Coupe Rear ARB options aka FARB on the rear of a coupe

[Rusty]

Torque = Force x distance from centre of rotation. Thus, the increase in length means a greater torque is applied. See the beginning of the thread for my calculations and assumptions. Feel free to offer some others but if you start introducing distance moved then that is talking about energy stored rather than the basic torsional modulus of the material and thus torsional stiffness.

[stiglet]

He's saying that because the drop link is attached further inboard the wishbone it has less leverage, but i thought the difference was neglible as the drop link on the normal roll bars are mounted on the macpherson which moves slightly less anyway.

[Error404]

well in my logic, when the subframe is taken as a reference point, the strut is levered by the balljoint essentially in terms of vertical movment, so the older ARB mounts 1/3 way up the wishbone would give less travel

[Rusty]

The front roll bar levers are 1.5 times longer I think. Best check the original calcs. They assumed one unit of twist. The lower movement will change things a little but the diameter^4 term is very strong. Work it into the orgincl calcs and see what happens as I'm on my phone so don't have the luxury.

[Error404]

maths isn't my strong point andy
if you were a real geek you'd use the formulas in Excel

[Audi_CQ]

He's saying that because the drop link is attached further inboard the wishbone it has less leverage, but i thought the difference was neglible as the drop link on the normal roll bars are mounted on the macpherson which moves slightly less anyway.

well in my logic, when the subframe is taken as a reference point, the strut is levered by the balljoint essentially in terms of vertical movment, so the older ARB mounts 1/3 way up the wishbone would give less travel

[stiglet]

Sorry, i was thinking because the strut is tanked it moves less for a given bump distance but it's irrelevant anyway, the older coupe bars are defiantely not as stiff as the newer ones attached to the struts so 2/3s is probably a good estimate.

In future i won't post when i've been drinking!

[Rusty]

I got thinking about this last night and basically taking the angle of twist from a theoretical horizontal position to a new position means that the ratio of the sines of the angles are proportional to the lengths of the levers. Using small angle theory for sine of an angle is approximately the angle, that means that the theoretical angle of twist is reduced by that ratio, so to properly compare front and rear bars the figures I've quoted need to be further divided by 1.5 for the front bars. As I mentioned earlier, the figures quoted were for one unit of twist.

The mounting to wishbones option means approx. 2/3 the movement so should be treated as a front bar as Stiggy posted earlier. I'll update the figures later for this and the spreadsheet too.

There's little point in getting too involved in the calcs as there are plenty of unknowns and assumptions. Things like the applied torque being a component of the applied force for larger angles, the bending of the levers and, such like.

Stiggy, posting when drunk is all part of the fun. I had a brilliant reply to a thread years ago, I think it was Tom. He must have been obliterated!

[Error404]

Right - got my parts back from the shot-blasters today and Al has kindly sent down some kinky drop links for me.

All mocked up it looks like a runner









The cup washers, new arb mount rubbers, droplink bushes and sleeves will come to about £30 all in I think, add another £20-25 for new drop-links too.

Andy, it would be useful to get your thoughts as you already have the bushes in your recent rear wishbones - does this look as if it would bolt up correctly to yours mate?

[Rusty]

Looks like it'll play! The only issue I can see is clearance on the corner of the subframe with the bar. Might just be a trick of the camera tho. Overall that looks gert lush!

[Error404]

Yeah i thought that when i was offereing it up, but theres plenty of clearance there. The wishbone fouls the subframe before the ARB does.

I hope the 22m rear bar will be a good relatively balanced match for the 26mm up front

[Rusty]

It'll be fine as it should be a touch softer than the 24mm option I ran and that was nice, 25mm is quite aggressive and I have a 26mm to try if the HnRs soften the handling a bit Its how they should have left the factory!

[Error404]

yup, i'm guessing about 5x stuff than the stock bar (assuming thats a 14mm bar - i couldn't remember )

the 24mm could be up to 7.5x stiffer, and the 25mm bar 9x stiffer.... !!

I'm going to try and find a 23mm, 24mm and 25mm older style bar so I can chop and change and find the best set-up, and also dial it in with future mods

[Rusty]

Original numbers:-

1.0 ...... Std S2
4.4 ...... Whiteline (soft)
4.8 ...... Whiteline (med)
5.3 ...... Whiteline (hard)
15.8 .... Non S2 Coupe Quattro - late type using S2 style drop links.
18.5 .... Std S2 FARB.
21.5 .... RS2 FARB.

22mm bar as per Error404 calculates at just over 11 times stiffer than standard but only moves by approx. 2/3 of the full strut deflection.

All these numbers are open to errors and are for 1 unit of twist so do not take into account the relative movements fo the bars. I need to go and measure the OE rear and front bar level lengths as they were guesses originally...

Going back to the thing about leverage, relative angles etc. If the lever length is irrelevant due to the

The FARB B4 options need to be further divided by

[Error404]

I've got a S2 type front and rear sitting at home i'll measure later mate save you crawling around on the floor in your state


....although you probably have too i guess