Aluminum vs Steel Skid Plates and Armor – Deep dive into vehicle protection

0 2,797

We decided to take a deep dive into skid plates and armor to check out some of the differences of Steel vs. Aluminum skid plates and how their properties and designs can protect your off road vehicle. From Rock crawling to trail riding skid plates are designed to protect your vehicle and give you a piece of mind. We dive in and dig into all the gory details to give you some guidance on gearing up for your off road mission.

Off Road Mission

If you’re like me and out researching choices for skid plates and armor for your vehicle, you most likely have a mission in your mind that you’re preparing your vehicle for. In my case, I love off-roading in the San Juan mountains and jaunts to other remote locations such as Moab or my nearby off road playground Hidden Falls Adventure Park.

For me, my mission isn’t that of hardcore rock crawling, but of more trail riding and back country driving. When out on these trails my experience suggests that one of the most common issues I’d like to defend against are shear forces.  I’ve noticed when out on the trails when I feel I’m bouncing around the most that lateral movement across a surface during acceleration/slippage or lousy line or sliding off a rock (or the rock sliding out from under me). I also seek out something that is tough and durable – so to achieve these goals I started to research the materials science quantification of what is considered sturdy and durable.

Skid Plate Engineering

When it comes to these materials, the engineering choices are what should drive your gear selection. When we did our research we found steel skid plate armor is often 3/16″ and aluminum is 1/4″ thick. In some measurements,  This thickness gives the materials, in essence, a “Same Standard” in that for some use cases the desired result is nearly the same. I’ve reached out to a few manufacturers and vendors to ask some questions on their engineering design decisions but until I get a response, I did some research on describing the material science differences of the two materials used in Skid plates.

I’ve broken down many of the properties engineers used in developing armor solutions and have tried to show how they relate to skid plates.

I have linked to products from a single vendor to show the weight/price differences. These are affiliate links and any purchases made through these links will support us creating more content.

Disclaimer: I’m not a materials science or certified engineer – This post is the result of research on the topic of aluminum vs. steel skid plates. Please let me know if you find any issues with our descriptions and or math, and we’ll update the article as needed. (Register to comment below!)

Engineered properties

Geometric Stiffness – the geometry of the shape and how it impacts stiffness – will the part bend?

Structural efficiency – the mass a structure can hold divided by the bass of the structure.

Hardness – the relative resistance that its surface imposes against the penetration of a harder body

Strength – the amount of force a material can withstand and still recover to its original shape

Toughness – the amount of energy that a material can absorb before fracture.

Stiffness – A mechanical property of linear elastic solid materials.

Deflection – Beam or load condition of a material with a fixed end.

Geometric Stiffness of skid plates

Without actually having the engineering diagrams and formulas for each type of skid plate to calculate against, I can only speak to this as a general reference that benefits both aluminum and steel plates. The geometry of the plates should be designed in such a way that the strength, toughness, geometric stiffness and structural efficiency are distributed not only amongst the armor but through the vehicle.

The geometric stiffness of the product should be designed equally into each product whether its steel or aluminum. Both will fail with improper geometric properties (and installation).

Steel has a slight benefit when it comes to parts that need to be thinner as the stiffness and strength will be higher than aluminum.

Structural Efficiency

With regards to structural efficiency – the ratio of a material’s stiffness to its density, aluminum is shown to have a structural efficiency much greater than steel. An aluminum plate of equal thickness will weight 57% the equivalent steel plate and designed to the same stiffness; an aluminum plate will weigh ~ 45% of the same steel plate.

If your resting on an object and the mass of your vehicle is resting on your skid plate then the structural efficiency by weight scales in the factor of aluminum but thickness it scales in favor of steel.

Hardness of skid plates

One way to measure the hardness of the material is based on the materials “Mohs scale.” The Mohs scale represents the resistance the surface imposes against the penetration of a harder body. Hardness is an excellent quality to have for something such as a skid plate for your fuel tank since your goal should be to resist puncture and have fuel to get home safely.

Mohs scale of hardness: 

The Mohs scale of mineral hardness is a qualitative ordinal scale that characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material

  • Aluminum Hardness = 2-3
  • Steel Hardness = 4 to 8

As you can see the hardness of steel is twice the hardness of aluminum at equal thicknesses. This is one area where aluminum plates are engineered to be thicker than steel.

When it comes to hardness, steel at same thickness is the harder material but aluminum being lighter can be made thicker and have a better weight benefit. The choice is here a ton of protection at a higher weight or some protection at a lower weight. If you’re mostly scratching your JKU tank because of the longer wheelbase, then Aluminum probably affords enough protection, but if you’re dragging across rocks in extreme rock crawling, I will go with steel.

Strength of Skid Plates

What would be some characteristic properties of engineering skid plates? For protecting a fuel tank, you may be interested in shear strength and impact strength and to some degree the yield strength of the material.

Shear strength:  Think of shear strength as the ability of the material to withstand being cut across like scissors, the strong the shear strength the harder it is to shear the materials in its cross sections.

Impact Strength: This is the amount of energy absorbed before fracture.

Yield Strength: The yield strength of a material is the point at which a material begins to deform plastically. Prior to failing in such a way said material would return to its original shape when the applied stress is removed.  Once the yield strength has been surpassed some fraction of the deformation will be permanent and non-reversible.

Shear Strength

Shear strength is the strength of a material or component against the type of failure where the material fails in a shear.  In structural engineering, the shear strength component is important for designing the dimensions and geometry of the materials used.

  • Steels  – 0.75 UTS (Ultimate Strength Relationship) and .58 TYS (Yield Strength Relationship)
  • Aluminum – .65 UTS and .55 TYS

UTS is the maximum stress above which the material will break

TYS is the stress at which a material deforms permanently materials sciences ultimate strengthIn the case of Aluminum vs. Steel skid plates the shear strength of the materials and geometry of the equipment seems to be the primary design difference as the materials are nearly equal in relational UTS and TYS when you figure in the thinner steel vs. thicker aluminum and geometry of the design.

Impact Strength

In general, for a given volume of material, steel will absorb more energy than aluminum.  For a given volume, aluminum weighs only 1/3 as much as steel. If you could afford 2/3 more aluminum (thickness), then you could exceed or meet the impact strength of steel. However, this isn’t optimal for clearance of off-road vehicles.

Yield Strength

  • aluminum yield strength: 35mpa (megapascal) – 5076.3195 PSI
  • steel yield strength: 205mpa =  29732.7285 PSI

Steel has nearly 6x the yield strength of Aluminum.  In this case, the geometry and thickness of aluminum would have to play a greater role to protect your vehicle in the case of yield strength tensions.

Stiffness

In simple terms, a stiff material needs more force to deform than a soft material. Youngs modulus is the measurement of a materials stiffness.

Youngs Modulus:

  • Steel 205 GPa
  • Aluminum 70 GPa

The Shear modulust represents a materials rigidity.

Shear modulus of 1018 steel vs. AA 2024 – Carbon Alloy vs. Aluminum Alloy

  • Steel: 11603 ksi
  • Aluminum 4029 ksi

A material having a large shear modulus is difficult to bend.  Differences in shear modulus can be alleviated by the geometric properties of the skid plate as well through better supports, transfer of stress and strain or even by a designed failure of the part.

Toughness of skid plates

When it comes to resisting fracture, steel has a higher tolerance as seen in the graph below. Aluminum can be made thicker to increase the toughness but has to be exponentially thicker to achieve parallel toughness.  On an off-road vehicle, you wouldn’t necessarily want to trade off thickness for toughness since often clearance of your vehicle over obstacles is superior to skidding over obstacles, to begin with.  If you already have great clearance and fracture isn’t a concern, then this property may not be part of your mission. I would also add that some people choose to accept fracture as a means of protection in that the skid plate failing is doing its job vs. having your vehicle disabled. Now if you do something that impacts both your plate and vehicle then steel would offer the best protection.

 

toughness of metals

Skid Plates – Deflection

One formula to understand the engineering of aluminum vs. steel is the beam deflection calculator. This treats the stress as if the item is a diving board, stress at the end of an item. This isn’t much an issue in most installations, but I have seen armor for drive shafts and axles that may fit this paradigm.To keep the math easier this was calculated on a 20″ by 20″ by .25 (1/4) inch piece of aluminum vs. the standard size of 20″ by 20″ by 3/16″ (.1875″) for Steel on a 4000 lb load.

To keep the math easier this was calculated on a 20″ by 20″ by .25 (1/4) inch piece of aluminum vs. the standard size of 20″ by 20″ by 3/16″ (.1875″) for Steel on a 4000 lb load. (calculated on WolframAlpha)

Aluminum deflection:

  • Deflection (inches) – 40.96
    Bending Stress (psi) – 384000
  • Energy (joules 9255.62

Steel deflection:

  • Deflection (inches) – 32.36
    Bending Stress (psi) – 682666
    Energy (joules) – 7313.0notes:

If deflection is a concern – such as armor that doesn’t connect at both ends but rather has a support structure that extends out then steel is by far the best and only choice.

Aluminum vs. Steel skid plate Costs

In comparing products from the same vendor (to remove variables of brand cost differences), aluminum skid plates have a higher overall cost than steel skid plates.

According to fueleconomy.gov, 100 lbs of weight can make your car 1% less efficient when it comes to gas mileage. On a Jeep which is already a brick driving against the wind most of the time, I believe this number is slightly larger.  Let’s say adding aluminum skid plates saves you 50lbs over steel skid plates and you plan on driving your vehicle for 150k miles.  Saving .02cents/gallon

Using EPA averages (which are hard to meet) let’s say you get 18MPG.

150,000 / 18 =  8,333 gallons of gas over lifetime = $167 dollars in gas savings

Over the life of the car, the 50lbs may not add up to much since the cost of aluminum just about surpasses steel by as much as the total savings. However, if you do the above math on a more reasonable and real ~15mpg efficiency of Jeeps, it will slightly favor aluminum even more. (or if fuel prices go up)

Where weight may have a bigger cost saving is in affording you weight in other areas such as bumpers/gear or reducing stress on components such as lift and shocks preserving lift heights and such.

Aluminum vs. Steel skid plate Corrosion & Fatigue

Aluminum is subject to fatigue failure (its endurance limit) wherein steel fatigue is not normally an issue with the exception that both have limits around engines. (vibration). One would normally mitigate this by providing cross supports and bracing/connections to reduce deflection & fatigue.

An advantage of aluminum is that aluminum will not rust and doesn’t need a touch-up painting, however, it will corrode, but such corrosion creates an oxide barrier which prevents further corrosion. If you don’t touch up paint on scratch steel, the steel will rust and corrode possibly leading towards early failure.

Aluminum vs. Steel skid plate Wrap Up

All said and done; I believe aluminum skid plates will work for my mission to help provide protection for critical components on my Jeep to reduce the possibility of being disabled while out on remote mountain trails.  If your mission is hardcore rock crawling you expect to drag/slide and bounce your vehicle over rocks, then steel plates may still be your best option.  The reduced weight of aluminum, the shear strength and improved hardness and toughness over not having anything to me is better than having nothing and optimal for my mission in weight savings and ease of maintenance.

I’m personally excited to see expansion into aluminum and hope material sciences will continue to improve to reduce weight, improve efficiency and safety of vehicles. I’m not entirely sure its worth the extra cost vs. steel, but I will say an engineered aluminum solution does offer protection and an overall weight saving and with a lower weight, ease of install is a nice thing too.

In the end though, its a tough call and more of a personal choice. In some respects, the steel seems to win hands down when it comes to overall engineering properties on paper, but in reality, it may be better to have a skid plate fail in some cases rather than have that potential energy transfer to the vehicle – for example it may be better to shear off a part of your skid plate at extreme stresses with an aluminum plate than it would be to absorb said forces on a steel plate and potentially break off bolts/connections/rods.  In other words, the failure of the skid plate may, in fact, be the skid plate doing its job.

Again, I’m not a trained engineer. I would love some feedback so please post your experiences and thoughts in the comment section below. Registration is quick & easy. I’ll also try and update this post with more direct links to Wolfram for the actual maths for those who want to experiment around.

One set of skid plates I wouldn’t go aluminum on but recommend possibly as your first set of skids for any just-starting jeep is the control arm skids.

Safe Trails!

Leave A Reply

Your email address will not be published.