How the Motorcycle Wheelbase Affects Handling

[Hugh Janus]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

There is a direct relationship between wheelbase length, which is typically in the range of 48 to 65 inches, and quickness of steering response. This is easy to understand. When you deflect the steering, the front wheel begins to steer off the vehicle’s previous direction at X inches per second. This sideways movement of the front wheel steers the rear wheel by using the wheelbase as a lever. The shorter the lever the more the rear wheel is steered and vice versa.

The same is true of stability. When a disturbance (bump, wind gust) gives the front wheel an accidental steer input, a long wheelbase reduces the degree to which this also steers the rear wheel. In the early 1970s, when engine power was rising steeply, high-speed weave became a problem (side-to-side swing of the rear wheel at two-to-three cycles per second). The standard “cure” back then was to lengthen the wheelbase.

In certain applications the quickest possible steering is required. Look at a classic Harley XR-750 dirt-tracker. What hits you is that front and rear wheels are as close to the front and rear of the engine/transmission unit as they can possibly be. Dirt track shaped that motorcycle over its successful 47-year racing career, so that short wheelbase is no accident.

A motorcycle like the Hayabusa is given a long wheelbase to increase stability, but that can make steering response slower than on a short-wheelbase motorcycle. (Suzuki/)

On the other hand, the lack of weight on the front wheel of certain designs has forced the adoption of a response-slowing longer wheelbase and “highly stable” (nice way to say sluggish) steering geometry. This describes Ducati’s classic 750 bevel-drive twins. The near-horizontal front cylinder pushes engine mass rearward, so to restore a degree of front/rear weight balance, the wheelbase was extended to an American-LaFrance-like 60 inches. Because wobble and weave oscillations are damped by the footprint areas of the tires, and because of the limited weight and smallish footprint up front, this bike was further stabilized by a large 31-degree steering rake angle (today something in the range of 23.5–25 degrees is usual) and a highly stable, and slow-steering, trail distance of 4.5 inches. This bike was successful in its day because it was stable when Japanese literbikes, with 27-degree rake angle, were definitely not.

Related Content: All About Geometry

When Kel Carruthers tested the Yamaha TZ750 prototype, descendants of which would win Daytona 1974–1982 inclusive, he found that the quick-steering short wheelbase wanted by Giacomo Agostini (53.5 inches) gave that 90 hp proto a scary weave at high speed. Kel called for an extra 3 inches to be added to the wheelbase by extending the swingarm.

If you take your new bike to the dragstrip, one of several things that reduce E.T. is to move the rear wheel back all the way, thereby lengthening the lever by which the bike’s center of mass acts to prevent snap wheelies from spoiling the run. You also pull yourself forward. The most extreme case of this is the drag bikes that run in Pro Street, which are given mile-long swingarms. Despite all that leverage, they can still wheelie thanks to the combo of turbocharging and pavement shiny with glue.

Just after 1900, Belgian arms maker FN produced a bike powered by an inline-four with its crankshaft parallel to the wheelbase. The resulting length made the machine A) very stable, and B) a real handful on city streets made slippery by four-legged horsepower by-product. Once a rider let that long wheelbase start to swing, recovery was iffy.

Related Content: Do You Really Know Your Bike’s Wheelbase?

In general, the better the rider, the more likely he/she is to prefer a short wheelbase. Dick O’Brien, long Harley’s racing chief, told me years ago that when riders first got on the XR roadracer (same engine as the dirt-tracker, but different chassis) they were most comfortable with a longer wheelbase. As they adapted to the bike and their lap times dropped, they found best performance with a shorter chassis. Part of the reason for this is that the quicker the steering, the less distance is consumed by the lean-in process.

Sportbikes built to win races are designed to turn and therefore have shorter overall wheelbases than a cruiser or standard streetbike. (Yamaha/)

Because there are just the two basic cornering styles, bikes tend to be designed for one or the other. A bike built for a corner-speed rider such as Doug Chandler or Jorge Lorenzo needs stability to stay close to the grip limit all the way around, so it has a longer wheelbase, lower build, and stabler steering. A point-and-shoot bike (the late Nicky Hayden’s Honda RC45) is given a short wheelbase and higher engine position to achieve instant weight transfer for right-now braking and acceleration, plus very quick steering to waste the least time in transitions.

Touring bikes and “production dragsters” (’Busas and ZX-14s) are given longer wheelbase, the former to civilize the riding experience, the latter to reduce taillight lens breakage. Sportbikes were given the shorter wheelbases they needed to win the Supersport races that used to sell so many bikes. Most bikes today compromise somewhere in the mid-50-inch range.

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[Pedro]

I was looking forward to the part about the GS's weird front suspension and it's very short wheelbase, but it didn't happen.

Where's Lone Amigo?

[Buckster]

On 14/05/2021 at 22:04, Pedro said:

 

Where's Lone Amigo?

 

Flounced.

[XTreme]

2 minutes ago, Buckster said:

Flounced.

What you say Clive?

[Buckster]

7 hours ago, XTreme said:

What you say Clive?

What you say Bruce?