Geometry Adjustment & Setup

A project log for SECApocalypse

Yamaha XJ750 "SECA" with adjustable Hossack front end and Mad Max style

sebwierssebwiers 05/22/2016 at 04:300 Comments

TCMaker was having an open house exhibit today, so I bolted the bike up for display and took the opportunity to measure the actual component dimensions and make adjustments to get a geometry I hope will feel good for test rides. This also gave me a chance to nail down the actual "as built" dimensions to enter into the motorcycle geometry program I'm using.

It turns out that as I had it set up at Maker Fair last week the trail was only about 2". I probably didn't notice that pushing it around / coasting down a hill because of the low speeds. This is actually a good thing - the design makes it very easy to increase the trail, but to reduce it I'd have to make some clearance cuts. Once I'd made the needed bolt adjustments, the wheelbase came in exactly what it was on the stock bike, and the front tire fit nicely snug against the shock.

The program I'm using was written by Tony Foale. It predicts various handling characteristics of a wide variety of "Funny Front End" designs based on user inputs for relevant dimensions and bike weighting. Using the software in the design phase was vary helpful, as it allowed me to determine what dimensions (roughly) I needed to shoot for, and showed me which ones would need to be carefully controlled to get the behavior I wanted, and which ones had little impact on suspension behavior (and so could be tweaked to meet packaging / structural requirements). Here's the setup screen and predicted kinematics for the geometry I have dialed in in the picture above. In a standard motorcycle magazine, the relevant stats would be as follows.