There are two lengths of swingarm on the Tonti frame – 390 and 470. My donor bike is a later model Cal EV, so it has the longer swingarm. Because of this additional length, the stock shocks won’t work given the upper mounts on the trike, so we had to do some geometry to figure out what to do. The primary objective is to ensure that the shock is traveling in the middle of it’s stroke under most conditions, when loaded. The stock shocks and springs given the new geometry would be operating over-center and with the wrong height. So something has to change.
Quick CAD assignment to work out the new length:
That works out to 15.5″. A quick scan through many shock providers, landed me at Progressive Suspension who has a gobsmacking selection of shocks and they publish their tech specs online. That led me to these:
https://www.amazon.com/dp/B000UKGJU0/
UPDATE: These units have far too much damping – the length and spring-rate are right, but the excessive damping causes the rear wheel to skip and crash. I’ve added a new post in October to outline the update made to fix this. All the rest of this post remains valid, however.
Fire up the lathe to make some inserts to adapt the upper and lower eyes to 12mm.
You will notice in this photo that the cross bolt at the top is carrying load on the threads. This is of course, wrong. I was only using the bolt that was handy to hold the suspension. Proper length bolts will have the shank (unthreaded) portion of the bolt body carrying load across the “ears” of the upper mount. This avoids carrying shear load on the thread.
Here you can see the before and after – left is the stock shock, right is the Progressive shock. The angle is quite different. One could imagine that, once fully loaded, the stock shock would be operating on the wrong side of horizontal. The Progressive side “looks” about right.
