The Culprit

Since February I have torn down the scooter and sent in the engine to AJ’s Cycles on Deer Valley Road in Phoenix as the case has to be split in order to get to the drive shaft and I don’t have the skill set to attempt such a task. However since the case is going to be split I might as well have the mechanic replace some wear items while in there. These items will be a full complement of seals, the CVT belt, a couple of plastic CVT motor gears, bearings, and piston rings. The total cost in parts and labor will run me about $2200 but hopefully when all is done I’m resetting the bike for another 30,000 miles of trouble-free riding before any further costly work is needed. According to official Suzuki documentation the CVT belt shouldn’t need replacing for the life of the scooter, whatever that means, and by personally inspecting the used belt myself I can understand the claim. However since I don’t know the history if the first 30,000 miles and the complexity involved in replacing the belt, I simply feel good about making the replacement and so I’m doing so. The original belt, for the exception of a single nick, doesn’t even look used! You would think it was brand new! Truly amazing technology and I would surmise that this is the same technology that is used in today’s cars that have CVT transmissions.

What you are looking at above is the drive shaft. More specifically one of the ends of the drive shaft. The other end of it looks exactly the same however the evident twist is in the opposite direction. The splines are in place to interface a gear and a cog that are used to transfer power to the rear tire and to buffer torque being transferred back up into the engine. The completed assembly looks like the image below.

The drive shaft is made with a hardened tempered steel and by rights shouldn’t bend or twist.
However as you can see, not only did it twist it had actually started to crack. It was only a matter of time, a quick twist of the throttle before it twisted in two. I’m truly surprised that it got me home that night when it all occurred.

So what went wrong? I can only surmise. However if you study the next image I’ll give you my thoughts. But first let me try to explain how the assembly below works. As you can see, there is the drive shaft that is in my hand. On the drive shaft you see a drive gear, a bushing and nut, a spring, and between the spring and gear, if you look carefully, a cog which aligns with splines on the shaft. The gear without the cog in place, will spin freely on the shaft. It’s the job of the gear to transfer the tongue produced by the engine to the cog which in turn will transfer the torque to the drive shaft which spins a drive gear not shown in the image to a series of final drive gears and rear tire. What is important to remember is that the purpose of this simple yet involved assembly is to absorb shock going the opposite direction, from the tire to the engine, into the spring. That is accomplished by the spring becoming compressed when shock is transferred back up to the engine. Looking carefully at the image and at the gear, you see on the gear two barriers to the cog on the left and right side of the cog’s spline. These barriers and the cog is manufactured with a slight curvature which allows for movement of the cog in parallel to the drive shaft. That is, as the tongue is applied, the cog can move away from the gear but resisted by the effects of the spring. However I believe that because I was exiting the freeway and was traveling at a high rate of speed and in need of decelerating quickly, I forced the cog to jump over the barrier due to a combination of being on the throttle and breaking hard at the same time, something that is not hard to do on a “scooter”. Once the cog jumped over the barrier on the gear, a substantial release of inertia occurred that was counteracted by the forward motion and inertia of the scooter in whole and when the rotation of the gear bet with the cog rotating in the opposite direction loaded with torque being produced by the engine, the end result aside from a bouncing and chattering rear tire was a twisted and cracked drive shaft evident with twists of opposite direction on either end.

I’ve been told by the mechanic and people the mechanic has spoken to that Suzuki nor themselves have ever seen anything like this before. Well Suzuki, here it is. I have the original broken part if you’re interested.

So now I’m waiting for the engine to be finished being rebuilt. The people at AJ’s are very nice and competent people, but efficient they are not. I’ve had the engine with them since late February and I hope to have the engine back in my possession before August so I can begin assembly for the fall riding season. Right now I cannot recommend them for complicated engine repair, but they seem to be very responsive to high turnover work like tires, breaks, old, ignition, and other similar tasks. In those areas, I can recommend them since it’s those sort of jobs that is slowing my work and they do good work. Since however this not a review of AJ’s, I’ll just say that I will follow-up with my impression of the complicated work they are doing for me now in a future post.