Chassis Geometry

Story by Costa Mouzouris//
June 1 2014

Motorcycle specifications can often be dizzying, especially chassis geometry. For many, the numbers are meaningless; however, deciphering those numbers and understanding how they affect motorcycle handling just might help you understand why your bike handles the way it does. Or they might help swing your decision when making a bike purchase. Here’s a quick look at what those numbers are all about.

Three basic measurements have an important influence on handling: wheelbase, rake angle and trail. Wheelbase is simply the distance between the front and rear axles. Rake angle, sometimes called caster angle, is the angle of the steering head (steering axis) from vertical, measured in degrees. Wheelbase and rake are readily visible when looking at the profile of a bike; trail is not so obvious. To obtain a trail measurement (in millimetres), you have to visualize a line going straight through the steering axis to the ground, and a second line going vertically through the front axle, also to the ground. The point where the steering axis meets the ground will be ahead of where the axle centreline meets the ground. In other words, the tire’s contact patch “trails” behind the steering axis. This gives the front end inherent self-centring properties that enhance stability. Just think of the front wheels of a shopping cart; the steering axis is vertical, but the wheels are offset to the rear. This pulls the wheels rearward, thus centring them.

These three measurements are closely correlated, and a change in any of them will affect handling. Wheelbase has a direct effect on steering quickness and straight-line stability: a short wheelbase is conducive to agile handling, and a long wheelbase enhances straight-line stability. Likewise, a bike with a short wheelbase will have a tighter turning circle than a bike with a long wheelbase, given the same steering angle. Moving the wheel back or forth in the adjusters on chain-driven bikes will alter the wheelbase a bit, and racers often do this to tune the chassis for specific tracks. Altering the wheelbase has no effect on rake or trail, but note that pulling the wheel back puts it farther away from the bike’s centre of gravity and will put more weight over the front wheel, thus making steering at least a little heavier. This is why drag racers extend the swingarm; it helps prevent front wheel lift.

As described, trail helps centre the front wheel, thus keeping the motorcycle moving in a straight line unless steering input, road camber or a crosswind causes it to change direction. The higher the trail measurement, the more force will be required to change direction. Sportbikes have little trail, somewhere in the 100 mm range, while cruisers have more trail, often upwards of 150 mm.

Rake angle also affects stability and steering response. The closer to vertical the rake angle is, the quicker the bike will steer and the less stable it becomes; the more laid back the angle, the slower the bike will steer and the more stable it will be. This is why you’re more likely to find a steering damper on a sportbike, which has a steep rake angle, than on a cruiser that is “raked out,” comparatively speaking.

The rake angle has a direct effect on the amount of trail – the steeper the rake angle, the shorter the trail. To achieve the chopper-like look of a stretched-out front end, bike makers often sacrifice handling by introducing copious trail. To counter this, some Harley-Davidson, Honda and Yamaha models cheat the fork angle by offsetting the forks in the triple clamps to achieve the look of an extremely raked front end while maintaining a reasonable rake angle at the steering head. The Yamaha Raider, for instance, has a rake angle of 34 degrees, and the forks are offset six degrees from that. This gives the bike an included fork angle of 40 degrees, while maintaining a sportbike-like trail of only 102 mm.

Now, the rake angle is measured on a static chassis, without a rider weighing down the rear suspension. As weight pushes on the rear of the bike, the rear end squats more than the front, which effectively changes the rake angle and trail measurement (in relation to a vertical line), increasing both and slowing steering down. Aside from the added weight, this is why it’s more difficult to steer a bike when taking on a passenger or loading on luggage. This is also why the rear shocks almost always have adjustable spring preload; it is there primarily to maintain ride height, and therefore handling integrity, when adding a load.

Road racers also use rear ride height to alter the steering geometry; they will raise the rear end, thus reducing the rake angle and trail, to improve turn-in on tighter racetracks. It’s a fine balance, however, because it also changes the weight bias of the bike, which might have undesirable consequences. Also note that changing tire sizes might also affect ride height, and therefore the handling characteristics of a bike.

When I was a technician, I was always reluctant to lower the rear suspension of a bike to accommodate short riders, as it had an adverse effect on handling. When making such alterations, always try to match the amount of the rear drop by sliding the fork tubes, if possible, up in the triple clamps – but check for clearance issues, and remember that lowering a bike will reduce cornering clearance. Any chassis modifications should be well considered and researched ahead of time, and don’t make any alterations that you cannot reverse – you may ruin a perfectly good bike.

Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.

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