One engineer’s home-stretched suspension
With the earliest mountain bikes assembled from bits and pieces from all sorts of cast-off bicycles and motorcycles, often modified to suit, bodging is a proud part of mountain bike history and culture. When Magura engineer Reiner Kuenstle wanted a suspension fork for his fatbike, he couldn’t see simply ordering up another brand’s offering. No, he decided to modify a dual-arch Magura TS8 29er fork to accept his 135mm hub and 26x4in tyre.
Shown below is the process used to create Kuenstle’s first effort, a second was started this week. One of the major goals in this unsanctioned project was to see if it could be done at home, without the need for special bonding or laminating tools. The short answer is yes, the long answer follows…
- Laying the groundwork (and using Magura CAD data to ensure a precise fit), a pair of aluminium inserts were designed to follow the inside contours of the arches while adding 35mm in width. A wider crown was also drawn up and the parts were machined on a multi-axis CNC mill.
- No doubt following multiple measurements, the magnesium lowers from a standard MT8 29er fork were split down their centreline.
- Using a dummy front hub and the new, wider crown/steerer assembly for spacing and alignment, the conforming aluminium inserts were bonded into the fork’s bridges using common two-part epoxy. Reiner has said that the inserts and epoxy provide the majority of the legs’ rigidity; who are we to argue with the project’s engineer?
- Bring out the carbon! Once the inserts were cured in place, the arches were wrapped in standard carbon fibre at varying orientations for additional support.
- The carbon fibre wrap used an aerospace-proven resin that reaches full strength after a long cure at room temperature, chosen to avoid damaging the already-coated lowers. The bushings remained in the legs throughout and a the tied-off innertube ‘sausage’ ensured good contact between the carbon fibre and extended arches.
- Ordinary auto body filler was used to cover the carbon fibre and smooth the arches- the most time-consuming part of the process. Once smoothed to satisfaction, the filler was sealed with resin.
- Standard white paint was used to cover the modifications and blended with the lowers’ existing powder coat and dried in the open mouth of the Kuenstle household oven.
- Wide stance aside, the fork was reassembled in the normal fashion. Without a close look, it would be hard to tell this fork from a factory product.
For his second go ’round, Reiner plans to use woven carbon fibre tubes to wrap the arches, which should cut down on the smoothing and filling work time. All in all, this looks like a well thought-out and cleanly executed project, calling it a bodge really doesn’t do it justice.
For those chomping at the bit, now that the concept has been proven Magura is working with composite suppliers to determine the feasibility of series production, more on that when we know. Many thanks to Reiner Kuenstle for the details and photos.
Posted on: January 23, 2014