Tech Torque: Building a Lambretta or Vespa Street Racer
Having a much more powerful engine means the brakes and suspension are under far greater stress, upgrading them is vitally important…
Stu Owen brings us Part 5 of his winter tech series, his guide to building a Lambretta and Vespa Street Racer. This month: Brakes and suspension
If your street racer started out in standard form chances are after it’s finished the power output will have at least doubled. That’s fine – the intention was to build a fast road going scooter all along. The brakes it was initially fitted with won’t have the stopping power to cope with the forces now going through them and neither will the suspension. The simple solution is to fit better brakes with stronger stopping power and a stronger shock absorber to alleviate the problem but unfortunately it doesn’t stop there. This is where the laws of physics take over and it can get quite complicated.
Understanding the law
When energy is produced it transfers from one component to the other. If the scooter is moving very quickly and the brakes are applied, all that energy of forward movement is now put through the brakes. As they slow the scooter, the force is transferred to the suspension and the tyres gripping the road. So it’s not just the brakes that need to be upgraded but everything connected to the chassis. Also remember the quicker you need to stop, the greater the force exerted on the components. How much stopping power you need depends on how much power
your engine is now creating the mass or weight of your scooter. The heavier the scooter, the more stopping power is required – which is why it is vital to make your street racer as light as possible.
A common mistake
Braking on any two-wheeled vehicle in dry conditions should be 70% to 30% in favour of the front wheel. So the front brake that needs significant upgrades, the back brake far less so. Both the Lambretta and the Vespa were fitted with drum brakes at the rear from the factory. Though you want to make sure they are in perfect working order upgrading them is not only complicated but you have to ask yourself is it worth it. There are aftermarket versions readily available for all Vespa models so for them it’s not too difficult.
For the Lambretta, it’s a different story. Not only are the conversions offered agricultural and extremely heavy they work on the theory of transferring the braking force through the side of the hub. The Lambretta rear hub was not designed to take extreme force in this direction and often failure can occur with dangerous consequences. When it comes to the Lambretta, the best option is to upgrade the drum set up including the cable and pedal linkage to the best available but just leave it at that. It can be argued that in the wet braking should be equally 50% on the front and the rear. Though this means the less efficient drum set up may not seem so good it must be presumed that you won’t be accelerating in the wet as much as you would in dry conditions. Therefore the brakes won’t need to work so hard cancelling out the drum brakes’ inferior stopping power to that of a disc.
Front-end perfection
Knowing that the front brake is the most important and most used, it’s imperative to upgrade it to a disc setup. Both the Vespa and Lambretta have options for this and for the latter a whole host of companies now produce their own version. What you will require depends what power your engine is now producing. If you are only raising the power modestly do you need a multi calliper set up? On the other hand, if you plan to upgrade the engine further it may prove to be a wise investment.
The next problem which relates to the transfer of power discussed earlier is the springs and dampers on the forks. If the damper and suspension setup is not strong enough you will suffer severe handling problems as the front end compresses down. It is vital to upgrade all these components but be careful in doing so. If the front end is too stiff, it will cause the front wheel to bounce as the force is dissipated between the tyre and the tarmac, often referred to as chatter.
The tried and tested Vespa set up uses a single stem fork and trailing link system to mitigate the braking effect. As the engine is mounted at the rear of the frame under heavy braking the mass moving forward isn’t as noticeable as on a Lambretta where the engine is situated much further forward. Therefore the single stem can cope. The important part is the damper while at the same time being able to fully adjust the hardness settings.
There are some extremely well-developed ones readily available.
If all that seemed too easy we now turn to the Lambretta fork system which can be an almost never-ending conundrum. The design uses two fixing points on either side of the front wheel. Again it uses a trailing link system with internal springs within the fork stems. There is a tendency to upgrade these with many being offered that do a better job than the original ones. Though the fork springs need to be in perfect condition they only do part of the job. It will be the damper that makes the real difference. Also if the fork springs are too strong they will possibly be part of the chatter problem already mentioned. Standard ones in good condition are fine.
The real choices start to mount up when it comes to dampers and it can often be confusing. The original pencil style ones are not strong enough to cope with a modern hydraulic disc brake and will cause quite a dive at the front end under heavy braking. Over that last few decades, the Kawasaki steering damper has been used which has four settings on its hardness. While this helps mitigate braking force they only operate in one direction. On their strongest setting the front end has the tendency to be too hard and on uneven roads can give very uncomfortable handling characteristics.
Luckily over the last few years with the advent of Lambretta product development, there are now plenty of good options available. All work in both directions with damping and rebound. Also with several settings and hardness, they can be adjusted to the rider’s preference depending on how hard they are on the brakes while at the same time knowing the rebound will cope on any road surface. There has been a tendency for fitting an anti-dive system which is often seen on race prepared machines. While this works very well on the smooth tarmac of a race track on the road the hardness on the front end makes for uncomfortable riding.
When it comes to sorting the Lambretta front end, start by choosing which disc brake you think will suit your machine best. Once you have that, the setup of the forks and dampers can be worked out afterward rather than the other way round. By planning this stage carefully, whether it is on a Vespa or Lambretta, you should have a front brake that is capable of stopping the machine safely while at the same time offering comfortable handling.
If not you may find you have to change components once it is built and out on the road. Remember your machine has its own unique power and weight – so don’t go off what someone else is using and trust your own judgement.
Having got your front end all set up and with much-improved braking the final part to remember is the fork bearings. Much more force is now being put through them so it is vital they are in perfect condition. It is easy to change the bearings themselves but what about their housings? If they are worn or pitted it won’t take long for a new bearing to quickly wear out too. As the build will be starting from a bare frame if necessary change the bearing housings. It is far easier to do at the build stage rather than later.
A state of suspension
Just because the rear suspension doesn’t become involved in front end braking or handling doesn’t mean to say it should be forgotten about. Again the choice of which shock absorber you use is vital. The one bonus here is if you don’t get it right first time or want to upgrade it then it’s simple enough to do. The only downside is you are paying out more money to do so. From a Lambretta point of view, be careful about changing the rear shock absorber due to its length. If its measurement is different to what you originally used then if a hole was cut out for the carburettor it may be out of line. Clearance for the inlet manifold is important because it may foul after changing.
The reason the shock absorber needs to be upgraded is both the Vespa and Lambretta use the pivot system where the engine is fixed at a forward point and mounted to the rear of the frame purely by the shock absorber. The more power an engine creates the more the shock absorber will have to soak it up as the engine tries to move on the front mounting. As you will now have considerably more power the standard factory absorber will struggle to cope.
For both makes, there are plenty of options but beware of buying a shock absorber adapted from another vehicle. It may have been designed to cope with far more power making it too hard to use. Also, it may have been designed to work in tandem as a pair. Both the Vespa and Lambretta use a single system so one that was designed to work in a pair may not be strong enough. It is best to only use shock absorbers that have been specifically made and tested to work on either make.
When thinking about which one to fit remember its not only about the power but also the road holding. If the suspension is too hard then it can cause the back wheel to bounce on uneven road surfaces. If it’s too soft then you can struggle to corner or accelerate out of one. Also, will the Scooter be used for two up riding more than a single use? The advancements in design mean most have multiple settings on the hard or softness so eliminating many of the problems in getting it right. The Lambretta has the benefit of quick and easy removal from the frame and engine. Some Shock absorber systems come with different spring options so if you want to change it, for instance, going two up on a rally with luggage to do so will only take a matter of minutes.
Pretty looking thing
All aspects of the brakes, dampers, and suspension are vitally important to make sure you have perfect handling and stopping power out on the road. These elements are by far the most overlooked when it comes to a scooter’s working parts especially those that are tuned. It’s okay having a tuned engine that can mean greater speed and acceleration but if the scooter is unrideable there seems little point in building it. That’s why there is such a contested market for systems that improve this area of performance.
Here is where it can get dangerous though as many make bold claims of what they do. Combined with nice finishing and packaging they can look appealing regardless of whether they can do the job or not. Don’t buy something because it looks pretty buy it because it works correctly under the conditions you are going to expose it to. This part of the street racer build is essential. Do as much research as you can and plan it to the smallest detail. Though you want to build the street racer of your dreams you also want to be able to enjoy it.
Next month: The final instalment – rev counters, gauges, and the finishing touches! Words & Photographs: Stu Owen