Scooter wobbles and almost becomes uncontrolabe at 50mph

[Geezerray]

Hi My son has one of the pair of Italian TV175s that I completely rebuilt about 3 years ago. Just recently we have started to take them out for a ride. But he contacted me the other day about the wheel wobble he has always complained about on his scooter. Is there anyone on this club page that may have some idea as to what's causing the problem, as I cannot physically get to see the scooter to diagnose what causing the wobble. Any ideas gratefully received.

[Storkfoot]

If it is coming from the front wheel, the first thing I would check is that the two halves of the front hub are tight. When I say tight, I do not mean really tight, just hand tight.

Also, spin the wheels and watch the shape of the rims and tyres. Are the rims damaged? Were the tyres properly seated ie was tyre paste used when they were inflated, allowing the rubber of the tyre to move easily to the edge of the rim?

But, if it is a resonance that kicks in at a certain rev range, or speed it could be all sorts - crank, petrol tank touching the frame, engine mounts, rear shock absorber etc etc.

[nastro azzurro]

Check the forks are not binding. Even slightly bent forks will emphasise a wobble.
Try loosening the top race and give it a go.
Don’t leave it loose though!

[Rich Oswald]

Check your links are a matched pair and that they are straight.

[Knowledge]

Ray, a simple check for the forks is to sit on the scooter (stand up) and place your thumb of your left hand over the gap between the bottom of the headset and the top of the legshields (under the ignition lock). Now, with the front brake on, rock the scooter backwards and forwards and feel with your thumb if the two are moving in relation to each other. There should be no movement.

Having said that, over tight steering bearing are just as bad as loose ones. You need to ensure you have the Goldilocks setting.

Finally, replacing the balls and races is not expensive, though it does take time. I think I wrote an Oily Rag article about replacing the bottom cup (the one at the bottom of the fork tube) a while ago.

[Geezerray]

Many thanks for the replies I had much appreciated. I rebuilt the scooter replacing parts which I felt needed doing, I cannot remember if I changed the front wheel bearings and reused the original links, but did change the top and bottom bearing races. I think the problem is the forks have become loose. As when he sits on the scooter he rocks it back and forwards with the front brake on, and he tells me there is a noise coming from the headset. I think that now its started to get used a few things have worked loose.

[Fast n Furious]

There are some upper bearing races out there for sale in the market place which are substandard.
Basically, the depth and shape of the well in the race is incorrect for the size of balls in the cage making it impossible to set a proper preload.
If this is your case, then it can only be remedied by replacing the upper bearing assembly with a known quality item.