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.

New LSD from Titan

My car was fitted with a limited slip differential when I bought it. I've never known what kind of LSD I had, and I've never had any problems with it. One time long ago I asked Caterham what type it was and they responded that according to their files it was a Quaife ATB.

When I took my rear suspension apart to change wheel bearings I took the opportunity to remove the half shafts and look into the diff. It had a cross in the middle, and according to this guide it was a ZF, or Titan! That meant it was a plate type diff, and on those the discs eventually wear out. So I took the diff out. (not very easy I should add)

It was a ZF diff. And it was not worn out. So now I had the options to put it back, refurb it, or replace it.

I decided to replace it with a Titan with different ramp angles. Mostly because I wanted to try something new, and it is said that the Titan is very good. And now when I know how to do it I can play with ramp angles and pre-load if I want.

I did the work myself which a first for me. It was not that hard but time consuming as I didn't want to mess up. I replaced the rear bearings and seals but left the pinion as it were. Tried to set 0.01mm backlash but that was not very easy to get precise. But I got close enough, I hope.

Here is a good guide. and here is the workshop manual.

The original ZF diff had 45/45 ramps and weighted 6691g.
The new Titan diff have 30/90 ramps, carbon plates, and weights 5902g. 40ftlb (54Nm) pre-load.

30/90 ramps equals no locking while trail braking, and heavy locking when accelerating out of corners. 45/45 means equal locking when accelerating and braking. When the rear wheels are locked the car will have difficulties to turn and understeer in tight corners. It also means the rear tyres will have a better chance to transfer power into the ground, both when braking and accelerating.

I'm looking forward try it, but I doubt it will happen before spring.
And hope I never need to take the diff out again.

Update: Two short test drives made. It works!

Rear wheel bearings

I was told the bearings had this number printed on it: "4EO 407 625 A SKF BAF-0076 C ITALY A 23 3 3347". Which after some Google magic indicated they originally belong to an Audi A6C6 2,0 T / 2,4 / 3,0. For us swedes it can be conveniently bought at Biltema for a nice price here. Or of course, the real deal at Caterham parts here.

I've read they're a pity to remove, but for me they came off pretty easily. The first one simply fell off, the other one was a bit harder. I got them off by using the bearing bolts as a puller by screwing them against the brake disc with a impact gun.

The cheaper Biltema version was identical to the existing bearings, except for the inner dust seal being made of metal instead of rubber. Metal seals are probably not as good but the bearings are easy to replace so I'll take the chance.

Unsprung weight

In order to get an estimate of the chassis unsprung weight, I weighted some parts. First I tried to put the wheel on a scale with the push rod disconnected but the result was a bit off (~18kg for the fronts). To be sure I weighted most of the parts individually instead.

Front

Upright + AP caliper + disc + hub assembly: ~10 450g
Wing: 1 725g (incl wingstays etc)
Wheel nuts: 202g
Wheel: 9 360g (Force Racing 7"x 13" with half worn Dunlop slick)
Damper: ? (Bilstein Race)
Push rod: ? (steel)
Rocker: ? (aluminium)
A arms: ?

Sum: ~21 737g + ????

Rear

Hub carrier: 3 595g
Caliper mount: 823g incl fittings
Pads: 369g
Caliper: 1 440g incl fluid and hose
Brake disc + hub: ~5 400g
Wheel nuts: 200g
Half shaft: ~3 000g (unsprung part only)
Wheel: ~10 800g (Force Racing 9"x 13" with half worn Dunlop slick)
Damper: 2 401g incl spring and everything else (Bilstein Race)
A arms: ?

Sum: ~28 028g + ?

3D printed GoPro mount

I've tried various generic GoPro mounts over the years but they all have had problems with vibrations and wind in high speeds, and I've always ended up with securing the camera around the roll bar with insulation tape.

A rough camera mount and a dirty
cage. The cable is to my data logger.

Now I've modelled and 3D-printed my own mount.

I'd love to show a video from when I tested it on track the other day, but unfortunately the memory card was full and nothing was recorded. But I think it will be robust enough.

I'll sand it and put some paint on it some later on.

Bronze bushing

Another try with the front lower rear a-frame bush. Now in oil filled bronze bush, thrust washers and a custom made crush tube. This solution feels rock solid without any play at all! Ahhh inner peace, finally.

So far I've tried standard rubber, Powerflex polyurethane, DIY moulded polyurethane and now bronze.

The upper image is a rendered cad drawing. I'm slowly improving my 3D skills. The image to the right is a photo.

I have an extra set of these parts if somebody would like to buy as a kit. Please contact me.

3D-printed moulds for polyurethane bushes

For a couple of years ago I bought Powerflex polyurethane bushes and replaced the rubber bushes in the front suspension. I was never really satisfied with the small bush in the lower rear a-arm as it felt like they're a tiny bit too small. Also, the design of the bushes does not do a very good job of handling lateral forces. It has always been on my agenda to replace them with something different and now as the play is worse I've decided to take care of it.

There are many ways to make bushes, and I decided to try to mould my own. Partly because I don't have access to a lathe, but mostly because I'd like to see if it would work out.

So I 3D-printed moulds and reused the steel tube from the old rubber bushes. I used some wax as a release agent, and moulded with a 2-compound polyurethane. I let it cure for 24-hours in room temperature, and then 14h in the oven @ 80°C.

The result was surprisingly good and I put it in the car and test drove it. No play and no binding! I don't know how it would hold on track though, but it would probably be fine.

The design of the bush is still not very good because it still doesn't handle lateral forces.

But I have a new plan...

Bad earth troubleshooting

My engine has been a bit "moody" when idling. It can idle smooth, and then suddenly change idling speed. Some of the dash gauges has also been flickering a bit. It haven't bothered me much but today I took my USB oscilloscope and started troubleshooting as I figured it must be electrical. My suspicions were the alternator as it use to haunt me.

But I was wrong. Here is the voltage between the battery negative and the engine loom ground:

I moved the loom's ground connector from a pedal box cover screw, to the engine block. Now it look like this:

The scales are different but you get the point. One could argue that bumps around 50mV won't do much harm but at least now the oil pressure gauge is more stable. It feels like the engine runs smoother too, but that might just be placebo.

Custom moulded ear plugs

I hate ear plugs. The sensation of being inside a bubble playing a video game has got me avoid them except when using power tools in the garage. I ride a motorbike to work everyday, and now when I'm getting older I can feel my ears has been taking a lot of beating over the years.

I've been testing different kind of ear plugs. From simple foam plugs to diy molded plugs. They either leak what can be described as a razor sharp noise, damp too much, or dampen the sound of the engine but not the noise from the wind.

So I decided to get a pair of custom moulded ear plugs and booked an appointment at Hörseltekniska Laboratoriet (Hearing Technology Laboratory) here in Stockholm, who often provide ear plugs for musicians and others with high demands of a good dampening curve. The plugs they recommended are from Bellman & Symfon, which sell a kind of plugs that are suitable for motorsport and that fits inside your helmet.

He filled my ear canal with some kind of foam and waited for it to cure. The silence was almost intimidating, nothing came through. Three weeks later I had the finished plugs in my mailbox.

The plugs are soft like silicone and goes in a long way in your ears. On the motorcycle they're great. I can clearly here the engine, but not the wind noise.

But in the Caterham on the other hand, the engine is just too quiet compared with the wind noise. When I drive with the plugs in at the motorway, I can't tell if the engine is running or not. I have to look at the rev counter to see if it's time to change gear.

As this year is a disaster from track day perspective, I haven't tried them on track, but I can imagine the wind noise is still so much louder than the engine noise, and no ear plug will help me there. I'll probably use them anyway, both for lowering the noise levels and for reducing the sensation of sensor overload.

UPDATE: They work great on track! I feel I even could use a bit more dampening.

3D printed cold air intake

Intake temperature has a direct correlation with engine power. For every 10°C rise in air intake temperature, engine power will be reduced by 2%. Last season I logged intake temperatures over 40°C on a 20°C day, which is really a waste of power.

So I need to duct cool air. I've been googling for different types of air scoopes and naca ducts but prefer not to cut a hole in either bonnet or nose cone. So I decided to try to get some air from in front of the radiator.

As I've written before, my 3D modelling skills suck. But where I work I have access to a 3D printer and wouldn't it be nice with an air duct that goes in the small space between the radiator and nose cone?

I started with a lot of measuring.  I used my kids' clay for the space between the nose cone and radiator, that I cut in pieces and measured. Then I modelled the constraints in the CAD program. I used the loft feature for a nice flow-friendly air duct and started printing.

3D printing is a slow process, and they can only print small objects. I had to split the part in five smaller parts and glue them together with epoxy. Each part took about 8-12 hours to print in medium quality! And it took a few tries before the outcome was good enough.

3D printing, first attempt.
I quickly learned that support stays should be avoided as much as possible.

I also did some CFD analysis to get a design that flowed all right.  I admit it is not perfect, but the first versions was worse...

CFD analysis of an early version

The theory is that the  higher air pressure in front of the radiator will force air into the duct. I've never had problems with high coolant temperatures and I hope it will still will be ok.

The parts glued together with epoxy.

Some filler and black spray can paint. As I didn't want to ruin the existing radiator alu frame I manufactured a duplicate and cut a hole for the duct. When I look at the result I'm amazed that I didn't put just a little more effort to make the end result better looking. But I just wanted to get it finished...

This duct has taken a lot of effort to produce. I've learned the hard way that 3D printing is not a mature technology and have a long way to go. The printer I used was far from a cheap entry level model.

Next step is a better suited air hose and how it will integrate with the filter. I have a temporary solution that works but could be much better. Also some back to back testing and see if there is any improvement.


References:
1. Comparison of Engine Power Correction Factors for Varying Atmospheric Conditions

Rear bump steer

Bump/roll steer check on the rears for the first time, using a laser level on the brake disc and a mirror. I don't know why I haven't done this before. It turned out they (the rears) had quite a lot of toe in bump steer dialled in.

Apparently it is common to set up road cars for understeer and stability by inducing a bit of bump steer - toe in on the rears and toe out on the fronts. On race cars close to zero bump steer is preferred if possible, for maximum absolute grip and less scrub.

I reduced the bump steer by simply switching place of the bump steer spacers and it was significantly reduced. Not quite zero but I leave it there. Now the larger 9mm spacer is up and the smaller 5mm is down - opposite of the top image. (the top image is from the CSR Assembly Guide Supplement document)

I did the fronts for quite some time ago but without the mirror. I must say the mirror method is both simpler and more accurate. The basic idea is to put the laser level on the brake disc and point it forwards or rearwards to a mirror that reflect the beam back to the same point. If the point moves sideways relative to the origin when you raise the wheel - you don't have zero bump steer. Adjust the spacers and repeat.

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.