A chain is as strong as the weakest link?

[John Gaines]

I was looking at the rear mount points for my roll bar when I was fitting an FIA bar to my ྜ live axle. The rear mount points seem amazingly weak when compared to the size of the cross-sectional size of the bar. When you have the boot cover on it looks strong, but when you look at the mounts you realise what is going to give way first ("O" level metalwork and "A" level D&T, Garelli Tiger Cross,

BSA Starfire" etc. etc!) Is there a way of giving more strength to the mounting points so that the force of a roll can be distributed without the bolts shearing their mounts or have I missed something?

 

[scooby dooby doo]

On my 96 supersport car (ie with a supersport rollbar) the bar broke first. The weld between the tube and trailing bit went...

 

So I guess its not as weak as it seems....

 

HOOPY CYCLE WINGS CUCUMBER

[Top Hose]

Ah, those were the days. If I recall, the only thing to break on my Tiger Cross was the exhaust. Not sure if that was before or just after I had whooped the a*se off the local Fizzie 😬

 

Come to think of it, the seat broke, the mudguard broke, one of the main bearings gave up the ghost, and the petrol tank was a bit dodgy, but it was a hell of a moped.

 

Apologies, I'm hi-jacking your thread.....

[scooby dooby doo]

Doh! read the ****ing question. you said rollbar and i thought you meant anti-rollbar....

 

ho-hum.... it was late etc etc...

 

Most of the load on an FIA Roll Bar is taken at the front mount (which is much more substantial). but you're right - its not that solid at the back is it. At least the SL cage has solid chassis monuts along side the driver and passenger. Although the pathetic unserscuttle ones can't do much.

 

HOOPY CYCLE WINGS CUCUMBER

[Andrew Walker]

I guess that's why the Petty Strut, which attaches to the chassis on the passenger side under the dash, is such an important part of the structure. I tend to leave mine fitted to the car even when it's not on a track.

[PeterS]

John

I think you raise a legitimate concern. There is the possibility that if you have inverted and still have some forward momentum, that those back braces will be in compression and the weakness of the members that the braces connect into will cause the bar to rotate backwards. The base plates at the bottom of the main hoop are capable of some kind of moment resistance, but I am not too convinced about their abilities. If you have an FIA bar, there is a holding down bolt, underneath, which must be in place, and this could help. When we bought our latest 7, the FIA bar did not have these bolts in place, so it is worth checking.

Obviously, having a full cage is much better, as you have a better structural resistance to forward or backward movement whilst upside down.

If you use a Petty strut, you prevent the passenger seat being used, and if the strut is in tension (i.e. a tie rather than a strut) there may be some couterbalance to the weakness you have identified. I would however worry about the forces that can be taken by the lower bolt of the Petty strut. Incidentally, I'm told that the FIA approval of the Caterham bar requires the use of the strut.