on older bikes, or across the steering head. The damper is there to control the speed of the steering and to prevent tank-slaps. But, even with a damper, stability might still be an issue over bumps at high speed, which means you’ll have to relax the steering head angle to improve stability again.
Wheelbase: Check out the diagram above. We have two wheels... In the middle of each wheel is a spindle; now from the spindle draw a line vertically down. Once the line hits the road, the distance between these two lines is the wheelbase, which is often given in bike specifications.
What does a change of wheelbase do? If you have ever seen a drag bike built specifically for ¼-miles runs it will have a huge wheelbase, which prevents the bike from pivoting, which mean fewer wheelies and therefore gives more drive. A long wheelbase will also give more stability as the bike will always want to go in a straight line (a drag bike doesn’t have to go around corners), therefore they can go radically long on wheelbase. Trying to turn a drag bike is hard work, if not impossible in extreme situations. The opposite happens when we reduce the wheelbase: we increase the chance of wheelies, and in extreme situations will quicken the steering and increase the chances of instability.
Should we change wheelbase? You may have done this already when you tightened your chain and moved the rear wheel back in the swing-arm (depending on the bike). A road bike is designed to have a ‘short’ or a ‘long’ chain, so you won’t have problems with stability. However, in racing, even on track day bikes or at club level, riders can notice the difference in wheelbase changes when you lengthen or shorter your chain length. This will affect the speed of the steering and the bike’s stability. In road racing, I’ve seen riders change the length of their chain, and come back in after one lap complaining of instability over the bumps.
Summary: Again, this is a very basic understanding of geometry and the terminology used – rake, trail, and wheelbase. We have not looked at fork offset, swing-arm position, etc. Again, you shouldn’t need to change these, unless you’re on a race track. And remember every small change can have a significant effect on the bike’s dynamics. It might be solid as a rock at 100mph but handle like a wheelbarrow with a flat tyre at 150mph. If you’re unsure, leave it to the experts – but hopefully now you have an understanding of the basics, enough to impress the kids or mates down the pub/café.