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Bike Review - Posted 20th March 2015
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Moto Morini 350K Sport – Front Fork Upgrade

Martin Gelder fits (with much assistance) a set of cartridge emulators to the Marzocchi forks on his Moto Morini Three and a Half. Classic improvement or pointless modern meddling?

The Moto Morini 'Three and a Half' 350 is a bike that handles well enough to outrun many larger engined contemporaries. What it lacks in straight-line velocity it can make up in corner speed and even ridden at sedate speeds more appropriate to today's traffic conditions, it has a poise and balance that many motorcycles of the seventies lacked.

In 1976 Bike magazine's Bill Haylock described the Morini has having “one of the best chassis/suspension packages you'll get from any manufacturer”. Dave Minton praised the “uncanny stability and sensitive handling” in Motorcyclist Illustrated in the same year. Even into the 1980s, Cycle World magazine were still saying that the “handling of the Morini is hard to fault.”

Moto Morini 350K Sport – Front Fork Suspension Upgrade

These steering qualities were achieved by combining a strong chassis - the Morini's frame and swinging arm construction is on a par with Japanese 750s of the late seventies and early eighties - with fundamentally correct frame geometry and common-for-the-era stiff and short travel suspension. In the seventies, stiff was good and if you wanted to go fast you needed a smooth road and faith in the grip of your tyres.

Climbing from the plank-like seat of a seventies Italian road burner (of any capacity; I'm looking at you, Ducati and Moto Guzzi) and settling into the surprisingly accommodating saddle of a modern sports bike is a revelation, if a slightly disquieting one. Everything is so... soft and supple. The suspension at both front and rear sighs softly as it dips under your weight and you can't believe that anything so yielding can possibly be safe and secure, let alone sharp and precise or stable and sensitive, at any sort of decent speed. And yet it all works uncannily well. The wheels track the ground impeccably, the tyres stay glued to the road and you can exploit the massive abilities of the engine and brakes at will.

Moto Morini 350K Sport – Front Fork Suspension Upgrade Original Marzocchi damper rod assembly in the middle, modified ones above and below.

The difference between then and now is the quality of the suspension. Frame construction has changed over the decades but geometry isn't that different and the fundamental suspension components perform the same function; the changes have come in the way that those suspension units damp out and control the movement of the wheels relative to the chassis. And it's the damping and control of the front suspension that I'm hoping to improve on my Morini.

The standard Marzocchi forks are sturdy but stiff, and previous experiments with changing the viscosity of the damping oil have either given me a set up which is smooth and comfortable for road riding but gets tied in knots when ridden on track, or a ride that works well when pushed hard on the (relatively) smooth expanses of Mallory Park or Cadwell Park but jars and jolts when confronted with a typical A or B road surface.

Moto Morini 350K Sport – Front Fork Suspension Upgrade A cartridge emulator, yesterday

It might be worth taking a moment to talk about what we mean by damping, and what effects it has on the handling of a motorcycle. The springs in suspension units support the weight of the bike and rider, and allow the wheels to track over bumps and the suspension to move when weight is transferred during braking, accelerating or cornering. Left to their own devices, though, those springs would allow the bike to bounce about all over the place in uncontrolled confusion, and so we calm their enthusiasm with rebound damping; as the springs recover from the forces fed into them, the rebound damping absorbs some of the force and allows the spring to return to its normal state in a controlled way, ready for the next suspension input. Rebound damping is what gives your bike stability as you ride over bumps, brake heavily, and so on.

Suspension units also have compression damping, which slows the rate at which the springs can be compressed. Compression damping needs to be weaker than rebound damping, as it's working with the springs rather than against them, and having the right amount of compression damping gives the rider a good 'feel' for what the tyres and suspension are doing. Early (or cheap) suspension units had rebound damping but no compression damping, and this can give acceptable handling; compression damping gives feel but rebound damping is essential for control.

To further complicate things, and to further confuse modern sports bike riders when adjusting suspension settings, we can also differentiate between high speed and low speed damping. The speeds in discussion here are the speed that the suspension unit is forced to move when dealing with an input of some sort, rather than the speed the bike is moving along the road or track. High speed damping deals with sudden, big movements like hitting a bump in the road; low speed damping deals with the more gentle movements caused by weight transfer when braking, accelerating or cornering.

'Classic' compression and rebound damping was done by forcing oil through holes of carefully selected sizes; to change the damping, you changed the oil. Modern damping achieves a much more precise version of the same thing by forcing oil through holes and past stacks of spring-steel shims that flex by different amounts. These stacks are assembled into cartridges which can be (relatively) easily changed to alter damping characteristics, and to alter rebound and compression damping separately.

Moto Morini 350K Sport – Front Fork Suspension Upgrade

Help is at hand for those of us who want to add cartridge levels of control and adjustability to our old orifice-damped bikes, in the shape of the cartridge emulators available from companies like RaceTech (more on their cartridge emulators for damper rod forks) and last spring (ahem) Paul 'EVGuru' Compton modified my forks to allow the fitting of a set of these to my Morini.

As is always the way with projects like this, I ended up putting the forks back into the bike very shortly before heading to Cadwell Park for the Morini Riders' Club trackday. Even after making up suitably shorter preload spacers to allow for the extra height of the emulator on top of the damper assembly and using the thinnest, runniest damping oil available, the forks felt much too stiff; I'd overfilled them with oil, reducing the amount of air above the oil and creating a much stronger air-spring effect as the forks compressed.

Moto Morini 350K Sport – Front Fork Suspension Upgrade

It's the first time I've started a track day by draining out and refilling the fork oil in the paddock, but once we found roughly the right level I was able to get a feel for the new dampers. Remember how I said I was hoping for supple and controlled rather than stiff and jarring? Well... I had stiff and controlled. I was using the thinnest oil I could buy – 2.5W – and the rebound damping was still much too firm; I'd optimistically brought bottles of 5W and 7.5W to the track but with the front end pattering on the way into corners it felt like we were an order of magnitude out with the rebound damping; the tyre was doing a lot of the suspension's work and although feel was improved from the previous damper-rod-only set up, the ride was simply too harsh to push any harder.

Moto Morini 350K Sport – Front Fork Suspension Upgrade
Bouncy Morinis on Now...

A trip back to Paul's workshop to drill out the rebound damping holes from 1mm diameter to 2mm (twice the diameter gives four times the area so four times the flow) gave a much plusher ride, although there was still a lot of stiction (a combination of friction and sticking) in the forks.

...And then the MOT ran out and family life got in the way, and Morini tinkering was shelved for a few months.

Fitting the cartridge emulators has definitely been a step in the right direction but I need to put some decent road miles on the forks now before attempting any more adjustments to the compression damping.


Further Reading: Want to know more about suspension and chassis geometry? Race Tech have produced a Motorcycle Suspension Bible, and Tony Foale is best place to start for motorcycle chassis information.

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