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. 

Penske dampers

When I bought my car it was equipped with Caterham CSR race spec dampers, non-adjustable Bilsteins. They're now 10 years old and not serviced once, and therefore I now think I finally got an excuse to replace them. They've always worked great but I've always been longing for adjustable dampers. Also, my rear droop travel has been limited.

After being in touch with different damper manufactures I chose a set of double adjustable dampers from american Penske - their 7500DA. The reasons I chose Penske were:

• Good reputation, high end dampers
• Very helpful and service minded
• User serviceable! And equipped With Schrader valves to allow control of nitrogen pressure.
• Affordable (but certainly not cheap)
• Custom built for my car, not an off-the-shelf product.
• Plenty of information about their products on their home page.
• Black and gold... What could possibly go wrong??

I was disappointed that the Swedish Öhlins couldn't help me. A lot more expensive (about twice), and still they couldn't tell me anything about their products that made them better than the others. In fact they couldn't tell me much about anything at all. They were not the least helpful. 

I sent them information about the car and let them decide what would work best. It took some trial and error, a bit of thinking and a mechanic workshop for the mis-alignment spacers before the dampers was finally mounted on the car with the adjusters accessible. Now I got adjustable dampers with a lot more droop travel without sacrificing bump travel. Not sure what it will do on the lap times. I guess it will depend on myself and how I manage with the setup.

Damn these salty roads. I can't wait until spring!

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.

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 + ?

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...

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.

DIY Cornerweight

I've been looking for corner weight scales for rent at a reasonable price for some time now without success.

At the local Biltema store 130 kg analogue bathroom scales was only €5 each, so I bought a complete corner weight rig consisting of 8 scales for less than what it would cost me to rent racing scales a few hours. Not as accurate of course, but I thought that hopefully they was accurate enough and I can do it any time I want.

Well, they weren't.

When I was standing on a scale, my body weight shifted +/- 10 kg depending on if I was standing on my toes or on my heels.

First I tried with a piece of plywood to even out the weight, but it did not help much. I got the same readings without it and easier to handle.

I repeated the measurements multiple times and the readings was a bit more consistent than I expected, within 10-15 kg. Still not good, but maybe enough for a very rough corner weight adjustment.

Anyway I don't trust them, even for my own body weight.

Front damper spherical bearing play

There was a play in the joint between the rocker arm and damper in the front suspension, noticeable when I jack the car up, removed the wheel and lift the suspension at the hub. I decided to change the spherical bearings.

• First step was to remove the dampers. Remove push rod from lower wishbone. Undo damper bolts starting with the lower bolt.
• The misalignment spacers on one of the dampers was tough to remove, but eventually they got out with some help from a blow torch. Warning! Heating bearings could give off hazardous fumes. These fumes can be harmful to the eyes and to the lungs, if inhaled. Source: SKF.
• Remove retainer circlip with a pointy sharp tool.
• Carefully remove bearing using a socket and a hammer.
• Bilstein part number E4-B46-610. Can be obtained from Caterham Parts or directly from Bilstein. You can also find similar bearings at your local bearing shop in different qualities. 
• When I fitted my new bearings I noticed a small play between the new bearing and housing. I used Loctite 648 retaining compound as glue. I don't want any movement there, or the shocks could easily be ruined. I hope the new bearings will outlive the shocks, as they probably is going to be tough to remove in the future. (Heating them might work)
• Reassemble. A good opportunity to replace the bolts and nuts as well. 

Note: I have the "race spec suspension" on my car. The procedure and part numbers may be different on the road spec dampers.

I changed the upper bearings on both sides as I thought that would be enough. Now when the suspension is reassembled I can still feel a small play in one of the lower bearings, sigh...

Replacing front bushings

I've noticed that my front wings move quite a lot when I brake, and that have become worse over time. I also have a brake judder on the front. My suspicion was the rubber bushings on the front suspension.

Working on cars are fun fun fun

Removing the old bushings was a real PITA. The worst thing I've done on the car so far. On the CSR the rearward lower wishbone bushings are located in the chassis in a very tight location. The front bushings are in the removable wishbones and a bit more manageable. But to remove the ones in the chassis takes lots of swearing and bloody knuckles.

I tried to press them out with a threaded rod, nuts and washers with no luck. At the end the only thing that worked was burning out the rubber with a blow torch and then sawing through the outer ring with a Dremel milling tool using an extension cable to be able to reach. The best thing would be to cut through on opposite sides of the bush but the tight location makes that (almost) impossible. Even then it was not easy to get them out but eventually they gave in. Be careful not to cut too deep and damage the chassis.

Bushings can be made up from many different materials with different characteristics.

• Rubber (standard)
• Polyurethane bushings (Powerflex among others)
• Delrin
• Nylon
• Rose joints

Except for completely stiff rose joints, Delrin is what racers seem to prefer because of their stiffness and self lubricating qualities. Unfortunately no complete kits are available in Delrin, and therefore I bought Powerflex bushings out of laziness. The opinions about Powerflex (and other poly-bushings) vary to say the least. If they doesn't work I can reuse the hardware supplied with the bushings and make up my own bushing material in Delrin.

Rubber bushings are not necessary wrong. But as I wrote, removing them is something I rather not do again. Other types of bushings are much easier to replace.

Despite of what Poweflex website says the correct part numbers for the CSR are four PF8-901 bushings and four PF8-902 bushings per car (front suspension only). They are sold in pairs.

The smaller PF8-902 bush comes with a special bolt as they're too small to have room for an inner sleeve, but I used the standard 3/8"x2.5" for the bottom rear bolt as the supplied bolt was too short.

Lets hope this works.

Mantorp - slicks vs CR500

Mantorp, again. A new personal best, again - 1.21,8!!! That is 1.5 seconds faster than last time, and I haven't changed anything except put on a new set of (second hand) rubber. I never thought I could reach laptimes like that!

At the end of the day I mounted a set of standard 15" rims with Avon CR500 tires just to see the difference compared to the Dunlop Radical slicks. 1.24,8 or 3 seconds. That is a big difference, no questions about it, but with the CR500 the car suffered tremendously from understeer and I think the difference would be less if I took the time to adjust the suspension to suit the tires better. Not to mention tires with better dry performance than CR500.

Slicks in red, CR500 in blue.

I also put zip-ties on the damper rods and that confirmed my suspicion that the front dampers bottoms out onto the bump stops. I don't know if it is while cornering or while braking, but that is something I should look into next time.

Riding on the bump stops is not necessary a bad thing. Read an article here.

Camber calculator

An easy way to measure camber is to use a spirit and a ruler. It is really important that the car is standing on a completely flat surface for this to work. Accurate? Well, accurate enough for me anyway.

Simple camber calculator:

Rim	mm
D1	mm
D2	mm
Camber	degrees

Rear hub nuts

Both sides loose - scary! Left side so much I could turn it with my fingers.

41 mm socket.
270 Nm (yes that is a lot!)

Adjusting Camber

Adjusting rear Camber
Rear camber is really easy to adjust, and can be done in just a few minutes.

1. Remove the wheel. 
2. Loosen the four small bolts (unlike the picture, normally two of them are hex bolts).
3. Add or remove shim plates as necessary. Silver = 1deg, red =0.75, blue=0.5 and black 0.25. You can buy them from Caterham parts.
4. Be careful not to over tight the bolts, especially the vertical ones. 

Beware that if having a cross-ply setup with not much camber, be sure you have enough threads left on the horizontal bolts, or the wheels may fall off!

Adjusting front camber
Adjusting front camber can be a pain, especially the first time.

1. Undo the camber lock nut at the end of the upper wishbone.
2. Remove the wheel
3. Remove the wing stay by loosen upper ball joint nut, and then removing the stub axle nut. The indicator wires prevent from complete removal, but tuck the wing stay out of the way and try not to scrape the wishbone paint.
4. Undo the top ball joint nut until the nut is on the end of the thread.
5. Now split the tapered joint. This can be done in several ways. Personally I use a cheap tool which fits perfect used up-side-down! See picture. I've read about others who use a hammer!? Some use a bolt with two nuts against the hub (never tried it).
6. Remove nut completely and adjust camber. One turn equals .25 degrees of camber.
7. If you feel lucky, put copaslip on for easier dismantle next time. But beware - copaslip can make it really painful to screw back the wishbone to upright nut!
8. Stub axle 82Nm
9. Upright top ball joint 52Nm. 
10. Wheel 74Nm
11. Tighten camber lock nut

Note that camber affects toe, so if you adjust camber, you must recheck your tracking!

Limited rear droop travel

I noticed my rear droop travel was way too limited and suspected that could explain some of the rear inner wheel spin I have been troubled with.

My car is fitted with a Quaife ATB, and one of the "features" of the ATB compared to other LSDs is that if one wheel is completely unloaded, the ATB won't lock! So, when in roll while cornering hard, and the droop travel is too limited, the load on the inner wheel is reduced and the ATB is unlocked with inner wheel spin as a result.



More info about how the Quaife works

Here is a good article in Mark Ortiz Automotive CHASSIS NEWSLETTER describing what's happening when the droop travel is limited.

So now I have lowered the car with three complete turns on the spring platform on each damper.

I was at Mantorp (where else...) testing last week, and yes, the difference was obvious. No corner entry over-steer I've complained about last time, and the corner exits where a lot less dramatic. I think the inner wheel spin was reduced as well, but not completely. I've read on Blatchat that inner wheel spin is a common complaint on the ATB and what makes it not suitable for racing, and the solution is probably to change to a real LSD such as the Titan.

During the day I also experimented changing the rake to change the under/over-steer balance and found it to be a very easy and convenient way to quickly change the car characteristics in a few minutes. Just jack the front up and remove the hood and turn them with your hands. As little as an half turn each made a real difference. I made the car a little more tail happy and it did affect the lap times in a positive way.

But as usual...

That stupid plastic piece of tubing on the inlet was cracked, again. I tried to fix it with some isolation tape but it still leaked and sometimes the engine hesitatate at 6000 rpm as well as a notable lack of power. So no personal lap records today even if the chassis felt good.

Wheel alignment check

My current wheel alignment, measured in my garage. Cross-ply setup.

	Front	Rear
Toe	0.5° out	0.7° in	per wheel
Camber	-1.25°	-0.25°	±0.25°
ARB	Hard setting	-
Tyre	195/45-15	215/40-15	ACB10 road legal
Pressure	19 psi	19 psi	warm
Rims	6.5"-15	9"-15

Rear left wear

Front left wear

The wear in the middle came after running a bit too high pressure on Mantorp earlier this year. I think 19 psi is just right. I know that sounds low, but it works for me.

My rear 9" rims are too wide for the ACB10 according to Avon, but it works ok.

Another lonely day at Mantorp

A nice and sunny day at Mantorp Park! The day started out well being the faster of the two cars on the track.

...then this guy came along with his brand new Formula Renault...

The driver is only 15 years old and compete at european level against adults! As a comparison he had lap times around low 1:15 and I got at best high 1:27. When I was 15 I had a moped...

Anyway the day went on well and I had time to experiment with some toe settings. Previously I had neutral toe both rear and front. This day I first tried some toe-in on the rears which really made the car more stable and predictable. I didn't had the same problems with corner exit over-steer as before and my lap times went down. Later I also tried some toe-out on the fronts which made the car easier to rotate on corner entry. No difference on the lap times but it felt good and I think it is an improvement. My top speed was really low this day but I don't know if it depends on the toe settings or the fact it was a bit windy.



I polished my personal best with a few hundreds, but my tyres are long gone and I think with a fresh set I can really improve the time!

The day ended when I ran out of battery and the engine stopped! Apparently I have a bad connector to the alternator.

Tyre pressure 19 psi warm all around.

Front bump steer

There is plenty of information about bump steer to be found on the internet. Basically bump steer is when the toe alignment changes during bump, ie when the suspension compress.

Here is an article explaining how to correct bump steer on Caterhams. 

There are lots of different techniques to measure bump steer. I used a construction laser level which projects a line when you point it on a surface.

I jacked up the car, removed the wheels and disconnected the link to the dampers on the lower a-arm making it possible to raise (bump) the wheels by hand.

Then I put my laser level firmly on brake disc with one hand and pointed the laser to it to the wall in front of the car. When I raised the wheel with my other hand and watched how the line moved on the concrete floor. With zero bump steer the line should not move sideways. To  be more accurate you can reverse the level and look backwards. The movement (if any) should be the same backwards as forwards.

There are many other ways to measure bump steer. One is explained in the article above, another one of the more clever ways I've seen is by using a mirror to remove the camber-gain effect.

You adjust bump steer by adding or removing spacers under the steering rack. It turned out that I had quite a lot bump steer on the front right and I ended up by removing one big 4mm spacer. For me it was a bit of a pain to undo the steering rack clamps because the dampers is making it hard to access the hex bolts from above, but eventually I got it apart.

If you change the steering rack height, you have to reset the toe alignment! I aim for zero degrees front toe. Some say a bit of toe out won't hurt and will give the car better turn in.

Note to self: Never undo the rod-end-to-upright nut! It's a real pain to get back!

Cross-ply rear camber

Crossply tyres such as Avon ACB10 don't need as much negative camber as radial tyres, CR500, 888 etc.

On the CSR you adjust the rear camber by using spacers. But what happens if your bolts through the spacers are not long enough?

The wheels fall off!!! Well, almost. This is what happend to me at the "NGK"-turn at Knutstorp last spring. I'm glad I stayed on the track!


This is how the rear suspension looks like:

What happened was the threads on the two horizontal bolts ripped, which caused the two vertical bolts to rip as well.

So if you have a cross-ply set-up with a bit less negative camber, check that you have enough threads left on the horizontal bolts, and maybe replace them with something longer.