resin infusion and front end chassis

Tuesday 29 January, 2013, 1pm

Things have been a bit dramatic in SE Queensland recently, cyclone Oswald causing unprecedented flooding (in our 30 years here) at Goomburra. The creek peaked at about 7 metres and took huge chunks out of our creekbank, also taking a large chunk of our concrete crossing with it. So mobility is a bit limited at present.

As for pospedal, I’d done a bit of work on it before all the drama.

 I infused the bottom bracket supports, using a complex matrix of plumbing –

This gave me the ability to inject or vacuum from any of the four ports that I set up (three on this face, one in the centre of the back). Infusing from the back to start –

Then switching to each of the front ports in turn. This technique gave me the ability to control just where I wanted the resin to run, and allowed me to vacuum all four ports when the infusion was finished –

The end result was good, I’ll put pics of it on the next blog.

I started playing with some M-board after this, cutting out a panel (14mm thick) to support the front suspension. Strips were routed so that I could bend it (22mm wide for 14mm board) into a U channel –

The wide strips are where the main frame tubes will run. It’s easily bent, with thickened epoxy spread on the joint before folding and clamping –

Wooden blocks keep it square. A jig was made in formply to locate the holes for the suspension pivots and frame tubes in it –

This is the finished jig clamped in place –

The large hole is for the frame tube. A steel washer has been machined to cut an accurate hole for the frame with a router. Top hole is the upper wishbone, lower hole is lower wishbone and left hole is a reference hole for the shocker mount. Cutting the frame tube hole –

And the member after drilling –

After a week off to let my eczema heal, I’ll be back at it shortly. Looking forward to assembling the front end.

Kingpins and Ackerman angle

Tuesday, 15 January 2013, 2pm

I’ve finished off the kingpin/stub axle units. Firstly I had to build a jig to drill the holes in the stub axles to house the kingpins –

At top is the stub axle, below it is the axle cradle, then the spacer I made to get the horizontal distance set precisely. By setting the horizontal and vertical distances precisely I was able to set the angle at the desired 14.196 degrees. Here’s the jig set up in the lathe –

I’ve added some nails to hold it in more firmly. It was drilled out with centre drill, drill and cutter, tricky and slow work. Here are the components of the kingpin after drilling –

The threaded high tensile studs and axle nut inserts (all steel) were screwed into untapped holes, 1mm smaller diameter than the threads. After gluing these parts together with epoxy/nanotube mix (pouring a couple of mils of it into each end of the axle to form a block around the kingpin as well), I made up a jig to set the Ackerman angle of the steering link pivot at the desired 20.18 degrees.

The Ackerman is the angle set so that the inner and outer front wheels go round corners at different angles, the axles aligned to a common centre pivot. This prevents the tyres scrubbing.

For those nerds interested in this (probably not many), I worked out a formula to calculate the angle myself. It goes something like this (angles upper case, distances lower case) –

Firstly you need to adjust for camber. Find the horizontal distance from the centre of the kingpin/stub axle intersection to the wheel axle centre (a) using

a = d cos A

Where d is the actual distance (from the centre of the kingpin/stub axle intersection to the wheel axle centre) and A is the camber. Then find the horizontal offset caused by the wheel camber (c), i.e. the sideways difference between the wheel centre-points at the axle and at the tyre contact point, using

c = b sin A

Where b is the wheel radius, then find the actual horizontal distance (from the centre of the kingpin/stub axle intersection to the wheel axle centre) (z), using

z = a – c (negative camber) or z = a + c (positive camber)

Then calculate the angle of the outside wheel at full lock (B) using

B = 1/tan [ w / (w/tan X + (t – 2z)) ]

Where X is the angle of the wheel from full lock to straight ahead, t is the tread width (distance between centres of front tyres) and w is the wheelbase (centre of front to back axles under load, with no people on board).

After those adjustments, we’re ready to calculate the Ackerman using this formula –

L sin C + L sin (B – C) = 2L sin (X/2) x cos [(X/2) + C ]

Where L is the chosen length between centres for the steering link pivot and C is the Ackerman angle. So X is constant (in my case 40 degrees) and 2L sin (X/2) is therefore also constant. Angles are substituted in for C until both sides are equal. Wasn’t that fun?

So here’s the finished pins –

And here’s how the whole shebang (passenger side) fits together –

This is what’s known as the ‘unsprung weight’- essential to keep it as low as possible to make the suspension as responsive as possible. The kingpin/stub axle weighed 180g, as opposed to 280g for the (smaller) steel item of the Mk 4.

I’ll be infusing the bottom bracket mounts next.

Kingpins and Crank brackets

Friday 11 January 2013, 4pm

I’ve been quiet on the blog, but busy in the shed. I started work on the kingpins for the front suspension. Here’s some components.

The small black (primed) tubes were hanglider tubing (7075 aluminium) and the larger tubes (internally primed) the more common 6061. The smaller tubes will be the kingpins themselves, while the larger ones will be the front wheel axles. I’m using unidirectional tape and two diameters of carbon sleeve. I was planning on using 4 layers of uni and three of the small tube on the inside of the kingpins and 4 layers of small tube and one layer of larger tube on the outside, in two infusions. The problem was that the fibre was packed so tightly inside the smaller tubes that I couldn’t draw the resin through, so I ended up putting all the carbon inside a piece of electrical conduit (16mm ID). The resin takes a long time to make its way through and just made it before the resin jelled. I used a piece of half inch wooden dowel in the centre of the axles, which was drilled out afterwards, to make the axles into tubes.

The axle used 2 layers of small sleeve, 4 layers of uni, 5 layers of small tube and 3 layers of large. Here’s how I pulled the carbon through –

and the dowels –

axles ready for infusion, kingpins ready for failure –

Second attempt at kingpins, ready for infusion –

And the infusion setup with gravity assist –

The trimmed components –

And with the conduit and dowel removed -

The conduit was turned down until very thin and the rest removed with a knife, the dowel was drilled out.

I also started work on the bottom bracket / cross-shaft mount. Here’s a jig I made up using a scrap of 17mm formply –

And the components being glued in place on the frame tubing (once again 7075 from a hanglider – powder coated as it corrodes easily) -

The upper tube is the bottom bracket, the lower the cross-shaft bearing mount and the frame tube (62.7mm OD) has been covered in fibreglass tape. And here it is after bogging, ready for infusion -

I’ve done some fancy embossing and made the distance between the mounting tubes longer, as I realised that my feet were going to hit the cross-shaft (woops!).

I also finished off some components –

And turned up some brackets to hold the frame tubes –

And a dropout for the rear derailleur –

That’s all for now.