Rake and Trail
-
Upload
riki-mandol -
Category
Documents
-
view
221 -
download
0
Transcript of Rake and Trail
7/27/2019 Rake and Trail
http://slidepdf.com/reader/full/rake-and-trail 1/8
10/16/13 rake and trail
www.chopperhandbook.com/rake.htm 1/8
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 I’ve 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 componentparts 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.
7/27/2019 Rake and Trail
http://slidepdf.com/reader/full/rake-and-trail 2/8
10/16/13 rake and trail
www.chopperhandbook.com/rake.htm 2/8
Figure 2.2
The rear wheel suspension system for the Softail frames consists of a pair of shocks mounted horizontally under the transmission, which
aren’t 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 Post4 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
7/27/2019 Rake and Trail
http://slidepdf.com/reader/full/rake-and-trail 3/8
10/16/13 rake and trail
www.chopperhandbook.com/rake.htm 3/8
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 a l and/or the shallower the an le of that force relative to the round line becomes the better the wheel tracks
7/27/2019 Rake and Trail
http://slidepdf.com/reader/full/rake-and-trail 4/8
10/16/13 rake and trail
www.chopperhandbook.com/rake.htm 4/8
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 i ts 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 weren’t 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 lookcool 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.
7/27/2019 Rake and Trail
http://slidepdf.com/reader/full/rake-and-trail 5/8
10/16/13 rake and trail
www.chopperhandbook.com/rake.htm 5/8
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.
7/27/2019 Rake and Trail
http://slidepdf.com/reader/full/rake-and-trail 6/8
10/16/13 rake and trail
www.chopperhandbook.com/rake.htm 6/8
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.
7/27/2019 Rake and Trail
http://slidepdf.com/reader/full/rake-and-trail 7/8
10/16/13 rake and trail
www.chopperhandbook.com/rake.htm 7/8
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 wellahead 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 ne ative trail condition increasin the fork offset will decrease trail while decreasin offset will increase trail. The
7/27/2019 Rake and Trail
http://slidepdf.com/reader/full/rake-and-trail 8/8
10/16/13 rake and trail
www.chopperhandbook.com/rake.htm 8/8
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 profi le 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. You’ll 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.
| Main Page | Copyrights | Terms of Use | Warranty Disclaimer | Security and Privacy | Contact |
Copyright © 2003-11, All Rights Reserved