Safety Officer’s Report:                                    Steve Coomer, Safety Officer

Suspension & Steering Basics, or…”Steve, why does your chopper ride so smoooooth?”

Last issue I gave you a rundown on tire data and promised to follow that with a suspension article. Due to the amount of information for a suspension article, I am breaking it down into digestible sub-sections so we don’t need to mail a small ‘book’ for the newsletter---I hope our Editor and the Postal Service can appreciate this.

One of the first questions I get about my chopper is “How much rake do you have?” I wonder how many riders asking me know what their stock bike has, or more importantly, why the angle is set the way it is? With that said, this issue I am going to enlighten you regarding rake and trail. That’s right--I said rake AND ‘TRAIL’.

Rake, in degrees, is the angle from vertical to the extended centerline of the steering head post. As an example, the rake on my bike is 46^o. This is the maximum angle that can still provide good steering and suspension (see next issue) characteristics while providing a car-like ride down the highway due to the increased wheelbase.

Trail, in inches, is the distance from the tangent point of the front wheel (measured perpendicular through the centerline of the axle) to the point where the extended steering head post centerline intersects the ground. The accompanying illustration should help you visualize this…

Your current (’02 & ‘03) stock Harleys will have rake and trail numbers as follows:

Sportsters:

XLH 883, XL 883R, XLH 1200, XL 1200S: 29.6^o x 4.6”

883 Hugger: 30.1^o x 4.7”
XL 883C & XL 1200C: 31.0^o x 4.7”

DYNA’s:

ALL Super Glides: 28.0^o x 4.1” 
FXDL, FXDWG: 32.0^o x 5.1”

SofTails:

FXST, FXSTB, FXSTD: 32.0^o x 5.0” 
FXSTS: 32.0^o x 5.3”

FLSTF, FLSTC: 32.0^o x 5.8” 
FLSTS: 32.0^o x 6.3”

Baggers:

FLHT, FLHR, FLHRC, FLHTC, FLHTCU, and FLTR:    26.0^o x 6.2”

…and for the V-Rod curious: 34.0^o x 3.9”

Now, let’s consider the effects that rake and trail have on how a bike handles. The smaller the rake angle to the vertical, the more agile (less stable) the front end is. This isn’t so much a function of the angle as it is an effect of the trail decreasing as the rake angle approaches vertical. As the trail becomes smaller, stability at higher speeds is sacrificed for improved low speed handling. Larger trail dimensions will give the bike excellent high-speed stability at the cost of cornering agility at slower speeds. Trail can be adjusted for a given rake by changing the length of the fork or by changing the offset. Trail can also be adjusted by modifying the rake angle for a given fork length. The ‘ideal’ trail dimension is a compromise. Minimum trail results in a front end that flops from side-to-side when the bike is upright and stationary. Excessive trail creates a front end that requires excessive effort to turn.

Offset is the distance between the steering head centerline and the imaginary or real line parallel to the rake angle where it intersects the front axle. As an example, the offset for a springer is accomplished by controlling the length of the rockers that hold the front wheel. Telescopic front end offset is determined by the triple clamp spacing of the fork tube centers relative to the steering head post center. This is how the V-Rod can have the largest rake angle for a stock Harley and maintain a short trail length, which in turn provides for excellent steering qualities at low speeds as well as good high-speed capabilities. From the previous illustration, note how offset and trail are related-decreasing the offset (moving the axle centerline toward the steering centerline) increases the trail. Conversely, increasing the offset will decrease the trail.

From all of this information it is easy to wonder why the baggers aren’t the best handling bikes due to their lower rake angle. Remember trail? Except for the FLSTS, baggers have the largest trail dimension. This, combined with their small rake angle, provides them with optimum steering and road handling for their weight. However, the FXD’s,  FXDX’s,  and FXDXT’s (known affectionately as Super Glides) have the best overall handling and steering characteristics because the geometry of the front end is the best combination for the weight and wheelbase of the bike—sorry Sporty owner’s, but riders being equal, the physics are in favor of the Super Glides.

Enough said for front end physics. The next issue will discuss front suspension, building on what we’ve learned about rake and trail. Until then, ride safe and have fun on your Harley!