PROJ.

Ramshackle

A double-wishbone suspended, Ackermann-steering electric longboard

Project Ramschackle draws heavy inspiration from the engineering marvel that is the Baja Board. A true aesthetic treat for the eyes, the Baja Board boasts a masterfully integrated gas-shock system and double-wishbone suspension that are as functional as they are beautiful.

While the Ramshackle can never hope to rival its shining idol, the intangible takeaways from the process of attempting it are the real good stuff. Creating the Ramshackle put me through the full gauntlet of prototyping: from part sourcing and speccing, engineering drawing and GD&T, designing for manufacturing and so much more.

A major regret of mine is not having taken more videos and pictures of this project before I decommissioned it. The media you see on this page don't tell its story all that well, and I apologise for this. Nevertheless, this wild ride truly lived up to its name, both pre- and post-development, and I hope you enjoy it as much as I did!

Design Overview

(Click render to toggle exploded view)

The Ramshackle's body consists of 2 tiers: the top tier being the deck, and the bottom tier being the chassis spine. The two tiers pivot with respect to each other to enable Ackermann steering: the top tier extends an arm per pair of wheels downward past and between the spinal extrusions to control, each, a set of steering arms. The entire body floats on a double-wishbone suspension system. All major structural parts are made of bent 5052 aluminium sheet-metal, and all linkages are 20mm-diameter 6061 aluminium rods with female threaded ends to accomodate M6 rod-end bearings.

Power flows from a re-purposed 22.2V 6S 10000mAh flat-pack LiPo battery through a 100A-rated on-off turn-switch to an also re-purposed HobbyKing X-Car Beast Series ESC which connects directly to the RC receiver and the rear-wheel Turnigy Aerodrive SK3 - 6374-192KV out-runner BLDC motor. Each driving wheel is connected by a HTD 3:1 pulley system to the motor shaft.

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In Action

Key Features

Safety Features

Spherical casters below the chassis spine allow the Ramshackle to roll away safely (or at least it's supposed to) in cases where the body ever makes direct contact with the ground. Such cases include excessive impact bottoming-out the suspensions or having a wheel so decide to rudely bail on you mid-ride. Ramshackle is a primitive first prototype, and budget constraints really put a damper on concerns outside of just making the whole concept work in the first place. In light of this, safety failsafes are absolutely critical.

Cost-effective Design

90% of all structural parts of the Ramshackle are made from bent 5052-H32 aluminium sheet-metal. Sheet-metal cutting and bending was (and still is) the most accessible and cost-efficient out-sourceable metal part manufacturing method available to me as a casual hobbyist. Designing everything in sheet-metal was a great challenge, as load-bearing capability and thickness bendability had to be delicately balanced. Static finite-element analysis (FEA) was heavily employed to optimise part thickness while ensuring strutural integrity.

Why 5052-H32 though? 5052-H32 aluminium lends itself well to sheet-metal bending as it is more formable than its ubiquitous 6061-T6 counterpart, albeit for a negligible compromise on tensile and shear strength and a slightly hiked price. 6061-T6 (yes, the temper makes a big difference to the alloy's properties) tends to exhibit small tears around sharp bends, which quickly develop into fractures under repeated impact. For the same thickness, a sheet of 5052 can be bent to smaller bending radii than an equivalent 6061 with minimal outer-bend tearing, making it the better choice here.

In more technical depth, 5052-H32 has a fatigue strength of about 120MPa, which is 30% greater than that of 6061-T6's 96MPa. This means that under cyclic loading, 5052-H32 far out-performs 6061-T6 in the long run, and cycling loading is mostly what the parts of the Ramshackle undergo.

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Custom Adjustable Shocks

Shock absorbers of suitable dimensions, stroke length and force are either way outside of my budget or non-existant. I figured that the working principle of a shock is actually pretty simple, so I hacked together some low-cost components to create a custom, adjustable shock assembly that actually worked amazingly well (for the abomination that it is)! Each homebrew gas-shock is comprised of a car boot gas-spring with removeable rod ends, a spring slipped onto the gas-spring's piston rod, and multiple M6 nuts used as spacers to control the spring's pre-compression and thus the shock's preload.

A major advantage of this homebrew gas-shock in this largely experimental project is in its affordable re-configurability. The gas-spring alone can be purchased commerically off-the-shelf (COTS) with different force and damping profiles, and the springs are available in granular specifications and swiftly procurable for less than a dollar a pop. This allowed me the budget room for 3 iterations before settling on a satisfiably optimal configuration.

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Ackermann Steering Geometry

Ackermann steering geometry allows each wheel to follow their own turning circles, reducing the axial forces the wheels endure during turning. This feature is especially critical for the Ramshackle, as its control arms are particularly suspectible to failure by such axial forces (which they ideally should not be).

One of the greatest challenges encountered in the Ramshackle's design was in figuring out how to translate the longboard rider's leaning motions into the action of an Ackermann steering system. The two-tier body design was developed as a reverse-engineering guess at how the Baja Board accomplishes this--a guess derived from ambiguous online pictures at the time when the Baja Board was still in its early pre-production stages.

The top deck is intended to tilt with the rider's lean and control a set of steering arms per pair of wheels, while the lower tier remains parallel to the ground at all times. Note that the entire body floats on the suspension system, making keeping the lower tier level at all times while accomodating for dynamic movement non-trivial.

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Retrospect and Revision

Suspension Control-arm Design

Shortcomings

A pretty major caveat of the current control-arm design lies in its impermanent assembly of multiple parts: the two struts on each arm are secured by anemic set-screws to a common block that holds them in the "A" shape at the apex.

During operation, control arms undergo large tensile and compressive forces. In this case, there is a real risk of the set-screws shaking loose and the struts slipping out of the common block completely--a catastrophic failure. If the control arm does not fail by that mode, the stainless steel set-screws shear the softer aluminum surface and allow the rods to slip bit-by-bit over time anyway.

However, it must be said that this design choice was (unfortnately) intentional within my constraints, and for good reasons: 1) Being unable to afford trial-and-error in production, capacity for adjustments and re-assembly was THE top priority; 2) The individual parts of the control arm are collectively very much cheaper to produce; 3) I had a plan-B safety feature designed into it to counter such a failure anyway, and I was willing to take this risk as the only one daring enough to ride this jank.

Potential Upgrades

With a generous budget, it is ideal that the control arm be milled completely out of a single block of raw stock. Not only will that allay the worries of spontaneous unintentional disassembly during operation, the design freedom it presents will allow for an integrated spherical joint within the control arms that interface with the knuckle, which is a more elegant solution to the current bolted rod-end bearing.

However, milling does entail a lot of material wastage, and even with a large budget, it is not very material- nor cost-efficient. A better option--and one that is actually more realistic given my circumstances--is designing it out of weldment members and welding them together. Not only will there be minimal stock wastage, the process of manufacturing a weldment control-arm requires access only to a metal-cutting circular saw, a welding machine (preferably a MIG welder for a newbie such as I) and a working space I can smith freely in without worry about endangering furniture. A weldment control-arm would be the way to go if those were at my disposal, and I do hope that one day they will be.

Going with the weldment option: 5052-H32 square tubes do exist, and material-wise it is easier to weld than 6061-T6, but for structural integrity at the compromise of added weight, carbon steel square tubes would be a better option. In particular, low-carbon steels would be ideal, as high-carbon steels are more prone to weld-cracking and require more pre-heating and post-heating processes to avoid this. The greater strength and stiffness of high-carbon steels is excessive for this purpose anyway.

Wheel Knuckles

Shortcomings

The wheel knuckles, having to be made out of sheet-metal, came with various troublesome design constraints: it is ideal to have the kingpin axis as close to the wheel as possible, but under the sheet-metal design constraint, there was a significant minimal distance the wheel had to be from the knuckle. Consequently, the wheel does not pivot about a vertical axis that is coincident with its plane, increasing wheel scrub and axial forces on the wheel and control-arm assembly.

The wheel axle also experiences larger-than-necessary bending moments due to awkward bearing placements, where the wheel axle extends further than needs to be from the knuckle, and is supported by two admittedly sub-optimal (but budget-wise very agreeable) UCFL bearings on either side of the knuckle. The large bending moments severely increase radial and moment loads on these bearings, decreasing their operational lifespan and hurting performance.

Potential Upgrades

Instead of a clumsy sheet-metal design, a generous budget would accomodate a milled, block-style knuckle. Unlike the control arm, this block-style knuckle can easily be designed for minimal material wastage when milled from raw stock, making milling the ideal method of manufacture for the improved design I'm holding in my mind. In machining this new design, 6061-T6 would be used instead of the current 5052-H32 alloy: 6061-T6 is more machinable than 5052-H32 as it has a higher shear strength (among other factors), making it less "gummy".

This block-style knuckle will present a significantly reduced spatial footprint and would be able to fit snugly up against and inside the rim of the wheel, much closer to the wheel's plane. This would allow the wheel to pivot about a kingpin axis much closer to its wheel plane. Despite a smaller size, it will allow for two in-set bearings spaced further apart along the wheel axle to reduce the radial and moment loads on each bearing. Instead of radial bearings, thrust bearings may be used as well, although that may be a bit excessive for the forces in consideration.