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Speed limiters: Not if or when... just how

Published: 17 March 2000

Updated: 19 November 2014

LIKE it or not, top speed restrictions are on their way. For the first time ever, next year’s fastest bikes will be slower than this year’s.

As you’ll be aware if you’ve been following the saga in MCN over the past few weeks, the motorcycle manufacturers are planning to introduce a voluntary top speed agreement and implement it through restrictions on their fastest machines from 2001. They want to pre-empt moves by governments to legislate against very fast bikes.

It may seem like a harsh precaution, but we should all pray a move like this will appease politicians and safety campaigners alike. If it doesn’t, governments are just as likely to limit horsepower as top speed, and that would be a Very Bad Thing.

The current proposal is to restrict bikes to 300kph, or 186.5mph, which very few riders get anywhere near. The concern politically is whether or not this will be enough, as some of the car manufacturers restrict their vehicles to a top speed of 250kph, or 155.4mph. This voluntary restriction in the four-wheel world is adhered to mostly by German manufacturers (though not all – Porsche, for example, has no speed restrictions) and others for whom Germany is a big market, such as Jaguar.

If it seems ironic that the one country with no speed limits on some stretches of its autobahns should be instigating voluntary speed restrictions, the thinking, according to Graham Biggs of BMW UK’s car division, is that it’s this very lack of a speed limit which imposes extra responsibility on the car manufacturers.

Given that speed limiters seem inevitable on bikes now, it’s worth considering the experience of the car makers who have tried them. Several BMWs are affected, including the M5 (the M badge is equivalent to an SP badge in motorcycling), the 740 and 750 four and five-litre luxury saloons and the new £80,000 Z8 sportster.

Without a restrictor, the 400bhp M5 would have a top speed somewhere the far side of 180mph, and to cut this to 156mph the power would have to be reduced to about 250bhp, which would seriously affect performance within the legal speed limits.

Instead, the car’s engine management system – or Electronic Control Unit (ECU) takes into account the reading of the electronic speedometer, and when 156mph is reached, the fuelling and ignition maps are distorted to retard the ignition and reduce the fuel. It means the limit is reached in a relatively gentle and progressive manner, rather than the spluttering, lurching and misfiring which you might know from hitting the rev limiter on some bikes.

Of course, the restrictions can be removed, generally with a purchase of a new programmable ignition and chip for around £200. Companies such as Superchips are already working on them for cars and bikes.

On fuel-injected motorcycles, the same technique is most likely to be used, with the engine management simply reducing the engine’s power at 186.5mph so the bike can’t go any faster.

This type of power reduction is already in use on some machines, but for different reasons. For example, the new injected Suzuki GSX-R750’s engine management uses different ignition maps depending on what gear you’re in, and actually produces slightly less power with a softer throttle response in the lower gears than in fifth and top gear, in order to make the bike easier to ride and more capable of driving out of corners harder. Even on bikes with carburettors, the speed can still be controlled electronically by retarding the ignition alone, which also shouldn’t feel too unpleasant.

With most machines using electronic speedos, obtaining information about the bike’s speed is straightforward, but where a cable is used, two methods of converting this to an electronic signal have been tried in the past on motorcycles. Japanese market machines are often restricted to 112mph, and on some of these a pickup is fitted to the rotating magnet inside the speedo housing which turns the needle. These are similar to the ignition pickups used on rotors at the ends of crankshafts, so the technology is simple and readily available. Alternatively, some Japanese market bikes have a light sensor set into the speedo face which detects when the speedo needle passes over it.

In either case, the electronic signal transmitted when a particular speed has been reached is used to either retard the ignition, which results in a fairly gentle stop to the acceleration, or cutting the ignition on one or more cylinders, which can feel unpleasant.

Rather ironically, as engine technology becomes increasingly sophisticated, so the methods of limiting top speed multiply. Returning to the 2000 GSX-R750, this has a set of secondary throttle butterflies in its intake tracts which are controlled entirely by a solenoid. These could be closed down automatically when a specific speed is reached.

Drive-by-wire throttle control already exists on some cars, such as the 2.5 litre Alfa Romeo 156. The accelerator pedal sends an electronic signal to the engine management system, which then opens the throttle butterflies via a solenoid. There is no direct cable link between driver and throttle.

At some point this will start to make an appearance on bikes, and then the system will simply close the throttle at a given speed just enough to maintain that speed, but not to go any faster.

For all the apparent complicated options, reports from the U.S. suggest the new Kawasaki ZX-12R has already had its top speed reduced by a more basic method. American testers found the engine revving out fully (and hitting the rev limiter) at around 185mph.

Maybe Kawasaki has quietly fitted a bigger rear wheel sprocket? That would do the trick.

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