T-Bar sizing Question

[Rodsrsr]

Since I no longer track the car , I decided I was done with the ultra stiff suspension, so I swapped out my 23mm front T-Bars for 21mm. Car also has Bilstein sports all around, 22mm thru the body ft sway bar, 28mm rear T-Bars with factory sway (18 or 19mm?)

The ride is noticeably better from a comfort standpoint. It pretty much feels like a normal old performance car, stiff but not obnoxious. Very pleased with the difference, however I did lose some of the go cart feel. The front end was super planted before, just point and it goes. After adding some pre-load to the front sway, I managed to get some of the feel back but nothing quite like having the bigger bars. Anyway, I understand when it comes to suspension everything is a trade off and I'm happy with the balance.

The car seems to handle quite well overall, it actually feels a little bit better, probably because this setup instills more confidence on unpredictable roads vs a track.

Anyway my question pertains to balance. I've often heard that 21mm front bars work best with 27 as that provides a better balance. I guess 26, 27, 28 is within range but 26 may induce understeer, 28's oversteer with 27 being the ideal size.

Any real world experience with this? Also, what can I expect in terms of handling if I were to go down to the 27's or is it even worth it?

Last point, not sure if it matters but the car is a wide body with 255/40/17 and 315/35/17. I read that the wider stance with significantly wider tires may have an effect of reducing the rear t-bar rating. Not sure if thats true. What does the Pelican braintrust think in regards to this and overall lowering to 27mm rear bars?

[Bill Verburg]

Originally Posted by Rodsrsr

Since I no longer track the car , I decided I was done with the ultra stiff suspension, so I swapped out my 23mm front T-Bars for 21mm. Car also has Bilstein sports all around, 22mm thru the body ft sway bar, 28mm rear T-Bars with factory sway (18 or 19mm?)

The ride is noticeably better from a comfort standpoint. It pretty much feels like a normal old performance car, stiff but not obnoxious. Very pleased with the difference, however I did lose some of the go cart feel. The front end was super planted before, just point and it goes. After adding some pre-load to the front sway, I managed to get some of the feel back but nothing quite like having the bigger bars. Anyway, I understand when it comes to suspension everything is a trade off and I'm happy with the balance.

The car seems to handle quite well overall, it actually feels a little bit better, probably because this setup instills more confidence on unpredictable roads vs a track.

Anyway my question pertains to balance. I've often heard that 21mm front bars work best with 27 as that provides a better balance. I guess 26, 27, 28 is within range but 26 may induce understeer, 28's oversteer with 27 being the ideal size.

Any real world experience with this? Also, what can I expect in terms of handling if I were to go down to the 27's or is it even worth it?

Last point, not sure if it matters but the car is a wide body with 255/40/17 and 315/35/17. I read that the wider stance with significantly wider tires may have an effect of reducing the rear t-bar rating. Not sure if thats true. What does the Pelican braintrust think in regards to this and overall lowering to 27mm rear bars?

here's a comparison of the effect of the t-bars you are asking about

it is only a first approximation guestimating weight and f/r track
there is no input wrt to wheels and tires which will greatly increasing under steer

ride freq is the most informative non aero track car want to be in the 1.5 -2.5 range

as you can see the bigger the rear bars the less under or more over you will see, this cancels the effect of the wheels and tires to a large extent

[Rodsrsr]

Thanks Bill! I was hoping you would chime in with one of your graphs. Great data points. Based on this, the 27's do appear to be a better match over the 28's. Does anyone have any anecdotal real world experience? I guess what I'm trying to determine is what the graph doesn't reveal, which is how substantial will this difference be. Is it enough of a difference to warrant the change on a street car?

[Bill Verburg]

Originally Posted by Rodsrsr

Thanks Bill! I was hoping you would chime in with one of your graphs. Great data points. Based on this, the 27's do appear to be a better match over the 28's. Does anyone have any anecdotal real world experience? I guess what I'm trying to determine is what the graph doesn't reveal, which is how substantial will this difference be. Is it enough of a difference to warrant the change on a street car?

I use and like very much 20/26 t-bars w/ 20/18 sways 225 on 8 and 255 o 9.5 x17 for street use

here's mine

[Mark Salvetti]

Originally Posted by Rodsrsr

Thanks Bill! I was hoping you would chime in with one of your graphs. Great data points. Based on this, the 27's do appear to be a better match over the 28's. Does anyone have any anecdotal real world experience? I guess what I'm trying to determine is what the graph doesn't reveal, which is how substantial will this difference be. Is it enough of a difference to warrant the change on a street car?

A few years ago I was emailing with Steve Weiner for help modestly upgrading the suspension of my 1979 Targa. He recommended 21/27, custom-valved Bilsteins to match, and the slightly thicker sway bars from the later Carreras. Keeping the stock 16-inch wheel and tire combo.

I was slow to get the parts together (found them all used), and unfortunately Steve got sick before I could order the shocks from him. Still plan to go this route, but I have a bunch of other stuff going on and I don't know when I'll be done.

Mark

[PeteKz]

The shocks have more effect on the felt stiffness of the suspension than the springs/T-bars. Bilstein Sports are pretty damn stiff shocks for street use, IMO. I use Koni Sport Adjustables, but on their softest setting for the street. Currently using 21/26 bars, although I have a set of 27 rears that I will put in the next time I have to do rear suspension work.

[Rodsrsr]

Thanks for the feedback everyone! I guess Ill be going for the 27mm bars.

[Jeff Alton]

A different point of view....

All things being equal, as sway bars, wheel width, tire size, bushing choice, dampening, alignment all play a roll....

But I prefer a bit more rear spring rate.... as in 21/28 or 22/29. Like most production cars, they left the factory with too much front vs rear spring rate, as that is safer/easier to drive when one is going faster than one's skill can handle.....

Like I said, just my preference, and NOTHING wrong with any of the "tried and true" suggestions.

Oversteer? Understeer? Well, what conditions? Street, track autocross? Tire pressures? So many variables...

As for ride quality- most affected by the lightest end of the car. So, your dampening choice at the front of the vehicle will have the largest impact on the quaility of the ride, which may or may not be in conflict with the best performance..... again, so many vairable.

Cheers

[Bill Verburg]

Just remember that there are 2 basic types of grip

1) mechanical: which comes from wheels. tires, bushes, suspension spec and setting, mass, location of mass, wheelbase, track, etc.

2) aero: from spoilers and basic car size & shape. a thru '89 911 has negative aero unless it has spoilers and even then it's effect is minimal w/ the factory supplied aero packages.

getting back to suspension, minimum suspension stiffness ie min ride freq. maximizes grip. But there are also negatives to min. stiffness such as increasing geometric changes to
suspension function w/ increasing force put through the system. The goal is is to have just enough stiffness to control the usually neg. aspects of roll on suspension geometry for the driving that you do.
This is closely tied to the wheels and tires installed, narrower wheels and tires w/ taller sidewalls are less negatively affected by roll induced geometry changes than wider shorter versions.
Shorter stiffer sidewalls and wider tires run at lower slip angles and increase grip at the expense of ride quality and warning near the edge.
all the other contributing factors to mechanical grip have similar constraints and compromises, for instance increasing mass also increases grip but at the same time it imparts negatives in that it sucks up energy to accelerate it.
acceleration being a vector quantity that means speed up, slow down, or change direction. Another is alignment specs, a track alignment and street alignment will be very different, use a track alignment on the street and it increases
wear and decreases grip when the lower street use forces go through the system, conversely a street alignment on track will do the same because of the larger forces involved.
This is closely tied to the wheels and tires installed, narrower wheels and tires w/ taller sidewalls are less negatively affected by roll induced geometry changes than wider shorter versions.

a list of mechanical grip contributors , all w/ constraints and compromises in any particular spec chosen
tires
wheels
tire pressures
ride height
suspension-springs, bushes and sways
camber, caster, toe f/r
shocks
front track
wheelbase
mass and distribution of mass
rear track

[silverlock]

As a datapoint: I dove into a bunch of Bill’s old threads/posts a couple of months ago when refreshing the suspension on my ‘78 SC (no A/C, no heater blower; 2450 lbs).

This one with front vs rear spring rates was particularly insightful: https://forums.pelicanparts.com/porsche-911-technical-forum/1131760-torsion-bar-help-2.html#post11880386

Running Koni Sports (half a turn from soft) + 21/26 T-bars + 24/25 sway bays (short front; long rear) on 205/55 and 225/50 tires. Needed something compliant enough for backroads events, rallies and getting myself out of/into NYC. Very happy with the setup.

If I ever fit wider tires (e.g. 225/245) I would probably adjust my rear sway by one stop shorter to dial in a little more oversteer to compensate.

[3literpwr]

I'm running 19/27 on my rally car with no sways and it's great. My sport / track car has 23/31 and it's too stiff, slightly understeer prone and not very compliant.

[Rodsrsr]

Always amazed at the wealth of knowledge and experience on this forum!