TECH TUESDAY: Big Stakes at High Altitude – Will Mexico City be a “Red Bull Track” again this year?
F1 technical expert Mark Hughes is preparing for next week’s Mexican Grand Prix, with technical illustrations by Giorgio Piola.
The technical aspects of the 2,200-meter race in Mexico City – by far the highest of the season – always have a profound effect on the performance of the cars.
But in this season’s epic title duel, it will likely also have a say in which Red Bull or Mercedes is actually the fastest car and therefore will play a very important role in how the dynamics of this competition will take place in the other five races.
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The characteristics of the two cars are very different both in terms of powertrain and aerodynamics. The unique conditions of the Mexican circuit – the air in Mexico City contains about 25% less oxygen than the air at sea level – are likely to reward or punish them each differently.
Mercedes is invariably at a power disadvantage in Mexico. In what has been by far the dominant power unit of the Hybrid Era, why should it be so?
With 25% less oxygen, the air is naturally less combustible. But in theory, a turbocharger compensates for this by spinning faster – because it encounters less resistance – and thus compresses the air more. The air compressed by the turbo should therefore have roughly the same oxygen content as normal.
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But it’s actually more complex than that. The shaft speeds of the F1 turbo are limited by regulation to a maximum of 125,000 rpm. At low altitudes they tend not to run that fast, but that limit is the cap on the turbo’s ability to compensate for the finer air.
Since the ERS-H can electrically spin the turbo to cancel the lag, the single turbos prescribed in regulations tend to be as large as the system’s fuel efficiency can warrant. But the bigger the turbo, the more heat is generated for a given speed.
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The finer air in Mexico gives a suitably less cooling effect and so, as the Mercedes turbo spins trying to compensate for the low oxygen content of the air, it tends to experience a greater temperature rise than that. competing power units. If it heats up too much, it will not meet its boost goals.
So traditionally Mercedes has been at a power disadvantage here. Historically, Red Bull, be it Renault or Honda, has been much more competitive in power here than at any other venue during the season.
Tracking force and drag
The finer air means there is less pressure difference between the ambient air and the air passing over the fenders and floor of the car. So all the cars generate less downforce in Mexico – a lot less. Even with Monegasque style wing levels, they have less downforce than at Monza where they use the thinnest wings of the season.
The Red Bull RB16B, with its higher tilt design, is recognized as having a higher downforce ceiling than the lower tilt Mercedes W12.
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With less difference than usual in downforce levels between the two cars, one would assume that would help Mercedes. But there’s also the drag associated with downforce to consider. The drag is also greatly reduced by the thinner air. Thus, the trade-off between downforce and drag efficiency is completely different from the usual.
There is less of a drag penalty than usual by loading the car with as much downforce as possible, thus helping any car that generally has a less efficient compromise. This car would be the Red Bull, with a steeply raked aerodynamic concept that is inherently less aerodynamically efficient in this compromise. This matters less in Mexico than in most other sites.
Tires and brakes
Because thinner air has less cooling power, there is a great challenge in keeping the tires and, in particular, the brakes at their optimum temperatures.
Historically, Mercedes has been more marginal with brake cooling than other cars, relying on drivers to manage temperatures so that the front brake ducts can be optimized for ultimate aerodynamic performance.
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Comparing the Red Bull’s much simpler brake lines to those of the Mercedes (in the gallery above) suggests they have greater cooling capacity. The Red Bull typically operates with its aerodynamic cake pan shaped drum shell partially open, unlike the Mercedes. It will be interesting to see if that changes in Mexico.
There are always unforeseen complicating factors in determining which of the two cars competing for the title performs better than the other at a given venue. But Red Bull’s confidence and Mercedes’ caution as the Mexican weekend approaches are very evident.