How twitchy will a rear sway bar make a '74 without one currently?

[tdskip]

Hi guys - My '74 does not have a rear bar on it currently but I have a 20mm rear bar on hand. This is a fast road and touring car, and I am wondering if I am asking for trouble by stiffening up the rear of the car. By trouble I mean the "I am not actually as good of a driver as I think I am and the rear bar might catch me out unexpectedly" sort of trouble.

Any coaching to pass along?

Thanks!

[fintstone]

Made a huge improvement on my '74. If anything, made it tighter and more predictable. The rear stays planted better.

[tdskip]

Well that is encouraging. Makes sense I suppose.

[ganun]

20 is bigger than stock so you might want to use the beefier aftermarket welded mounts not the stock ones.

[tdskip]

Thanks - she doesn't have rear bar at all at this point. Think going with a 20 is too big of a jump?

Thanks for the reminder on needing to reinforce the rear mounts.

[newms]

What size is the front bar? 6x15 all around?

[fintstone]

I am running upgraded torsions as well (22 front and 28 rear.)..but the factory Konis and 7x15 inch wheels. I think the sways are stock. Handles much better than my previous '74.

[Charles Freeborn]

What Flintstone said. When I put mine on it all got more predictable. Mine had lost one of it's mounts, so I retrofitted to Wevo's and an SC bar. Chase the link in my signature line on the process.
-C

[moneymanager]

Don't worry about it. Put it on, drive it slowly. It won't bite you at under 75. Think about how it feels when you go around a corner. In short, start conservatively and build your confidence over time. Some people will be comfortable in ten minutes. Others will take a few drives. You'll like it.

[74-911]

Originally Posted by tdskip

Thanks - she doesn't have rear bar at all at this point. Think going with a 20 is too big of a jump?

My '74 came with the "S" suspension option and the rear sway bar is 18MM. Don't know what a '74 drives like without a rear sway bar but I can't imagine that adding one would make it "twitchy"... just the opposite, more planted and stable.

[tdskip]

Thanks Jack, I am sold

[spuggy]

Originally Posted by ganun

20 is bigger than stock so you might want to use the beefier aftermarket welded mounts not the stock ones.

And I would use the matching front sway bar.

[tdskip]

Originally Posted by spuggy

And I would use the matching front sway bar.

Let me double check what I have on the front.

Thanks for the coaching.

[78-911SC]

Contemplating doing the same except my chassis does not have the mounting brackets. Anyone have the exact locating for the mounting brackets? See the approximate location in pictures but don't want to weld in wrong position.

[chrismorse]

When Dad's 74 passed to me, it had no rear bar at all. One of the rear mounts looked like it had been trimmed off with a sawzall.
Driving the car home, gently, (no sliding through turns, the car had 20 year old Pirellis on it) it seemed to handle just fine.

The original window sticker said the car was optioned with "Carrera" bars, that is 20mm ft and 18mm rear. The rear bar mount probably failed and my Dad's mechanic probably said "you're not driving the car hard enough to need this so we will just take it off".

So, I bought Wevo rear mounts and installed them an a used bar, links and poly bushings. I also installed Dunlop direzzionales, so it is hard to put all of the handling improvement to just the rear bar.

The car seems to corner flatter and feels more poised.

I am going through a front end, (brakes, steering and suspension), overhaul and just a couple of days ago decided to upgrade the torsion bars too.

If you are going to drive the car hard enough to get the tail out a bit, you might want to consider a bigger front bar, preferable one that is adjustable, so you can tune the oversteer/understeer balance.

The stock 74 comes with 18.8mm front T bars and 12.4 rear T bars. The strength the bars is calculated by taking the diameter to the fourth power, so going from the stock 18.8 to a whopping 21mm shows a 55% increase in stiffness.

Looking at the Carrera bars 20 to the fourth divided by 18 to the fourth give us a relative front to rear stiffness of 1.5 to 1, that is the front bar stiffness is 50% stiffer.
So, if we carry out the Carrera bar analysis, you might need a 22mm front bar to balance the 20mm rear bar. If it is adjustable you can shorten up the effective arm length and crank up the front roll stiffness.

All of this balance is assuming you have stock T bars and that the factory got the 20/18 Carrera bar option well balanced.

Alas, all of this is still theoretical, because I haven't had the 911 back on the road much less on the track, but I have been around a few turns on the track.

Best,
chris

[fintstone]

Originally Posted by 78-911SC

Contemplating doing the same except my chassis does not have the mounting brackets. Anyone have the exact locating for the mounting brackets? See the approximate location in pictures but don't want to weld in wrong position.

These might help.

Pelican Technical Article - Porsche 911 (1965-1989) - Standard and Adjustable Drop Link Replacement

Pelican Technical Article: Replacement of Sway Bar Bushings and Brackets - 911 (1965-89) - 930 Turbo (1975-89)

If you are missing rear brackets, I would have the the WEVO reinforced ones as the factory ones tend to break. Also, remember that if you use factory sways, they changed in '78 so you would need to mount later ones a bit differently. Described here: http://blog.dietersmotorsports.com/?page_id=172

[78-911SC]

Fintstone thanks for the info but I'm trying to locate the exact position the rear brackets need to be welded to the frame. I have a 70T and it came without them. I plan on using the 78 rear control arms so if needed I can use a heavier sway bar. Just need to be sure where to weld.

[Craig_D]

I've got a set of SC front/rear sways with end links if you're interested. PM me if you are.

I'm running Carrera sways and Tarrett end links plus sport hardness bushings...I've tracked my car and done thousands of canyon carving miles with the stock rear sway mounts, so it's definitely not a requirement to upgrade those mounts (assuming you already have stock mounting points on your chassis)

Cheers

[fintstone]

Originally Posted by 78-911SC

Fintstone thanks for the info but I'm trying to locate the exact position the rear brackets need to be welded to the frame. I have a 70T and it came without them. I plan on using the 78 rear control arms so if needed I can use a heavier sway bar. Just need to be sure where to weld.

Although I am not an expert on this, I am pretty sure that you hang them and then bolt up the brackets (no bushings), mark where they are, tack them on, drop the bar and finish welding the bracket and then install.

[911pcars]

Random thoughts:

Add'l rear roll stiffness increases oversteer. Thus, the front end bits also enter into the proper equation.

A 911 with factory sway bars (some or none) and torsion bars handles differently than one with upgraded versions. Suspension mods for a 911 follow a proven path that includes torsion bars, sway bars, shocks, ride height and alignment specs.

Doing so gradually is fine. Merely doing one item may create some negative results and isn't recommended.

Sherwood