Switching tack

Electric Power Steering


As part of the implementation of the Vauxhall Corsa C Electric Power Assisted Steering (EPAS) into the car, I need to consider switch gear. The ignition switch only carries signal currents, indicating the Body Control Module (BCM) the switch position. The electrical ‘heavy lifting’ is handled by BCM controlled relays. I need to retain the ignition switch as-is, so I will have to find a way of replicating this function. I could wire a relay for ignition and engine cranking to the ignition switch, but there are other areas to consider:


Control Stalks


The Corsa C uses a series of resistor ladders in the control stalks (indicator, main beam, cruise control (where fitted), wiper and washer). I managed to track down the following internal schematic from the Locost Builders forum.

S229.1 Switch - High beam
S229.2 Switch - Turn signal
S229.3 Switch - Cruise control


Basically, this results in a resistance for each switch position which is nice and simple (read cheap) when engineering the switches themselves, but adds some complexity to the electronics. The control stalks appear to be physically interchangeable with the Vectra B counterparts, so it is possible to retain the Gardner Douglas approach and pass the bulb and wiper currents through the switches. However, this would not allow the use of a ‘lane change’ flick to flash the indicators, dictate the use of manual wipers (no rain sensing), remove the ability to use a modern hazard switch, and makes automatic wipers a non-starter. Electronic engineering is the way forward!


One way the indicator could be connected to a ‘new’ BCM is to apply a voltage to switch pin 3 and add a suitable resistor between pin 2 and ground. The BCM can then read the voltage across the pull-down resistor and determine the switch positions – easy!


Not quite. The BCM has to, among other things, try to catch quick switching events (e.g. a lane change ‘flick’ of the indicator) so it would have to continuously poll the voltage, and still may miss events due to the time involved in the Analogue to Digital Conversion. In addition, this then doesn’t leave time for other jobs in the BCM such as measuring the other switch positions, managing the bus communications, or working out the required light configuration.


I needed a new plan.


A better way to react to asynchronous events is with an interrupt. Some microcontrollers have the ability to interrupt when the voltage at a pin changes level when compared to a reference voltage. Upon a switch event, the change in voltage at the output of the switch would change the output of a comparator, invoking an interrupt. Perfect! Unfortunately, the number of comparators available on my options of microcontrollers for the BCM brain (Arduino / Teensy / Pyboard) is limited (one or two in some cases), and I would like to apply this across at least five inputs in the BCM, it is not a reasonable route. Oh dear!


Plan C…


I can take the principal of the comparator interrupt and build an external circuit to achieve much the same effect. Since a Pin Change Interrupt is available on most digital pins of the micros, I have a way forward. Joy!

Long time no see!

Whilst it has been some time since I have updated this blog, I have still been working on the car. Family commitments, house purchase and subsequent move, and life in general have all got in the way, but I have still made little pieces of progress albeit in a greatly reduced capacity. The loss of a camera with the photos of this progress didn’t help, so I’ll try to fill in the blanks.

Power assisted steering

Should this car ever actually make it to the road, I would like to make it as ‘everyday driveable’ as possible, which for me, means it needs power assisted steering. There is an ‘unofficial’ hydraulic option where the steering rack from (I believe) an Impreza is substituted in place of the standard part. I am concerned that this will negatively impact the steering geometry including bump steer, so I have shelved that idea. The other possibility (which I have elected to try) is Electric Power Assisted Steering (EPAS)

After some trawling of the internet, the EPAS from a Vauxhall Corsa C seems to be a good choice. It is column rather than rack mounted, has simple electrical needs (i.e. no CAN bus messages to replicate), and is popular in other car building communities so is readily available. The biggest drawback is that it needs a vehicle speed signal in order to adjust the amount of assistance it provides. This takes the form of a square wave where frequency is proportional to speed. There are a number of ‘little box of electronics’ available on eBay that provide this signal which come with a dial in order to alter the frequency. This is a manual operation and only therefore allows for a fixed level of assistance which more than likely ends up with the steering too heavy at low speed (a little redundant then!), or too light at high speed (more than a little dangerous). The challenge is achieving a speed related assistance so I will need to develop a way of getting the Cobra’s speed signal into the Power Steering controller. Unfortunately, the controller is expecting a speed signal from the Corsa which almost certainly has a different number of pulses per mile than the speed signal from the transmission in my car, so some frequency conversion will be required. Fortunately, this opens up the possibility of an amount of tuning of the assistance vs. speed map. An awful lot of the groundwork for this has already been carried out here so at the risk of ‘standing on the shoulders of giants’ I need to integrate this information into my own box of tricks. Sounds simple!

corsa epas power steering kit car

With the electronic side of things in hand, I need a way of mounting the column into the car. A ‘standard’ Gardner Douglas mk4 uses a Vectra B steering column, but this is wildly different to the Corsa design. Therefore I need to design my own installation, bearing in mind any new brackets will have to absorb the torque reaction from the column. After a flurry of activity with calipers, height gauge, inclinometer and numerous bits of steel, I have been able to re-create the Corsa column as a 3D CAD model. This then allowed me to design a bespoke bracket arrangement that picks up on the windscreen mounting points on the body in a fashion similar to the GD design. At the other end, I am mounting to the corner of the transmission tunnel as this makes for a short cross car tube for maximum stiffness. I ordered a kit of laser cut steel parts from my CAD models and have welded up the assembly. Some adjustment is needed for final alignment, but it sits roughly where it needs to so I’m taking this as a win! Connecting the column to the rack is the next step on the installation journey. When procuring the Corsa column, I also obtained a lower joint which would normally connect the output spline to the steering rack. This has an extendible portion to aid in assembly as well as provide collapsibility when in a crash. Since the steering rack in the Cobra has a different input spline, the bottom end of this lower shaft was pointless and so was despatched with judicious use of a drill and grinder. I then turned up an adaptor to join the remaining part to the steering shaft extensions in the car.

Pedal Box

Gardner Douglas supply an adjustable pedal box which, since I have unusually long legs, seems like a good idea to allow others to drive the car (again, predicated by the ‘finished car’ status, whenever that may happen!). Unfortunately, all the messing round I have done with the steering column means a key structural joint intended to support the pedal box is now not feasible. I could have hacked together a bracket to pick up on an alternative hard point, but where would be is the fun in that? I elected to design my own pedal box and take the opportunity to make it electrically adjustable as reaching under the dash and turning a knob manually seems so last century!

The pedal box is a safety critical item as failure of components would likely be catastrophic, so some care had to be taken. Some digging around indicated that the maximum effort that a strong person may apply to the brake pedal in an emergency (with safety factor) is around 1500N, so this was my target. The confines of the foot well provide some interesting constraints, so I measured and modelled the environment to help with the design. I used a standard Jaguar brake master cylinder (the LHD version was more convenient for hose routing) and an aftermarket clutch cylinder. Ensuring sufficient movement of the clutch slave cylinder whilst achieving the target pedal ratio of 6.5:1 (since these are non-power assisted brakes, this ratio should give a reasonable feel) proved to be a little tricky, but a few iterations to get the concept refined meant I could turn to Finite Element Analysis (FEA). This allowed me to ensure that under the maximum load, no undue deflection or excessive stresses would occur, and normal operation would see no fatigue issues. When I was happy with the design, I sent the flat pieces off to the laser cutter and set about some bar stock with a lathe for the round bits.

Using a vee block, a hydraulic press and an inclinometer and a little bit of maths (to estimate spring back), I formed the joggles in the pedal arms.

I machined up a matched die pair from scrap aluminium that I used in a press to curve the pedals as needed.

I still need to make a small fixture in order to jig the pedal arms to their respective pedals and pivot guide tubes.

It might be a car one day!

Despite the lack of updates since my last milestone, I have been making progress in a number of areas on the car.

I have started fabrication of the lower radiator pipe (to the water pump) from a collection of stainless bends and straight sections - this is currently tacked up and test fitted to check the clearances and it is ready for fully welding

Installed the four rivnuts which will hold the fan brackets to the radiator

Test fitted all of the metalwork for both doors (hinges, hinge brackets, intrusion beam, lock mounting etc.), bonnet (‘hood’ for subscribers across the pond) and boot.

Most of these were a simple fit, but I had to adjust the boot hinges somewhat. Unfortunately, the only way to tell exactly what adjustments were needed was to fold my 6’2” frame into the boot, then lift the loose panel into plate (with wedges around the opening to make sure it was centralised). Now inside the boot (along with a torch), I could offer up the hinges to their respective mounting points on the boot and see what needed adjusting. It turned out that I needed to adjust the angle of the mounting plate on the end of the metalwork which involved cutting a slit most of the way through the arm, bending it closed then welding it up. This seemed to do the trick so I could disassemble everything and send them all off to be powder coated along with the radiator fan brackets.

Trimmed and filled the remaining seams on the bonnet, boot and door panels. These are substantially complete, but there are a few areas that need more attention.

Moulded the brackets that will mount the bonnet and boot panel gas struts. These were made by setting up a polypropylene barrier in the four locations (2 on each of the panels) and applying a layer of release tape to the fibreglass skin of the bonnet or boot.

I could then gel coat and wet-lay the carbon fibre tape against the barrier and release tape to create a perfectly fitting bracket.

Once cured, I trimmed to shape and bonded in the threaded metal insert that accepts the strut end ball stud. These can then be bonded to their respective panels along with installing the captive nuts in the bodywork to allow the gas struts to be fitted.

Machined the slots for mounting the windscreen / windshield to the body, along with drilling and tapping the mounting points for the two wing mirrors into the two screen pillars.

After test fitting the pillars and screen though, I have noticed quite a large gap (~10mm) on one side at the bottom of the pillar which tapers to essentially no gap where the pillar meets the body. I need to address this as the mounting bolts will no doubt clamp it up tight, but this will stress the windscreen which won’t do it any favours. I am not sure I can straighten the pillar enough without cracking the chrome plating, so a tapered ‘wedge’ may be required

In addition, given the significant lead time, I have now ordered the trim (seats and carpets) – but you will have to wait until the big reveal to find out what colour(s) I have chosen!

Short post, big milestone!

Short update, but big milestone reached this week. With Herculean assistance from my brother (thanks Neil), we were able to turn the body and trestle around to the correct orientation, then use an engine crane to take the rather significant weight of the fibreglass and dismantle the cradle from underneath. The body could then be lowered into position and with a little pushing and levering, get it settled into its final position on the chassis.

Its starting to look like a car now!

Catching Up

I have been slowly making progress on the replacement bonnet catch. The last update I had made a mould from the region of bonnet inner where the catch would be mounted - I then proceeded to make a part from this mould which would form the basis of the new plug.

I pressed my CNC machine into service to machine some tooling resin to shape which then had to be ‘let in’ to the fibreglass, the gaps closed off and a good fillet of filler applied to the base to minimise any stress from opening, closing and driving vibration. 

This new plug could then be covered in a plug finishing resin, sanded to a good finish with ever finer grades of wet and dry, before the final step of 6 wax coats as a mould release.

I was then able to make a new mould from this plug which needed minimal finishing before it received its own 6 coats of wax

I was finally able to pull the actual part in the same colour as the bonnet itself – quite a lot of effort to get to this stage!

This replacement piece was then trimmed to size and used to mark up for the far more scary job of trimming my pristine bonnet inner skin to accommodate it. With some careful cutting and sanding I was able to get a reasonable fit between the two parts – almost done!

Since fibreglass is not great at accepting the threads for the bonnet catch mounting screws, I had a metal insert machined (thanks Colin) which would be sandwiched between the upper and lower panels. This was bonded onto the upper skin by screwing it to the new piece then mixing resin and micro balloons (tiny glass beads) to make up an adhesive paste which could then be applied to the insert and the whole thing put in place. Once cured, the adhesive held the insert in place well enough to remove the ‘cap’ and reinforce the joint with resin and fibreglass.

The cap could then be finally installed using more resin and micro balloons paste on the insert and all around the opening in the bonnet skin. Quite hard to describe, but the pictures should explain it! Once this was cured, I ground a small groove along the join line, and applied gel coat to fill it in. This could then be cut back and polished to form an apparently seamless joint.

With the bonnet modification complete, I was able to hinge it to the body and install and adjust the bracket which mounts the other side of the catch. This will be getting a cosmetic cover in due course and I still need to decide on the most appropriate position for the release handle along with routing of the cable, but having this job done clears the way for finally mating the body to the chassis!

Progress and everything!

Despite my lack of updates, I have still been progressing the build on a number of fronts and there is a bumper load of pictures for your perusal!

Since my last email, I have cast all of the wax cores for the intake runners though things were hampered slightly by the first set of moulds ‘drinking’ a couple of cores worth of wax, which meant I had to order some more wax! In addition, two of the carbon fibre lay ups failed to meet the required standard - I managed to blow a couple of holes in the heatshrink which turned out to be unrecoverable after curing so I also ran out of carbon sleeve.

Still, I now have 10 runners (8 good ones!) from which to build up the required manifold parts, though there are only 8 shown in the picture! Unfortunately, they will take a little more finishing than originally hoped due to the less than ideal surface finish from the heatshrink. I will have to wet sand them smooth before applying a finish coat of resin, which will get post-cured along with the bulk of the layup when I melt out the wax.
Onto the body work and probably the most dreaded part for fellow Gardner Douglas builders electing to carry out the finishing work themselves – the engine bay bulkhead. This is where two parts of the moulding meet and the join is usually a gap in the gel coat that needs to be filled.Some people choose to use body filler and paint, but I wanted to use the factory recommended method of using gel coat for a seamless and hard wearing finish.

Things went a little down hill from the start – the first problem encountered was the join itself was not so much a gap as a step in the moulding. Holding a straight edge against the panel showed around a 4mm gap at the worst point!

I persevered though and proceeded to key in the surface and mask off the entire panel with three layers of masking tape. The idea is to apply the gel coat and use the masking tape as a depth stop when skimming the surface then peel it off before curing. The top layer of gel does not cure in contact with oxygen (allows for a better bond for subsequent reinforcement when using it in a conventional layup), so this is removed with acetone before rubbing back the cured material to the desired level and finish.

Given the depth of the mis-match I had to use a number of coats to build up the required thickness, but I got most of the way there. I decided to flat back to a level before applying the finishing layer and this is where I noticed a colour mis-match between the gel I was applying (which matches the body) and the inner engine bay moulding. Apparently they are two similar, but different greys!

Since I would not be able to apply and feather out the correct grey across the whole bulkhead, and after a short bout of tourettes syndrome, I elected to use the body filler and paint approach.

This progressed quite quickly thanks to the warm weather and after a good period for properly hardening, I was able to polish it back to a reasonable finish.

The next job on the list was the footwell extension. This sits on the driver’s side and provides a wider area, particularly for the clutch pedal. This is supplied as a fibreglass moulding which needs to be trimmed and fitted with a peripheral neoprene seal and jack nuts to allow installation. I also elected to paint it body colour, just because I had a can spare from painting the bulkhead!

The only other area in which I have been working is the bonnet catch. I wasn’t keen on using the typical quarter turn locks that are used in the two rear corners of the bonnet, so I managed to track down a (slightly) more modern solution in the form of a unit from a mk2 golf. This requires an addition to the bonnet inner skin moulding to allow for mounting and fastener threads, so I need to make a small inlay of the correct form.

To achieve this, I have taken a moulding from the underside of the bonnet in the region of the new catch mounting and from this I will be able to make a copy of the skin. This can be built up the region in question, before using this as a plug for a new mould. It will all make sense shortly… I hope!