Questions about VW1302RSis ride

[VW1302RSi]

Originally Posted by Mikey

Hmm. that took me a second, but I believe I have an answer.

Both. The brace is there to help stiffin the rear suspension as a whole. The frame horns are not supported, and the shock towers are not strong enough to do what we're asking of them with coilovers. But, if you tie them together, and tie it back up to the torsion housing, it gives everthing the support it needs.

Similar to a roll cage. If it was separated right in the middle of the car, the front and back are strong by them selves, but since you tie them both together. They make the whole car stronger. One part isn't relying on the other for streingth, they more or less, compliment each other.

Does that make sence?

Hey Mikey


Your right in that the brace is there to stiffen the rear suspension and transmission mounting points. But I don’t understand your logic, and I was talking about a 3 bar not a 5 bar set up.

This is how I think a basic 3 bar set up works and why but it’s only my opinion and I have been wrong before, after all I am married.

The basis of my opinion,
I have never known a beetle to break a rear shock tower on the road, (if any knows better let me know) I have however seen bent and damaged frame horns, in fact if you jack your car up on the end of the frame horns you will see them bend quite a bit, try doing the same on the shock towers and you see no movement (well I haven’t), the only 2 times I have seen broken shock towers was once in a dune buggy that was airborne and came down hard on that rear wheel and it broke the shock tower and bent the shock, the other was in a car that spun out and hit a lamp post sideways.
If you look at the design of the shock tower it was designed to take all its force in an upward direction and not side ways. Under heavy acceleration, load, wheel spin or whatever the frame horn flexes, this causes the engine and trans to move, this movement can introduce shudder in the driveline and wheel hop.

How I think a 3 bar works,
So if you look at the rear of an IRS beetle with a 3 bar kafer cup brace you have the two bars going from the frame horns up and outwards to the shock towers, if there is any load that would have caused movement either up or down in the frame horn this load will transfer to the shock tower.
If the load is trying to push the frame horns up then the load will travel up in to the shock towers, as we know the shock towers are designed to take load in the upwards, the problem begins now the shock tower was not designed to take this sort of load in an upwards and out wards direction, in theory the distance between the two towers would increase with the load and allow the frame horns to move under load and the opposite would happen if the frame horns moved down the shock towers would get closer to each other and the frame horns would still move.
That is where the third bar comes in, this bar is the key to the system, it stops the shock towers from trying to clap hands or jump out from under the fenders.
I would ague that the shock towers would support the weight of the car with coilovers and modest spring rates, however if the shock was applying any side load (across the vehicle not fore and aft) to the shock tower then you would need to think about some sort of brace, If I was going to make a brace to do this I would go from (looking at the rear of the car) both top shock towers to the centre of the torsion bar housing seeing as this isn’t possible due to that pesky transmission. I would the top of the right hand shock tower across to the point where the left hand shock tower is attached to the torsion bar housing and of course the same for the other side, of course this would be used with the 3 bar set up as well.

That’s enough dribble from me, I’m just playing devils advocate or is that being a prick??

Scott

[zeroaxe]

What I like about logical explanations is that I can actually understand them! I am not by any means choosing sides here, but that explanation sounds about spot-on to me. But I would also like to know if it is wrong in any way :agree:

[Mikey]

I like your explanation better. Sounds good to me. I took a shot in the dark.

I am by no means of the term an expert on suspension.

[zeroaxe]

Ok, so I have been thinking about the explanation and things.... It was mentioned that when the bars are installed, there should be a SLIGHT 'preload' on them. Now exactly how/what order should the preload be applied?

Here is my take on it, but correct me if I am wrong:
The two diagonal bars should be installed with a 'negative' preload. Thus pulling the top of the shock mounts closer to the frame horns. Because of the framehorns flexing downwards when you excelerate hard, you want the 'free play' to be as little as possible by 'pulling them up' slightly. Then the top bar should be installed with a slight 'positive' preload. Meaning pushing the top shock mounts outwards. This way the top bar will cancel out the 'pulling inwards' force that the diagonal bars are causing, and tie the whole lot in nice and tight.

This is my version of a shot in the dark

[VW1302RSi]

Hi Zeroaxe

Originally Posted by zeroaxe

Ok, so I have been thinking about the explanation and things.... It was mentioned that when the bars are installed, there should be a SLIGHT 'preload' on them. Now exactly how/what order should the preload be applied?

Here is my take on it, but correct me if I am wrong:
The two diagonal bars should be installed with a 'negative' preload. Thus pulling the top of the shock mounts closer to the frame horns. Because of the framehorns flexing downwards when you excelerate hard, you want the 'free play' to be as little as possible by 'pulling them up' slightly. Then the top bar should be installed with a slight 'positive' preload. Meaning pushing the top shock mounts outwards. This way the top bar will cancel out the 'pulling inwards' force that the diagonal bars are causing, and tie the whole lot in nice and tight.

This is my version of a shot in the dark

Ok I’ll correct you, cause I think your wrong but as I have stated before this is my opinion and not gospel, I have been wrong before, I’m bound to be wrong again, just ask my wife.

I don’t know were you got the bit about preload from, as a rule I wouldn’t preload the bars but make sure there was no play in any of the joints when they are installed. There is bound to be some play between the bar ends, mounts, and bolts, that is the play I would remove, but I wouldn’t wind any preload on to them, other than just the amount required to remove the play.

Also what I would do is if you are installing the bar on a car that’s is all reedy going then I would adjust up the bars with the car sitting on all 4 wheels, so if your not real skinny then you would need ramps or if you’re a bit like me then a 4 post hoist wouldn’t go a miss. If the car is still in the building stage then I would just loosely fit the bars, but once you are ready to drive it I would remove the bars and get a few miles on the car to settle every thing down, then recheck the engine/trans mounting bolts and fit the bars with the car sitting on all 4 wheels.

The reason for this is if you install the bars with out the engine and transmission in place then the frame horns may not be in there correct place and buy fitting the bars once you have installed the engine and trans you may be inadvertently preloading the bars.

And because I don’t like looking like a goose I just checked CSP’s website and looked at the instructions for there “torque bar” and they actually say there is no need to preload the bars, Its quite a good set of instructions. http://www.csp-shop.de/technik/pdf/deeng/20917.pdf
Anyhow I must go, the wife is calling

[zeroaxe]

Sorry, when I said "slight preload", I was actually referring to the free play. But at least we agree on that

[ricola]

Great extra pics and video on your site. Sounds nice :-) Any reason you didn't just reverse the coolant manifold?, look like a lot of work went into remaking it like that!
Rich