The past three months have been marked not so much by what construction has occurred, but rather by the number of final – if they are truly final – decisions that have been made.
The previously acquired Ford Escort steering rack has been replaced by a Toyota rack. The Escort the rack sat in front of the axle line, whereas in the donor Mitsubishi 380 the rack sits behind the axle line. What this means is that the rack moved in the opposite direction to that desired! This error was simply due to my inattention to the basics of rack operation, although I might halfheartedly plead that a full time job and two young children, in addition to running Race Magazine distracted me somewhat!
The other previously stated option of the Suzuki Mighty Boy rack was not taken further, as upon inspection it was not felt to be sufficiently strong enough. This was also the case for racks from other light cars such as Getz, Charade etc. Why consider these racks, you might ask? Well what I wanted was a rack that was light, had a straight-ish pinion angle and was relatively narrow between the inner tie rod ends. The location of these inner ends was to set the spacing of the inner wishbone bearings as the rack was to be mounted low and very close to the lower inner suspension bearings. Why exactly is this fact important? Well, for the intended amount of suspension travel, the simple rule is that the longer the suspension arms the better potential camber control over the full range of travel – a very desirable feature given the desired amount of suspension travel on Godiva!
In any case it was back to the Quaife catalogue and to the various wrecking yards to see what could be acquired. Why Quaife? They make a few different high ratio rack and pinion sets and it is always cheaper to buy a commercially available product, instead of developing your own. In
addition, should anyone else wish to build a Godiva they will be able to readily access exactly the same components, this is a consideration still, given the expressions of interest of some readers. The Quaife catalogue gave a couple of options and the steering rack from a Toyota AE71 (which is the same rack as the AW11 MR2 in the catalog) was chosen. Fortunately I was able to borrow and subsequently buy one from a fellow enthusiast (thanks Glenn) when it was found to be suitable. The rack is more solidly made than the Escort rack and most importantly has a pinion angle and placement that is very suitable for the column placement in Godiva. Unfortunately the rack has a different size pinion spline to the 380’s column and this is a problem I have yet to find a solution to…let me know if you have one! As I did not want to have to modify the column if at all possible, the pinion length and rack height then also set the column position/driver reach length. The actual angle seemed to be working out well, but the reach was a little more than desirable, with a fairly straight arm position required.
To my great advantage I had then a visit from a knowledgeable friend, Tyrone, who suggested that I move the rack so that the tie rod ends enter into the steering arms from the top. This rack location apparently has advantages, but I will research this some more before I commit it to print. I admit that I was initially reluctant to move the rack, as it meant drilling out the steering arms to a straight hole instead of their tapered hole and machining up a conical insert to support the tie rod end. However mocking up the higher steering column position showed that it not only moved the column back, making a more comfortable driving position, but also made the existing column mounts more useable for the planned structure, as well as adding some addition room for feet and pedals – a win in every respect. I will be sourcing a mill/drill for the workshop as I feel that this drilling and some of the possible machining of suspension mounts will require such a tool.
This change in rack position meant that I could reconsider using the 380’s dash tube, with all of the bracketry cut off. I had seen this used at the cross tube in a roll cage constructed by Adam Russell at Russell performance and it seemed like such a logical thing to do if you could make it work. Unfortunately I could not, so I will have to cut off the column mounts, modify them and attach these to the cross tube used.
You can see from image that the struts were dismantled and one of the strut bottoms was mounted to a flat board. This was done so accurate measurements could be taken which will be then used to design the suspension in the near future. The second use of the mounting board was to mock-up the upright location relative to the planned ride height and planned chassis structure. Why do this? Well I want the lower wishbone to be level at ride height and with the tyre diameter marked on the board I could adjust the height of the upright to suit. This was a valuable exercise as it showed that the lower tube will need to be raised to meet the inner end of the wishbone, otherwise the suspension mount, and thus the suspension forces, will be offset and would induce a twisting motion to the tube, which is very undesirable. This is even more undesirable with the planned square tube at this point, as square tube is less able to deal with torsional loads. Of course structure/material could be added to the chassis to compensate for an offset mount if the tube was not moved…but then we are simply making a bad design heavier!
Moving this tube up approximately 30mm will also mean that the cabin section will also need to be modified to suit, though the modification is small and only affects the base level tubes under the driver’s feet. It will however make the cabin structure slightly more challenging to make.
Since we are talking about this area you will see that the cabin area has yet again been changed. The diagonal tubes are no longer straight along their full length. This change was caused by the steering rack change, with the new steering rack being narrower. However the lower tube position is possibly going to change again, at least as far as the distance from the centerline is concerned. This is because the current layout as pictured was completed with the low rack height in mind…now it is up higher the inner wishbone pivots can be moved further inward if required.
The chassis change to accommodate the new rack has left a rather ordinary structural point at the outer front corner, near the wheel well. Ordinarily the emphasis would be placed on good spaceframe practice, whereby tubes meet at load points and good triangulation is employed. Clearly this is not the case at this point, so the suspension mount will incorporate some added bracing. Also remember that the cabin bulkhead will be made of honeycomb panel, but why keep it this way? If I narrow this point then I will have to offset the pedals to the centre of the car, which I do not want to do. The pedal arrangement would also be compromised with respect to the ability to use the 380 accelerator pedal, something I do not want to modify to fit the car as the 380 uses an electronic fly by wire throttle. Lastly this structure is quite simple and easy to make when compared to the alternatives. Yes, it is not ideal from a theoretical spaceframe structure standpoint, but it is an acceptable compromise for Godiva’s stated purpose.
I had some interesting and relevant information regarding suspension come my way from none other than Steve Randall, who was the suspension engineer for the McLaren F1 Supercar and who now runs Randle Engineering Solutions in the UK. This interaction came about because I was very fortunate to make the acquaintance of a McLaren F1 owner, who was working with Steve to tune the suspension on his car. After a brief discussion the F1 owner agreed to pass on some questions and an overview of Godiva to Steve and the advice was as follows:
“Before you start worrying about suspension settings and layout, the wheel and tyre sizes and the front/rear balance are the most important issue to settle Once you have estimated the weight distribution… will be key to controlling your lift-off oversteer and limit grip.
Your suspension layout is best off with directly driven dampers. Bell cranks and pushrods are for open-wheelers with skinny chassis. On anything else, it’s just weight and lost motion in bushes and bearings.
Roughly speaking, try to keep scrub radius below 30mm, and running four to five degrees of castor with the axis running through the wheel centre in side elevation will be about right. In the order of 8 degrees KPI will also be fine. Front roll centre should be 60-80mm, rear about 90-120mm”
Of course the disclaimer for this information is that it is based on specific data relating to Godiva and should in no respect be taken as a starting point for any other vehicle. This is also the appropriate point to indicate that I will not be designing the suspension for Godiva. Instead I will be employing the services of a highly skilled professional in the area of suspension design, though I will still report any information that comes to hand. There are three primary reasons for my decision to not do it by myself: One – the more I read about suspension design the more I realised that I have neither the time nor knowledge to complete the task to my satisfaction. Two – I am completing Godiva in the public domain and I am very keen to promote the best possible practice, which clearly would not have been mine had I chosen to go it alone. Three – A powerful mid-rear engined car is apparently more difficult to equip with relatively benign handling at the limit…which is what I want.
The last thing of note was the rear knuckle/upright has been selected. By chance I was in Russell performance when Adam Russell was preparing a new Mitsubishi Evo 8 for a customer. After looking at the steel rear hub and spline, I noticed that the shaft and spline size seemed the same as the 380. This was later confirmed by Adam, when a 380 driveshaft proved a perfect match. This rear upright/hub may not be the lightest component, but importantly for Godiva it incorporates a handbrake mechanism and has very similar disc sizes to the 380 – which means that the front/rear brake package should be reasonable well matched. In addition the components are from an ADR compliant car and it has the same stud pattern as the front. The weight of the upright, bearings and hub (not including disc) is 8.8kg, which is within 50gm of the front assembly. The front calliper weighs 4.5kg and the disc is a very heavy 9.7kg. The wheel that I plan to use is 6kg for a 17 x 7 alloy and the tyre is a further 8kg for a 225/50/17. This makes the basic corner weight 37kg (not including the percentage – estimated to be 50% – of the suspension arms and damper that gets added to the total) but I will round it up to an even 40kg. Some weight can be taken away from this total in the future, but I thought it was still quite a high figure. However it is worth noting that the McLaren F1 had an un-sprung weight of 42kg at the front, so perhaps it is not too bad!