Das Beast: My E30 track / street build

[dvallis]

This is a wheatstone bridge. Something you will rarely encounter when messing with E30s, except we need one in this case.

The linear pot is the equivalent of a strain gauge. It exhibits small changes in resistance when measuring the physical property of interest. Wheatstone bridges are the signal conditioning circuit of choice for these applications.

Why? It's not because of sensitivity. If you look closely the bridge only produces about 150 mV/in of voltage change. Same as the simple voltage divider. Difference is in DC offset. Resistor divider has a large DC component, on top of which there is a small DC change. If you put that into an amplifier, the DC offset will just drive it to the rails. Example: If R1=2500 and the pot equals 2500, Vout is 2.5V. Amplify that by more than 2X and the output saturates.

However, the bridge can be balanced, such that when R1=R2=R3=Rg, DC output is zero. This means we can put the output into an amplifier and get more gain on the signal, i.e. get better measurements. We'll use a gain of 6.

The AIM system accepts 0-5V analog input. By setting R1=R2=R3=2K the bridge will balance out at a six inch draw on the pot. With an "at rest" pull of nine inches, we'll get three inches each of compression and rebound within the 5V range.

NOW I can build the damn thing. :devil:

[ForcedFirebird]

I would still make that loop on the switch not "looped and make 2 runs to battery. Perhaps over safe, but anytime I've ever looped them, they cause issues. All battery terminations at switch "should" be isolated IMO

[dvallis]

Not seeing the issue. That short loop is on 1/2" copper posts with hydraulically crimped solid copper lugs. Same as a #2 direct from starter to battery. It makes those two posts a copper bus bar. Per ETM stock E30 also makes direct connection from starter to battery, avoiding inrush current through any switch or relay. Rest of car systems are electrically isolated though the 175A rated DPST switch.

[ForcedFirebird]

Loops are loops. Anything not isolated to the battery post is in danger of RMF. You have to remember there are dynamics in a running engine and back feeds. Not challenging the thought, information, or logic, just the final results of the system.

See my previous post about the client car and the 5v USB charger he installed ;)

Nothing worse than showing up on race day and failing tech....

[dvallis]

If it fails tech I'll let you know. LoL

[dvallis]

Building this wheatstone bridge amp for each of the linear sensors.

[Nader393]

You're doing some interesting things here. I think this thread is a bit lost in this engine subforum. Should really be moved to the "member rides."

I'm a little concerned about using metal conduit for your electrical runs. It's fine for a house, but a car is vibrating, leading to chaffing and electrical shorts within the grounded conduit (impossible to track down when electrical gremlins arise). I'd use plastic, or leave them bare and zip-tied every few inches, and clamped to the chassis with rubber Adel clamps.

Here's a general hint, having built a racecar myself: For ultimate durability and safety, look at aircraft suppliers for bits and pieces (switches, relays, etc). I've also learned that it pays to spend extra on name-brand AN fittings, to avoid leaks in the future. I've had to go back and replace hardware store brass fittings and cheapo AN fittings with the real deals, because of leaks.

[dvallis]

So, good news and bad news.

Good news: I was able to create a custom sensor in AIM Race Studio, modify the dash layout and get data in through the analog channel. I'm driving I-BAT with voltage from a power supply, simulating the current sensor output. The other two are floating inputs.



Bad news is the 5V reference voltage output on each analog channel has pitiful current output. It was not able to power the Hall Effect sensor. Will have to build a DC-DC and cut it into the cables.



No big deal really. It's just the guts of a USB car charger.



Also modified the suspension sensors to add the wheatstone bridge amp. Will be building them this weekend. Amp only draws microvolts, so the 5V reference should be able to power it.

[dvallis]

Fuel system fully installed and pressure tested. One step closer to engine install.

[dvallis]

Considering a side exhaust to make room for the diffuser. Something similar to this except exiting through the rocker panel on passenger side to avoid fuel lines.



Good side exhaust execution. Might do this with a removable turn down tip insert to meet 103 db at 50' if necessary.

[dvallis]

Cluster wiring complete. Final fiberglass on the dash is next, then flocking, mounting the cluster and we're that much closer to go time.

[dvallis]

Spent today doing metal fab. Takes longer than you'd think with measuring, cardboard templates, cutting, filing, drilling, riveting, test fitting, etc. Also, nothing is at right angles.

Here's the dash ready for bondo and sanding. Maybe flocking tomorrow.



Replaced the end bit that got chopped off during my overly enthusiastic fiberglass fab. LoL. That was a while ago. Turned out nice. Won't know the difference once it's flocked.



Finger brake paid for itself again today. Was able to do a nice pan cover for the switch panel.



Looking good. Will get pained flat black to match the switch panel.

[wworm]

Side pipes are cool. Cant wait to hear this thing...cuz I plan on peer pressuring you into posting exhaust/burnout clips lol

[Digitalwave]

Don't most rule books (NASA/SCCA) require bulkhead fittings for fuel lines when passing through sheetmetal?

[dvallis]

NASA no rule. SCCA only applies to fuel lines passing through cockpit which must be solid tube or braided hose. We don't penetrate driver compartment.