| Written by Costa Mouzouris on Thursday, 27 May 2010 |
Words: Costa Mouzouris. Photos: Honda/Bill Petro, unless specified otherwise.
As a motorcycle rider, you probably know that the engine in your motorcycle has at least one piston that goes up and down, that is connected to a crankshaft (via a connecting rod) that converts the piston’s reciprocating motion into the crankshaft’s rotating action. Or at least you should know it.
This conversion of motion is important because we need a spinning shaft from which to drive the spinning rear wheel, unless you want to have a vehicle that hops instead of rolls.
We also need to include a system for converting torque and speed (the transmission) so that you can climb hills and actually go faster than a bicycle.
But we’re not done yet because between the crankshaft and the transmission we also need something called a clutch, which engages and disengages the constantly spinning crank (as long as the motor’s running) from the transmission and rear wheel.
Without a clutch, power would always be engaged, and taking off from a stop, shifting gears or stopping without having to kill the engine would be very tricky at best (you know what I’m talking about if you’ve ever had a clutch cable break).
TRANSMISSION 101
Big wheel front and small at the back = higher gear and visa versa.
photo: Scott bicycles
To best understand how a transmission works, all you have to do is look at a bicycle; you shift onto lower gears to make it easier to climb hills, higher gears to go faster.
A small front sprocket working on a large rear sprocket offers better leverage (torque) through a lower ratio on the driven wheel. But you don’t get owt for nowt, and the speed is reduced meaning that you’ve got to pedal more to cover a given distance.
In contrast a larger front sprocket working on a smaller rear sprocket provides a higher ratio meaning you’ll be pedalling less but covering more distance per pedal stroke, and travelling at a higher speed. Of course, higher speed means less torque, so good luck keeping in a high gear when you hit the first hill.
This is a very simplistic view of a transmission, but it is the basis by which all power transmissions operate.
CVT in action to show extremes of attainable gear ratios.
photo: unknown
I don’t know about you, but I did away with pedals as a means of transportation when I got my first motorised two-wheeler. With that went the derailleur and in its place a transmission took care of the various gear ratios I needed.
Motorcycles and scooters use a variety of transmissions to vary the ratio between the crankshaft and the rear wheel. First, let's take a quick look at the two most common types used on two-wheelers; the automatic CVT and the manual, and then the Dual Clutch Transmission used on the new VFR1200F.
THE AUTOMATIC CVT
The most basic of transmissions is the continuously variable transmission, or CVT, which you’ll find predominantly in scooters, as it’s fully automatic (i.e. no need for manual gear changes).
CVT uses a belt and a pair of pulleys with movable faces; on the drive pulley they come together as engine rpm increases, thus pushing the belt away from the pulley’s centre (like selecting the larger front sprocket on a bike) and apart during retardation (effectively reducing the diameter).
CVT from a Yamaha Grizzly ATV, with expandable pulleys and belt to allow for seamless ratio changes.
photo: Yamaha
These combined actions seamlessly change the ratio between the crankshaft and the rear wheel, which also allows the engine to run mostly at a fixed rpm — where it develops optimum torque — regardless of road speed (a telltale sign of a CVT is the steady RPM the engine maintains while accelerating).
Clutch duties are taken care of with a centrifugal design, which uses a set of sprung weights to engage or disengage drive when the revs rise above or fall below a certain speed (just above idle). This too is automatic and requires no input from the rider.
The CVT has several benefits, including light weight, a compact, reliable design, smooth operation and infinite ratio possibilities between its highest and lowest gears.
The downsides include some loss of engine power (due to frictional loses between the belt and pulleys), and limited engine braking as there is no positive engagement between the crankshaft and the drive wheel.
And a lot of people simply don’t like the feel of a motor stuck at one RPM as you accelerate away.
THE MANUAL TRANSMISSION
A two speed gear box to keep it simple. Remember only one of the two gears can be selected or the input shaft will try and spin the output shaft at two competing speeds (= box goes bang)
The most common motorcycle transmission is the manual gearbox. This consists of an input shaft that connects to the crankshaft via the clutch, and an output shaft that drives the rear wheel via a chain, belt or shaft.
On these two shafts are several gears of different ratios, all constantly meshed together, but with only one pair of gears engaged to their shafts at any one time (the other gears always have one of the pair freewheeling so as to not engage two ratios at any one time – otherwise the gearbox would go 'bang').
The operator must manually select the preferred pair of gears, with each gear change requiring the use of a manual clutch, to uncouple one set of gears before the next ones can be selected.
You know the drill: select first, slowly release the clutch while applying some throttle to get going, then roll off the throttle while pulling in the clutch to change gears. Release and repeat.
On this BMW K motor, you can see the crank drives the clutch, which then spins the input shaft. One set of gears at a time are engaged to transfer power from input to output shaft and then to the wheel – in this case by shaft drive.
photo: BMW
It is a very efficient method of transferring power to the rear wheel, as frictional losses are much reduced over a CVT, and gears are positively engaged offering slip-free operation. The drawbacks are added weight, bulk, and the need for a manual clutch, which adds another operation for the rider to handle.
And herein lies the biggest drawback; the manual clutch not only delays the shifting process, but it also interrupts power delivery to the rear wheel which results in ‘shift shock’ – the jerk you feel every time you upshift (though an experienced rider can keep this shock to a minimum).
A manual transmission also relies on the operator to select the proper gear; an inexperienced rider might not know which ratio to select for the task at hand and either be in a gear that’s too high, thus lugging and potentially stalling the engine, or he might leave the machine in a gear that’s too low and over-rev it.
However, one of the biggest factors cited by experienced riders in favour of the manual transmission is that it leaves all the control, decisions and riding experience to the rider – for better or for worse.
THE DUAL CLUTCH IN THEORY
Study carefully as this is your best chance of understanding how this thing works!
For 2010, Honda adopted the dual-clutch transmission (DCT) on its new VFR1200F, though the technology was first seen in race cars and then performance street cars.
DCT cutaway - Colour coding of gears match diagram above.
The dual clutch transmission is essentially two manual gearboxes in one, combining the ease of operation of a CVT with the efficiency of a manual gearbox.
The VFR’s system incorporates two separate clutches (both housed in a single unit), mounted on dual concentric input shafts (a solid shaft running inside a hollow outer shaft).
One of the input shafts holds gears 1, 3 and 5 and the other gears 2, 4 and 6 with a separate clutch to engage or disengage its shaft.
The gear changing system is the same idea as on a manual gearbox, except that this system allows not just first gear to be engaged, but second as well, though it freewheels because its clutch is disengaged.
Another cutaway, this time showing the dual clutches.
Upon upshifting, the first gear clutch disengages and engine power is transmitted to second gear as its clutch is automatically applied – all within a matter of milliseconds.
While this is happening, the computer controlled shift mechanism moves first gear out of engagement and engages third gear, which freewheels as its clutch (the same clutch as first gear) is disengaged, ready for the next upshift.
The process repeats for all six gears, and it just reverses when downshifting.
Because operating the two clutches simultaneously would be impossible for a rider to do, the machine’s computer takes care of the job with the help of solenoids and hydraulics. As a result, gear changes can be completed almost instantly and all without the shift shock that comes with a manual transmission.
VFR1200F DCT comes sans gear lever.
The electronic shift management also allows engineers to program almost any shifting map they want. On the VFR, you can choose between two automatic modes; Drive for fuel-efficiency or Sport for aggressive riding, and Semi-auto for manual gear selection via a handlebar-mounted switch.
You can also override the preselected changes by use of up and down buttons on the switchgear, which tell the computer when to do the change.
The DCT is renowned for smooth, quick shifts, as engine power is never uncoupled from the rear wheel when shifting gears as it is in a manual gearbox. The constant power transmission can also significantly improve fuel-economy by up to 10%, though we don’t know what the figure is with the VFR.
And finally, since there is a solid link between the engine and the drive wheel, it also allows for higher torque loads than a CVT, and of course, it offers the convenience of clutch-less operation.
Dual clutch means a slightly sticky-out clutch housing and a little more weight.
The main downside of a dual-clutch transmission is that it adds weight. On the VFR1200F DCT, claimed wet weight is 278 kg (613 lb), 10 kg more than the standard VFR.
It’s also more costly, as reflected in the dual-clutch VFR’s $19,999 retail price, a $1,700 increase over the manual-shift Viffer.
But the real question is what’s it worth to you? If you’re intimidated by clutch use, a 150-horsepower sport bike probably intimidates you too, so the VFR maybe isn’t a machine for you, but if you have tendonitis in your left hand, or suffer from some other condition that limits use of this appendage, then you’ll find the VFR DCT highly valuable.
Hey, maybe you’ve just had enough of manual changes!
THE DUAL CLUTCH IN PRACTICE
Costa was impressed during a day at Mosport.
I rode the dual-clutch VFR recently at Mosport during a media introduction hosted by Honda Canada and was very impressed with its operation. It shifts buttery-smooth in either of its automatic modes, and allows you to shift when you choose, if you choose, in manual mode.
In Sport mode, shifting points weren’t exactly where I would have chosen them for a fast pace, sometimes coming either too soon or too late depending on what I was doing with the throttle, but gear selection was spot on for each of Mosport’s 10 turns.
In manual mode, gear changes were as smooth as in automatic mode, but I chose when they happened. Riding a clutch-less bike at speed took some getting used to, and my left hand often reached for a ghost clutch lever.
Being able to do manual changes with a foot shifter would have been better.
The only criticism I have with the manual gear change setup is the lack of a foot shifter, which is more intuitive for aggressive track riding; I wouldn’t miss it all that much on the road, especially since I’d leave the bike in one of its automatic modes most of the time.
Granted, the only time I tried Drive mode was while exiting the pits, and I found it short-shifted all the way to top gear by the time I was going about 60 km/h.
I’d need more seat time in real world conditions to make a proper evaluation of the VFR’s dual-clutch transmission, and that’ll be coming later this season when we get a medium-term dual-clutch VFR test bike, but so far, auto-shifting is everything Honda claims it is ... and less.
WATCH THE VIDEO
Honda have put together a handy video of the DCT in action ...
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