Jeff in Kentuckyoriginally printed @ beginnerbikes.com, August 2003
from a longer article by Richard Rose, with 7 Honda Shadow VLX bikes, some stock and some customized.
Here we shall look at a topic many of us slept through in school: geometry. Specifically front end geometry concerning motorcycles.
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. 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 25º lean, a Vulcan Mean Streak with it’s massive 67.1″ wheelbase would take the same curve at the same speed but with a sharper lean of 30º (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. 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. 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 wheelbarrow in a 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.
Let’s look at two different motorcycles that are compatible in terms of size and weight, Kawasaki’s EN500 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 33º 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″ for wheelbase, and has a 35º 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 a bit more ease compared to 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, because its longer trail dimension offers better stability at higher speeds than the Vulcan. I’ve ridden both machines and can attest that these are in fact true.
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.
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.
(Shadow Shack answers email at an addy that can be found in his forum profile
Chop 2 It!)
One word of caution on rigid/hardtail rides like stock Harleys built before the 1960s (Indians had back shocks much earlier); forget what you know about proficient cornering. Curves and corners will need to be taken slowly, as any bumps in the road can send your rear skipping and skating along causing a loss of traction. If you habitually try to beat every yellow light in right turns, this isn’t the mod for you. Bumps and potholes can send your tail skywards too, and rough roads can rattle your fillings loose. All in all this is a barhopper’s mod, but if you’re willing to sacrifice good handling in the name of style, nothing looks cooler than a slammed rear end and a fat tire stuffed up into the fender. Rear lowering kits can be had from numerous manufacturers including Highway Hawk, Scootworks, and Cobra. One good source for modifying OEM shocks is Sons of Thunder Metric Cycles, and SOTMC along with Scootworks carry hardtail struts for some models.
The most extreme choppers out there have frames that are modified for more rake, raked trees are added for looks and to make the bike more manageable, and the rear is often lowered.
Check out Seeger Cycle Accessories for info on chopper kits, they have them available for both Harleys and Metrics.
Check out the various HR3 kit bikes in the Custom Chrome catalog, you can get a complete kit that works right and looks good starting at $13,000 and up, and oftentimes that’s less than the asking price of a used stock Hog. Getting a title on a kit bike is easier than a custom build, but not as quick and painless as it would be with a factory production model.
In closing there are a few things I’d like to point out about the chopper mod. Do the research before buying your first part(s). Lots of it. Especially concerning rake and trail. Make sure you have everything you’ll need for the transformation before turning that first wrench, after all you don’t want to have the major stuff done and then find out you need a longer brake and speedo line. This means you’ll be forced to drool all over an unrideable bike while those parts are awaiting delivery somewhere in Podunkville. Having a second bike to ride is always beneficial, I have no qualms about the “one to ride and one for show” principle. Also of noteworthy mention, aftermarket parts designed to spec doesn’t neccessarily translate into quick and easy bolt-on simplicity. Expect to run into snags along the way, one company I went with (Rizoma) was so far off in every respect with my Shadow I think I would have had an easier time adapting Harley parts. Aftermarket parts compatibility isn’t the only issue to contend with, in some cases your kickstand ends up being too long or too short and the bike will be parked in a near upright or waaaaay leaned over stance, so be prepared to modifiy or exchange that too. If you can, purchase the most expensive parts first as they won’t get any cheaper in the next year or two thanks to inflation.
Finally, while this kind of modification can be easily performed on a beginner bike, it is not a beginner mod. A chopper is to the new cruiser rider as a 600+cc supersport is to the new sport rider. They handle much differently. They brake much differently. They corner much differently. In other words, get some good saddle time in on your first bike before considering a chopper. Besides, with a long wheelbase you’ll never pass the DMV riding portion of the license test when it comes time for the slalom through the cones. And leave early for work, because while the front wheel may be on time you’ll be five minutes late.
Keep those raked knees in the breeze.
