Modern Vespa

I noticed some weird sense of slight wobbling on the way to work today, like wind pushing the scooter slightly. Couldn't figure it out, though I was imagining it. I had recently adjusted the bitubo front suspension to make it softer, and maybe that was part of the issue. It would be most noticeable when coming on or off throttle, but I still couldn't be sure it was for real or if I was going crazy, having a stroke....

I checked the front wheel at work, it was solid and all the nuts on the rear wheel were tight. On the way home the wobble got worse. Finally came to my senses, stopped, and tugged at the rear wheel. It wiggled back and forth very noticeably. The rear hub nut had backed off and the wheel was held on solely by the cotter pin! I still had 2 miles to go, and rode as gently as I could. The cotter pin held!

I popped off the hub in the garage expecting the worst, but the hub splines check out fine. Lucky on at least a couple of fronts, esp. as I only had my half-face helmet on and had been doing 50 mph on the way in to work when the wobble started. Could have been a disaster.

This was the motor I rebuilt from an eBay salvage, and to be honest I never bothered to check the rear hub nut during the rebuild. I assumed it was on tight. Big mistake! Now that it's off I will spray it black, and torque it back up and install a fresh cotter pin.

It is a castle nut, Correct ?

Was the cotter cap in place, or is it deformed.

How does the nut back off if a split pin was in?

Is the castellated insert in place? I'm thinking it must be missing.

Pictures help.

How did you rebuild the engine without removing the rear hub nut?

So many questions.

Crikey. you guys are right. No castellated nut. Just a flat washer and a 24mm nut on top. I wonder what in the world happened to it?

Other answers: The 24mm nut was backed off to the cotter pin, which is through a hole in the axle.

I never had the hub off for the rebuild (I just replaced the main bearings, crank, and top end), not the rear wheel bearing.

This is why people who know next to nothing about scooters, like me have to be extra careful (I'm not saying the OP is like that - just sharing my story). I randomly had a friend notice a wobble in my rear tire as I drove by. I never felt it. After staring at the rear tire I decided to see if the nut was tight. It was loose and had no cotter pin in it. I posted pictures here and was told what to do from there! Vintage scooters are fun, but I have to over compensate for my lack of knowledge by being vigelent in my safety checks and reading this forum as often as possible so I can learn about things I don't even know that I should know! Thankfully this is a hobby for me and I don't ride over 15-20 mph maximum and stay local and out of traffic.

Don't be Cotter Pin Guy

2nd ride after rebuild...wheel fell off.....went to hospital

I definitely knew better. The engine was missing the dust cap, so the un-castellated nut was clearly visible, if I had looked.

To resolve, it looks like there are two options:

1. Castellated nut that the cotter pin is bent into: ]https://www.scooterwest.com/pub/media/catalog/product/cache/ecd051e9670bd57df35c8f0b122d8aea/0/0/002314-vintage-vespa-hub-nut-castleated_10.jpg

or

2. a special metal cover (not the dust cap) that fits over the hub nut: https://www.scooterwest.com/cap-for-rear-wheel-nut-194423.html

Any thoughts on which solution is better?

The 'special metal cover' (castellated insert) would be the best and easiest solution. The split pin hole in the axle is in the correct place for this.

Awesome, thanks much. Just ordered one.

New split pin.
New split pin cap.
Torque wrench - it wants a lot.
Try to position cap and pin so that if nut does comes loose it won't back out hardly at all, saving your splines.

FWIW I ordered a castellated nut (as linked above) along with the locking cap, just to see the differences. The castellated nut must be for a smaller diameter rear axle than on the LML motor. So the cap it is.