Braking

My wife recently bought a Pashley Picador tricycle. It has two rear wheels and one front wheel; the same arrangement as a Reliant Robin; and is about as stable in the corners as…a Reliant Robin. It comes with two brakes as the law requires: both operate on the front wheel.

Braking a single-front-wheeled tricycle significantly enhances the cornering instability as the forward weight-transfer very rapidly puts the centre of gravity outside the triangle of the wheels. This is exacerbated by going downhill as that also moves the centre of gravity forwards; and when are you most likely to need to slow down? That’s right, while descending a slope.

Most of these attributes I can do nothing about, other than waiting for Cerys to get bored of the cuts and bruises and switch to a proper bike where you can at least lean the whole thing so that you have some control over the dynamics in the corners.

What I can attempt to improve is the efficacy of the braking systems. One of the brakes is a drum-brake which is quite good at stopping the trike and has a useful latch to lock the brake on, so that you can use it as a parking brake. The other brake is a large-reach single-pivot calliper brake which probably wouldn’t squash a fly if it sat betwixt pad and rim while you hauled on the lever.

We’ve removed that brake entirely, despite the law, as it was too ineffective to do any good if left on the trike. I would have replaced it with a modern dual-pivot calliper brake but they are not readily available with a large enough reach. Our plan is to replace it with a Magura HS33 hydraulic rim brake which will require retrofitting cantilever brake bosses to the fork somehow. We could have used a medium-drop dual-pivot calliper brake with a drop bolt but fabricating a drop-bolt seemed as hard a job as retrofitting brake bosses (possibly harder) and the braking performance would not have been quite as good.

We will attach a horseshoe-shaped frame to the existing brake mounting point at the crown which will hold the brake bosses in place and transfer some of the braking load to the existing brake-mounting-point. The ends of the horseshoe will be clamped to the tines of the fork. This frame and the two extra frames which are part of the HS33 brake system will handle the outward-pushing resultant force from braking against the rim without transferring it to the forks: I did not want to subject the forks to a tine-separating force they were not designed to cope with. I am imposing a new forward-pushing force on both tines, at the point the ends of the horseshoe frame are clamped-on; I do not think this will exceed the load applied to the same parts by use of the existing drum brake, so it is not a concern.

We chose to design and fit a frame because the alternative methods of welding, brazing or soldering bosses onto the existing fork would require repainting the fork: at the moment it matches the frame very nicely, and we felt it was unlikely we would be able to do as good a job. A secondary issue is that the bosses need to be 80mm centre-to-centre and the inter-tine distance is approximately 90mm at that point, so it would have been a non-trivial job.

I designed the horseshoe frame in OpenSCAD and have ordered a print from Shapeways in White Strong & Flexible. When it arrives we will see how it looks/works, refine the model as needed, possibly reprint it in plastic to confirm the redesign if the differences are significant, and finally find a supplier who can mill it out from an aluminium block. Obviously one can design shapes which are printable but not millable; I am taking care not to do that. If Shapeways add a wax material before we get to that stage I will consider using that to create a positive for a lost-pattern cast final part.