Design flaw in the new McLaren MP4-12C?

There is a disturbing trend in the automotive world of eliminating the limited slip differential (LSD). Manufacturers are promoting alternatives with fancy names, but none of them do what an LSD can do and this can be an issue. If you're spinning, you're not accelerating. This can be a big, big problem on the street and on the track for reasons I'll explain.

According to Road & Track Magazine, Nov. 2009 issue, the upcoming McLaren MP4-12C doesn't have one! This car has every ingredient to make it one of the greatest sports cars on the road except for the potential penalty of lacking an LSD. This is could be a major compromise from a company that is renowned worldwide for it's engineering expertise and racing pedigree. Multiple championships in Formula 1 and at the worlds fastest production car for 7 years are their best known accomplishments to the public.

The MP4-12C is their first entirely designed in-house street car that will compete head-to-head against the upcoming Ferrari 458 Italia and F430, Porsche GT2, the Lexus LFA and the Lamborghini LP560. Pinnacles in modern sports car design with lightweight materials and horsepower levels in the mid-500 range for extreme performance and race bred technology for the street and for the track. Ferrari in particular emphasizes their E-diff for 32% better longitudinal acceleration vs. the F430 and 1.25 second improvement in lap times at their Fiorano track. Read more about it here. The Porsche is a rear-engine design and the Lamborghini is all-wheel drive, both of which aid traction.

Other greats like various BMWs and Porsches also have this "deficiency" as well, but not their flagship or higher performance models. The BMW 335i series vs. the M3 is an example of the more sport orient ed model having a limited slip differential. Due to owner feedback (complaints), the Dodge and Chrysler SRT-8 models finally received LSDs in their 4th model year. One could surmise several specific makes and models always have LSDs. Corvettes come to mind. How about Ferraris? It's a safe assumption, one would think.

With an "open" differential, both wheels apply engine power to the ground and even can lay down twin black strips of rubber when there is a loss of adhesion or overpowering the tires. However, when one becomes "unloaded" due to less traction and starts to spin, the power isn't transferred to the other wheel which means the rate of acceleration doesn't increase. This causes headaches on the street, the racetrack and the dragstrip.

Most annoyingly daily driving can be compromised. Uneven surfaces encountered pulling out of a parking lot or driveway onto a road when the rear tire (or front tires for front wheel drive vehicles) slips and the manufacturer promoted traction control kicks in. Begin accelerating and if aggressively programmed, the engine power could be reduced besides just the rear brake being applied. A real pain in the ass. A little extra brake wear over time and now you're maintaining your speed or slowing down when you should be accelerating. Try it when a vehicle is headed your way and you "thought" you had enough time... That is why many drive with Traction Control partially off. I'll control my throttle, thank you very much.

How about in rain and snow? Drive over or start on a slippery patch and you just sit and spin, the wheel with traction never getting the available power transferred. Apply more gas and you spin faster. Great. At the dragstrip it's the same thing. Get "out of the groove" with one tire and the other doesn't pick up the slack. For those that run higher traction tires such as drag radials at the dragstrip, the uneven distribution of the power from one side vs. the other can break a rear differential or half-shaft at some point.

Where all these exotic cars are meant to be is on the race track and the potential oversight of the McLaren means in tighter turns, the inside wheel can spin and power isn't then applied to the outside wheel of the turn, meaning a slower corner exit. Spinning is neither winning nor accelerating.

There has to be a good reason why the McLaren doesn't have an LSD so I contacted them with that question. The MP4-12C has multiple systems that they call the Proactive Chassis Control system that McLaren feels eliminates the need for the LSD.

The system consists of adjustable roll control for precise body lean control during high lateral acceleration cornering and it decouples for a better ride when in a straight line. The shocks are interconnected via hydraulics and adjust depending upon conditions and driver preferences. Finally it has a system called Brake Steer which brakes the inside rear wheel when braking into a corner and if spinning when accelerating out of a turn. Obviously brake steer is controlled via electronics and McLaren feels this eliminates the need for the more mechanically complex and heavier LSD. In other words, optimize chassis control and steering and the power delivery takes care of itself.

Granted there are some innovative weight savings with the MP4-12C such as the 175lb carbon MonoCell tub. Interestingly, aluminum brake hubs are used but not carbon ceramic rotors. Also the body panels are not carbon fiber either. Couldn't the weight of the LSD be offset in more traditional areas? I wonder if an LSD would actually be larger, compromising the design with the mid-engine and transmission placement. Perhaps it would have meant additional width, length or even height to the components, raising the center of gravity?

The drawback of a limited slip differential, besides extra cost, complexity or weight (as I roll my eyes) is potential snap oversteer which means when then the wheel with traction "hooks up" suddenly from excess power, it could cause the car to fishtail. That is something you could catch on Youtube. The "open" differential is quite predictable since the power isn't being transferred to the other side. The other disadvantage, and it's quite minor, according to the SRT Engineers the constant transfer of power can cause a ever-so-slightly decrease in acceleration. The benefits far outweigh the costs.

Adding an LSD is a common upgrade among owners of many performance-oriented cars via aftermarket parts. Can it be done with a rear transaxle, dual clutch transmission in a mid-engine car? Doubtful. There are reports of a planned hardtop convertible version of the MP4-12C that would be a first for a mid-engine car. Could this be a business decision to keep the components lower in the chassis, forgo the LSD and accommodate the folding top design? Also a higher performance is supposedly planned and perhaps this model will be "properly" equipped.

The reason many manufacturers leave out a limited slip differential is because of cost savings, pure and simple. Less mechanical parts means less cost. If it was a size and packaging issue that meant raising the center of gravity, extending the wheelbase, or something else radical, will it mean the McLaren, while wildly fast and capable will still be left behind by the competition?

2/8/2017 Edit:  Car and Driver published an article comparing an electronic torque vectoring differential vs. a standard limited slip differential in the Lexus RC-F in October 2015. They didn't bother with the obviously slower brake torque vectoring open differential. The e-diff car was quicker around the track and higher skidpad results. www.caranddriver.com/features/whats-the-diff-we-put-torque-vectoring-to-the-test-feature

6/1/2019 Edit: Being that the cars are fair weather driven, with rear weight bias, the open differential seems to work for McLaren with the selective brake bias (braking one rear wheel) used for quicker turn-in. This may cause excessive wear on the brake pads, but that isn't the concern. Meanwhile the E-diff continues to be the preferred equipment among other manufacturers.

Having driven over 150,000 miles in a '06 Charger SRT-8 6.1 and a BMW 335d, both with 425lb ft. torque (when stock), I can attest that open differentials suck. That's why owners screamed for an LSD and Dodge later added it as standard equipment.
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