Build #2, No huge car progress...but...cool new toys! Options

[jd74914]

Hello all.

I figured I’d throw up a progress thread for my car’s second rebuild. For those who don’t know me my name is James and I’ve been a 914 addict since age 13 (now 26) and used to hang around here a bunch more. It seems like a good start to this thread would be some background information. Its first build took place when I was in high school (age 13) and ended as a freshman in college. When I originally purchased the car it didn’t run and had some pretty serious external rust problems. I rebuilt the motor, fixed tons of electrical issues, replaced all of the rotten metal with new (all hand-formed since I didn’t have the money to pay for reproduction pieces), and repainted. Everything was done in my garage with the exception of turning/balancing the flywheel and I learned how to MIG weld and paint from my dad, some books, and through a lot of practice.

After reassembly, it was my daily driver for 3.5 years during my undergrad degree. I pretty much drove it hard and put it away wet for the entirety of these years and it never saw a garage. Something about getting a mechanical engineering degree, dating a few girls, working throughout the year as a design engineering intern, and finding FSAE cars really limited the amount of time I spend on my own car. By the end of my undergrad degree there were some pretty rough spots, mostly in terms of the suspension/brakes and a pesky ignition switch (replaced 3 or 4 times and it kept failing), which pushed me to taking it off the road and fixing everything correctly. I thought it might take about a year-that was 3.5 years ago! (IMG:style_emoticons/default/wacko.gif)

Just like after the first rebuild, life got in the way and the car sat as I went through a master’s degree, worked full time, continued to play with FSAE cars, and starting working on friend’s real racecars. Now I’ve finally finished my MS (and know way too much about fluid dynamics and heat transfer (IMG:style_emoticons/default/laugh.gif) ), am applying to schools for a Ph.D., still haven’t stopped [advising] FSAE design, and really want to drive her! The play was to start and finish rebuilding the suspension last summer (I saw Chris Foley-Racer Chris in the grocery store one day and told him this), but I got carried away and a bit behind. This thread is to chronicle the build back to the road.

We’ll start with a few pictures from when it was originally completed in 2006/7 (well, it's missing the plates and still has the original windshield but...).

[jd74914]

Over the course of last winter, I couldn't stop thinking about lowering the rear of the car. Unfortunately, I wasn't comfortable dropping the roll center super low just by cranking down on a set of threaded spring perches without making any kinematic changes. I actually drew up both the front and rear suspension in a kinematic software I have access to and looked into modifying the pivot points, etc. Through this modeling I decided to raise the rear trailing arm mounts (just as a note changing from trailing arm to multi-link suspension was considered, but in the end I decided to stay semi-trailing arm since an suspension genre switch is really easier in a full custom car).

Additionally the car needed new bushings. As this project was moving more into the realm of questionably street-able I decided to try something different and switch to spherical bearings in the rear pivots. I'm still a little worried about how they are going to work, but my rationale was that in the worst case I would just throw away my modified arms and cut the inner pivot out of the car for replacement with something a different. The cost to change is pretty low so I'm not too worried; the most disappointing part would be throwing away the machining necessary to make the new "pivot shafts."

Just for size reference, the sphericals have a 3/4" bore. I went with this size because it most closely matched the size of the existing pivots. I don't remember the exact numbers, by the factor of safety for the spherical (radially-loaded) is something like 8 if the car experiences a 3g bump, 2g brake, and 2g lateral load all at the same time with the entire weight of the car on a rear wheel. (IMG:style_emoticons/default/laugh.gif)

Anyways, the next pictures show a little suspension modeling (no real kinematics unless some really wants to see plots) and then the trailing arm inserts. They were machined as such because I did not want a stress concentration for the bolt at the trailing arm/spherical interface. As designed, the bolt shank goes through the spherical and into a flat in the trailing arm and the the threaded portion starts. The tube everything is combined in gets pressed into the trailing arms and the edges welded. I don't have pictures of the part but it all went pretty smoothly.