Overcoming Dyna Instability

Ever since Harley-Davidson introduced the original Dyna in 1991 it has been plagued with stability problems.

The common assumption both within and outside the factory was that the frame lacked sufficient rigidity.  That in fact is not the case.  The frame itself is more than stiff enough.  The redesign of the frame in 2006 increased its weight considerably but produced very little improvement in stability.

Many owners have attempted to improve stability by upgrading the tires, fork and rear shocks, installing fork braces and steering dampeners.  The improvement is noticeable but the problem is definitely not solved.

In 2005 Alan Sputhe devised a set of brackets that allowed stabilizer links to be installed at the front of the engine and the rear of the transmission utilizing the existing isolator mounting points.  The Dyna already has a stabilizer link at the top of the engine.  Sputhe is the first to admit that the concept didn’t originate with him.  Eric Buell invented and patented the idea more than twenty years ago. It was first employed on the ground braking Buell RR-1000 Battle twin.

Sputhe considers Buell a genius and one of the great motorcycle engineers of all time.  According to Sputhe “Buells concept is elegant in its simplicity.  Three points define a plane.  The engine is free to pitch in the frame so the isolators can absorb the engine vibration but it cannot roll or yaw, which would allow the contact patch of the rear tire to oscillate laterally.”

Before installing the PosiTrac on our test bike: a 95 inch 2004 Dyna Low Rider with less than 4000 miles on the clock, new tires were  installed and dynamically balanced.  The front and rear isolators were also replaced.  Wheel bearings, fork bushings, steering head and swing arm bearings were all checked for tightness. Careful attention was paid to wheel alignment.  The Dyna was given a test ride of about 250 miles to get a solid impression of the stock factory handling characteristics.

Installing the PosiTrac is quit straightforward. Sputhe supplies every part needed. It is not necessary to put the bike on a lift but it does make it easier since one is basically working on the bottom part of the bike.  The front assembly is the easiest to install since everything is exposed.  The rear unit is buried behind the primary case, frame and mufflers so the bolts are harder to get to.  The banjo bolt on the rear master cylinder must be loosened and the brake line swung to the side.  On our bike a wiring bundle also needed to be moved aside.  While the front unit can be installed in less than ten minutes the rear unit will take three or four times as long even with the bike on a lift.  Add ten more minutes to check alignment.  The whole job can be done in less than an hour.

A test ride after installing the Sputhe PosiTrac stabilizer immediately and convincingly demonstrates its effectiveness.  Without PosiTrac our test bike was noticeably twitchy at about 90 mph.  Above 120 or so the instability was positively nerve racking.  After installing the PostTrac the bike tracked like it was on rails up to an indicated top speed of 133 mph.  The sensitivity to rain grooves and cross wind was greatly reduced.  High-speed sweepers no longer required constant correcting.  The PosiTrac equipped Dyna is much more relaxing and enjoyable to ride fast because you are not constantly being nagged by feeling that the second you stop paying attention 110% the bike will go into a tank slapper.

There was no discernible increase in engine vibration with the PosiTrac. With the PosiTrac installed a small amount of chassis oscillation remains.  We asked Alan Sputhe why.  “The PosiTrac is very effective at eliminating lateral flexion in the isolators so the rear wheel is always inline with the frame.  As Sputhe explains “The mass of the power-train is not centered in the frame.  The engine itself is offset to the left and the heavy alternator, primary drive and clutch are all mounted far outboard on the left side. The heavy mufflers are mounted far back and outboard on the right. Think of it as a large dumbbell resting diagonally in the motorcycle. When the power train moves forward it twists the chassis to the right and when it moves back it twists the chassis to the left. Without the stabilizer the mass of the entire motorcycle is utilized to resist power-train yaw. Eric Buell understands this so he centers the engine in the frame and places the muffler directly under it on all of his motorcycles. The only way to eliminate the last bit of oscillation in a Dyna is be lightening the offending components or adding counterweight to bring the center of mass of the drive train into alignment with the center of the chassis.

Since the Positrac is so effective why hasn’t Harley-Davidson installed it at the factory? This question brought a sly smile to Sputhe’s face. “They should have.  Maybe the bean counters didn’t want to pay Eric Buell a royalty, or maybe the product-cheapening department overruled the engineers.  Harley’s lawyers probably won’t let them do it now because it would be admitting that there is a serious problem and opening a product liability can of worms.”

The Sputhe PosiTrac is not cheap but it is well made and definitely worth every cent in terms of the increased enjoyment that you will get from your Dyna.

Jace Kendall