Convert blade rear roll bar to T-post

[Lyngengr]

Has anyone converted the semi-adjustable rear anti-roll bar setup back to the original T-post configuration? Mechanically it is simple, but I'm wondering how the stiffness of the T-post compares to the blade setup. The post comes in two different diameters, 1/2 and 5/8, I have both but I will probably use the smaller diameter one. Short of making some kind of bench-top test rig to measure stiffness, is there any other way to determine how stiff the T-post setup is compared to the blades?

[Lotus7]

I would suggest there are too many assumptions and approximations in trying to calculate the stiffness of the blade design in its various angled settings. Set up a simple comparative rig.

Be aware that the T-post design used by VanDiemen has a propensity to fail at the upper internal weld, and the failure can't be seen "as installed", so disassemble it and check it regularly.
Cheers

[Rick Kirchner]

I wanted to use the late model "T" type bar on a '94 to get the attachments off the rear wing to facilitate gear changes. I always left it full soft anyway. We calculated the stiffness and then designed this bar to approximately the same stiffness. My dead a$$ can't tell the difference. We used this design because the early cars don't have the spine that runs down the center of the engine cover to accommodate the bar.

We designed the levers to have no effect on the bar's stiffness, and the connecting tubes to have minimal effect, but I suppose it would be easy enough to design them to act like one of the original chopstick bars. On a 90-96 car the length is limited by the need to go under the spider but on a 97 and up car the limitation would probably be in the heim joint angular limits.

Basically, you have a bar of given length and diameter and a given lever length to get the desired stiffness. The pivot points are brought to the center through two thick tubes made from bar stock and they are considerably stiffer then the bar inside them.

[BeerBudgetRacing]

Has anyone converted the semi-adjustable rear anti-roll bar setup back to the original T-post configuration?

Lyn, Why are you considering this? What problem are you having with the blades?

[Lyngengr]

Lyn, Why are you considering this? What problem are you having with the blades?

Only one of the blades is adjustable, the other one you just set for what you think is where you want it and hope it stays at that angle. I have had the "fixed" blade not stay in the set position and wonder why the car either became loose or started pushing. The retention mechanism for the "fixed" side is really a poor design, relies on friction to keep it in position. I resorted to saddle washers to keep it locked down and that sort of works but again is relying on friction. The adjustability is sort of a wish, really, because it is very stiff and I tried once to change it while on track and couldn't get it to move. The cockpit adjuster likes to get tangled up in my arm restraint. And most recently, the tab that is on the adjustable blade broke off so I'll have to get that welded back on and hope it doesn't break again. Basically, the adjustable blades are a PITA and since I don't mess with it on-track there is no good reason to keep it. I guess if you like a loose car there is some logic to keeping it, but I don't. I'd rather be in the set it (soft) and forget it mode with the rear roll stiffness. I'll get the tab welded back on and then keep the whole thing as a spare. Save a pound or two in weight. I'm pulling off the side panels this winter to get both repainted so now is the time to remove the cable and tidy up the other two lines (brake & clutch) that run alongside the adjuster cable.

[SV@RHC]

Talk to Bill, We welded arms to each blade and then connected them together with a short rod with ends. That way when you made a bar adjustment it rotated both blades at the same time. I made the move after years of using the stock cross bar like you are thinking about switching to, I liked the blade set up better.

Scott

[Mike B]

If you do get rid of the adjustable bar I'll take it off your hands.

[alangbaker]

Talk to Bill, We welded arms to each blade and then connected them together with a short rod with ends. That way when you made a bar adjustment it rotated both blades at the same time. I made the move after years of using the stock cross bar like you are thinking about switching to, I liked the blade set up better.

Scott

Scott, there's a blog post about that...

...or there was before thinkfastengineering.com went offline.

But basically, he made the case (convincingly) that simply rotating the blades by linking them had problems of its own.

[Spengo]

Scott, there's a blog post about that...

...or there was before thinkfastengineering.com went offline.

But basically, he made the case (convincingly) that simply rotating the blades by linking them had problems of its own.

https://web.archive.org/web/20220525...2/04/farballs/
Is this the post you are thinking of?

[SV@RHC]

Scott, there's a blog post about that...

...or there was before thinkfastengineering.com went offline.

But basically, he made the case (convincingly) that simply rotating the blades by linking them had problems of its own.

Thanks Alan good read! For whatever reason I still liked the adjustable blade setup, I'll have to chalk it up to one of those great mysteries.

[DaveW]

..., he made the case (convincingly) that simply rotating the blades by linking them had problems of its own.

IMO, that statement depends on the way the blades are installed, oriented, and guided.

For example, if:
o the swaybar links are fore-aft
o the 2 blades' flat sides are parallel to each other
o the connecting link between them keeps them that way
o bar at their base is straight across
o the blades are essentially perpendicular to the swaybar links in side view
o the blade ends are free to deflect horizontally perpendicular to the links,
then the opposing swaybar link deflections (forces) in roll cause the blade end deflections to laterally mirror each other. With reasonably small deflections, that will not cause any strange behavior.

My front swaybar is set up that way.

[alangbaker]

https://web.archive.org/web/20220525...2/04/farballs/
Is this the post you are thinking of?

The very one!

[DaveW]

https://web.archive.org/web/20220525...2/04/farballs/
Is this the post you are thinking of?

IMO, that statement depends on the way the blades are installed, oriented, and guided.

For example, if:
o the swaybar links are fore-aft
o the 2 blades' flat sides are parallel to each other
o the connecting link between them keeps them that way
o bar at their base is straight across
o the blades are essentially perpendicular to the swaybar links in side view
o the blade ends are free to deflect horizontally perpendicular to the links,
then the opposing swaybar link deflections (forces) in roll cause the blade end deflections to laterally mirror each other. With reasonably small deflections, that will not cause any strange behavior.

My front swaybar is set up that way.

The major difference in performance in these 2 setups is that Spengo's referenced version is much more linear throughout its adjustment range than my setup. However, if the blades and their connecting bar are appropriately sized, the middle portion of the 0-90-deg somewhat sine-wave-shaped range, say 20-70 deg, provides enough stiffness variation to make the roll stiffness variation useful.

Another difference is that my setup is much simpler and lighter than the referenced assembly.

As an aside, in my experience, the front roll stiffness is more useful in setting the overall roll stiffness, and the rear is more useful for affecting the F-R balance during a session.