Brake pressure sensor

Logging driver inputs is a way to get quicker, they say. I don't doubt that for a second. For me though, more frequent driving would probably be more effective. But that didn't stop me from putting in another sensor in the car.

When I actually do drive my car on a track I often find the lack of consistency very frustrating. I want to know why I'm faster in the morning than in the afternoon. I could look for excuses such as track temperature etc. Or I could stop fooling myself and start looking closer to the steering wheel. Can my braking be improved? (yes of course) How and where? That's what I want to find out. Also, in order to make sense of the other data collected from the car's sensors I need to know what the driver was doing.

"Aggressive application of the brakes is something that separates good drivers from average drivers. Most driver inputs to the car should be smooth, [...] but in big braking zones a driver should be very aggressive with the brakes." - Optimum G.

"Some of the most useful information to be had concerns driver performance and coaching. Braking is the one aspect of car control that goes against the accepted wisdom of smooth driver inputs."- Competition Systems.

Pressure sensor

For measuring brake pressure you need a pressure sensor in the range to about 200 bars, preferably with a DIN/ISO brake fitting. As there are plenty of production vehicles with brake pressure sensors you can find them at a decent price on eBay. I bought a second hand Bosch 0265005303 for about €25. The tech sheet was a google away and the electrical connector was available from Conrad.com or Farnell with part numbers:


Tyco PSA 2x3 pol
967 642-1 housing
965 907-1 contact pins
967 067-1 seals

1. GND
2. Output
3. Supply voltage +5V

The sensor was connected to one of the analogue inputs on my DL1 data logger.

Brake lines and flares

I installed the sensor on the front brake circuit right after the master cylinder. I made a new brake line with a tee coupling for the sensor. It was not hard, but a bit messy with the brake fluid dripping.
 
The standard Caterham brake lines are 3/16" with an ISO bubble flare. The nut on the master cylinder side is M10x1. On the front left/right tee split, Caterham thought it would be nice to use a UNF 3/8" thread instead.

There are at least three common types of flares and it seems to be massive confusion about ISO/DIN flares. Note that the DIN flare is flat on the underside with a matching nut. With some patience, you can make flares yourself using a simple €20 tool available from almost everywhere. Youtube is your friend!

Front suspension travel sensors

For a while I've had suspension travel sensors on my rears. Now finally I have sensors on the fronts as well.

The rears with 3D printer mounts.

Variohm PZ12A 75mm linear position sensors. (100mm on the rears)

This time I didn't have access to a 3D printer so the brackets were made of alu sheets.

The lower mount is a 90 deg bracket fitted to some existing unused threaded studs on the frame, right behind the steering rack. I have no idea why the studs are there in the first place. 

The upper mount is hold in place by the roll bar linkage ball joint.

Damper position sensors

I've written before - I have a thing for measuring things. I also have a new set of adjustable Penske dampers and wanted to see how their behaviour changes when turning those adjusters.

So, I've bought a pair of position sensors and made 3D printed suspension mounts for them. So far just for the rears but plan to do it all around.  They're wired into my data logger.

For reference - rear right.
The same five runs with Mid/mid setting.

Rear left, five runs with different settings.

I've made an attempt before with strain gauges on the pushrods. I'm looking forward to combine the two methods and see what that gives.

This weekend I got the chance to do a few test runs. I drove the same piece of road five times and adjusted the damper on the left rear between each run.

The X axis is the damper velocity (input voltage derived by time). The Y axis is the probability.

There are theories that the best suspension settings are when the diagram is symmetric around the zero. Regardless if you believe those theories or not, one can note that the rebound adjuster moves the curve sideways, and the compression adjuster mostly changes the peakiness of the curve. The Mid/Mid setting, which is the default setting from Penske, is very symmetric and that is probably a good sign?

The usefulness of this can be discussed but at least this gives me something to look at, as I don't trust my butt-dyno at all. 

Pushrod strain gauges

As I have a thing for measuring things, this summer I did an attempt to put a strain gauge on one of the push rods. A strain gauge is like a thin film with an embedded wire. You can glue it on objects and when a force is applied to the object, the electric resistance of the wire changes and from that you can measure the strain. Strain gauges has many applications, such as measuring the weight on a crane or the torque on a shaft. Or for the electronic bathroom scale.

At first the plan was to glue the strain gauges on the springs instead of the push rods. I bought one car spring with the lowest price and lowest spring constant and started experimenting on the kitchen table. I used a cyanoacrylatec glue (like super glue) for the gauge and put quick epoxy on top as protection.

Strain gauge pair glued on a spring

OP amplifier experiment

The resistance change from a strain gauge is very tiny so it has to be amplified a lot. I used a Wheatstone bridge and a 100x amplifier. It worked as a charm. The output reacted nicely even with small loads on the spring.

I drew a small circuit board and ordered it from a Chinese PCB manufacturer. Amazingly easy these days, and costs like $15 for five boards. One board contains two amplifiers so two boards would be necessary for logging data from all corners. I used to work with doing things like this, but it was a long time ago and my soldering skills are not what they used to be. Neither are my eyes.

PCB board with dual amps, bridges and power regulator.

I decided to do a test by putting the strain gauge on one of the push rods instead of the springs (see the top picture) and wired the output to my data logger.

Steering back and forth on the road.
The lower signal is from the strain gauge.

It worked!

Still, as the push rods are quite thick the signal was very weak and contained lots of noise compared to the signal I got from the springs.

Now the question was, what to do with all this new data? 

One of the goals was a way to measure aerodynamic lift. The other was if I could find any undamped suspension frequencies. I took a drive on the motorway when there wasn't any traffic.

I started to play with Matlab. I don't know much about signal processing, or Matlab, or even maths in general. Only having one signal instead of four also made things harder. I did what I could to compensate for weight transfer using the logger's accelerometers and got this graph which with some biased imagination could confirm the front end lift. 

Speed vs front downforce (in this case lift)

I also tried to do some FFT analysis but couldn't get any interesting results from that.

Then I took the car to a track day.

Blue = track day, red = road

This is a histogram of the strain gauge signal derived over time (ie the rate of the signal change). The red bars are from a calm road test, the blue bars is from a track day session.

High end data acquisition systems (such as Motec) have this kind of analysis functions built in for suspension travel sensors. The theory is the histogram should ideally follow a certain shape, and be symmetric along the centre. As this signal is from the pushrod and not suspension movement, I assume a low rate of change is a good thing as it reflects the tyre's contact patch. I don't fully understand what conclusions I can draw from this.

So what have I learned?

Well, not very much new from the gathered data. Also that I'm good at starting projects and quite bad at finishing them. But I knew that as well.

Still a very fun project.

Oil temperature sender

I'm reading a book right now that among other things has a lot to say about oil. One of the things it says is:

"An engine should never be driven hard until the oil reaches 70°C". 

And later:

"In a race engine, bearing failure is a possibility any time the oil temperature goes past 130°C, and generally hp is lost over 115°C".

That is interesting. The first quote goes without saying but the second one makes me concerned of something I've neglected.

The CSR already has a 1-pol oil temperature sender on the oil return line from the dry sump pump, but it goes nowhere, at least on my car. Now when I have this fancy logger I can easily log the oil temperature and display it on my Dash.

As the sender is 1-pol I used a 2.2kΩ pullup resistor to the reference +5V. So far so good, now it only needs to be calibrated.

Right...

As I'd already done the wiring I thought I do it in the garage right next to the car. I put some engine oil in a stainless steel cup and heated it on my camping stove while watching the voltage on the dash and the oil temperature using a kitchen thermometer. The idea was that it would give me real world values despite all tolerances and the wire's resistance. The problems were that the oil temp sender did respond much slower on temperature changes than the thermometer. Also the temperature differed very much between different locations inside the cup. I ended up heating in steps of about 20°C and then waiting a few minutes until the temperature has settled before reading the voltage and temperature. (No, the smell of hot engine oil was not very pleasant). Maybe a better approach would be to measure the sensor's resistance in the oven instead.

°C	Volt
11,7	3,260
37,6	1,882
57,7	1,070
79,2	0,600
97,7	0,358
116,5	0,204
136,5	0,126

I entered the values in the Analysis software that came with the logger and it gave me the formula:

-12,2 * x^3 + 74,2 * x^2 - 155 * x + 149

It is quite cool that the logger can calculate this real time and display it on my dash! But is it accurate?

Well not quite. It will give me a hint within maybe 5-10 °C and maybe that is enough. Regular interpolation would probably give better results. But why stop there?

Steinhart–Hart equation

(aka I got too much time because it is holidays and the children are sleeping)

The Steinhart–Hart equation is a model of the resistance of a semiconductor at different temperatures. The equation is:

After testing some different values I found that A=1,47E-3, B=2,40E-4 and C=8,77E-8 gave me a curve that looked more like my measured values.

Logarithmic chart of temp vs voltage.

Temp in °C = 1/(A+B*LN(R)+C*(LN(R))^3)-272,15
Where R = V*2200/(V-5)

So the final equation I use for my DL1 logger is:
1/(0,00147+0,00024*LN(-x*2200/(x-5))+0,0000000877*(LN(-x*2200/(x-5)))^3)-272,15

I think it works.

Race Technology DL1 + Dash4Pro

Early Christmas this year when I got myself a Race Technology DL1 Club logger and Dash4Pro dash to go with it.

I've been using a Racelogic Performance box for many years but always longing for a system like this. One big reason for finally upgrading is to be able to see engine performance counters while driving. It can also show alerts when one or a combination of signals are out of range.

It can also show predictive lap time on the fly, and one side of the shift lights can be programmed to show time slip rate so you immediately can tell if a line is faster or not. It also syncs all logged data with my old GoPro camera and can play back the video inside the analysis software.

The display can be set up to display four screens with any of the parameters available. That means all CAN bus data from the ECU as well.

I intend to add a few sensors in the future, such as steering wheel angle and brake line pressure. If feeling extreme I could add wheel speed sensors and suspension movements. The throttle angle I can already get from the ECU.

Instead of using the supplied bracket I drilled two small holes in the steering wheel to mount the display. I wired the power from the windscreen wiper switch so I can turn the system on and off easily.

I'm really looking forward to try the system out this spring!