Front pivot wobble on my 4WD buggy

[DryMycologist6432]

Hey all,

I'm building a teleoperated 4WD patrol buggy as a personal project and hit a wall with the front-end stability. Hoping to crowdsource some "what would you do differently" wisdom before I start cutting more metal.

Build specs:

• ~150 lbs fully loaded (frame, 4x hub motors, batteries, electronics)
• Chassis is 4040 aluminum extrusion
• 4x brushless hub motors (one per wheel), ~20" wheels
• Wagon-style front pivot steering
• Linear actuator drives the pivot
• Pivot bearing is currently a lazy susan between two aluminum plates, bolted into the 4040 extrusion with T-nuts and through-bolts
• Runs on mixed terrain (pavement, grass, gravel) at low speeds (~5 mph)
• Tooling constraint: I only have a pillar drill and a miter saw. Would prefer solutions that work with off-the-shelf parts + aluminum extrusion.

The problem:

The front end wobbles badly. I've narrowed it down to a few behaviors:

1. FWD is the worst, RWD is slightly better. When I drive with only the front hub motors, the pivot plate visibly twists under load. Every bit of motor torque tries to rip the pivot sideways instead of propelling the vehicle. It genuinely looks like the bracket is going to snap.
2. The whole front subframe has some flex in addition to the pivot itself, I can see and hear the rectangular extrusion cage skew slightly under hard turns.

My questions for the community:

1. Are there any quick fixes to drastically reduce the front-end bend that stay within my tooling constraints (pillar drill + miter saw, aluminum extrusion, off-the-shelf parts)?
2. Is this rabbit hole of improving the pivot worth going down, or should I switch to Ackermann steering? If so, what's an easy way to do that with this frame?

Trying to be scrappy here, I want to ship the simplest thing that works before over-engineering. But I also don't want to get stuck in purgatory iterating on a doomed design.

Appreciate any and all takes, especially unsolicited roasts of the design choices.

Thanks!

 

[RunForTheHills]

I don't see how the lazy susan pivot with a single bearing for the steering wouldn't have a lot of slop compared to a steering tube going through a longer sleeve with bearings at the top and bottom like a bicycle headset. The forks should be one piece or welded with the crown instead of separate pieces held together with a single bolt and friction preventing them from moving. The whole frame is just not very rigid.

 

[DryMycologist6432]

I don't see how the lazy susan pivot with a single bearing for the steering wouldn't have a lot of slop compared to a steering tube going through a longer sleeve with bearings at the top and bottom like a bicycle headset. The forks should be one piece or welded with the crown instead of separate pieces held together with a single bolt and friction preventing them from moving. The whole frame is just not very rigid.

Yeah I see your point- thought aluminum extrusion would be pretty easy to prototype with but running into all these problems now so not too sure

 

[RunForTheHills]

Maybe go to a single wheel in the front using a bicycle fork? You could cut the headtube off of a cheap Craigslist bike or get a piece of pipe with the same inside diameter and mount a bicycle headset in it. Or you could use two bicycle forks with a tie-rod to sync the steering. You might need to get someone to weld some mounting brackets onto the headtube for you.

The rear forks could be reinforced with diagonal braces.

 

[robkl]

Instead of center pivot steering, you may want to explore side-pivot steering with tie-rod setup where you can adjust the caster angle of each side-pivot ( L and R ) to induce more self-centering. Followed with the correct Ackermann steering angles to reduce steering scrub as the buggy turns a corner. Some wagons sold at the Home Depot or Lowes use side-pivot steering for design ideas. And Ackermann setups can be found on go kart sites.