Modern Vespa

I put Bitubo shocks on my 2005 PX150 a few months ago. The front is the YAV with spring preload and compression adjustment. The scooter only has 1400 miles, and about 400 are with this new shock. It was a perfect fit bolting in, and the only issue is that the spring is a little too stiff to meet the minimum sag reccomendation. In other words, I have the preload adjuster all the way down. I have been riding with the compression damping at minimum as that is most comfortable.

When I drive over sharp edged bumps I hear a knocking from the front, and it is more noticeable if it is many in a row or a long stretch of bad road. I usually don't hear anything on isolated bumps. I have tried adjusting damping to both extremes and several points between, and it doesn't seem to change much. I really can't tell if the noise happens on the compression or rebound because it only does it on short/sharp cracks and edges.

I thought it might be chassis jacking and hitting the upper internal shock stop on rebound, as I am tall and ride pretty far back on the seat. That might be the reason i can't set the minimum sag. Anyway, riding much further ahead on the seat, which compresses the front shock more, didn't change the knock.

I finally put the stock shock back on to verify, and sure enough, it is now quiet. Do I have a defective shock or what?

Many aftermarket shocks are just plain too stiff...along with poor valving. It wouldn't surprise me if the shock spring is so stiff that it's overriding the rebound damping. When that happens you'll hear a "thunk" from when the top of the inner shaft tops out inside the shock (at full rebound).

The thunk is a lack of rebound dampening. Lots of performance shocks are notorious for this. They handle braking well but are either too stiff or don't allow for enough preload adjustment to absorb bumps and potholes.

Well crap, That's kinda what I thought it was, but expected the knocking to go away when I slid forward. Guess it's just to far out of the adjustment range I need.

Will try to pursue this with SIP fwiw.

I contacted SIP to see if they had a softer spring, and never heard back. Contacted Bitubo directly, who told me to go through a service center. Dan at Traxxion Dynamics got hold of Bitubo, and they said they do not have a softer spring to fit the YAV/YEV.

So. I pulled the stock spring from the OEM shock and made it work. Had to add a 1-3/8" machine bushing at the bottom, as the stock spring does not taper down as Bitubo's does. Also, printed a plastic bushing to keep the spring centered and protect from rubbing. I still ended up with the spring at minimum preload, but it did the trick. It is a little softer, and no longer hitting the top stop on rebound.

A side note, the stock damper with less than 1000 miles, had almost no compression damping but decent rebound damping. The Bitubo has very strong damping both directions, an I am running lowest compression setting. It is giving a very stable and controlled ride.