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.

Pic 13: Universal link on the steering

The moded final model is much better. I wonder if I can motorize this with a Power Function.

Pic 14: My version of Lego Technic 9392

Wan Khairil Reza

My first two-speed (then four) Lego Technic gearbox design

Last weekend I took to myself to design my very first Lego Technic gearbox. I set a simple goal; a two-speed gearbox with 1 input axle, 1 output axle, and no gear grinding allowed.

I'm still at the very early stage engaging in this Lego Technic hobby and I'm only beginning collecting my Technic parts. As such, unfortunately, I don’t have enough parts available to build an actual Technic gearbox. Luckily, Lego provides a useful tool to help people virtually design their Lego projects. It is the Lego Digital Designer (LDD) and it is downloadable totally free from Lego's website. Even though LDD is not as flexible as I wish it could be but it is good enough for most design tasks. The learning curve is not steep, provided you have some foundation in Lego building. Good mouse dexterity would come in handy as well.

In this blog post I want to share my process in designing my gearbox, using LDD for now and hopefully with actual Lego Technic parts soon.

First, being a newbie, I figure I need to study Lego gear pairing. The screenshot below shows what I did in LDD. The objective here is simple; to see how various pair of gears can be positioned, since Lego designing is generally constrained by the number and position of the studs and holes of Lego bricks. So, I tried out a few combinations and roughly got the idea. I also found the site that helps in Lego Technic gear pairing.

Pic 1: Gear pairing exercise

 

Next step, I went straight to designing my gearbox. By the way, if you need to understand how gearboxes work, refer to How Stuff Works or here for Lego gears tutorial.

The screenshot in Pic 2 below shows what I did. I decided to lay it flat first, in order to see how the gears would be arranged and linked. As I've mentioned, I don't want gear grinding during shifting, so I use the driving ring that allows me to lock on to either one of the free wheeling gears.

Pic 2: The gearbox, flat

Basically, the gearbox provides a gear down from a 12 teeth gear to a 20 teeth gear (1:667) shown with green gears and a gear up from 20 teeth to 12 teeth gear (1:0.599) shown with grey gears. Because my design is laid flat, with very little effort you can trace the gear and axle movement of each speed setting, as shown in Pic 3.

Pic 3: Gear one and gear two

Now I know what I needed, next would be to simplify the design. Just like most things in engineering or computer programming, some components can be reduce, reuse or shared. After a bit of fiddling, I got this:

Pic 4: My two-speed gearbox

Pic 5: My two-speed gearbox, bottom view

Simply, the flat gearbox is ‘folded’ so that the gears and axles are layered. I've also combined the output axle, and made it compact. And just like that, I've completed my task. I’ve designed a two-speed gearbox. I can now go and watch The Big Bang Theory.

But I didn't.

This is too simple and too easy. I also looked at my gear ratios and it was clearly unrealistic.

If I can make a two-speed gearbox, surely I can make a four-speed version. The simplest thing to do is just to replicate my two-speed gearbox, and change the gear pairing to have different ratios. And that's exactly what I did.

Pic 6: Two two-speed gearbox

Now, the available gear ratios are:

1st gear 1:3
2nd gear 1:1.667
3rd gear 1:1
4th gear 1:0.599

But by having two two-speed gearbox, I now have two input and two output axle. So, the simplification cycle starts again. With a little rearrangement, and a few extra gears, Pic 7 is what I got:

Pic 7: Final four-speed gearbox

The final design fulfills all my requirement. It's a 4 speed, 1 input, 1 output, absolutely no grinding while changing gears. Can’t wait for my Lego Technic parts to arrive!

Am I done? Yes for now, but I can see a lot of gaps in my design. I'm sure seasoned Lego Technic designers would take one look at this, smirk and point out what I should have done. And I hope they do, so please, share your thoughts.

- Wan Khairil Reza -