Too Tech Suspension

Fork Geometry and the hidden effect of “trail” changes

“Trail” is the distance at ground level between a vertical line intersecting the front axle and a line drawn from the headstock Center Line to the ground along the steering axis angle. Stable handling demands that the line through the headstock contacts the ground in front of the vertical line dropped from the wheel axle.

"Trail” determines the amount of self-straightening a vehicle has. A good hands-on illustration of trail is noted when pushing a shopping cart. As you push it forward, the wheel will always follow the steering axis. The tendency to follow the steering axis makes the wheel stable and follow behind the steering axis. As the wheel follows behind the steering axis it points forward. Without trail you could not take your hands off the wheel (without crashing).

CLASSICAL DEFINITION OF TRAIL AFFECTS

Trail affects front-end stability and the ease with which the bike steers. The longer the Trail distance, the more stable the bike is. This is because the farther behind the steering axis the wheel sits, the more likely the wheel will follow directly behind the axis and point forward – thus stability. The only ‘classical definition’ tradeoff to large Trail dimensions is that the steering requires more effort and more rotation of the handlebars to initiate a turn. Conversely, a shorter Trail distance creates lighter & quicker steering but less self straightening effect.

THE CLASSICAL DEFINITION MISSES THE INFLUENCE OF LEAN ANGLES AND BUMPS

I cannot find a description that includes how trail affects handling in the bumps and lean angles that we encounter off road or on MX tracks. The closest I came to someone insinuating another affect is shown below:

“A higher mechanical trail is known to make a bicycle easier to ride "no hands" and thus more subjectively stable, but skilled and alert riders may have more path control if the mechanical trail is lower”.

My explanation goes as follows:

We know that the tire touches the ground behind where the steering axis intersects the ground. When the bike is upright, the tire is behind the steering axis and wants to stay behind (like a shopping cart). The weight of the bike is directed downward (vertical) and reacted by the ground which pushes directly upwards. This upward vertical force is directly in line with the steering axis, so there is no turning force imparted to the forks. When hitting bumps with the bike straight up and down, the instantaneous load increase comes directly upwards through the forks and does not create any turning moments.

HOWEVER, when we lean the bike over things change. Imagine the bike leaned over until the pegs almost touch the ground. The weight of the bike is still directed straight down at the tire contact patch, and the ground still pushes directly up at the same point. BUT this time the forks are not in line with the upwards force, but instead the force is sideways (almost perpendicular) to the forks. Since this sideways force is behind the steering axis, it uses the Trail distance as a lever arm to rotate the forks around the steering axis. SO the larger the trail dimension, the longer the lever arm available to create this ‘self steering’ rotational force as the bike leans over. Now on a street bike driven on a flat road, this self steering force is steady and predictable so it does not create a handling problem. Large trail dimensions do require slightly more rider strength (but still insignificant), but with no bumps the effects are calm and predictable. To prove the effects of trail; take the bike off the stand and hold it at the front of the seat. Now lean the bike over to one side and watch the front wheel turn into that side. This occurs because the tire contact patch is behind the steering axis.

BUT in a dirt bike the results are dramatically different as we lean the bike over and hit bumps. When we hit bumps we momentarily load the tires far more than the weight of the bike (illustrated by pinched tubes on square edges). This instantaneous additional weight at the tire is reacted by the ground and pushes back at the tire contact patch with an equally large force.. SO, when we are leaned over and hit a sharp bump, the instantaneous weight at the tire goes up creating an instantaneous and unexpected ‘self steering’ effect. The more we lean over and the sharper the bump, the more violent the self steering force becomes. ADDITIONALLY, the longer the “Trail” dimension, the longer the lever arm and the more violent the ‘self steering’ event becomes. And of course all this instantaneous and unexpected self steering comes when we are leaned over and the most vulnerable to any steering disturbances. 

Since “Trail” is designed into the bike’s geometry, we can only make small changes to it by altering the race sag or fork height. The 2008 & 2009 Honda’s have a steering damper which has the potential to minimize the effects of self steering’. Unfortunately Honda did not put enough damping into the steering damper to do the job. Too Tech is changing the oil, repositioning the piston on the shaft, and changing the valving to produce more on center damping and a noticeable progression as the handlebars rotate. The welcomed result is that the wild and unexpected steering inputs are gone. Amazingly, the rider is unaware that the damper is even on the bike.

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