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 Chassis - Motorcycle 2
The first motorcycles, of course, had no suspension, and for much longer, the rear had none than the front. Alternatively, on the opulent seat of a Harley, which can accommodate two (rather lightweight) people and is
blessed with seemingly endless suspension travel. BMW didn't introduce rear-wheel suspension until Hitler himself pressured them to do so during an auto show.
And, like the Lancia Lambda mentioned earlier, it was a sleeve-shaped valve with a relatively short stroke. There just isn't much room left, but the cardan drive apparently doesn't allow for any more bending.
Later on, they kept this design and added trailing links.
There are always one or two trailing arms, although the term 'trailing link' is more appropriate due to its restricted freedom of movement compared to a wishbone. Such links can be single- or double-sided, with the former
being more common, for example, in racing, likely due to their lighter weight.
In terms of the structural mechanics of a road-going motorcycle, this means that stability must be provided in duplicate, extending from the pivot point of the rear trailing link to the respective extremities; for instance, there
must be separate structural support for the section reaching from the seat to the license plate, and for the link itself (image above). Aside from the bench, just think about whether you might want to attach a suitcase. The
length of the links has a significant impact on a motorcycle's wheelbase. Their deflection often causes it to change considerably.
Wheelbase and caster determine driving stability, which increases significantly here as speed rises, so much so that at very high speeds, it appears to be difficult to move the handlebars at all, let alone jerk them sharply. I
don't think anyone has really tried it yet, and if they did, they probably wouldn't have been able to tell us about it afterward.
The center of gravity also plays a significant role, both in terms of its height and its position within the wheelbase. The farther back and the higher it is, the less stable the two-wheeler becomes. The long distance from the
tire's contact patch to the steering head creates a considerable lever arm, which is somewhat mitigated, for example, by the Duolever system shown in the previous chapter. This long lever arm is subjected to bending
stress during heavy braking, which, due to friction, also makes it difficult for the suspension or damping system to respond smoothly. Given a tractor-trailer's superior braking ability, you should never follow too closely
behind one.
The image above shows the resultant force of centrifugal force and weight from the center of mass. A torsional moment develops around this point, which increases as the inclined postition grows, at the very least due to
the increasing lever arm. That said, many aspects are still simplified here, and factors such as the condition of the road surface and suspension travel, especially at the rear, have been omitted. By the way, did you know
that on a motorcycle, too, the front wheel follows a different turning radius than the rear wheel?
Unfortunately, a description of the fascinating evolution of frame construction, for example, toward complex cast lightweight alloys, and its impact on design would go beyond the scope of this book. A motorcycle's
suspension is much more closely integrated with the frame or chassis than, for example, in a car or even an SUV. The picture above will illustrate this.
| Some manufacturers prohibit drilling even a single hole in them. |
To wrap things up, let's take a look at the rear link and the drive system installed there. Did you know, by the way, that a motorcycle with significant rear suspension travel and a well-blocked drive tends to move backward
slightly? To be more precise, this only happens when it moves from below into a horizontal position, that is, when the center of the wheel is shifted slightly backward. Furthermore, it's actually rather the other way around.
Long links cause less displacement of the wheel center than short ones for the same spring travel. Their bearings should then be moved forward as far as possible, past the engine, which would look strange and would
further increase the size of each of the strings described above, each with its own stability. But there is another reason against such a design.
After all, since the chain tension remains constant, the axis of the front sprocket should actually be aligned with that of the front link bearing. In that case, the chain drive is still not independent of the link changing position,
but at least the chain play remains the same.
However, because, for design reasons, the transmission's output shaft (start of the secondary drive) very rarely aligns with the link bearing, chain slack is unavoidable, for example, when the link is deflected downward.
However, owners of motorcycles with shaft drives shouldn't get too comfortable and assume they don't have these or similar problems.
This is because, in this situation as well, the motorcycle moves when the drive is blocked and the rear suspension compresses significantly. Even more so than with a chain drive, this is a torque that results from drive or
load-change reactions. This results in the link rising at the rear when accelerating and suddenly dropping when boost is suddenly applied.
BMW motorcycles feature a design in which the rear end of the link is decoupled and connected directly to the frame via a reaction linkage (see image above). However, this requires the cardan shaft
to have an additional universal joint with tapered roller bearings exactly at the potential bending point. On newer BMWs, however, the outer linkage runs above the split link.
Here is the entire powertrain:
| Continuous Damping Control System for motorcycles |
ZF illustrates the monumental task that a motorcycle’s chassis must perform compared to that of a passenger car. The weight, including the driver, is assumed to be 280 kg, and any luggage, including a second
passenger, is assumed to weigh 100 kg. That would be 47 percent more for a driver weighing 70 kg, which would not be acceptable for a passenger car.
This gives rise to specific challenges in tuning a motorcycle's suspension. Sachs, now part of ZF, has always been involved in suspension technology; since 2013, it has also been working on the semi-active,
electronically controlled damping system, which was originally designed for the passenger car sector (adaptive damping).
Of course, data from sensors is used. For example, while the acceleration of the assembly is important, it can be incorporated into the control system of a conventionally designed damper that has been supplemented
by a proportional valve. This can increase or decrease the oil flow within the damper in a matter of milliseconds. In the rebound stage, a smaller range is utilized, while in the compression stage, higher damping is
possible and probable.
Despite improved ride comfort, the aim is to achieve better handling and to make even the heaviest payload as imperceptible as possible, so including braking and safety features. However, there is also the benefit of
reducing tire wear.
The days of 'screwdrivers' for adjusting the suspension before a drive or during breaks are over. In general, you can also preselect the CDC while driving; with ZF, you can choose between 'Sport', 'Comfort', and
'Passenger Mode'. However, to prevent, for example, excessive nose-dive during braking, the system can also work in conjunction with other onboard systems, such as ABS and ESP, if available.
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