more t/bar stuff - anyone got experience/comments on 21/31 combo?
 

originally i had 22/28 ordered for my '86 daily driver. however, since its theft/recovery it has become a second car, for fun/weekend/occasional track use only.

i have been advised on the 21/31 combo, and given the change in status of the car wondered whether i should take the advice.

this is smaller in the front than i was going.

i guess 21/31 promotes better turn in/oversteer?

in another thread i read that bigger bars on the rear dont give such a harsh feeling as bigger in the front.

anyone got experience? on the street?

as long as a get a big grin from driving it i'm all ears...........:D

Bruce Anderson would suggest 22/28 for your car, but he mentions in his book that many track guys believe that for track use you want to go much firmer in back. He goes out of his way to acknowledge them in his book.

I ran 21/30 on my car, and was very happy with it. I sometimes wished it was 21/31, in fact.

The differences in handling produced by these different sizes are more subtle than you might suspect, given the amount people worry over them. I drove Thom Fitzpatrick's widebody 3.6 with 22/29, and the handling didn't feel at all unfamiliar to me.

jack,
i was hoping to get your attention - thanks. i know you do loads of track work, but whats yours like to drive on the street - fun?

My guess is 22/28 would be slightly harsher on the street than 21/30, since you feel your front suspension hit things first.

In any case, I thought mine was fine on the street. I imagine a cobblestone street would have been a little tooth-rattling, but the car was more go-kart like -- which is exactly what I wanted.

The new version of my car has coil-overs with even stiffer spring rates.

thanks jack, i'm gonna take the plunge.

Dickster:
I'm providing you with this to be an "informed consumer"...
Your 86 was likely delivered with a 18.8 front bar and a 25 rear bar. The equivalent spring rates are 110 lb/in front... 140 lb/in rear. Stiffer rear ( only) will promote oversteer....stiffer front ( only) will promote understeer. Here's a breakdown of front and rear TB sizes and their spring rates, after my exhaustive investigation on sizes, bar lengths, pivot arm lengths, etc:

Front:
18.8= 110 lb/in
21= 173
22=210
23=250

Rear:
24=120 lb/in
25=140
26=165
27=191
28=221
29=254
30=294
31=332
33=427

Notice some interesting trends.... many people say to upgrade to 21/27...or 22/28. The 21/27 combo equates to 173/191...and the 22/28 combo equates to 210/221. In both cases...fairly equal. This maintains the front/rear balance Porsche designed into these cars, but some say (me included) that we can go proportionately stiffer in the rear to dial-out some unwanted low speed understeer. So...maybe 21/28 ( 173/221)....or 22/29 ( 210/254). But look what you're contemplating:
21/31 ( or 173/332). IMHO...this is too much rear bar...and the overall stiffness for the rear is over 2.3 x as stiff as stock.... !

You may want to rethink this my friend......:)

--Wil Ferch

More....
However, the advice you got from the "ride" standpoint....that overly harsh front bars are really the issue ( not the rears).. is true. You'll be amazed how well the weighty rear end of our cars softens the blow when you run big rear bars. Bumps are mostly felt through the fronts.
My other post is only to help you get in the proper ballpark with regard to front/rear balance ..regardless of how overall stiff you go.
---Wil Ferch

wil,

thanks for all the info.

i'm not sure what i've got, a mish, mash more than likely. the fronts are fairly new, unknown origin. the rears could be oe.

the handling is awful. severe understeer. its gonna be a fun only car now, so i just want the handling sorted. i'll try the 21/31 combo and see how i get on. jack's got experience of 21/30 on the street and has positive comments about it. jacks car is probably lighter than mine as well.

surely if you maintain the balance as you are describing you are going to retain the stock cars understeering tendencies?

there seeems to be a concensus that the oe's weren't optimum, but lots of variation on actual sizes to go for. i'll post my opinion when i've tried them, and maybe change the fronts for 22's later.

I'm with Wil's observation on the additional stiffness in the rear that the 31S would give you with the 21s in front.

My experience (with 22s and 29s, BTW) is that more rubber in the front is the best cure for that dreaded understeer. Wider tires really make a bigger difference than spring rates....

Mike

Dickster:
I think you may have missed a critical point in my post..
The often-recommended 21/27 .... or 22/28 ... pretty much maintain the balance that was originally there...granted. I go on to say that 21/28 or 22/29 might be better to sort out low speed understeer. Some take this thinking a bit further with 21/29...or 22/30. Look at the equivalent spring rates.
However...even from this more extreme starting point...you're suggesting 21/31....this is a bit too much front-to-rear spread... IMHO.

IROC...yep, that's what I did, but mainly because I am so cheap that I'll squeeze a penny until Lincoln's eyes pop !!! I went with 7 x 16 Fuchs front using original 205's ( kept the 7 x 16 rears with 225's)...gained an inch or so of front track width, planted the shoulders of the front tires more squarely..and solved about 75% of my unwanted low speed understeer. Can also do this with bigger rear bars but this gets more involved...

--Wil Ferch

This is the Bruce Anderson quote:

"Some people run much stiffer spring rates on the rear of their cars than what I recommend, and they swear by it, but I have no personal experience with this setup. They are running with the 22mm front and 31mm rear torsion bars. These same people also are using a much stiffer rear sway bar."

JacK:
True enough, but....
1.) These "people" are likely track people.
2.) 22 and 31...still isn't as extreme as 21/31. From Dicksters' posts it appears he wants a fun weekend car...having driven the 86 stock car before. What he would end up with using 21/31 is the "most extreme" of all examples. I normally wouldn't recommend "end-of-envelope" type settings to anyone except perhaps seasoned veterans.
---Wil Ferch

wil

you're right, its a fun weekend car.

i realise i'm going to extremes here, but just so i understand what you're saying i looked at the figures.

stock the rear is 27% stiffer than the front.

to dial out understeer that figure should be higher?

21/28 is 27%
22/29 is 20%

i don't see how this achieves the goal?:confused:

21/31 is 92% (i can see thats a huge difference).

am i way off off here with my understanding of this?

I think it all depends on the car and the intended purposes. Jack has a 3.6L in the back of an early 911 that is really built for the track, so 21/30 worked for him (although I did caution him about this at the time he was considering it). An '86 Carrera daily driver is an entirely different car - I would not go 21/31 personally. It will likely hop around and exhibit snap oversteer unless all the shocks and sway bars are upgraded to race set-up, and then it isn't a daily driver anymore. Just my .02...

Dickster:
What complicates this whole thing is that Porsche also altered the anti-roll bar sizing..which can be considered an "overlay" to this whole scheme. The front/rear balance is not totally borne by the torsion bars. Good practice, however, would say do the rough tuning / balancing with the torsion bars...and "fine tune" additional understeer/oversteer characterisitcs with adjustable anti-roll bars.
I didn't want to complicate matters more than than they are..so I focused on the TB's. As to the numbers you quote.. I must say you're correct in your math...but you need to look at the whole spectrum to see where you're headed. Stock is known. "Commonly quoted" upgrades are known ( 21/27... 21/28). All these maintain "roughly" the same balance. Going progressively stronger in the rear ( increasing the rear size an additional 1-2 mm) is the ticket to dial-out a bit of the original car's understeer. Just look how much change one size of rear bar compares to another 1-2 mm larger. It's a big increase because the stiffness increases to the FOURTH power with diameter ! You don't need a sledgehammer approach to get the rears fully 2x as stiff as the front to then say.."Yessir....only *now* do I feel a difference!". You can attack this in a less aggressive manner. Don't forget, you have the added complication ( with *any* of these sizes chosen)...that the car will understeer more at low speeds...and less at high speed corners. IMHO...a 21/31 set up will get you too close to high speed oversteer, which would be an irresponsible recommendation unless you were REAL experienced AND real good !! Maybe Jack Olsen ...with his bountiful track experience...is such a guy...but I wouldn't do this for a street car that may be driven at highway speeds.
I'm not trying to scare you...just asking you to realize where this set up is heading. You know..if you *think* you're driving your car very hard, but in actuality are not...this set up might not be as bad as I make it out to be. Certainly I thought I was driving aggressively ( on the street), until my first track experience and found out that my street driving and getting to the "limits" was actually pretty lame !!

---Wil Ferch

Now that we have TB's and adjustable sway bars on the table, Have we neglected the effects of damping on the oversteer characteristics. For street, you have people running combinations of sport and HD shocks. For the track, I've seen some running custom valved shocks or sports all the way around. I would think that an awful lot is going to depend on how fast you are loading and unloading the springs. So, couldn't you see radical differences in handling depending on the valving of the shocks?

I seem to recall that most professional race teams spend more time tuning bump, rebound and tire pressures to acheive the optimum setup. Is that just because they show up so close to spot-on?

ok, when i hear snap oversteer i get scared!:eek:

now i'm heading towards 21/29. 47% stiffer in the rear.

i don't want to upgrade everything at this point, and its got to be fun to drive on the street.

as for shocks they control the springs and need setting up for different conditions/tracks/etc.

jacks comments about thom's car are interesting. thoms setup has 21% more stiffness in the rear (6% LESS than stock), whereas his own is 70% - yet he finds the handling in thom's not unfamilar. i hear what you are saying about the weight of the 3.6 but thom has the same.

A data point: me swapping ends in turn one at Thunderhill at sufficient speed so that the air pressure -- as I shot backwards -- was enough to blow the windshield out of the car without it ever touching the hood.

But that was really more my fault than the car's. :rolleyes:

LOL

yeah, i'm a little concerned now. william knight suggested the combo after i asked for 22/28 and he said he doesn't supply that combo often, and so doesn't stock them - would have to order in. i asked what was common and he suggested 21/31 maybe 22/31. he knows its a street car, i gotta trust my supplier - right?

i'm tempted to suck and see, but i'll only know if i made the wrong decision when i'm travelling backwards into a hedge.....:(

since the cars just for fun now i want an aggresive street machine that will go well on the track. i want neutral or oversteer character but not a dangerous unpredictable car (my girlfriend will drive occasionally).

i'm getting conflicting advice. :confused:

This is a very nice discussion going on in here guys keep it up :) I am where Dickster is now,i don,t know what to buy either. I am thinking about the 22/29 setup myself because i,m with Wil about the springrates. But it,s hard to decide like Dickster said you have one change only :( or buy others but that,s expensive. I would use the car on track mostly and i dont care about a stiff ride on the street. Advice is welcome :)

Cheers,

Ed

this seems to be quite subjective with the different opinions etc. and people (including racers) do like their cars set up completely different as well to suit their own driving style.

in think theres some truth in what jack said in that any of these combos will probably improve the handling over stock - and its probably only at the limits that the differences will show up. ie snap oversteer.

thats all i can conclude at the mo.:)

When looking over all these posts....some things now become more clear that we didn't stress before...

How hard will you drive the car ( how close to .."at the limits"...)?

So...maybe try the 21/31 combo Dickster...and provide feedback. The one thing I've learned is never to lose an "open mind" attitiude about anything.

The only caution to Dickster or Dutchie or anyone else contremplating this:
- be honest about your own capabilities and whether or not you're truly going to get near the car's limits ( one way to find out is to do several DE's with a nearly stock car...you'd be amazed how much "more" the car has than you thought).
- if you're driving "hard" and find out you're only at 7/10's....maybe go for it. The discussion so far is valid if you're near "at the limit" handling.
- be careful in the rain , though ;) :)
I personally think an agressive street up of 22/29...or even 22/30 would be just fine, and you'll notice a difference.
---Wil Ferch

Lothar's point about shocks being dampers of the springs is a good one and goes to what I was trying to say: a 31mm rear torsion bar on a 911 that is insufficiently damped (read anything less than custom valved shocks- Bilstein, Penske, JRZ, Moton) is going to behave unhappily. At the limit on an uneven or slippery surface, this means rear wheel hop and (with a 21mm front) snap oversteer. The problem with saying that this is not a problem if you drive 7/10ths is that you do not always get to pick the time you will be driving over this threshold - what about a blind decreasing radius corner on a downhill section you started taking at 7/10ths. Not a time you want to be lifting or braking to get back under your limit, if you have time to do that...

Wil this is only the end of the envelope for torsion bar cars. When the 31 bar is not stiff enough 911 owners go to much stiffer coil over springs. A 31 bar is not that stiff in the racing world.

You can theory to death the effects but real in the seat feedback can't be discounted.

My theory;) is that from the factory these cars have some under steer engineered into them. Combined with rear weight bias and tendency to snap spin if you lift in a corner would lead the factory to create a more forgiving set up. So trying to maintain the factory balance is not the hot set up for track racing.

Where you race should be a consideration too. What type tires, street or R rated DOT?
Softer set up for parking lot events with street radials.
Firmer for high speed events and R rated DOT tires.

Adjustable sway bars can help improve confidence in your car too. With the 31/21 torsion bars adjust the front sway bar for full stiffness and race it. Soften the front sway bar for more steering as you become confident with the new set up.

That said I went from a 21/26 to a 21/30 set up on my 2200lb 911. The firmer rear bar suits me perfect. Next I would like to have my Bilsten shocks re-valved to handle the increased rebound from the stiffer torsion bars.

I did drive it 350 miles the other weekend while sitting in a Sparco race seat, the seat was not so plush but the car's suspension seemed acceptable on the street to me.

My car weighs 2200lb so a 31 rear bar in a heavier car appears acceptable to me.

Ted:
Points all well taken....
but Dickster ( it appears)...is not a racer and the car is not set up for coil-overs...I do understand you can go even further. I felt that was way "OT" for this type of discussion.
Nonetheless...what kind of "general" messge can we give Dickster?
First I say that this set up is extreme..then I recant and say fine, as long as you don't get near max limits...then someone says you can't "choose" when 7/10's occurs ( true enough)...yet others go on to say you can stay with 21/31 as long as you use adjustable anti-roll bars.

If I were Dickster...I'd probably say right now...WTF do I do now ???

I'll maintain that if he hasn't experienced any of these combo's yet...he wouldn't be disappointed with something just under the 21/31 extreme...wouldn't you agree? :)
--Wil Ferch

I'm NOT a racer...
 

My 88' Carrerra is set up with 31/22 hollow torsions, with monoballs and sway aways in the rear. Tyres are the Victor racer series...and although I haven't taken to the streets of Willow nor Button, I seem to hang very close to Dave Buhaines white RS look, race car while heading through Angeles Crest Hwy. My car probably weighs a bit more than Mikes rocket or that matter Daves 3.6, but handling wise it seems to do just fine when it's time to perform.

I'm sure if Randy Wells were to drive it, he could determine more accurately his very experinced opinion. It appears that suspension symetry on these cars can be tweeked in many ways, but it does not compensate for poor-average driving abilities VS seasoned vets ala: Jack or Randy W...

These are guys have done their do-deligence and have invested alot of time "on the track" and, IMHO, can offer very good advice on this thread...

True, and I assume his desire for a stiffer set up is based from experience with his current set up.

So yes your set up would provide an improvement.
In fact I think your setup would be preferred if racing on radials in a parking lot AX/DE/TT. His tire choice and racing location are still not clear to me??

Hopefully it would not leave him searching for a stiffer set up.
My .02 is I have what he is speaking of and it works for me.
It might be a bit stiff for the parking lot tracks but much more comfortable at the high speed events with race tires.

I would prefer to error on the chance that a car is a bit stiff. A soft car at high speed is less desirable to me. This just gets into personal preference.
Personal preference becomes the issue when spitting hairs between a 29 and a 31 rear bar.

Most of us want to avoid spending money for incremental changes that we redo more aggressively every year. Perhaps this is part of the reason this person wants to take such a big step.

Either way it's expensive if you go to shop and have this done correctly. Buy and install new bars and set ride height, four wheel alignment and corner balance could set you back a grand easy.

I am very interested in Jack’s set up experiences and I’d die for a chance to take Randy’s car for a couple hot laps. These guys are fast.

Good talking with you Wil.:)

Curtis how come Dave B has not got you out to the track yet?
You like to drive fast, your cars are very well prepared.
I think you would have a BLAST!
Your 3.6 monster would eat'm up on the Fontana straight too.
Give it a try once.:D

Curtissaa:

So...your advice is for Dickster to do what?......

Only Dickster can gauge his capabilities to detemine whether he goes for an aggressive or conservative set up....no? The info on this thread...from me or anyone else...is to get him to make a more "informed" decision..
--Wil Ferch

loads of great info/advice here.

LOL - actually things are getting clearer!

i'm realising from you guys and some reading that i'm gonna have to do these at some point.

very true, i was at the cars limit at the track because of the understeer.

spot on;)

my driving is mostly street, but i want to go to the track and not wish i had a hotter setup. track is where i would push the car, on the street, well, its usually not possible.

it is more obvious now that experienced guys have different opinions based on their preferred setup. i can always increase the fronts pretty easily (the rear is more of a pita), and i'll get the sway bars.

its interesting that 21/31 should be less harsh on the street than 22/28 because of the fronts.

i feel happier with the decision now, thanks for all the advice.:)

Just for U Wil...
 

BTW...dickster...put some miles on your car "as-is". Learn the handling symetry, then "baby steps" so your car will be comfortable when driving on the streets...

Looks like everyone is happy...I'm glad...
--Wil Ferch

To all,

This was a really cool thread. There was a great exchange of ideas and opinions. I am pleased to have been a part of it.

There is a lot of expertise and knowledge here. It's great when it bubbles to the surface in such a constructive way.

FWIW...I opted for 22/29's with an adjustable rear sway bar (stock up front) and Koni adjustable sport shocks all around (set at mid point). Sway bar is set fairly neutral. Ride is firm with little or no body roll...gets a bit choppy if the pavement is rough, but that's rare in this part of the country. I enjoy the 'go-cart' feel.

On the track, with R-compound tires the balance is about right (for me). I can use controlled oversteer to get through turn 10a at Road Atlanta while not feeling the threat of unexpected rotation going through 12 or 1. Overall, it's a great track/street set-up....highly recommended Dickster.

Wil,

Where did you get those numbers for the effective spring rates of the T-Bars?

Those numbers are different than the ones shown by Steve Timmons here:
http://instant-g.com/Data/911CoilConv.html

Thanks,

Tom

Hmmmmm!
Interesting! A couple thoughts to add. I heartily endorse adjustibility! By all means, go the adjustable sway bar route.

Secondly, check out some of the books like Fred Puhns "Make your Car Handle" (I think thats the title!) and read about the theory of frequency. That is, each spring will create a natuaral frequency, and the weight and distribution of that weight have a lot to do with it. After you feel good with your frequency balance, then make sure your shocks are capable of handling it.

If at all possible, adding adjustable dampers (2 way if possible) will really help fine tune the car. Often shocks can cause problems which are attributed to spings or sway bars.

By creating an adjustable set up, you can go stiff (which will help avoid the "I want more" scenario later) and then start at a safe middle point, and dial in gradually to the perfect set up for you.

Last, don't forget to corner weght it after all the changes!

All, of course, IMHO!

Adjustability is the ultimate hedged bet!

Ill help confuse things even further :p

I have 350/550 lb coil overs in my 1900 lb 911

http://www.pelicanparts.com/support/smileys/wat6.gif

We run our weekend track / AutoX car on 21/26 torsion bars with 22mm ARB's front and rear.
Although this is a lighter early 70's car, we are pretty speedy with a 2.4 911E motor.
I subscribe to the process by Wil Ferch. With your heavier car, and modest power and rubber, go with 22/28. If you get to the track need more roll stiffness, do the anti roll bar upgrade next. If you are running street rubber, you probably wont find the grip to explore the extra stiffness from more torsion bar.
The 22/28 combo will allow you to run low enough to take advantage of the lower CofG too, without always being on the bump stops.

Regards

Hayden

So where does one LSD (or lack of) fit in this discussion :D

Emcon5:
I calculated the TB equivalent spring rates myself..and am quite sure of the results. It's in the Rennlist Tech area that I can't seem to cut 'n paste. Go to Rennlist.org, then click Tech forum / 911-964 / suspension...and then click "torsion bars".
I did note that the "relative" spring rates ( compared to one another as a percentage)...compares well to Timmin's notes..but all I can say is that I know what dimensions I took to come up with my numbers...I don't know Timmin's basis.
---Wil Ferch

Wil,

Doing the calcs, did you take into account that the longitudinal tube, forming the inboard edge of the Front Winshbone, is also a torsion spring?
Being in series with the torsion bar, it has a significant impact on the torsion bar installed rates.

Regards

Hayden