Being part of an amateur motorsport team means you tackle the same engineering problems as the big dogs like Formula 1, but with a measly 0.1% of the budgets. This means making some very tough decisions regarding what materials can be used. Carbon fibre monocoques are not within the realm of possibility, nor are Inconel exhaust systems. This means being smart with the budget when some compromises must be accepted.
Budget motorsport means that, when possible, standard materials should be used. Most amateur race cars will be constructed primarily of mild steel, especially when it comes to the roll cages and chassis tubes. For a few pounds a meter you can buy mild steel tubes, and a trained welder can work with it. Sure, it is going to be heavier than the chromoly steel or aluminum, but the cost saving is worth it. It would cost about £500 in materials, plus a day’s worth of welding, to build a complete roll cage out of mild steel. Doing that in chromoly would triple the price and require a welder on the more professional side.
Chromoly steel (4130 alloy) has certain advantageous applications. The AMS 6370 specification addresses chromoly tubing of aircraft quality. While expensive, chromoly steel tubing offers approximately double the strength to weight ratio of mild steel. This practically means that thinner walled tubing can be utilized, ousting the need for tubing construction that is heavier. For instance, with chromoly tubing, a front suspension upright or a highly stressed chassis member can spend 200 pounds on chromoly tubing, as opposed to 50 on mild steel, and save a few pounds in critical places. You save weight, and pay for the weight savings as opposed to strength that is unrequired.
When building non welded or non welded construction, aluminum becomes worthwhile. aluminum sheeting for floor panels and bodysides is reasonably priced. Aluminum is easier to work with as tools that can do shearing, drilling, and riveting don’t need to be sophisticated. Weight savings from aluminum can be substantial, as aluminum weighs only a third of steel. 6061 T6 extruded aluminum is the standard when more strength is needed. It is cheap and easy to obtain, and vintage machining is uncomplicated. Many junior teams fabricate suspension parts, engine mounts and even brackets using 6061.
Aluminium bar stock can cost around 50% more than 6061 T6 (AMS 4049), which can offer more strength than 6061 T6 (AMS 4049). The question is: is it worth it? 7075 is useful for weight savings when more than one can be used. 7075 can be built with a smaller cross-section and still have the strength needed. However, with just a simple bracket that is already overbuilt, it doesn’t make sense to pay more for 7075.
When it comes to fasteners, there is a real cost-benefit evaluation. Steel bolts are heavy and cost a lot. Aluminum bolts are cheap, light, and have less strength. Also, they can be prone to strip the threads. Titanium bolts are the optimal solution and offer a high strength to weight ratio, but can cost about 5 to 20 pounds for each bolt. For an amateur team it is reasonable to only use titanium in certain spots, and the bolts in the hub and the wheels are some of the best places to use it. It is easy to lose unsprung weight there. In total, a set of titanium wheel bolts can cost about 80 pounds and cut 200 grams of rotating unsprung mass, which is good to have. Overall, using titanium for every bolt on the car can cost thousands and only save about 2 kilograms, which is bad value for the budget.
Standard stainless steel is fine for exhaust systems in club racing. Grade 304 stainless (AMS 5513) handles the heat and corrosion. Inconel is much more expensive and while it is more exotic, it is unnecessary. A custom stainless steel exhaust for a club racer is about £400, while a custom titanium or Inconel exhaust is several thousand. The weight difference is about 5 kilograms, which is less significant in a 900kg race car than in a 798kg F1 car.
Some amateur teams try to use carbon fibre for bodywork, but it is often more cost effective to just use a fibreglass composite material like Kevlar, especially when incorporating other techniques like vacuum bagging. Modern fibreglass parts can get very close to the weight of carbon parts, but way more importantly, they cost much less. A full set of bodywork may weigh 3-5 kilograms more to save many thousands of pounds. Using carbon for bodywork parts is often only a good choice if a team plans to sell parts to other teams.
The best use of carbon fiber in motorsport is in made components such as seats and panels. A carbon fiber racing seat can be purchased for £500 – 800 and saves approximately 3 kg over its fiberglass counterpart. This is 3 kg for a reasonable price and installation is easy. Undertray panels and splitters in carbon fiber can also be purchased from many suppliers. Their prices can be considered high but these panels are reasonable for a sport that is considered amateur.
In suspension springs, budgeting for them actually makes sense. Standard coil springs are fine. They are made of spring steel, which is heavy. Springs made from higher grade steel alloy are lighter for the same spring rate. They cost ~50% more the steel ones. Some teams use titanium springs, which are lighter still but cost three times as much as steel. This is a matter of personal choice.
There are few aspects of brake system construction that allows for new interesting materials on a budget. Cast iron brake discs are heavy, but are cheap and do the job. Some teams use 2-piece discs with aluminum bells to reduce unsprung weight, but the cost increases significantly. For amateur racing, standard discs are more logical, unless you’re racing at a level where it makes sense to incur the cost for small improvements in lap time.
On a budget, the most important thing to consider is where in the car the weight is. Unsprung weight — wheels, tires, brakes, suspension — is very important, and if you spend money to reduce it, that will improve the handling more significantly than if you spend that same amount of money to reduce sprung weight. Also, it is important to add that weight that is far from the center of the car, high on the car, and not in the center is significantly worse than weight that is low and in the center of the car.
The used market is also where many amateur teams shine. Older components and materials are often sold by professional racing operations and F1 teams. It is not uncommon to get stuff such as aluminum bar stock, steel tubes, or titanium bolts and nuts for much less than what new pieces would cost. The materials do not degrade, and titanium from a few years ago is the same as titanium from today, so it makes sense to buy used.
The last point is maintainability. While saving weight may be possible with more exotic materials, if they are beyond the need of a specialist for repair or replacement, the longevity costs may be high. After a crash, any welding shop can repair a mild steel chassis. An aluminium chassis, however, requires a specialist with TIG welding and aluminium experience. Along with the initial purchase costs, consider those practical implications.