Unravelling the Mysteries of Rake, Trail, and Offset
Rake, trail, and offset are probably some of the least understood terms in motorcycling – particularly trail and offset. We often see these numbers in specification charts, but do you know what they mean?
Rake and trail terms refer to motorcycle steering geometry, and, along with offset, have a great effect on the machine's handling and steering effort. Rake, trail, and offset are inter-related. As such, changing one will affect the others.
Rake is the angle in degrees of arc which the headstock of the motorcycle is inclined when compared against a vertical line drawn perpendicular to the ground. The smaller the rake angle, the less effort is required to turn the steering. Though, the motorcycle will be less stable in a straight line. Conversely, a larger rake angle requires more effort to turn but tends to make the motorcycle more stable at high speeds and helps maintain a straight course.
Sportbikes and dirt bikes tend to have lower numerical rake angles, while cruisers tend to have numerically larger angles. Sport touring and adventure bikes tend to be in the middle range. To throw everyone off, BMW specifications are typically measured differently than other brands.
Trail is determined by a straight line drawn through the centerline of the frame's headstock axis and through the centerline of the front axle, extended to the point where it meets the pavement surface. Trail is then measured as the distance between where this theoretical line intersects the pavement surface and the center point of where the tire actually contacts the surface (directly below the centerline of the axle). This measurement is generally offered in inches, millimeters, or both.
Less trail reduces steering effort and high-speed stability, while more trail increases straight-line stability while increasing turning effort. As with rake, sportier tend to have less trail, while cruisers tend to have more trail.
Steering Offset is the distance between a line drawn through the centerline of the steering stem/ headstock axis and the centerline of the front fork tubes. Typically, if offset increases, trail decreases.
Relationships Between Rake, Trail, and Offset
Modern motorcycles are carefully engineered to work well just as they come from the factory, with a blend of good handling and stability. Owners who want to alter their motorcycle's cornering ability or straight-line stability sometimes adjust rake, trail, and offset. This should only be done, however, if a person has a thorough knowledge of the principles involved – along with requisite mechanical skills. Owners should not alter steering geometry on a whim. However, it is fine to optimize available adjustments, as long as you fully understand the principles involved and the potential consequences. Even small changes can significantly alter handling and, as such, safety.
If rake is increased, trail increases. If trail is increased, rake increases. If you increase offset for both triple clamps, trail decreases. If offset is increased for only the top triple clamp, trail will increase. If offset is increased for the bottom triple clamp only, trail will decrease. And so on and so on. You need to have a good idea of what you're aiming for before taking on the task of adjusting any of these things.
Steering dampers are often used on sporty motorcycles – which have a very low rake angle – to keep the machine stable, preventing oscillations from becoming a severe speed wobble. Aftermarket steering dampers can be added to machines that don't have them if the owner feels that one is needed.
Changes to steering geometry can be made in several easy ways. Some of the things that can be altered:
- Front fork: Shorter fork tubes decrease rake and trail. Some owners slip the tubes upward into the clamps somewhat to quicken steering.
- Rear suspension: Lowering it will effectively increase rake and trail. Conversely, raising rear suspension will decrease rake and trail and quicken steering.
- Spring preload adjustments: As noted, these can change ride height to fine tune handling via rake and trail changes.
- Wheel diameter: a smaller diameter front wheel/tire assembly will decrease rake and trail. A smaller rear wheel/tire assembly will increase rake and trail. Conversely, larger diameters will have the opposite effect.
- Aftermarket triple clamps: These can be installed to alter offset; it's possible to change trail without affecting rake.
Braking and suspension travel also affect rake and trail when the motorcycle is moving. For example, under heavy braking at the front, the fork will compress (dive), decreasing trail and rake. This can potentially induce a speed wobble. A heavy passenger or luggage compressing the rear spring(s) also affects rake and trail, increasing them and slowing down steering. This is a major reason why rear suspension preload should be kept properly adjusted.
Be warned that changes, while seemingly minor at low speeds can have major effects at higher speeds, such as decreased stability!