Roll Cages - Myths And Fallacies

[R31Nismoid]

What i posted about, around $650-$750 for a cage that keeps the rear seats and therefore keeps the car as a 4/5 seater with Vicroads.

As the cross points are bolt in, same as intrusion bars.

Otherwise you can get it welded and re-rego the car as a 2 seater

Weighs about 40kg's

Both require engineers to be road legal if you want to go that path. But they can be built to the RWC spec.

[Rowdyr32]

Thanks Ash. I'll have to drop in and speak to them. Thats a really affordable price.

[R31Nismoid]

PM Bris

He is working out a possible group buy.

And thats another $100 or so off if we get enough heads

[Snowman]

I was thinking about this further last night and there might be some other considerations if going for this (which I still will).

But I presume - especially with a half cage - that the weight balance of the car is going to be totally pharked.

i.e. I know with the GTR's they have a 63f - 37r % ratio of weight and consequently suspension springs are matched accordingly (hence why 650 fronts are matched with 380 rear springs). But I guess a half cage would add weight especially to the back half of a car. Would it be enough to require corner weighting and re-springing do you think?

Also with the extra rear weight distribution would things like trail braking become more of an issue and would something like a brake bias become needed? Or am I over analysing the effects of the extra rear weight?

[Roy]

Sure it will add weight. But more then balance front to rear i woudl be most concerned about the weight of the cage you are throwing up around the roof line as this weight is above the centre of gravity (BAD)

The front to rear things isnt as bad as you imagine. The weight is being added to between the wheels, so forgetting all the physics, basically if you are going to throw weight at your car then you want to be doing it to the middle of the car. I cant remember the exact numbers but throwing an extra say 10kgs in front of the front axle (those bigger intercoolers and raditators that hold more volume and oil coolers) would do more to upset the handling of the car. (Can do the numbers but you get the idea)

Its because of the polar moment of inertia. Basically the polar moment of inertia is related to the mass around (in front and behind) the front and rear. You can imagine how this works by grabbing a 100cm bar with 5kgs on either end. Hold the bar in the centre above your head and rotate it forwards and backwards using your wrist. Now imagine doing the same thing with only 2kgs at either end but with a 6kg mass where you hold the bar in the middle. Same weight but a whole lot easier to rotate forwards and backwards...Its the same thing you are doing when you add weight to a car.

Cars naturally have a pretty high polar moment of inertia because of engines, diffs etc and the need tohave nothing in the middle of the car because of passengers. So technically forgetting the overall weight for a second, having bigger wheels and brakes and increasing unsprung weight will do more to upset the handlign of the car. Ditto throwing stuff off the front bar, intercoolers, oil coolers, radiators etc etc as this increases the polar moment of inertia.

So thats the theory anyway...to know exactly you have to run some pretty simple calcs...but a general understandign is all thats generally needed so that you have an idea of where its ok to add weight and where you shoudl try to avoid adding weight. LOL, if there is such a thing as where to add weight

[R31Nismoid]

I think its a valid point... If the cages were quite heavy.

At around 40kgs, its probably similar in difference between say Towie Jack & Aaron say.

You could say there would be maybe a 20-25kg difference between the two of them? (not implying anything, just eg's)

If you wanted to offset that you could just remove the passenger seat, although its not over the mid-rear But then the cage is 100% over the rear either., more mid-front

Its split almost down the middle and *reasonably* evenly. Its not sitting either side of the Front/Rear Axles as Bris said. Its in between them

Obviously if you went a really serious cage with more metal then it would be noticeably different.

Its probably something that might cause a problem. But it would be very much a minor fix i would imagine if anything.

Just thought there

[SABBAi]

Think of it as having half a passenger with you in weight gain.

[Snowman]

I was more worried about it being in the back half of the car. I remember early on I had issues with the rear of my car espeically at turn 1 Sandown.

Anyways - advice has been taken on board and I'm happy to give it a crack without changing things.

[R31Nismoid]

It will change it a bit... its just a question of is the 40kg's noticeable or not.

If it was a 40kg steel slab in one corner, i would say for sure.

But when its spread over the cabin space, it might not be that bad.

We will all be in the same boat though (if everyone gets a cage)

[Roy]

In an R34 it may not be as noticable as say an R32, (which are pieces of poo). But you may find in a car like yours with so much adjustability in your shock that you can tweak it even further. As the cage will tie in the B-pillar and rear inner guards/shock tops together (dependign on the cage you want) So more then the weight you may find that previously you ran stiffer suspension to allow for body flex. So with the stiffer body you now find the suspension to firm.

Fark i dont know.

[djr81]

It will change it a bit... its just a question of is the 40kg's noticeable or not.

If it was a 40kg steel slab in one corner, i would say for sure.

But when its spread over the cabin space, it might not be that bad.

We will all be in the same boat though (if everyone gets a cage)

40 kg is very noticeable - chuck a couple bags of cement or something in the car one day & go for a drive. It doesn't have that much of an effect on lateral grip, but will reduce the longitudnal gees your car can kick out. You will easily tell the difference.

If you want a lighter roll cage you need better quality steel. This was doubtless covered at the talk fest/night thingo but the general principle is as follows:

The different grades of steel make little difference to the stiffness of the cage - the design is the important factor here. But the different grades of steel can make a big difference to the weight of the cage. A stronger (higher yield strength) material can be made as strong without being anywhere near as heavy. Essentially this is because the yield strength of the steel changes much more than the youngs modulus.

With regard to spring rates front & rear they are almost always softer at the back to give the car traction. It isn't really related to front/rear weight distribution as much as you might think.

[djr81]

In an R34 it may not be as noticable as say an R32, (which are pieces of poo). But you may find in a car like yours with so much adjustability in your shock that you can tweak it even further. As the cage will tie in the B-pillar and rear inner guards/shock tops together (dependign on the cage you want) So more then the weight you may find that previously you ran stiffer suspension to allow for body flex. So with the stiffer body you now find the suspension to firm.

Fark i dont know.

Think of the chassis as basically an undamped torsional spring linking your front & rear suspension. Chassis stiffness is measured in Nm per degree, ie how many newton metres of torque do you need to put into the thing to get it to deflect one degree. Typically it is in the realms of the 10s of thousands of Nm/degree for a car like a Skyline. Compare this to spring rates of a few kg/mm & you should see it has less effect than what you may think.

The reason that race teams chase chassis stiffnes is simply that it makes tuning their suspension easier. Sometimes it makes it work BETTER, but most commonly it makes it easier to tune. Again it goes back to the chassis being an undamped torsional spring. It is a bit difficult to allow, say, the rear suspension to transfer load to the front by a change in roll centre if it has to go through a heap of sloppy chassis. Also it is harder to get the dampers working correctly if they can only control some of the movement of the vehicle.

[NickR33]

I really doubt you guys with over 300awkw are going to notice 40kgs by seat of the pants... lap times maybe... but a small price to pay for added safety?

I'm keen on the: "around $650-$750 for a cage that keeps the rear seats and therefore keeps the car as a 4/5 seater with Vicroads.

As the cross points are bolt in, same as intrusion bars." mainly so I can mount the harness and a camera and have some side impact/rollover protection... and lets face it R33 GTSt resale values are in the toilet already, its not going to make much of a difference lol

[R31Nismoid]

40 kg is very noticeable - chuck a couple bags of cement or something in the car one day & go for a drive. It doesn't have that much of an effect on lateral grip, but will reduce the longitudnal gees your car can kick out. You will easily tell the difference.

If you want a lighter roll cage you need better quality steel. This was doubtless covered at the talk fest/night thingo but the general principle is as follows:

The different grades of steel make little difference to the stiffness of the cage - the design is the important factor here. But the different grades of steel can make a big difference to the weight of the cage. A stronger (higher yield strength) material can be made as strong without being anywhere near as heavy. Essentially this is because the yield strength of the steel changes much more than the youngs modulus.

With regard to spring rates front & rear they are almost always softer at the back to give the car traction. It isn't really related to front/rear weight distribution as much as you might think.

I wouldnt chuck a couple bags of cement, i would need to spread it out all over the cabin.

The cage doesnt load up one particular point, so why should any other ballast.

I notice *to an extent* with a 80kg passenger in a 300+rwkw car (1400kg) a difference... but its not that much, and a cage is half that again and i could offset that by removing a seat to boot.

Like i said, when in some cases the weight will only be a *comparative* 20kg's to someone else's car due to driver weight.

And most of the GTRs are running similar times taking into account they are all different weights.

Proof is in the pudding there for me.

As for the weight, im just going by the approx's provided by the biggest cage maker in Oz.

He will make it, thats the weight it will be for the price he quoted.

From that, what he tells me is good enough for me taking into account a few other things aswell.

Im not saying your wrong or you dont have a valid point, im just leaving it in the hands of the cage maker himself (and then my suspension man need be).

This is not an environment where 10'ths of a second concern me. I'm more worried about my personal safety, any problems/quirks from a cage install ill deal with as a post upgrade scenario.

[Roy]

LOL...too much science. My point is its all theory, the only way to know for sure is to do it with a rough idea of what to expect. My R32, if i crack the window 10mm and go for a drive in the passenger seat with my fingers jammed in there i can feel the flex. I jact my car up and it slops everywhere. I may have a tired chassis...but its sloppyu. I am pretty sure a cage will change dramatically how my car uses its suspension

[Beer Baron]

well I in my car I have used:

no cage

half cage (with 1 diagonal crossbar, and one horizontal bar between rear towers)

and full cage (with the stupid bendy legs)

I didn't drive the car much with the full cage in as I found the front part very intrusive. I did drive it a lot with no cage, and I now drive it all the time i with rear cage. Firstly, I have no doubt that the aussie made one will be far better than my cusco one. In mine I have noticed a better feel in the rear end, I would say it feels "more direct". as it's only a half cage and is therefore 'half arsed' i can't really say how much better it is overall.

40 kgs I guess is the difference between a quarter tank of fuel and a full tank. I know I can feel that difference. BUT the extra rididity and the safety aspect I think makes this sacrifice well worth it. The price mentioned is very good. I'm almost considering driving down and get one for my nugget!

[djr81]

Yeah, look, please don't get me wrong. I am not for a moment saying don't install a cage. Infact quite the opposite. They are of huge benefit in making a car safer on the track. That & a good seat.

But I am simply saying that 40kg of weight can be felt & measured in your lap times. If you want to save the kilos start by chucking out your fuly sik subwoofa.....whatever that is.

In summary I would suggest spending the extra bit of money for the higher strength steels & to think carefully with regard to where the tubes go near the driver, ie your proximity to them.

The other point is that roll cages which pick up on the suspension towers (or subframe mounts etc) are massively more effective than what a half cage ever could be. The diagonals in the cage are the important bits for giving you torsional rigidity.

[phunky_monkey]

I'll be in for this

[SQUIRREL]

Definatly agree with putting a cage in if you are serious about safety. If any one is iterested I have an R32 for sale complete with Cams approved roll cage and seat and harness. The cage is a work of art and built to compete in Targa and cost $5,000.00 on it's own. I want around $5,500.00 I will through in the car with seat, big slotted rotars etc. All that it needs to complete is motor and box, wheels tyres and brake calipers. The body has been stripped and lightned and will make a very competitive car if someone has a bit of time to spend on it. I live in Tassie but will assist in shipping. Call if interested 0409004259

[Roy]

Mild steel cage? Bugger about the side intrusion bars being the way they are...otehrwise i would have almost handed over the cash straight