My Serene Rage: Lego Technic 9392 Quad Bike Review (and Mod)

If you've read my previous post, I've recently made it back to world of Lego. My first come-back set is the 2012 Lego Technic quad bike (9392).

Pic 1: Lego Technic 9392 Quad Bike

It is a decent Lego Technic model. Building it was without any problem. I didn't time myself, but I'm sure it was less than 10 minutes.

Overall, I like it. It captures the essence of a quad bike. The big wheels, the body panels, the bouncy high suspensions and this model even has a chain linking the dummy engine to the rear axle. What I love the most is the front suspension; good travel and works nicely.

But one thing that I'd rate poor is the steering. It is loose with lots of free play and is not center balanced. Makes it hard for the quad bike to stay straight. This is due to so many loose joints in the steering design.

Pic 2: Loose joints that contributed to lousy steering

Another thing that I consider a design mistake in this model is the location of the swingarm’s pivot axis, which affects the tension of the chain. In real world situation, a chain's tension is kept constant by ensuring that the distance between the drive sprocket and the wheel sprocket is kept constant. Otherwise, it might cause the chain to become loose or break. To achieve this, the drive sprocket must be on the same pivot axis as the swingarm. But this is not the case for Lego Technic 9393 quad bike. Pic 3 shows the location of the drive sprocket axle and the swingarm pivot point.

Pic 3: Default 9392 Swingarm setup

Pic 4 and Pic 5 show that the chain becomes loose when the rear suspension of the quad bike is pressed.


*Pic 4: Chain in normal condition*	*Pic 5: Chain becomes loose wen rear is pressed*

I have to solve this two issues.

First, the swingarm. I modified it so it pivots at the same axis the drive sprocket. I added or removed as few pieces as I could and I reused as much as possible. Pic 6 shows the moded design. And as can be seen in Pic 7 and Pic 8, the chain's tension is maintained when the rear suspension is pressed. Success.

Pic 5: The redesigned swingarm


*Pic 7: Chain in normal condition*	*Pic 8: Chain doesn’t become loose when rear is pressed*

Next, the steering. I noticed the toe of the front wheels are going slightly outwards and it gets worse when you push the bike forward. I used this to my advantage. What I did was I flipped the whole front part of the quad bike, as shown in Pic 10. By doing this, I turned the toe inwards. The steering arm is also now at the back.


*Pic 9: The original steering design*	*Pic 10: The modified steering design*


*Pic 11: The original steering design, with outward toe*	*Pic 12:The moded steering design, with inward toe*

The inward toe actually straightens when pushed forward and does make the bike go in a straight line! Nice. to finish off the steering, I also used a universal link to give the handle bar a slight angle. Success.