Tech Torque: Power: The Chain reaction – Part 1

Tuning a Lambretta engine is fine if you want to go faster – however the transfer of power, and its burden on components, is often overlooked.

Tuning a Lambretta engine is fine if you want to go faster – however the transfer of power, and its burden on components, is often overlooked.

Without getting too technical or going back into the classroom for a physics lesson, the power delivery through any engine can be quite complicated to explain. The physics surrounding a force being created and exerted on the next object could have you scratching your head. Explaining it in simple terms, the expanding flame front in the cylinder sets the piston moving up and down and starts the motion off, the resulting chain reaction reaches its final destination when the surface of the tyre meets (and turns) on the road. When Innocenti produced the LI engine in the mid to late 1950s it was a mechanical work of art. So much so, that apart from a few simple upgrades it stayed largely the same right u up to the end of production of the GP in 1 1971. The purpose of this article isn’t to t tell you which named brand to buy, but t to consider how strong you need to b build your engine, therefore using the b best components you can.

If you look at the LI Series 1, it was producing 6.5bhp (at the crank and around a 5hp at the back wheel) and compare it with the later GP 200 which produced 11.70bhp (at the crank, and around a 9-10hp at the back wheel) then thet initial LI S1 power figure had almost doubled. It is testament to the work of Innocenti I that they produced an engine so good in the first place, and when designing the components a safety margin wasw clearly built in. It’s no good producing ana engine that is only just within its limits

otherwise reliability will be thrown out of the window. Summarising all of that, and based on the GP 200 power, then all components must be able to withstand at least the manufactured crank measured power of 11.7hp (9-10 rear wheel), plus a safety margin… but how much?

These days, and for a considerable amount of time, it’s been possible to easily exceed the factory figure, and certainly many components have been exposed to have weak areas as the power figure climbs. When the TS1 first arrived on the scene more than 30 years ago, 18bhp was easily achievable even on the worst set-up engine. These days 1820bhp is considered to be pretty tame and more than 30bhp is easily achievable without too much effort. That is around three times the size of Innocenti’s upper development figure. While most engine internals are upgraded the reasons why this is necessary are often misunderstood. Look at it as a chain reaction and the energy being transferred from one component to the next. As the transfer of energy/power occurs, any weak link is exposed by way of failure – not just of that part but the engine itself.

Top-end

The energy in a Lambretta engine starts off as chemical energy in the form of the air/fuel mix. It is then converted into heat energy through the burn and finally mechanical energy when the piston starts moving, so let’s start there. There are various forces exerted on the piston, so depending on your power range, you need one which is up to the job. The piston rings then create a frictional force along the bore, the standard three-ring piston creating far more friction than a much narrower two-ring piston.

The power from the piston then moves on to the little-end bearing and the con-rod. Those who have used standard rods in a tuned Lambretta may have had one snap at some point, with devastating consequences. Thankfully con-rod technology these days has been greatly improved, many Lambretta engines using Japanese versions. From there the force is transferred through the big-end bearing and gudgeon pin into the rotating mass of the crank webs. It’s easy enough to uprate bearings, but the gudgeon pin has always been one of the weak points of the Lambretta crankshaft with the interference between the pin and the web unable to cope, due to the narrow design of the webs. The result often ends with spinning/twisting webs and the crank runs out of true. Welding the pin has been a sort-of solution in the past but even that can fail. Again, advances in build techniques and product design have vastly reduced this problem. But anyone building a tuned engine needs to keep in mind that even just the top end alone should use the strongest components to be able to cope with the extreme forces put upon them.

Another area to note here is the flywheel taper. Innocenti upgraded it significantly when the GP came out, making it much wider. Not many people still use the LI/SX

taper crankshaft these days but even with an engine with moderate power you will find it difficult to stop the flywheel from coming loose. That is why that style of crank shouldn’t be used on any tuned engine as, regardless of anything else, the taper simply isn’t strong enough for inertia created as the flywheel spins up, no matter how tight it is fitted. That is why even with a GP crank, both its taper and that of the flywheel need to be perfect. Any poor fit will be exposed in an instant, potentially causing irreparable damage to both components.

Lastly, in regard to the crank, and the bearings that hold the crank in position on the main and flywheel side, it should go without saying… use the best bearings possible as they have great force put through them. Cheap bearings will wear out much quicker and cause all sorts of issues.

Cush drive and front sprocket

The power is next passed to the transmission via the cush drive. This acts almost like a shock absorber to prevent too much force being exerted on the transmission. The design by Innocenti makes it very difficult to change. Therefore, the only thing that can be done here is to make sure every component is in perfect condition. Never use a cap that has been stepped and may not sit correctly when tightened up. The biggest wear is between the sliding dog and the front sprocket. No matter how good a condition the tension spring is in as the force is transmitted through these to components, they can begin to overheat with friction. Always check for wear and replace it if it occurs. Also, the sprocket itself can wear on the teeth. This will be in the way of curving the ends and show wear is happening through the force exerted. Always use a good make of sprocket to help eliminate this problem.

Chain

The chain is next along the line and being duplex is usually very strong. In saying that, its main downfall is stretching and because the side case needs to be removed to access it, it’s often overlooked. Always use the best quality one possible to reduce the problem. Just because a chain looks sturdy doesn’t mean it is strong. The power put through it will soon stretch it if it’s poor quality.

Clutch

The poor old clutch is the one component that suffers more than anything else. The problem is that it’s not a fixed component and just the tension of the springs and the surface area of the plates allow it to do its job. The more power put through it, the harder it has to work. Springs alone are not enough no matter how strong they are. The other way to help is by having a larger frictional area. By having more plates the surface area is greater. The plates can’t be wider because of the space in the casing so by adding more it is done that way. There are so many different types of clutch now available for the Lambretta, it can be confusing. Just remember that this component suffers wear more than most others. Don’t go for one that is ‘just about good enough’ – get one that is more than capable of withstanding the power that the engine creates. Also keep in mind, any future power upgrades usually mean an increase in bhp at a later date so always ‘overbuild’ in this area, to save buying twice. That philosophy should be applied to all areas of the engine, budget permitting, as often ‘economy based purchases’ can be a false economy, when

you have to buy again later. Build in the biggest safety margin possible.

The crown wheel is the next place the energy is transmitted to via the clutch, even though it forms the housing of the clutch. Again, like the teeth on the front sprocket, they can wear. The choice will be governed by what clutch you buy as usually, it comes as part of its make-up. If fitting a clutch into an existing crown wheel there is an option to make sure it is as good as possible. Never use one that is old and worn – the power increase will expose its weak points immediately. Lastly is the clutch spider, all the energy created then going through the centre boss. Innocenti held it in position with five rivets that were up to the job for 22.7bhp, sadly not so with a much higher bhp engine. A standard spider simply isn’t up to the job. Thankfully many on today’s market will be, but always go for the strongest possible, again… over-engineer.

So the energy first created by the movement of the piston has gone through the top end and transmission. From here it goes into the gearbox and beyond where it doesn’t get any easier to work with, but that is for Part 2.

The next point is the con-rod and as the bhp and revs are increased, a weak con-rod could fail by snapping The point where the energy transfer all starts, going from chemical to heat then mechanical as the piston begins to move in its cycle.

The crank webs are held in position by the gudgeon pin which, if it doesn’t have a good enough interference fit, can cause the two halves to move out of line.

On the flywheel side, the taper needs to be in perfect condition and match the flywheel boss perfectly. If not, the force can cause the flywheel to come loose.

It was common on Indian flywheels for the rivets to come loose and the mechanical energy always finds the weakest spot to cause the component to fail. A solution to this was to weld the rivets in place on high-powered engines.

A well-made Italian flywheel which has strong rivets holding the centre boss in position; some newer flywheels are a one-piece unit.

A four-plate clutch has ess surface area than a five plate, so if all else is equal, the heat build-up in a five plate clutch is dissipated more readily due to the larger surface area, and provides more surface area for grip, to handle increases in power

Following on from the chain, the crown wheel is next in line. The old-style Innocenti crown wheel was a two-piece item riveted together. Regardless of make always use a one-piece like this to eliminate the weak point of the rivets.

The final part of the transmission pathway, before the gearbox, is the clutch spider. Again the weak point is the rivets. Thankfully it is an easy part to engineer and there are now plenty of robust options when it comes to fitting one in a tuned engine.

Front sprockets need to be changed depending on what gearing option is chosen. Wear can occur on the ends of the teeth which in turn damages the chain rollers. Always use good quality and the best-fitting sprockets to stop this from happening.

A, B, C, and D show where the mechanical energy continues along its path through the transmission.