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Trevitt's Blog: Can Two-Strokes Make a Comeback?

KTM and Husqvarna recently announced that they will be producing two fuel-injected two-stroke models for 2018, as part of their respective enduro lineups. (The Husqvarnas will be essentially rebadged KTMs, as are most of the company's models.) While most manufacturers, including KTM and Husqvarna, offer a range of two-stroke dirt bikes, there have been few two-stroke street bikes of any note since the demise of the Yamaha RZ350 in the mid-nineties. The KTM and Husqvarna announcements, even though covering enduro models, could be the beginning of a trend that sees two-strokes making a comeback in the on/off-road market, and perhaps eventually street bikes.

Two-strokes offer less weight, fewer moving parts, less friction and - of course - twice the number of power strokes than four-strokes, for substantially more performance in a given displacement. On the downside, however, emissions and fuel economy are significantly worse. There are two issues here: One is that in the two-stroke cycle, the transfer ports (that "transfer" the fuel/air mixture from the crankcase into the cylinder) are open at the same time as the exhaust port, and for a significant portion of the cycle. During this time, unburnt fuel can go directly out the exhaust, affecting emissions considerably. The second issue is that, because the fuel/air mixture in a conventional two-stroke passes through the crankcase, the lubricating oil for the big-end and main bearings ends up being burnt along with the fuel, also affecting emissions. The writing was on the wall for two-stroke street bikes in the early eighties, with increasingly strict emissions laws being more and more difficult for the manufacturers to comply with.

In the mid-nineties, Bimota manufactured the V-Due, a 500 cc two-stroke V-twin street bike. The V-Due worked around the emissions issues by using fuel injection and forced lubrication for the bottom end. Ideally, a two-stroke would use direct fuel injection, where fuel is injected into the combustion chamber (rather than the throttle body) after the exhaust port is closed, to minimize emissions. This technology has issues of its own, however, and the V-Due used transfer port injection. While not an optimum solution, in this setup only air goes through the throttle body, into the crankcase and up the transfer ports; the fuel is finally injected in the transfer ports, where it can't pick up the lubricating oil. As well, the exhaust port can be almost closed when the fuel is introduced, minimizing how much goes directly out the exhaust unburnt.

With a separate lubrication system for the V-Due's bottom end, and only air going through the crankcase, the amount of oil that made it to the combustion chamber was also minimized. While the bike did pass US emissions standards at the time, it had significant issues with rideability attributed to the fuel injection, and eventually the system was scrapped altogether in favour of carburetors. Even then the model had continuing issues, and was largely blamed for the company's bankruptcy.

While little was revealed in the KTM and Husqvarna press releases, the KTM version did indicate that the new bikes use transfer port injection, like the V-Due. Certainly the technology has progressed significantly since the V-Due's time, and KTM promises "a completely new experience in terms of power delivery and rideability." KTM, and other manufacturers, have surely been working on two-stroke fuel injection for some time, and the technology is very common in the marine and snowmobile market. If the new KTMs deliver on those promises of rideability and power delivery, it may open the floodgates for the other manufacturers to follow suit.

What will the holdup be for street bikes? The RZ350 and V-Due had a difficult time meeting the relatively relaxed emissions standards of their time, and the current Euro 4 standard is extremely difficult even for clean-burning four-strokes to meet. Load up a two-stroke with direct injection, an elaborate lubrication system, exhaust valves and other extras to meet today's standards, and cost, weight and complexity quickly approach the four-stroke realm. (Note on the image above all the extra equipment tacked onto the cylinder of the KTM engine.) Additionally, in the last 20 years since the V-Due, four-stroke technology has improved considerably and closed the gap to two-stroke performance.

The media launches of the new KTM and Husqvarna models are mid-May, at which time we'll know more about the technology used and if it could potentially be applied to street bikes. It may be wishful thinking on my part, but I don't think I'm alone in wondering how something like an up-to-date RZ500 would compare to a current four-stroke litrebike.

Friday, 07 April 2017 12:16 Published in Andrew Trevitt


Trevitt's Blog: Cornering ABS and using the rear brake

One rider aid that is available on an increasing number of new motorcycles is cornering ABS, sometimes incorporated into a stability control package. In its basic form, a cornering ABS function takes into account the motorcycle's lean angle to adjust brake pressure, increasing the ABS effectiveness beyond a simple straight-line stop. However, as I outlined in a previous Inside Motorcycles article ("Smarter ABS," Dec. 2015), these systems offer much more than just improved ABS function.

One basic handling characteristic of practically all motorcycles is the tendency to stand up when the front brake is applied in a corner. Due to the front-end geometry and the relationship between the front tire's contact patch and the steering axis, using the front brake when the motorcycle is leaned over causes the steering to turn further into the corner, inducing countersteer that stands the bike up and causes it to run wide. This, of course, is exactly what we don't want when we encounter a fallen tree limb or some similar hazard in the middle of the corner.

The various cornering ABS functions, sometimes part of a stability control or cornering management feature, counter this by sensing that the motorcycle's lean angle is decreasing as the front brake is applied, and transferring brake pressure from the front brake to the rear. This reduces the induced countersteer but retains overall braking pressure, so the motorcycle still brakes as the rider wants but doesn't stand up.

Noted tuner Kaz Yoshima uses the analogy of driving a car with a trailer when it comes to using a motorcycle's rear brake. Just as using the trailer brakes alone can stop a trailer from uncontrollably swaying side to side, using the rear brake on a motorcycle can add stability when entering a corner.

On a road racing machine, riders brake so hard that the rear wheel is often in the air, and the rear brake has minimal effect through the majority of the braking zone. As the rider releases the front brake and arcs into the corner, load does transfer to the rear and more rear brake can be used during this brief transition. That said, this typically can be managed using engine braking, either through electronic controls or an adjustable slipper clutch, and many riders do not use the rear brake at all on track.

The situation is much different for street riders, however. Data shows that even at a "spirited" pace for most riders, braking forces on the street are considerably less than those seen on the track. This means that there is typically much more load on the rear tire under braking, even in a straight line and especially entering a corner. This additional load can tolerate significantly more braking than the engine alone can provide, and now the rear brake is more effective. Using additional rear brake and less front brake will reduce the chance of the front tire locking up, and at the same time - using the trailer analogy - will add stability to the situation.

Take a step back from the sport bike realm, and the effect is even more noticeable. Standard bikes, sport touring bikes, and especially cruisers and touring bikes don't have the front-end bias of a sport bike, leaving plenty of load on the rear tire that can be put to good use for braking.

The takeaway here is that while riders on the track may use little or even no rear brake, on the street it is a much more effective tool for not only increasing safety, but also influencing the handling of the machine. Cornering ABS and stability control functions use this to look after the safety aspect should you get into trouble some day, but using the same concept, pro-active use of the rear brake has both safety and performance benefits.

Wednesday, 11 January 2017 14:53 Published in Andrew Trevitt


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: What's a Qualifying Tire Worth?

At the final rounds of this year's Mopar Canadian Superbike Championship series, held at Canadian Tire Motorsport Park, Dunlop offered qualifying tires to the top 10 riders for a separate, 15-minute final qualifying session. This was new for many of the competitors, as qualifying tires have not been used in the series for several years. Jeff Williams was one of the riders in the top-10 session, and I was able to get some data from the Accelerated Technologies Honda CBR1000RR that he was riding for the weekend.

Tuesday, 06 September 2016 11:23 Published in Andrew Trevitt


Andrew Trevitt's Blog: Transfiormers - Daring to be different in a sea of Moto2 uniformity

Last month, Spanish rider Ricard Cardus won an FIM CEV (Campionato Espagnolo Velocita, a European Championship) Moto2 race at Catalunya in Spain. That in itself is not overly surprising; Cardus has been racing in the Moto2 World Championship for several years with a handful of top-10 finishes to his credit. What's more noteworthy is the motorcycle he was aboard: Transfiormers.

Friday, 08 July 2016 16:31 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: Ducati's MotoGP wings

Over the past several seasons of MotoGP, Ducati has occasionally experimented with wings on the side of the Desmosedici to influence its aerodynamic characteristics. Yamaha also used wings in the latter part of last season on the M1, and wings have sporadically appeared on other bikes in the past. Until late last year the wings in use have been quite small, but during testing this year the Ducatis have sprouted two very large wings on each side.

Friday, 26 February 2016 17:21 Published in Andrew Trevitt


Trevitt's Blog: Identifying (and quantifying) smooth riding

We know that riding smoothly is one aspect of quicker lap times at the track and being safe on the street: Gentle, precise throttle inputs, fluid body movements and steady lean angles mid-turn are just some of the characteristics of what you'd consider a smooth rider. Jorge Lorenzo is a perfect example, with a glass-smooth riding style that looks like he is going much slower than he actually is.

Friday, 11 December 2015 12:16 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


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