Clo5e Ratio


Cluster p/n

Cluster Teeth

Loose Gears

Gear p/n

no. teeth

Primary Ratio

Drive Ratio

Ratio Jump

AF Clo5e Ratio






47 / 18



5 Speed







































Alt 5th






The AF Clo5e Ratio Gearbox ensures the jumps between gears are close and get smaller with each successive gear change. This makes it easier to stay in the 'powerband'. It is also kinder to the crankshaft when shifting back down the gearbox as the sudden increase in rpm of selecting a lower gear is not as abrupt as it would be with larger jumps. The jump between 4th and 5th (with either 5th fitted) is a smaller jump than any other production Lambretta gearbox has between two gears. This is very deliberate, and makes 4th a tall gear. The reason this is desirable is 4th is now a gear that is comfortable to stay in for large amounts of your riding. Once the bike is up to speed the large overlap between 4th and 5th means it is easy to leave the bike in 4th when there is a head wind or hill and it shouldn't feel like it is 'screaming'. Whilst 5th is still there to carry the bike to an acceptable top speed without running out of rpm. Essentially these two gears replace the single 4th previously relied upon to cover a large speed range.

We offer two variants of 5th gear. A standard 35 tooth gear (9% jump), and the alternate 34 tooth gear (12% jump). There are two main considerations that should affect your choice. First is the type of riding you are doing, and the second is the cost benefit of having to buy another crownwheel to attain your desired final drive.

The Standard 35 T (9% jump) is best suited to when your riding speed is often dictated to you. By this I mean if you ride in a group or an area with many speed restrictions. This kind of riding is characterised by frequently dropping out of top gear because the rpm is too low and the bike is chugging, but often finding that 3rd gear is too high up the rev range, and not comfortable to sit in. This is very common, especially for riders of powerful bikes that comfortably cruise at 70mph, being forced to sit at 50mph. The tall 4th gear should now sit comfortably in that slightly lower speed range, making the bike nicer to use and more efficient.

The Alternate 34T (12% jump) is better suited to more aggressive riding, or riding where you are not being held back. This kind of riding tends to be solo riding where you are more likely trying to leave everyone else behind than pootle along with them. Reving out each gear making use of the whole powerband, not short shifting up the box. The larger jump then means that when 4th has hit its crescendo selecting 5th puts you back in to the center of the powerband rather than much of the way through it.

The secondary reason would be you have a high spec clutch with an expensive crownwheel, it seems a shame to spend another £80+ on another crownwheel. If you look through the gear chart, most popular sizes still allow you to get most desired final drives by choice of front sprocket and 5th alone. We'll use a 46 tooth crownwheel to demonstrate as they are very popular.

17-46 (81 Chain) std 35 5th FD = 4.98

17-46 (81 Chain) alt 34 5th FD = 4.84

18-46 (82 Chain with a pull down guide) std 35 5th FD = 4.71

18-46 (82 Chain with a pull down guide) alt 34 5th FD = 4.57

In most cases choosing the correct final drive for your Clo5e Ratio Gearbox will mean selecting the appropriate sprockets to get either the same final drive you previously ran, or placing your new 4th and 5th either side of your previous 4th. E.g. An RB20 running an NK Road would typically run a 5.0 final drive, with the AF Clo5e Ratio, you may choose to run a 4.8 final drive, placing 4th at 5.2 and 5th at 4.8.

Remember the 5th's are available separately, if you've made the wrong choice or your use of the bike changes. You only need to purchase the other 5th gear.

Although most of the ratios are familiar the profile and clearances of the gears have been altered to reduce friction and noise. The metallurgy, and surface treatments are also new to Lambretta gearboxes.

We've been running samples of these gearboxes since November 2016 in some of our RB test engines, a group 6 race bike, and several powerful road bikes. We've been exceptionally pleased with the results. The highest mileage sample has covered just over 4500 miles mostly in a 28bhp RB ridden as a daily B road commute by our top mechanic Scott, Scott's riding style is best characterised as aggressive.

I'm confident this new gearbox will enhance the enjoyment of riding a Lambretta for all those who choose to fit one.

Fitment is straightforward, they are supplied with instructions to highlight the checks required, but if you are competent to fit a standard gearbox this will offer no surprises. However the skill of a good mechanic should not be underestimated, there are a lot of checks in building a standard gearbox, if in any doubt, have it fitted by your local dealer. It is cheaper to get it right, than risk your safety and money on 'it'll be alright'.

Click for full fitting instructions