There are many factors that can affect the safe operation of a motorcycle. Good brakes and tires are very important, but there is something fundamentally more important and it has everything to do with the stability and control of any motorcycle. What we are talking about is having the proper amount of trail for any given amount of front-end rake.
Bikes with a high amount of rake, like say 38 degrees and up, will give even a relatively light machine a heavier feel at the handlebars due to weight of the chassis trying to self-straighten the fork as it hangs in the steering neck. A higher degree of rake, with proper trail will give better straight-line performance, but it will feel heavier and a bit sluggish during tighter turning.
Lower rake numbers, take for instance the 26 degree frames used on Harley's big touring bikes, make the heavier machines feel deceivingly lighter than they really are. The big bikes are much more maneuverable than, say, a raked out softail, but these bikes do not track as well at very high speeds and that's why you'll find maximum recommended speed limits for them actually listed in the Harley owner's manual.
Another thing you may notice about some Harleys upon closer inspection, is that they use raked triple trees right from the factory. In fact, the springer models have a specific rake angle for the 21-inch wheel, and one-degree less rake built into the trees for the Heritage Springers with the larger 16-inch wheel. During their exhaustive testing procedures Harley found the fatter-tired bike needed to be de-raked one degree to stabilize the handling.
Either way you look at it, selecting your desired rake and trail is going to be a compromise in one direction or another. You'll either have quick handling and a little less high-speed stability, or you'll have excellent directional control at salt flat speeds with a machine that feels a little heavier as turning radii decrease.
Knowing how rake and trail affect the way any bike handles will automatically make you ask yourself another question. You'll have to think about the way you enjoy riding most, to know if a short and tight rake suits your need for quick handling, or if you're more of an open highway rider, in which case you'll want to opt for a longer, stretched out rake for easy cruising.
In cases where a frame might have too much trail (see diagram 1 for explanation of trail) due to excessive rake, the steering can be uncontrollably heavy and needs to be held at all times at low speeds to keep the front end from "flopping" over on it's own. Although your bitchin' new frame might be the coolest thing you've ever seen, unless set up properly, it can be very dangerous to ride. Cases like these can usually be cured very easily with the use of fixed-rake, or adjustable-rake triple trees.
We stopped by to see the long-bike specialists at Custom Cycle Creations, where owner Mark Cohen and his righ-hand man, Anders, showed us how to properly rake one of their limo-length bikes under construction. The frame has 43 degrees of rake built into the neck and we used a set of CCC's 10-degree adjustable trees to arrive at the correct amount of trail. CCC recommends between two and five inches of trail for safe handling. The nearer to two inches, the quicker the handling and the closer to five inches, the better the stability. Cohen wanted his final trail figure for this bike to be four inches. They did this in various stages using a set of their own adjustable triple trees. Markexplained each step as our photographer took notes and picked up a few cool tips.
Mark and Anders start by clamping...
Mark and Anders start by clamping the frame with rear wheel and tire installed, into a jig. The frame is leveled with a set of wooden blocks underneath the down tubes and a CCC-made tool, OK, it's really just a fancy pointy thing, is dropped down through the neck with the tip pointing to a scale drawn on a strip of masking tape on the lift. The line we see the pointer at here will be our zero-inch trail mark.
Now Anders is installing a...
Now Anders is installing a set of their adjustable trees, set at zero-added rake in the neck of the frame.
A 14-inch over fork leg is...
A 14-inch over fork leg is slid into the trees to help illustrate the changes in rake we'll try today shorten the overall trail.
The trail is measured from...
The trail is measured from the centerline of the axle, straight down, forward. Mark knows exactly how much this front end will collapse on the finished bike, so he compensates for that using a tradition square marked with masking tape to represent the centerline of the axle on a 21-inch wheel with tire installed. The distance from the leading of edge of the square, to the line pointed at with that nifty pointy tool in photo #2, will be our trail measurement in inches.
Well, our first measurement...
Well, our first measurement at 43 degrees of neck rake with zero degrees in the trees gives us an unacceptable eight inches of trail. If we kept the bike like this it would be quite a chore to ride at low speeds and handling would be very poor.
Anders knows he needs to add...
Anders knows he needs to add a little rake more rake to the trees. He adds roughly four more degrees by loosening the hardware on the trees and sliding components in the necessary direction.
A check with this really cool...
A check with this really cool digital angle finder reveals Anders added exactly 4 degrees, bringing our total rake to 47 degrees. Yes, Anders was as surprised as we were that he had hit the mark perfectly.
A quick check with the measuring...
A quick check with the measuring tape shows that we're getting closer by hitting the six-inch mark, but we still need to add in a little more rake to the trees to near our desired four inches of trail.
Once again, Anders loosened...
Once again, Anders loosened the hardware, adjusted the trees-this time more than once-and we end up at the 50-degree mark.
Now we've hit the four-inch...
Now we've hit the four-inch mark which is exactly what we were shooting for. With this set up, the bike will handle very well for having such a long front end, and the rider won't be completely worn out at the end of a long ride.
We were curious to find out...
We were curious to find out exactly how much the trail could be shortened with the CCC trees, so Anders opened them up and flipped over this ball-socket on the left to give us the maximum amount of rake built into this set of trees. At 10 degrees of added rake in the trees, our overall figure will be 53.
The stainless steel ball that...
The stainless steel ball that holds the top of the trees is tightened down...
...and we're ready to see...
...and we're ready to see just how little trail the CCC trees will let us adjust for. You can see in the photo that we're just a hair over the two-inch mark on our tape measure. Although at parking lot handling would be very good and the front end would feel very light, anything approaching highway speeds would most likely result in a speed wobble, leading to possible injury-definitely not recommended.
Mark disassembles the lower...
Mark disassembles the lower tree to give us a better view of the interlocking adjustment "teeth."
CCC also offers triple trees...
CCC also offers triple trees in various degrees of fixed rake. We sat trees with three different amounts of rake to better illustrate the difference. On the left, is a CCC tree with seven degrees of rake built-in. In the middle is a set of CCC's four-degree trees and to the far right is a set of trees with zero-degrees built in.