Are lightening the 911 and torsen bar size/rigidity related?

[dd74]

I read an article where a 911 owner decided to not put heavier torsion bars on his car because of the weight saved from the fiberglass bumpers he installed. Could enough weight be saved to offset the need to increase torsion bar sizes? Or, do torsion bars "increase" in rigidity as the car is lightened?

Thanks.

[Superman]

Torsion bars are what holds the car up. They fight against the weight of the car. Less weight means they don't have to fight as hard. 29mm bars might feel very firm on a light car, whereas 31mm bars might feel soft on an impact bumper car with A/C and a 3.6.

[KTL]

Ditto what SuperJim said.

My buddy Mark lightened his '86 Carrera enough that the suspension looks like it "came up" a couple inches. This is with stock 19/25 F/R t-bars.

And of course if you put in stiffer t-bars and indexed your spring plates/A-arms to the same position as they were with the old bars installed? You'd have a Cayenne-looking ride height.

So the stiffer bars resist the weight of the car more than the smaller bars. The lighter the car gets, the less it will be able to load the suspension. Get too light (or too much spring resistance) and you can't effectively transfer the car's weight. Contrary to popular belief, in a car you want some weight transfer for good handling.

[dd74]

Ah ha! Kevin, you bring up a valid point. So there is a point where a car can be too light and the torsions be too stiff? And with that, erode handling more than benefit it? Might most of this depend on how good or poor a road surface is?

What about tire and wheel combinations. I've also been told the larger the wheel/tire combination, the more likely it is one will need larger torsion bars, irregardless of the car's weight.


NOTE: sorry I spelled "torsion" t-o-r-s-e-n. I knew it didn't look correct?

[KTL]

Road surface & shape is an issue. Example, a simple go-kart doesn't have suspension because they don't care too much about bottoming out and so forth. Object for them is to be as low as possible and stick like glue. Even if they don't stick like glue (like the ones you get with the rock hard tires that are supposed to be equally prepared for everybody at the group drive....... ) at least it'll be low enough not to flip and will slide instead

A road car needs to semi-absorb the bumps so it doesn't unsettle the car (and the driver) when one least expects this to happen.

Tires are certainly a factor in the equation. The more grip you have available, the more weight you're going to transfer (up to a point). Bigger wheels and the resulting bigger tire, with a stiffer sidewall, transfer more load into the suspension as well. Bigger grip loads require stiffer springs or sway bars to address this.

And that right there pretty much exhausts what I know about suspension stuff...............

[Randy Webb]

You want the wheels to be able to track any bumps in the pavement -- too stiff and they cannot do so.

I don't understand the rationale for the stmt. that some wt. transfer is good. Max. stick is when each tire is well loaded -- wt. transfer reduces that.

The important issue is to what extent any effect apply under your real world conditions -- review data on car wt. vs. t-bar size (stiffness) over model years for what PAG thinks is right for a street car.

[dd74]

So is there a chance these guys who are running street SCs with a weight of 2,500 with, for example, 22f/28r torsions, might be too stiff to provide maximum grip. I know it's almost an unanswerable question; that much depends on road conditions, but given 2,500 pounds and the thickness of the torsions, I would say that might be a bit much torsion bar, particularly since turbos had 19f/26r.

Or another question might be were turbos sprung light for ride comfort?

[KTL]

Don't think of weight transfer only in side-to-side frame of reference. Front-to-rear is important as well. Like braking, corner exiting,..........

[KTL]

I can't answer the 22/28 question as far as stiffness/spring rates and ultimate grip is concerned, but I can say that the 22/28 or 22/29 combo for a mostly street, occasional track SC or Carrera is what always seems to be recommended by those in the know.

You'll find that some guys have chosen to run 23/31 on cars that are street/track cars. 89911 and dickster are a couple of fellas that come to mind.

[Grady Clay]

dd74,

Superjim, Randy, and Kevin are right on.

The real issues here are:

Overall weight; That is the mass you are trying to accelerate (ahead, breaking, cornering, up & down.) The less mass the better.

The ratio of total weight on a tire to the un-sprung weight. BIG deal. This is why Porsche has gone to the trouble for ceramic brake rotors, alloy calipers, light weight wheels & tires, and even alloy lug nuts. The race cars have alloy hub carriers, Titanium parts and hardware, etc.

Rotational mass; All the stuff in the engine, transmission, axles, rotors, wheels, and tires that rotates needs to be as light as possible in the rotational sense (angular momentum.) That is a physical equivalent to the mass of the car.


Now, to get back to torsion bar size.
The issue is what kind of driving are you doing? You know that off-road 911s (Paris-to-Dakar) have very soft springs, soft balloon tires, soft shocks, and huge amounts of suspension travel. Road race 911s (935s & GT3-RS) have exceedingly stiff springs, rigid low-profile tires, firm shocks, and very little suspension travel. You are talking about a street car compromise between these two extremes. Porsche delivers 911s as appropriate street cars. If you want higher smooth road performance, you can change in the direction of road racing. However you tend to give up ride comfort and rough road handling. Almost everything is a compromise.

Best,
Grady

[Randy Webb]

"Don't think of weight transfer only in side-to-side frame of reference. Front-to-rear is important as well. Like braking, corner exiting"

- Ah, Ok -- and BTWE the rear engine confiuration works really well for those conditions.

Try this: Calculate the wt. of your car rel. to an earlier (lighter) one. Look up its t-bar sizes and compare to yours. If it had smaller ones, then think strongly about replacement (for a street car). You may need to try various ones (with adj. to ride ht & corner balance each time) to get it really dialed in for what you want. Perserverance furthers...

[dd74]

Well, in my case, I'm about 2,300 pounds and have torsions equivalent to those on the 930.

Two thoughts I have for my car this year is to firstly shed additional weight, then secondly do suspension work which could entail larger torsion bars. I only wonder if the larger torsion bars are necessary provided that the car will be lighter. Hence, can the lighter weight of the car's unsprung weight "act" as if it has heavier torsion bars.

I apologize if I'm not being clear, or that I'm not "catching on." Maybe this is just a bit beyond my ability to communicate it effectively...

Please bare with me.

[Randy Webb]

DD - your plan of attack (sequence) makes sense.

When reposting state your intended use, and how happy you are with the car's feel. You may want to wait until you lighten it or have calculated how much you are going to lighten it.

[surflvr911sc]

Originally posted by dd74
Hence, can the lighter weight of the car's unsprung weight "act" as if it has heavier torsion bars.

Dave, unsprung weight will not change unless you change things "below" the torsion bars; i.e. wheels, brakes, etc.

The lighter weight of the chassis (sprung weight?) will act the way you referenced, how much per pound I have no idea.

Did I tell you I lost over 100lbs during my conversion?

[dd74]

Originally posted by surflvr911sc
Did I tell you I lost over 100lbs during my conversion?

Conversion? I didn't even know you got baptized.

No, seriously: tell me how. I'm always into weight loss stories.

Me and Jenny Craig, you know...

[Tyson Schmidt]

I know that if your wife or girlfriend loses weight, it usually increases your rigidity.

[Superman]

Didn't have time to see all the posts, but thought I'd throw comments in just the same. 22/28 on a 2500 lb car is nothing. If you're gonna "over-spring" a car, it'll have to be way lighter and way stiffer.

If you have stiffer-than-stock Tbars, and your car is going on a diet (starting at 2300 lbs), then you might want to keep those Tbars until the diet is over. They might be stiff enough.

Imagine a car with springs so stiff that the wheels are not really allowed to move. When you hit a bump, your entire suspension system boils down to just the air in your tires. What does a basketball do when you compress it? It bounces back. The point I am making is that, if a suspension is effectively too stiff, this means that it cannot absorb the energy that would otherwise compress the tires. The car would bounce and skip across the road.

But also keep in mind that if you're going to talk about "too stiff," you'll be talking about a colossal amount of stiffness. There are plenty of track cars out there with 23/31 with under 2000 lbs. for street driving, you wouldn't want that much stiffness of course, but mostly because of the impact on comfort. a 2000-lb 23/31 car would feel like the axles are welded to the tub. Seriously.

[dd74]

Originally posted by Tyson Schmidt
I know that if your wife or girlfriend loses weight, it usually increases your rigidity.

Now that's just gosh darn EEEEVILLLL !!!

[surflvr911sc]

I think Tyson has been into the German beer again...

I don't think I've done anything that you haven't done, weight wise. I've lost 150 lbs since this time last year, most of that during the conversion.

Spare tire replaced w/ AAA card
All interior insulation removed
Rear seat back rests removed
Heater blower removed
A/c (compressor only) removed
Stock exhaust replaced w/ B&B

But I’ve also added a few things-
Elephant Racing Triangulated strut bar (nice and light)
Two front Carrera coolers
C2 retractable tail
’93 964 3.6

Everything else but the suspension is stock
22/28 T-bars, 22/21 Carrera S-bars, bushings, Turbo T-rods, HD/SP shocks (need to send old ones in to be custom valved), 7X8X16 Fuchs (need 9s).

[dickster]

There are plenty of track cars out there with 23/31 with under 2000 lbs. for street driving, you wouldn't want that much stiffness of course, but mostly because of the impact on comfort

super,

i have 22/31 on my (heavy) 3.2 and they're great - no loose teeth yet!