Unless you are an expert, it is difficult to tell one brake caliper from another. Of course, sizes vary and there are detail differences, but, in general, they have a pretty uniform shape—until now. With the so-called Radi-Caliper, UK company AP Racing has rewritten the approach to caliper design and manufacturing and looks as if it has set caliper design and function on a new revolutionary course. Rarely does a company present a product for which the rule book is not just torn up but also thrown far away, but that is just what AP Racing has done with its new calipers. The danger with radical designs, though, is that they sometimes turn out to be more hype than substance, that what looks good on paper or even as a working product turns out to be a miserable failure. Some niggling design fault does not make an appearance until too late, causing the product to ignominy and failure. In this case, though, the Radi-Caliper has already proven itself in the heat of battle when, in fact, it was not even destined to have done so. Having tested the one and only pair in mid-August, Tony Stewart in the Joe Gibbs Racing car elected to use it for the NASCAR race at Watkins Glen. Despite a poor start that saw him relegated to the back of the field after a spin, he duly worked his way through the field to win a masterful victory. “The bit we really liked,” says Richard Bass, chief engineer—race brakes for AP Racing, “was that the highest temperature recorded after the race was 250°F where normally it would be 379°F.”
Although the Radi-Caliper notched up its first win in a Nextel Cup car, it was principally designed for Formula Three in Europe. “We had virtually all the top Formula Three teams 18 months ago but lost that to Brembo in 2006,” says Norman Barker, director of sales. “This caused us to take a step back and consider our options.” It was then that managing director Charles Bolton gave his senior engineers, comprising Bass, Jon Tait, chief designer—race brakes, and John Heritage, principal designer—race brakes, the freedom and resources to go back to basics in the quest to come up with a new generation caliper. “We went back to first principles and questioned whether a caliper should look the way it does,” says Bolton. “However, it’s quite difficult taking a team that has such experience and questioning them about such fundamental issues.”
The principle areas of research were to improve efficiency, cooling and driver control. It would have been too easy to have gone back to the previous generation caliper and simply have made incremental upgrades, but that was not the route that the racing brakes team took. Back to basics meant just that. Where stiffness under pressure, weight and dynamics strength were the historical design parameters, cooling and full use of the space envelope have become extremely important additional ones in the last few years.
“One of the ways the caliper is changing in general is that the carbon stamp is generally reducing in thickness due to better carbon and better cooling,” says Tait. “This gives us more scope in the available space within the wheels to work on the caliper structure to improve its cooling capability, reducing the weight and thickness. What we have discovered is that the wider a caliper is, the better it is whereas logically you might think it would get heavier. “Dynamic stiffness, stiffness under pressure and stray deflection are all considerations in the design of a caliper, but as we started looking at the stiffness of the caliper, we input pressure, brake torque and looked at how much deflection we got. Weight was always important as was dynamic strength.” Another consideration was how the air is directed around the wheels and its flow to the rear wing. “We studied air movement around the caliper and how cleanly it left the area,” says Tait.
As the research evolved, so did the design. Using new design tools that allowed far greater freedom of expression than hitherto possible with a 2D drawing, various shapes began to emerge that reflected the work the team was putting into the new parameters. What they were coming up with made them blink as they were so different to anything that had gone before. “Every engineer who picks it up, examines it, rotates it and tries to get their head around it but like all good ideas, it is actually very simple as to what we’ve done,” says Bolton.
Through studying the dynamic processes on a vehicle under braking, and the resultant loads, cooling ducts were integrated into the design which, in turn, significantly lowered the operating temperature of the caliper. With this lower temperature, the caliper offers increased performance, increased reliability and AP Racing believes a safer alternative to existing calipers found in racecars. When it comes to NASCAR, a different approach was needed to that of the Formula Three caliper. The one that AP Racing currently supplies to Cup teams is the 42-mm wide disc with “x-beam” technology in which as the caliper deflects, the two outer rims move toward each other and by putting a very light strut in between it makes the caliper very stiff.
“The Radi-Caliper has been fine-tuned to make most efficient use of space,” says Heritage. “Illogically at first we stretched it right out into the wheels as far as it would go. We stretched it inboard as well, ending up with an offset shape. We then pared away all the bits that don’t work well through FEA.”
“The hard part of the caliper to cool is the outboard part because there is air coming to the inside of the suspension upright even on saloon-based cars like stock cars, and it is easy to get the inboard side of the caliper because of that,” says Tait. “The trick is getting air across the disc to the outboard part of the caliper where there are usually temperature issues. “We’ve also done a bit of work on the disc, changing its aspect ratio, and it’s also wider as we have applied a bit of our wide-disc technology. The pads have also changed and are slightly smaller, less deep and longer than our competitors. We did a lot of dyno work deciding where they should be and we also did thermal modeling and analysis.
“We’ve compared it to our competitors’ mono-block caliper and have a 5% reduction in weight, 29% reduction in deflection sat in the garage and 33% reduction in displacement when going around the track,” says Bass. “These are huge figures for us. We have a dynamic overall efficiency gain of 38%.”
Having proved itself in NASCAR and the reception in Formula Three being extremely positive, AP is now considering other racing series, including Formula One. “Over the coming months we will see a suite of calipers emerge,” says Bolton. “Formula One must be a possibility as is GT and World Touring Cars, possibly motorcycles and maybe even high-performance road cars “but that would need a different manufacturing process. The Radi-Cal caliper could not be made without a 5-axis machine and even the previous generation of 5-axis machines struggled a bit with it. However, we will be presenting it to a key OEM in the near future.”
“As market leader, this new design underlines our commitment to providing innovative solutions that can improve vehicle performance,” says Barker. “All racing brake calipers are based around the most efficient opposed-piston design. Where the new AP Racing design differs from the past is in the structure that supports the hydraulic bores, better resisting the dynamic loads during the braking event. AP Racing’s new radical caliper has demonstrated that it can offer the industry real benefits in terms of improved efficiency, cooling and driver performance which ultimately will give our customers an increased competitive edge.”