Tech Torque: Lambretta Performance review – Part 5: Aerodynamics
The quicker your Lambretta tries to go, the more wind resistance increases and tries to slow it down. This is where aerodynamics play a crucial role in improving performance…
Can you translate success on the track to performance on the road? Stu Owen, one of the UK’s most successful scooter sprinters, continues his winter series on Lambretta performance analysis.
Tuning the engine to gain more speed and acceleration is probably what every Lambretta owner wants to achieve. The problem is the quicker you accelerate up to your top speed the more wind resistance starts to slow you down. It is often called the drag coefficient and to put it in easy terms is a force that works in the same direction as the airflow. The faster the vehicle is travelling the stronger the force gets. This is where the amount of power an engine creates helps to overcome it but the problem of a Lambretta is the limited amount of power available. As an example, a good sprint Lambretta with 35bhp can accelerate as quick as, quite often even quicker than, a large cc motorbike over the 1/8th of a mile. By that time the Lambretta is travelling in excess of 80mph but as the speed increases, the rate of acceleration decrease as the drag coefficient gets greater. Whereas the motorbike has more power to battle the force, hence why it will win over the full quarter mile distance. And the motorcycle has another weapon in its armoury to combat this, in the form of a fairing to cut through the air more efficiently. Streamlining will help a Lambretta accelerate quicker, certainly at higher speed, and allow it to go as fast as it possibly can.
Frontal area
The ability to cut through the air as efficiently as possible starts with the frontal area of a two-wheeled vehicle. Just as important though is how the air flows over the body of the rider and how it exits off the rear of the machine. The question with a Lambretta is, what can you do to make it as aerodynamic as possible without altering the overall original shape?
The answer is, not that much, but there are some things that will make a subtle difference remembering that, as noted earlier in this series, small increments in performance do add up. Firstly, look at the range of Lambretta models. The wider the frontal area, the less efficient the drag coefficient is, so a Series 1 or 2 Lambretta will perform worse than a Series 3. With the Series 3, an Li or SX has slightly more frontal area than a GP, therefore the GP is the most aerodynamically-shaped Lambretta.
Presuming you are not going to cut the front end down in any way, then you are stuck with the original shape. There are some things that can be done though to improve it slightly. Getting rid of any ridges impeding smooth air flow will help. The leg shield trim can be taken off and the lip removed, certainly if you are in the process of building a street racer. Over the last few years, several racing Lambrettas have appeared with this modification. Also, it is possible to remove the horncast rubbers and blend in the bodywork, making the front surface almost like a one-piece shape with no ridges. By doing so you are also slightly reducing the frontal surface area.
Next are the handlebars and headset, which originally would be fairly streamlined, but can get cluttered without you realising. One of the commonplace add-on accessories is a wing mirror. No matter how small it is, it isn’t helping as it creates drag. Though you wouldn’t believe it, so too is the hydraulic master cylinder, quite often with its big square shape. There are a lot of much smaller and more compact master cylinders on the market which will allow better airflow over them, compared to the standard and much bigger Yamaha type ones that are usually fitted. If you have a headlamp grille fitted, this will be considerably disrupting airflow as it passes through all the small holes so it’s best not to fit one in the first place.
Finally, there is the option of fitting a front screen. It must be presumed that the traditional fly screen will not be fitted as this has a large frontal area and sits almost upright. There have been one or two compact screens available in the last year or so which mould around the headset top and are also angled to cut through the air. Though small, they do make a difference in deflecting some air over the rider. This type of screen was first seen believe it or not on the Vespa T5 and worked extremely well. Fitting a screen may not be to everyone’s taste but it is worth looking at to improve efficiency.
Lastly at the front is what options you have if building a cutdown. If you leave the front completely bare, with just the frame tube then the air flow will be hitting your body, which is extremely unaerodynamic. The options are to either fit cut-down leg-shields or more fashionably some from a Vega. Of the two, the cut-down leg shields will be far more efficient as they have considerably less
frontal area than the rather square and upright shape of the Vega style. This is far more noticeable when going past 55mph where resistance starts to increase dramatically and as the bhp starts to tail off, certainly in with a lesser powered engine. The last point is the front mudguard which quite often is left off or has a small narrow one fitted. Using one that covers the whole distance across the forks and is as low as possible to the front wheel will be the most efficient. Remember this is the first contact point with the bodywork and deflecting the air around it as smoothly as possible before continuing over the rest of the machine is vital.
Middle area
The middle area of a Lambretta doesn’t have too much that can be altered to improve aerodynamics but there are a couple of things which can help, and one of them considerably so. The main component is the splash guard which has quite a big area in size and sits almost upright against the air flow. Its job is in its title – to stop slashes of water – and to a certain extent small stones and dirt hitting the underside of the engine. While it’s good to prevent this from happening it is not essential, and removing it won’t cause any problems. If it’s left there, its shape, size and angle severely hamper air flow and it is often named the Lambretta air brake, so if possible… remove it. As an added bonus it is also removing unnecessary weight from the machine.
The other way of smoothing out the lines where the air goes underneath the frame is by passing the cables through the frame. This leaves the frame tube bare because not only are the cables removed from the outside but also the straps that hold them in place. This leaves you with the smoothest flowing line possible on the frame tube. It may not seem like much but it all adds up to the end result. Fitting the outer cables through the frame is a bit of a tricky process but once done feeding the inners through is just as easy as if they were on the outside. It is also possible to put the wiring loom inside the tube but if you are doing this then it is best to make up a narrower lightweight loom compared to the ones that are currently available.
Back area
While it is vitally important to make a vehicle as aerodynamic as possible, to cut through the air, it is just as important to make sure the airflow exiting at the rear is as smooth as possible. The reason being, as the air leaves the back of the vehicle it needs to leave from a tapering shape. If it doesn’t then it leaves a vacuum or hole which creates a vortex and in theory is sucking the vehicle back in slowing it down. A good example is a high sided lorry which is square at the back. If you ride behind it you speed up as you get close to it, which is where the vacuum is being created.
Again the problem arises – how do you make the rear end on a Lambretta as aerodynamically efficient as possible without altering the looks? Well to a certain extent the rear of a Lambretta, certainly a Series 3 has quite a good tapered shape in the first place. Compared to a Series 1 or 2 Lambretta the Series 3 is much more aerodynamic at the rear as well as the front. Against the Vespa with its rather bulbous side panels it’s even better. To prove the point even further, the GP 200 was the fastest accelerating and highest top speed Lambretta ever produced even though it had almost identical power to that of all the other 200cc models. One of the reasons for this was its superior aerodynamic form.
While the tapering shape of the rear of the frame is good, what doesn’t help is the shape of the rider. As the air flows off the rider’s body it is quite turbulent, which isn’t helped by the square shape of the traditional Lambretta seat. By fitting a seat with a small tapered back, this will help matters considerably. There are several versions of this type of seat shape available already or for those having one made, consider your options first. When it comes to a cut-down or racing Lambretta then the options are far greater. The Honda or Aprilia RS 125s are perfect examples to use. These have the best shape possible to aid airflow from the rear