Modern Vespa

I know this is a topic that has come up before, but I'm interested in finding a solution to the wobble.

Not coming from an engineering background, but from one in mathematics and logic, I'll have to rely heavily on the engineers here for help. I welcome all suggestions and comments.

My theory: the fender is too light and flexible and causes a turbulence that is carried as a wave at certain conditions.

Thought for solution: make the front fender heavier and more rigid by means of an endoskeleton mounted to the underside of the fender.

Thoughts?

That would account for the GTV version of the series.
Do they have a tendency exhibit a wobble too?

dmpawley, I think you are way off on your theory. Adding weight to the front fender would only increase unsprung weight. That isn't a good thing. All cycles have a natural frequency (or speed) where they will tend to oscillate. The best thing that you can do is make sure BOTH tires are in good shape and are properly inflated and balanced and that the steering head is properly torqued. Sometimes adjusting the air in the tires "just so" will help. Heavier bar end weights will help too. In my experience some tires tend to cause a wobble while others(sometimes the same make and model of tire) won't. I often wondered if anyone makes roller bearings that fit the Vespa steering head.

Just change the tyres and save a whole bunch of time.

Bill X

Till "something" makes it occur. Again.

To an extent I agree with Bill Dog. Several years ago I had a new front tire mounted and balanced on my 1800 Goldwing and immediately noticed a low speed wobble upon deceleration in the 25 to 30 mph range. I tried several different air pressures but the wobble was still there. I took it back to the shop and told them something was wrong with the tire. They installed another tire(same make and model) and the wobble was gone. Sometimes it's just the tire.

I don't think it's the fender at all. I think it's the design of the fork, and the specifics of the rake and trail. In fact, I'm certain that it's not the fender, because it only happens at specific speeds, and only while slowing down. The aerodynamics of the fender could not account for that behavior.

For me old tyres on GTS's make them wobble and new ones don't, be it Pirelli or Michelin.

Simple fix.

Either do that or buy a BV.

Bill X

I really havent noticed this yet, but I am still breaking in with only 434 miles on the GTV. I havent pushed it to the higher speeds yet. I do ride with a passenger 90% of the time though. Haven't noticed it then either.

From what I have seen in other posts. Tires and non-stock shocks seem to improve all kinds of things. Perhaps a new shock would help you out.

Tweaking the rear unladen weight suspension to eliminate as much sideways movement as possible helps enormously.

If the oscillations aren't from aerodynamics, one good way to tell would've a wind tunnel. Anyone have access to a wind tunnel?

OK, so with everything in top shape and in balance, the wobble is minimal. However, heavier bar weights can also correct it. Is this an affect on the suspension or the center of gravity on the bike? Albeit a very small change on the center of gravity, so that's doubtful.

If it compresses the suspension ( extra weight ) and we originally see the affects on deceleration, then can we assume the wobble comes from the suspension compressing while the tire is in motion?

All kinds of different brand tires, various air pressures, different sized tires a 120, 130 & 140 in front. Really loading the front rack on a trip. New steering head bearings. Having special bar end weights machined, for more weight and attching them with Jettin' heavy bar end weights.

What works for me......a set of OEM Sava tires, no windshied, no rear trunk and riding solo = no wobble. Currently I have a tall Vespa cut down windshied, Vespa trunk with attached top rack, front rack, 140 tires F/R and do a lot of 2-up riding = wobble. So I ride mostly with both hands on the bars.

Would be interesting to have some type of steering stabilizer made, but I'm not sure that would stop it........I think it's the nature of the beast.

Mild wobble on deceleration.

Never escalates, certainly never into a tank-slapper. Again, many PTWs (under the right conditions, probably most) experience this to some extent.

It just is.

If your tires are good, properly inflated, and your steering bearings maintained, I say move along.

If you know you're ok, then you're ok. And you're ok.

P.

The bar end weights act as a damper. The added mass isn't there to change the center of gravity of the bike, but to alter the oscillation frequency seen at the bar ends.

Ok, I'm still reading evidence for the cause of vibration. A windscreen and/or top case create drag. This drag could cause the front suspension to rise/expand. Normal riding could also do this to a lessor extent. The expanded suspension would travel/contract on slow down/breaking. When this happens, (not knowing if there is a piston) could the piston or spring vibrate and the vibrations travel to an asynchronous point and be redirected at an angle that would convert a spring compression into a spring oscillation?

I don't want to dampen your enthusiasm, I really don't. Enthusiasm is good. But the core issue of the wobble is pretty well understood, even if all the variables that go into the wobble (and the way they interact with each other) are somewhat more opaque. And while it's true that top cases create drag, simple suspension dynamics almost certainly introduces far more variability into the equation than aerodynamics does.

When you consider the effect of aerodynamics at 30mph vs 20mph vs 60mph, it's hard to come up with a logical scenario where aerodynamics causes this problem at 30mph and nowhere else. Add to that the fact that it's only deceleration, and aerodynamics seem even more remote.

The suspension, on the other hand, goes through significant changes when the bike transitions from acceleration, to steady state, to deceleration.

Final thought: What are the aerodynamic effects that cause a shopping cart wheel to wobble while you're pushing it?

I'm past the the aerodynamic affect on the fender; sorry if I didn't make that clear.

When there's a huge load in the back then the wobble is evident (on the Hornet at least) on acceleration as well, even with both hands on the bars. Same speeds though, centred around 25mph.

I understood your revised statement to be addressing aerodynamics on the top case. Was that correct?

I've never experienced that, but maybe I haven't had a sufficiently large load.

Your problem sounds more like a bent fork.

As I understand it, an objects weight is less the faster it goes on a plane (Neutonian). Would the vibrations have a different affect based on weight/gravity as opposed to mass?

This would tell me that the shopping cart wheel would either stop vibrating or pop off. I believe those vibrations are caused by friction (stop - skid). When enough force is applied it overruns the friction.

The larger issue with a shopping cart wheel is the rake and trail of the wheel fixture. Which is the same underlying problem for the GT/GTS fork.

Like a Sopwith Camel's rear tire?

While I'm familiar with the Sopwith Camel, I'm not familiar with the specifics of its rear wheel.

I agree with the rake and trail issue...
Im interested in following the Yourban 13" wheel post which will change the rake and trail dynamics since he has only changed the front so far... after he gets more seat time I would like to post a question on wobble with that setup...
Besides the fact I LOVE those wheels!

Yep. That should be an interesting data point.

My BV has "done it".
I think ANY/EVERY PTW has the inherent capacity to.

I get the wobble at two points regardless of the weight in my top box.

30kph & 300kph.

Aren't we over thinking this a bit ?

Some GTS's sometimes experience slow speed wobbles on some tyres.

It's an inherent trait of this particular scooter.

My BV hasn't

Exit.

Proper tire pressure, proper tire choice, tight head bearings, good heavy bar ends and minimal weight on the rear (heavy top-case) will keep the wobble in check.
.

I recently started to experience a fair amount of steering wobble with my 250gts. I removed, cleaned and repacked the bearings and torqued the steering head assembly and eliminated the wobble. I am able to ride hands free and she's smooth as silk. As Jess stated earlier, I also feel the wobble is due to the steep angle of the front fork. Just my $$.02

Bill, It's not about the nail!

Willie - Is this an easy job to do? Is this something that should be done as a part of normal maintenance? I haven't seen anything showing how to accomplish this. I'm now at 12,000 miles on my GTS and have done nothing with the steering?

thanks

On my GTS 250 I have found that 35psi in the front tire and 36 rear equals no wobble, 34 psi and I wobble. I know that is over inflating a bit, but I am a bit of an overweight at 200 lbs with gear and I have a top box so I don't think I am over doing the psi.

Anyway it works for me.

Rick G

You need the appropriate spanner, a torque wrench and patience. I had researched this subject and weighed everyone's voodoo remedies for so called fixes. The steering head bearings made the most logical sense and that's the route I took. My logic panned out and I would recommend this for anyone who's experiencing considerable wobble / headshake. I have attached a link to a thread that discusses this in detail. BTW, I do agree with running the tires a bit over inflated. I ride mine very aggressively and have found this makes the bike a bit more stable when cornering hard and straight line stability especially under hard braking. The trade off of course is a bit firmer ride. Tire wear doesn't seem to suffer much unless you're one those that likes to get 8K miles out of your tires. If so, they generally dry rot before the tread is gone.

Adjusting steering head bearings on a 250 GTS

Malossi never made steering dampener/stabilizer for racing duty??

Seems strange that steering dampener doesn't exist for the GTS model...

Or, presumably, a post-2014 GTS/GTV. (No issues with my '15 GTS.)

Did you blow the dust off this post when you pulled it out of the attic?

My 2008 BV250 has a bad wobble. Not bad with both hands on. But Sunday on the freeway the rain grooves amplified it. Scared me with a 70mph shake. New Pirelli GTS tires, DOT listed PSI, tall wind screen, side cases and trunk. Large bar end weights. No signs of any crash damage.

My 06 BV has the same thing. Not at 70 mph though.