Rubber band bust Mate? How does your twist & go go? | FEATURE
If you want to buy a brand new scooter now you are faced with literally hundreds upon hundreds of choices of manufacturer, engine size, colour, style, whatever – but only one transmission. Yep, its twist & go or you join the bus queue. Actually, before you write in and tell the Ed you know differently, there are still a few geared scooters sat in dealers, like the LML Star and Vespa PX but now we’ve got Euro 4 when they’re gone, they’re gone.
Like the two-stroke engine, geared scooters make it more difficult for manufacturers to meet the latest emissions legislation when compared to twist & go, or CVT – Constantly Variable Transmission to give it its proper name. However, having said that, KTM/Husqvarna have released a brand new two-stroke enduro bike with TPI (Transfer Port Injection), the 300 EXC will meet Euro 4 so maybe the two-stroke isn’t dead after all.
What, Why and When of CVT
Back to scooter transmissions and a look at the ‘what’, ‘why’ and ‘when’ of CVT.
The ‘what’ is basically two pulleys, one driven by the engine and the other driving the rear wheel with a rubber/Kevlar internally toothed belt connecting them (some industrial applications even use multi-strand chain). Each steel pulley runs on a splined shaft and is split so that the two halves can slide closer together or further apart from each other. Oh, and the inner surfaces of the two halves slope outwards towards the top, forming a valley in which the belt runs. The belt is V-shaped, matching the pulley halves on each side and slides up and down the faces of the pulley.
When the throttle is opened the two halves of the engine-pulley are pulled closer together by the ‘variator’, the belt is forced outwards as the valley gets narrower and runs faster and faster around the bigger circumference thus formed. At the same time, the pulley halves at the rear wheel are moved apart and the belt slips down towards the centre, running around a smaller circumference. The variator works on the principal of a speed governor, bob weights are thrown outwards by centrifugal force engaging springs, which then pull the two pulley halves towards each other. The faster the engine turns, the more force generated by the weights and the larger becomes the circumference of the pulley which the belt has to run around.
If you’ve followed this so far you’re halfway to your Doctorate in Engineering, so well done. We’ll continue; each revolution of the front pulley can now make the rear pulley turn many more times, the ‘go’ part of the twist & go. One final bit of exceedingly clever stuff is that when you reach a red light and want to put your feet down without the engine stopping, the rear pulley has a centrifugal clutch which automatically disengages the drive below a speed of about 5 mph. Simples! The traffic lights go green again, you twist your wrist, the centrifugal clutch re-engages, the front pulley closes up, the rear one opens making higher and higher gearing and the Audi which was beside you at the lights disappears into a tiny middle-management speck in your mirrors.
Sorry, no illustrations as I can’t draw for toffee, so to save us all a lot of bother there’s a link to a video above which will show you exactly how CVT works.
Our survey says
Let’s turn to the ‘why’. Quite apart from having to meet EU emissions laws, manufacturers love CVT because its a lot cheaper to make than a gearbox and as its fairly trouble free and easy to use. You, dear new scooter buyer like it too. Oh yes, you do, you know you do, all the market surveys say so.
The downsides are that if you rode a Lambespa in the ’70s (or still ride a geared scooter now) you miss the smell of two-stroke in the morning and you NEED gears. Whoever thought of making the left handlebar lever work a brake for goodness sake? Unnatural innit? Admittedly CVT is slightly less efficient than a gearbox (ie. loses a bit of power between the engine and rear wheel) and as the engine tends to run at higher speeds than maximum efficiency, torque then fuel consumption is slightly increased. However, the advantages to the rider outweigh these considerations as the engine is always ‘in the right gear’ and acceleration tests have shown that a CVT scoot achieves 5% quicker times than any similar spec geared model – saves all that frigging about with the clutch and twisting the left grip to get the next gear!
So now to the ‘when’ and this is the surprise. What do you think – Suzuki Burgman 1998? Vespa PK50 Automatica 1984? Noooo – way before then. The first CVT used on a two-wheeler was designed by Zenith Motorcycles Ltd’s Chief Engineer Freddie Barnes, patented in 1905 and it appeared on the 1911 6HP 770cc Zenith Gradua. The front pulley halves moved in and out just like the modern system, at the same time the rear wheel was pulled backwards in the forks to keep the drive belt taught as the driven pulley was a fixed diameter; all mechanically controlled by a handle alongside the tank.
The Zenith set a number of speed records and was so successful that after WW1 the ACU banned it from racing. Thereafter the nameplate carried the word ‘Barred’ in red letters across the logo and the company reaped huge rewards in ‘reverse’ advertising “So fast we beat the competition outta sight and got barred!” During WW1 the Zenith and Rudge Multi (as in ‘multi-geared’) which used an improved version of the Gradua belt drive, were favourites with the DR boys as it got them out of many a scrape when chains and fixed-ratio drive belts gave way in the glutinous mud. Funny thing is, a lot of old-timers still refer to ‘Gradua’ meaning belt drive; just the other day I was speaking to a good ole boy on a Phillips moped which he’d had brand new in 1957 and he said to me “Its dead easy to ride, I don’t have to pedal or nowt as its a Gradua.”
The first scooter to fit CVT was the American Salsbury Motor Glide in 1938, copied by Cushman with their Varimatic model. Salsbury stopped scooter production in 1948 but Cushman continued until 1965. As a matter of interest, for the last four years of Cushman’s life, the company was the Vespa importer for the USA.
In 1946 Fuji introduced the Rabbit scooter series, a few months before the first Vespa. One Rabbit model, the Superflow together with the Mitsubishi Silver Pigeon both had CVT and both continued in production until the mid-’60s. Silly name perhaps, but the Rabbit scooter was the father of Japanese two wheel production and now they rule the world and Fuji make jolly good cameras, one of which I use all the time. There was then a gap in CVT scooters for a while, although car companies like Daf in the Netherlands used the system during the ’60s and ’70s. Incidentally, I had a Daf 55 back in the day and it would go as fast backwards as forwards! Ford, Mini, Toyota, Subaru, Nissan and other car makers still offer CVT today in different markets around the world, as do several tractor manufacturers such as Fendt.
Back in fashion
The return of CVT on scooters after a 20-year gears-only lull came with the Vespa Automatica which was announced at the 1984 Berlin Motorcycle Show. A couple of years later Honda offered the Helix with constantly variable transmission, followed in 1998 by Suzuki’s Burgman, with Benelli and Malaguti fitting the same powertrain, which was designed and manufactured by Moto Morini at that time. Soon returning to a store near you folks, in the shape of the new Scomadi 400, albeit with EFI. The twist & go era was upon us as CVT was quickly adopted by every manufacturer east of Suez hoping to get a piece of the action in the burgeoning new scooter market, with a maxi, commuter and more recently retro models. Euro 4 which came into force on 1 Jan 2017 is the final nail in the geared scooter coffin and now twist & go is all we can buy. Unless of course, you know differently…
Words: Nigel Shuttleworth
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