RaerDesign

Suspension Technology (Bicycle)

With such light frame construction on bicycles the ratio between sprung and unsprung weight on them makes it challenging to produce high quality suspension action because suspension loads are fed into a very light frame reacting to rotate it. Riders feel this as a pitching motion, or in extreme cases as a ‘kick’, in the frame from partially absorbed bumps. Sprung weight being the frame cranks pedals handlebars etc, and, unsprung weight being the wheels swinging arm telescopic forks etc - basically the pieces which move with suspension motion. With motorcycles the mass of sprung weight is greatly increased with the engine whereas with bicycles such a central balancing weight is missing. The only ways of circumnavigating this loss of counterbalancing weight are to have;
fully active suspension : - not possible on a bicycle due to complexity vulnerability and extra weight;
provide a substantial extra weight to the frame : - not viable;
or, feed an amount of suspension motion and load from one suspension system to the other, front to rear, and, rear to front.

Directly connected suspension does this. On hitting a bump with the front wheel this reacts to extend the rear suspension a little, and likewise with the rear suspension on hitting a bump this reacts to extend the front. Suspension forces are fed back and forth by means of the interposed shock absorbers, which, at the same time, dampen out the load received by the opposing suspension system.
A bicycle, with telescopic forks and rear suspension, on hitting a bump that is 120mm high will have the forks compress around 90mm and the handlebars rise up by the remaining 30mm. Unknown to the rider, the rear suspension will have also compressed about 6mm as the force acts to rotate the frame around its centre of gravity (polar moments of inertia). With a distance of 720mm from the handlebars to the rear swinging arm pivot point on the frame this has caused a sudden rotation of 2.9 degrees in the frame, and, a lifting of the centre of gravity by 12mm. A longer frame therefore is beneficial to reducing pitching motion but this is at the expense of increased weight and decreased strength and stiffness.

With directly connected suspension, on hitting a bump of 120mm and having the front suspension compress 90mm where the handlebars rise up by the remaining 30mm, now the rear suspension does not compress but extend slightly. With an extension of the rear suspension of 10mm on hitting this bump and at 720mm from the handlebars to the swinging arm pivot point the sudden frame rotation is now only 1.6 degrees (a 45% reduction in frame pitch), and, the centre of gravity rises up 20mm upon impact.
Given these calculations, what was missing from the original equation (when this design was first calculated and then tested on a motocrosser) was the lifting of the centre of gravity with directly connected suspension from 12mm to the higher 20mm combined with the reduced rotation (pitching) of the frame, both of these factors react to force the front suspension into being more effective on hitting the original 120mm bump. So instead of the front suspension absorbing only 90mm of a 120mm bump it will now absorb around 105mm of the bump. Readjusting the original calculations the handlebars have now only risen up 15mm and the rear suspension has further extended 2mm from 10mm to 12mm giving a rotation of the frame of only 0.2 degrees and a lifting of the centre of gravity by 13.5mm, a figure similar to the telescopic framed bike. This is now a massive 92% reduction in frame pitching motion!!!
The above figures were only used as an example to explain the self-levelling effect of directly connected suspension, where the bicycle now has the potential to remain remarkably parallel to the ground as it travels where any bumps are better absorbed and with so little violent pitching motion in the frame registering with the rider. In short, directly connected suspension reacts like a mechanical ‘active’ suspension without the complexity or expense.

With the dynamic condition that leads to the rider being thrown over the handlebars on hitting a square-edged bump, such is mitigated with directly connected suspension. This is because after the front wheel becomes airborne on passing over the bump the front suspension extends causing the rear suspension to squat downwards altering the projectile of the bicycle from level to upwards thereby throwing weight rearward onto the rear suspension thus assisting the rear suspension to absorb the approaching bump. On the rear wheel hitting the bump with the front wheel still off of the ground – the most dangerous moment – compression of the rear suspension extends the front suspension dissipating the energy which would otherwise produce the ‘kick’ from the rear suspension.

To explain why there is no energy loss when peddling it is the stated lifting of the centre of gravity with the principle of not having suspension forces, including them experienced with the rear suspension when peddling, rotate the frame around its centre of gravity. Basically, under no circumstances does the centre of gravity want to lift, and with directly connected suspension, as described above, there is a desire to keep the frame parallel to the ground in all circumstances further increasing this effect. The force you apply at the pedals is less than that required to lift the combined mass of bicycle and rider, therefore all pedal force is translated into driving motion.
Finally, given that directly connected suspension alters the angles to which the shock absorbers are attached to the front control arm and to the rear swinging arm by displacement of members of the opposing suspension system this alters the leverage ratio where it gives stiffer front suspension action when the front is compressed under braking and lighter rear as it extends under braking, plus lighter frame design, therefore directly connected suspension gives benefits electronic active suspension is not capable of.

The picture (Photo 6.) has the steering head angle in dark red where this is determined between the two ball joints (one beside the handlebars and one just above the front tire), and the front suspension arrangement (control arm attachment points and distances between them plus the shock absorber) highlighted in white, with the rear suspension shock attachment and distances marked in yellow.