Chassis Choices: Making Sense of Motorcycle Dimensions

  • AUTHOR
    Honda Riders Club of America
  • POSTED
    May 10, 2010
  • POSTED IN
    Offroad

By: HRCA staff

Why are motorcycles built the way they are? Nowadays, virtually all sportbike chassis have wheelbases close to 55 inches, while numbers for touring bikes and cruisers stand closer to 60 inches and more. Dragster chassis are much longer and lower yet. Doesn’t that mean sportbikes are throwing away valuable acceleration ability with their higher center of gravity (Cg) and shorter wheelbase? MX bikes have 11 or 12 inches of suspension travel, while sportbikes have less than half as much, and touring bikes less again. How can these hugely disparate chassis numbers all be the correct choice?

It all comes down to the requirements of specific applications. For instance, a sportbike combines into one vehicle features needed in four different circumstances. First, when approaching a turn, the rider brakes to get down to turn-entry speed. Theoretically speaking, an ideal configuration for motorcycle weight distribution in light of braking chores would look something like a dragster with the engine swapped to the back end—it would have a long wheelbase with major masses located to the rear, giving the bike an increased ability to brake hard without lifting the rear wheel.

Next, when entering a turn, the rider countersteers in the opposite direction of the turn, causing the machine to lean over into the turn. The longer the wheelbase, the longer the lever by which the steered front wheel acts to steer the rear wheel—and the slower the bike responds. Quick steering responses for sporting applications require a short wheelbase.

As the machine leans over, it needs to have its chassis, pipes, engine cases, and rider footpegs situated high enough to clear the pavement. With tires as good as they now are, that pushes the Cg wide, especially with a wider engine. Yet at full lean in the corner, major masses should be located down low to make best use of the grip of the narrower front tire and wider rear. Suspension affects cornering clearance: the softer it is and the more travel it has, the less clearance remains. So the sportier the bike, the shorter its suspension travel must be.

Now the rider begins to open the throttle to exit the turn, and the machine begins to roll upright. As less tire grip is needed for turning, more throttle can be used for acceleration, and soon the front wheel grows lighter, decreasing grip and potentially causing the machine to run wide. To prevent this, we’d again like a more forward and lower position of major masses—engine, fuel, and rider. As the machine comes fully upright, full acceleration can be used. Now a long, dragster-like wheelbase and low engine position would make best use of all of the engine’s power.


Steering geometry presents another set of variables. If we could project the steering axis onto the pavement, we would see that the center of the front tire’s footprint trails behind it—necessary to make the steering self-centering. So the distance between projected steer axis and center of front tire footprint is called “trail.” For most street and dirt bikes, this dimension is usually around four inches.

The issue of steering rake is more complicated; for a fuller treatment, look in Tony Foale’s book “Motorcycle Handling and Chassis Design.” To combat the tendency of a sportbike’s forward weight distribution to cause heavy low-speed steering, rake has been reduced to around 23 degrees. If the rake angle were zero, the height of the steering head would not vary with steer angle, but as rake is increased, the steering head falls as the front wheel is turned to either side.

The trend toward reduced rake has come from road racing and has brought lighter steering to all production pavement motorcycles, touring bikes included. In the past, very large rakes and trails (31 degrees and 4.5 inches or more) were applied to help machines with excessive chassis flex or very little weight on the front wheel. Today, in the field of cruiser bikes with modern chassis, very large rake is primarily a matter of style.

Modern fighter planes locate guns and engines in the fuselage rather than out on the wings to reduce the planes’ inertial resistance to rapid roll rotation. This concept likewise holds true in motorcycles and is best exemplified in Honda’s CBR-series of sport bikes and high-performance off-road CRF-X and motocross CRF-R machines, which stack engine, fuel and rider as close together as possible to enhance maneuverability. The same is true for the fore-and-aft locations of other masses: everything possible is moved near the machine’s Cg, and parts that cannot be (fenders, taillight, passenger seat in the case of street bikes) are made as light as possible. Looking at specifics, the CBR1000RR features an engine that is about 5 pounds lighter than the previous generation, and the engine is smaller and more compact for enhanced mass centralization. In addition, the exhaust system is positioned almost entirely beneath the engine to situate it as close as possible to the motorcycle’s center of gravity and mass to reduce the inertial effects of the system’s weight. Net result: a 1000cc sportbike that weighs in and handles like a 600cc machine.

In parallel fashion, the current new-generation CRF450R is endowed with an all-new, lighter engine that has a lower overall height than the previous design, and it has been positioned closer to the front wheel, resulting in a lower overall Cg. The CRF450R also features a clever exhaust system that exits from the left side of the head to allow an increased length of the head pipe to reside closer to the engine. This, in turn, allows the new titanium/aluminum muffler to mount dramatically farther forward, much closer to the bike’s center of mass. In all, such changes result in a machine that turns in and changes direction more easily and quickly.

In terms of general suspension design, the telescopic fork at the front and swingarm at the rear have by steady improvement held off all alternatives for years. By use of linkage between swingarm and rear suspension unit it is possible to vary spring and damping rate over the range of wheel travel. This is especially useful when the load varies a lot, as with road bikes such as the Gold Wing that may often carry a passenger plus assorted baggage, or when very large energy forces must be absorbed in limited travel, such as with sport bikes at track speeds. Linkage can thus provide a comfortable ride for a single rider, yet stiffen to also carry a passenger without bottoming, or with a CBR, work well at street speeds as well as on track days. Off-road bikes are already very tall because of their long suspension travel, so further increases in bump absorption must come from suspension linkage that stiffens as it compresses. This makes it possible to land in control after spectacular Supercross jumps.

The geometry of rear suspension strongly affects sportbike handling. As the bike accelerates, tire and chain forces try to extend the rear suspension, but acceleration weight transfer tries to compress it. If compression wins during a corner exit, the rear of the bike squats down, robbing the front tire of grip so the bike runs wide. If extension wins, rear suspension extends against its up-stop. Once again, we see the need to balance settings in a middle ground.

If you’ve thoroughly digested all of the information above, hopefully you now possess a deeper understanding of the many variables that must be integrated into the design of a new motorcycle. So the next time you eyeball a bunch of spec charts and find a variety of chassis figures, maybe now you’ll have a better appreciation of how much thought, engineering and science go into making each and every Honda function as well as it does.

Originally published by Honda Riders Club of America.

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