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Vespa Suspension - how to set up

Created by Jesco at 16:03 on March 8, 2021

The vast majority of mechanics and tuners are aware of the importance of the suspension for the safety and enjoyment of riding scooters. An ideal suspension shortens the braking distance, increases the riding comfort and with it also the riders safety while negotiating bends in the road. The suspension includes a multitude of components, as all parts involved in connecting the scooter to the road are included. First a short description of the spring and damper elements of an articulated wheel suspension. In the case of vintage scooters this is usually taken care of by a spring and shock absorber strut assembly.

These components have an especially important function, enabling essential tyre and wheel contact to the roads surface. They should compensate for uneven road surfaces. Whether they are exposed to long or short impacts, major or minor surface irregularities or even the rocking movement experienced during braking and acceleration – all these forces must be evened out. This is important as the wheels themselves can only transmit the negative, positive or lateral acceleration optimally. The suspension is also a deciding factor concerning the overall comfort of the rider and pillion.

In order to satisfy customer demand for improved suspensions there are many manufacturers offering aftermarket components enabling customised set ups. Even mechanics with years of experience are faced with quite a task when fitting and adjusting these parts: The better the shock absorbers are, the more complicated they become to set up properly due to the additional adjustment possibilities. These added functions may seem advantageous, but some people tend to avoid the challenge of correctly adjusting every parameter. This is mostly due to the many half-truths and misunderstandings surrounding this subject.

With this blog I would like to attempt to provide an element of structure and give guidance to the basics, along with a few tips to assist in the correct setting up of your own scooters suspension.

 

The two basic elements of a shock absorber:

  1. Spring: This enables an articulated wheel suspension. It enables rider comfort and even contact of the tyres and wheels to the road, also when travelling over uneven surfaces.
  2. Damper: These are included to reduce the rebound movement of the wheel when encountering bumps. Without a damper unit the rebound energy of the spring would prevent sufficient contact to the surface of the road. The riding comfort would also be negatively affected.

What are the purpose of the individual parameter adjustment possibilities?

  1. Spring rate: The profile of the spring coil included determines the rate at which the spring is compressed.
  2. Progressively-wound springs: The springs special coiling construction enables a rising-rate of stiffness with increasing compression.
  3. Spring pre-load: The length of the spring without load can be adjusted with a staged cam-ring or linearly with an adjuster ring and locknut.
  4. Rebound damping: This can mostly be adjusted with a manually-operated wheel, located near the end of the damper rod and helps determine the rate at which the springs rebound energy expands the shock absorber.
  5. Gas/Oil external reservoirs: These provide additional volume for the hydraulic damper medium contained within the damper cylinder and also has the effect of the temperature remaining more constant, while also increasing the adjustment possibilities.
  6. Compression damping: This determines the force with which the compression of the spring is controlled.  This can mostly be adjusted with a manually-operated wheel located on the external reservoir.
  7. High/Low speed damping: High and low speed compression damping can be separately regulated and compensated for.
Correct adjustment of the chassis is worthwhile

The basic construction of a damper unit:

There are many ways to construct a shock absorber, but most of the modern adjustable types share the same basic structure. A common single-tube gas-pressure shock absorber is constructed as follows:

A hydraulic cylinder is attached at one end to the wheel suspension assembly, which contains a piston the rod of which is then attached to the chassis side of the wheels suspension. This cylinder is filled with a hydraulic oil. When the piston moves along the cylinders internal length, the hydraulic oil has to be transferred from one side of the piston to the other. This is achieved with openings in the piston. As this movement of oil requires energy, the movement of the wheel in relation to the chassis is decreased, or dampened.

 

This damping effect can be varied by altering the size of the openings in the piston. These holes only allow the oil to flow in one direction, enabling the rebound and compression damping to be separately regulated and adjusted.

Alongside the hydraulic oil there is also a gas-filled pneumatic chamber contained within the shock absorber cylinder. This is kept separate from the oil with the use of a further piston. This chamber compensates for the volume of the piston rod during compression. As the piston travels up into the cylinder it occupies a volume previously taken by the oil, which compresses gas within this pneumatic chamber. As the piston leaves the cylinder this gas pressure is once more relaxed. This causes the effect seen when an unmounted shock absorber is squeezed together by hand and then returns to its normal length. This effect can vary according to the construction of the shock absorber involved. This is a demonstration of how this pneumatic gas-filled chamber adds to the function of the shock absorber.

What effect does the adjustment of the spring pre-load have?

The adjustment of the spring pre-load is quite simple. Sadly though, there seem to be stubborn misunderstandings about the effect. Rule number one in every serious article concerning the pre-load adjustment: The spring pre-load does not affect the stiffness of the shock absorber, only its length! It is mainly useful when adjusting the suspension to suit the total weight of the rider, or on the race track to alter the suspension geometry slightly.

Although most scooter fans are also aware of this, it is still quite frustrating to discover that they remain sceptical and refuse to understand the principle of these facts! This is the reason for the requirement for repeated explanation that only the height of the scooter is changed, not the stiffness of the suspension! Absolutely zero! Really!

Beforehand a diagram for visualisation. This displays a spring rate performance, or the extent to which it is compressed by increasing force. The 'X' axis the spring travel in centimetres, on the 'Y' axis the riders weight in kilograms.

This displays that a rider weighing 100kg compresses the spring by 80mm (lets ignore the vehicle weight for now). A 400mm long shock absorber would be 320mm long once the rider is seated. If this shock absorber was equipped with a 300mm long spring, only 220mm of spring travel remain available. The 80mm, by which the spring was compressed, or sag, is only available as negative spring travel. This sag is the distance that a wheel can compensate for the abrupt reaction of the suspension when, for example, negotiating a pot-hole in the road.

What happens, when the spring has its pre-load adjusted?

When the adjuster is wound up, the potential travel of the spring without the riders weight is decreased. As an example lets take 40mm. If it is tightened by 40mm, or in other words, the spring has been compressed by 40mm. As the spring is less compressed as it will be when the 100 kilogram rider is taken into account, with this weight added the shock absorber will be compressed identically as before, leaving 220mm. As the spring has already been compressed by 40mm due to the pre-load, the vehicle and shock absorber can only sink to the value of the difference between the pre-load and the weight of the load. In this case only 40mm, making the negative spring travel just 40mm. Simultaneously, the scooter sag is reduced by 40mm, making it 40mm higher than before the spring pre-load was adjusted.

Most important: Following the pre-load adjustment the riders weight causes the scooter to sag by exactly the same distance as beforehand. The spring is also exactly as stiff, absolutely nothing has been altered, apart from the height of the scooter once the rider is in position. Another factor has also been affected, the relation of the positive to the negative spring travel.

As can be seen in the table above, a number of factors are altered by adjusting the pre-load of the spring. Without load the shock absorbers length remains identical while only the springs length is changed. When load is applied the length of the spring is not changed although the length of the shock absorber does. Once the rider is in position, the stiffness of the suspension has not changed, only the height of the vehicle.

This is also true for shock absorbers equipped with progressively-wound springs. It is the same physics here: A rider with a certain weight will always further compress the spring once seated by a certain distance, no matter how the spring pre-load has been adjusted!

Without perfectly adjusted suspension, such cornering is not possible

How can the spring pre-load be correctly set up?

To set up the spring pre-load accurately it is first necessary to determine the amount of spring travel available without load. This can usually be established with the help of the scooters centre stand. Once the scooter is on the centre stand the load can be taken from the wheel. Then, taking the centre of the shock absorbers axle as a reference point, find a part of the chassis that is horizontally aligned to this point and mark the height of the axle in relation to the scooter provisionally with a piece of tape. Now take the scooter off the stand and allow the rider to take position. Now you will have to hold the scooter in a vertical position while the rider has both feet up, maybe a third person could help. Now once again make the measurement from the axle in relation to the chassis, again marking the point with another piece of tape.

Now you have two measurements, the first without load and the second with the combined weight of both the rider and scooter added. The second value can then be subtracted from the first to provide the negative spring travel distance. This should in theory be between approximately 25-30% of the complete length of potential spring travel. In real-world numbers that means if a shock absorber has 200mm of spring travel, its negative spring travel should be between around 50-60mm.

These ideal values are however not so easily realisable where vintage geared scooters are concerned. The potential spring travel is almost always insufficient. A compromise in this case is an acceptable negative spring travel of 20mm.

This means the springs pre-load should be adjusted until the difference between the measurement without load and the measurement with load is as near to 20mm as possible. This leaves the springs pre-load suitably adjusted.

What is the effect of adjusting the rebound damping?

When the wheel encounters an obstruction, the road surface irregularity is compensated for by the compression movement of the spring. This is now under tension and will rebound, returning to its original shape and position. This rebound energy is then arrested by the damping function of the shock absorber. The strength of this force regulating the stiffness of the damping is the parameter that can be adjusted.

It is essential that this damping force is not too suddenly applied to keep a settled suspension. On the other hand it should also be reacting rapidly enough to be prepared for the next obstruction in the smoothness of the road. If the rebound damping is too stiff it can lead to a lasting contraction of the shock absorber following a series of repetitive bumps, leaving the suspension ineffective and uncontrollable. The shock absorber is only further compressed, with no chance to recover through extension, becoming even stiffer. This causes the tyre to lose full surface contact and begin to skip across the road, without any kind of grip!

How is the rebound damping correctly set up?

To provide a feeling for the set up of these suspensions, take an uninstalled shock absorber and compress it between your hands and then release one end. The period required for it to return to its full length should be about one second.

Sadly, once again this simplified measurement technique cannot be easily transferred to the set up of a vintage scooters suspension. An optimal recovery period depends a lot on an adequate amount of spring travel. As these vintage scooters typically have a lot less spring travel available than the average road-going motorcycle, this ideal recovery period is effectively reduced to around half a second. The measurement of the ideal recovery time for your scooters suspension can be determined with the use of a timer, stopwatch or any smartphone.

If the chassis rebounds too quickly, the rebound damping should be wound up in a clockwise direction. If the opposite is true and its rebound is noticeably sluggish, the rebound damping should be wound down in an anti-clockwise direction.

The external reservoir

An additional facility allowing adjustable compression damping is a function normally reserved for high end aftermarket shock absorbers destined for race orientated suspension set ups. This can only be provided with the addition of an external reservoir. These are normally situated in a piggy-back position on the shock absorber at the lower end of the gas cartridge. To allow the exchange of the damping medium they are connected with a solid pipe or, in some cases, a flexible hydraulic hosing. The gas reservoir and its separation body are also housed within the external reservoir. This arrangement ensures a free-flow of gas and oil to guarantee operational requirements.

The main function of an external reservoir however remains to ensure a more stable and constant temperature of the hydraulic damper oil. The larger volume of oil it enables, remains cooler for longer, with the additional surface area of the extra reservoir enabling more rapid cooling. This is especially relevant as the viscosity and damping qualities of the oil being reduced with increased temperature.

A further advantage of an external reservoir: The point at which the damper cartridge and reservoir are joined is an ideal position for the location of a special valve, enabling the regulation and adjustment of the rebound damping stiffness. This is similar in function to the compression damping regulation. There are, again, openings with one-way valves and those that allow oil flow to the reservoir can have their size adjusted. This means that the oil flow during compression of the shock absorber can be regulated.

What is the effect of compression damping?

The one-way valves that deliver oil towards the reservoir can be closed and this dampens the spring compression considerably. If this valve is fully-opened the spring compression is increased. Their adjustment regulates the speed at which the shock absorber compresses. Increased compression damping stiffens the suspension noticeably, exactly the parameter that is not affected by adjustment of the springs pre-load! This stiffness remains constant throughout the springs travel, it does not increase with compression of the shock absorber.

How is the compression damping correctly set up?

Unfortunately the basic set up of the compression damping is not as straightforward as the correct adjustment of the rebound damping. The set up requires a certain feel for potential improvements through experience of individual suspension adjustment. In the end there is nothing that beats actually riding the machine to be set up, do a little fettling and then repeat as often as is necessary to find the correct combination for each scooter!

Once you have approximately adjusted the parameter, a dry-run can help: The rider positions themselves over the machine holding the handlebar grips. They should than exert intermittent bursts of pressure with all their weight down through the handlebars to the shock absorber requiring adjustment. If this action causes the shock absorber unit to almost or even fully compress, the compression damping has to be increased. If the whole weight of the rider has little or no result, the compression damping is too stiff and should be reduced. An acceptable method of training a feeling for the potential of any particular suspension set up, is to ride the machine a few miles with the compression damping minimised. Then, for comparison, reverse the damping to the stiffest setting and repeat the test route. This gives you a chance to realise the effect this has on the ride directly, with more effective adjustments being realised more rapidly as a result.

Dual rate compression damping

The force applied by the damper mechanism remains linear, or constant, during the damping process. It can change though, according to the speed at which the damper rod enters the cylinder during operation. A High/Low damping speed adjustment allows this resistance to movement to be separately set up, for either low or high speed movement.

The regulation of this parameter is usually also located somewhere on the external reservoir. Mostly these adjustment screws are arranged coaxially, sometimes however also separately positioned next to another. The two stages of compression rebound to be adjusted are labelled either 'Low speed' or 'High speed'. Firstly you should try and imagine what these terms really indicate. They are intended to cater for the compression adjustment, whether the shock absorber is being compressed at a higher or lower rate, this term does not refer to the scooters road speed! A shock absorber is only effective when a bump in the road causes the larger proportion of the springs potential travel to be exploited. If the spring travel is minimal, this movement remains almost entirely in the low speed region. It is by no means a simple task to correctly adjust this parameter. This can be made a little easier, by first understanding the exact consequences its set up have on the behaviour of the suspension. The suspension usually operates in the low speed region, even the reaction to short, hard road surface irregularities seldom cause operation in the high speed region. Every time the spring is compressed, the beginning of the process always starts in the low speed region of operation. This can peak momentarily in the high speed region, before rapidly returning to operation in the low speed region and coming to rest. Even short, and rapid movements caused by, lets say, cobbled streets do not cause operation in this high speed region. The spring travel remains relatively short and so the damping occurs in this case in the low speed region.

Hard braking manoeuvres cause a rapid relocation of the load between the axles and also extended piston travel in a relatively short period. This causes the piston to travel in the high speed region. If the braking is more progressive this also causes extended movement and compression, but spread over a longer period. This leaves the damper operating in the low speed region.  

Load in the high speed region occurs when the scooter makes use of its entire spring travel suddenly while negotiating very uneven surfaces, causing the damper piston to accelerate to compensate. The high speed compression damping is nothing more than a kind of pressure relief valve that, with the help of a hydraulic mechanism, prevents the shock absorber becoming fully compressed and the spring coil-bound.

The low speed compression damping stabilises the scooter in dynamic riding circumstances, such as moderately uneven surfaces, braking and accelerating. It also supports the spring operation, which can sometimes compress to easily without the resistance provided by the low speed compression damping parameter.

How is the High/Low speed compression damping correctly set up?

If the compression damping in the low speed region is over-increased, the high speed operation has to compensate by functioning prematurely to reduce the excess pressure quickly enough. This means that the high and low speed damping strength should not be too far apart. A value that can help basic set up is that the high/low speed values have five 'clicks' between them on the available range of adjustment. These values should also remain five 'clicks' apart during adjustment.

The set up of the high/low speed compression damping can be considered as the next level of suspension tuning and adjustment. To perfect this skill there is nothing that beats pure experience. A small tip – When adjusting the high/low speed damping for the first time, always keep the two values within five adjustments - parallel to each other on the scale, no matter whether increasing or decreasing the resistance similar to more simple damping adjustment. Once you have found an acceptable setting, observe the scooters behaviour, especially in riding situations where the suspension operates in the high speed region, such as hard braking before negotiating a curve. If the front end of the scooter dives, the high speed damping require stiffening. If, on the other hand, the rear shock absorber bottoms out regularly, forcing the rear vertically. The full potential spring travel is not being used, the high speed damping should be reduced.

Conclusion:

If anyone wishes to set up a suspension effectively, they should first provide themselves with a basic understanding of how the various components function and also realise the consequences of each adjustment parameter.

Then the only thing that deepens this knowledge is the most enjoyable! Lots of riding scooters and testing the various settings to get a feeling of how to compare and improve every separate adjustment! To get a suspension set up perfectly only time and lots of patience are the keys to success.

Even before you invest in any aftermarket components it should be kept in mind, that a hard suspension is not automatically a good one, or indeed always the best option for racing purposes. Any suspension must allow the tyres to compensate for uneven surfaces and maintain perfect contact with the road. Only this can be considered advantageous in sport. This means the stiffness of the suspension should always be set up to suit the potential riding circumstances. An extremely stiff suspension only provides advantages on the most perfect and freshest of race circuit surfaces! In my many years working as a mechanic, I think I can say that most vintage scooters equipped with aftermarket suspension components, have a set up so stiff that they can only be effective on absolutely smooth racing surfaces. If you are then riding over cobbled streets and see everything double or blurred, you can also be sure that your tyres have virtually no effective contact or grip!

For most uses only two adjustment possibilities are fully sufficient. The pre-load on the spring and the rebound damping of the same item. If you have higher and tougher demands, compression damping regulation can also be of advantage. The high/low adjustment parameters are something for the more experienced tuners with racing ambitions.

In the end its worth taking care of a decent adjustable suspension. A good suspension set up not only increases riders safety, but also intensifies your riding experience enormously. It is still astonishing the improvement that can be achieved with correct suspension regulation and adjustment! After all, its hardly rocket science, why not give it a try?

Click here for our SIP Performance shock absorbers in the shop, available for almost all Vespa models.

Jesco
Jesco

Jesco is a technician and product developer at SIP Scootershop. Many people know him from videos on the SIP TV Tutorial Channel, where Jesco explains complicated stuff in a simple way.