Crushbox, Attachment idea

[Roux]

My crushbox is made with Carbon Fiber and Nomex Honecomb. The attachment method I will decribe here can be used with Aluminum or Carbon crushbox designs. My goal was simplicity in attachment and removal of the crushbox to access the master cylinders. For years I lived with cutting my hands up from working down a hole in the crushbox to try to open, check, fill and reclose the master cylinders. Current solution is based on simple parts not requiring more than a lathe and bridgeport. In a pinch I think you could make my design with a pistol drill and round files.

Nose/crushbox attaches with 4 fasteners. Bottom two are high strength capscrews 5/16 inch diameter. These are long enough to pass through 1.25 inch long tubes that are integral to the chassis bulkhead . The upper two attachments are threaded studs, 7/16 fine thread. The uppers could be smaller in diameter. The studs are quite short since they pass through steel ears on the bulkhead that are 1/4 inch plate. The locking device is a steel bar, 1/4 inch thick and 1 inch wide. At the bottom there is a key hole feature that slides over the head of the capscrew and at the top the bar has a 1/2" hole that contains a captive barrel nut with female 7/16 threads and a 3/8 hex for ratcheting. The system requires two of these bars. One for the right and one for the left.







Attaching procedure: Poke the cap screws through the tubular holes in the bulkhead/frame near the bottom of the car. Poke the upper threaded studs through the holes in the frame at the top of the front bulkhead. Push the crushbox back against the car, exposing the heads of the capscrews and the 7/16 threads on the footbox side of the frame bulkhead. Take one attaching bar and slide the large part of the keyhole over the capscrew head and then slide down to engage the head of the capscrew with the keyhole slot. If fully seated the barrel nut at the top of the bar will be aligned to the threads of the upper stud. Start the thread, and then install the second bar on the other side of the car. Ratchet the two barrel nuts tight and the nose is attached.




everything is done from the top of the car with no need for extreme dexterity. Nuts are captive. Takes only a piece of a minute to get the nose on or off.


Still to do: make the bars lighter on the mill. Consider upgrading the bottom capscrews to 3/8 to get strength margin

[Roux]

Good. The nose crushed instead of buckling near the base


The Ugly: Epoxy sheared along a plane where the upper threaded stud was laminated into the nose. This was predicted and will be addressed in the repair and the next version. The threaded stud should be welded to a steel plate and that should be through bolted and bonded to the nose to spread the load and positively engage the carbon fibers without relying on the epoxy which failed due to a peeling load

[Roux]

This was 16 or more years ago. Royale RP31 frame actually gave way and saved the crushbox from crushing. Fortunately impact was with a tire wall that did the giving.


Conclusion: You are building a system and everything needs consideration including loadpath into the chassis

[BeerBudgetRacing]

Clever. I like it. I'll have to see of I can adapt it to my VD.

Q: Does the bottom stay a bit loose? Since you slide the keyhole over the bolt it can't be snug.

I had a similar idea I've never tried where the bottom "stud" was a slotted post and you could drive a wedge into it to create tension. Yours is simpler to do with regular parts.

The other method I was thinking of used a couple springs on the bottom to create tension and provide a little give and return when the nose gets knocked. That could be adapted to your method.. Hmm.

Thanks for sharing.

[S Lathrop]

I too like the idea of having studs that pass through the front bulkhead and are secured on the back side. That is how the Dallara Indy cars are done. Your steel bar is replaced by by a can type mechanism that draws on the studs tightens the nose box against the bulkhead. To release or lock the nose box only requires a turn of the cam mechanism.

One thing that Dallara does is have a barrel nut that sets the length of the stud so when the cam draws the studs tight, there is even tension on all 4 studs. The other thing the nut offers is a much larger shear area at the inter face of the crush box and the front bulkhead.

For my next iteration, I am thinking of having the bosses through the frame 5/8" ID or maybe larger.

My first experience with composite nose boxes came with the FA 40/41 Ralt. That nose box was done in 2 sections, the front half was a simple carbon shell, maybe .060 or .080 thick. It was riveted and bonded to the main section which was built more like the tub itself. Most accidents only damaged the front section and were relatively inexpensive to fix. One really bad crash was when the Ralt part was replaced with an after market item that was a single piece, That part was knocked off after a first impact with a barrier and the front wing. The second impact was the front bulkhead and a concrete barrier. That left my driver in the hospital with a severely mangled left foot were the steering rack crashed through the front bulkhead and into his foot.

That experience is why I am so concerned about how the crush box deforms in a crash. The other issue is with winged cars, the wing has enormous leverage on the mounting structure and especially in impacts where that impact force produces a resultant force at the bulkhead closer to later. The nose box has to have failure points other than the mounting point at the front bulkhead. On DW's car I had an extension on the nose box that the wing mounted to. In most instances that extension would fail and allow the wing to separate from the nose box. In DW's case, it appears that the wing mount did fail but the impact to the box itself was largely lateral and that the box mountings failed with a lateral deformation. In the future, I want something that will fail much further from the front bulkhead and do a better job of absorbing the initial crash energy. Still looking for answers.

[Purple Frog]

Just an opinion, not a lecture...

I think if it takes 4 minutes longer to check the fluid levels in the master cylinders, its time well spent if the safety structures are improved.

What if the nose and crush structure were two separate structures?
Maybe the nose carrying the wings with all that extra leverage could break away easier without affecting the crush structure fasteners.

[S Lathrop]

Interesting Roux, in the picture of the Royal there are 2 Zink Z16s.

Mike; I am planning on building the nose box with 4 separate sections. Each one would be stronger that the one in front of it. The intent is to get a more progressive collapse of the nose box.

It is the lateral impacts that are the issue, I believe. and the longer the nose box, the more likely the impact will be lateral. I am thinking that most accidents start with a front wheel striking something. That in turn rotates the car into say the barrier. The next part of the car that is impacted is the nose box. Ideally we want this impact to absorb energy and further slow the car down. Often this happens but the nose box is stripped away in the process.

If we have a wing on the front, that front wheel impact is very closely follower by the front wing and that force really pushed the mounting structure side ways. What I am thinking is that the front nose cap will leave with the wing and from the wing spar back, we still have 3 more sections to deal with the forces to come.

I have not build this yet. The first step, I hope, will be to simulate this in a confuser.

[SEComposites]

Its probably worth putting some thought into a rear impact structure as well.