Modified 944's

[944 man]

ORIGINAL: John Sims

My comment was on the suggestion that bolt in cages are deliberately designed not to increase structural rigidity.

And I stand by that comment. The purpose of a six point bolt in cage is to strengthen the cabin area of the vehicle to make it more resistant to impact. Its purpose is only to reinforce the cabin area and suspension pickups are prohibited in many classes to specifically prevent the structure from being used as a stressed member.

 

[Hilux]

Anyway; if you really want to stiffen a car (where a proper multipoint cage isnt permitted by the rules) you seam weld it.

..............................

.........................and tie the front and rear suspension longitudinally and laterally utilising the cage and strut/tower braces. All of the above and aforementioned will stiffen a car if used together but a half cage wont make a blind bit of difference IMO.

Only thing I would add is that I dont trust the simple bucket seat floor supports regardless if they are OMP or Recaro et al. They wont come away from the floor provided strengthening/spreader plates are welded in but will and do allow movement side to side so therefore will your head but at an increased rate as most are just simple `L` shapes with multiple holes in them. As long as you have at least 5 point harnesses to stop submarining and good well mounted seats the fore and aft movement is naturally limited. The complication is lateral movement therefore you are far more likely to hit the top roof rail and door frame with your head (even more likely with a crash hat) than you are to hit the front cage protrusions so I personally would (and will to my current project) add seat back stays/braces. They keep skinny bucket seats from flopping about laterally and therefore your head.

 

[Fen]

I think we're probably in agreement. What I am saying is that a bolt-in cage has to add rigidity however I doubt it would be noticeable and I do not believe it is any reason to fit a half cage. You however said in post 89 "Do you honestly believe that a simple six-point bolt in cage adds structural rigidity?" which I contend it will, hence your statement there is incorrect.

In terms of your experience, fine, it hadn't come across in your previous posts.You have a depth of experience in this area, but I bet many others do also (some I know do). I made comment on this not because I thought you didn't know what you were talking about, but because I didn't see anything in your posts to confirm that you do, and that that is also the case for others that you are implying with "There is a lot of repeated opinion in this thread, but very little real knowledge" in post 91 don't know the subject.

Now I don't want to get into an argument with you because I know from your posts here and on TIPEC that you have a lot of experience and knowledge and I respect you for that, but I am going to pick you up on what I see as sweeping generalisations that are incorrect, and I think suggesting someone is naieve to believe a bolt in cage can add any structural rigidity is just plain wrong.

Moving on to the desirability of adding rigidity or what it is used for I think things might get silly. Are you really going to suggest that stiffening part of a chassis is a bad thing? Sure I guess it's possible that stress could be concentrated by removing the damping effect of flex elsewhere, but I don't believe that is a problem we'd be likely to face with a 944 running suspension that can be used on the road (which is the specific scenario we are discussing here, remember). What it is used for is barely even worthy of a response. Any flex, anywhere in the chassis will obviously upset handling by allowing suspension components to move relative to one another and therefore outside the ideal relationship. Possibly not significantly in a 944 coupe and possibly to a lesser degree than even a minor mal-adjustment in geometry or tyre pressure, but again clearly undesirable. At this point I might also state that the cumulative effect of tiny improvements that individually are hard to detect can be very worthwhile.

On reflection I think the biggest negative with a bolt-in rear cage is that if the time comes to rely on it that it might just punch holes through the bottom of the floor.

In terms of tyres I won't disagree. I believe I am correct in saying that on TIPEC we two are the only people who have advised the guy who has a totally standard Lux not to buy R888s as his first trackday modification as they will overwhelm the chassis.

 

[944 man]

ORIGINAL: John Sims

ORIGINAL: 944 man

....... You havent added any worthwhile strength, because the suspension mountings are still all unreinforced, but you have reduced the area of the structure which is able to absorb the still present flex/movement...

You best take that up with the MSA then as they are obviously deliberately putting drivers at risk.[]

.....Its a good job that I didnt mention anything about not running List 1B or slick tyres in an unprepared chassis..... []

Why's that? We could then have something to agree on. []

No they arent, theyre deliberately preventing competitors from increasing the rigidity of their chassis by connecting it to the roll cage, which is only intended as a safety feature. We are only talking about six point bolt in cages here, remember? 2004 onwards FIA / multi-link weld in typs cages are completely different. The MSA arent putting anyone at risk: people binding a six point bolt in cage into their bodyshell with excessive tie-ins (thereby making the cabin area alone more rigid) are accelerating the wear on the rest of ther bodyshell, as the ability to pass the movement over the hwole structure has been lost...

Slicks only belong on fully prepared bodyshells. By increasing the stresses on an unreinforced bodyshell, you will eventually cause it to fail. Ive seen a Evo5 used principally for track days and run on slicks; which now has the structural integrity of a blamange. It was so bad that I dont think that seam welding it would have been enough. People dont want to hear about this though.

 

[Fen]

ORIGINAL: Hilux

Anyway; if you really want to stiffen a car (where a proper multipoint cage isnt permitted by the rules) you seam weld it.

..............................

.........................and tie the front and rear suspension longitudinally and laterally utilising the cage and strut/tower braces. All of the above and aforementioned will stiffen a car if used together but a half cage wont make a blind bit of difference IMO.

Only thing I would add is that I dont trust the simple bucket seat floor supports regardless if they are OMP or Recaro et al. They wont come away from the floor provided strengthening/spreader plates are welded in but will and do allow movement side to side so therefore will your head but at an increased rate as most are just simple `L` shapes with multiple holes in them. As long as you have at least 5 point harnesses to stop submarining and good well mounted seats the fore and aft movement is naturally limited. The complication is lateral movement therefore you are far more likely to hit the top roof rail and door frame with your head (even more likely with a crash hat) than you are to hit the front cage protrusions so I personally would (and will to my current project) add seat back stays/braces. They keep skinny bucket seats from flopping about laterally and therefore your head.

I tend to agree - I would fit back braces to my seats running off the cage if I had one as the seat shell has very noticeble flex. My side mounts use the OE holes in the floor though so I believe that that aspect will be strong enough.

 

[944 man]

Fen: the principal point, I think, is that an increase in the rigidity (less in my opinion, than in others, but varying depending upon how much further tying-in is undertaken) in the cabin area, is of no real benefit. All you have achieved is to make the cabin less able to flex, but that doesnt practically help you. If youre able to take the more-rigid cabin area and connect it to the fore and aft suspensions mountings, then and only then, will you see a real benefit. Then, you have gone way beyond the remit of a simple six point bolt in cage and youre firmly in territory of a multi-link welded in triangulated bad boy.

I stand by my point that making the cabin area alone more stiff (through further plating and tying-in) is a bad move. The stiffness offers no practical benefit and it does, in fact, cause undue stress to the area of the chassis ahead of the firewall (and behind the rear firewall, although not in our cars, of course).

 

[944 man]

ORIGINAL: Fen

ORIGINAL: Hilux

Anyway; if you really want to stiffen a car (where a proper multipoint cage isnt permitted by the rules) you seam weld it.

..............................

.........................and tie the front and rear suspension longitudinally and laterally utilising the cage and strut/tower braces. All of the above and aforementioned will stiffen a car if used together but a half cage wont make a blind bit of difference IMO.

Only thing I would add is that I dont trust the simple bucket seat floor supports regardless if they are OMP or Recaro et al. They wont come away from the floor provided strengthening/spreader plates are welded in but will and do allow movement side to side so therefore will your head but at an increased rate as most are just simple `L` shapes with multiple holes in them. As long as you have at least 5 point harnesses to stop submarining and good well mounted seats the fore and aft movement is naturally limited. The complication is lateral movement therefore you are far more likely to hit the top roof rail and door frame with your head (even more likely with a crash hat) than you are to hit the front cage protrusions so I personally would (and will to my current project) add seat back stays/braces. They keep skinny bucket seats from flopping about laterally and therefore your head.

I tend to agree - I would fit back braces to my seats running off the cage if I had one as the seat shell has very noticeble flex. My side mounts use the OE holes in the floor though so I believe that that aspect will be strong enough.

Ive advocated a back brace for years and people just point at me and stare! This is the first time that I have ever seen anyone else agree.

 

[homesea]

I would like to talk from my experience of owning a fully prepared car. The car has slicks, bolt in cage, shell seem welded every so many inches and many suspension upgrades. A bolt in cage offers massive amounts of structural rigidity because of how it's fitted. You don't just bolt two bolts through the floor of the car and also I would like to see the difference in strength between bolts and weld. Yes a multi link weld in cage would have extra benefits over a 6 point bolt in due to the extra links. But to say a bolt in cage doesn't offer any extra rigidity is madness. My cage is for strength 1st but you can't add strength without added rigidity. My car has been on its roof (new shell and cage now) and the bolt in cage did its job.

 

[Fen]

I actually think we agree on the vast majority of points.

 

[Hilux]

I have a Roll Centre cage in my golf and its a well made bit of kit, the harness bar is not part of the cage.

Mk 1 ??

Ive advocated a back brace for years and people just point at me and stare! This is the first time that I have ever seen anyone else agree.

Belts and braces me.................

I simply consider the aftermarket seat supports and look for rigidity. Funnily enough I dont see any lateral rigidity so you know they WILL move side to side, they have to.

By the way, I`ve been there (hanging in a harness) after a major side impact and roll so am ever more thoughtful about what I put into a car I`m building.

On reflection I think the biggest negative with a bolt-in rear cage is that if the time comes to rely on it that it might just punch holes through the bottom of the floor.

I am in complete agreement. Thats why a weld in cage to FIA spec etc has strengthening plates welded on BEFORE the tubes are welded on. A couple of bolts are ok and will probably be ok but will stretch/shear/deform the metal far more so if regularly thrashing it a weld in cage is much better.

 

[zcacogp]

ORIGINAL: Hilux

I have a Roll Centre cage in my golf and its a well made bit of kit, the harness bar is not part of the cage.

Mk 1 ??

That's what I thought. And drooled ...

Sorry to interrupt the row chaps ... as you were ... []


Oli.

 

[944 man]

This isnt a row: this is enriching and enlivening the forum: and youre learning something too...[]

 

[333pg333]

Some interesting points to come out of all this. I still haven't come to a conclusion about a half cage being worth it. Personally I don't think some of the bolt in cages would help much but I can imagine others perhaps offering some assistance in some circumstances. Are we all at risk doing trackdays without any form of cage? How many tens of thousands of people around the world do this? Is the likely hood of rolling severely reduced by not actually participating in door to door racing as opposed to timed events in file?
You also raise an interesting point about the choice of rubber being detrimental to our chassis/bodies. So what happens? Does this introduce metal fatigue? Most of us are talking about trackdays where we circulate for 3-6 laps and come in. We do that maybe 4-6 times a day and go home until the next trackday or sprint in a few months time. Are we doing any damage to our cars by using R spec or stickier rubber? Again, thousands of people do this. The case of the Evo on slicks may demonstrate what happens to a car that does a lot more miles on track AND lets face it, is not up to the build standard of our vehicles. I am intending on moving up to a Hoosier type tyre next season so I hope that my car doesn't fall apart around me.

 

[zcacogp]

ORIGINAL: 944 man

This isnt a row: this is enriching and enlivening the forum: and youre learning something too...[]

Actually, although I suspect you wrote that in jest, it's true. I am learning as I read this. A lot of what I believed to be true about cages has been mentioned on here, and has been challenged, so it is educational - thanks.

FWIW, I can't believe that a cage can't fail to strengthen the car, regardless of how it is bolted or welded in. It stands to reason that any extra bracing anywhere in the car will affect the bend stiffness of the monocoque. The question is whether the degree of assistance it provides is beneficial. In your Volvo case, I can see that even a slight improvement will make a big difference. In the case of something naturally stiffer, such as a 944, a similar improvement would be much less noticable as the starting point is much higher (i.e. the Nm/degree rating of the shell is much higher to start with.)

The point about strut braces is interesting. When I put a bottom brace onto my Mk1 GTi (Golf), it made a staggering difference to the stability around corners, the turn-in alacrity and the steering feel. When I rigged up a crude top brace for the same car it made no difference at all, so I abandoned the project. But then, the Mk1 Golf was always known for having weak lower chassis arms, and a relatively sturdily attached pair of strut tops (as opposed to the Mk1 cabrio, which is meant to benefit hugely from the top brace as well, and has a rear brace welded in by VW in the factory.)

However, I cannot make any contribution concerning cages from a position of experience, having never even sat in, let alone driven, a car with any form of cage in at all. And I fear I won't ever be in the lucky position to contribute from experience, so I'll pipe down right there.


Oli.

 

[sawood12]

OK, lets attempt to put things into perspective here. To say a bolt in half cage (correctly installed of course with welded spreader plates) wont add any stiffness to the body is clearly not correct even if it is not the primary objective of the device. How do you expect it to work as a safety device if it doesn't add stiffness to the structure of the car? The two aspects are mutually inclusive. You make structures stronger by stiffening them up, so you will benefit from a stiffer chassis at all times.

The reality is that the vast majority of us who dabble with trackdays are more likely to roll our cars on the journey to and from the track through being involved in an RTA caused by a half-asleep foreign truck driver. And no-one is advocating the use of full FIA spec roll cages in street cars.

 

[rickware]

wow. i think i may have started this by saying i noticed a difference....to bennyboy who first asked me about installing a cage if your just building a track day car like mine i would say go for it as having a cage around you certainly makes you think the car holds better!!! now i suspect it is just because it feels like a racer and nothing more but hey isn't this the point?

 

[edh]

ORIGINAL: 333pg333

Some interesting points to come out of all this. I still haven't come to a conclusion about a half cage being worth it. Personally I don't think some of the bolt in cages would help much but I can imagine others perhaps offering some assistance in some circumstances. Are we all at risk doing trackdays without any form of cage? How many tens of thousands of people around the world do this? Is the likely hood of rolling severely reduced by not actually participating in door to door racing as opposed to timed events in file?
You also raise an interesting point about the choice of rubber being detrimental to our chassis/bodies. So what happens? Does this introduce metal fatigue? Most of us are talking about trackdays where we circulate for 3-6 laps and come in. We do that maybe 4-6 times a day and go home until the next trackday or sprint in a few months time. Are we doing any damage to our cars by using R spec or stickier rubber? Again, thousands of people do this. The case of the Evo on slicks may demonstrate what happens to a car that does a lot more miles on track AND lets face it, is not up to the build standard of our vehicles. I am intending on moving up to a Hoosier type tyre next season so I hope that my car doesn't fall apart around me.

Lots of good points Patrick (or can I call you Partick? [] )

There are fewer crashes on trackdays compared with races - But I have seen a few cars that have rolled on trackdays at Donington (digging in at the gravel trap at the Old Hairpin). None of them were flattened in a way that would have threatened driver safety.

You may be increasing the stresses on your shell by running r-spec tyres, but I can't quite see it being significant. It's just something else to worry about, but way down my list. I wonder how Andrew Sweetenham's 944 is holding up? - he runs it pretty hard on trackdays on slicks. With the money you've spent, a re-shell every few years would be negligble anyway....

That talk of blancmanges reminds me of my first track car - A v6 Jelly Mould that I ran on s/h slicks (because it was faster & was fun to go past cars in a £500 shed []) That had a bolt in cage, but I was probably in more danger in a crash on the road from the wheels & tools I used to cart around in the back. I didn't worry too much about it falling apart, & it didn't, even though it was a Ford. (we blew it up eventually after turbocharging it ).

btw - 3-6 laps? - I thought you Aussies were real men? []

 

[sawood12]

I think the point about slicks or R spec tyres is that they increase cornering speeds and therefore the liklihood of you coming a cropper, and if you do come a cropper you're probably carrying another 10mph or more into the scene of the crash.

I guess if you roll in the gravel, because the gravel yields, there is not as much force being taken by the car. Its a bit like if you were to forward roll on a foam mattress rather than a hard floor.

It's all about how the structure transmits and absorbs energy. A cage is a stiff structure so transmits energy to the softer parts of the structure to absorb the forces - i.e. the rest of the car. That was the main point of my first post about the front end of the car being asked to suddenly deal with more force than if you had no half-cage - i.e. the forces normally absorbed by the rear of the car would be transmitted to the front if you have a half cage, thereore the front part of the car has to deal with more force.

But I guess crashes range from minor to pretty major. I'm sure a half cage would be fine in a minor crash/roll. If you have a major stackup involving multimple rolls/flips then the half-cage could very well introduce complications.

 

[Fen]

The tyres consideration is more to do with putting more force through the suspension and to a lesser degree chassis than it is designed for in my view.

20-25 year old road car suspension is so far out of its depth on track that it's not funny (even if it's brand new) and that situation is massively exacerbated if tyres that have far stiffer sidewalls and massively more adhesion than was available 25 years ago are nailed on.

 

[Neil Haughey]

I do recall a few years back reading about heavily tracked E30 M3's suffering from cracks in the chassis. Never heard of any 944 suffering from this though? What is the chassis longevity like for cars in the PCGB championship like for example.

I agree that massively stiffening the passenger compartment whilst adding no extra stiffness/bracing to the suspension pickups fore and aft will increase the amount of flex in those areas and hence increase the odds of getting the dreaded cracks.