Workshop : Lighten that Flywheel

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Everybody loves to cram every last hp out of their racer. Consequently, snapping the crank is a huge downside of this. Buying billet crank isn’t in everyone’s budget. Cheap insurance on your crank is to lighten up the flywheel area. There are multiple ways of doing this. Lightening the flywheel assy will change the behavior of your motor as there is less rotational mass to slow down. The weights shown below are not exact, they are mainly there to show the gains that can be made by these making changes.


Above we have the stock weights of 99-02 and 03+ SV650 flywheels assys. The 99-02 ( right) weighs in ~2328 grams(5.132 lbs) and the 03+ is 2048 grams (4.515 lbs). This includes the the stock sprag, bearing (98g), and sprag bolts(12g each).



The easiest is going to a total loss/no charging system in which you can remove all magnets and machine the flywheel down to a bare minimum.   The Sv650 has 6 magnets around the inside of the flywheel.  Each magnet weighs ~60 grams. So simple math, you will save 360 grams. You can lighten the sprag which we will touch on later. After it’s all said and done, your flywheel assy can be around 2.5-2.8 lbs. The main issue with taking this course is that you will have to have your battery on charge at all times and the bike will only run for a limited amount of time. This route for many is too much of a pain and not worth the hassle. However you can remove 3 of the 6 magnets and retain charging abilities.

Sprag Bolts

Next is the simplest way to drop a bit of weight. Replacing and/or removing sprag bolts. A single stock sprag bolt weighs in at ~12 grams. You can safely run 3 bolts. Which means cutting 36 grams comes at a very cheap price with very little time spent. These bolts can be replaced by titanium ones which are around 6 grams each. 18 grams total for the 3x Ti bolts vs 72 grams total for the 6x stock bolts.


Starter Sprag


Above are the stock SV650 ( 2nd generation left, 1st generation right) starer sprags. As you can see from the photos the 1st generation starter sprag is over 200g heavier than the 2nd generation. Compare those weights to the lightened sprags below, 198g, 268g, and 278g, there can be plenty of weight saved by modifying this part. The 198g sprag was CNC’d and could have 3 of the mounts removed to drop the weight even more. You can even as far as having a titanium one produced but this will come at a cost.



The flywheel itself can be machined around the outside to loose about 220 grams (roughly 1/2 lb). See photo at bottom of this page of how it may look after being machined. Below are the stock SV650 fly wheels, 2nd generation on the left and 1st generation on the right.


Race Generator

The best option is going with a full HRC/BDK style race generator. This eliminates the flywheel and is replaced with a new one. The stator is also replaced. BDK offers one for the SV650 at the price of £595.00 ( which is around $1000 USD ). The race gen available for both generation SV650 and ninja er6 650 (£690.00). With the BDK set up you can still use a lightened sprag and 3 bolts. This will still allow your battery to charge while keeping weight off the end of the crank.

SV Alternator

My personal flywheel has been lightened with CNC’d sprag and titanium bolts. The whole set up is almost 1.5 lbs lighter than the stock setup. Any weight off the end of the crank helps with your longevity.

This post has already been read 4280 times!

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