Swarf Full Suspension, a brief history.... Part 2

In the first part of this brief history of Swarf full suspension frames we looked at the high pivot bikes, in this second part we are going to have a detailed look at the more recent short travel 29er frames.

Part 2, the short travel 29ers

Shortly after I made the decision to stop working on the Curve project I started messing about with 29 inch wheels. I really wanted to try them out so I made myself a hardtail. This bike was a revelation, it was nothing I expected it to be and everything I didn't expect. All the negative talk I'd heard about big wheels just fell by the wayside, it was fun, easy to corner, fine in the tight stuff and went uphill like a rocket. It was also fast. Even on choppy trails I was only marginally slower than on the Curve, despite feeling loads slower through the tricky stuff. Getting back on the Curve it was clear to see why, all of the sections you don't consider, the linking flat sections, that boggy short uphill, the short sections between corners, all these sections felt so slow and hard work on the full suss. The only downside to the hardtail was that it was punishing, riding rough stuff for any time was tiring and getting off line was scary! Every time I rode it, the thought was there, 'this would be amazing with a small amount of rear suspension'. So I sat down and started designing.

My Spline 29, riding this set the FS 29er project in motion

​I approached the new design with a different mindset than I did with the Curve. As I was intending the bike to be simply a forgiving hardtail, I didn't worry about coming up with the most efficient suspension system and instead concentrated on simplicity and ease of manufacture. I ended up going with a single pivot design with a shock extension yolk to give a mildly progressive leverage rate curve rather than the linear or even regressive rate that you get with a more conventional single pivot layout. 

The bike had the following geometry

Head Angle  67
Seat angle 75.5
Chainstay 445mm
Effective Top Tube 595mm
BB drop 45mm

It had 110mm of rear travel from a 200 x 51mm shock and the frame weighed 3.0kg without the shock.

It was a great bike, rode really well and I was more than happy with the geometry. However I went and completely redesigned it! There were a few reasons for this
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• I wasn't totally sold on the looks
• The linear nature of the rear suspension meant it would bottom harshly in certain trail situations.
• There were a couple of other designs coming out that used the same layout and I wanted to be able to differentiate myself
• No water bottle mounting
• The back end was tight on clearance and wasn't as stiff as I'd have liked (tyre rub on chainstays and seatstays!)

Contour 29er FS

​The new design addresses all of the issues I had with the original layout, using a rocker allowed the leverage rate to be tuned to something much more progressive, it freed up space on the downtube for a water bottle, it provides loads more stiffness to the back end and gave me that hardtail aesthetic. Winner!

I decided to go for broke and use a flexing seat stay arrangement to drive the rocker to eliminate the need for a pivot at the dropout (I'd always wanted to do a flex stay design after I saw the Lahar DHV M9 years ago). Fortunately I was able to minimise the flex required by optimising the layout to the point where the stay flexes a total of 6mm (3mm up and 3mm down), the steel stays have no issue whatsoever with this small amount of flex.

​Frame weight 3.2kg without shock
Rear Travel 115mm
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I made two prototypes and we've been riding them for well over a year now, they've been absolutely faultless. I've done a second design loop with very minor changes and it's almost ready for production. The alterations were as follows
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• Rocker made narrower by 4mm each side, as some riders found their knees gently touched the bearing housings on the seatstays. Reducing the width here has helped.
• Dropped the chainstays 8mm at the chain ring just to give slightly more chain clearance, we found that when using small oval rings the chain got quite close to the top of the chainstay
• Knocked 0.5 degrees off of the head angle 
• Removed the brace between the seat stays. Testing with the prototype frames showed us it simply wasn't needed, It was an awkward area during welding (created some annoying distortion) and it was a tight spot for mud clearance.

Carbon rocker

When I removed the brace from the seatstays I perceived a reduction in stiffness. Instead of the brace taking the scissoring effect that side loads on the rear wheel create in the stays, the rocker now had to deal with these loads. At the rocker the two sides are trying to rotate in opposite directions, the 3 rods linking the rocker halves are not really the ideal engineering solution to this kind of torsional loading. Ideally the two halves of the rocker would be connected by a tube. 

I spent ages trying to design a new aluminium rocker. I looked at a one piece machined part but the cost of manufacture was way too high, it's an incredibly complex part to machine to tight tolerances from a single piece of material. I also looked at a two piece aluminium design either bolted, welded or bonded together. Again the machining cost was looking to be the sticking point plus none of these seemed like the right solution. Bolting requires precise interference fits between the two halves to ensure the bolts don't become loaded in shear during use. Welding would have been a disaster, distortion and heat treatment worries being the main concerns. Bonding could have worked. I figured that instead of machining complex aluminium parts over and over again why not just machine moulds once and make the parts in carbon fibre? So that's what I did. The rocker was a two piece moulding, bonded together with aluminium bearing housings and threaded inserts also bonded in at the same time. The joint was then reinforced with further plies of carbon.

It made no noticeable difference to the stiffness of the frame at all! Sometimes the less than ideal solution is good enough, and this was one of those cases! I swapped between the original rocker and the carbon one and couldn't detect any differences at all, both by flexing the frame by hand and during riding.  The carbon rocker was way more involved to manufacture and it weighed the same as the aluminium one. With no real benefit I decided to just re-work the design of the aluminium rocker to improve it slightly and stick with that for production. There is a lot to be said for ordering machined parts and having them appear a few weeks later in a box ready to use!

Testing

Out of all the one man band frame builders out there I reckon I spend more time than any personally testing my frames.. Two reasons for this 1, My years in the Aerospace industry means I'm paranoid about releasing something before it's ready and fully proven. 2, I love riding my bikes! 

I have ridden my own Contour prototype exclusively for well over a year now, it's covered 2500km and survived a Scottish winter. Bearing life has proven to be exceptional and the original set of pivot and linkage bearings lasted 2200km before I replaced them. The bearings were graunchy but there was no play in any of them. 

Dave has been riding the Large prototype for a month or so longer than mine and also has had no troubles at all. 

The frame has recently passed BS EN ISO 4210-2 which is the industry standard testing for mtb frames.

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