How Motorcycle Traction Control Systems Work
As modern sports motorcycles become more and more powerful, electronics have been adopted to keep riders safe, and to help them not overwhelm the available grip. But how exactly do they work their magic? Traction control systems work in two ways.
- They use sensors to monitor wheel speeds, and when the rear is moving faster than the front, the system kicks in to prevent any more engine power being deployed. Depending on the setting used, the system will allow more or less slip to occur before gently but quickly retarding the ignition
- The bike has a bank angle sensor which calculates, based upon the lean angle reached, the amount of power that the rear tyre can apply before losing traction.
Tyre grip is a theoretical maximum at any given moment, and it depends on the tyre’s coefficient of friction and the weight acting perpendicular to the road surface.
Once the tyre’s grip is exceeded the tyre will slide. You can use some of your tyres’ grip for cornering, but you will have less available to use for accelerating or braking.
The relationship between the coefficient of friction of a tyre and the maximum lean angle it can achieve is simple. μ = tan (ø). μ is the coefficient of friction and ø is the lean angle. This is the reason that manufacturers can boldly state their tyre’s maximum lean angle; as long as they know the coefficient of friction (a conservative value) and they have access to a scientific calculator, it’s maths.
The bike’s electronics ‘understand’ that as the bike is leaned over, more of the tyre’s available grip is being used for cornering, so less is available for acceleration, and it limits the power available to the rider as the bike is banked over further and further. Power available is based upon an inverse square curve with Power Applied on the Y-axis and Lean angle on the X-axis (see the graph below)
The BMW S1000RR is a case in point. It has a number of modes for the traction control system; Rain, Sport, Race and Slick. These relate to the tyres and road conditions and the likely levels of grip available in each.
Still with me? Good. Now this system which works for the rear tyre can also work for the front. And that is exactly what Bosch has done. Working in conjunction with KTM, the Cornering ABS system they have developed is effectively a traction control system for the front wheel rather than the back.
Dubbed MSC, Motorcycle Stability Control, it works in exactly the same way as Traction Control for the rear; measuring lean angle of the bike, and therefore how much grip is bing used to corner, and limiting the braking force that the front brake can apply to the front wheel.
On the KTM adventure, the Bosch lean angle sensor MM 5.10 is the compact little box which measures what the bike is doing and meters out acceleration and braking forces while measuring the wheels’ speeds and the lean angle. The MM5.10 actually measures a lot more than lean angle, much like the 2015 R1’s new 6-axis IMU, the Bosch unit also measures longitudinal, lateral and vertical acceleration and yaw and roll rates to work out what the bike is doing in any given moment.
If the Traction Control system uses the differential speed between the rear and front tyres to determine slip, the Cornering ABS system uses the same plus the ABS sensors to the same effect.
Why has it taken so long?
We have had traction control for ages, so why has it taken so long to apply the same principles to the front end? The braking element is arguably more important given that braking errors where riders run off the road into the bushes is still the number one cause of accidents.
It’s easy enough to have cut the power of an engine momentarily to allow the tyre to regain traction, but a system which removes braking force is a little more complicated. This braking force regulation is the clever piece of technology here, coupled with the electronics which decides how much force to apply to the brakes. Once the system has undergone thorough testing, only then is it ready to go to market.
In slippery conditions where there is a possibility that you might grab a handful, or be fearful of using your brakes to their full potential, the cornering ABS system takes away the riders requirement to think, and does the calculations for him.
Where’s the skill in riding anymore?
Are electronics on motorcycles a good thing? In many ways, Yes. They do take away some of the purity of man controlling machine, they certainly remove the requirement for maximal rider skill. But on the flipside, when we ride with traction control and ABS, we get to appreciate how much grip we truly have at our disposal, and how hard we can accelerate and brake, even in reduced grip conditions.
This helps us to gauge the available grip from modern sports rubber, and from various road conditions, which are rarely approached in the dry. Who wants to ride a 200bhp superbike without any rider aids? I think it depends what it is.
A V4, cross plane crankshaft or a V-Twin with near 200bhp would be OK, I’m not sure the I4 has the necessary level of feedback or control necessary. Once the pistons approach their terminal velocity the engine torque pulses are considerable (check out Kev Ash’s article on the subject). In this case electronics aids are a must.
What electronics won’t necessarily help is if a rider climbs aboard an older machine without any such electronic gizmos. It’s a bit like normally riding a supersport bike with plenty of acceleration and cornering clearance, and then one day riding a less capable bike in the same fashion and getting caught out A) overtaking and B) going round corners too fast and scraping bits off the bike.
All in all, electronics makes for a safer machine, and I do appreciate the step forward in technology. There are those who have the tactility to feel what a bike is doing beneath them, but they have usually learned this by crashing.
For those of us who don’t like pain, the electronics can be a useful way of achieving the same effect.
Motorcycle Traction Control and Cornering ABS