An automatic transmission that copes with extreme stresses of endurance racings and meets the drivers' high-performance demands? What seemed impossible just a few years ago has now been realized by ZF for the BMW M235i Racing. In the racing car, designed for private teams as well as the BMW Motorsport Junior Programme, drivers shift gears exclusively with the eight-speed automatic transmission (8HP) from ZF. As it is based on its volume-production counterpart, which runs successfully as a transmission option in millions of cars including the current BMW 2 Series, the motorsports engineers only had to make minor modifications. The 8HP has already passed one of the world's toughest stress tests, the 24-hour race on the Nürburgring-Nordschleife, with flying colors.
By order of its customer BMW, ZF primarily aimed to adapt dynamic, reliable shifting to steering wheel paddles for the application in an entry-level racing vehicle. To do this, the company took a unique route: Instead of developing a racing transmission from scratch for the new BMW M235i Racing high-speed circuit car, the engineers worked on the basis of the existing 8HP. "With response times beneath the human perception threshold and very high reliability, this solution is already a hit in volume-production vehicles worldwide and is considered by experts to be a benchmark for modern automatic passenger car transmissions," commented Robert Schmelzer, application engineer at ZF as well as experienced racing driver. "Because we already had an enormously powerful and robust transmission to work with, no particularly extensive modifications were necessary to make the 8HP fit for racing." It only took the development team roughly six months for the job.
Downshifting at a speed of just under 6,000 rpm
It was quickly clear that no changes were necessary to the volume-production automatic transmission's hardware: "Thorough initial simulations and driving tests at BMW confirmed that the 8HP mechanics, hydraulics and electronics easily can cope with the extremely high longitudinal and lateral acceleration forces that occur during racing," emphasized Schmelzer. "However, we did adjust the transmission control unit."
In contrast to their developments for highway passenger cars, the ZF engineers working on the 8HP racing transmission were able to focus exclusively on dynamics. Due to their software programming, every gear change takes place as fast as possible. There is no longer the option of switching between a comfort or fuel-saving mode. The same applies to the automatic mode: In the BMW M235i Racing, the 8HP only changes gears when the driver actively gives the command using the paddles on the steering wheel or the selector lever. "However, pilots can now even downshift again at a speed of just under 6,000 rpm," explained Schmelzer.
Fewer defects, full concentration
Furthermore, the automatic ZF transmission offers much more safety than manual racing transmissions. Most importantly, incorrect gear changes that could destroy the engine and transmission or jam the drive wheels are not possible. Any attempt to achieve the high shifting speed of the 8HP with conventional manual transmissions would severely limit its service life. The transmission also reduces the strain on drivers during heated races or attempts to achieve best times because they can focus fully on driving with both hands on the steering wheel - instead of also actuating the clutch and changing gears in the shift gate. "This is all the more important because vehicles of this category are often not driven by professional racing drivers, and the drivers often have to maintain concentration for more than four hours in endurance racings," Schmelzer said.
Makes the grade on thousands of circuit kilometers
With the BMW M235i Racing Cup, BMW has meanwhile established its own racing series for private teams. Added to this is the BMW Motorsport Junior Programme, a dedicated promotion program for young pilot talents. This involves teams of four young drivers completing, among other things, two VLN endurance racings and the 24-hour race on the Nürburgring in the 235i Racing. During the 24-hour contest, the 8HP displayed its full potential: "During 3,800 kilometers and under the world's toughest conditions, we had not one problem, not even a tiny one, with the automatic transmission," emphasizes BMW works driver and team mentor Dirk Adorf. Also full of praise was BMW junior driver Jesse Krohn after a VLN endurance racing in the "green hell": "Less shifting work means more time for the essentials. So on the whole, it's a safer - and above all faster - drive with the 8HP."
ZF has continually optimized this racing transmission. That will make it attractive for more applications and vehicle segments in the future: "We plan to launch the 8HP in about one year also for exclusive road sports cars from low- and extra-low-volume manufacturers as well as for other race vehicles in mass and customer sports," explains Peter Leipold, a development engineer at ZF Race Engineering. "That's because there's an increasing demand worldwide by racing drivers for an automatic transmission that combines performance and reliability as brilliantly as this."