One of the hot topics in the sector is the one concerning the engagement angle: let's see, thanks to DT SWISS, how it affects the effectiveness of pedaling and we reflect on the truth or otherwise of the widespread opinion according to which "the wider the better".
WHAT IS THE ENGAGEMENT ANGLE?
The engagement (or engagement) angle corresponds to the maximum rotation amplitude that the freewheel body can make before the teeth of the system engage with each other, imparting acceleration to the hub. This angle is calculated by dividing 360º by the number of freewheel system engagement points.
AND ALSO HOW MUCH PLAY DOES A CRANK HAS?
Backlash is the maximum freewheel travel that the crank can make before the freewheel mechanism engages and converts the force on the crank into acceleration of the wheel. The free play depends on three factors: the length of the crank arm, the gear ratio and the engagement angle.
The length of the crank arm and the engagement angle are directly proportional to the free stroke of the crank, while the transmission ratio acts inversely proportionally on the play.
Therefore, the lower the gear ratio, the greater the maximum free travel of the crank.
Basically, when is a small game important? When climbing hills and in situations where immediate acceleration is required to overcome certain obstacles.
KICKBACK PEDAL
Pedal kickback is a phenomenon that only occurs on full-suspension bicycles. It is due to chain tension during suspension compression, which causes the cranks to move backwards. Furthermore, chain tension prevents the suspension from moving freely (if you want to learn more about this, I refer you to this link). But what is the relationship between the kickback pedal and the engagement angle?
To describe the effect in question in detail, we analyze two different hypotheses. In the first case, let's assume we have a fixed rear wheel, which cannot rotate either clockwise or anti-clockwise. In the second case we instead assume a static situation, i.e. we do not take the speed of the bike into consideration.
What happens if the rear suspension compresses?
- The rear shock absorber of the bike compresses.
- The rear wheel moves around the pivot point.
- The distance between the bottom bracket and the rear wheel axis changes. This would not happen if the pivot point of the low chain stay was located exactly on the bottom bracket.
- As a result, the length of the chain also changes, which in turn causes the crank to rotate backwards.
What influence does suspension have?
If you prevent the crank from turning, the chain tension keeps the distance between the bottom bracket and the rear axle constant; this, in turn, prevents the low frame from moving freely and the suspension from absorbing the impact.
KICKBACK PEDAL
ENGAGEMENT ANGLE
What is the relationship between the kickback pedal and the engagement point? To understand the effect of the engagement angle on the kickback pedal, it is best to first consider the two extreme cases, i.e. instant engagement and no engagement.
INSTANT ENGAGEMENT
∞ POI
Instant engagement is achieved with a freewheel system that offers an infinite number of engagement points (PDI). Regardless of the situation, the freewheel body engages in the appropriate seat on the hub. The freewheel body can rotate counterclockwise, but not clockwise.
As the suspension is compressed, the distance between the bottom bracket and the rear axle increases (depending on the kinematics of the bike).
The resulting variation in chain length is then compensated for by the gearbox or chain tensioner. Since the freewheel body only moves counterclockwise, the chain tension also acts in this direction, causing the cranks to move backwards.
If the cranks were fixed, no change in the distance between the bottom bracket and the rear axle would be possible, and this would prevent the suspension from moving freely.
NO GRAFTS
0 POIs
The other extreme case consists of a freewheel system without engagement points (PDI), in which the freewheel body would therefore rotate freely in both directions.
With this starting position, the freewheel body can also move clockwise. Even if the crank is blocked by the weight of the pedaler's body, the rear can function freely, since the chain can move clockwise over the cassette.
EXAMPLE OF PEDAL KICKBACK
Let's assume that a bike has a 3° pedal kickback in the event of an impact at 50 mm, with a gear ratio of 32/14. What does this imply for the freewheel system?
To understand the effect of the engagement angle on the kickback pedal, you must first determine the rotation angle of the freewheel. To do this, simply multiply the kickback pedal by the gear ratio.
3° on the crank equates to a 6,8° rotation of the freewheel body.
In a system without engagement points, the freewheel body can rotate freely with the appropriate engagement angle in any position, without generating any pedal kickback effect. Conversely, a system with infinite or large numbers of engagement points has a higher probability of pedal kickback.
Referring to the previous example, the probability of having a kickback pedal becomes high with an engagement angle lower than 6,8°.
The greater the engagement angle, the less likely you are to have a pedal kickback.
More engagement points = greater likelihood of pedal kickback*
*Depending on the kinematics of the bike and the pedaling speed.
FURTHER OBSERVATIONS
While pedaling, the hub rotates at a certain speed. To accelerate, the freewheel body must engage the hub and this happens if the cassette rotates faster than the hub. If the angular velocity of the hub is greater than that of the cassette, the freewheel body cannot engage the hub and the wheel does not accelerate.
In terms of pedal kickback, this means that if the angular velocity of the hub is greater than that of the freewheel body generated by chain tension, there will be no pedal kickback or any negative effect on the suspension. However, it is difficult to reach this critical speed, because the faster you pedal, the greater the probability of exerting a marked compression and the greater the speed reached by the freewheel body under the effect of the chain tension. If it were easy to achieve this speed, World Cup cyclists would avoid having to remove a sprocket to achieve freewheeling and thus eliminate pedal kickback.
However, it is also worth noting that the kinematics of some bikes exclude the phenomenon of pedal kickback.
PROJECTED IMPACT
To create an optimal freewheel system three factors must be taken into consideration: reliability, weight and engagement points. But we must also keep in mind that there is a trade-off between these three aspects.
If we wanted to develop a system with multiple engagement points, we would have to enlarge the hub structure, and this would result in significantly greater weight.
However, rather than minimizing the engagement angle, it is a good idea to optimize the overall performance of the system. According to DT Swiss, the system has 36 engagement points represents the best in terms of reliability, weight and engagement points, which are the three key factors for an optimal freewheel system.
Damil produces 60 and 120 (here for example the FLOW MK4 test) while Industry Nine, in 2019, staged a real exercise in style with 690 points of engagement in his Hydra model!
CONVERSION KIT
For many Trial enthusiasts it is important to have a reduced engagement angle, so DT Swiss offers a 54 tooth conversion kit that allows you to reduce the engagement angle to 6,7°.
FURTHER LINKS
