Skid Plate Mounting

At Gorman Hakan took a jump at 85 mph, and from what I heard bent his subframe. This made me wonder, it seems that most people mount their skid plates solidly to the subframe/frame. However, if you nose dive at speed into the ground after a jump, I would think you would want to have some give, so that the shock is not directly transmitted to the frame. Which brings the question, why don't people mount the skid plates via some sort of poly bushings (or maybe even stick a piece of tire between the subframe and skid plate)? This way, if you have a serious shock to the plate, rather than transmitting all the shock directly to the frame, a lot of the energy would be absorbed by compressing the centimeter or so of rubber/polyurethane. Is this stupid?

klim

It's an idea, but there's not room for any serious shock absorption. You'd drag your plate over every rock. Really a solid mounted skid plate should strengthen everything and distribute the shock?

I have seen hockey pucks used as a spacer somewhere on this very forum, cant remember where tho. and some people build a seperate bar, or frame to mount the skid so as to NOT tie into the sub frames and other components.

---

I be sorry as a muthafucka I did, still sorry I did n' hustled ta peep what tha fuck I holla'd a lil' bit better, or at least try to.

I used swaybar mount bushings on mine between the plate and the frame.

But that was just to gain some extra clearance rather than any "impact absorption"

Quote
frumby
It's an idea, but there's not room for any serious shock absorption. You'd drag your plate over every rock. Really a solid mounted skid plate should strengthen everything and distribute the shock?

Exactly right; there is precious little room to space the skid plate below the oil pan or trannie to allow for the plate's deflection when you hit rocks and not get into the pan. So soft mounts just make that worse; you would literally need an inch or 2 of mount deflection to achieve anything, as the car weights involved are the same weight as the front suspension has to deal with, and you know how long the suspension travel has to be....
As for spreading the load, that is all extremely variable and dependent on the plate shape, the mount locations and how you hit. It is very common to have the plate mounted so that all the vertical motion just goes right on up into the front subframe area, and little or no shock load goes into the body. You would probably have to extend 1.5" tubing minimum back under the body to achieve any good results in carrying the load into the body; I have tried 1" tubing and it just bends so easily. (And then you end up with endless straightening of the pan support tubes after even a mild hit, just to get the darned thing aligned with the mount holes again!) I would venture to guess that for the incident in the original post, it was either the subframe or the engine pan destined take the damage regardless.

What was the car?
Better to bend a subframe than dnf!

---

Grant Hughes

dsm?



(more at http://learmiller.smugmug.com/Sports/Gorman-Ridge-Rally-2013/31470775_wHMmTV#!i=2725866440&k=WzCScqm )

---

KF7RWG
http://www.utahrallygroup.com

who hasn't? if you are mid to rear pack at NEFR you can spend most of Icicle Brook dragging the skid plate on the high center in 4th gear... something will bend eventually.

---

-KTurner
Stomp down on the exhilarator and hold on to the wheel.

I've seen a few skid plates that are mounted in the rear by a pin system, like a couple of hitches of sorts. That way the skidplate has room to move back and forth a bit under hard hits, without moving the subframe much. Finding room under a modern, fwd based chassis can be difficult though. I started on a system for the VW I built, but never did the pins on the skidplate, just the tube pockets on the front of the subframe.

-Jon

Quote
jrally
I've seen a few skid plates that are mounted in the rear by a pin system, like a couple of hitches of sorts.

that's how mine is, 2 bolts in the front with big fender washers and the back is two studs that slide into a piece of angle welded to the transmission subframe.



you can see the rear skid mounts in this picture.

---

-KTurner
Stomp down on the exhilarator and hold on to the wheel.

Somebody needs to find a lighter navvie!

Oh dear, is one allowed to reference what serious high buck teams beginning with F did without a pile of trolls getting snippy?

See the blue crossmember-y looking thing just in front of the aluminum crossmember? Thats the sumpguard center support. Sump guard mounts to the red bar at the front and then to just aft of where the compression struts mount.


It is not bolted to the center support. there mouse pad glued to the bottom for rattle..


Its a brilliant design because it works on the priciple hat its hard to bend metal on edge, but easy when its flat..(Likewise tube can be strong but WAY strong in compression and tension)

Here a better view of the thing:
Genuine Ford part in the back, locally made part in front


Thet were available in "tarmac" normal and 'Acropolis spec" the difference being the part leaning on the vertical part: normal had a plate on the front, and "Acropolis" had it on both sides, tarmac had neither.



Bending on edge is a function of height so this thing is strong--and light.

Seems the principle about difficulty of bending something on edge escapes most people..

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

I'm gonna have to loom under my impreza and see if that type could be adapted.

Hakan landed at that same angle pictured. Broke control arms and ruined a handful of other parts. When I saw it loaded on the flatbed ( retiring after SS10) both front and back co-driver wheels were bent a ways underneath the car. He hit so hard he blacked out and shattered his elbow. We were all glad it wasn't worse and that Hakan was OK. I guess it probably wouldn't matter how the skid plate was bolted or what brackets were used if your car is 10 feet in the air and lands on it's side stuff is going to break. The picture really doesn't do justice to the true height and distance he went. In short don't do that and mounting to the subframe is probably fine. DRP ( Dirty Racing Products ) Just relased a sweet new skid plate set up where tubing is bolted to different parts of the frame then a skid plate on top of that... for subaru's of course.

Ok, the idea of bushings is not going to work. It is just too insignificant to make a difference.

Using the values for the bushings from here:http://fcpk.pl/data/mp/1335_elastomery_2.pdf
and using little calculators from here:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

One can calculate, if we completely disregard the forward velocity of the car, what would be the energy of dropping 1500kg car from the height of 1m. Its 14700 Joules.

If we take the hardest bushing from the pdf above, with a diameter of 40mm, length of 32mm and max compression of 8mm, then it will take 12 Joules to compress a bushing like that to its max compression. So, even if you have 6 of them, you absorbed about 72 Joules, which is 0.5% of the total kinetic energy of the impact. And thats dropping a STANDSTILL car down from a height of 1 m. The forces experienced will be WAY larger if you are going at 100km/h and nose dive at 45 degree angle.

Its insane that the struts don't go through the mounts every time you land a car after a jump!

Klim