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

[dickster]

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...........

[Jack Olsen]

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.

[dickster]

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?

[Jack Olsen]

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.

[dickster]

thanks jack, i'm gonna take the plunge.

[Wil Ferch]

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

[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

[dickster]

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.

[IROC]

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

[Wil Ferch]

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

[Jack Olsen]

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."

[Wil Ferch]

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

[dickster]

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?

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

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

[Randy W]

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...

[Wil Ferch]

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

[Lothar]

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?

[dickster]

ok, when i hear snap oversteer i get scared!

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.

[Jack Olsen]

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.

[dickster]

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.

[Dutchie]

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