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XB superlight primary drive

The steel parts of the XB primary are pretty tough and will last for a long time, but they heavy and significantly reduce the response of the engine when the throttle is opened fast. Here are a few numbers and thoughts that I have put togehter to explain my the reasoning and the intensions for the changes of the primary drive.

A rotational weight reduction at the crank side, will give you 1.6 times per gram the effect as weight reduction at the clutch side (the calculation is based on the 1.6 primary drive ratio/35 teeth sproket). So stripping weight from the engine sproket gives you a lot of performance for little effort and money. For this reason, reducing the diameter of the flywheel to 175 mm (as shown here - arround 2.5 Kg of weight has been taken of from the flywheel of my 1208 engine) makes a huge difference, but is also a lot of work due to the changes that are required.

Reducing the mass of the alternator rotor has an eaqually big impact. Due to the much larger diameter of the rotor and the heavy magnets mounted inside its rim, this reduction, despite less total weight than the sprocket creates more impact.

The clutchbasket turns once for every 1.5 (38 teeth sprocket) 1.6 (35 teeth sprocket) revolution of the crank. Consequently, it is accelerated at 40% of the rate of the crank. By reducing the weight of the starter ring we managed to bring this down in weight to pretty much minimum and this was a lot easyier to do then the modifications on the crank side.

In combination the above measures have a significant impact on the bike corners and overall bike agility.

But there is a downside to all of this. Reducing the weight of the crank and clutch will often adversely affect drive train smoothness (unlike lighter wheels). For this reason these modifications make more sense for the short stroke engine and do not recommended themself for slower reving big stroke engines.

Here you see what I have build for my bike. The modified Millenium Technologies Aluminium parts with additional custom components that I have constructed and had build for myself have allowed me to get rid of 2 Kg of rotating mass from the engine sprocket and 1.7 Kg from the clutch basket. Durability of these parts is very good so far (9.000 km).

These Alumininim Clutch plates allow a further weight reduction of arround 300 gramm)

The modified starter ring has given us the opportunity to strip arround 1/3 of its weight over a fairly large diameter. To make this work even better than the original, I have modified the clutch hub and the nut that seccures the hub.

Sine the last weekend (06/2007) I also have an XB 9 of a friend of mine running this configuration and it makes an immediate difference to how this bike revs up. This is a bigger change in performance than the porting of heads and from my point of view a lot less painful when you want to implement this. The engine stays in the bike and its a straight swap of components. No juggeling with the ECM and all the fun and games that this causes.

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