Luiz stated that he got a new 28mm rear sway bar recently....I noted that this one was for armoured cars and the part number he posted was for a 20mm sway bar. It ends up, it was a 20mm bar.

He would like to get an upgraded rear bar, and I would too because I made the mistake of putting on the 17.4mm bar instead of the 20mm.

Anyway, there is a company, <a href="http://www.hellwigproducts.com/" target="_blank">Helwig Products</a> that stated they would make a custom bar for Luiz for $450 plus shipping. He wants to order one.

I am also interested. It's 8-10 week delivery, so it will be here for track season in April. It appears it will be cheaper with more than 1.

I have a doner A8 bar that will be used as a template.

<b>Need Help with Technical</b>
<i>Thanks for the info, however, at this time I cannot justify tooling a new kit without some sales up front. We can manufacture a custom sway bar for you if you provide us a sway bar as a pattern. Cost for a custom bent sway bar will be $450 USD FOB Visalia, CA. You will need to arrange for shipping and export fees. The custom bar will require 8-10 weeks lead time as it requires hand forming by one of our R&amp;D technicians and we have quite a backlog right now. This bar will be made from solid 4140 heat treated steel in whatever diameter you specify. You will also need to provide end links and bushings to match the new sway bar.</i>

So the question is, since the stock bar is hollow, and the new one will be from 4140 heat treated steel, what bar should we get to achieve 150% stiffness as compared to a stock S8 20mm bar?

Or would you specify a different percentage?

Randy? Bueller? Anyone?

 

If so, the calculations shouldn't be too difficult. I would carefully select the percent stiffness increase, since balancing the front and rear handling characteristics is just that- a balancing act. You don't want to over do it and dial in oversteer. I'm NOT the guy to answer that question, however.

 

From Randy's previous post:

13mm ---- 14 lb/in
17.3mm -- 39 lb/in
20mm ---- 67 lb/in
28mm --- 211 lb/in

These are for hollow stock tubes.

pw

 

Here's my calculations for 47" long sway bars with 10" lever arms.

Hollow bar with an assumed 3mm wall thickness,

13mm ----- 14 lb/in
17.3mm --- 39 lb/in
20mm ----- 67 lb/in


Solid bars,

20mm ----- 88 lb/in
21mm ---- 107 lb/in
13/16" ---- 100 lb/in
7/8" ------ 134 lb/in

If the 3mm wall thickness assumption is correct for the hollow bars, a 20mm solid bar would be "safe" upgrade; but I expect that Hellwig is using English unit bar stock. I'd be willing to try even the 7/8" (it wouldn't be for the faint of heart), but I agree with you and would prefer the 13/16" bar.

As skiracer said, though, we need to confirm the wall thickness of the 20mm bar. Since Luiz has a brand new uninstalled bar, can he get a measurement?<ul><li><a href="https://forums.audiworld.com/a8/msgs/78735.phtml">Earlier sway bar calcs</a></li></ul>

 

How shoudl I do to measure the wall thickness of my rear sway bar? Perhaps should I saw it? If so, I will saw the old one

 

The strength highly depends on the surface. The hollow has almost double surface. The solid bar is stronger if you want to bend it, but for twisting the hollow is least as strong as the solid bar. Also the solid bar is probably 10 lbs heavier.

 

The end of a hollow bar is just squeezed flat before drilling the holes for the end links. You should be able to see and measure the wall thickness there without any problem.

 

You're absolutely correct, the center of the bar does almost nothing except add weight. <A HREF="http://www.h-sport.com/cgi-bin/EDCstore.pl?user_action=category&category=Shop%20b y%20Make%3BAudi">H-Sport</a> makes hollow bars for just about every Audi ... except the A8. Perhaps we need to talk to them?

 

Yes, take total thickness and divide by 2

 

FWIW, the 2 ton "pig" urS6 avant has sported the 28mm "Happersized" rear sway bar, aka HRSB (dont ask) for well over a year now and it is quite simply a radical transformation. Some notes and pic can be found here:
https://forums.audiworld.com/s4s6/msgs/19991.phtml
Would you have some idea as to the strength of the frame mounting tabs for the bushings on the A/S8?
I have not heard of any breaking but it could be a concern on the less than perfect city street pot holes.
IMHO this could be a grA8 mod with a lot of bang for the buck. Even though I swear the A8 is grA8 enough as it is, I would sign up for one of these in a heart beat. (and please someone stop me if I want any additional mods Thank you very much!