King Blackfoot Pro-Mod

[Mad Ax 9312]

1 minute ago, Frog Jumper said:

[OldSchoolRC1 983]

It can be done. No hammer needed.   I'm waiting on a few bits, but so far it seems promising.

All kinds of crazy going on here. ....

[Mad Ax 9312]

7 hours ago, OldSchoolRC1 said:

It can be done. No hammer needed.   I'm waiting on a few bits, but so far it seems promising.

that is awesome - and something along the lines of what I was thinking.  I was going to try some TL-01B front arms on there next time I have a workshop day, but didn't want to install hexes up front if I could help it.  I've also got a complete front end from something else that might fit on with some modification, or I've got another front end with the Tamiya name on it which could work.  I like how your TL setup brings the steering cranks back on top of the chassis and retains the stock front bulkhead.

[Mad Ax 9312]

Hey folks!  It's been a while since there was any update to this thread, my weekends have been a bit disjointed for one reason and another and it's been quite some time since I was able to get a full day in the workshop.  It's been a bit too cold to really enjoy the workshop without the fire burning, but it's not worth burning the wood for an hour here or there (especially since it takes an hour to heat up the stove).

Here's a catch-up of the little bits I've got done here-and-there over the past few weeks.

30th Jan began with a coffee, from the mug I designed myself and had printed through my wife's company, http://amekyacreations.co.uk/

After that, I was back to investigating possible front end options for the King Blackfoot.  I had this lying around from the Fifty Fifty build - the front end from a WT-01.

The width is not a million miles away from the KBF transmission, so in theory, with some chopping of the plastic lugs and some careful creation of spacers, I could make a plate chassis similar to Fifty Fifty to house the WT-01 front end.  But that was quite a lot of work.

I also looked at fitting the WT-01 front arms to the KBF front bulkhead, but they're far too wide, the track width would be ridiculous.  I then went scrounging around through a bunch of other cars trying to find something that would work.  I pulled out the Blitzer Beetle looking for something different, and found this:

[Mad Ax 9312]

Now I'm pretty sure this is what @OldSchoolRC1 posted above - Blitzer Beetle bottom arms!  They're almost a perfect fit on the KBF bulkhead, they have much better geometry at the C-hub and are roughly the right width.  I think they're probably a few mm wider but we can worry about that later.

Wheelbase doesn't seem to be affected much (if at all), so these have got to be the perfect donor part!

Next plan was to order a new set of Blitzer arms so I can keep my Blitzer running (it's something of a personal fave and I don't want to cannibalise it for parts).  I already had a set of front C-hubs going spare, as I remember having loads of trouble sourcing the parts tree in 2020 after breaking a rear upright at Tamiya Junkies, and I finally got the parts from Racecraft RC.  All I'd need are some new uprights to replace the horribly bent ones on the KBF.  I found some alloy ones with alternate Ackerman holes and placed a long order from China.

The weather was seriously nice on this day, as you can possibly tell from the photos - the first properly warm day so far in 2022, I was working with the workshop door open and bathed in sunlight, and I was really happy to have finally unblocked myself on this project.  I'd have gone ahead and started modifying the Blitzer parts for this truck, but unfortunately a family issue beckoned and I spent the rest of the day on the sofa watching Disney films and trying not to think about the good weather I was missing.

The following weekend I'd agreed to go to a Sunday Roast Club meet with some friends in Bristol, so by the time I'd made my coffee and tidied the workbench it was pretty much time to get changed and set off, and by the time I got back it was cold and raining, and I opted to go hide in the studio where it was warm, so nothing got done.

Sunday 13th was much better, although I knew I had to end my workshop session at 1pm to go deal with family things, at least I had some new parts to play with, so I got right up in the workshop nice and early with several layers of thermals on in an effort to save some wood and some polar bears.

New Blitzer front arms, and step screws I fortunately found in my parts stash:

This little nub on the back of the arm needs to be removed, as the KBF mounting is very slightly bigger than the Blitzer one.  I chose to remove the one from the rear face of the arm, to extend the wheelbase, but the difference will be marginal.

[Badcrumble 4203]

Nice to see your progress @Mad Ax and I totally understand why progress has been disjointed, I only worked on my CR-01 yesterday because I had the lounge to myself (took the opportunity to watch Convoy on DVD).

Really like your mug!

 

[Mad Ax 9312]

I'll use proper turnbuckles for the finished article but for now I cut a 34mm length of 3mm allthread.  It needs some tweaking but the geometry is way better than stock.  RC4WD rod ends aren't very elegant but they're tough, also I'm running out of Tamiya ball studs and rod ends.  At full extension there's quite a lot of camber, right now it goes neutral at full compression but that will need to be adjusted.

After that, progress on the steering was stalled because a) I'm still waiting on those new uprights, which will be a while on their journey from China, and 2) it seems I only bought one of the hub parts trees in 2020, so I don't have any spares to make the other side.  Another parts tree has been ordered for next weekend.  At a glance, it looks like the steering angle should be a lot better but I'll know for sure once I have a go at fitting the new uprights.  Hopefully the aluminium arms will allow me to put some spacers or tall ball studs on so I can bring everything into line.

[Mad Ax 9312]

One man's challenge is another man's opportunity, and the opportunity I faced next was the front shock mounts.  The stock suspension uses a mounting hole on the C hub for the lower shock mount.  In most off-road RCs I've seen, the shock mounts onto the bottom arm - sometimes with various mounting points on the bottom arm to give different setup options.  What this means on the standard KBF is at the lower shock mount is way higher than usual, increasing the overall height of the chassis, plus it's also further out than usual, which decreases the leverage that the wheel has over the shock spring.  In effect this stiffens up the front end, which is a notable problem in these trucks - there's very little weight over the front and there tends to be very little compression when they land a jump or hit a bump.

Doing away with the stock C-hub means we must either use the Blitzer C-hub's turnbuckle hole to mount the shock, or use the lower shock mount in the Blitzer's bottom arm.  Figuring a conventional setup was likely to give better results than sticking with the KBF's bizarre configuration, I opted to use the bottom arm mount.  This isn't perfect - actually these bottom arm mounts are a pet hate with a stock ball stud, because the stud's thread isn't really long enough and tends to pull out after some hard running.  I found some ball nuts and some long grub screws, which would go deeper into the arm for a more robust connection, but the grubs were too long.  I solved this problem on my Blitzer by drilling the hole out deeper, which I'll eventually do here - but for now I'll use the standard ball stud just for testing on the bench.

Now all this means that the mounting point is much lower, so the standard shock towers are too tall (or, alternatively, the truck will have very low ground clearance even on full extension).  Also the standard top mounts are spaced wide to accommodate the wide bottom mounts, so using the same geometry as the standard towers would mean vertical shocks, which isn't ideal on a lightweight truck which really wants a nice progressive front end.

All this added up to one thing: a custom shock tower!  And this is where challenge became opportunity: now I could create a shock tower with a variety of mounting options, so I can tune the truck once it's ready to run.

Fortunately, the standard shock tower is very flat, and therefore easy to make a template from.  There are some nubs behind the shock top mounts and body post mounts which prevented me from making a full template, but I didn't actually need the top part of the tower anyway, and they helped keep the tower level against the 3mm FR4 sheet that I would use to make the new tower.

[OldSchoolRC1 983]

Looks great! Glad to see this project coming along 

[Mad Ax 9312]

With the holes drilled, I could secure the shock tower to the material to draw accurately around it

and cut a block out so I could test-mount it

I might have to cover up the hole in the bulkhead with something later, to stop all manner of debris filling it up from inside, but otherwise it's a neat fit.  The radio cover on the KBF makes a very effective tower brace - I think the early ORV chassis monsters didn't have these, to the shock tower had to withstand a lot of force in a roll over.  Fortunately the stock shock tower is flat, so the new one mounts right up to the radio cover with no hassle.

[Mad Ax 9312]

I wanted to give myself a lot of options for shock mounts, so I dismantled a click-top pen to mark the shock radius.  For this arc, I had the suspension fully extended and the shock also fully extended.

After drawing this arc, I realised it would actually be better to measure the radius with the bottom arms flat and the shocks half compressed, as that marks the theoretical best midpoint for the suspension and where it would best sit when fully laden.  After drawing a new arc, I picked a spot 6mm in from the radio cover so the shock mounting nuts won't interfere with it, then marked off 3 more points, 12mm apart on the arc.  Then, to allow for a shorter shock, more static sag, or more ride height, I measured 8mm in from each of the mounts.

Then I had a few stabs at drawing around the mounting holes to get an attractive shape

[Mad Ax 9312]

One side cut out and mounted on the truck to test.  I think these shocks are from a TXT-1, and they were assembled with a spacer under the piston to limit downward travel.  I removed it to allow the max possible suspension movement for now, although it can go back in again later if I need to control the weight transfer.  The stock spring is too short for full extension without any collars fitted, but I've got a lot of options for tuning.  It's very compliant on this hole, even with no electrics in the chassis.  Hopefully the front end will actually work properly on this truck now  

 

 

[Mad Ax 9312]

The tricky part with custom-making something like this is making it symmetrical.  One tiny inconsistency in the top mounts will cause chaos setting up the springs.  The way I do this is to screw a piece of scrap material under the good side...

Then use the existing holes as a template to drill holes in the scrap piece, also draw around the outline and rough-cut it with a jigsaw

Screw both parts together, then file the scrap part down until it matches the good piece

Then, unscrew the scrap piece, flip it over, screw it back on...

Drill holes, draw around the outline, rough-cut with jigsaw...

Then screw back together and file until the profiles match

[Mad Ax 9312]

And that was as far as I got.  That took me neatly up to 1pm, when I'd promised to come in, cook lunch, and look after my daughter while my wife got on with some coursework.  A bit of a shame, as another 10 minutes would have seen this piece neatly filed down and test-fitted.  FR4 is a good material but the filing dust is pretty noxious and I didn't want it all over my good clothes, so once I'd got changed to go in, I knew I wasn't going to pick it up again until next time I had my workshop clothes on.

As it happens, it looks like I've got a free Saturday coming up because my daughter is staying away for the weekend, we were supposed to be having a late Valentine's / Date Weekend, but my wife has a lot of coursework to finish, so I've got a free day, provided plans don't change again  

I'll probably be fitting Blitzer arms to the rear of this truck too, although the stock rear end is less bad than the stock front end.  Rear shock mounts are half-way along the upper arms, and on my truck it's looking like it's damaged in that area, so I want something a bit more robust.  So, a new shock tower for the rear end is in order as well, lowering the overall height of the chassis and possibly giving me some new body mounting options.  I need to see what the final wheelbase is before I make a decision on a body.

[Mad Ax 9312]

I don't usually do workshop on a Saturday, but as the daughter is staying away and my wife was busy with diploma stuff, I had the morning to myself.  I started my day by laying down some buckets to catch the roof leaks after Storm Eunice took the felt off the workshop yesterday, although this turned out to be an exercise in futility, I had to stop the clear the storage level around mid-morning and use the big tubs that I normally use to store my monster trucks in to catch water streaming through in 5 different places.  Hopefully won't have to wait too long for the roofers to get in and replace it.

Then back to the KBF.  A bit more filing and the front towers were in nice shape.

[Mad Ax 9312]

A thought occurred to me as I was finishing up last week.  By relocating the shock mounts to the lower wishbone, and lowering the shock towers, I will be able to lower the body for a sleeker, more racey stance.  If I had a narrow body, it would fit between the wheels so I don't have to cut the arches so much.

And then I realised.  By chasing better geometry, balance and tunability in the name of a nicer-handling truck, I've basically done exactly what people were doing back in the late 80s, which led to the development of the RC10T and the now-common stadium truck format.  Now the argument here is that the original Blackfoot and all the trucks that followed on the ORV platform were not true monster trucks at all, because they were RWD with independent suspension.  But they were built to look like monster trucks, they were top-heavy and cumbersome and hard to drive, and visually impressive.

I don't really want to create a stadium truck.  I've already got a Blitzer Beetle (albeit with a HPI stadium truck body on it).  What I want is a King Blackfoot that handles a little better, has some adjustability, and is fun to hustle around a track without being a low-slung stadium racer.  Most of this will be down to how I mount the body, but it was an interesting (and slightly arresting) thought.

Anyway, back to the build!  Having mounted the front tower, it was time to look at the back end.

[Mad Ax 9312]

Lest there be any doubt, this truck has seen action on the astro

This is the state of the rear upper arm, and the reason why I was so keen to convert to a conventional shock mount.  First, the ball stud is pulling out of the top arm.

Second, the tension on the arm has bent the step screws and also the arm itself

[Mad Ax 9312]

I spent a long time toying with Blitzer bottom arms, for two reasons:

1) the Blitzer suspension up-front is slightly wider than stock KBF.  Fitting Blitzer arms and hubs on the rear would widen the back end to match

2) the Blitzer bottom arms have reinforced bosses to mount the shocks on.  The KBF arms don't have that.

In the end, I decided to stick with stock, for the following reasons:

1) the Blitzer uprights require a different drive stub, and I don't have one

2) the overall position would mean drying to make or source an oddly-sized dogbone or CV joint

So - instead the KBF parts got cleaned up and re-installed, utilising some Duratrax turnbuckles that I've had in my parts bin for over a decade

Then I had to investigate a new shock tower.  I started by looking closely at the old one, and found it bent where it slides into a recess in the gearbox housing.  Obviously this part takes a lot of impact in a roll - it's tall, only fixed by two screws, and has those big webbing structures to try to spread the load - I can't help thinking some screws in those webbing parts would help.

[Mad Ax 9312]

I took an educated guess at how high I'd want my towers to be, and marked out some crazy lines on 3mm FR4 sheet.

I used a combination of electric jigsaw and mini hacksaw to cut out the recess where it fits over the gearbox housing

marked and drilled

and an arc drawn with the shock and the arm at mid-point

[Mad Ax 9312]

I drilled 4 holes, plus 4 lower ones to allow for more sag or ride height, then test-fitted

 

[Mad Ax 9312]

After that, it's back to my usual antics of making a dummy to transfer the shape and holes to the other side.  Except in this case I decided not to make a dummy.

The stock tower is 5mm thick, and fits into a recess on top of the gearbox case.  This adds a little more robustness to the design.  I figured if I made my dummy out of 2mm FR4, and filed it to the same shape, it could be installed as a reinforcing piece.  It was just as easy to cut it to the full tower shape as it was to just make a reinforcing piece around the gearbox mounting point.

My initial measurements were a bit off - this piece doesn't quite go to the end.  No big deal tho, it works fine.

 

[Mad Ax 9312]

Finished rear end

Chassis reassembled.  Just waiting on the new uprights before I measure the steering links.

With the wheels on it now looks sleek and aggressive.

Plenty of sag on the front, possibly too soft for bumpy terrain but lots of options to stiffen it up

No sag at the rear with these old blue springs installed, but there's no motor or battery installed yet

Battery and motor give some sag

[Mad Ax 9312]

So there it is - not much to do before the chassis is complete.  I need to decide on a body, source and paint it, recondition the motor and put it all back together.  I'm kinda hoping I'll get to a Tamiya Junkies meet in March (depending on what weekend it happens and if I can get childcare) so it would be nice to have a deadline, but there's not so many weekends left and I might have to clear the entire storage level of my workshop to have the roof replaced.

Exciting times - I'm really keen to see how this handles now  

[ThunderDragonCy 7878]

@Mad Ax This is excellent. Looking forward to seeing it at RHR. 

[Mad Ax 9312]

11 hours ago, ThunderDragonCy said:

@Mad Ax This is excellent. Looking forward to seeing it at RHR. 

Still waiting on a date for the March event.  If I'm free on that day, I'll do my best to get there - although my work may well be cut out clearing the workshop for the roof replacement, depending on when the roofers can book us in.  I've just thrown another tarp over some stuff on the storage level but it's now pouring off onto the rubber mat next to my main workbench, so no more Workshop Sundays if it's raining until we get it fixed.