CAT D7R LGP tracked dozer

Radio controlled

A radio controlled bulldozer; that's what I wanted to build for a long time. During a Lowlug meeting I played with a radio controlled truck and because I would like to have my own I wanted to buy set 8366. But this set wasn't I LEGOs assortment anymore. Fortunately I found a copy shortly after the meeting in a large toy store. However first I was quite disappointed about this set because I actually wanted to build an RC-truck. The wheeloutcome of the steering unit is quite poor and you have to make a very big turn to make a corner. I stopped the project after a few days and let the RC-stuff for what is was.
Later I got another idea; the set shouldn't be worthless so I had to do something with it. A bulldozer would be one of the possibilities and within a few hours I built a demo to test if it would work properly. I just created a simple undercarriage and didn't fit the receiver unit otherwise I would spent too much time to something which might be useless. It worked very well and it became a new work in progress and also a new challenge.

Demo

I built a bulldozer in 1999 already. At that time I was very pleased with the result and it had much attention during the Modelshow Europe. But when I look at it now I think lots of things were not like it what it should be. The targets are different nowadays so this means you have been growing during the years. Points of improvement were the undercarriage and the dozer blade. The tracks were made off wide chain links which are in the todays sets again after some years of absence. At that time I didn't put 1x6 plates on the tracks to give it a more massive look wether is was possible or not. In a meanwhile I saw the possibilities of the gears of the early 70s and I had sufficient chain links left in my collection as well as those big yellow gears. It looks more massive but moreover more realistic. While the D9R was a static model the D7R wouldn't be similar definately. I had to make a stronger dozer blade because it should be adjustable.
The D7R is quite a rare machine and I found out while I already was building it. In the late 90s I received the necessary documentation about tracked dozers. From those brochures I built the D9R and also the lighter D7R. As regards the capacity it's slightly heavier than the D6R but personally I think the D7R looks much better. Searching for suitable pictures on the world wide web provided me with the information necessary but sometimes it still wasn't enough. Especially when it comes on the details.

The techniques

The undercarriage was more difficult than I would expect. The concept was made from scratch and the dimensions were far from it what should be. But though it was relative easy to rebuild it to something which is more similar to the real thing. But the tracks where too tightened. An empty undercarriage drove well but when I put something heavy on it changed into a shocking moved model. By putting pneumatic cylinders at the front of the tracks you could adjust them into the right position but it also became a very weak point especially while turning the model. After a while I removed the cylinders and made rigid track layers.
Something which was very interesting to build was the steering systeem. First I had a lot of problems with that. I used two polarity switches, but when you move forward you had to use both controls of the sender. And while turning the model I didn't knew the difference between left and right anymore so actually it was useless. Via the Brickbash forum I saw method which would be ideal. Therefore you also need two polarity switches controlled by two seperate handles. You have to connect the electric plates on a certain way but eventually the system works very well, thanks to 'Defecto'!
Both 'RC motors' were replaced for two times already. It was because off the mountage of the undercarriage and later also because off the position of the steering mechanism. While the LGP is not equiped with a ripper it was troublesome the motors were placed at the back underneath the operators cabine. And moreover it is quite easy to disconnect the electric plates now if it's be necessary. I possitioned the receiver somewhere in the middle and you need long fingers to reach the on/off button. But as long as I can turn the switch on and off I'm satisfied with that.
I put the project aside for a few months. During this periode I built the operators cab but when that was done I postponed it for a while again. Other things had more priority and the inspiration was gone also. After two months I picked it up again and than it passed a rapid. The dozer blade got the right shape and the mechanism to raise and lower it had found it's space.

Dozer blade

The finishing touch took more time than I thought on forehand. Off course it takes a while before I'm satisfied and some parts didn't had the right colours so I had to order them in yellow. The dozer blad is adjustable with the help off a old style 9V motor. The cylinders are partly made off the long cylinders of the old pneumatic system. They actually don't have a function except from keeping the dozer blade in the right position while moving it. A disadvantage of the cylinders is they do need some airpresure and when you're not operating it for a while you have to help them a littlebit to bring them into movement even when you lower the blade. The long cylinders from the 'XXL mobile crane' are not suitable because you can't connect them anywere properly. Besides that the cylinder itself is not fixed (no stop bush inside). The rope which pulls the blade is hidden behind the front grill. It works very well but when the batteries loose power the blade raises slower and slower. Driving the machine itself works reasonable then.
Once supplied with the necessary stickers it is a conspiciously model about which I'm more than satisfied. This kind of bulldozers are one of my favorite machines. Nice to build but they also fit on different kind of trucks. One thing I would like to remarks is that when I started to build this model in April 2006 LEGO didn't announced it's 2007 sets. So I didn't knew they would come with set 8275. Maybe LEGO copied a littlebit?

Other data

Length:	43.5 cm
Width (over blade):	30.5 cm
Height:	28.9 cm
Length undercarriage:	33.0 cm
Construction:	???? many hours
Date:	April 2006 / April 2007

Sources

Lkw-Stefan, chuckfull of construction machinery

Bouwmachineforum, topic about the D7R

Telepathy

Jan van Himbergen has also build a CAT dozer, a D9R. Without knowing of each other we were busy with building a dozer at the same time. Jan's creation has also been loaded onto my lowloader.

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