Crash protection/Size of Caterham fuel tank

[Alaskossie]

Re the subject of fuel tank protection on the thread about crashed Sevens, I glued the Caterham honeycomb panels to the sides and bottom of my fuel tank with silicone caulking, which seems to provide a strong bond. Caterham does not supply a pre-cut honeycomb panel for the right end of the tank (where the fuel pump/sender unit is) for some reason, so I ordered an extra left- end piece for the right end, and cut a 5-inch circle in it to permit access to the sender unit.

 

I plan to always carry the spare tire in its rear mount, which should offer some additional rear-end crash protection.

 

I have been discussing a possible larger-capacity fuel tank with Andy Wiltshire in UK (http://andywiltshire.com/index.htm). I have some ideas for a larger-capacity Seven tank without losing any boot space. Andy has purchased a Caterham JPE saddle-tank arrangement (3 separate tanks) as a mock-up for making a larger-capacity tank, but it looks pretty complicated.

 

If I do go for a larger custom tank, I would like to be able to obtain some of the Caterham honeycomb panels in bulk, to cut for my individual needs. Don’t know if this is possible.

 

Tom Meacham

Alaskossie

 

[bsimon]

McMaster-Carr

 

About $25/square foot, 3/8" and 1/2" thickness.

 

Looks like 24"X48" is the largest sheet.

 

It will probably have to be cut on an abrasive waterjet table, although it might be able to be cut with a saber saw or router. I can picture the saw or bit grabbing the thin cell structure and evacuating it from between the face sheets in a most spectacular manner.

:cry:

 

Maybe someone here has some experience cutting the stuff with low tech tools?

[Alaskossie]

Bob,

 

Thanks! Don't know why I didn't think to look there, instead of for a UK source.

 

McMaster-Carr seems to have virtually everything a tinkerer needs (or thinks he needs)......

 

Tom

[slomove]

I don't know who ever got the idea of using aircraft-style honeycomb sandwich panels for impact protection. Especially the aluminum and Nomex/phenolic cores have very little resistance to impact (maybe a little better than nothing, though....). I think some people even use the honeycomb core material without skin sheets which is bizarre. In a lab where I used to work we had aluminum honeycomb that got damaged by just moving it from one shelf to another one.

 

Sandwich panels with polypro honeycomb, high density PVC foam or end-grain balsa core (with aluminum or steel skin sheets) are supposed to be better in impact situations but specialty products, not available off the shelf.

 

Gert

 

[Alaskossie]

Slomov,

 

What is the intended benefit of honeycomb aluminum panels for aircraft? Rigidity only?

 

Caterham uses the honeycomb both for fuel tank protection and for cockpit side protection, as options.

 

Better than nothing, or worse than nothing -- or somewhere in between??

 

Tom

[Kitcat]

I started a discussion a few days ago on the Sevens list about replacing my stock 36 liter Caterham tank with something from ATM or Fuel Safe. The downside of the stock tank is it is in a vulnerable spot and has little impact resistance.

 

The downside of fuel cells are cost ($4K+ for the 55 liter Caterham unit, about $1.5K for the 7 gallon Fuel Safe unit). They also have a limited useful life and should be inspected annually & replaced every 5 to 7 years (more cost). They also reduce tank capacity and there can be the usual fitment issues

 

Both Nathan at RMSC and Chris T at Sevens and Elans recommend the honeycomb stuff. Nathan said that the kit supplied by Caterham (for about $400), provides a very tough, nearly impenetrable surface protector.

 

I am inclined to try it. The tank has to be partially removed to glue the pre-cut panels but otherwise it is simple/cheap and, no loss of tank volume, annual inspections/fitment issues.

[southwind25]

We have run fuel cell's in our vintage race cars,,and will proably heavily consider this in the birkin we are in the process of buying.

Over the years we have seen some wicked accidents racing as well as right in front of our house (we live on the outside of a curve).

The fuel bladders do have there cost...but burnt skin is much more expensive. The other advantage is with the foam you dont get fuel slosh, they usually have roll over valves, and i have always saved on the cost simply building my own box (we have a tig welder). i Am not so sure the honeycomb will offer much protection for the cost or effort involved.

[bsimon]

I'm surprised that Caterham or Birkin don't offer a "next best" solution for fuel tank safety. Most of us would like to see improved safety for track days and general street use, but not necessarily want the expense and service problems of a full blown ballistic racing cell. Obviously if one has to meet the scrutineers standards for organized racing, an expensive bag tank would probably be required.

 

I believe a cost effective approach would be a cross linked propylene tank housed in an aluminum enclosure with replaceable anti-slosh foam. A JAZ type tank with the added protection of an aluminium enclosure, if you will.

 

The stock Corvette gas tank is a good example of this type of construction. It has one of the best safety records of any stock vehicular gas tank.

 

Here's what a JAZ tank looks like.

 

Here's some interesting reading on tank safety. The Corvette tank is mentioned in brief.

 

Here's an interesting study done by ford when Police departments made requests for fuel cells in radio cars. Seems product quality and longevity were major issues with the specific brand of bladder they tested.

[JBH]

I am not a safety engineer, so I preface my comments by saying I may not correctly understand the construction of the fuel tank in the Caterham or the dynamics of a rear end collision. But, from my perspective...

 

In the Caterham, the fuel tank is sandwiched between the rear sheet metal and the differential. I am afraid there are not a lot of good options. Those old enough to remember, will recall the horrible videos of the exploding gas tank in the Ford Pinto. This actually pre-dates Chevrolet truck side saddle. Here's the problem:

http://www.fordpinto.com/images/kboomdrawing.gif

 

You can see the fuel tank was located between the rear bumper and the rear differential. To quote the article: "...Pinto accident reports revealed conclusively that if a Pinto being followed at over 30 miles per hour was hit by that following vehicle, the rear end of the car would buckle like an accordion, right up to the back seat. The tube leading to the gas-tank cap would be ripped away from the tank itself, and gas would immediately begin sloshing onto the road around the car. The buckled gas tank would be jammed up against the differential housing (the large bulge in the middle of the rear axle), which contains four sharp, protruding bolts likely to gash holes in the tank and spill still more gas."

 

Looking at the placement of the tank in the Caterham, there is little distance between the tank and the differential. Perhaps wedging honeycomb panels between the tank and the differential would help - I'm not sure. I don't see how a fuel bladder in the same location helps much since it will undergo the same deformation and with enough pressure will rupture too.

 

The Side Saddle tanks in the article referenced in bsimon's post correctly points out that the best location is above the differential 8" or so from the rear bumper. Unfortunately, the Caterham has very little distance between the seats and the rear of the car - placing a fuel bladder in the boot right up against the seats would be best - I think this is the way they fit the Caterham race cars

 

Edit: I believe ATL makes a fuel cell for Caterham that is suitable for fuel injected engines. Placed in the boot area, it may not be such a pain to replace the bladder. I have heard of fuel cells filled with non-corrosive wire wool instead of foam.

[slomove]

Slomov,

 

What is the intended benefit of honeycomb aluminum panels for aircraft? Rigidity only?

 

Caterham uses the honeycomb both for fuel tank protection and for cockpit side protection, as options.

 

Better than nothing, or worse than nothing -- or somewhere in between??

 

Tom

 

They are used for very lightweight, rigid, mostly internal structures and typically laminated with thin composite sheets outside. That way the panel can take some bending loads (which the thin composite alone can not do).

Optimized for this purpose the sandwich core is pretty resistant to vertical pressure when distributed over a large surface area and as long as the comb structure is intact.

 

However, when a local impact penetrates the thin face sheets, or the foil making up the honeycomb begins to crumple under load, the entire structure crushes right away.

 

I would admit some effectiveness if the impact is blunt and distributed over the rear panel. But most impacts hit with a corner or an edge of the colliding vehicle. So, IMO it is a little better than nothing but I would rather invest this money in a fuel cell or a good dinner.

 

 

We should not have the illusion that a Seven is even remotely safe at today's standard and any rear impact over 10 mph is really bad news. I had a rear-ender at pedestrian speed and that was bad enough: $3000 damage and a punctured tank. The offending Honda Civic had barely a scratch on the bumper. I guess even the infamous Pinto was probably much safer in comparison.

 

But that is why I have a fuel cell which may help me up to 20 mph ;-)

 

Gert

[EburgE]

FWIW, I just got the FuelSafe 7 gal. tank for under $1300.

 

The tank does not need to be replaced, but the foam filling does every 2-3 years. And it does not require returning the tank to the manufacturer.

[Alaskossie]

Slomov,

 

Did you have your spare tire mounted at the time of the crash? Just curious.....

 

Tom Meacham

[slomove]

Slomov,

 

Did you have your spare tire mounted at the time of the crash? Just curious.....

 

Tom Meacham

 

Well, that was part of the problem. I still had the spare carrier mounted at that time but no spare tire on it. The center mounting stud being pushed in was what caused the puncture (a nice clean 10mm hole). With the tire in place that may or may not have happened. Otherwise, the damage would probably have been similar.

 

I think the issue is just that there is NOTHING but a thin ali sheet and a few flimsy steel tubes that are trying to prevent another car from invading the boot/tank area. It takes a bit more to stop 3000-4000 pounds moving at street speed. We are always bragging with how light a Seven is but there is an obvious downside to it.

 

But, I consider the Seven still much safer then e.g. riding a sports bike and take the remaining risk for the sake of fun.

 

Gert

 

 

[Kitcat]

EburgE.

 

What is the estimated delivery date? How do you replace the foam without returning the tank (cut it open yourself)? Is it your understanding that it just bolts right in? I am v close to making the same purchase, but may wait to hear your experience as to quality and performance issues.

 

Mike

[EburgE]

EburgE.

 

What is the estimated delivery date? How do you replace the foam without returning the tank (cut it open yourself)? Is it your understanding that it just bolts right in? I am v close to making the same purchase, but may wait to hear your experience as to quality and performance issues.

 

Mike

 

Estimated four (4) weeks delivery. Purchased through this dealer.

http://www.hrpworld.com/index.cfm?form_prod_id=875,100,50_649&action=product

 

Spoke with the manufacturer, FuelSafe, and they explained that the tank has an opening, around the filler, that can be unscrewed to replace the foam. Overall the manufacturer was very helpful in explaining this. I encourage you to call them yourself.

 

As far as "bolts right in", it should. FuelSafe cannot vouche for any design changes in the seven you are looking at. They suggest measuring the dimensions of the original tank that you are replacing. I took a chance and ordered the tank (still waiting on the car kit), because I believe the dimensions have not changed.

 

FuelSafe has two options: ordering their Lotus cell (the link above) or sending your tank to them for conversion. The first option is cheaper, and that is the one I went with. If you are worried about fit, then you could send your tank to them.

 

Also, I am thinking of adding a clear in-line fuel filter between the tank and the pump as others have done. My rationale is that I want to see if any particles from the foam enter the fuel line, 1-3 years down the road.

 

BTW, I have about two more weeks to wait for the car kit! The wait is killing me!

 

 

:cheers:

[Alaskossie]

Two additional thoughts on Seven rear-end crash protection:

 

1. Could the possibility of the spare tire mounting stud punching a neat hole through the tank in a rear-ender be reduced by cementing a 3/8' thick square of tempered aluminum between the stud and the tank? Something that would have more impact resistance at that location than honeycomb aluminum sheet?

 

2. Could the same thing be done on the tank panel facing the differential?

 

Tom Meacham

[slomove]

Two additional thoughts on Seven rear-end crash protection:

 

1. Could the possibility of the spare tire mounting stud punching a neat hole through the tank in a rear-ender be reduced by cementing a 3/8' thick square of tempered aluminum between the stud and the tank? Something that would have more impact resistance at that location than honeycomb aluminum sheet?

 

2. Could the same thing be done on the tank panel facing the differential?

 

Tom Meacham

 

That is probably a good idea, i.e. removing any protruding features that may damage the tank skin in an impact and some kind of plate to blunt what can not be removed. The fools who assembled my car even let the bolt stick out an inch.

 

Gert

[bsimon]

1. Could the possibility of the spare tire mounting stud punching a neat hole through the tank in a rear-ender be reduced by cementing a 3/8' thick square of tempered aluminum between the stud and the tank? Something that would have more impact resistance at that location than honeycomb aluminum sheet?

 

2. Could the same thing be done on the tank panel facing the differential?

 

That's along the lines of Ford thinking. Of course, it was not implemented.

 

The technology was available to make the Pinto a safer car. An inexpensive lightweight plastic baffle was placed between the front of the gas tank and the four protruding bolts on the differential housing. This piece of plastic prevents the bolts from puncturing the gas tank and was used in one of the only successful crash tests the Pinto underwent. In another successful test, a piece of steel was placed between the tank and the bumper to add support against the crumpling back end.

 

BTW...I'm driving my Corvair to the park today for a spirited game of lawn darts...