Kraton Project "Front-Butt" (4-wheel steering)

[NeverDun]

I searched 4 wheel steering on this forum awhile back, and didn't really come up with anything besides people saying "Why bother, it's just more stuff to break". I agree. But idea just wouldn't leave me alone.
I wanted to make this as simple as possible for anyone with basic tools to accomplish if they wanted to repeat what I did. This means use as much existing off the rack Arrma parts as possible. So I assembled all of my spare parts, and a number of others collected from kit breakers.

I tried a variety of different parts and configurations and came to the conclusion that the only reasonable way to do this without a significant re-design and multiple custom components, was to leave the suspension and steering geometry alone. Any switching out of lower hinge pin blocks in an attempt to retain the locking features that mate with the chassis resulted in terrible bump steer characteristics.
The only acceptable results came from literally taking the front end as a whole and sticking it on the back. It's surprisingly easy to do with very few permanent modifications to a couple of parts:

-Either the chassis or hinge pin plate (FF) need to be carefully notched out with a file to lock into one another.

-The rear droop tabs need to be cut at an angle to fit in the arm pockets. I used a Dremel with a fiber reinforced cutting wheel for this.

-Front shocks and shock tower need to be used unless you make a custom rear tower. At least with the Kraton, you can't drill holes in the rear shock tower for the upper hinge pins. The holes partially interfere with the existing tie rod holes.

-A plate mimicking the front of the chassis is needed to brace the pick up points for the steering posts, and servo mount. The rear of the chassis is narrower than the front, and the servo would hang off of the side without a plate. The rear has no kick-up, so everything is angled upwards towards the center diff. The plate can be easily made by using a transfer punch through the existing mounting points at the front of the chassis on to sheet metal, and tracing the general shape. Then bend the opposite angle (making the servo mounting surface parallel to the chassis) and cut away excess material to lighten it up.
I drilled one hole on center and bolted through the horizontal section of the adapter plate (next to the servo) with a spacer (a nut) between the nut and the chassis to keep the plate stiff and level.

-The rear chassis brace will have to be custom. I found no simple solution for the original rear brace. Some custom solution will be necessary.

I am almost finished, except for the rear shock tower and rear brace. For the rear shock tower I have concluded I will just have to make my own, which is easy to do with a transfer punch and some aluminum plate.

I am currently waiting on a Futaba 3PV to arrive with servo mixing so I can actually hook everything up and start playing with it. In my mind, the ideal situation would be to still have steering authority under full acceleration even with the front wheels off the ground. Of course there are a variety of other fun things you can do in the air, and with crabbing and such. This may be one of those things that just works better with an air transmitter, so you can do anything you want any time you feel like it ?.

 

[SrC]

Nice project. Good creativity. I'm all for the effort. A $custom$ chassis would solve this. A mirror image of the front at the rear without the kick up. Then all the OE parts would fit. ST top plate, bellcranks, etc. Good luck.

 

[NeverDun]

Thanks SrC! At the beginning I am trying to stay as low tech with this thing as possible to see how far you can get with basic tools most people would be able to afford. To be completely honest, even a custom chassis could easily be made using tools I have used in my garage for very little money:
Workbench
vice (preferred, but not required)
Clamp(s)
Transfer punch set (and hammer)
Metal scribe
Drill
Bandsaw (or hack saw)
File set
jig saw or coping saw (for inner cut outs)
Even with the mirrored chassis, the lack of kick-up means that the FR hinge pin block, steering posts, and steering brace all tilt up at 8.3 degrees:
https://www.arrmaforum.com/threads/proper-kick-up-angle-for-k8s-chassis.16519/.
That means they are floating in mid-air. So a custom chassis either needs a wedge block to do what my metal plate does, or be 3 dimensional and have the wedge machined into it.

 

[NeverDun]

I got the custom rear shock tower done last yesterday.
It just takes some patience, and good layout technique. Anything flat with holes in it is very easy to duplicate. Came out pretty decent. The most difficult part is getting the interface between the tower and the differential mount just right, because it keys in pretty tight and provides support for the upper hinge pins and shock mounts.
I clamped the stock Kraton shock tower down on an aluminum plate, and used a transfer punch to locate the holes for the shock pick up points, and diff mount holes. then I scribed the profile of it onto the plate.
I then switched the rear tower with a front tower, and punched the locations for the upper hinge pin holes. this gave me a hybrid front/rear shock tower.
Once I got the shape roughed out, I filed up to my scribe lines, and drilled the holes that were transfer punched with the correct drill sizes.
then I bolted the stock shock tower and custom one together through the diff mount holes, and carefully filed the diff mount contour to match just right.
I put it all together, and had a bit of a face palm moment. Shock hole location on the front arms is much further inboard than the rear arms. So the rear shocks are still too long. The shocks bottom out about 1mm before the chassis hits the ground. So I may be in the market for some front shocks at this point. I don't really want to run a taller rear tower, because it already sticks out pretty far and is susceptible to damage. But at the very least, maybe this will help inspire someone to make their own towers. You could do this with 7071 T6 sheet and have significantly stronger shock towers for very little money.
I can still drive it with the shocks that are on it for testing, and order some front shocks later. looking at the goemetry, it looks like the rear will be too soft now, and require heavier oil and springs to compensate. This will put more load on the shock bearings and mounts. Oh well, on with it!
My next test is seeing if the front chassis brace from the talion (longer) will be an acceptable replacement for the rear kraton brace. My Kratalion will be donating it's stock brace, and receiving one from hot racing

 

[CrashBoom]

Really cool, can't wait to see how it turns out!

 

[LibertyMKiii]

Nice project. Good creativity. I'm all for the effort. A $custom$ chassis would solve this. A mirror image of the front at the rear without the kick up. Then all the OE parts would fit. ST top plate, bellcranks, etc. Good luck.

I believe the kick up angle is necessary to keep the suspension geometry original. Maybe a 3d printed wedge of some sort would solve the issue.
I ran into this same problem on my Unlimited build with the flat carbon chassis. My solution was some aluminum cut at an angle and some bearings to support the steering posts that were floating in the air.

I am currently waiting on a Futaba 3PV to arrive with servo mixing so I can actually hook everything up and start playing with it. In my mind, the ideal situation would be to still have steering authority under full acceleration even with the front wheels off the ground. Of course there are a variety of other fun things you can do in the air, and with crabbing and such. This may be one of those things that just works better with an air transmitter, so you can do anything you want any time you feel like it ?.

I believe you could pull this off with the higher end radios like the 4px or 7px (for a price)

 

[NeverDun]

I even thought about just kicking up the rear of the chassis too (it would have made my life a heck of alot easier), but it gives you the same amount of caster in the rear as you have in the front, but positive instead of negative. It would also give you a bunch of anti-dive in the rear which would make the car transfer weight to the front really well under braking, but probably make the rear end squat really bad under acceleration. As it sits now, there is still some positive caster in the rear, but it is lessened by putting all of the caster shims to the front, and mounting the assembly flat on the chassis. I searched for quite awhile on the internet and there is little to no information on caster settings on rear-steer cars. I think any caster positive or negative just puts more stress on the steering system without any real benefits to traction. I dunno, I'm just guessing here.
I thought about using one of my plane TX's for this car, but throttle control is much better with the trigger than the thumb. My mind wanders to a trigger style TX with one more control axis for the rear steer using the thumb on your trigger hand.

 

[NeverDun]

I tried to make the front talion chassis brace work, but there's to many variables that get in the way of it being effective. It needs to be modified to clear the rear driveshaft, the rear sits really low and at the wrong angle, and it interferes with the servo mount. I may just run a brace to the differential mount. My Futaba 3pv came in, and to my disappointment the receiver it comes with isn't compatible with the 3PV's 4WS mode. The description on the website is pretty vague about what combo of TX/RX are needed for specific functions. Turns out after reading the manual, I have to purchase the Futaba R304Sb 2.4G FHSS Telemetry Receiver to have 4WS. So another 60 dollars for another RX ?. I guess I'll just put the original R203GF in the Kratalion and stop using the FLYSKY GT3B all together.
I also ordered some front shocks from Jenny's RC, and a diff to diff brace just to see what it looks like. It was only like 3 bucks.

 

[SARRT8]

Great progress can’t wait to see more.

 

[NeverDun]

I finally got a chance to mock up and make the rear brace this afternoon. I started with good ol cardboard and scissors, and cut out a shape that seemed to line up and clear the driveshaft. I cut a notch in it with the proper angle for the rear kick up. That was transferred to 1/4 inch aluminum plate, and roughed out with the bandsaw and hacksaw.
I just kept cutting/filing away more material, check fitting, and repeating until it fit just right. Once it fit, I matched drilled the steering brace hole and drilled and tapped that to M3. With the top mounted, I could center and mark the spot for the M4 from the bottom of the chassis. The bottom hole was drilled and tapped, and the bracket was ready for final sanding and buffing with a Scotch Brite pad. I'm pretty happy with the result for an initial prototype. Now I'm waiting for the fancy RX to show up in the mail so I can start fiddling with setup!
More pics

 

[NeverDun]

Sorry for the lack of order with the pics. Does anyone know if this interface has the ability to re-order the attachments?

 

[Slevin]

This is quite interesting. Can’t wait to see how this runs!

 

[slick2500]

Sorry for the lack of order with the pics. Does anyone know if this interface has the ability to re-order the attachments?

You have to upload them in the order you want unfortunately.

And this is a nice build.

 

[NeverDun]

You have to upload them in the order you want unfortunately.

And this is a nice build.

Thank you!

 

[Mongoose]

Finally! Someone is doing 4WS!!! That’s how I had envisioned it too. How does it handle in the air?

 

[NeverDun]

I'll let you know as soon as it's running!

 

[Sixes]

This is a very interesting build
Well done so far?

 

[NeverDun]

Gracias!

 

[Mastaa]

Finally! Someone is doing 4WS!!! That’s how I had envisioned it too. How does it handle in the air?

Already seeing flips with a 360° rotation combined mid-air

 

[NeverDun]

Yeah, I have no idea what will happen mid air. I'm also a bit embarrassed to admit that I'm new enough to this that I'm sure I'm building something beyond my driving skill level. I've messed around on some tracks with the Kratalion, but I haven't even tried to flip it yet. I guess I need to find a good ramp somewhere and practice.