A quick blog post which may hint at my next big kit that I will be building (along with a hint from the end of my previous LEGO post here – can you guess?).
My dastardly-evil (just kidding) other half gave me this recently: kit 8259 with the chain-link treads which I was after (nyuh nyuh how did she know!?).
I rarely if ever build LEGO kits “to the letter”, preferring instead to dump the bits into my collection and go DIY (the most fun!), but I thought I’d give it a go this time around and here’s some quick pics of how that turned out.
Of course, the main thing of interest (for me at least) in this kit is the treads which will be used in my next big build.
There’s still more parts to acquire before I can truly get started, but stay tuned for more on that front soon! In the meantime, enjoy…
What you’ll see here is the latest and greatest iteration of this truck of mine – and probably the last one at that – alas, I’m looking for bigger fish to fry. More on that later though, let’s start by getting down and dirty with all the changes and upgrades which have happened to this vehicle since Mk1 back in 2009…
Point me in the right direction
One of the things I could have done better with the design of the Mk1 was that of the steering system; the steering gear ratio didn’t deliver as much steering torque as I would have liked. I solved this by adding an additional gear-set to which increased the gear reduction by another 3:1. This means more turns of the steering wheel is required to get the same amount of steering action, but with it comes a welcome increase in steering force and realism.
I’ll let the following video do the rest of the talking…
Chain me up (again)
Another new feature with Mk2 is the addition of a “chain brake”. This is basically a form of “handbrake” which serves to hold the vehicle stationary and stop it rolling away, or off a table onto the floor for example.
Normally the gearbox serves as a sufficient braking mechanism when either 1st or 2nd gear is selected and the motor stopped, however if the gearbox should slip into neutral, the vehicle has the tendency to let gravity take over…
The chain brake operates by sliding a “friction gear” into mesh with the gearbox final drive gear. The friction gear is keyed to a stationary shaft which cannot rotate, meaning that once in mesh with the gearbox, it resists movement of the drivetrain and thus, the rear wheels. The only disadvantage of this system is that if one rear wheel (or both, somehow) are off the ground, the vehicle can still roll even with the chain brake engaged due to the action of the rear differential.
The picture below highlights the location of the chain brake and its associated linkage system and lever (red). Keen observers may also notice the chain tension gear I’ve added to the rear chain which helps avoid the chain jumping links and slipping over the rear differential crown gear under load:
Trippin’ on LSD
That somewhat controversial heading is somewhat (fully?) matched by the sheer innovative genius that could only describe the limited slip differential (LSD) that has made its appearance with Mk2. /brag
A bit of background…
The design of this LSD is purely my own and took a bit of experimentation and tweaking to get right, but I must say, it works quite swimmingly in its current form. For those who aren’t quite familiar with what an LSD is in “vehicle-speak” (you’re still reading this?? just kidding!), it’s a differential system which attempts to overcome a traditional differential’s one major short-coming: the tendency to transfer ALL torque to the driving wheel with the LEAST amount of traction.
Let’s step back one step further: the purpose of a differential is to allow one wheel on a common axle to turn faster than the wheel on the opposite side during a corner. This ensures effective steering action and reduced tyre wear. I won’t go too far into this, as there are a lot of articles and info on this on the Internet already (see the end of this post).
“Most” cars and vehicles are fitted with what’s known as an “open” or “standard” differential. This works fine around corners and during general operation on stable, high-grip surfaces where both wheels on the axle have equal amounts of grip. However, if one should venture into a loose surface, and one of the wheels connected to the differential should slip, or become raised off the ground by a bump, etc, all torque is immediately transferred to that wheel. Meanwhile, the wheel on the ground on the other side of the axle (in most cases with all the traction) sits motionless – when it could be driving and moving the vehicle.
An LSD works by re-routing the torque that normally takes the “path of least resistance” and transferring it to the wheel which actually has traction, i.e, the “loaded wheel”. Thus how the system earned its titled “limited slip”. A good measure of how well an LSD works, is to raise one driving wheel off the ground, and seeing how much torque gets transferred to the “loaded” wheel on the other side (an “open” differential in such a situation would spin the lifted wheel uselessly and move the vehicle nowhere). You’ll see me perform this test in my LSD test video below shortly.
How it works!
So we finally get to the nitty-gritty; just how the LSD system on the Mk2 functions. Originally, I was going to trial a geared version but belts and bands have always intrigued me for their smoothness in operation and the fact that they are generally more forgiving under load and perhaps even more variable in setting than straight-up gears.
So…I went with a “dual-band” setup for the Mk2’s LSD system. I’ll do what I always do and attempt to explain in text, follow with a vague pic, and finally show you a video.
On each half-shaft of the differential, I’ve mounted a large pulley. Now these two pulleys on their own don’t achieve anything special, but to these pulleys I’ve added a “band” each, which then connects to a “common shaft”. This common shaft is mounted upon an adjustable mechanism which moves the shaft back and forth (adjustable by the user using the “torque wheel”) so that it’s either closer-to or further-from the pulleys and thus band tension can be almost infinitely adjusted.
Here’s that pic:
Increasing band tension has the effect of making the common shaft “connect” both of the large pulleys (and the two halves of the differential) together as a single unit. What this ultimately achieves is making the differential unit behave less like an “open” differential and more like a solid axle. This naturally means that when the LSD is set to its maximum torque transfer setting (maximum band tension), differential action (allowing one wheel to turn faster than the other around corners for example) is limited, but this is where the magic of using bands rather than solid gears comes into play: no matter how tight the band setting, there is always a little bit of “give” in the system.
Time for a video…
Some additional notes…
As mentioned earlier, the use of bands rather than “solid” meshing gears gives this system its own type of “elegance”.
With this so-called elegance however comes one disadvantage over the use of traditional gears: band wear. It must be said however that under “normal” operation on solid ground and with minimum tension, the bands experience almost no wear as the pulleys go about spinning by their own free will. It’s only when the bands are tightened and the differential subjected to prolonged slip condition (where one wheel turns faster than the other), band wear starts becoming evident as they try to rotate the common shaft and transfer torque (band wear is generally visible in the form of bits of rubber residue along the bands). Having said all that, I’ve gotten several hours worth of LSD operation out of the bands currently fitted to the truck, and they haven’t let go…yet!
Now slip ratios are not an easy thing to determine with a band-drive LSD system as there are many factors and variables which could affect it, such as band wear, band tension and perhaps ground surface type.
From my observations so far though, I’ve noticed that under full band tension, the LSD unit comes reasonably close to achieving a 1:1 slip ratio…meaning that the loaded wheel in a slip condition almost turns as fast as the wheel up in the air (doing nothing). Obviously getting as close to 1:1 as possible (or even over-driving) is desirable, but with a band system, it’s not as easy to achieve this (as compared to a geared setup). I’d hazard a guess and say that under band tension, this LSD transfers around 50% to 75% of the torque to the loaded wheel, which is not a bad effort indeed and as you can see in the video above, seems to work quite well in practice.
One of the other major advantages I’ve seen with this LSD system is the fact that the system is very compact and hardly takes up any more space than the standard differential unit. The large pulleys mounted on the differential half-shafts hardly protrude from the vehicle and the common shaft is tucked neatly up into the chassis of the vehicle too.
Some of the other additional features I’ve added to Mk2 that I thought worth mentioning (but not exactly worth their own heading) are as follows:
Rear flat-bed-mounted hoist (adjustable for tilt via linear “hydraulic” system)
Headlights! (yes, that actually work)
Various miscellaneous chassis strengthening tweaks
Here’s a bit of an image gallery showcasing the Mk2…
I’m quite happy with the way Mk2 turned-out in the end. It’s certainly come a long way since I laid out the bare chassis rails on my building bench and started dreaming for the first time. I would have loved to add an RC system to this truck and maybe even a PTO (Power Take-Off) unit to the rear-end, but alas, some things just weren’t meant to be it seems.
Where to now? Onwards and upwards onto bigger and better things! My next model could be a U.S. Army Half-track, or a robot. Who knows?