Half Track build

[MadInventor 3888]

This is something I've been thinking about for a little while now. I'm going to have a go at building my own design 'heavy duty' half track. I'd got an old king tiger left over from another build I had butchered the top of and wanted to do something with, and wanted a model with tracks that would have a bit more speed than the normal tanks. As my bodyworking skills aren't the best, I'm not going to attempt to replicate a scale model of a half track, rather draw on influences from other designs and full on tanks to make the build process easier and give me room for inventiveness. In my head at the moment is a rear enclosed hull based on an entwicklung E25 hull shape, with king tiger tracks (I've got a pair kicking around and they are the only cheapish tracks to easily get hold of). Eventually I plan to make my own idler wheels, but for now will make do with the king tiger items. The front end I hope will end up looking like a german half track, but with oversize off road tyres.

For power, I'm going with a 540 size motor powering the tracks through a king hauler axle, I debated whether or not to put in a 3 speed box, but after the 6x6 experience I think I will make do with a simple reduction gear transmission.

Here's the pics of what I've done so far (I've only been working on it for a few days so far)

Completed drive axle with King tiger sprockets fitted.

It was not a simple process to get to this stage . I had to use mad bull axles to get an axle with the length required. The circlip on the longer axle had to be fitted inside the diff housing, otherwise the drive shaft would drop out, and I did not want to have to cut another recess for the circlip on the axle on the outside of the housing. The axles also had to be cut to the right length, turned down to 4mm for an M4 thread, and have flats machined on them for the grub screws to locate from the sprocket adapters.

The assembled diff assembly. Note the lack of circlip on the one side of the diff. Shims are not required as the pinion gear prevents float in this direction.

Assembled axle with sprocket adapters fitted.

Sprocket adapter close up.

I had to skim the inside of each sprocket as the hole is tapered and the box wrench would not fit all the way down to tighten the wheel nuts. Normally KT sprockets are held in place with a screw, screwed into a much thicker axle component

Axle with sprockets fitted.

[bromvw 744]

You have me thinking now Martin . In my spairs I have a pair of T Rex 60 axels and a killer transfer unit . My Heng Long Walker Bulldog is becoming a little boring . Metal tracks and sprockets though . i'll watch and learn and maybe do something similar

[30YrsL8r 97]

You are, indeed, doing the opposite to me. I'm trying to figure out how to mount Rough Rider rear wheels to output shafts, with flats and threaded holes, that aren't the same diameter as the Rough Rider ones. If they were, I'd just put RR UJ's on the ends.....

[IBIFTKH 1374]

Watching with interest, this is going to be fun

[MadInventor 3888]

You are, indeed, doing the opposite to me. I'm trying to figure out how to mount Rough Rider rear wheels to output shafts, with flats and threaded holes, that aren't the same diameter as the Rough Rider ones. If they were, I'd just put RR UJ's on the ends.....

That's going to be awkward. If you could remove the shaft you might be able to turn the end down to 5mm, then put an M5 thread and an oversize nut on the end, but you'd need a lathe for that, and you'd still need to be able to drill the shaft to fit a pin for the drive hub for the rough rider wheel. A more modern wheel with a 12mm hex might have been a slightly easier proposition. Could you butcher the old tank sprockets to fit tyres on, or attach wheels to the sprockets, as this is just a testbed ?

[30YrsL8r 97]

That's going to be awkward. If you could remove the shaft you might be able to turn the end down to 5mm, then put an M5 thread and an oversize nut on the end, but you'd need a lathe for that, and you'd still need to be able to drill the shaft to fit a pin for the drive hub for the rough rider wheel. A more modern wheel with a 12mm hex might have been a slightly easier proposition. Could you butcher the old tank sprockets to fit tyres on, or attach wheels to the sprockets, as this is just a testbed ?

My facilities are very limited. Last night I drilled out the existing hole and put in a new thread so that I can screw in some TB-02 axles I have, so the output shafts now have drive cups for dog bones. I'll update my build thread later.

[MadInventor 3888]

Got a bit more done this morning:

Butchered the front end of the hull to let the drive axle in. I might have to cut the front off entirely as the axle is a bit high at the moment.

Sanity check to see if the fit is about right.

Shortened hull with 40 degree slope at the back.

This was the bit that got chopped off.

Another sanity check. Balance is pretty good, with the hull slightly front heavy with the battery in the back. Hopefully that track tensioner and rear idler wheels should help with neutral balance. Being front heavy will help with the effectiveness of the steering anyway. Should have plenty of room to fit the track tensioner and the rear idler wheel.

Yes, that's a 3S LiPo in there

[teddyt 17]

looking sweet !!

[biggamehunter 127]

martin , i have been waiting for this since we spoke about it a couple of weeks ago !!! i almost wondered if it was going to happen at all , it looks amazing so far , cant wait to see the rest of the build , following intently , great job

[mr crispy 2297]

This I'd getting very interesting. You know, stick a motorbike front end on there and you'd have a 1/6 scale Kettenkrad!

[MadInventor 3888]

Last update for the weekend now, I've got the chassis rails fitted:

Didn't turn out quite as well as I had hoped mind, as the rails don't touch the rear armour plate, which I hoped they would butt up to.

This is how the rails are attached to the hull:

and this is how the drive axle is attached to the chassis rails.

In the end I removed most the front lower glacis plate from the hull, as well as cutting a slot to make it easy to get the axle in and out of the hull.

I haven't got any form of adjustment yet for the track tension, this one is too loose:

and this one is too tight:

I didn't get the holes for the idlers in the hull in quite the same spots, so the shaft the rear idlers run on is not square to the hull. It won't matter anyway as both holes will have to be enlarged to accommodate movement for adjusting the track tension. I'm still debating whether to wait doing any more on the track tensioners until I'm certain that this the position I want for the idler wheel. If I can tuck it inside the 4th road wheel I could get the idler closer, shorten the track by 2 links, and then shorten the rear end of the hull as well, although this would be awkward as I would have to straighten and re-bend the rear lower hull plate.

Other problems surfacing are that the chassis rails are too far apart for the front of the vehicle, so I'm going to have to chop them off and get them closer together. They're also too low, so they will need to be raised somewhat as well. I'm also thinking about what to do for the front suspension. I'm thinking solid axle for definite, although the original design of a centre pivot axle spring on a leaf spring will be very awkward to implement, so I'm thinking a 4 link setup with an undriven front axle, with a mini servo mounted onto the axle for the steering.

[mongoose1983 3362]

Oh, WOW. What a project you have there. Really awesome!

[BiggusDitchus 38]

Another project Im going to enjoy watching. Top work.

[MadInventor 3888]

Alloy one piece wheels, using the same roadwheel layout as the panther. From front to back layout will be:

Sprocket, inner, outer, inner, outer, thin inner idler wheel.

4 of 8 done so far, will probably sleeve them with steel tube to start with, so I can run them on 4mm axles. Adding roller bearings will be a pain, although I might give it a try on the inner thinner wheels.

[mongoose1983 3362]

By the pictures I suppose the front wheels will also have traction. I don't know if that was the way these vehicles worked in real life, but as an RC vehicle, I must say your work is frankly awesome.

[SteveU30 1011]

I think this is my favourite build of yours so far Martin, watching with great interest!

[MadInventor 3888]

By the pictures I suppose the front wheels will also have traction. I don't know if that was the way these vehicles worked in real life, but as an RC vehicle, I must say your work is frankly awesome.

I was actually planning on having the front wheels not driven, I just liked the look and size of the F350 tyres for the front end. Having them driven will promote understeer, which I think will be a problem with the half track anyway, hence the reason for an undriven front axle. I'm also liking the look of the idler wheels so much I might make a 2wd set for the front wheels to fit the F350 tires.

[MadInventor 3888]

Another update, I've now got all the wheels made, and one of the track tensioners completed:

Track tensioner

Spacer to get idler in the right place

Ballraced idler wheels

Wheel layout

I don't like the wheelnuts. I'll try to come up with a solution that doesn't require them at some point, but to get me going they'll do for now

I had to move the drive axle forward slightly, as it fouled on the foremost idler wheels when the suspension moved a lot. This also necessitated removing the whole of the front of the hull.

I've learned a lot going through this process. In hindsight it would have been a lot easier to cut the front off the KT hull, rather than the back, and then use the standard king tiger tack tensioner system, rather than having to come up with my own. Also, some of the suspension arms appear to be bent, or not as straight as the others, so now I have wheels with little slop, a couple of the wheels have a tendency to ride up over the track guides. I also had to move the axle forward as the KT standard dimensions don't leave hardly any gap between the drive sprocket and first idler wheel on one side of the tank. However, a couple more days and I should have idler finished for the other side, and will then be ready to think about what to do up the front. I've decided to press ahead with this first before messing with the rear hull design, just in case it drives like a pig and I decide not to pursue the project further.

[biggamehunter 127]

great work martin !!! following this with great interest

[biggamehunter 127]

any updates on the track martin ?

[Psylence 25]

Wow, just looking at the beautiful machined parts.... so lovely. Fantastic work!

Can't wait to see the finished article!

[MadInventor 3888]

any updates on the track martin ?

There will be another update soon. I've started machining the front axle beam. I also got some more done on the 6x6 a couple of weekends ago, got the log bed welded to the chassis, got the new headache board welded and painted, and also got a number plate welded to the rear of it. Also repainted the rear end of the chassis post welding. Will be putting pics of that on soon. On the whole though, it's been too hot for the last couple of weeks to do anything.

Edit 23/07/2013

The start of the front axle:

Not much to show at the moment, I'll be using F350/TA04 uprights to hold the wheels. I've got loads to do on this still to remove weight out of it, mount the servo for the steering, and work out what suspension to use (I'm leaning towards a 4 link system and oil shocks.

[MadInventor 3888]

Got the gearbox done:

The gear used is an unmodified thundershot diff gear (Well I drilled out the screw holes to 3mm, but that hardly counts). The motor plate is large enough to take a 4274 size brushless motor. The output shaft comes out both ways in case I ever want to put the gearcase in a model with multiple driven axles. Output shafts are 6mm to fit TXT-1 / REVO, etc drive shafts. The aluminium gear plate the thundershot gear is screwed to is fixed to the main drive shaft with Loctite 638.

[MadInventor 3888]

Got the gearbox fitted now. I had to put it in the back, as it was getting in the way of the front axle and suspension arm links. However, pleased that the transmission is now finished, I just need to find 2 circlips (If anyone knows what the part no. is for these please let me know) for the UJ.

Also got the front axle fitted. Still thinking about front shocks and mounting method, as well as the steering servo.

[sabreuk1 56]

this looks awesome I cant wait to see the finished product bud and im loving the pics