Rake and Trail

Basic Frame Terminology

The world of choppers and chopper building like most other fields of endeavor has it's own unique terminology. In most cases people throughout the world use the common chopper terms but you will find that there are sometimes terms that are unique to a particular region or locality. There are several on-line 'chopper' dictionaries and glossary's one can refer to but most Iíve seen are pretty elementary and cover the common slang phrases and words but don't go into technicalities.

For this reason we thought it might be a good idea to include a short section in the manual that describes the basic parts of at least the frame on a typical chopper. For those new to the entire world of motorcycles you can pretty easily pick up the lingo for the component parts that bolt on to the frame as you follow through the rest of the manual.

Figure 2.1 below illustrates a typical rigid chopper frame for the style that many people call 'Old-School', meaning it represents the style or design that was popular back in the seventies, sixties and fifties before the arrival of wide tires.

Figure 2.1

This is what's called a 'rigid' or 'hardtail' frame meaning that it has no suspension system of any kind for the rear wheel and tire which has it's axle rigidly bolted to the frame.

Figure 2.2 illustrates a typical suspension type of frame. In this case the diagram depicts a 'softail' frame where the rear tire 'swingarm' pivots about a shaft that runs transversely through the main portion of the frame.

Figure 2.2

The rear wheel suspension system for the Softail frames consists of a pair of shocks mounted horizontally under the transmission, which arenít shown in the drawing.This diagram illustrates the so-called 'classic' rendition of the Softail which is intended to look like a rigid frame but you can also buy or build this frame type using a more conventional looking swing-arm but still utilize the inboard horizontal shock system.

Figure 2.3 depicts a 'conventional' swing-arm frame type very similar to a stock H-D that almost everybody is familiar with. The suspension for the rear tire in this case consists of a pair of coil-over shocks mounted outside of the frame-rails, canted at an angle that runs from approximately the midpoint of the swing-arm to a pivot point just aft of the rear edge of the seat. This type of frame makes up about 85% of all cycle frames, both modified and stock that are on the roads today.

 

Figure 2.3

At one time, from the standpoint of building real 'hardcore' choppers, this last frame configuration was at the absolute bottom of the barrel from a desirability standpoint but today there are many builders going back to this style but chopping it down significantly from the stock factory dimensional configuration.

The outboard coil-over shocks provide a far superior ride to the inboard softail shock configuration. This type of frame is significantly lighter than a softail design and actually much easier to build so it's coming back into favor by knowledgeable riders and builders everywhere.

A list of the various component parts of the typical frame is shown in the following table

Number

Item Description

1

Steering Neck

2

Backbone or Top-tube

3

Seat Post

4

Backbone Brace Tube

5

Wishbones (left and Right)

6

Fender Mount (Wishbone cross member)

7

Lower rails (Left and Right)

8

Seat Post Cross Member

9

Rear Transmission Mount Cross Member

10

Axle Plates and/or Axle Adjusters

11

Down Tubes (Left and Right)

12

Front Transmission Mount

13

Rear Motor Mount

14

Front Motor Mount

15

Top Motor Mount

16

Forward Control Mounts (left and Right)

17

Steering Neck Gussets

18

Rear Stanchions or Boomerangs

19

Swing Arm Assembly

20

Swingarm Pivot Shaft

21

Coil-Over Shock Absorbers

 

 

Rake and Trail 101

Rake is a term used to describe the angular relationship between the bikes steering stem and an imaginary vertical line dropped down from the centerline of the frame neck to the ground. A cycle with zero degrees of rake has a stem that is perpendicular to the ground or in other words straight up and down.

Such an arrangement doesn't work to well because as a pushing force is applied to the wheels axle the wheel has a tendency to track along a course that is exactly opposite the direction of the applied force. If the force is coming from directly overhead the wheel simply wants to revolve around a pinpoint spot on the ground directly below the force being applied and it will just spin around about its vertical axis getting nowhere as seen in Figure 2.10.

Figure 2.10

However if we change the angle of force from directly overhead and incline it slightly the wheel will want to follow along an imaginary line opposite the force being applied and it will roll along in a straight line tracking from a point that lies directly beneath is vertical axis to an imaginary point opposite the line of force as depicted in Figure2.11.

Figure 2.11

The more force we apply and/or the shallower the angle of that force relative to the ground line becomes the better the wheel tracks ahead in a straight line. If the force is lessened however or the angle of attack becomes steeper the wheel will have a tendency to start revolving about its true vertical axis since the force of gravity acting downward will eventually overcome the angular forces as illustrated in Figure 2.12 below.

Figure 2.12

This is the reason that your bike becomes unstable at very low speeds. As the motive force is reduced gravity will eventually take over and you end up trying to balance the wheel assembly on a pinpoint spot directly below the tire contact patch. The opposite also holds true and as the speed increases the angle of force also increases as it overcomes gravity regardless of the steering stem angle. At extremely high speeds, like you might see at the Salt Flats for instance, this angle can become almost parallel to the road surface and the wheel becomes very unstable.

To overcome this phenomena all motorcycles, and bicycles for that matter, have built in a mechanism to keep the forces applied to the front wheel at an angle of attack that provides relatively good low speed maneuverability while providing high-speed stability.

This mechanism is called the steering stem rake angle.

Figure 2.13

Two examples of this rake angle are shown in Figure 2.13 above. The illustration on the left represents a fairly typical stock steering head situation while the one on the right represents a more radical design usually found on the more extreme chopper frames.

Offsetting and inclining the steering head behind the front wheel forces the bike to track along in a relatively straight line even if you're just pushing it along manually. Without this rake the front wheel would just want to spin in circles when you werenít under power.

Almost all stock motor driven cycles have a steering stem rake angle of somewhere between twenty-four and thirty-five degrees measured relative to an imaginary line perfectly perpendicular to the ground. Modified bikes, bobbers and choppers on the other hand can push this angle another five to ten degrees and some extreme choppers have fifty degree rake angles.

As a general rule of thumb the less rake angle favors low speed maneuvering and the greater angles favor high speed cruising at the expenses of loosing low speed handling agility but be warned that this statement is very general in nature since there are other factors that effect handling such as the location of the bikes center of gravity, the bikes weight, travel speed, tire size, pavement composition, spring rates, rigid or softail, fork length and even the type of forks you're planning to run.

Since this guidebook is about chopped bikes however we need to say right off the bat that raking the steering head beyond stock angles is done purely for the sake of appearances except for drag bikes which are intended to go in only one direction to begin with. To look cool you're going to have to sacrifice some handling agility whether you want to or not. How much you give up depends on how cool you want to look.

The following illustrations depict a single frame with four different degrees of rake angle to visually show how much impact neck rake has on the overall profile of any given bike. In descending order the bitmaps show 30, 35, 40 and 45-degree rake angles applied to the very same frame.

The more or less stock bike with a rake angle of 30 degrees can turn around at low speed within a circle having a five foot radius but on the extreme opposite end of the spectrum the bike having the 45 degree rake angle needs another six feet of room to make a 180 degree turn. While this doesn't sound like much of a difference in tight traffic or parking lot situations it can mean the difference between making a simple turn or having to jockey the bike around in a hammerhead maneuver.

The term 'trail' is used to describe another variable that greatly affects the handling characteristics of our motorcycles and in fact proper trail is far more important than rake in determining how well any given frame and fork geometry combination handles on the road.

Trail is expressed as the distance measured horizontally along the ground level between a point that lies directly beneath the wheels axle and an imaginary line extended through, and at the same angle as the steering stem as shown in the hypothetical geometry of Figure 2.14 below.

Figure 2.14

In this particular example which represents a fairly conventional stock situation the trail is 3-inches and the neck rake angle is thirty degrees and the bike is designed to handle reasonably well at both low and high speeds. If we leave everything else stock and simply rake the neck out to about forty degrees and add some extended forks to keep the frame level the trail distance increases to 9-inches as seen in Figure 2.15.

Figure 2.15

Now we have a bike that is extremely stable at very high speeds, perhaps even too stable, with sluggish handling characteristics, while at low speeds it requires constant attention to the handlebars to keep it going straight and the turning radius has increased significantly.

Most authorities agree that the ideal situation is to keep trail somewhere between 2.0 to 4.5-inches regardless of the rake angle but in my opinion this generalization is far to broad and this dimensional range should be treated only as a starting point to be used in the development of your front end geometry. There are many bikes out there with trail measurements over five inches that still handle reasonably well at all speeds and conversely there are bikes out there with little or no trail that also handle well. One noted Springer designer sets his front-ends up for nearly zero trail and they handle superbly.

When a bike is running fairly large neck rakes and mounting telescopic forks it is extremely difficult to keep trail measurements within reason without resorting to what are called 'raked triple trees' that in effect move the axle of the front wheel forward thereby reducing trail while leaving the neck angle untouched. These trees are typically available with 3, 5 and 7 degrees of rake and are intended to be used exclusively on modified frames that have neck rake angles in the range of 37 degrees and greater.

Unfortunately raked trees are relative cheap and some people have used them in a cost-cutting attempt to get the raked chopper look on an otherwise stock frame which can lead to disastrous consequences and the rider can end up with what is called 'negative trail' where the extended stem angle line is actually behind the extended vertical wheel axle point as shown in Figure 2.16.

Figure 2.16

This is an extremely dangerous situation as the bike appears to be handling perfectly at low speeds typically encountered in city traffic but as the speed increases the cycle becomes more and more unstable but appears normal to the rider until some roadway irregularity sends the whole package into the ditch.

Unless the builder deliberately creates a negative trail situation by using raked triple trees it is seldom seen in Telescopic or Springer fork front end setups but it is very often seen on frames with improperly installed girder forks which we'll discuss in another section.

While we're on the subject of rake and trail it's probably a good idea to talk about 'Flop'.

What the hell is 'Flop'? Is it contagious?

The term 'Flop' is a very descriptive and accurate word to describe what happens on motorcycle and bicycle forks when gravity overcomes the effects of trail. Remember we described earlier that trail was a dynamic attribute and as speed increased trail became more visibly effective and as speed was reduce trail had less effect on the behavior of the wheel and forks. All fork/wheel combinations will experience Flop at some point and that point varies from bike to bike so it's not a set value and that's why older bikes had adjustable forks stops. Fork stops are used to prevent the front end from swiveling around the steering stem and crashing into the frame or fuel tank if the forks reach the 'Flop Point'.

As you turn the front end of a two-wheeled vehicle more and more in one direction or another the wheel and fork combination will try to reach a point where their center of gravity seeks equilibrium. At the 'Flop Point' the front end wants to swing all the way over towards one side to a position where the front wheel is at a 90-degree angle to the frame. This is simplistic explanation but if you have a bicycle or a motorcycle you know what I'm trying to describe. The point being that you should set your fork stops at some arbitrary position that is well ahead of the bikes Flop Point. Generally bikes with hydraulic forks have the stops set at 45-degrees on either side and bikes with Springers or Girders have the stops set at 35-degrees.

The entire theory of rake and trail geometry when its applied to real bikes and not just mathematical calculation is complicated to say the least and the vast majority of data available to the designer and builder is largely empirical but that data does suggest that one can alter trail fairly significantly before the effects of a change are noticeable to the rider. For example an increase or decrease of up to 1.5-inches in trail makes very little perceivable change in the handling characteristics of any given frame and in fact even changes of rake angle from stock to as much as five degrees in either direction have little impact on overall handling within the speed ranges most bikes are operated.

For example changing the neck rake from 30 to 35 degrees only changes trail by slightly over 1-inch on the average Big Twin. Going from 30 to 40 degrees changes trail by 3.3-inches but if the fork crown offset is increased by an inch the effective change in trail is only 2-inches which is within the realm of manageability and the bikes overall handling won't suffer nearly as much as anti-chopper proponents would have you believe.

In summation then its my opinion that if you really want to rake your ride then do it big-time and don't mess around with little changes that have very little visual impact but having said that I also don't believe that there is much to gain from an appearance standpoint by using rake angles over forty degrees, forty-two maximum. Between forty and forty-five degrees or over, the engineering becomes far more complicated than it's probably worth unless you're building show bikes.

Offset

On the cheapest and simplest ways to alter trail on motorcycles is to use triple trees with different offset dimensions.

Fork offset is the distance measured between the steering neck centerline axis and the axis of the fork tubes as shown in Figure 2.17 below which represents a hypothetical top clamp viewed from above.

 

Figure 2.17

Unless the bike has a negative trail condition increasing the fork offset will decrease trail while decreasing offset will increase trail. The reverse holds true if you're trying to correct a negative trail situation. Figure 2.18 illustrates the effect offset has on a typical cycle when seen in profile where increasing the offset moves the axle of the front wheel forward relative to the extended axis line of the steering neck.

 

Figure 2.18

Another method commonly used to decrease trail is to use forks that have the wheel axle offset forward of the fork tube axis as seen on many racing bikes using hydraulic forks.

Springer fork systems are another good example of offsetting the wheel axle and in the late sixties and early seventies we'd be running Springers with 12-inch long rockers in an attempt to keep reasonable trail on radically extended and raked bikes.

In the examples above we have used one way to calculate trail but there is another way as well. Both methods provide 'relative' results and are equally valid when used to provide comparisons between different configurations. Youíll sometime hear the expression 'True Trail and False Trail' used to describe the differences but in reality there isn't any difference in the results of the two methods used as both give an accurate indicator of relative trail values. Use whichever method is easiest for your particular application.

Before we leave this section I think that it's important that you understand that no raked Chopper will handle like a Road Bike regardless of the trail figures. Trail has little to do with how 'nimble' a bike is when negotiating curves in the road unless the bike is a road racer. Reducing trail will not reduce the turning radius of a Chopper and conversely increasing trail will not increase the turning radius. The turning radius of any bike is a function of the wheelbase.

If trail was so critically important to the design of 'general-purpose' motorcycles the 'magic' figure would have long ago been chiseled into concrete and it hasn't been in over eighty years of cycle development. In fact there are thousands of bikes on the roads today that have trail values ranging from as little as 'zero' to as much as ten-inches and they all behave well within their normal operating parameters.

Handling characteristics involve hundreds of variables as we mentioned earlier and the trail value is but one of those factors. For more definitive information about trail as it effects road bikes I urge you to read the book entitled 'Motorcycle Handling and Chassis Design' by Tony Foale.

For the average Chopper builder Trail is strictly an indicator of relative 'forward' or 'dynamic' stability at various speed ranges and if your bike is within the 2 to 9-inch range you're in the same group as about 98% of all other Chopper owners.

 

 

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