You may want to look into 1/4 to 3/8 inch thickness bulk sheets of UHMW polyethylene that can be bought relatively inexpensively.

This is the material commonly used for underbelly protection on rally cars.

 

You may want to look into 1/4 to 3/8 inch thickness bulk sheets of UHMW polyethylene that can be bought relatively inexpensively.

This is the material commonly used for underbelly protection on rally cars.

I agree, UHMW or HDPE are viable choices. Nothing wrong with aluminum, but with the power and load-carrying limitations of an automobile the weight savings of a "plastic" undertray would be welcome. I guess a heavier plate would better anchor your vehicle upright though. ;-)

Alternatively you could run full-length steel "skid rails" under the car and only cover up vital areas with full plating.

 

I would imagine your aerodynamics would improve, however slightly, because placing skid plates on the under carriage would create a smoother surface for air to flow over.

 

I've seen Impreza bash plate assemblies - they're an easy google. But I was under the impression alloy was more commonplace than a plastic composite.

How is it attached? Is it bolted, drilled, what? Are the plates reinforced to the chassis in such a way that it cold cause damage if not done properly?

It's it's simply a matter of getting some plastic sheets and attaching them with cable ties, I'm wondering why even bother?? What I want to do is sposed to be the real deal.

Tough. Bolted. Metal.

Does anyone have any specs on weight?

Cheers

 

Tough. Bolted. Metal.

Does anyone have any specs on weight?

If your auto shop is making it custom, I don't see why you couldn't have it bolt up to as many points beneath the vehicle as you care to pay them for! Obviously the attachment points should be structural. One main reason I have not purchased an off-the-shelf skid plate for my car is that the attachment points don't seem appropriate to me as far as strength. This is something I will pursue further after I finish a few other projects on the car (suspension, cargo rack, ham radio install).

Use of plastics for skid plates is not uncommon. UHMW/HDPE are of course much weaker than aluminum by a factor of ~7. But if you are only going to put full skids under smaller, vital areas of the vehicle, you can use 12-25 mm thick sheet; apply enough thickness and you can match the strength of a thinner aluminum plate. And once a plastic skid plate wears out, you can use common hand tools to build another one. (Try that with aluminum.)

But if you have the $ for such a task, aluminum all the way imho.

You can easily calculate the weight of your proposed designs; just figure out how many cc of material you will be using and multiply that by the material density.

Strongest to weakest (all specs from matweb.com):

Density of 6061-T651 aluminum = 2.70 g/cc
Density of acetal = 1.41 g/cc
Density of UHMW = 0.93 g/cc

For example, if you have a skid plate that is 400 mm long, 200 mm wide, and 10 mm thick, that is 800,000 cu. mm. or 800 cc.

For aluminum, 800 x 2.70 = 2160 g = 2.16 kg
For acetal, 800 x 1.41 = 1128 g = 1.13 kg
For UHMW, 800 x 0.93 = 744 g = .75 kg

 

Superu - thanks for your input. I'm using alloy plate, just coz the weight difference is small enough but the toughness difference is significant.

Neutron - thanks again. You've been a great help to me in the past.

 

A - have you done anything to strengthen the plastic liners in the fenders? They seem rather frail to me, for slinging mud or making big puddle splashes.

 

A - have you done anything to strengthen the plastic liners in the fenders? They seem rather frail to me, for slinging mud or making big puddle splashes.

No, I have not. And I am not sure what you are referring to, to be honest. The fenders are where? the underside of the body, or the wheel well? I am losing the plastic insulating the underside of my front left wheel arch coz I've come down on that side rather hard, and more than once, I believe. I need to get that re-attached.

I have an optional extra in the under-engine, plastic splash/rock guard. It's as old as the car, and so I've had it re-attached more than once just with cable ties. Last time a new anchor point had to be drilled into guard to fasten it. It's got to go!

Be please enlighten me - what specifically can you show me RE the fender liners?

ThanksLizard

 

Skid plates

Hi Abadonn,

What are you trying to protect besides the obvious (oil pan, diffs...)

I'm running 3/16" aluminum front and rear plates from Primitive Racing.

Subaxtreme makes a pretty nice sump guard down your way and I'm sure the Backyard Boys over at Ausubaru have some ideas for other options.

 

I am losing the plastic insulating the underside of my front left wheel arch coz I've come down on that side rather hard, and more than once, I believe. I need to get that re-attached.

That's it, and you answered my question. They don't seem to be made for extreme use, I wonder if there's any way to improve on them - do the rally cars run with OEM or does some vendor make anything tougher?

 

Superu has just given me some places to start looking. Thanks!