• Frame Geometry 101

    reprinted from BB.com , August 2003
    copyright owned by author

    Frame Geometry 101

    Welcome to this month's installment of Customarily Minded. This month we'll look at a topic many of us slept through in school: geometry. Specifically front end geometry concerning motorcycles. For those that have been here at Beginner Bikes for a while, you may have read some of my mumbo jumbo concerning frame geometry and using wierd terms like "rake" and "trail". For those just joining us, and for those of you that have been mulling over motorcycle specifications in magazines or websites and/or simply just wondering what the heck those two terms mean, this article will deal with the subject of rake and trail, and what it means in the real world for your decision on your next motorcycle purchase. So strap on that DOT approved thinking cap and let's get right to the heart of the matter.

    Illustration 1: Note the three color coded measurements, yellow for rake, red for trail, blue for offset

    First off we'll look at rake. Rake (also known as Castor Angle) is the angle of the steering head (and consequently the forks as well as on most bikes they are parallel) in relation to a perpendicular line going from the ground through the steering head. See the yellow lines in Illustration 1. A steeper angle yields a shorter wheelbase, a larger angle yields the chopper-esque longer wheelbase. Of course the frame/swingarm design also has a lot to do with the wheelbase, but the angle of the front end will ultimately seal the final figure. Wheelbase will ultimately determine how much lean a motorcycle requires to handle a turn at a given speed. Take two different bikes regardless of weight and power into the same curve at the same speed, the one with the longer wheelbase will have to lean at a greater angle than the shorter wheelbased model in the curve. This means a 250 Rebel with its diminuative 57.1" wheelbase will be able to take "Deadman's Curve" at a speed of 45mph with a 25 lean, a Vulcan Mean Streak with it's massive 67.1" wheelbase would take the same curve at the same speed butwith a sharper lean of 30 (these figures are simply for reference concerning this imaginary situation) This means that the faster both bikes go, the Vulcan will be dragging parts sooner than the Rebel as it will be leaning more to maintain the same speed as the Rebel in the same curves (further proof of the falacy concerning "outgrowing a beginner bike"). This is why sport bikes have shorter wheelbases by design than cruisers, it enables them to take curves at higher speeds.

    Next up is trail. Trail is a somewhat confusing concept. The measurement itself is taken by drawing an imaginary line through the steering axis to the ground (angled yellow line in Illustration 1). Next drop a perpendicular line from the front axle to the ground, which in any production motorcycle should fall some distance behind the point where the steering axis line touches down. The distance between these two points is your trail dimension (see the red portion of Illustration 1). Trail ultimately defines the handling characteristics of a motorcycle, how stable it will be at higher speeds, how easy it will flick through S-curves, and how easy it will be to control at sub idle parking lot maneuvers. Bikes with a short trail will be real easy to handle at slow speeds and quite responsive when the road gets occupied with esses, but at higher speeds these bikes will respond more to the road conditions and feel a bit twitchy. On the other side of the coin, bikes with longer trail dimensions may handle like a wheel barrow in the parking lot, require a bit more encouragement to tackle an S curve, but track straight and true like an arrow at freeway speeds, offering little response to the road until the handlebars are activated by the rider. The trick for the motorcycle designer is to find some happy medium for the bike he/she is designing, something that will match the role the bike is intended for.

    Do rake and trail go hand in hand with each other? Typically bikes with greater rakes have longer trail dimensions. However, the design of the triple trees is what eventually defines the trail dimension. Offset is measured by drawing an imaginary line from the center of one fork tube to the other, then dropping a perpendicular line from the center of that line to the center of the steering stem. The distance of this perpendicular line is the triple tree offset. What this does is position the front axle to a point where the trail can be made to a suitable position. This means that the closer the forks are to the steering stem the longer the trail can be modified, as the axle is being positioned further back from the steering axis to ground contact point. The farther apart the forks are from the steering stem the shorter the trail.

    Now then, with all this in mind let's look at two different motorcycles that are compatible in terms of size and weight, Kawasaki's 500cc Vulcan and Honda's 600cc Shadow VLX. The Vulcan tips the scales at 439 pounds (dry weight), has a 62.7" wheelbase, and sports a 33 rake and 5.9" trail dimension. The Shadow weighs in at 439 pounds dry (or 445 with the chrome Deluxe package), stretches out to 63.2" in wheelbase, and has a 35 rake/6.5" trail dimension.

    Without riding either one, we can presume the following handling characteristics based on what we now know about rake and trail: The Vulcan should be able to negotiate slow speed maneuvers with relative ease over the VLX thanks to a shorter trail, and at the same time it should be able to handle curves and corners slightly faster than the VLX as it won't lean over as far at any given speed due to its half inch shorter wheelbase. The Vulcan also shouldn't require as much force to negotiate an S-curve. On the freeway leading out of town, take both bikes up to the posted 75mph speed limit and the VLX will handle better as its longer trail dimension offers better stability at higher speeds than the Vulcan. I've ridden both machines and can attest that these theories are in fact true.

    As you can see, a motorcycle's handling characteristics are yet one more design that is of a give and take nature. Just like a larger more powerful engine consumes more fuel over a less-peppy smaller engine that conserves fuel, so it is with rake and trail. You just can't have it all.

    One trick that is being performed these days is the use of raked triple trees. Some manufacturers are beginning to utilize this design from the early days of choppers to make their bikes easier to handle. Harley Davidson is one example, their new liquid cooled V-Rod sports a most generous 38 rake (the largest OEM rake available currently). Harley was shooting for the dragster look in the V-Rod style wise, and if you've seen pro dragster motorcycles they have some serious rake (and trail) in order to maintain stability at the higher speeds. However, with that greater rake comes a longer wheelbase and a higher trail dimension (depending on the triple tree offset), and Harley wanted this bike to be a decent handler as well as a rocket. The engineers at Harley opted for the raked triple trees. A raked triple tree is designed such that the lower tree sticks out slightly more than the upper tree, thereby creating a greater rake on the forks in relation to the steering head. Harley used a +3 triple tree on the V-Rod, added to their 35 raked frame to get the new 38 degrees of rake. Like a longer offset measurement, the raked triple tree positions the front axle closer to the steering axis to ground contact point, all the while retaining that cool drag/chopper-esque extra rake. In the end, the sub-big bike sized 1130cc V-Rod has a trail dimension that measures a crisp 3.9 inches, very close to most sport bike measurements. Coupled with as much if not more horsepower than the Japanese big bore V2 powered musclebikes that sport 4.5" to 6" trails, and less weight, the V-Rod simply mops the floor with them all around. As Jedi Master Yoda said in the Empire Strikes Back, "Size matters not."

    Illustration 2: Note the shorter trail dimension on this diagram (same scale as Illustration 1) with raked trees

    Okay I know what may be going through your mind right now. I have a cool cruiser and I want to make it into a cool chopper. So I'll just rush right out and grab up a pair of raked triple trees for it, not only will it give me some of that cool looking extra wheelbase it will also help my handling. Wrong. It just isn't that simple. For starters you need to know what your base rake and trail dimensions are before even contemplating a chopper. Just because something looks good in a magazine doesn't automatically mean that it will work on your particular bike. Those killer showbikes seen in Street Chopper and other magazines have been carefully researched long before the first wrench was turned during assembly. Because if you end up with too short of a trail dimension, you get what is known in the chopper world as a "bar slapper." Meaning as you get going faster and faster, the front wheel becomes more unstable until the point where the handlebars start slapping the tank. Chopper builders add raked trees to their raked frames to shorten the trail. Add raked trees to a stock frame and you guessed it, the trail gets shortened. Chopper builders do it and get away with it because a raked frame already yields a longer trail, making it shorter benefits them. Make a shorter trail even more short and it can cause more harm than good. There is a very fine line amongst frame position, frame rake, triple trees, and fork length that defines how a chopper will handle. If you don't do the research and just start throwing things together from aftermarket catalogs, you could end up with a Franken-bike sporting an unholy handling pact that can bite back.

    So with all this in mind, next time you're browsing the spec pages of a given motorcycle and you come across the rake and trail info, you'll begin to get a better idea how said bike will handle without even riding it. And when you do ride it you'll have a better idea of what to expect. Once again to summarize it all: bikes with longer rake and trail dimensions can be expected to be quite stable on the freeway but will feel more sluggish in curves and slow speeds, those with shorter rake and trail will be easier to control at slow speeds and in the curves but will feel more twitchy on the freeway.

    That wraps up this month's edition, and unlike other geometry classes there will be no quiz on this material aside from your experiences and discoveries during the upcoming ride. Keep your knees in the breeze.

    When Shadow Shack isn't drawing imaginary lines through his motorcycles, he answers email at an addy that can be found in his profile

    "Customarily Minded Machine of the Month"

    This month's featured bike is an over exaggerated depiction of rake and trail. This machine was designed and built at Denver's Choppers in Las Vegas (yes, the same chopper company that was started by the late Denver Mullins in California). It features an EVO powered rigid frame built by Denvers, upswept fishtail exhaust, and DC's second longest front end ever fabricated, a whopping 30" over springer. That's a lot of flex in that front end too, you should see it when the owner grabs a handful of throttle, it'll leave you wondering if the springer mechanisms are actually ever in use. I was offered a ride on it a couple years ago at a rally by the owner (who worked at Denvers at the time) but I make it a personal policy to turn down offers for rides on $45,000 motorcycles...

    This article was originally published in forum thread: Frame Geometry 101 started by Shadow Shack View original post
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