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.

Airfoil behind roll cage CFD analysis

Does an airfoil generate downforce when placed behind a roll cage? Since I'm starting to get along with the CFD analysis software I did some more simulations.

The airfoil is a NACA 2312 at 160 km/h with 20° angle of attack. Length ~170 cm, width ~20 cm. As I suck on 3d modelling, none of the models correspond much with the real world. Neither the results probably, but can give an hint of what you could expect.

Without roll cage

Scenario 1 - No roll cage, airfoil 40 cm above trunk
Airfoil downforce: 791 N
Airfoil drag: 282 N
Total downforce: 1810 N
Total drag: 1230 N

40 cm

Scenario 2 - Airfoil 40 cm above trunk
Airfoil downforce: 542 N
Airfoil drag: 174 N
Total downforce: 1482 N
Total drag: 1296 N

50 cm

Scenario 3 - Airfoil 50 cm above trunk
Airfoil downforce: 857 N
Airfoil drag: 268 N
Total downforce: 1742 N
Total drag: 1438 N

Scenario 4 - No airfoil
Total downforce: 738 N
Total drag: 1051 N

Conclusion:

A wide airfoil behind the roll cage does generate downforce, but raising it just a bit increase the effect dramatically. To no surprise, the outer parts of the wing are the most effective regions. Real world experiments are necessary to find the optimal location.

Engine bay undertray aerodynamics

Some time ago I started to do some CFD analysis to see if I could do any simple changes that improved the aerodynamic properties of the car, without altering its appearance too much. It turned out that I had too much faith (or more probably - too little knowledge) of what the CFD software could provide me, and after putting in a great deal of work I finally gave up.

It is said that from an aerodynamic point of view, one of the most important features of a car's body design is how it looks underneath, and even if my previous CFD adventures failed it pointed me in the same direction. To fit an engine bay undertray is not very complicated and doesn't affect appearance much, but before I started I wanted to confirm if it would improve things or just be a waste. This time I made a much simpler 3d-model of the car, and had much lower expectations of what the CFD software could help me with.

Standard under body

The results should be taken with lots and lots of salt!

Standard
Drag: 879N
Downforce: 427N

Undertray only
Drag: 793N
Downforce: 455N

Undertray and skirts
Drag: 809N
Downforce: 590N

Undertray, skirt and splitter:
Drag: 858N
Downforce: 604N

All calculations are in an air speed of 160 km/h. The standard design does generate a bit of downforce, because the car body is at a small angle compared to the direction of the air flow. The ground clearance is 15 cm, minus the engine oil sump. The design measurements are just guesses as I didn't take time to go out to the garage and measure. (it does look a bit short). No rounded edges. The body is solid on the upper side.

An underdray reduces drag but doesn't give much improvement in downforce. On the other hand, extending the undertray with a 5cm skirt improves downforce quite a lot. I also tried to add a small splitter in the front and see if that made a difference (see top banner image) but the small gain in downforce is not worth the effort.

One apparent issue with fitting an undertray is of course heat. I have a plan for lowering intake temperatures (more about that later), but if the heat from the exhaust primaries will be just too much and start to melt things I don't know.

Caterham has a ready made undertray on their website, but I've already bought a sheet of 1 mm aluminium and started to cut out the under tray. I'll post more when (and if) I finish it.

Rear diffusor and front splitter inspiration

I more or less gave up on my CFD simulations. At least for a while. I could not get consistent results and the obstacles were just too many to keep me carry on. But before I gave up I did some simulations with an engine bay under tray and a rear diffusor, and according to the simulations it did quite a big difference on both drag and downforce.

Today I received some inspirational pictures from Paul Cardy on his previous CSR. I think I'll try to do something in the same lines.

Paul writes:

"The diffuser was attached to the chassis using these brackets with threaded inserts as per picture 1, the rear end of the diffuser was held using these hanger brackets in picture 2, and the main body was held in position using P type clips with steel surrounded by a rubber sleeve and clamped round the chassis tubes. 

The diffuser was made by Freestyle in fiberglass and I had him glass in some carbon on the last section for the look. I made up the front mounts and cut up the diffusers to fit as this was the first ever to be fitted to a CSR so its was a make it up as you went along. 

The front splitter is a sheet of carbon cut to shape its fixed to the chassis with the P clips again. The front upright section is aluminium folded and cut to follow the underside of the nose cone and fixed to the splitter its wrapped in black vinyl."

I also would like to cover the engine bay. But before I do that I must take care of the already high intake air temperatures. But that is a different subject.

Aerodynamics - CFD simulations

After some vacation reading of the book "Race car aerodynamics" I have had lots of thoughts of how to improve the car's aerodynamics, especially the lack of down force.

The common approach on this subject is that the seven is a hopeless case and don't even bother. That might be true, or could it be the other way around? Since it does have the aerodynamic properties of a brick (or worse) - even small mods can make a great difference?

One problem is that you don't want to change the classic look of the seven. That is also true for a majority of seven owners. But I believed something like a flat underside could make a big difference. You can also add removable elements that you only have on the car when you're really going for it.

The question was, how much can be gained, and is it worth the effort?

Autocar did an article a long time ago (?) about wind tunnel testing of a Caterham. You can find it here:
#1 #2 #3

Some time ago I did some simple simulations of how much some downforce would affect lap time. The result was that 160N more downforce @100km/h would shorten the lap time with over a second! As a reference, F1 cars generate more than ten times of that, at the same speed. (source: the book mentioned.)

Wind tunnels are not accessible for most of us. At first I thought I would do road testing with string potentiometers measuring the suspension compression and connect it to the data logger. Straight roads are not that easy to find where I live, and going 200+ km/h on public roads are not that great either. The number of variables are endless and if something works or don't work I probably won't know why.

The book mentioned above briefly talks a bit about CFD - Computational fluid dynamics - and concludes that it is very expensive, complicated and not something for others than high end race teams. Well, time has passed since the writing of that book, and now the CFD software is not that hard to use and 30-days trials can be downloaded over the internet from many different software companies.

It turned out that the book was quite right. After experiments I now understand why top race teams still use wind tunnels. CFD simulation is very complicated, and it takes lots of effort to get accurate results. But for rough estimates it could still be very useful!

Baseline

So I made a rough simplified CAD model of my car. It may sound easy, but for a complete 3D modelling newbie it took quite a lot of time learning.

I started with high ambitions, but after a while I lowered them just to get some results at all. In these simulations the wheels are not rotating and the ground is not moving.

@160 km/h
257N front downforce

1935N drag

Note, these are rough numbers. 

I use to complain about front end lift when I drive, but this is pretty much the opposite. 

Anyway I plan to do some changes to the model and see how it affects the output. I also plan to investigate some individual parts in detail with finer meshes and higher accuracy.

Nose cone aero whiskers

Mega downforce nose cone whiskers for less understeer in fast corners. These beasts will add at least 50g of downforce even at low speeds.

I glued them on with melt glue until I was satisfied with the positioning.

Then I drilled three small holes through each whisker, removed the glue and pop riveted them in place.