Motorcycle news, Motorcycle reviews from Malaysia, Asia and the world

Making Sense of Engine’s Bore and Stroke

We were treated to an all-new Yamaha YZF-R1 for this year. Poring over the specification sheet we found that the engine’s bore and stroke has changed i.e. larger bores and shorter strokes. And yes, it revs higher.

What is bore and stroke?

To put it simply, the bore is the hole the piston sits in. Stroke, on the other hand, is the length that the piston needs to travel between its highest and lowest points.

But why is that?

The relationship between an engine’s bore and stroke determine, to an extent, how it makes its power. For a given capacity, ‘long stroke’ engines – ie those with a relatively long stroke in relation to the bore size – will tend to be relatively low revving but with strong low down power, while ‘short stroke’ or ‘oversquare’ motors – short stroke with a wide bore – will be able to rev higher. And, because more revs equal more horsepower (horsepower = torque x rpm divided by 5252, so increase the revs and the bhp increases too), manufacturers are always looking at ways of safely increasing the upper rev limit of their motors.

One of the major factors determining an engine’s upper rev limit is piston speed. For every revolution of an engine, the piston moves up from the bottom of its stroke (bottom dead centre or BDC) to the top of its stroke (top dead centre or TDC) and back again. So in the case of the ’04 R1, the 77mm wide piston goes from a standstill, travels 53.6mm up, stops, and comes back down again. At 10,000rpm it makes this journey just over 166 times each way every single second, at an average speed of 17.9 metres a second.

The stresses on a piston and conrod at high revs are massive. If the piston is forced to travel too quickly something’s going to break. Put very simply, if you reduce the distance the piston has to travel – ie its stroke – it doesn’t have to travel as fast and can make that journey more often. So that’s what Yamaha chose to do with their new R1, reducing the stroke by 4.4mm and adding 3mm to the bore. Last year’s R1 redlined at 11,750rpm, while this year’s redlines at 13,750, and makes its peak power 2000rpm further up the RPM scale.

Another way of reducing driveline stresses

Another way of reducing stresses is by using lighter materials for the pistons and connecting rods. Every moving part has momentum, and momentum is calculated by acceleration multiplied by mass. So, the more mass a moving object has, or/and the faster it moves, the higher its momentum. Lightening these parts will reduce stresses and also lets the engine rev faster.

How does an Assist Clutch Work?

We have covered the subject of the slipper clutch, now let us look at the assist clutch function. The assist function is an evolution of the slipper clutch and is fitted to an increasing number of motorcycles these days.

Why do we need the assist function?

Previously, harder clutch springs are required for high powered motorcycle engines in order to force the clutch plates and friction plates together, in order to maximise power transfer. Cutch springs that are too light can cause the plates to slip past each other, especially under hard acceleration.

Problem is, the clutch lever will feel very stiff as we need more finger pressure to overcome the springs’ force. It becomes even worse when the bike is accelerating hard and at speed, as the clutch’s centrifugal force pushes the plates in. Not only that, the gear lever can also feel really when using a quickshifter.

So how does the assist function work?

As with the slipper clutch, there are also ramps on the clutch’s pressure plate. However, these ramps face the other way, which cause the pressure plate to push inside onto the clutch plates for more positive engagement. In other words, less of the engine’s power is wasted from clutch slippage.

The takeaway from this is we can now use lighter clutch springs, allowing for a lighter pull on the clutch lever. It is especially useful when your motorcycle does not have a quickshifter. Additionally, shifting with the quickshifter can be potentially faster and the gear lever feels softer.

How does a Slipper Clutch Work

The slipper clutch is a common feature in road motorcycles nowadays, compared to when it was used in racing exclusively. Even some “performance” kapchais are equipped with it.

Why do we need a slipper clutch?

We are familiar with that deceleration when we shut off the throttle, or when we downshift. That is called engine braking or back torque. It is especially strong on four-stroke motorcycles with bigger engines that produce higher torque. The higher the engine’s torque, the higher its back torque too.

This engine braking can become intrusive, especially when we downshift to aggressively or we accidentally downshift to a gear that is too low. It can cause the rear wheel to hop, or even lock up momentarily. It is not something we want as we are tipping the bike into a corner, and certainly when the road is wet.

How does it work?

That is exactly why the slipper clutch was developed for: To reduce the engine’s back torque through the clutch plates to the transmission and to the final drive.

Slipper clutches usually consists of ramps that would cause the clutch basket to disengage or in other words, “slip” when the rear wheel tries to drive the engine faster above a certain deceleration threshold.

These angled ramps let the clutch faces which are normally meshed together under acceleration and normal riding to pull apart and disengaging the plates when there is too much back torque. Consequently, the rear tyre continue to rotate. It also decreases wear and tear on other transmission and engine parts due to the engine overrevving.

However, on some more sophisticated bikes (read: expensive), the slipper clutch works in conjunction with an electronic feature called engine braking control which regulates the engine speed to avoid clutch hop altogether. But that is a story for another day.

Triumph Thruxton – Where did the Name come From?

Following the news of an upcoming Triumph Thruxton 400 to replace the Bonneville Thruxton R 1200, let us take a look at the origin of the Thruxton name.

The Thruxton name plays a significant part in the marque’s history, just like the Daytona, Bonneville, Speedmaster, Trophy, Speed Twin. This is why Triumph decided to keep the name running albeit in the 400cc range, after stopping production of the 1200cc model.

What is Thruxton?

More like where, actually. It is the name of the Thruxton Circuit, in Hampshire, England.

It began as a military airbase, known as RAF Thruxton in 1942, and was home to troop carrying aircraft of both the RAF and USAAF. In fact, some of the airborne troops who took part in D-Day, 1944, took off from from this airbase.

The circuit was laid out along the perimeter of the runway since the 1950s, but the longer and present track was only fully established in 1968. History, it has since hosted the Thruxton 500 endurance race (1960-1964, 1969-1977), British Formula 3, British Formula 2, and many more. The circuit remains part of the British Superbike Championship (BSB) and British Touring Car Championship (BTCC) calendar until today.

The Thruxton 500

However, the most relevant and important race in this story was the Thruxton 500.

It all began as a 9-hour endurance race for motorcycles in 1955, followed by another two in 1956 and 1957. This 9-hour race evolved into the famous Thruxton 500 mile (800km) race from 1958.

The Thruxton 500 was a production motorcycle race, meaning the motorcycles being raced must be available to the public, not unlike the current day FIM Endurance World Championship. Each bike entered will be ridden by two riders on rotation.

Anyway, Triumph entered the inaugural endurance race in 1958 with a Bonneville T120, entered with Mike “The Bike” Hailwood and Dan Shorey as riders. They won.

The race soon caught the attention of the public. As such, motorcycle manufacturers were keen to win it to showcase their products’ performance. “Win on Sunday, sell on Monday,” even before Soichiro Honda made those words famous.

Triumph won again in 1961 with Tony Godfrey and John Holder after being runner ups to the BMW and AJS in 1959 and 1960. It was this win that spurred the Triumph factory to build the T120R Thruxton, which was hand-built by a team of Triumph technicians using specially picked components and precision-machined cylinder heads and crankcases. Peak power was increased and each ‘Thruxton’ engine was bench tested to deliver around 53 bhp (40 kW) at 6,800 rpm with a safe rev ceiling of 7,200 rpm. Only 52 of the Thruxton T120Rs were built in 1964/5 to meet homologation requirements for production racing. About 100 more machines were subsequently manufactured and supplied to selected dealers and riders.

It was one of the rarest Triumph motorcycles.

Rise of the modern Triumph Thruxton

In 2004, the new Triumph factory based in Hinckley, England introduced the Thruxton 900. The engine was derived from the Bonneville lineup, but has new cams and pistons 90mm pistons, taking capacity to 865 cc and power up to 70 bhp. The crank was a 360°, which meant both pistons rose and fell together. Triumph was smart to sell the Thruxton 900 as the café racer of the Bonneville lineup. (Tom Cruise rode it in The Edge of Tomorrow, by the way.)

The company then introduced the new Thruxton in 2016. It used the new 1200cc, 270º, liquid-cooled engine. There were two variants, one the standard while the “R” got Ohlins shocks, Showa forks, and Brembo brakes.

The Thruxton has since enjoyed good sales the world over, although the later-launched Boneville Bobber pipped it as the best selling Triumph.

Then, in 2024, Triumph announced the Thruxton Final Edition as they moved the performance Bonneville to the Speed Twin.

Closing

Long live the Triumph Thruxton!

Triumph Thruxton 400 Prototype Spotted

There have been many rumours about an upcoming café racer for the Triumph 400 range. It looks like it is true, after all, as a Triumph Thruxton 400 prototype had been spotted undergoing road tests in India.

Triumph had stopped production of the Thruxton 1200 (Final Edition in the main picture), and instead moved the high performance Bonneville ‘task’ to the Speed Twin. That leaves the company without a model with the Thruxton name in their lineup. To us, the Thruxton 1200 was the best Bonneville. Ever!

However, it now seems confirmed that the name will live on, albeit in the 400 range. We have ridden both the Speed 400 and Scrambler 400X, and they were just amazing. On top of it, they are sold at very competitive prices. It is why both models are selling like hotcakes.

So, over to the Thruxton 400 prototype. The pictures show a café racer clad in a half-fairing, although it was an accessory for the Thruxton 1200. The handlebar or clip ons are also placed lower than the Speed 400. The rest of the bike appears the same apart from these two details.

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The engine will of course be the 398cc, single-cylinder unit built together with Bajaj Auto. It produces 39.5hp and 37.5Nm of torque, channeled through a 5-speed transmission. While it does not seem high on paper, it is entertaining enough.

Well, we cannot wait for more news about this bike. The Thruxton 1200 and its Final Edition were out of our financial reach, so maybe the Triumph Thruxton 400 could be the one!

How to Tell if Your Riding Gear is Genuine (Part 2) – RS Taichi

Continuing this series, let us check out another brand, the world-renowned RS Taichi.

To recap, we wanted to know how the makers of motorcycle riding gear take steps to ensure that you and I buy are genuine. These major manufacturers have put in much effort and cost to ensure that their products conform to safety standards in order to protect us.

However, the advent of online shopping platforms have created the problem of dishonest sellers parading copied and fake wares. And these shopping platforms are doing nothing to stop them.

Thus the onus falls back on our shoulders to ensure that we are spending our hard-earned money on genuine products.

**Courtesy of Hodaka Motoworld Singapore**

Who is RS Taichi?

The brand was founded my a professional motocross racer, Taichi Yoshimura in 1975, in Osaka, Japan. Yoshimura was raced for the Suzuki and Honda factory teams in motocross, and had the distinction of giving Honda their very first win in motocross racing.

The first RS Taichi product was a gear bag, before going on to making racing leather suits in 1978. Now, RS Taichi makes the complete range of motorcycle rider gear and accessories, from helmets to jackets, gloves, pants, shoes, inner wear, leather suits, bags, protectors, and more.

How can you detect a genuine RS Taichi product?

1. Look for the yellow Taichi tag in your jacket, gloves, pants, and bag.

2. Use a torchlight or your phone’s flashlight and shine through the tag.

3. You should see many Taichi logos.

Counterfeit RS Taichi products may have the tag, but the hologram will not show.

Conclusion

Alright, we have covered Komine and now RS Taichi, and we shall continue to cover other brands. Again, buy your riding gear from official channels. It is not worth saving a few bucks but compromising your safety.

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