Since the i-track suspension platform can be implemented in so many different ways, it was quite difficult to decide on the particular embodiment that P1 would take on.
For our first prototype, the design objective was simple: To develop a ‘proof of concept’ prototype, that demonstrates how the i-track suspension platform can be used to create an advanced acceleration response on a long-travel bike with a rearward axle path.
An 'acceleration response' is a graph showing how the amount of
anti-squat varies throughout the suspension travel. The amount of anti-squat (%) on the y-axis, and the amount of vertical wheel travel (mm) on the x-axis. Basically it shows a curve that describes the pedalling behaviour of the bike, and allows the pedalling performance of different bikes to be quantitatively compared.
We consider an 'advanced' acceleration response to be one that includes certain deliberate features that will make the bike perform in a desired manner (some of these discussed later).
At the same time, the design should be kept simple and relatively easy to fabricate.
With this brief, we opted for a single pivot design (axle path governed by a single pivot), a directly actuated shock, and a pair of links to control the idler. The front idler link is attached to the main frame at the same location as the lower shock eye, and the idler axle is at the same location as the floating pivot between the two idler links. This minimises the amount of hardware required for this design, and keeps it looking as 'clean' as possible.
The frame was constructed from 4130 Chromoly, as it was readily available, and does not require any post weld heat treatment.
Travel: 203mm vertical wheel travel
Total axle displacement 230mm
Shock: 8.75" x 2.75"
Head Angle: 64deg
BB Height: 20mm above axles (roughly equates to 360mm off ground – depending on tyres).
Sagged: 33% F&R
Head Angle: 63.7deg
BB Height: 44mm below axles (roughly equates to 296mm off ground – depending on tyres).
The main benefit of the i-track suspension platform is that it allows the pedalling performance to be tuned independently of any other design variables. Another benefit is that it allows the pedalling characteristics to be tuned with greater flexibility than other suspension platforms.
It was obvious that the Acceleration Response of P1 should demonstrate these capabilities.
The graph below shows the Acceleration Response of P1.
There are two curves: ‘Total Anti-Squat’ and ‘Frame Anti-Squat’.
‘Total Anti-Squat’ describes the pedalling behaviour observed by the rider.
‘Frame Anti-Squat’ serves as a reference line, to illustrate where pedal feedback exists.
There are a number of noteworthy features of this Acceleration Response:
ZONE A: In the early stages of travel, the Total AS curve passes below the Frame AS curve.
This means that there is no pedal feedback in this zone, giving excellent small-bump compliance when pedalling.
ZONE B: Throughout the middle range of travel, the Total AS is increasing. This provides a very supportive pedalling plaform, particularly towards the bottom of the pedal stroke (where most suspension systems exhibit excessive compression - especially when standing and pedalling).
ZONE C: From about 140mm to 200mm travel, it is unlikely that pedalling is going to occur in this zone. Therefore, it is no longer necessary to have an increasing amount of AS. Instead, we can tune the AS curve to overcome some other related issues. In this case, the AS curve takes a sharp downturn to reduce the pedal feedback on big suspension compressions. This decrease also serves to manage the overall chain growth, so that the derailleur arm doesn’t have to move as far.
P1 clearly has a very high pivot. This provides a very rearward axle path which allows the wheel to react to rough terrain more easily. In more technical terms, it provides a greater amount of what we call 'wheel rate separation', which is the difference between the wheel rate observerved by the rider due to rider inputs (generally vertical in direction), and terrain inputs (more rearward in direction).
The wheel rate curve for P1 is digressive.
It is generally accepted that many riders prefer a progressive wheel rate curve, however with the limitations imposed by the design brief, it was not possible to achieve this.
With such a high pivot, the P1 exhibits quite a high amount of Anti-Rise.
It's not as high as some other high pivot suspension systems, so we're happy to run with this initially.