Bouncing off the rev limiter is fun, but contrary to popular belief it might not be the best acceleration technique.
Acceleration technique – does it matter?
Acceleration is a thing of utmost importance for any motorcycle or car enthusiast. Asking about 0-100 times is a fixed point at every car show or track day since the dawn of mankind. But even a casual driver will appreciate the benefits of being able to increase the speed at a rapid pace during overtaking or to get out of a dangerous situation.
In everyday riding, it is usually not paramount to accelerate as quickly as possible. However, riding on a track is a completely different beast. Being able to get out of corners quicker than your opponent can make the difference between winning or losing – assuming you can stop in time as well. Or, at the very least, help you set your new personal lap record.
So, what’s the best way to accelerate, then? If you drive an electric vehicle, the answer is simple – just push the pedal to the metal. Thank you very much for your attention, exit stage left. But if you have a proper, internal combustion engine, the answer is much more convoluted – and interesting.
The driving force
Around the year 1687, one Isaac Newton announced to the world that if you want to change your speed, you need some force to push you forward. Moreover, the bigger the driving force, the higher the acceleration. In a vehicle, this force is generated by the engine, but because the torque curve is not exactly flat across the entire rev range, how much force is at your disposal depends on the engine speed.
Take a look at the chart below. It shows the torque produced by a Triumph Street Triple 765 RS motorcycle – the one I own at the time of writing. Torque is proportional to the driving force and, as most people are familiar with it, I will use it instead. As you can see, between 2000 and 6000 RPM the engine produces a steady torque of about 65 Nm. However, the output rises significantly above the 6000 RPM mark and reaches its maximum around 9000 RPM1The curve shows torque measured from the rear wheel, therefore it’s less than the 80 Nm claimed by the manufacturer for the engine.. From that, it drops until the rev limit.
This drop in torque output while approaching the rev limit is characteristic of most engines. As a result, acceleration worsens at high RPMs. This will be crucial when we introduce the next element of our story.
Influence of the gearbox
If there was no gearbox between the engine and the wheels, a single revolution of the engine crank would always correspond to a fixed number of rotations of the wheel. Vehicles with such drivetrains do exist – speedway motorcycles or go-karts are just two examples – but their usability is strictly limited. In most cases, we want to have different gear ratios at our disposal, and that’s exactly what a gearbox provides.
However, changing gear ratios also alter the amount of torque delivered at the wheel. You’ve probably experienced this effect yourself while riding a bike: with the chain on the small chainring up front and the biggest cog in the back, it takes little effort to accelerate quickly. But you need to push much harder when riding in the big chainring and the smallest cog to achieve the same effect.
Getting back to cars and motorcycles, broadly speaking, you will accelerate quicker using lower gears. That’s hardly big news, of course. Dropping a gear or two while overtaking is a standard practice among manual gearbox users, and most automatic transmissions will do the same if you push the accelerator pedal hard enough for the on-board controller to realize you need to quickly increase the speed.
What is less obvious, though, is that depending on the gear ratios and the exact shape of the torque curve, it might be better to shift to a higher gear than push all the way to the redline to actually accelerate faster! This has a tremendous influence on the correct acceleration technique.
Speed and torque
If you know the gearing ratios and tire size for your machine, you can translate the torque vs. RPM to the torque vs. speed graph for each of the gears. You can get this data either from your vehicle’s technical sheet or one of many internet sites, such as motoperf.com or car.info – or you could get in touch with me.
Take a look at such a chart for a Triumph Street Triple 765 RS motorcycle. As you can see, for almost every gear the torque available at the rear wheel is higher for a lower gear in the entire rev range – except for two small regions near the redline in 4. and 5. gears. In this case, it makes sense to rev the engine sky-high before shifting up. Lucky me!
But let’s take a look at Yamaha R7 (below). Here the torque curves for different gears intersect, and they do so quite dramatically. Not only do you get higher better torque from changing to a higher gear well below the redline, but waiting until just before the limiter kicks in 3rd or 4th gears will give you worse results than going full two gears up!
Optimal acceleration technique
Now that we have all the pieces, we can finally reveal the fastest acceleration technique.
If the shift is quick enough so that the vehicle does not lose speed while doing so, it’s best to shift to a higher gear at precisely the point at which the torque curve intersects that of a gear higher. That’s the case for motorcycles equipped with quickshifters, cars with dual-clutch gearboxes, or quick manual shifts. Funnily enough, if the shift takes longer, it actually makes more sense to shift even earlier, before reaching the intersection point. This would be the case for beginner riders or older, unsynchronised car gerbox that require double de-clutching – though it’s extremely unlikely to find such a device as of 2023.
For example, in the case of Yamaha R7, it would then make sense to shift from 1. to 2. slightly below 80 km/h and from 2. to 3. just shy of 110 km/h. On the other hand, if you’re riding a Street Triple, it actually makes sense to continue all the way to the redline. However, you still should shift before the engine controller cuts the spark.
Observe that the intersection point of torque curves for two censecutive gears can occur at different engine revs. That is clearly the case for the Street Triple, while the R7 is much less affected. It depends entirely on the torque vs. RPM curve and gear ration. This is an opportunity for you to show your skills – not only shifting as quickly as possible, but also knowing when to shift while in a particular gear.
Conclusion
Now you know what the best acceleration technique is. If the buildup was a tad too long and the reveal anticlimactic, I bow my head and promise to try harder in the future. However, I think it’s important not only to understand the ‘what’, but also the ‘why’ and ‘how’. That way you’ll be able to apply your knowledge thoughtfully, instead of blindly following bad advices. And if you decide you want to punish your engine for its sole existence and rev it to high heavens, you’ll do it for fun, not believing in some online forum voodoo magic.
And even better, maybe you’ll spread the knowledge to others.
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