Is converting Lambrettas to a single fork leg an act of heresy or engineering genius? To find out, Sticky rode Casa Performance’s winner of the Pomposa 500km scooter endurance race; the one-armed bandit that stole the show…
It’s ‘small bike’ Sunday practice day on the recently-enlarged Pomposa race circuit, somewhere on the Adriatic coast between Venice and Rimini. On one circuit karts are practising, on another motocross bikes are tearing up the dirt. In a small area of the paddock minuscule Italian kids are being taught to ride mini race bikes around in tiny circles with their knees on the deck, leaning further and further until they tip over and roll around on the floor laughing.
Out on the main circuit is a mix of pit-bikes, mini-race-bikes, full-sized 125cc and 300cc 4-strokes and also scooters. Some of the kids on these little bikes are demons through the corners. One of them may well be the next Valentino Rossi.
Out in the mid-morning fog, amongst the underwater farting sound of 4-strokes and screaming automatic engines you can clearly hear manual-gearchange scooters, firing out of the mist.
Luca Zani: Production Class Lui
The first is Casa Performance’s new CP One35-kitted Lui; built for their version of Production Class; which requires scooters to be restricted by carb size, exhaust and to specific cylinder kits. With an out-of-the-box CP135 kit and box-shaped exhaust the Lui would be hard-pushed to compete with the smallframes that rule this class. Nevertheless, the point of this session was for rider Luca Zani to test how the chassis handled with its single-sided Polini fork and to improve its set-up for racing.
Paolo Birtele on Crimaz’s Vespa endurance racer
Next out of the mist, on one wheel, came Crimaz’ Quattrini-powered endurance racer. It truly was a sight to behold as it leaned at ridiculous angles and powered down the straight trying to flip its rider like a bucking bronco.
Luca Fuschini: Lambretta SST265
Finally, from out of the gloom emerged Luca Fuschini on the SST265, making it look unbelievably easy to pilot a 40hp race Lambretta – sideways.
What do all of these scooters have in common?
The simple answer is a single-sided front fork. Not only single-sided, but with the leg on the left unlike almost every manual-gearchange Vespa ever made, except the very earliest.
Such a fork seems un-natural in a Vespa smallframe, but it looks completely odd under a Lambretta front mudguard. Not having a fork leg on each side seems like something is missing.
Gemini theory (twin must be better)
In theory, it makes sense to have twin-legged forks. Pretty much every motorcycle has them after all. There is something inherently balanced about having one fork leg on each side of the wheel that should work better, shouldn’t it?
Well, for motorcycles that might be true. Most are made with high-precision machined telescopic forks held rigidly both above and below the headstock. As a rule, fork springs are carefully matched for length in any quality motorcycle.
Scooters are different. Forks are built on the cheap using the minimum of materials to save weight and money. What could be an advantage in terms of alignment can actually become a hindrance if the people making the forks don’t do it accurately and you end up with more force on one fork link than another.
Let’s be straight; Lambretta forks are a bad design, made badly for good measure. There’s not enough suspension travel, components move on plastic bushes rather than bearings and you’ll often find that the welded-in stops for the fork springs are at different depths in the fork legs. Countless articles, Spanner’s manual chapters and videos have been set aside to sorting out what should be relatively straightforward, but isn’t.
When you have a two-leg design that isn’t matched from one side to the other the wheel will tend to tip to one side under suspension travel or braking.
Like everything Lambretta, there are a thousand ways to spend money to improve all this stuff: fancy dampers, redesigned stainless steel fork links, anti-dive brake conversions, dual-rate fork springs. All of it can make your scooter handle better. None of it can turn Lambretta forks into a good design.
If you really want to do this job badly then you can forget strong steel tube and make it out of bits of folded steel with exposed moving parts. Hey presto, you’ve invented the Vega/Lui fork!
Pirate theory (one leg is enough)
Meanwhile, let’s move across to the opposition. The beauty of the single-sided Vespa fork is not only that it weighs less – because it’s missing a leg – but also because it’s made with only one piece of kinked tube it can be pretty rigid.
It’s a good job too because if you have a wheel that’s only supported on one side it adds massive loads trying to twist the fork link.
All the PX and PK forks retain the use of interchangeable tube-type wheels which is a core trait of the traditional Vespa. They also use the fork on the right, on the same side as the engine.
If you look at all three of these scooters, the fork leg is on the opposite side. Why is that?
Automatic for the people
From the mid-90s onwards all the big Italian tuning brands – and their German competition – concentrated development efforts on the automatic scooter market.
For racing, the main machine in Europe was the Piaggio Zip SP. As far as styling goes, its a run-of-the-mill plastic ‘ped with no discernable character. What it did have however was a lightweight chassis, a single-sided front fork with a decent disc brake and the best motor of the time for tuning; which was Piaggio’s 50cc water-cooled reedvalve motor.
At one point automatic scooter racing got so big that it attracted guest rides from the MotoGP stars of the era such as Max Biaggi and Loris Capirossi. Meanwhile, in the UK I raced Zips in a Malossi-promoted UK series against British GP and Superbike legends Leon Haslam and James Toseland.
As soon as you get to high-level racing, you get high-level investment and equipment, so the Zip SP quickly gained upgraded brake discs, calipers and shocks with intricate damping adjustments.
Such sophisticated suspension was never made available for the traditional Vespa market, so pretty soon Vespa smallframe racers began fitting modified ZIP or ET4 forks in order to use the upgraded brakes and shocks that were available only for left-sided front-ends.
Another advantage of the Zip set-up, beyond the speed of wheel-change, is that the Vespa tube-type split rim wheel and hub was abandoned in favour of an aluminium tubeless wheel. Gone was the heavy, wonky-running steel wheels in favour of a lightweight, true-running cast wheel that could be changed in an instant.
Enter the Polini Evolution Fork
The problem with the Zip fork is that it was designed for a 50cc scooter where weight saving and economy of manufacture are paramount. While it functioned well – like a mirrored upgrade of the Vespa PX design – it’s not exactly robust. You can see quite how flimsy it is here in this innocuous-looking video of John ‘the Greek’ Chitoglou’s crash at Lydden Hill. The Zip forks fitted to the smallframe he was riding bent very easily.
In Italy problems were discovered with the Zip forks in the 100cc class where outputs exceed 30hp. The forks were being machined at the bottom bearing seat to give the scooters a nose-down stance; which helped with handling. On at least two occasions, some teams went a bit too far with the machining and actually weakened the forks. Weakened to the point that their riders went to brake at the end of the straight and the fork leg snapped completely and disappeared under the scooter while the riders disappeared over the handlebars. Next stop: hospital. Snapping forks at 90mph is never a good look.
Thankfully Polini stepped into the breach by producing a totally redesigned mono-fork using two separate sections of straight tube welded into a custom-made steel casting. Everything about the design is beefed up so it weighs 1.1kg more than the Piaggio original.
Other advantages of the Polini Evolution Zip SP fork include:
- The ability to use different spacers under the lower steering bearing to change how low the front of the scooter sits
- The possibility to fit taper roller steering bearings instead of the original ball type
- Special clips to hold the top shock bolts in position while the front suspension is fitted or removed
- The ability to adapt the Zip fork to smallframe Vespa by cutting the steering stem and welding in Polini’s replacement top section
Enter Casa Performance
What became quickly apparent to the Casa Performance boys was that the Polini fork – with its straight stem – could be modified to fit Lambrettas by getting replacement stems produced and welding them in. Polini have collaborated with Casa Performance in supplying the Zip fork without the original stem welded in position, so all CP have to do is press out the Zip tube, fit the Lambretta tube into place and secure it by welding.
So far, Casa Performance have produced a small batch of steering stems to suit LI/SX, GP and Vega/Lui frames. Each is CNC-lathed and milled and supplied with a screw-on steering stop to offer the correct amount of fork rotation inside the frame.
Advantages specific to Lambretta use include:
- Increased front suspension travel
- Reduced overall weight
- Massive choice of brake and suspension upgrades available
- Standard Zip 10″ wheel is tubeless and round
- Single-sided wheel replacement is quick and easy
- This fork fits under any type of fixed Lambretta mudguard
- In the future it may be possible to fit taper-roller steering bearings instead of the original ball bearing
- The price of a high-end brake, fork and suspension set-up is greatly reduced compared to upgrading Lambretta forks to the same level
Disadvantages of this fork are:
- A single-sided fork looks odd in a Lambretta chassis
- Currently, this fork design is outside the rules for use in Lambrettas in many race series
- There is no facility for fitting a mechanical speedo drive
Sticky gets duffed up on track whilst testing the handling characteristics of the CP modified Polini front end.
Text, photos & video: Sticky
Additional images: Mihaela