Rear suspension bike summary

 

 

 

 

 

 For several months, I’ve been making and trialling a new recumbent bike. Its unique feature is a rear fork suspension with rubber blocks. The blocks rest inside the rectangular frame and compress over bumps. This is a new concept, but the rest of the bike – seat, tailbox, front end - is based on front wheel drive leaning trikes I have been riding for years. As with all my cycles, aims with this bike were to be aerodynamic, light and have good load capacity.

The rear fork has no pivot point and finds its own position. The fork stem is from brazed steel tubes stacked as a rectangle.  With 3d printed plastic spacers each side, a pin to hold position, and the rider’s weight, the fork can only rotate in one plane and acts as suspension. The suspension blocks are cut down portions of Mckay industrial dampers. Although other rubber material can be used (pedal rubbers, cycle tyres and tubes), the industrial dampers are designed for suspension of this type, so are stable over a wide range of temperatures.

It’s taken a long time to come to this design as I started building front wheel drive bikes with the rectangular hollow frame that could use it in about 2012. Here is a blog post from back then.

The 2012 bikes and some of the trikes I’ve built since have used Capral rectangular hollow 82.6 x 28.6 x 2.3 wall aluminium beams with a combined tailbox and seat. The tailbox is aerodynamic and has about 50 litres of storage. It clamps to the beam and slides up and down the beam to adjust for leg length. It’s removable and doesn’t need any frame holes or bosses for support or grip. The trick to using this aluminium section is that with the front wheel drive I use, there’s no frame chain stress, so stress is mainly from body weight. This means a relatively deep and thin section of tube can take the stresses and not flex too much.

Bike rear triangle circa 2012

 

2012 bike on Murray tour

The first frames of this type were fabricated by Michael Rogan from  MR Components in Hastings, and they included reinforcements and pivot bosses at the back. But since then, I’ve worked out how to do leaning trike frames at home with some custom cast parts, some 3d printing, handsaws and a drill press.  A plan for the leaning trike is on thingiverse at https://www.thingiverse.com/thing:4201871 . This latest rear fork borrows from the design of the front frame join: body weight under gravity helps keep parts together.

So far the new bike works well and I’m happy with it. For me it takes more shed time to produce than the equivalent leaning trike, but the bike back end weighs less and should have less rolling resistance. A nice modularity is also possible, with the same tailbox and front section accepting a bike or leaning trike rear end. The rear wheel and fork detaches quickly from the bike by removing one screw, then some jiggling. That should be good for fitting the bike in trains or cars.

The bike’s tailbox is still a work in progress, but I am confident I will end up with a light design. At the start my shoulders were too far back, and I’ve made  a temporary wedge that pushes my shoulders forwards but is also a small glovebox for keys, wallet, phone, camera etc.

 

Here are the parts for the back of the new bike and the equivalent parts for a leaning trike.

 

Bike back frame & wheel

Parts: 2 Custom rubber buffers, 1 custom fabricated fork, 4 3d printed guide plates, screws, custom wheel build includes dynamo (4 sets of parts)

Frame drilled one place only

Rear frame & parts weight: 4.6kg

 

 

Trike back frame and wheels

Parts: Unicycle cranks, YST bottom bracket, 2 custom bearing housings, 2 custom wheel axles, 2 standard pedal prix trike wheels from MR components. (5 sets of  parts)

Frame drilled 4 places.

Rear frame & parts weight: 5.2kg

 

 Forces and movement of back wheel

With 2 separate elastomers working at the same, the rear wheel dynamics aren't immediately obvious.  But each elastomer can be considered a pivot point  as well as compliant suspension, and when this is done some motion / suspension equations can be worked out. In the diagrams above, the top line is the resultant of the two lower motions. The simple version has equal spring rates and distances, and the general version has unequal distances and spring rates.

 

 

Regards

Steve Nurse