Rear Swaybar Drop Links

[Steam Driver]

I have a set of Tarett Engineerings rear sway bar links to install on my SC. However I have what may be a stupid question.

What is the function of the offset hole in the metal bushing? Obviously the offset is there as an adjustment function but for what and why? It doesn't seem to have enough offset to have much effect. Is this a way to offset preload on the bar?

Any suggestions will be appreciated.

[dannobee]

Rear camber adjustment. Or toe. One or the other.

[Steam Driver]

No. The sway bar has nothing to do with either of those.

[winders]

Originally Posted by Steam Driver

No. The sway bar has nothing to do with either of those.

Sure it does....if you mount the drop link to the spring plate like many do today.

[winders]

See here:

Tarret Drop Links install

Tarret Rear Sway Bar - Drop Link Issue

[jjeffries]

Same on a 944. Standard Porsche design of the era, right?

John

[garment]

I installed these earlier this month. You’ll need to install the links finger tight on the bolts, and then rotate the spacer in such a way that the upper ball joint can rotate back and forth and doesn’t bind as the sway travels under load. Easier seen than explained.

Good luck!

Originally Posted by Steam Driver

I have a set of Tarett Engineerings rear sway bar links to install on my SC. However I have what may be a stupid question.

What is the function of the offset hole in the metal bushing? Obviously the offset is there as an adjustment function but for what and why? It doesn't seem to have enough offset to have much effect. Is this a way to offset preload on the bar?

Any suggestions will be appreciated.

[911pcars]

As I mentioned in the referenced link, there needs to be rotational space on both sides of the spherical bearing. If none or very little, the joint cannot freely transmit angular movement as needed. The available space may not accommodate a rod end. In that case, an alternate method has to be employed (e.g. rubber bushing).

Sherwood

[Mahler9th]

Not much benefit from adjustable drop links on stockish street-use-only cars.

As others have pointed out, the eccentric facilitates spring plate mounting. For some cars, trailing arm mounting is possible.

Removing ARB pre-load requires car to be at proper weight, including in driver's seat. The rest is straightforward.

Eccentric gadgets for aftermarket rear ARB drop links do not hold as well as the "locking" scheme I described in previous threads. Of course that scheme holds toe in place an eliminates the need for a custom eccentric of any kind.

Again, I have found the Puhn book to be the best reference... added bonus: the cool car on the front cover was likely raced professionally out of Mountain View, California.

[Steam Driver]

Thanks everybody.

[911pcars]

Originally Posted by Mahler9th

Not much benefit from adjustable drop links on stockish street-use-only cars.

As others have pointed out, the eccentric facilitates spring plate mounting. For some cars, trailing arm mounting is possible.

Removing ARB pre-load requires car to be at proper weight, including in driver's seat. The rest is straightforward.

Eccentric gadgets for aftermarket rear ARB drop links do not hold as well as the "locking" scheme I described in previous threads. Of course that scheme holds toe in place an eliminates the need for a custom eccentric of any kind.

Again, I have found the Puhn book to be the best reference... added bonus: the cool car on the front cover was likely raced professionally out of Mountain View, California.

Fixed length drop links precludes removing preload from the sway bar due to whatever the case with the chassis. A preloaded sway bar (at a standstill) affects corner balance if that's an issue of concern for good handling.

Sherwood

[Walt Fricke]

I don't believe Porsche ever used the rear alignment hardware for sway bar attachment. For sure not on 911s. That was an aftermarket approach starting, perhaps, with Automotion's bars.

[winders]

Originally Posted by 911pcars

Fixed length drop links precludes removing preload from the sway bar due to whatever the case with the chassis. A preloaded sway bar (at a standstill) affects corner balance if that's an issue of concern for good handling.

Yes, of course Mahler9th knows that.

The important words to notice are "stockish street-use-only cars". Obviously Porsche didn't care about preload issues with "stockish street-use-only cars" since none (I think it is none....but certainly virtually none) of their air-cooled Porsche 911 cars with sway bars had adjustable drop links.

How many air-cooled Porsche 911 "stockish street-use-only cars" ever get corner balanced?

[chrismorse]

Originally Posted by dannobee

Rear camber adjustment. Or toe. One or the other.

As long as i was making things adjustable, i decided to try drilling the stock (85) 18 mm rear bar flat sections to get a little adjustability out of the rear bar when using the adjustable drop links.
The bar is harder than the hinges of hell, so cobalt bits are a must. This is a fairly small change in resistance, less than a bar size change

I chose the Elephant ones because they had a boot on each end to keep most of the road dirt/water out of the joint. The adjustable spring plate sports a camber max from elephant also, I have heard it is not a huge improvement, but it helps.

It looks like when getting an alignment, rotating the cam or toe eccentric, (which ever has the drop link)will also load the drop link, so before alignment, it would help to loosen/disconnect the top link bolt. Post alignment, the drop link can be adjusted for 0 preload and reinstalled. Probably best to have the alignment tech do this as he will have the car on a (Hopefully) flat alignment surface..

[dannobee]

I know that from first hand experience, too! Once it's been tempered, it's just like a spring.

You need to do the math on the changes in the arm moment. Each hole can be a big difference in bar stiffness. I would bet that a half inch movement of the hole will make a bigger change in stiffness than a 1mm change in bar diameter.

[Walt Fricke]

Right - the change between hole 1 (front hole) and 2 (center) is much larger than the change between 2 and 3, even though they are equally spaced. There is an exponent in the formula. As a practical matter, if you start with the rear hole, and decide you have too much push, so move to the center, but conclude that though it is better, why not try the front hole, you might find you are now too loose (this all being without adjusting the front sways, if they are adjustable).

[chrismorse]

Originally Posted by dannobee

I know that from first hand experience, too! Once it's been tempered, it's just like a spring.

You need to do the math on the changes in the arm moment. Each hole can be a big difference in bar stiffness. I would bet that a half inch movement of the hole will make a bigger change in stiffness than a 1mm change in bar diameter.

Check this out Danno; see if i slipped a digit or just had a senior moment, further degraded by cerveza and using a ruler for on car measurements.

Original drop link mounting hole is roughly 6.1 inches to centerline of antiroll bar. First hole is roughly 5.23 inches from bar CL, so 6.1/5.23 = 1.17, for a 17% leverage advantage.

Hole #3 is 4.35 from bar cl, so 6.1/4.35 = 1.40, or a 40% increase in bar effectiveness.

Keeping the bar length the same and only changing the diameter, we can use the Diameter to the 4th power for a relative comparison, (also keeping the lever arm the same).

18x18x18x18 = 105K
19x19x19x19 = 130K so, 130/105 = 24% stronger than 18mm bar
20x20x20x20 = 160K so, 160/105 = 52% improvement over 18mm bar.

So, if i've got this right hole #1 is stock
Hole #2 is a 17& improvement and
Hole #3 is a 40% bump in effectiveness, somewhere between the 19 and 20 mm bar.

Praying for guidance from the cognoscenti,
chris


p.s. I have heard a vicious unsubstantiated rumor that a HD Polyurethane bar chassis mount will, by reduced compliance, increase the effectiveness of the bar by a one size increase.

sorry for the off topic side track, (but it was close :-)

[dannobee]

Recheck your math. Just doing a back of the napkin calculations, changing the arm length from 6.1 to 5.23 makes the bar about 40% stiffer and changing the arm to 4.35 will double the stiffness. But again, I'd need to know the exact measurements on the bar (distance between the bends, exact diameter).

Changing the arm from 6.1 inches to 4.35 inches would in effect be the same as changing the bar diameter from 18mm to 22mm. FWIW.

Now, how does this all fit together? It is one data point when computing the front roll couple. You'd compute front roll couple by calculating wheel rates and adding bar stiffness (don't forget motion ratios) to each end of the car, and throw in the rear weight and hp as variables. Higher hp cars require higher front roll couple, for instance. A miata might handle best at 50% front roll couple but a high hp Nextel Cup car might want 92% front roll couple, even with the same f/r weight distribution.

[winders]

Originally Posted by dannobee

Now, how does this all fit together? It is one data point when computing the front roll couple. You'd compute front roll couple by calculating wheel rates and adding bar stiffness (don't forget motion ratios) to each end of the car, and throw in the rear weight and hp as variables. Higher hp cars require higher front roll couple, for instance. A miata might handle best at 50% front roll couple but a high hp Nextel Cup car might want 92% front roll couple, even with the same f/r weight distribution.

Ah, good ol' roll couple distribution (RCD).

I don't think most people that install sway bars on their 911 cars have any idea as to the actual front and rear wheel rates. Most certainly they don't know how much the sway bars effect the wheel rates in roll. There is no way in hell they have any idea what their actual RCD is or what it should be if they did.

Even if you did know all the wheel rates, the sway bar rates, and the weight distribution of the car, there are a bunch of other variables that make calculating the "ideal" RCD problematic. Here is a short list that I could come up with off the top of my head:

Tire choice
Tire pressures
Camber settings
Toe settings
Aerodynamics
Ride height/Rake
Moment of Inertia
Driver preferences

I am sure there are many more.

Aerodynamics is probably the variable that can make the biggest difference.

We change roll couple distribution to affect the handling balance of the car. On the most basic level, the average 911 driver doesn't need to know anything about wheel rates or how much the installed sway bars add to those rates in roll. The driver just needs to know if his car has too much understeer, too much oversteer, or is just right. Depending on the car and the situation, you may need to adjust aero instead of RCD or both the aero and RCD to affect the correct balance change.

Air-cooled 911s have been around for a long time and are generally well understood. For just about any model with just about any normal torsion bar combination, the sway bar choices are also well understood. If you call up Tarett or JWE and tell them what your car is and how you plan on using it, odds are they know exactly what sway bar sizes will work well.

[Walt Fricke]

VanValkenburg gives the formula for sway bars. As a practical matter, the bar diameter is what it is, the constant determining the bar's deformation is fixed, and the bar's length is fixed. The only variable at the track is the arm length. And that is a "length squared" term. So you can't just use teeter totter lever calculations to determine the effect of changing the arm length.

As a practical matter, all you need to know is that the changes are non-linear, because you have only three holes, and you will want to experiment with all three of them to see which works best for the rest of your setup. In bars with slider adjustments you just keep moving one direction or the other to optimize.