Vespa Shock Absorber | Chassis Adjustment

Created by Jesco at 10:03 on March 1, 2021

Most mechanics and tuners know how important a chassis is for the safety and riding pleasure of a scooter. A good chassis shortens the braking distance, increases riding comfort and provides more safety in curves. The chassis includes several components: actually, everything that connects the vehicle to the road is part of the chassis. Here, we will focus on the spring and damper elements of the movable wheel suspension. In the case of classic scooters, these are usually "suspension struts" or"shock absorbers".

These components have a particularly important task when it comes to enabling the wheels to maintain even contact with the road. They are supposed to compensate for the most diverse road irregularities. Whether long or fast impacts, whether small or large bumps or rolling movements caused by acceleration and braking - the movements must always be compensated. This is of essential importance. Only when the wheels have even contact with the ground can they optimally transmit negative, positive or lateral acceleration. The chassis is also a decisive factor for riding comfort.

Many manufacturers are happy to comply with the wishes of many two-wheeler riders for an improved chassis. There is a broad market of suspension technology for all requirements and budgets. Even experienced mechanics face a hurdle: good suspension elements in particular have a multitude of adjustment possibilities. While this is nice, many shy away from the challenge of adjusting all parameters correctly. This is also due to the fact that many misunderstandings and half-truths are widespread on the subject.

In this blog, I would like to try to structure the individual functions and provide a guide with both basics and tips on how to adjust them on your own vehicle.

The two basic components of a suspension strut:

  1. Spring: It enables flexible wheel suspension. This enables driving comfort and even contact of the tyres with the ground, even on uneven road surfaces.
  2. Damper: It serves to dampen the deflection movement of the wheel. Without the damper, the energy of the spring would result in poor or no road contact when the wheel springs out. In addition, the ride comfort is impaired.

What is the purpose of the individual setting options?

  1. Spring rate: The characteristic curve of the installed spring determines how much a shock absorber is compressed at a certain force.
  2. Progressive spring: The spring force of such a spring increases withprogressive compression. Helpful for suspensions with low spring travel.
  3. Spring preload: Adjustment via a nut, which limits the length of the spring when unloaded.
  4. Rebound damping: Is usually adjusted via the adjusting wheel at the end of the inner damper rod. The rebound damping determines how high the force is that counteracts the rebound movement.
  5. Compensation reservoir: Gives additional volume to the damping medium from the damper cartridge. Ensures consistent damper function through better temperature balance and improves adjustment possibilities.
  6. Compression damping: This determines the force with which the damper counteracts the compression movement. It is usually adjusted via a wheel on the shock absorber's reservoir.
  7. Double compression stage or High/Low setting: Here the counterforce to fast and slow compression movements can be adjusted separately.
Jesco sitzt auf der Vespa
Das korrekte Einstellen des Fahrwerks lohnt sich

The basic structure of the damper

Structure of a shock absorber

height=416" alt="" width="120" height="416" data-udi="umb://media/684be43023d9457d8892e2998a37de31" />There are many different ways a shock absor ber can be constructed. However, there is a basic design that most of today's shock absorbers that have adjustment capabilities follow. I would like to briefly explain this "single-tube gas pressure shock absorber" schematically:


A cylinder is connected to one side of the wheel suspension. A piston runs in this cylinder and is connected to the other side of the wheel suspension via a piston rod. There is oil in the cylinder. When the piston moves in the cylinder, the oil must move from one side of the piston to the other. This is only possible through holes in the piston. Since a force is needed to push the oil from one side of the piston to the other, the movement of the wheel is damped in relation to the chassis.

The effect of the damping can be varied with the size of the holes. Oil can only flow through the holes in one direction at a time. By using different diameters of the holes for the inward and outward spring movement, these movements can be damped to different degrees.

In addition to the oil, there is a chamber filled with gas in the cylinder. This is separated from the oil by another piston or "separator". The chamber serves to equalise the volume of the piston rod when it enters the cylinder. When the piston rod enters the cylinder, it takes up volume in the cylinder that was previously available to the oil. This compresses the gas in the compensation chamber. When the rod leaves the cylinder, the gas in the chamber expands again. This leads to an effect that is familiar when compressing a dismantled shock absorber: The shock absorber moves apart again. This effect is different for different shock absorbers. This is the reason why the pressure in the gas chamber also influences the behaviour of the chassis.

What effect does the spring preload have?

The spring preload is actually a very easy parameter to adjust. Unfortunately, there are persistent misunderstandings about what this adjustment does. Therefore, I would like to start with a piece of wisdom that is found at the beginning of every article about adjusting the spring preload: I do not adjust the hardness of the chassis with the spring preload, it only adjusts the height. This is mainly to adjust the suspension to different rider weights, but it can also be used to change the geometry, e.g. on the race track.

Almost everyone has heard this before. But apparently it's hard to really believe it or take it to heart. After all, the spring is compressed when the spring preload nut is tightened. Therefore, we will explain here why it is really only the height that is adjusted and not at all the hardness of the suspension. Not at all! Really!

On the left is a diagram to illustrate this. It is about a spring characteristic curve. That is, the distance a spring compresses under a certain force. On the X-axis, the spring travel in mm. On the Y-axis, the rider's weight in kilograms.

It can be seen here that, for example, a 100 kg rider compresses the spring by 80 mm (we disregard the vehicle weight here). For a 400 mm long shock absorber, this would mean that it would still be 320 mm long when loaded. If this strut has a spring of 300 mm, 220 mm would still be available when loaded. The 80 mm by which the rider's weight compressed the shock would now be available as negative travel. The negative spring travel is therefore the distance by which the wheel can deflect in order to compensate for a pothole, for example.

What happens if the preload of the spring is adjusted? If the nut for the preload is tightened, then the length available to the spring in the unloaded state is reduced. As an example, let us take an adjustment of 40 mm. This means that the spring would be compressed by 40 mm. Since the spring is now less compressed than it was under the load of the 100 kg rider, the 100 kg rider would compress the spring to the same length as before, i.e. to 220 mm. But since the spring was already compressed by 40 mm, the strut and the vehicle will only lower by the difference between preload and weight force. In this case, therefore, by 40 mm. This would mean that the negative spring travel would only be 40 mm. At the same time, the vehicle has lowered by 40 mm less, so it is 40 mm higher in the loaded state than before the adjustment of the spring preload.

Very important: Even after the adjustment, the weight of the driver has compressed the spring to the same length. This means that the spring is just as hard as before the adjustment. The only thing that has changed is the height of the vehicle when it is loaded with the driver. Something else has changed: the ratio of positive to negative spring travel.

Tabelle
In der Tabelle ist gut zu sehen, was sich durch die Verstellung der Federvorspannung verändert

The table clearly shows what changes when the spring preload is adjusted. In the unloaded state, the length of the spring strut remains the same and the length of the spring changes. In the loaded state, the length of the spring remains the same and the length of the spring strut changes. If the driver sits on the vehicle, the hardness has not changed, only the height of the vehicle.

By the way, this also applies to suspension struts that are equipped with a progressive spring. Here too, a rider with weight X means that the spring compresses to length Y, no matter how the preload is set.

Roller liegen in der Kurve
Ohne perfekt eingestelltes Fahrwerk sind solche Kurvenfahrten nicht möglich

How to adjust the spring preload?

In order to set the spring preload correctly, it is important to first determine the spring travel in the unloaded state. In many cases, this can be done easily using the main stand. Place the vehicle on the main stand and take the weight off the wheel. Then measure from the centre of the axle to a point on the frame in vertical alignment. Mark this point with a small piece of tape. Now take the vehicle off the stand and sit on it, or the driver, in the driving position (it is best to warm up the shock absorbers beforehand). At this point you need assistance, as the vehicle must be straight, the driver should have his feet in driving position and measurements must be taken on the chassis. Again from the centre of the axle to the mark or measuring point.

Now you have two readings, one unloaded and one with the weight of the driver and vehicle. If you subtract the second value from the first, you get the negative suspension travel. In theory, 25-30 % of the total suspension travel should be the negative travel. This means that if a shock has a travel of 200 mm, the negative travel should be 50-60 mm.

Unfortunately, this value cannot simply be transferred to the conditions of a classic gearshift scooter. The spring travel there is usually too short. A good reference value here is approx. 2 cm.

The spring preload must now be adjusted until the difference between the two measurements is this 2 cm. The spring preload is already in a good basic tuning.

What is the effect of adjusting the rebound damping?

If the wheel rolls over an obstacle, the unevenness is compensated for by a spring compression movement. The spring is now under tension and moves back to its original position. This spring-out movement is damped by the shock absorber. The strength of the force that the shock absorber exerts against the spring in this direction can be changed with the rebound stage.

It is important that the rebound movement does not take place too quickly so that no unrest occurs in the chassis. On the other hand, the wheel must not be allowed to rebound too slowly either, so that the full spring travel is available again when the next obstacle is encountered. If the rebound damping is set too hard, a kind of "wind-up effect" can occur. The tyre compresses on several obstacles in succession without being able to rebound in between. The shock absorber sags further and further and becomes hard. The tyre bumps and loses contact with the road.

How is the rebound damping adjusted correctly?

A guideline for a good basic setting: the chassis should take about one second to spring out again. This means that the shock absorber or the chassis should compress to the stop and then be jerkily released. The time it takes for the frame to spring back to its original position should be one second.

Unfortunately, it is also the case here that this rule cannot simply be transferred to classic scooters. Of course, the optimum of this time is strongly dependent on the length of the spring travel. But since classic scooters have much less suspension travel than an average road motorbike, the time here must be about 0.5 seconds. The best way to determine whether the time for the rebound is correct is by "counting" or saying the number "twenty-one". This takes pretty much exactly one second. To determine the half second, therefore, just "one and a half". If the chassis rebounds faster, the rebound damping must be closed further, i.e. adjusted clockwise. If the chassis rebounds more slowly, the rebound damping must be adjusted upwards, i.e. anti-clockwise.

The expansion tank

An adjustable compression stage is an option that only high-quality shock absorbers with mostly sporty ambitions provide. The prerequisite for mounting a compression adjustment is an external reservoir. Such a reservoir is located at the end of the oil cartridge and is connected to it via a small tube or hose. The damper oil can flow from the damper cartridge into the expansion tank. The gas reservoir and the separator are also located in the expansion tank. So only the oil flows from the cartridge to the reservoir and vice versa. The main purpose of the expansion tank is to keep the temperature of the oil at an even level. The larger volume heats up more slowly. Also, the additional container provides more surface area to dissipate temperature. This is helpful because the oil also changes its damping properties with its temperature.

Another advantage of the compensation reservoir: a valve can be perfectly positioned at the transition between the cartridge and the reservoir, which can be used to adjust the compression stage. This works in a similar way to the compression stage: there are holes that allow the oil to pass through in only one direction at a time. The size of the hole through which the oil flows into the expansion tank can be adjusted. This means that the direction in which the oil flows during the compression movement can be adjusted.

What does the compression stage do?

If the valve of the bore to the expansion tank is closed, the damper applies a lot of force against the compression. If it is open, the compression is dampened only slightly. The speed at which a shock absorber compresses is adjusted. The compression damping produces something like "hardness" in the spring behaviour, i.e. the parameter can be adjusted which cannot be adjusted with the spring preload. This counterforce remains the same during compression, i.e. it does not increase with increasing spring travel.

How is the compression damping adjusted correctly?

Unfortunately, the correct basic setting of the compression damping is not as easy to find by rule of thumb as that of the rebound damping. The right setting requires experience and feeling. In the end, there is no getting around riding, riding and riding to find the right setting. If a pre-setting is to be made, a dry exercise helps: the rider stands over the scooter and jerkily presses, with all his weight, on the respective shock absorber. If the shock absorber dips far or completely with the rider's weight during quick jolts, the compression damping is set too soft. If hardly any suspension travel is used during fast, powerful shocks, the compression damping is too hard.

A tip to get a reference value for the compression damping: Turn the compression damping all the way up and ride a few kilometres. Then set the compression damping to the other extreme, i.e. turn it all the way down and ride a few kilometres again. This helps to develop a feeling for what is happening with the adjustment in the chassis and how it feels better. Then slowly and with a lot of driving tests, get closer to the right value.

Double pressure stage

The force that the damper exerts against the compression remains constant with the spring travel. It is therefore linear. However, it changes with the speed at which the piston rod plunges into the damper. With the double compression stages, the resistance can be adjusted separately for fast and slow dipping movements. The double compression adjustment is also usually located on the reservoir. Usually the screws are coaxial, i.e. arranged inside each other, sometimes side by side. The two pressure levels are called "Highspeed" and "Lowspeed". It is important to remember what high and low speed mean in this context. What is meant is the speed at which the shock absorber is compressed, not the speed of the vehicle. A shock absorber can only be compressed quickly if a movement takes up a lot of suspension travel. If little suspension travel is used, the movement is almost always in the low-speed range.

Adjusting a dual compression damping is not easy. Therefore, a few considerations to help understand what is being adjusted:

The suspension works mostly in the Lowspeed range. Even short, hard bumps seldom produce movements in the high-speed range, since every spring action starts at low-speed, accelerates only briefly into high-speed and ends again at low-speed before the reversal point. Even short and fast spring compression and extension movements, for example on cobblestones, do not take place in the high-speed range. The spring stroke is too short and the damping is predominantly in the low-speed range. Strong braking manoeuvres cause a rapid shift of the axle load and thus a large stroke movement in a short time. The damper enters the high-speed range. If the brake pressure is built up slowly, a large lifting movement does take place, but over a longer period of time. The damper works in the low-speed range.

Loads in the highspeed range occur when the scooter compresses quickly over the entire suspension travel, for example in the case of violent bumps, only then can the speed of the damper piston build up properly. The highspeed compression damping is nothing more than a pressure relief valve that prevents the suspension from going "on block" hydraulically when a hard load is applied.

The Lowspeed compression damping stabilises the vehicle in dynamic driving conditions such as long bumps, braking or accelerating. It supports the spring when it would bend too much without the counterforce of the compression damping.

How to adjust high and low speed?

The more Lowspeed compression stage is set, the earlier the Highspeed valve opens, as the excess pressure cannot be dissipated quickly enough. Therefore, the settings of the two stages should not be too far apart. A value that can occasionally be found as a guide is five clicks. Furthermore, the setting of the high-speed stage should not be too far away from that of the low-speed stage.

With the setting of the double compression stage we have finally reached level 2 of the suspension adjustment. To do this perfectly, you simply need a good bit of experience.

A tip: At the beginning, always adjust the high and low speed in parallel to the same values. Proceed as with the adjustment of a simple compression damping. Once a good value has been found, observe the vehicle closely in driving situations in which the suspension enters the high-speed range. E.g. fast, heavy braking before a curve. If the front end buckles too quickly, the high-speed setting must be increased. If, for example, the rear wheel hits a block during a strong, fast bump and pushes the rear end upwards without using the suspension travel, open the high-speed stage.

Conclusion

If you want to adjust a chassis correctly, the first thing you should do is internalise how it works and get to grips with the effect of the individual adjustment options. Then the only thing that helps is a lot of driving and testing to develop a feeling and to be able to compare different settings. A perfect chassis always requires time and patience.

Even when buying the individual components, you should remember that a hard suspension is not a good suspension and of course it is not sporty. A chassis must enable the tyres to compensate for unevenness and always have perfect contact with the road. Only that is sporty. So the hardness of a chassis must fit the road. Only on very smooth circuits does a slightly harder suspension make the car faster. From many years of experience with my own scooters and those of my customers, I can say that many classic scooters equipped with tuned chassis have a much harder set-up than would be advantageous even on the smoothest racetrack. If you rumble over cobblestones with your everyday scooter that you see everything double, your wheels will hardly have any contact with the road.

For most purposes, two adjustment options are sufficient: Spring preload and rebound damping. Those who are a little more demanding can also quickly achieve good results with the compression damping adjustment. The double compression damping is something for experts with sporty ambitions.

At the very end, it is worth dealing with the suspension. A good chassis is not only safer, it increases the driving pleasure immensely. It is amazing what can be achieved with the right adjustment. It's not that difficult, just try it!

Click here to go to 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.