Torsion Bar Sizes

[Pittspirate]

Not to belabour the point of my original question….

if I wanted to stick with 215/45/17 up front, and 245/40/17 in the back (I really like the look of the wheels I have), with 18.8 bars up front what might be the optimum bar diameter in the rear be for a car that will probably only ever be road-driven? Stock anti-roll bars.

I don’t have to stick with 27’s in the back; particularly if they are going to make the car less user-friendly.

I’d rather make something closer to right decision now. Budget is not a primary concern.

[Bill Verburg]

Originally Posted by Pittspirate

Not to belabour the point of my original question….

if I wanted to stick with 215/45/17 up front, and 245/40/17 in the back (I really like the look of the wheels I have), with 18.8 bars up front what might be the optimum bar diameter in the rear be for a car that will probably only ever be road-driven? Stock anti-roll bars.

I don’t have to stick with 27’s in the back; particularly if they are going to make the car less user-friendly.

I’d rather make something closer to right decision now. Budget is not a primary concern.

Again, it just depends on how hard you drive and how much chassis roll bothers you, and whether stock over/under needs to stay or go one way or the other.

Many are perfectly happy w/ stock, in your case that was 18.8/24
18.8/25 imparts a little less under, 18/26 goes a little further
20/26 imparts a little more under and controls roll much better
20/27 is a little more under than the above 18.8 fronts and controls roll

18.8/27 you have now imparts a lot less under partially compensated but the unusually large split between the 205 & 255 tires

going to 215/245 puts the tires back in the more usual split and 18.8/25 puts the car back to stock handling but w/ more grip, that can lead to more roll

To keep the handling more or less stock w/ 215/245 use 18.8/25, to reduce roll use 20/26 or 20/27 one is a compromise a little one way the other a compromise the other, bot control roll well and aren't too harsh,

The front is way more sensitive to harness than the rear, and if it's too harsh for you put a set of twin tube shocks on it.

[PeteKz]

Bill, FYI: I find that the 21/26 installed in my car at present causes the inside front wheel to lift off the ground pretty easily on the AX course. As I've said before, I have a set of 27's for the rear that I haven't got around to installing yet (this winter?), and which I expect to reduce the tendency of inside front wheel to lift, albeit, not eliminate it. Yah, I realize that front wheel lift in hard cornering is just a "feature" of the weight distribution in these cars.

[Bill Verburg]

Originally Posted by PeteKz

Bill, FYI: I find that the 21/26 installed in my car at present causes the inside front wheel to lift off the ground pretty easily on the AX course. As I've said before, I have a set of 27's for the rear that I haven't got around to installing yet (this winter?), and which I expect to reduce the tendency of inside front wheel to lift, albeit, not eliminate it. Yah, I realize that front wheel lift in hard cornering is just a "feature" of the weight distribution in these cars.

The op isn't asking about track or A/X,

As I stated the faster you go, the more force goes thru the suspension and the bigger the spring rate to control it. Also best for street is not likely to be best for track or A/X, very different criteria. Even the factory o.e spec was based on criteria that many more performance-oriented owners disagree w/, hence the large aftermarket.

Even when the unloaded wheel is on the ground it's not doing much. In high speed maneuver at most 2 wheels are loaded and capable of doing much, That's why you do most of the braking in a straight line, then transition to the corner, then transition to the gas, The tires friction circle is the arbiter of what can be done and when.

[PeteKz]

Bill, yes, I know the OP was not asking about track or AX, and that street use is different from track use--remember, I frequently ask people who post questions about changing their suspensions what they plan to use their car for. That was just an FYI for you and others about my experience in my car with the 21/26 combination.

[Bill Verburg]

Originally Posted by PeteKz

Bill, yes, I know the OP was not asking about track or AX, and that street use is different from track use--remember, I frequently ask people who post questions about changing their suspensions what they plan to use their car for. That was just an FYI for you and others about my experience in my car with the 21/26 combination.

another way to encourage 3 wheeling is to use shorter shocks and raised spindles

[PeteKz]

"Encourage" or "discourage"?

[joe88911]

Originally Posted by Bill Verburg

Here's a comparison of some common street oriented t-bars used in a 2600# 911 w/ 40/60 weight distribution, column 11 summarizes the effect from a base '74 Carrera, the higher the % the more over/less under steer is induced by the t-bars. Wheels , tires, sways differing weight distribution will also affect this


The 18.8/27 set induces a lot less under or a lot more over depending on how you look at it. It's a set up that an A/X'er might want to induce quicker turning on a short tight course but a big track guy would hate because of it's high speed twitchiness.

The thing to keep in mind besides over/under steer characteristics is that the higher the wheel rate the lower the mechanical grip, the sole reason to use higher rates is to minimize the geometric changes seen in a loaded suspension,

Bill:

Help me out here. When looking at the 26mm rear bar specs, the percent looks off as does the rear wheel NM rate. It seems like the 21/26 % should be less than than 21/27 rate. Am i reading this right?

[Bill Verburg]

Originally Posted by joe88911

Bill:

Help me out here. When looking at the 26mm rear bar specs, the percent looks off as does the rear wheel NM rate. It seems like the 21/26 % should be less than than 21/27 rate. Am i reading this right?

Yeah I just went back through it, there were numerous errors where pointers went to the wrong cell, I think I got them all

this model is for a 2600# car, 40/60 weight distribution w/ Bilsteins and rubber bushes

[joe88911]

Originally Posted by Bill Verburg

Yeah I just went back through it, there were numerous errors where pointers went to the wrong cell, I think I got them all

this model is for a 2600# car, 40/60 weight distribution w/ Bilsteins and rubber bushes

Bill:
I've got a stock 88 3.2 Targa i'm autocrossing a little. I'm really new to any actual performance driving. The car seems to push pretty bad but i've also spun it a couple of times (on lift).

All my bars have wear from metal/metal contact and/or corrosion so I have to replace them all. I'm doing all the bushings while I'm in there. I'm thinking 20/27 and will run 205/225 200 tread wear tires on the 16" Fuchs when I get it all together before spring. I have Bilsteins, now.

I also do a lot or pleasure drives so having it not too harsh would also be good. This is why I thought 20 up front.

What do you think? Will it have too much bias toward oversteer?

Thanks
Joe

PS: you are my favorite in all the forums. You seem to back up all your contributions with data. I like that!!

[Bill Verburg]

Originally Posted by joe88911

Bill:
I've got a stock 88 3.2 Targa i'm autocrossing a little. I'm really new to any actual performance driving. The car seems to push pretty bad but i've also spun it a couple of times (on lift).

All my bars have wear from metal/metal contact and/or corrosion so I have to replace them all. I'm doing all the bushings while I'm in there. I'm thinking 20/27 and will run 205/225 200 tread wear tires on the 16" Fuchs when I get it all together before spring. I have Bilsteins, now.

I also do a lot or pleasure drives so having it not too harsh would also be good. This is why I thought 20 up front.

What do you think? Will it have too much bias toward oversteer?

Thanks
Joe

PS: you are my favorite in all the forums. You seem to back up all your contributions with data. I like that!!

Never driven a 911 that couldn't be made to snap spin, if you lift at the wrong time

and yes a stock 911 will understeer most of the time, this tendency is speed related, the faster you go the less evident. It is related to weight transfer and aero, so you drive to put weight where you want grip, gas on the straights, brake in a straight line w/ a little trailing throttle on the transition to turning,

tires are huge, they are you only contact w/ the rest of the universe, 205/55 on 6 and 225/50 on 7 is a very poor choice for performance

if staying w/ 16" Fuchs I'd want 7 & 8 205/50 & 225/45 min

the suspension choices are made to keep geometry changes related to suspension travel to a minimum, stiff suspension and geometry changes to suspension both kill grip, snap spin comes from the sudden transfer of weight from the rear + suspension geometry changes where camber and toe revert to less grippy realms.

for A/X you want more over/less under than for street or track purposes.
Here's an array of street oriented t-bars

𐤃 f/r is the best indicator of basic over under, higher numerical is more under lower numerical is more over(red)

18.8/27 is the most A/X oriented but would likely be too far for regular street use, 20/24 is the most under and most drivers would hate it. choice of bars is related to choice of wheels, tires and suspension setup



if I wanted to A/X but also use the car normally I'd probably go 20/27 w/ your stock sways

I'd also lower the car a lot , raise the spindles, lower the steering knuckles, use digressive Bilsteins(Vonn)