Triumphrat

InParkSlope · 08-23-2005

Sorry that title sucks I couldn't think how else to describe it.

Is it normal for this sprint (or all sports touring) bikes to offer up a trampoline effect resonance in the front forks, over certain unusual kinds of roads? I've noticed this more than once. Sometimes I have to shake my head to clear the impression that my tire has become oval or the front wheel is about to fall off!

It happens when the road surface has, for whatever reason, waves in it of a certain key frequency. And I'm travelling at a normal clip. The front starts to undulate out of proportion to the actual road, and gives the impression that the undulations could even start to increase to the point of control problems. By then I am annoyed enough to slow down or speed up sufficiently to reduce it.

I've noticed this across the brooklyn bridge, across slab with joins a certain distance apart, and over a freeway that for whatever reason has 20 foot sections that are sunken slightly. Sometimes I can't even SEE the problem with the surface but the forks are trampolining .. I've never noticed this effect in a decent car, at any speed - ie, the sense that the suspension is actually increasing the undulation of the surface, rather than soaking it up.

What is the simple way to reduce this? tighten up damping? Is it just another of the joys of motorcyle riding concentration?

thfwsf · 08-23-2005

I've had this happen once to me on my ST. I've had it happen to me on a car on occasion too. However, if it's causing you issues. Just speed up!!!

SpiderTex · 08-24-2005

Yep, can happen.
I have a stretch of highway just out of Brussels which does it to the Sprint AND to my Honda Prelude.

Any system, construction with a certain flex in it (so your front and rear apply

) have a proper frequency (don't know the exact technical term - dang that's twice this week).
If now the irregularities in the road cause your "system" to pulse near its own frequency it will start to oscillate more and more. Enhancing the movement. Theoretically, if no dampening etc would exist, the oscillations would get so big that it would rupture the system. Like that huge bridge which started to oscillate with the thunder. Tough for the bridge it got tickled enough on its own frequency and broke apart.

Iggy · 08-24-2005

I knew exactly what your title meant.
I work in vibration testing and recognized your problem.

I'm no expert, don't have a bike right now either, but....It almost sounds like the fork oil is shot or low. If the front end starts oscillating over expansion joints and seams then something is wrong.
The right harmonics will start an oscillation that could quickly increase to an out of control condition. Even if the wheel is slightly out of ballance, that too could compound the porblem.
I'd start by changing the fork oil, if that doesn't stop it then maybe a heavier weight oil.

Just my .02.

[ This message was edited by: Iggy on 2005-08-24 08:05 ]

SpiderTex · 08-24-2005

Iggy, I think that InParkSlope doesn't mean the vibrations when driving over a pot-hole and the wheel keeps bouncing (no dampening) which as you rightly points out is a shot su^spension dampening. But just vibrating due to the a unevenness in the road surface. I had it with my bike brand new, and on the car also.

Iggy · 08-24-2005

Hmmmm.....I though he said it would oscillate out of sync with the road surface. Some call it the "PO-GO Effect"??

It's always tough to diagnose a problem like this without experiencing it ourselves.

InParkSlope · 08-24-2005

yeah slightly out of sync otherwise the road surface bumps would not be reinforcing the pogo-ing.. it is like a swing where you kick at the right time to swing higher.

I did a google search and it there is conflicting advice. The advice that makes sense to me is that a bike lacking rebound damping will act as though it is just a spring - and you will pogo over undulations. The ease with which the front dives under braking makes me think this front end isn't well damped.

But another bit of advice says TOO MUCH rebound damping causes pogo-ing (I don't see how).

I'm not sure if its fair to judge a front shock based on undulating surfaces of a resonant frequency, though.

[ This message was edited by: InParkSlope on 2005-08-24 10:59 ]