Jeep driving over rocks

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 Aluminum vs Steel Skid Plates 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.

 

Aluminum vs Steel Skid Plates toughness

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!

7 COMMENTS

  1. Great evaluation between the Aluminum and Steel. I have been doing similar research but don’t understand why the mfr’s don’t run testing on the two products. I don’t want to add to large of a lift and upset the factory settings on my 2017 Rubicon so keeping it as light as possible really makes a difference. In the end, I haven’t made a decision between the two, and I might just do a combination of both. Steel to the areas that receive the most impact (after a few more trips out I should have that figured out) and aluminum where little to no impact occurs.

    • Thanks for the feedback! I’m in the same boat, I’m not as interested in massive changes to my vehicles geometry and I don’t really want to absolutely kill my fuel efficiency either. I think going aluminum for some protection and steel for other pieces is a good mix. I’ll plan on updating this post as I add pieces to my vehicle and gain experience.

  2. I like that you pointed out that although aluminum plates may be thicker, they still weigh less than steel, which can help give your a weight benefit. I also like that they area less expensive. My uncle lives in the country, so he’s been wondering which type of metal plates would best fit his needs. I’ll share this article with him, so he can make an informed decision.

    • I haven’t seen much consistency in the market. 2024 looks good because of its specs, but I see a lot of companies seem to use 5052 – I figure a lot of aftermarket companies are looking to try and keep the price competitive with steel pricing so their choice is based upon that. Aluminum is really starting to enter the market huge and it makes a lot of sense to minimize weight. Seems beyond weight, its cost cost and cost.

  3. I think your comparisons are heavily biased towards steel and you aren’t using the correct material numbers. First off, the 1018 steel any normal person is going to get their hands on will have a tensile strength of 63,000psi and a yield strength of 53,700psi. Even that’s not easy to find though and it’s more expensive so people are more likely to use A36 steel which has 58,000psi and 36,300psi respective strengths. Next it looks like you’re using numbers for an untempered or really weird aluminum alloy and not 2024 at all which is a pretty unfair comparison as well since your average consumer will be hard pressed to find 1/4″ plate that’s not heat treated. 2024-T3 which would be the most popular condition for 2024 has a tensile strength of 62,000psi and 40,000psi yield which actually makes it stronger than A36. Now to be more fair to steel, most people will use a heat treated version of 6061 or 5052. I’ll quit quoting numbers but 6061 is slightly weaker than A36 and 5052 is slightly weaker than 6061. When stepped up to equivalent thickness, aluminum will be much much lighter than steel though.

    Fatigue is pretty irrelevant for skid plates since heavy impacts will be the critical load. Unless you’re thinking about using sub 1/8″ thickness on a really wide full size frame, fatigue driving around won’t even play a factor in that vehicles lifetime. It’ll be that big hit on a rock that will limit the skids life.

    I’m not advocating one material over the other as they both have their place. However, this article is using a lot of misinformation to come to a conclusion that’s not very accurate.

    Unless you’re comparing flat skid plates, the design of the skid plate plays a bigger role than the material itself and unfortunately, most vehicles can’t run a flat skid without heavy modifications.

    • John, Thanks for your feedback, I enjoy hearing from readers! First of all, I don’t think your numbers or your point actually disagree with anything I said, I just may have spelled it out differently. Even amongst the different steels – the equivalency factor for steel vs aluminum doesn’t change by much (1.5x in general) and I’ll happily update the charts to reflect A36 vs 1018 or spell both out. (fair point, off shelf skids will be 1018!!). By this I mean even if you use 63/53 vs aluminum or 58/36 vs aluminum – the equivalency measured in measuring units is almost the same. I wasn’t meaning to be absolute, but show the materials properties. If I made this any more detailed, I’m not sure people would read it 🙂

      When I wrote this piece I largely assumed Off the shelf aftermarket components and standard Vehicles. There aren’t many design choices available since they’re often designed to fit the constraints of the vehicle and capability to add them without having to cut/weld/hack. If you want to engineer your own system and have a mission in mind then you can design to your heart’s content and change whatever you wish to have optimal performance for either material.

      I will be going aluminum + steel. Steel on control arms and diff cover (those are what i “bang”) and aluminum everywhere else. I know off the shelf aluminum without breaking the bank may mean I have to replace specific pieces that may fail – but if they do fail they may have failed doing their job (them breaking instead of my tranny or whatever) so I’m OK with that. I made an informed decision, knowing the materials properties, the reality of the aftermarket parts and made a better decision. it’s also nice not having to paint them and their lower general maintenance and rust-free convenience (which i can expand upon more than i did)

      Again, thanks for your feedback. If any of my data is wrong or there are some cool new skid plates or tech i’m unaware of, please link it!!! Otherwise, I don’t think we actually disagreed on anything. I went back and re-read my entire post and while I give some use-cases, I don’t think they spread misinformation or came to a conclusion that wasn’t very accurate.

      My gut says you work for a company that sells aluminum skid plates – if so, hook me up 🙂 I’ve been reaching out to companies to field test gear and even if I don’t get any offers I”ll be jumping on buying my own gear in the near future and you bet i’ll blog about it and update this post too! thx!