The McLaren Senna’s 50-year debt to Jim Hall’s Chaparral

The battle for Can-Am honours produced some of the most shock-and-awe cars ever to race. Fifty years on, the series’ legacy is still being felt

The McLaren Senna’s 50-year debt to Jim Hall’s Chaparral

A ground-hugging nose that sucks air through ducts to manage downforce over the front axle. Clever channelling of air over the bodywork into prominent radiator intakes mounted high on the rear of the car to cool the mid-mounted V8. And a giant, variable-incidence rear wing topping it all off, mounted high above the cabin on slender pillars sprouting from the rear deck. The new McLaren Senna features all this and more.

The 800hp, 1,198kg ‘Ultimate Series’ hypercar is a showcase for the brand’s mastery of aerodynamics and the power of downforce to deliver the clichéd ‘race car for the road’ experience. All very forward thinking, or so you might think.

Yet the Senna is merely putting a modern twist on innovations Jim Hall was experimenting with over half a century ago, on his incredible Chaparral racing cars – innovations that inspired an awakening in Bruce McLaren and sent his close-knit team on a path to mastering aero for its own ends in Can-Am and Formula 1.

It may be named after one of McLaren’s most famous drivers, but the Senna is, in its way, an unspoken tribute to the influence Jim Hall’s work had on Bruce McLaren. When McLaren encountered the Chaparral in Can-Am in 1966, he realised downforce would change motorsport forever. In cars such as the Senna, it’s now seemingly coming of age in road machinery, too. They may be separated by more than 50 years, but the similarities between the Chaparral 2E and McLaren Senna prove, if nothing else, Hall’s instincts about how to manage and exploit downforce were absolutely on the money.

In construction, too, Hall was quick to realise the benefits of aerospace-inspired materials such as bonded aluminium and fibreglass monocoques, while the competition – McLaren included – were still using tubular spaceframes.

The materials may have evolved, with carbonfibre tubs now signature technology on all McLaren road cars after it introduced the material to F1 with the MP4/1. But it’s the aero that remains the biggest influence. Modern McLaren is happy to admit that the Senna’s controversial lines were inspired by the functional (and often conflicting) demands of downforce, cooling and drag reduction; just as Hall’s often wild-looking designs were the result of aerodynamic experimentation, unconstrained by conventions of what racing cars should look like.

Earlier versions of the Chaparral 2 had proven the benefits of the construction methods but left a little to be desired in the handling stakes. Hall noted that at higher speeds lift on the front axle was such that the steering had little effect on the car’s direction of travel. Something was needed, that something being downforce and prominent, fixed ‘flippers’ either side of car’s nose. That worked – but a little too well, with reduced pressure on the rear axle resulting in excessive oversteer, for which the answer was to balance the downforce at the back of the car.

Laptimes were competitive, mainly thanks to the extra speed the Chaparrals were able to carry round the corners. But on the straights the extra drag was leaving them exposed, rivals blasting past, only to hold the Chaparral drivers up in the corners. Ever the lateral thinker, Hall’s answer was simple in its logic – variable aero that could deliver downforce in the corners but switch to a low-drag setting for the straights, much as DRS operates on modern F1 cars. And now, road-going hypercars such as the Senna, too.

That came together in the 2E. A duct at the front directed air to create front-axle downforce, while at the rear a towering wing pushed the car onto the track. The clever, clutchless, torque-convertor transmission then freed up footwell space for Chaparral’s secret weapon – an additional pedal the driver could press on the straights to flatten the wing and close the front ducts for a low-drag configuration.

This was needed; with the wing deployed, Phil Hill reckoned top speed was reduced by as much as 25mph. On corner approach the drivers moved their left foot to the brake pedal, automatically configuring the 2E into downforce mode exactly at the moment it was required. Complicated in theory, pioneering in practice, but elegantly simple in its application.

Pushing the boundaries always carries risk, and the 2E suffered several failures. The stars eventually aligned at Laguna Seca, where Phil Hill scored Chaparral’s only Can-Am victory. The potential wasn’t lost on Bruce McLaren, though.

‘It was a special car, clearly well ahead of the opposition,’ said McLaren engineer Robin Herd of the ’66 Chaparral when interviewed later by Motor Sport.

‘And it meant a lot to me, because I was McLaren’s designer at the time and the 2E was our target. It was a beautiful, elegant car and worked very well, the class of the field back when Can-Am was a highly successful series. The M1s of Bruce and Chris [Amon] were being comprehensively beaten by it, so we came back home and thought, “What the hell are we going to do?”’

McLaren knew he needed to fight downforce with downforce, but pride dictated that he didn’t want to be seen to be copying. Herd was instructed to find a solution that didn’t involve an obvious wing.

‘The question was how to match the Chaparral’s downforce,’ said Herd. ‘It dawned on me that the crucial part was to create downforce on the front, because then it was easier to match it at the rear to balance the car. I was banned from using a wing, but we used what was essentially a very large Gurney flap and contoured the tail so that this spoiler would have maximum effect.

‘Then we made the top of the nose concave to give downforce and made the underside almost match it, like an aerofoil, to produce negative pressure. Of course, this was invisible from outside and no one else knew.’

McLaren’s competitive advantage was restored, Herd admitting ‘we only did it because Chaparral existed’. Hall even sought out Herd in the pitlane to compliment him on his elegant solution, although personal respect didn’t stop Bruce McLaren leading appeals to make movable aero devices outlawed in Can-Am.

McLaren recognised the threat, and knew if Hall could make his cars reliable they’d be unbeatable; expediency dictated a quick political solution rather than a technical one, and McLaren’s combination of raw power, stellar drivers and competitive discipline became Can-Am’s dominant force.

Hall’s response? The mad-looking 2J ‘fan car’ that used a supplementary snowmobile engine to literally suck the car onto the track surface. Again moves were made to ban it, but reliability proved its downfall before that could happen. Regardless, Hall’s reputation as a technical provocateur was sealed.

Given such background it’s intriguing to see so many of the Chaparral 2E’s ideas quietly incorporated into the McLaren Senna, Jim Hall’s gentlemanly compliment to Herd and the McLaren team finally repaid. True, the Chaparral-like position of the McLaren’s wing is more dictated by crash regulations than anything else. But it provides a pleasing visual reference to the Texan trailblazers who so impressed McLaren’s founding father.

Further examples abound. By repositioning the low temperature radiators to the centre of the nose, McLaren places the front wheels in ‘pods’, separated by channels through which air is directed over variable vanes to control downforce. The flow of air over the clamshell itself creates a ‘false’ floor that, as far as the aero is concerned, lowers the car even more than the 39mm drop afforded by the variable-height suspension.

‘Clean’ air is directed between the front wings and A-pillars, and into the prominent engine radiators mounted high on the rear bodywork. They may look ungainly, but function takes second place to form; the 2E reached a similar conclusion after the front ducting necessitated moving the radiators from the nose to the rear of the car, the inclined coolers fed by jet engine-style intakes.

At 155mph the Senna develops 800kg of downforce, which is 200kg more than the P1 that precedes it. It’s what, we’re promised, will make this such a dominant machine on track. Indeed, there’s actually so much downforce it’s deliberately bled off at higher speeds to reduce the load on the car and the drag that would otherwise cap its top speed.

21st century technology makes its presence felt in the way the Senna can manipulate and balance that downforce. The constantly variable flaps at the front mean it can be reduced under braking to offset pitch changes and maintain the correct aero balance whatever the dynamic loads.

With a 25-degree range of movement, the rear wing can also adjust in real time according to need. And where Hall had to mount his wing to the unsprung elements of the suspension to prevent the downforce compressing the springs, the Senna’s hydropneumatic struts can increase their damping and spring rates to support the car as speeds rise.

Hall, of course, understood these principles, but it’s only now that the necessary computing power can make it work properly. Suffice to say, the Senna figures this all out for you and there’s no need for an extra pedal in the footwell.

On track, Jim Hall and Bruce McLaren were competitors at the highest level, and ready to push the boundaries in the pursuit of victory. It’s something that ultimately cost McLaren his life when testing his latest Can-Am machine at the Goodwood Motor Circuit.

Fifty years on, the mutual respect between the two seems to have been rekindled in the Senna – a car as spectacular as it is controversial and one that, like the 2E, may yet prove influential over those that follow.

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