By Mark Zimmerman
Bogie and Bacall, Lennon and McCartney, bacon and eggs, appealing on their own, as combinations they’re just about unbeatable. And so it is with Rake and Trail. While most of us are conversant with the terms, obtaining a complete and coherent explanation is sometimes difficult. What follows should help you understand what they mean, how they work and why you should care.
Defining Rake & Trail - What is Rake?
Rake, which is sometimes referred to as steering head inclination, steering head angle or steering axis inclination, is defined as the rearward inclination of the steering head as measured from the vertical. Rake is always described in degrees. Although there are exceptions, the accepted practice is to consider a vertical steering head; one placed at 90-degrees to the pavement, as having a zero-degree angle. Consequently, the rearward inclination, or rake is normally in the 22 to 36 degrees range.
Although the fork angle is normally identical to the steering head angle, it doesn’t have to be, and because of that when we discuss rake in the normal context what we’re actually talking about is the steering head inclination. Since the major factor in the calculation of trail is the steering head angle it’s important that we’re all on the same page.
Defining Rake & Trail - What is Trail?
If we draw two lines, one through the center of the steering head, (following the same angle,) and the other vertically through the center of the front wheel, we’ll see that they intersect the ground some distance apart. The measurement between those two points is called trail, for the simple reason that it’s the distance that the center of the front tire’s contact patch trails behind the steering axis. The trail dimension is given in either inches or millimeters dependent on the manufacturer’s whim, and varies from about 4- to perhaps 7-inches. Trail also affects the rear wheel, but since its affect is less pronounced, I mention it here only in passing.
How Does Rake & Trail Effect Handling?
Coming up with a formal definition of handling is tough, in part because as opposed to the race track where such things can be quantified by telemetry or a stop watch, on the street handling is largely subjective. So rather than a formal definition let’s just say that handling might be described as how much effort it takes to get the bike to do what we want it to, when we want to do it, and how stable the bike is while it’s doing it.
Playing The Angles: Why We Need Rake
In the nascent days of two-wheeled, self-propelled locomotion, early velocipedes like the boneshakers had their forks positioned vertically. At a walking pace, or maybe slightly faster which was about as fast as a boneshaker could travel, the upright fork worked passably well. Unfortunately, anytime the pace picked up or the front wheel was deflected by an obstacle the fork exhibited what we now call caster flutter, shimmy, or high-speed wobble; which is an uncontrolled oscillation around the wheel’s vertical axis.
Many of us have experienced a mild form of this and it’s unnerving. Imagine how much fun it was when the fork might rotate 180-degrees, with the predicable unpleasant outcome of being pitched to the high-side. This nasty turn of events could even be initiated if the rider’s attention lapsed and he relaxed his grip on the bars at the wrong time, which as you might imagine often made riding an early two-wheeler, way more exciting than it had to be.
In any event, some bright engineer discovered that inclining the fork rearward a few degrees induced stability and voila, we have Rake. Once Rake was introduced, the wheel no longer tried to change directions at the slightest provocation. The extreme riders of the day soon realized you could even ride the newfangled designs with your hands off the bars, a stunt that no doubt greatly impressed the young lovelies of that bygone era.
So why did inclining the forks rearward stop the shimmy? The simple explanation is that when round objects, like wheels, have a force applied to their axis, they prefer to follow a course that’s in opposition to that force. In this case, since the force applied by the rider was coming from directly overhead, the wheel had tendency to rotate around its axis, rather than move forward.
However once that force changed direction and was applied in a forward direction the wheel preferred to move in the opposite direction. Furthermore, because the steering axis now contacted the ground at some point forward of the wheel’s vertical axis, it also created a levering affect on the wheel that tended to stabilize it when it was in motion. That lever is what we call Trail.
Trail: Why We Need It - What it Does
Although the words “Rake and Trail” tend to roll off your tongue, as compared to trail and rake, which sounds like a gardening term, of the two, Trail is arguably the more important factor in how our bikes steer and handle. Indeed experiments by chassis designer Tony Foale, have shown that a bike with zero Rake handles and feels just fine, as long the proper amount of Trail is present.
Essentially, Trail is what gives our bikes directional stability at anything above a walking pace. Without it, the front wheel would try to revolve around itself like a broken shopping cart caster at the slightest provocation making the bike unrideable.
Because the center of the tires contact patch is behind the steering axis a self-centering force, an imaginary lever so to speak is created. (Figure 2) It’s this lever that enhances a motorcycle's stability, and helps restore it when it’s disturbed. In basic terms, it works like this: When you’re riding in a straight-line, the wheel is held in the straight-ahead position by a combination of Rake and the Trail “lever.”
If something turns the front wheel, be it an obstacle or the riders input, the lever instantly tries to turn the wheel back to the straight ahead position, but that’s not all it does. The length of the Trail lever determines how much effort it takes the rider to make the bike turn, and how much effort it takes to hold in the bike in a turn and it’s also largely responsible for the way our bike “feels” as we’re steering it, this is because the Trail acts directly against the front wheel, and that input is transmitted via the fork and handlebars directly to the rider.
Reducing the Trail dimension provides more input; and everything the front wheel does is telegraphed directly to the rider. Increasing the Trail dimension tends to dampen-out the feedback, and may make the front end feel a little vague. As you can see, while the basic concept of Trail is easily understood, the nuances can get quite involved.
The Practical Application of Rake & Trail
Rake and Trail enhance stability, so bikes with lots of both, like modern cruisers and heavy touring bikes, which typically have rake angles close to 30-degrees or more and a trail measurement of maybe 5- to 7-inches, tend to be extremely stable. They generally have impressive stability in a straight line and are able to maintain their composure through long sweeping corners.
On the downside they are often less maneuverable than we might prefer, especially at low speeds. These type of bikes usually require a stronger hand on the bars to initiate and maintain a tight turn. As an aside, this is one reason why cruisers have such wide bars, because the leverage reduces the steering effort. Obviously, other factors including the bike’s weight and of course, wheelbase also plays a role here but all things being equal, the more Rake and Trail we build into a motorcycle, the slower it will respond to rider inputs and the more stable it’ll be in a straight line.
As Rake and Trail are reduced, the bike will become more maneuverable and quicker to respond to steering inputs. It’ll also develop a lighter feel at the handlebars.
Unfortunately, it will also lose some of its stability. Pure sport bikes with their steep rake angles and short trail measurements, Rake measurements of 24-degrees (or less) and Trail measurements down around 4-inches aren’t uncommon, tend to turn and handle extraordinarily well. But their overall steering demeanor might best be described as nervous, twitchy, light or darty.
Though highly maneuverable, they also have a tendency to feel a little twitchy or unstable at high speed, so a steering damper is a good way to prevent things from getting out of control. But, let me digress here and point out that the last sentence shouldn’t be construed to mean that sport bikes aren’t stable at high speed they are in large part because while Rake and Trail play a critical role in the way a bike handles, they aren’t the only things that affect it. A well-developed chassis and properly designed suspension are also crucial. As are things like centralizing the bikes mass, engineering the right wheel base and so on, so in and of itself, the steering head numbers - while highly important -obviously aren’t the only things that impact a bike’s handling characteristics.
The bottom line here is that the more Rake and Trail we have the more stable the bike will become, although both steering and maneuverability may suffer for it. Conversely, when Rake and Trail are reduced, the bike will steer quicker and become more maneuverable, though it’s usually at the expense of some stability.
Manipulating Rake & Trail
As a general rule as Rake increases so does Trail, but there are times when more or less of either is desirable for a given application, and though we’ll come back to this, I’ll point out now that as far as Rake and Trail goes, there are no hard and fast rules as to what dimensions work best for any given situation.
Fortunately, you can adjust either dimension separately. Most motorcycles have their fork tubes placed at some distance from the steering stem, this is known as Offset and it allows the frame designer to move the wheel forward or backward in relation to the steering stem, thereby increasing or decreasing the amount of Trail.
As the fork tubes move forward in relation to the steering stem, the angle between the front wheel axis and the point where Rake and the pavement intersect narrows, so Trail decreases. As you’d expect decreasing the offset, which moves the wheel closer to the steering stem increases Trail. At the risk of belaboring the obvious, when the steering stem and the fork lays in the same plane the fork has zero Offset. When that’s the case, Trail becomes a function of Rake and tire diameter.
Offset can also be incorporated into the front axle mount, placing the axle carrier in front of the fork, as is done on many off-road bikes (Refer to the three images above) has the same effect as increasing Offset.
What else effects Rake and Trail? In short anything that affects the way the bike sits in relation to the road. For example: Installing larger diameter motorcycle tires at both ends of the bike adds Trail because raising the bike, even slightly, increases the angle between the steering head and the road. This adds Rake, which increases Trail. Installing a larger front tire or wheel will cause an even larger change, because as the front of the bike raises in relation to the rear a proportionally larger increase in the Rake angle occurs. This is a particularly important factor to be aware of when you are adding aftermarket wheels to your DIY custom motorcycle project.To decrease Rake, which also decreases Trail, you can install a smaller diameter tire, or slide the fork tubes up in their clamps giving them a steeper angle. Back in the good old days, this was a trick many racers used to make their bikes turn quicker. This effect could also be attained by installing longer rear shocks. If you’ve ever looked at a World Superbike or MotoGP prototype and wondered why they had a nose down “stinkbug” look to them it’s because the forks are shorter than you might find on an OEM sportbike or the rear ride height has been increased in an effort to enhance their cornering ability. Sliding the forks down in their clamps, which gives them a shallower angle relative to the pavement increases Rake and Trail, as does installing a shorter rear shock to lower the rear of the bike.
Changes to Rake and Trail can also be made inadvertently. Something as simple as mounting a tire with a stiffer sidewall or different profile will have some very slight affect. And carrying a passenger and luggage always produces a change unless the suspension is adjusted to compensate for added weight.
The lesson here is that Rake and Trail can be adjusted to suit an individual rider or situation, or even unintentionally if you’re not careful. It’s important to realize that any changes you make to, or that affect your bikes suspension, and that includes fitting non-approved accessories or replacement parts, (including tires) to your motorcycle can have unintended consequences.
Harley Put The Fork Tubes Where?
At low speeds, the effect of Rake is more pronounced than Trail. This is one reason why cruisers are sometimes such a handful when riding in town and during parking maneuvers. In 1980, Harley-Davidson wanted to reduce the steering effort of the then new Tour Glide so they reduced the Rake. That had the desired effect, but it reduced the Trail to the point where the bike became twitchy at high speed. Even with zero offset there wasn’t enough Trail, so in a bit of inspired engineering they placed the fork tubes behind the steering stem, which gave the front-end an unusual negative offset, but it pulled the wheel back, increasing Trail. Viola, they now had a bike that was maneuverable at low speeds, yet very stable at high speed. It looked odd but the fairing hid the unusual fork and the design eventually migrated to the rest of the HD touring bikes.
Summing Up The Angles?
In the simplest terms, Rake and Trail are the major force in determining the steering and handling characteristics of our motorcycles. Rake might be best be described as the steering component that makes a motorcycle directionally stable, while Trail can be thought of as the component that restores stability when it’s disturbed.
Although the principles behind Rake and Trail are well understood, and it’s easy to generalize, there are no hard, fast rule’s concerning what the right Rake and Trail dimensions are for a particular bike. Engineers spend countless hours experimenting with steering geometry, and even then don’t always get it right, in which case any fine-tuning is left to the rider.
By the same token, if you understand how Rake and Trail interact and how each affects the motorcycles handling even on a relatively superficial level, then picking the right bike or performance modifications for your type of riding becomes that much easier as does correcting any deficiencies in its behavior. In this instance, it all boils down to playing the angles.
A special thanks to my good friend Dr. John Wittner for clearing up some of my long held misconceptions.
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