So, again, it has been longer than I anticipated, and wanted, between writing a post. Partly due to the fact that I have been out most weeks testing a new driver with Richardson Racing, which means I have had less time than I would like to prepare parts and run them down to the powder coaters. The other reason is I haven’t had the cash to buy everything I need to re-build the axles, basically, I needed roughly £500-600. A quick run down of the things I need; 2x swivel bearing housings; 8x wheel bearings; 4x brake discs; 4x swivel bearings; MANYx paper gaskets; 2x swivel oil seals; 4x axle oil seals; and 4x hub oil seals, I have probably forgotten some things, but you get the idea. Happily, within the next few weeks I will have got all the bits and bobs I need to make some big progress. All I will need to do in the mean time is get the casings blasted and painted. As an aside, I have decided not to powder coat the casings as there is a chance they may distort whilst curing in the oven. It’s better to be safe than sorry.
Anyway, as you might imagine, there hasn’t been much progress in the past few weeks. So, in the latter half of the post I have included photos of the Formula Ford after a little off-track excursion.
Right, let’s get on with it.
The above is a picture of one of the radius arms, which connects the front axle to the chassis, it also travels right under the engine. It has an accumulated a nice layer of engine and road grime, as you can see in the picture below.
I spent about 10 minutes an arm with a wire wheel, the gunk went all over the place. But, the radius arms cleaned up quite well.
Not a lot of rust on them at all. Maybe the best way to prevent a component rusting is to leave it under a Rover engine…. Maybe not.
The above shows the radius arms after they had been powder coated, again in satin black (RAL 9005 if you are interested). Below is a close up, I think they look pretty nice. They look all shiny and new.
Below is a photo showing the general colour scheme, the calipers are fairly vivid. At the moment, I am not 100% sure how I will colour each part, but the general colour scheme is nice.
I have also got round to rebuilding my calipers, brand new pistons and seals all round. Below is a picture of a rear caliper, looking like new.
Anyway, unfortunately, that is all the progress I have to report on the Tomcat. It is still progress nonetheless.
So on to the Formula Ford.
The pictures below show the damage.
A very bent lower front wishbone, a slightly bent upper wishbone, a bowed pushrod, and a bent steering adjustment rod.
A snapped front upper wishbone bracket.
A bent front upper wishbone mounting rose joint.
A bent rear upper upper wishbone.
Along with two cracked alloys and a delaminated front wing.
The damage to the front wing only became apparent when it was bent. There was absolutely no structural rigidity to the wing, it was very flexible, which is not right. The photo above shows the rear of the front wing. When bent downward, a gap opened up in the carbon fibre along the point of the trailing edge, presumably where the upper and lower halves of the wing profile are (or were) bonded together. There was also a fine crack on the underside of the wing.
Later I found more damage, which included a split CV boot, a snapped CV tripod bearing retaining ring and a gouged drive flange. Effectively, an entire CV joint that was also damaged beyond repair in the accident. Just for those who are interested, the single most expensive piece was the front wing.
“How did it happen?” I hear you ask. Well, whilst at Snetterton testing, we had told the driver that we would do a race simulation. This means we wanted him to chase a lap time and stick to it for as long as possible. Just when he was getting confident he spun at the final corner and ended up on the grass. In the process of driving back onto the circuit he drove through the grass, so we called him into the pits to check the side pods for grass clippings (of which there were none), as if you don’t remove the grass the engine will overheat (due to blocked radiators), resulting in warped cylinder heads, engine seizure or other catastrophic engine failure mechanisms. We then sent him immediately back out to continue with the simulation. He completed an out-lap, another lap and on his 3rd lap after the first spin, the red flags came out.
The onboard footage revealed that as he went into the first corner, the car twitched a bit, which he caught. In doing so, he ran wide and put two wheels onto the grass, at which point he lifted off and jerked the steering towards the track in an effort to get back on to the track. Unfortunately, the back torque from the engine (engine braking) and the weight transfer forward due to lifting off the throttle, coupled with the lower coefficient of friction on one side and the steering input caused the car to do a complete 360 degree spin, across the track and into the wall. Fortunately, the driver was fine, as was the car… in the grand scheme of things. All the parts of the car that are designed to crumple, crumpled, leaving all the expensive bits (chassis, gearbox casing, driver etc) untouched.
Here are a couple of photos after a couple of hours work:
A new front corner.
A partially new rear corner.
The above picture shows the various ways these wishbones have been designed with the safety of the driver in mind.
Some of the points above are obvious, like routing the brake pipe inside the wishbone. This protects the pipe, which prevents the pipe rupturing and loss of braking as a result. Additionally, there are dual brake circuits (two master cylinders), one for the front and one for the rear.
The check strap is a kevlar re-inforced rope, its purpose is to prevent the wheel and hub assembly bouncing down the road should they get knocked off in a crash, preventing accidents such as the one that killed Henry Surtees. The length of the rope is such that the is very little to no slack along the length of it, i.e.: there is just enough rope to reach and secure the hub, and no more. This means there is a clearly defined spherical voulme within which the wheel will be able to move, obviously this should be nowhere near the driver (or any other driver/marshall/spectator for that matter). Again, it becomes obvious to see that the eyelets are there to prevent the rope getting damaged (by holding it still).
The side intrusion bars are obvious as well. If the wheels/suspension assembly is hit in such a way that both inboard mounting points snap so that the wishbones move inward towards the chassis, the movement through the chassis is stopped by the intrusion rails banging into one of the chassis’ main bulk head hoops. This significantly lessesn the chance of impaling the driver.
The mounting brackets that hold the wishbones to the chassis/gearbox are all made of Aluminium. The wishbones are steel, and the chassis is steel (the gearbox casing is cast Aluminium). Aluminium is a weaker, less tough material than steel for the same physical dimensions. So, we can logically assume that the brackets have been designed to break upon a large enough impact. This is for two reasons: 1) to isolate the driver from the impact, by removing load pathways into the chassis. 2) to protect the expensive parts of the car from damage. The expensive parts being the chassis and gearbox casing. With that being said, in the case of a perfect side impact (one were the car is stationary and something runs into the wheel), chances are the brackets wouldn’t snap, as they are butted up to the chassis and have more material on the chassis side. Not to mention the wishbones only have a few mm of travel before they hit solid metal. They are designed to break away in the most likely case, which will be wheel to wheel contact in a braking zone.
At the inboard mounting points of the wishbones, there are some necked regions. They are necked in the xy and xz planes.
If you assume the face we are looking at is the XZ plane, then the XY plane would go into the screen. The smallest cross-section of the wishbone is shown by the blue line, the necked region is shown by the two red dotted lines. The wishbone has a bend into the screen as denoted by the cracked paint (the surface we are looking at has been subjected to the highest tensile stress of the bend, which has stretched the paint and cracked it off). The bend has occured at the point of minimum cross-section. So, again, you can logically assume the neck is there as a bend initiation point. The only reasons I can think of are either: to absorb crash energy, reducing the loading travelling into the mounting bracket (and hence chassis); or to bend the rose joint so as to lessen the chances of it reaching the driver (in the case of cockpit penetration); there is also the possibility that you could cut the end off the wishbone and weld on a new rose joint (obviously after some fairly thorough NDT).
The final point is that the wishbone bent as opposed to snapped. From this we can deduce that the material used is very ductile, whilst still being very strong. Obviously this is a massive plus as it means that no sharp edges are created in the crash.
The graph above shows the tensile stress/strain graph for a typical ductile material. A few things should be noted from this graph line OA is the elastic region of a material, whereby the material acts like a spring and obeys a Hooke’s law (F=kx) type relationship. Applying the equation of a straight line to the linear portion, y=mx+c, setting y=stress, x=strain, m=Young’s modulus of elasticity (E) and c=0. We get stress “equals” Young’s modulus of elasticity “multiplied by” strain. It can then be seen that the Young’s modulus of elasticity is the stiffness of the material and is analagous to the spring stiffness. This means any force applied to the material (as long as it is within the linear range) will cause the material to stretch. When the force is removed, the material will shrink back to it’s original length.
Point A is defined as the yield strength, i.e.: if you apply additional force, the material will stretch, but it will not return to it’s original length. The material is said to be plastically deformed, thus the yield stress can be described as the stress above which plastic deformation will occur instantaneously. Generally speaking, in engineering terms, if the yield stress is exceeded, the part is thought to have failed. Therefore, components are designed such that the design stress is below the yield stress (usually, but not always).
Right, with that said, we know that the wishbones are extremely ductile, since they didn’t snap during an impact with the barriers (they will permanently bend). During the impact, work is done on the material, i.e.: a force is applied, and the material moves a certain distance. Remembering that “work” is a mechanism of exhanging energy, therefore, energy is transferred to and stored by the wishbone as potential energy. If this happpens in the elastic region, the energy is stored for as long as the force is applied (as above). If this happens in the plastic region, the material is permanently deformed, and the potential energy is converted to heat (by the work done against the internal friction of the wishbone) and noise.
Relating this back to the graph, the area under the graph for force vs extension gives the amount of energy absorbed by the wishbone for the given elongation. The area under a stress vs strain graph gives the amount of energy absorbed per unit volume for the given strain. It would be prudent at this stage to introduce the concept of toughness, which is defined as the amount of energy a material can absorb before fracture. Since the material stores and returns the energy in the elastic region, the energy is not absorbed. Thus the toughness of a material can be defined as the area ABCDE on the graph above, as the energy that is used to permanently deform a material cannot be retreived.
So using all the information above, we can guess that to achieve the results we want (wishbones that bend and don’t snap), we must use a material that is very tough. And for a material to qualify as tough it has to have a high yield stress (i.e.: be very strong), while being able to withstand large amounts of permanent deformation before it fractures (i.e.: be very ductile).
Relating all this back to the car, the necking on the wishbones, the break away mounting brackets and the material of the wishbone, all serve to minimise the effect of a crash by absorbing as much of the energy involved as possible. They do this by bending permanently. It is logical to assume that the suspension arms themselves are designed as crumple zones. They are not crumple zones in the truest form of the word, as crumple zones seek to minimise the acceleration (or deceleration) of a crash by deforming at a desired/optimum rate. I doubt the suspension would do that, as there are too many failure modes that a suspension system poentially has to deal with, but they would certainly absorb some of the energy in a crash. Although having said that, the suspension does have the same down side as a crumple zone, being that a lot of damage is caused for a relatively minor bump.
I find it amazing that so much thought and work goes into something that is as simple as a wishbone. I mean, the above discussion only deals with the safety aspect. There are many other aspects to it, including how to manufacture the things, the performance (weight, shape, size, geometry, resultant suspension geometry etc), the cost etc etc.
Anyway, enough wittering on about wishbones. Next time I will hopefully have a lot more progress to show off. Until then….
