FLHT SPEED WOBBLE, another "they all do it" topic.....

The dreaded speed wobble is a panic-inducing temporary loss of control of the front forks. You know you are in one when you are trying to remember the Lord’s Prayer. A speed wobble may last only a few seconds, but it strikes without warning……or does it? I’ve heard from old codgers that they can get so bad that the handlebars slap the gas tanks but a modern motorcycle in good repair would never get to that frightening stage. Accelerating will get rid of the dreaded wobble although the majority of riders would slow down which in my experience also cures the problem allowing the bike to stabilize. I have owned Harleys of every shape and form, but I had never experienced a speed wobble until I was riding a few years ago along a deserted highway at maximum speed. The bike, a borrowed FLHT, began to do the shimmy while I was passing a tractor-trailer. There’s no better way to convince a rider to slow down.

Long rides encourage lots of thinking so I immediately began to dwell about design differences between this touring model and other Harleys. Frame neck angle has a big influence on handling characteristics at different speeds and riding conditions. Increasing the neck angle on a frame will cause a bike riding in a straight line on good pavement to be more stable. That is why street bikes have more rake than dirt bikes, which have to turn on the proverbial dime. Rake is a measurement of the steering axis from vertical, given in degrees or in plain language, the fork angle measured from an imaginary 90-degree line. Less rake, as on the dirt bike, will make the front forks light, which is desirable for that type of riding but not for high-speed highway trips. Imagine riding a Softail on motocross trails where extreme turns had to be made immediately. Well, we both know that the rider would wind up on his rear end. Same thing with a motocrosser riding at 80mph! The ensuing speed wobble caused by too little rake would deposit the rider on the ground. Harley got fancy with the FLHT model to make the big touring bike handle well in the city, which it does. The factory tried to achieve the best of all worlds. Notice how the front of the frame is stretched out, much like a gooseneck, compared to a more conventional Harley design like the Softail. This is done to accentuate good highway touring handling characteristics, as is the rake of the frame that has been increased. So what’s the problem?

Well, the crowns have been reversed and de-raked relative to traditional Harley designs. That, in turn, makes the front end lighter, which makes it handle well on the city roads. That by itself, however, does not necessarily cause the speed wobble you described. You said that you ride the bike at high speeds, which is part of the combination needed to de-stabilize this particular model into a speed wobble condition. Let’s compare some stock Harley rakes to accentuate this point. The net FLHT rake after goosenecking and reversing the lower triple tree, (crown), is 26 degrees which is substantially less than the traditional figure of around 31 degrees. Next comes the Dyna narrowglide at 280 while the Sportsters vary between 29.6 and 30.1 which is consistent with being sportbikes. The Dyna Wideglide is 320 while the Softail is 340 which explains their highway prowess. The FLHT forks will have a tendency to fibrillate (shake) alter hitting a bump or crossing a truck rut because of their light handling characteristics required for such a heavy touring bike in city traffic conditions. Why does this get exacerbated with higher speeds? More speed equals more wind pressure, creating an updraft underneath the faring that makes the front end even lighter. Now, add a crosswind against the tour box, saddlebags and inside windshield and we’re getting close to a wobble. Combine these factors at 90 plus MPH, and the formula is there for a dandy speed wobble. If that isn’t exc