I am investigating the crash of a Sym euro jet x 50cc moped that started vibrating in the front wheel and could not be steered. The road was dry, smooth and flat asphalt and the speed was 25 mph. What could cause this to happen? the bike was a rental and only a few months old.

Front wheel wobble is common on full sized motorcycles. It is very similar to a front caster on a shopping cart starting to spin in the caster axis instead of the wheel spinning. Some cases of "wobble"get so bad at higher speeds that you actually get "slapping" where the handle bars violently move back and forth so fast the rider cannot hold onto them and they swing from bar stop to bar stop ( sometime hitting the gas tank if barstops are bent or removed). It can be caused by a bad bike design but most bike designers after 1990 have taken care of this issue. Small font tire, almost vertical forks, poor weight distribution (not enough weight in front) can all cause wobble. It usually gets worse or more likely to happen the faster one goes. Some things cannot be accounted for at design so some extra parts are available to further eliminate wobble - most common is a fork damper. Basically a shock absorber between one fork and the frame and are usually mounted above the gas tank on top of the fork tube. These are necessary when carrying heavy loads that are mostly in the rear of the bike (passenger plus saddlebags). Also, some high speed bikes with windscreens / farings can shift the effective weight distribution with down forces caused by the wind on the faring. Finally, worn or under inflated tires can cause the wobble. Once the oscillation starts, it is difficult to dampen with brute force. In most all cases, the solution is to lighten the grip on the handlebars and coast until speed decreases to a level were the rider is strong enough tO overcome the oscillation. Scooters with passengers are ideally set up to oscillate and wobble with the small wheels. If the built in speed regulators are disabled on the 49cc model, a passenger is involved and / or under inflated tires could all create a perfect storm to create wobble. Good luck -Mark I h