Introduction
A couple of months ago, I purchased a Transition Patrol MX Alloy. As standard, it is supplied with a 160mm fork paired with 160mm of rear travel provided by a 205x60mm stroke shock. The geometry of the bike is fairly progressive, with size-specific chainstays, a low standover and a 63/63.5◦head-angle. There are many disciplines of mountain biking and the Patrol sits somewhere in the ‘enduro’ category; a ‘do-it-all’ bike. This is their workhorse of a bike, designed to be able to pedal to the top of the hill, but still capable of riding the most challenging trails. Interestingly, the frame is also compatible with a dual-crown fork. These forks generally feature 200mm of suspension travel. A greater amount of suspension travel increases the bicycle’s ability to absorb impacts. The forks are generally found on ‘downhill’ or ‘freeride’ bikes. These are bikes specific to descending and designed for the most difficult terrain. I have always wanted a downhill bike, but it is impossible to justify spending the money on such a niche bike with the type of riding available in the UK. This got me thinking: What would it take for me to get downhill-bike-level descending performance from the Patrol?
Within this article, I will record my approach to creating a custom rocker link for the 2021-2024 Transition Bicycles Patrol MX Alloy. This redesign will give the suspension platform an increased amount of travel, along with a greater leverage ratio. This link will aim to give the bike improved performance in descending and match up more closely with a more aggressive riding style. This will, without a doubt, void the warranty!
It is worth noting that the kinematic graphs of any frame only paint half the picture of how the bike will respond on the trail. From a suspension perspective, the linkage characteristics work alongside the fork and shock damping. The tyres and frame in particular cannot be treated as rigid members either. They are components with damping and spring characteristics in their own right. Combined with a constantly shifting centre of gravity and a multitude of different riding styles, it is far beyond the scope of this report to attempt to identify an accurate picture of the full system; Perhaps a focus for the next project I work on!
System Dynamics
Fig 1. Kinematic Model in PTC Creo
While some of the frame geometry can be extracted from the manufacturer's website, I needed the exact pivot point locations on the frame. To do this, I took 3D scans of the bike and combined that data with traditional photogrammetry techniques. From this data, I was able to build a kinematic 4-bar model in Creo. Using the Mechanism functionality in Creo, I was able to obtain the motion ratio between the rear axle and the shock stroke. This allows me to calculate the leverage ratio and total travel. With the geometry extracted from the 3D scan, I was able to recreate the leverage curve provided on the Transition Bikes website, thus validating my model.
Fig 2. Reproduction of Leverage Curve
From here, I was able to adjust the locations of the rocker pivots until I achieved my initial specification - 200mm of travel and greater progression. I was also able to adjust the pivot point locations to steepen the head angle back to 63.5 degrees. The longer, 200mm dual crown fork placed on the front of the bike to match the increased rear wheel travel would raise and slacken the head tube in an undesired way.
Design Factors
A key aim of this project is to make it as simple as possible for the user to change between the stock and redesigned rocker links. Because of this, the redesigned rocker link should have all the same interfaces as the original part. It is important that the rocker link could be cut on a 3 axis mill to keep costs down. This meant the part should be cut in three sections: Two mirrored plates and a central stiffener. With the x and y coordinates of the pivots set by the mechanism analysis, and the z coordinates dictated by the existing component interfaces, it was just a case of joining the dots!
Topology Optimisation
Static structural analysis in ANSYS mechanical was used to access the component’s response to a range of forcings. It was particularly important to prevent side loading of the shock to prevent binding and increased wear. Deep pockets allowed the second moment of the area to stay high and achieve satisfactory lateral stiffness whilst removing unnecessary material. A symmetric model around the midplane allowed for a better mesh resolution without high computational costs. Stress concentrations were removed in an effort to improve fatigue life and reduce the chance of crack initiation.
Fig 3. Mesh of an early iteration of the design.
Testing
Testing will take place initially in the UK before the bike is subjected to a season of riding in the French Alps! The component has now been machined and is awaiting anodising. This was meant to take place earlier this week, but unfortunately, there was an issue with one of the machines in the University lab where I was going to do it. The control sample I anodised first came out great so I am confident that link will also! The three parts will be subsequently bonded before the installation of bearings, circlips and then installation on the bike.
Fig 4. One of the plate sections post-machining.
Alongside the development of this link, I have been building a supplementary telemetry system. This will allow measurement of the accelerations across the sprung and unsprung masses. This system was created with support from RS. Thanks so much to the team there for supporting projects like this!
Fig 5. Early breadboard prototyping of telemetry system
Comments