Kevlar skid plate

[02wrxRally]

So I've been looking at composites for model airplane building lately and it got me thinking about a skid plate. The only discussion of kevlar I found was here:
http://www.dirtyimpreza.com/forums/showthread.php?t=1027&highlight=Kevlar

I was thinking of sandwiching fiberglass between kevlar felt fabric (the kind used for canoe skid plates) using a block of pink foam from home depot as a mold in a vacuum bag to make a flat composite sheet with beveled ends. I figure the FG will give it most of the structure cheaply, and the kevlar outer skins will give it the impact resistance I'm looking for.

I've used fiberglass and resin before, so the process is familiar, but i've never worked with kevlar or made a stand-alone sheet; always used it as a skin. I'm wondering how thick I need to build it up, or if I should mold structural creases/indentations into the sheet, or use another material like foam for a core..


Anyone do this yet? I'm not building it for stage rally, just as a DIY that will hopefully come out cheaper or on par with a primitive plate - and will let me say I have kevlar under armor 8)


This is where i've found the kevlar felt:
http://sweetcomposites.com/Kevlar.html

[KevinWelker]

Kevlar works well, but is not very resistant to abrasion and blunt forces, like large rock impacts. Cool project though. I would be a bit concerned about how it holds up to water once scratches penetrate the resin coating.

[MConte05]

I know ACP has a kevlar skid plate on his Evo. There are some european suppliers that sell Kevlar skid plates and fuel tank protection.

[WAGONLOVE]

so it has to be pretty impact and blunt force protectin if they sell it as fuel tank protection and skid plates right? i wonder how well it would work.

[KevinWelker]

Check post #11 in this thread. Pretty well sums up the issue.

http://forums.nasioc.com/forums/showthread.php?t=841740

Personally I would not run kevlar, but if I had the skills of the OP, I would give them a try for sure. Take a look around at WRC under car shots to see where they use kevlar and where they use metal.

[WAGONLOVE]

there you have it. i love my primitive skid...
good point about checking out what where wrc uses metal and kevlar..

[thewizard]

I have a kevlar/fiberglass/carbon skid plate that came with my car. It is german made and not originally intended for my car. So far so good (I m sure my ground clearance helps) but I would rather have an aluminum skid plate.

[lorenkb]

You can forget about doing it as cheap as an aluminum skidplate. Try more like double the price. Kevlar is expensive, resin is expensive, and all of the vacuum bagging supplies just add to the cost as well.

I would stick to woven fabric (plain weave) in place of the felt. You will achieve a much stiffer structure with the same amount of material with the woven fabric.

Adding a core (foam, balsa, etc.) will create a much stiffer plate for minimal weight gain, but won't deal with impacts as well. You could make the core out of fiberglass too, which would negate the weight savings but might hold up better to impacts.

Kevlar works well, but is not very resistant to abrasion and blunt forces, like large rock impacts. Cool project though. I would be a bit concerned about how it holds up to water once scratches penetrate the resin coating.

I'm calling shenanigans on this. Kevlar is exceptional in its tolerance to blunt force (impacts) and is more than adequate in its abrasion resistance. When cutting requires specialized shears and sanding is an act of futility, I think the material just might be up to the task of a skidplate.

Although Kevlar is hydroscopic, something tells me the life of the plate is limited by other factors :mrgreen:

[Ruh Roh 1]

I have a carbon/kevlar front skidplate and a carbon rear skid plate.

As of right now I am only doing rallycross with it, but it holds up very well.

I am a compsite technician, so I had all the resources on hand. Their is a lot of variables that go into laying these materials up.

[KevinWelker]

I'm calling shenanigans on this. Kevlar is exceptional in its tolerance to blunt force (impacts) and is more than adequate in its abrasion resistance. When cutting requires specialized shears and sanding is an act of futility, I think the material just might be up to the task of a skidplate.

Although Kevlar is hydroscopic, something tells me the life of the plate is limited by other factors :mrgreen:


So why does every plate or underbody piece that I've seen show signs of long whiskers of kevlar hanging down and looking like they are wearing through?

Edit, here I am answering my own question. Since Kevlar is very hydroscopic any break in the resin that lets in moisture allows speedy degradation. I've seen some that look like damp cardboard after being soaked. Now, would I want something that is susceptible to that or some nice aluminum that you can wash down and not have to touch up every scratch and gouge so it doesn't turn to mush?

http://www.rallyanarchy.com/phorum/read.php?5,33861,page=1

[lorenkb]

So why does every plate or underbody piece that I've seen show signs of long whiskers of kevlar hanging down and looking like they are wearing through?

Edit, here I am answering my own question. Since Kevlar is very hydroscopic any break in the resin that lets in moisture allows speedy degradation. I've seen some that look like damp cardboard after being soaked. Now, would I want something that is susceptible to that or some nice aluminum that you can wash down and not have to touch up every scratch and gouge so it doesn't turn to mush?

http://www.rallyanarchy.com/phorum/read.php?5,33861,page=1

I'm with you on that. Until A) Weight is a major concern and B) only the skidplates are left as a weight saving option, I don't see the point of a composite skidplate either.

[KevinWelker]

But they are so damn cool, i really wish i was a gazillionaire to do stuff like that.

[02wrxRally]

Thanks for the replies guys, I was away at a TSD all weekend and just got back. I'll read this over when I'm not hungry and sleepy..

[Tommy Gun Rally]

No one in this forum is rallying at the level where the weight of the skid plates should be of any concern (except maybe one Jackson brother :) ).

Typically Kevlar is used to reinforce areas where skid plates are either too bulky or it needs to be much more form fitting. Top level teams layer sheets of it under the car directly onto the floor pan to prevent cabin intrusion from rocks and other sharp items that may penetrate. I used to own a Prodrive prepped GC and the underside of the shell was covered in the stuff. I've also seen it as gas tank protection. But for a front or rear skidplate, go with aluminum, and do the rest in HDPE.

[02wrxRally]

OK, I'm not 100% unconvinced, but I'm not gonna rush out and build it right away now either.

I'm putting together the vac bag suplies, have my airfoil templates and hot wire foam cutter built, and will start building foam core fiberglass skinned wings with carbon tow re-enforcement (they - the germans - call it a disser wing) very shortly. So I'll have resins on hand too. I was also going to get a yard of kevlar to cut strips for the leading edge of the wing and after a few FG fuselages I'll give a molded kevlar one a try. Then move on to kevlar skinned wings once I've gotten more familiar with it.

The reason kevlar is chosen instead of FG and CF is it's abrasion resistance. The guy in the nasioc post warning about high heals was talking about carbon, and is right - carbon splinters and cracks. But from what I've read, kevlar is more dent resistant (catching model by the wing, hard nose-in landings on rocky fields, etc..) and wont splinter on impact. I've read the fibers stay intact on impact, so the sheet could crack, but not tear or puncture. Of course I also realize my source for this is very light weight thin structures, not bullet proof vests. This kevlar is not the ballistic type; that's not easy to get it seems. I don't know the forces required to break the resin and fabric bond and I assume a canoe hitting a rock in the water is less of a force than what my car would encounter, but it does work for those guys. Then again boat guys just assume a high level of regular maintenance, don't they?

I also think that a lot of the cost of composite body panels is the labor. Also, weight wasn't my objective, I just thought it would be a good exercise in my new exploration of carbon and kevlar composite fabrication. But I'll start with the planes and go from there.

[lorenkb]

Do yourself a favor and buy a pair of kevlar shears before you get around to building your sailplanes. Do not cut ANYTHING but kevlar with them, ever. I speak from personal experience. You're thinking about using them to cut carbon tow as well aren't you? STOP THAT!:flamed

The forces a skidplate can/will encounter aren't even comparable to the canoe guys. Multiple orders of magnitude different. You have a 3000 lb car getting into a high speed fight with large chunks of rock that are solid enough to withstand the tests of time for however many million years they've been around. The rock is going to win.

I don't think you quite understand the costs involved yet. Don't worry, your wallet will understand soon enough :tongue: For ****s and giggles, here's my SWAG:
-Lets assume 4ft X 4ft for your uber skidplate. Could be completely different dimensions, but 16 sq ft is reasonable for overall coverage including excess material wasted on the sides of the mold.
-Now, for the fabric. Kevlar really isn't widely available in too many weights (oz/sq yd) so your choices are pretty much 1, 1.7, or 5 oz fabrics. 1.7 and below is some really light stuff, not up to skidplate duty. So that leaves 5 oz fabric. Plain weave is fine, which runs anywhere from $7 to 13$ per foot of material (standard width of 38") depending on how much you buy at once.
-How many layers are you going to use per skidplate? Rocks scare me; I've seen what they do to 1/8" aluminum and chrome moly tubes, so I'm guessing 6 layers of 5 oz fabric? Maybe 4 if we're lucky?

So, for a 4 layer plate, we need a total of 64 sq ft of cloth. Works out to just about 20 ft of 38" fabric. Not quite enough to get the full bulk discount, so we're at $9 per foot: $180 for fabric alone. No resin, no tools, no supplies. How about a 6 layer plate? Totals out to 30 ft of fabric at $7 per foot (woohoo bulk pricing!) for a magical $210 just for fabric. Now add in the rest of your supplies...

Oh yeah, and random note (for skidplate or sailplane). Don't use polyester resins with kevlar. They don't play well together. :thumbsup:

[02wrxRally]

I see 38" for $14.20 - $15 per yard for 5 oz. if I get 7 yards....about $100.

and yeah, I'll need scissors up for the task, and I've read about resharpening them every few cuts too. Tough stuff...which is why I'm here asking these questions :)

I was wondering how many layers....

This if fun to think about regardless of if it's a good idea or not.

[lorenkb]

Where are you seeing it for $15 per yard? Aircraft Spruce has it at $21.75 per yard, and CST has it at 7.10 per foot if you buy over 25 ft. You sure that isn't the price per foot for an entire bolt/roll?

I totally agree on the fun factor. I really enjoy composites, and it would be a lot of fun to put in the work in designing and testing a composite skidplate. To determine the layers I would make 1x1 ft test coupons of various layer count. Fab up a steel tube frame (to simulate mounting) and go to town with a sledgehammer (or perhaps a more scientific rack of falling weights...) for some practical destructive testing. :twisted:

[02wrxRally]

Check the link in my first post for sweet composites.