Related articles

Trevitt's Blog: Electromagnetic Valve Control (video)

While variable valve timing has been a hot topic over the last few weeks with the introduction of the updated GSX-R1000 featuring Suzuki's innovative mechanical VVT system, many companies are working toward the next step in this area: eliminating camshafts completely and controlling the valves directly using electromagnetic actuators. There are many stumbling blocks to electromagnetic valve actuation (EVA), but a research group in the Control and Automation Laboratory of the University of British Columbia has created a new type of actuator that may make EVA a very realistic option in the not-too-distant future.

Monday, 31 October 2016 13:47 Published in Andrew Trevitt

Trevitt's Blog: Backing it in

We see all the time now road racers "backing it in" to corners, sometimes with the rear end of the motorcycle out of line with the front just as much as dirt track or supermoto racers. Using engine braking or the rear brake, these riders are skidding the tire just enough that the rear end kicks out a certain amount under braking. Done properly with careful manipulation of the controls, this manoeuvre starts the motorcycle turning before the corner, effectively reducing the arc of the turn that must be completed.

How sideways the bike goes on the entry is determined partly by how much load is on the rear tire, and partly by the amount of slip (how much slower the rear tire is turning that the bike's actual speed). The wheel's load depends on how hard the rider is using the front brake to unload the rear end, and while some of the rear braking force can be provided by the rear brake itself, most modern four-stroke race bikes have more than enough engine braking for the desired amount of slip. While a very good rider can modulate the front and rear brakes and the clutch all at the same time for a beautifully controlled slide all the way to the apex of the corner, the goal is to have close to the right amount of skidding provided automatically so that the rider need only fine-tune the action.

One method to accomplish this is to use a slipper clutch, which reduces the effect of the engine braking to manageable levels so the rider does not have to be so precise with letting the clutch out. On most units, the amount of clutch slip and the point at which the slip initiates can be adjusted by changing the rate of and the preload on the clutch springs. Another method is similar to the old racer's trick to reduce engine braking by increasing idle, and current electronics systems slightly open a ride-by-wire throttle on deceleration. These systems can offer anything from a simple 1-2-3 level of adjustment to fully variable engine braking based on rpm, gear position, and more.

In series that don't allow elaborate electronics for engine braking control, however, we have to look to the chassis and ways of adjusting rear tire load in order to control how the motorcycle behaves on corner entry as the rear tire slips. In general, with more load the rear end will track straighter entering the corner, with less it will kick out more sideways. One way to address this is to raise or lower the whole bike, which affects how much load transfers under braking; with the motorcycle lowered slightly, for example, less load transfers to the front on corner entry and the rear tire stays more in line.

Another option is to make changes to the rear suspension to adjust the rear tire's reaction to a given load, rather than trying to adjust the amount of load. As an example, consider a rear shock setup with an amount of preload such that it takes 20 kg of load to even move the suspension; under braking, the rear tire will begin to skip and float across the pavement and go sideways if load goes below that 20 kg, because the suspension is topped out. By adjusting the spring rate or preload, we may be able to change that value so that the suspension begins to move with less or more load, altering when and how much the rear tire skids. Going a step further, we can even get creative with the top-out spring inside the shock to make that adjustment and influence corner entry behaviour without affecting the remainder of the suspension travel.

It all becomes a juggling act, with the slipper clutch, electronics package and suspension setup each playing a part. In an ideal setup, the bike backs into the corner just the right amount without requiring those precise clutch and rear brake inputs, allowing the rider to focus on more important aspects of the corner entry.

- By Andrew Trevitt

Friday, 27 May 2016 17:47 Published in Andrew Trevitt

Trevitt's Blog: Anti-Squat in Theory and Practice

In previous blogs, and in the print version of the magazine, I have discussed anti-squat and how it relates to chassis setup. Anti-squat is a very important tool in making a motorcycle lap quickly at the racetrack, especially a powerful superbike, but at the same time it's one of the least understood setup parameters.

Some people claim that the rear end of the motorcycle must always compress, or squat, under acceleration to properly transfer load to the rear wheel for better traction. Others claim that the rear suspension must extend under acceleration, to "push" the tire into the ground and increase traction. People in the second group point to the experiment of putting the front tire of the motorcycle against a wall so that the bike can't move; when the clutch is gently released, applying power to the rear tire, the suspension extends significantly. But what really happens when the motorcycle is on the road or track and accelerating?

Here is what we know about anti-squat in theory: The three forces involved in compressing or extending the rear suspension as the motorcycle accelerates are the driving force, chain pull, and load transfer. Driving force refers to the rear wheel pushing the motorcycle forward, and generally acts to extend the rear suspension because of the swingarm angle. Chain pull is the force of the top run of the chain on the rear sprocket, also trying to extend the rear suspension under most conditions. And load transfer refers to the additional weight on the rear suspension due to acceleration.

There are a couple of key points to consider here: First, the load transfer component will occur whether or not the rear suspension compresses; in other words, acceleration will add weight to the rear wheel even if the rear suspension extends during that acceleration. The attitude of the motorcycle does affect the amount of load on the rear wheel, but to a very small extent. Second, the experiment of putting the front tire of the motorcycle against a wall removes load transfer from the equation; the rear suspension rises because only the chain pull and driving forces are present, the forces which serve to offset load transfer - which is eliminated here because the motorcycle is not accelerating.

Sum the three forces, and the math shows that the anti-squat effect decreases with more suspension travel, mostly because the swingarm angle changes through the stroke. At the top of the travel, acceleration will cause the rear end of the motorcycle to rise; at a certain point, equal to approximately the static sag setting on many bikes, the forces sum to zero and the suspension will neither compress nor extend on acceleration. As suspension travel increases, the anti-squat effect reduces further and the rear end will tend to squat on acceleration.

What happens in practice? Data that I have from Jodi Christie's superbike shows that in some corners, the rear suspension compresses during acceleration; in others, it extends; and in others, it remains constant from the moment Jodi applies the throttle to the end of the succeeding straight. The amount of compression or extension depends on traction, camber, elevation changes, and any number of variables.

The takeaway here is that, by adjusting various setup parameters as they relate to anti-squat, we can make the rear suspension do what we want on corner exits - extend, compress, or remain constant. This is usually a compromise to find a setting that works for the entire track, and we most often look at rear suspension in conjunction with other data, not on its own, for guidance on what that compromise should be.

Friday, 01 April 2016 15:25 Published in Andrew Trevitt

Trevitt's Blog: The Ideal Line

Watch any road race practice session and you will see riders trying different lines almost every lap, as they search for the quickest way around the track. By race time the experimentation is done and the top riders rarely stray more than a few inches from their chosen lines, but sometimes you will still see significant differences between riders. The best line through a particular corner or sequence of corners is not always the obvious choice, and depends on a number of factors.

Friday, 23 October 2015 15:20 Published in Andrew Trevitt

Touring Essentials Blog: Riding in the Rain

R. Bruce Thomas' latest Touring Essentials blog entry features advice on riding in the rain...

Monday, 13 April 2015 10:06 Published in Touring Essentials with R. Bruce Thomas