Wheelbase, rake, trail, tires, and frame rigidity

Because motorcycles are built for many applications—touring, cruising, sport, and off-roadmotorcycle handling must in each case deliver the kind of performance riders in that discipline need.

One way of looking at motorcycle handling is to consider stability versus control response. The sporting rider, wanting a highly responsive machine, soon discovers that response cuts two ways. A bike designed to respond promptly to the rider’s control inputs will respond just as promptly to other disturbances, such as bumps in the road or crossing railroad tracks at an angle.

The touring rider traversing the great flat states of Nebraska or Kansas, with the sound system playing favorite tunes, does not want to be constantly correcting. That motorcycle’s wheelbase will be longer, providing room for rider, passenger, and luggage without crowding. Wheelbase is also the lever or “tiller” by which steering the front wheel swings the rear to a new heading. The longer the wheelbase, the more slowly a steering input at the bars (or the effect of any accidental disturbance) becomes a whole-vehicle change of direction.

Sportbikes’ wheelbases have grown as power has increased, but are still relatively short in comparison to less “hyper” segments.

Measure touring bike wheelbases and you’ll find numbers around 65 inches.

The sporting rider, being more entertained by curves and changes of direction, wants quicker steering and direction changing because in that way, maneuvers eat up less time and distance, and there is a tighter connection between cause and effect. Wheelbase is therefore made shorter. Stanley Woods, one of the great roadracers of the between-the-wars period 1919–1939, liked best a wheelbase of 52 inches. Giacomo Agostini in the 1960s, riding MV Agusta factory bikes, liked 53.5 inches. And today, racers in Superbike and MotoGP are most often on 56 inches.

Why the trend toward longer? As engines progressed from the 40 hp of the mid-1930s to double that in the late 1960s, more and more time could potentially be wasted in wheelies. Better to extend the wheelbase a bit to use more of that ability to accelerate.

Comparing MX and other off-road designs with those of 50 years ago is a shock. Back when suspension travel averaged 3–4 inches, MX bikes could afford a short wheelbase in return for quicker steering. But now that bikes with 8–12 inches of suspension travel are so tall—and much faster across our planet’s undulations—wheelbase has had to grow in order to keep the front wheel on the ground at least occasionally.

Longest of all wheelbases are those of dragsters, which are from 65 inches upward—again, to turn horsepower into acceleration rather than waste it in wheelies.

Acceleration is the goal of dragsters’ long wheelbase; turning is low on the list of priorities

Where does the engine belong in the frame? Forward? Back against the rear tire, “for traction” as designers of the 1950s claimed? Higher? Lower? Some time in the early 1960s came a piece of research published by the Japan Society of Mechanical Engineers (JSME), relating stability to engine position. Their conclusion, based on testing on a motor-driven artificial belt roadway? Move the engine forward for stability.

Today, have a look at most kinds of motorcycles and see that practice and theory agree. There are Honda’s Gold Wing and Harley-Davidson’s grand tourers, both with engines well forward (OK, part of the reason is to get them out from underfoot!).

The same is true of sport and racing pavement bikes; what use is increased engine power if the major result is wheelies that constantly interfere with control? Moving the engine forward makes wheelies less frequent and less sudden.

The big exception here is dirt-trackers. When I first saw a classic Harley XR-750 up close, I could see that its front and rear wheels were essentially pushed as close to the V-twin engine as possible, implying the shorter the wheelbase, the better.

Touring and cruising bikes have their engines set so low that 30–32 degrees of lean in cornering are about all she wrote (any more and bright sparks stream from hard parts hitting the pavement). Riders need to have the weight of those engines set as low as possible so they can confidently manage it when stopped at a light or maneuvering at low speed. The higher the engine, the worse the feeling that “I’ve gotta watch it or this thing will pin me to the pavement.” Likewise for the very low seat height—as little as 24 inches; it lets the rider plant his or her feet firmly on the ground to manage that weight.

Low and long is the mix most used for cruisers. 

Sport and racing motorcycles, their performance dependent as it is on rapid roll response, need to centralize their major masses (engine, rider, fuel). This makes the bike act more like a 24-pound cannonball than like a 24-pound ladder when responding to forces trying to change its orientation. Mass centralization lies at the heart of quick response.

One hundred twenty-five years of tire and rubber development has given us fabulously grippy tires, but in order to use that grip to the full we need cornering clearance (the ability to lean bikes to tremendous angles) as great as 63 degrees from vertical, in the case of racing slicks on good pavement. That too requires engines set quite high in the chassis, and made as narrow as possible (which is part of the reason why the cam drives of in-line four-cylinder engines were moved from the center of the crankshaft to one end; it allowed deleting the extra width of one of the six main bearings formerly used).

Another matter is steering geometry, which refers to two numbers:

  1. The angle at which the steering axis is set to the vertical (the rake angle)
  2. The distance by which the center of the front tire’s footprint trails behind the projection of the steering axis onto the pavement (the trail).

Back in the 1970s rake angles for pavement sport and racing bikes were in the range of 27–31 degrees. A good reason for these numbers (unfashionably large today) was the greater general flexibility of the steel tube chassis of that time. The more flexy the chassis is, the more vulnerable it is to instability, requiring more stable steering geometry to make it behave.

Bottom line? The more bendy stuff there is between your hands and the front tire, the less responsive your ride will feel.

– Kevin Cameron

As stiffer chassis and forks came into use in the 1980s, smaller rake angles and somewhat shorter trails became usable. Today’s sportbikes have rakes in the vicinity of 23.5 to 24.5 degrees, with trails in the range of 3.75 to 4.0 inches. Because of this, older bikes with rakes up around 30 degrees look decidedly vintage to me!

Another thing to mention in regard to chassis stiffness is steering delay. Years ago CW’s former resident off-road editor Jimmy Lewis described riding two makes of 125 dirt bikes with nearly identical rake and trail. Yet, he said, one of them delivered right-now steering while the other was sluggish. Compare the fork and bars on a 60-year-old Triumph 650 with one of today’s middleweights. You could hold the Triumph’s front wheel between your knees and turn the bars through scary large angles. Today’s bikes have at least two lower crown pinch-bolts per fork leg, and maybe three—these new front ends don’t budge when you apply the above test. The Triumph, having a high-vibes parallel-twin engine, was given squidgy rubber-mounted bars that added even more steering vagueness.

Bottom line? The more bendy stuff there is between your hands and the front tire, the less responsive your ride will feel.

The aesthetics of the chopper movement glorified large rake angles up around 35 degrees or even more. They probably originated in the large rakes found in the 1950s to give good stability at Bonneville or on the dragstrip. Occasionally a stylist today will “go for the look” by angling the fork legs more than the steering stem, thereby combining a bygone look with reasonably modern steering.

Source: cycleworld.com