Engineering Question

[Shaun @ Tru6]

I want to create a crush zone on a round Al tube so it crushes straight. I've seen crush zones on square tube that are created with small dents or cut-outs at each corner on the square plane.

I want to replace the bumper shocks on impact bumper cars so tube is about 12 inches long, 2 inches in diameter and I would like to use 1/8" wall thickness.

I was thinking of drilling holes in a honeycomb pattern for about 3 inches.

Any thoughts?

[aigel]

I am no expert, but if you look at steering columns, that should give you a good idea.

This one looks like expanded metal. It also appears like it was failing.



Source: Suspension Steering Column Crush Tube Cracked

[LWJ]

If you mill the cut outs it could be done. Don't ask me what the pattern would look like. That would take a real engineer.

[john70t]

(not an engineer)
For a gradual impact zone, I'm guessing the most holes/cutout would be at the very front for light taps.
Then a small section of blank.
Then a section of less holes for harder hits.
The big energy starts transferring there.
But the real problem with that design is the first part is already broken and doesn't transfer anymore.
Ok. Ignore all the above.

There should probably be a few safety wires in case the whole bumper randomly falls off and starts rolling under the car due to unseen damage.

Another approach would be pipe cylinders inside cylinders.
A section breaks and slides inside the next.
Then another section breaks.
But the problem with that design is all that overlap starts to add up in weight.
Back to zero.

To make it feasible, a few designs should be run through a CAD program and crunch the #s.
The weight losses may be negligible and the potential damage to the rest of the car immeasurable.

It might work well for bumper parking, but the insurance company might not cover any greater loss.

[1990C4S]

Why not make a tube within a tube? Slip fit and tack weld...the tacks break and the inner tube slides in...

P. Eng.

[David]

That's an interesting problem. I thought about something like that for the back of my 930 which pretty much has no bumper right now unless you count a fiberglass bumper cover with a big turbo crush zone.

I could model something and run a finite element analysis to see how it would deform but my software may not accurately represent actual crash buckling.

[herr_oberst]

Do you even need to create a crush zone in the tube? Won't the tubes be the path of least resistance by their very aluminum nature? (Assuming you will use all of the original mounting points with similar brackets)

How about a slot (about a quarter inch wide) running the length of the tube at the bottom to create an area for the tube to expand in case of impact? Or 4 horizontal slots.

I always thought an elegant solution to the bumper shock dilemma would be many pieces of thin solid aluminum rod welded together in multiple triangles. Think of stage light rigging, but on a much smaller scale. Light, strong, capable of withstanding stresses in many planes, and could also be engineered with a crush zone to save the tub. Probably too labor intensive to be practical.

[javadog]

My thought is that you are over-thinking this. I see no point in creating a crush zone.

In a light impact, like in a parking lot, any crush ruins the tube. Need a new one anyway, so what have you gained?

In a big wreck, the chances of the forces being in line with your tube are minimal, so your tube is likely to bend anyway. You will also need a new tube. What have you gained?

JR

[Shaun @ Tru6]

Good thoughts and questions, thanks. here is a little more detail on my thought process.

Front shocks are 4,6 oz each. Rears are 4,13 oz each, so about 19 lb total weight. My goal is a 1 lb/mount for a total 15 lb weight savings at the ends of the car. But I also want to stay as close to stock bumper dampening in light hits while not damaging the unibody in harder hits. Basically, like the shocks, I want transmit as little energy from an accident directly to the unibody in light, medium or hard hit situations.

They just look heavy.

[Shaun @ Tru6]

Originally Posted by herr_oberst

Do you even need to create a crush zone in the tube? Won't the tubes be the path of least resistance by their very aluminum nature? (Assuming you will use all of the original mounting points with similar brackets)

How about a slot (about a quarter inch wide) running the length of the tube at the bottom to create an area for the tube to expand in case of impact? Or 4 horizontal slots.

I always thought an elegant solution to the bumper shock dilemma would be many pieces of thin solid aluminum rod welded together in multiple triangles. Think of stage light rigging, but on a much smaller scale. Light, strong, capable of withstanding stresses in many planes, and could also be engineered with a crush zone to save the tub. Probably too labor intensive to be practical.

Your stage light rigging would be a very cool and very light idea but probably not practical to make.

I was also thinking of a series of horizontal cuts around the tube using a miter saw so they would just collapse on impact.

[Shaun @ Tru6]

Originally Posted by David

That's an interesting problem. I thought about something like that for the back of my 930 which pretty much has no bumper right now unless you count a fiberglass bumper cover with a big turbo crush zone.

I could model something and run a finite element analysis to see how it would deform but my software may not accurately represent actual crash buckling.

David, you just described a variation on my idea. Glass bumpers on 911s are like my 2001 Tundra, the bumper is purely aesthetic. I was thinking a band of Al bar that would go in between the glass and bumper mounts. It wouldn't give you the protection of the 50 lb chunk of steel that is the actual bumper in my truck, but better than 4mm of fiberglass.

Modeling something would be great! Sending you an email.

[javadog]

Okay, in anything other than the lightest of love taps, your aluminum tubes are going to fail. So, minor bumps will require replacement of the tubes. The forces transmitted to the tub will likely be less than those that would have been transmitted by the stock shocks, as they have less strength than the stock ones. It takes pretty decent hit to move those.

In a big impact, the damage to the tub won't come from forces transmitted though the tube. The forces will be high enough that the tube will fail and the bumper will soon be pushed into the tub. It's the bumper that's going to do the damage, assuming whatever hits you doesn't just go right over it and directly into your hood and fenders. Look at any 911 that's had a front impact, epecially those that are offset on one side. There's a fair bit of damage.

JR

[Shaun @ Tru6]

Failure of the tubes is a given.

Controlling the failure and how it impacts (no pun intended) the tub, is my goal.

[aschen]

I agree the energy absorption is going to have negligible effect in the context of saving the chassis. Just regular aluminum should do what you want.

OTH if you want to make them weak enough that they cant transfer too much force to the bumpers, which could be useful for saving bumpers in parking lot incidences, you could do alternating milling pattern on either side of the tube. It would sorta turn the tube in an S shape when viewed from the top. This will ensure the tube bucles right away and will use the whole length for deformation.

I are an engineer, but this isn't really my area, so grain of salt and all that

[MBAtarga]

If making something from scratch - start with the flexible exhaust pipe - but stretch (lengthen) it prior to attaching end plates for mounting surfaces.

[javadog]

Originally Posted by Shaun 84 Targa

Controlling the failure and how it impacts (no pun intended) the tub, is my goal.

I'd suggest reviewing the photos in the dozens of "my 911 just got hit" threads here to see what gets hurt in real world impacts, with the end goal being to see if the bumper tubes were much of a factor.

The point I'm trying to make is that they are largely irrelevant.

JR

[Arizona_928]

Here's from a 928... Stolen from xchop's build on rennslist

[island911]

^ ha! --not an engineered solution

Originally Posted by john70t

...
There should probably be a few safety wires in case the whole bumper randomly falls off and starts rolling under the car due to unseen damage....

There's an add-on fix for that - kachi-casters.

[Arizona_928]

Eh practical solution.

[Shaun @ Tru6]

Originally Posted by javadog

I'd suggest reviewing the photos in the dozens of "my 911 just got hit" threads here to see what gets hurt in real world impacts, with the end goal being to see if the bumper tubes were much of a factor.

The point I'm trying to make is that they are largely irrelevant.

JR

I get your point JR. It only takes one time. I simply want to plan for that one time and it's a worthwhile goal of reducing impact on the tub. And it makes for a lighter part, so that's a benefit as well.