F1 Carbon Pedals

In 2001 for the Prost AP03 car we were charged as always with the job of pushing forward the extremes of engineering, in particular weight saving on the mechanical aspects of the car.

I was looking after a lot of the brakes and cockpit components that year, and we fancied taking a look at the pedal arrangement. In F1 you have a funny situation of spending a lot of time designing parts to be a few grams lighter than the year before, and the same or better stiffness. Every year you believe that you have pushed it to the boundary of what is possible and yet the very next year you some how manage to rearrange things to pinch another few grams. In order to really make a sizeable weight saving you have to take chances and go to the extreme. At the time no one on the grid was prepared to take the chance and use carbon for the brake pedal. It's hard to explain why really when most of the rest of the car is made from the stuff.

The pedals on all of our cars had always been fabricated Titanium pieces. If you read my last blog you'll know the processes involved in making fabrications from Titanium. It's very labour intensive and tricky stuff to work with. Carbon provides you with a lot of ways of putting material exactly where you need it, without having to carry around material where you don't need it. You can vary the thickness through the part, you can orientate it differently to get different characteristics, and you can use different cloths and resin systems to get different material properties. The base fibres and resins are also stiff and very light, so ideal for making lightweight high stiffness parts so long as you can design the shape in such away as to physically make it in a mould(s).

Each driver has the cockpit laid out to their preference. So for each driver the pedals were different, shapes, positioned differently in the cockpit, so had side fences on to trap the foot in the pedal, others had a fence on one side but not the other, or like Alesi (picture above) no side fences. In this era the two pedal arrangement was just becoming popular, with the clutch on the steering wheel. Alesi preferred the three pedal arrangement pictured above, and so the brake pedal has no side fences because he needed to move his foot from one pedal to the next. Most other drivers on the grid used left foot braking and so there feet were held into the pedals to prevent them slipping off.

For the spare car you needed to be able to quickly change the cockpit to suit which ever driver was about to jump into it, so we had removable pedal pads that could be swapped over. At Prost the money was an issue and so in the later years we had a lot of different drivers paying for their seat, and each driver needed a new design of pedal arrangement, which drove me mad.

Mid season we decided to experiment and make a carbon brake pedal to retro fit into the car if it was successful. So the pedal set you see in the picture was born. The brake pedal is made in two halves, with a split line in the middle of the front and back faces. The pedal pad was a bolt on arrangement. The layup was varied to give a strong/stiff pedal face and basic neck of the pedal, and thinner in the less well loaded parts. The bottom was solid carbon, which we then machined and sleeved with Titanium, into which the pivot bearings were fitted. There was also a Titanium plate and insert where the master cylinder push rods attached either side. The two halves of the pedal were made so they socketed together and were bonded together.

Like all Carbon parts, you have a lot of things to make before you get to make your final component. You have first to make a pattern, which in this case the two halves of the pedal shape made in aluminium. Then these patterns get laid up with a tooling Carbon fibre pre-preg material, and "cooked" in an autoclave to produce the moulds. Now aluminium expands a fair amount with the heat of the curing process so the patterns have to be made smaller by the exact amount that they will expand. The contraction after curing does help release the patterns from the moulds. One you have the carbon mould tools, you can then make your final brake pedal, by laying up the carbon pre-preg in the moulds, and curing them in the autoclave. These mouldings then go to be machined which often meant making mould shaped jigs so they component could be held correctly whilst being machined. The final part of assembly is then to bond in the Titanium fittings and the two halves together.

Once we'd made the first one and tested it by applying loads to the pad face with an enormous hydraulic test rig we had, we knew that it was both considerably lighter and very much stiffer than the previous Titanium one. On subsequent pedals we were a lot less cautious with our layup reducing weight further. The unnerving thing about Carbon in these applications is that when you first apply the considerable load, the component makes large cracking sounds, as the fibres start to align with the direction of the load. You think it's going to snap, but, after repeated loading cycles it goes quiet. McLaren at the time had very sophisticated sonic measuring equipment and kept track of these noises on all components, and they used it to "life" each part.

Below is a picture of a Coulthard pedal that we went on to design and make for that years McLaren. David was very unsure about using a Carbon brake pedal and apparently took a lot of convincing. Like all things these have become a bog standard item now. In the picture below the two recesses in the pad face take gripper tape, which is essentially like a sticky backed sand paper to give the driver added grip.

Titanium upright design and manufacture

In what seems a previous life now I used to design Formula 1 cars. I thought you might be interested in some of the details of these extreme parts that don't come to light normally.

This picture is of a rear upright from the 2001 Prost AP03 car. It's fabricated from Titanium and a pair of them cost around £15,000 to produce.

The centre of the upright is made from a solid billet of Titanium 6Al4V, the first stage in the process is to wire erode all the triangular shape cut outs in the billet. These cut outs are used to pass cooling air from the brake duct through to the brake disc. The wiring eroding took about 20 hours from memory. This upright has a steel axle in it made in California by Metalore from a special steel that undergoes a special surface treatment. You can probably just make out that the tracks for the CV joint are machined directly into the axle.

The upright has a pair of pretty slender bearings that on this car featured ceramic balls. The ceramic balls reduced the rolling friction by an incredible amount and they are also considered self healing in that the balls are so hard if there's any damage to the bearing races the balls will smooth it out.

The outer parts of the upright were fabricated from Titanium sheet, normally 1.2mm thick, often formed over knocking blocks to get the shapes that we required. Brake caliper pickups, lower wishbone pickups, top wishbone and track rod pick ups are all Titanium machined pieces that are welded into the sheet.

Titanium has to be welded in an inert atmosphere or else is gets contaminated and can fail so we built special chambers where the Titanium parts were placed and the air removed and replaced with pure Argon. The welders had to weld them like you see Biologists in labs, buy placing their arms through in-built rubber gloves and looking through a window equipped with auto-darkening filters.

After welding the uprights were heat treated to remove any internal stresses.

The accuracy required means that any distortion due to welding would be unacceptable, so each critical part of the upright will have a stock of material on that after welding gets machined back to where it should be. The whole upright assembly would therefore be put on a lathe first and have the bearing features machined and then go onto one of our 4 axis milling machines to have all the other pickup points and mounting points machined in exactly the right places.

After machining these uprights were then shot peened and where then inspected and finally fitted to the car. The design of the rear upright system, including the upright itself, axle, wheel, brake package, carbon brake duct and tooling and all the data logging sensors for things like brake temperature took approx. 12 weeks each year and a pair of these uprights could just about be made in a week if two fabricators worked on them. I think we made 5 sets for the first Grand Prix each year and then another 5 sets during the season dependent on damage etc.

Below is a picture of the AP04 front upright, that shows the Titanium axle, Aluminium wheel nut and the full front brake duct assembly.

Design of monocoque chassis

I've been asked to explain how monocoque chassis are designed. It's a bit of a challenger because the details of how a chassis is designed is dependent on the materials used and the application.

However, the overall basic principle is the same. A ship, an aeroplane and most race and road cars could all be described as monocoques. They are all made from relatively thin walls formed into shapes, or supported by other elements to provide strength and stiffness.

A cardboard box is a form of monocoque, in that it had thin walls, and is only a rigid structure when the ends are folded to close the box off. What happens in the case of the box is that you have a shear plane on each face of the box and therefore you have a shear panel in each of the principle directions so that the box can't easily deform. In vehicles you will have a floor, sides, front and rear bulkheads and in a lot of cases a roof to form the enclosed stiff structure.

The problem comes when parts of the system feed loads directly onto these thin walls, obviously there is no stiffness there, so in order to stiffen the panels at the points that these loads are applied, by adding in bulkheads (to form an extra shear web). These don't always need to be full bulkheads, but they can be box section elements to help stiffen the panel, and direct the loads into surrounding panels.

For example an average car is made from sheet steel about 1mm thick, and if you ever try to handle a 1mm thick sheet of steel you'll see it's incredibly hard to handle. But fold a simple return flange around the edge even only a few mm's thick and it's stiffness increases dramatically. The floor of a car is essentially a sheet of steel, so it's not at all stiff until they fold it around the edges to form the sills, and they will form rib shapes into the footwell areas, and normally form or spot weld on a tunnel section where the gearstick and hand brake lever will fit. Normally they'll be a boxy section under the rear seats. There are plenty of pictures of floors and other car chassis parts being stamped online and no doubt on YouTube to give you a clearer idea of the sort of shapes and forms they add to the floor in order to make it stiffer. The rest of the cars chassis is similarly designed so that panels, folds and stamped forms add stiffness to the basic panel, before it all comes together to add stiffness and strength to each other sub-structure.

To simplify it further take a look at the common beam bending equations. Far and away the most important aspect of how much a beam bends or twists is the sectional properties of the shape. In bending that's I and in torsion J. If a car is to be stiff in beaming, then the general cross section needs to have a large Ixx value, and large Ixx values come from having the walls a long way the neutral axis, and from having lots of walls that are in the shear plane of the load. So for a car, you have a large approx. square shape, with a two extra shear webs from the tunnel in the middle and another couple of small shear webs formed by the hollow sills on the outside edges of the floor.

In a F1 car you have the same principles at play, except the Carbon fibre material used gives you a greater degree of control of the material properties. An F1 car is not actually all that stiff in comparison to a road car, and racing car with a roll cage in a production shell will often exceed the torsional stiffness figures of an F1 monocoque. This is primarily because of the Ixx value that you can achieve from the much larger cross section of a typical road car. However where an F1 monocoque wins is in it's stiffness to weight ratio.

Carbon monoques are have walls made of two skins of carbon fibre sandwiching a honeycomb material, which is usually aluminium. The aluminium honeycomb is nothing more than a thin foil maybe less than 0.5mm thick formed into hexagonal cells. The honeycomb comes in sheet form and typically F1 cars use about 10mm thick honey and 2-2.5mm thick carbon walls. Carbon fibre cloth is approx 0.2mm thick so you have to lay up around 10 sheet of carbon (termed plies) to form one wall. This sandwich structure forms exceptionally light and yet stiff walls for the monocoque. Far stiffer than the thin sheets of steel, and so you don't always need to make use of shape and bulkheads and other supporting structures in the same way as you would with steel. However, like any material if you try to put point loads directly into the walls of the carbon monocoque you'll have problems. Carbon is very brittle and you'll crack the skin of the monocoque unless the skin and wall is properly supported.

In a F1 monocoque in areas where they need to attached components like the suspension directly to the outside of the monocoque they be a solid carbon or metal insert replacing the honeycomb in the wall structure. In the case of suspension parts the walls will need added support and you'll find some bulkhead structure behind the inside wall to provide support. There is a great YouTube clip showing the layout of a Sauber F1 monocoque where the chassis is cut in half so you can see everything inside. This clearly shows where the bulkheads and other shapes and features are that support the outer walls when there are loads that require the added support.

A company called Hexcell composites have a great website where you can download a lot of details about working with their pre-made honeycomb sandwich boards. A lot of the techniques they describe in that documentation are used in vehicle composite structures.

With carbon composite monocoques you have the advantage that you can vary the material properties at any point on the chassis to aid stiffness, strength or support for other elements. You can thicken the area by adding more plies of carbon cloth, you can vary the orientation of the cloth so that more of the carbon fibres are aligned with the direction of the resulting load thus stiffening that area, or you can use Uni-direction cloth which has all the fibres runnin in a single direction, thus meaning that you can put all of the carbon to work in the direction of the load. Usually a carbon monocoque will be made of a mixture of cloth and uni-directional (UD) fibres. The UD plies are often wrapped around the chassis at approx 45 degree angles to help provide extra stiffness in torsion, and the woven cloth will be orientated differently through each ply and sandwiching the UD in order to support the UD fibres and provide a more universal load carrying capability.

Saying all of that, everything basically boils back down to the simple cardboard box concept, of thin walls supporting other thin walls.