Skid Plates--Why Not UHMW Polyethylene?

[Dante]

They make UHMWPE skid plates for 4x4s. The stuff is very impact resistant and is very slippery. It's also lighter and more corrosion resistant than either steel or Aluminum. The only downside I see is UHMWPE skid plates tend to be pretty thick. You'd probably want 1/2" or more for a TDI plate, which is a lot when you are only starting with 4 1/2 inches of clearance.

 

[FlyTDI Guy]

It's not real good in the heat. Weight is .034 lb per in³ vs .095 (avg) for aluminum. At twice the thickness of alum, it'd still be lighter. It's definitely more flexible than alum. Though it may not break, I think it would probably deflect and impact the pan more often than steel/alum. The steel ones have extrusions that add structure and really strengthen things up. Pricing could effect things too.

 

[Powder Hound]

At 1/2" or more, it sounds more like a rubbing block than a skid plate. But the real concern is that noted by flytdi guy. The desire is to keep a road object from breaking the oil pan.

Such a conundrum!

 

[Ski in NC]

That stuff is like coagulated grease. It has very low strength. I don't know what the strength numbers are, but they are super low. Even if thick, an impact in the middle of the plate will cause it to deflect upwards and impact the pan. Pan still will get crunched.

 

[ravent]

What about simple HDPE...besides, if the impact was strong enough to react with all the above, you should have seen it coming and NOT go there. HDPE is what most of those plastic panels inside the wheel wells are and if they were thicker they would resist most impacts. The skid plates are NOT made to be bullet-proof, just to protect from occasional rocks and road debris that at 70 mph (or even 130 without the speed limiter removed) is hazardous to our cars.

If you made it 1/2" thick (I don't it's needed) there isn't enough weight on the car to deflect it.

 

[K5ING]

What about simple HDPE...besides, if the impact was strong enough to react with all the above, you should have seen it coming and NOT go there. HDPE is what most of those plastic panels inside the wheel wells are and if they were thicker they would resist most impacts. The skid plates are NOT made to be bullet-proof, just to protect from occasional rocks and road debris that at 70 mph (or even 130 without the speed limiter removed) is hazardous to our cars.

If you made it 1/2" thick (I don't it's needed) there isn't enough weight on the car to deflect it.

It's easy to say "you should have seen it coming and NOT go there", but in the case of road debris it's not that easy. I busted my oil pan on a very busy 4 lane boulevard a number of years ago. The object I hit had successfully passed under a few pickups and SUVs ahead of me, but my car was too low (stock height) to miss it. The road was busy, running along at about 45mph, and I had nowhere to go by the time I saw it pass under the SUV that I was following. Not without causing a major pileup that is. In the case of this particular object, I don't think a plastic skid plate would have been much help. Needless to say that I now have an aluminum skid plate on my car.

 

[scurvy]

I worked for a company what supplied UHMWPE parts to the railroad industry, including designing & engineering some of those very parts. I can see several issues, although lubricity is not one of them - UHMWPE is very slick stuff.

IMO the biggest issue is how you would make it. UHMWPE doesn't flow well enough when liquid to injection mold it. You could post-process already molded sheets of the material (Quadrant sold 4'x8' sheets, just like plywood) but that may compromise the other material properties. Price out a compression mold big enough to make a skid plate and let me know how many hundreds of thousands of dollars it would be (hint: many). Not to mention the processing time required to get it fully cured with little porosity, you might be able to mold two skid plates a day.

Another issue worth mentioning is that while it is truly amazing, excellent stuff for wear resistance, it doesn't have a high fracture toughness - you'll never wear through it but a sharp nick could set it up to violently crack. Consider what happens to the normal skid plate - you hit small stuff fairly frequently which make longitudinal scratches & scars in the plate, and eventually hit something big. The scratches already in the plate could be a large enough stress riser to cause it to neatly crack in two if subjected to an impact load, such as K5ING's trophy.

Also when subjected to a compression load, it creeps and cold flows over time, so the areas captivated by fasteners would eventually squash right through and the plate itself could sag in the middle.

UHMWPE is an excellent material for wear resistance in a compression loaded, preferably captivated environment, but for a skid plate it's hard to beat steel or aluminum.

 

[PaulGiz]

It is a very good material for a skid plate. That thing we add to our VWs aren't really skid plates even though we call them that. We're not looking for slidey-slidey, we're looking for impact deflection (bumpy-bumpy). Two different functions.

It is a very bad material for impact deflection. It will deform and impact the oilpan, only it will act as a spacer allowing damage from objects 1/2" shorter than before.

P.

 

[Dante]

To clarify, I have had the OEM steel skid plate on my car for nearly as long as I've owned it. My interest in UHMWPE arose from the idea of using UHMWPE skids as a sacrificial layer to save wear and tear on my steel plate.

The idea of using UHMWPE for a TDI skid plate came from seeing UHMWPE skid plates on ATV, UTV and 4x4 web sites. However, as the poster above has noted, those skid plates are intended primarily to make the vehicles slide over obstacles more easily. TDI "skid plates" are really sump protection/armor.

That said. I still think a TDI skid plate could be made out of UHMWPE--it would just have to be reinforced with metal (probably steel) to make it stiffer and provide the impact resistance required to protect the sump.

UHMWPE may not be the right material for this application, but it is pretty incredible stuff. I have a few 4" cubes I use as reactive targets, and they hold up to being shot repeatedly with a high powered rifle remarkably well.

 

[Ski in NC]

You could screw strips or a thin layer to the bottom of the steel or al skip plate. It would definitely help you slide over offending objects. Seems like alot of work for marginal benefit. That plastic will get torn up often and drag on the road.

 

[Nihilator]

To clarify, I have had the OEM steel skid plate on my car for nearly as long as I've owned it. My interest in UHMWPE arose from the idea of using UHMWPE skids as a sacrificial layer to save wear and tear on my steel plate.

Ah, well, there's your problem. You're looking at it the wrong way. You should be showing off the dents and scrapes on the OEM steel skid plate. Battle wounds are sexy.

I agree with the previous posters that UHMWPE would be a poor substitute for steel, from a defensive perspective. I, too, have run over a large, solid, steel object and destroyed my oil pan. I, too, have installed an OEM steel skid plate to prevent that from happening again. That damn thing feels integral to the frame now, and I would be shocked if it ever lets the oil pan take a hit again. UHMWPE would NEVER defend as well as steel.

--Chris