Suspension Oxymoron

[Superman]

We all know about "jumbo shrimp" and "military intelligence," and most of us understand to some degree the problem with the phrase "combination street/track suspension setup."

Now, many of use faintly understand how a track suspension helps a track car on a track. The car leans less, and weight transfer between the four wheels is minimized. The tires maintain better traction due to this better balance, and the car is less apt to become upset. But we also know that tracks are as smooth as glass compared to Western Washington freeway sections. In fact, if a track is smooth enough, suspension travel can virtually be reduced to zero.

But what if you take a zero-travel suspension onto I-5 just North of Fife? I suspect that, in addition to being a nearly unbearable ride, the tires may not agree to staying on the pavement all the time. That is, the horrendous problems with the road surface on certain terrible patches of road, may in fact allow a "street" suspension better performance than a "track" suspension.

That is my question. On certain terrible road surfaces, will a "street" suspension provide a more effective level of performance (traction, I mean) than a "track" suspension? Aside from the comfort considerations.

[Chuck Moreland]

Absolutely. The general rule is the coarser the road surface, the more complance you need in the suspension.

A stiff track setup will bounce over bumps with the tires becoming unweighted or even losing contact with the surface. Not good for traction.

[MrKJBlue]

Taking it to the other extreme from track cars, have a look at the rally cars, extra height for more suspension travel.

[island911]

Yes. . .you've got it, supe.

I believe racing-karts still hold the track record at PRI (SIR). . .w/o active suspension members, they stick like glue. (a good example of a "suspension" extreme.)

I happened, one day, to get the bad idea of taking a race-kart up a twisty country road. (don't try this at home) Not a terrible patch of pavement; but in a kart at ~55mph. . .OMG that thing was down-right scary. . ..hopping and skipping all over . .. rattling my brain like a paint-can in a shaker (i hAveNt bIn da saMe sINce . ..though I hIDe iT wEL )

So, yeah, "street/track suspension" is a bit strange, until you start talking active suspensions. Though, of course there are those whom will believe a stiffer suspension is always a good goal. . .or sell.

[jluetjen]

The car leans less, and weight transfer between the four wheels is minimized.

Actually -- you got this wrong. Stiffer suspensions transfer more weight faster. One of the reasons that stiffer suspensions seem to work is because they can keep the tires in the optimum alignment with the road better (by limiting wheel travel). In many cases faster weight transfer and better geometry is better then slower weight transfer and poor geometry. If the geometry were better across the entire suspension travel, then in fact a softer spring set-up may be faster around a given track. A sign of a car with good suspension geometry is that it generally can run a softer set-up then a similar car with poor geometry. (Note that this assumes that the spring rate is sufficient to keep the car off of the bump-stops. Once you hit the bump stops your suspension goes hard really fast which upsets the handling and may spit you off the track.)

Yes karts are fast, but more often then not because of superior power to weight, low weight, relatively lots of tire and their small size compared to the dimensions of the track allow them to use the width of the track more effectively. If they had a real suspension they'd be faster still.

The fastest set-up isn't the stiffest that will keep the tires on the road but rather "Just right" -- not too soft nor too stiff. And the geometry counts a lot.

Corrections per Chuck's post in italics. - John

[ischmitz]

You could go with adjustable or double-adjustable shocks to have some quick way of tuning for track or bumpy HW roads.

[Scooter]

So what is the optimum setup then for a street driver that wants compliant suspension, a little lower than euro height, and wants good cornering without a lot of body roll?

[Chuck Moreland]

John I agree with most all your comments but take issue with your opening comment on transfer and stiffness.

For a given lateral acceleration, total weight transfer is entirely a function of the position of the center of mass relative to the wheel base (yes this will shift with body roll, but that is a very minor consideration).

Suspension can be tuned to distribute the weight transfer between front and rear, but cannot alter total weight transfer (accept by changing height and therefor center of mass).

You can also widen the track to alter the position of center of mass relative to wheel base.

[ischmitz]

Front 22mm sway bar, rear 20mm sway bar, 21/26mm or 21/27mm front/rear torsion bars with Koni-S adjustable shocks. I would go for about 1.5 deg neg. camber both front and rear. Voila

EDIT: And make sure your bushing are in tip-top shape or get some street hardness plastic busings.

Ingo

[Randy Webb]

remember wt. transfer really needs to be analyzed both at steady state and during transients.

for Scotter - the best thing would be to start stock (with NEW rubber or Eleph. parts) and then try out different mods -- can get spendy tho.

If you do this, pick a bumpy and twisty road and drive it enough to memorize it and the car's configuration. I was able to do better than this as I had 2 911s for a while - one stiff and one stock. the latter could beat the former on the bumpy twisties every time - I was able to drive one, then test the other within 5 minutes of the first, so I got a good comparison. I also had a technology bubble Boxster S for a while at the same time -- it's modern suspension absolutely killed both 911s (a 73 and a 75) -- and did it with almost too much comfort.

[Superman]

Suspension is my next big project, so this is why I am asking suspension questions. Even though I am an old fart that uses his 911 as a daily driver, I am not particularly concerned about ride stiffness/quality. If the ladies don't like it, I'll go to the next one on the list or they can meet me at the opera house. But I do want a suspension that will hold four tires against the road, all the time, at the track and on I-5 just North of Fife (locals will know what I mean).

So, perhaps 23/31 Tbars and shock absorbers that resemble solid bars may not be the perfect setup for me. On the track, I would like to kick ass, but on the freeway my interest is in staying alive. I need good traction for both those goals.

[Grady Clay]

Its all a compromise.

Race 911s on a smooth track can utilize stiff spring and shock rates, rigid suspension mounting, almost non-compliant tires, etc. Take the same 911 on Paris-Dakar and you need soft everything and lots of suspension travel.

Can you take a stock 911 and improve its handling? Yes.

If you live in rural Colorado and drive corduroy roads you want to raise the suspension, use softer shocks, taller profile tires, etc.
If you live in Denver you can use stiffer torsion bars, firmer shocks, lower ride height, and low profile tires.

When you go to the Second Creek track, guess which one performs better?

Its all a compromise.

Best,
Grady

[Superman]

Okay, so on some roads the go-cart suspension (none) is best. On others, rally suspension (2' of travel) is best. Hmmmmm......

So, then does it logically flow that the stock suspension on a 911 is ideal for a particular kind of road?

[Grady Clay]

No, It is a compromise. Even the best set up race cars work better in some corners than others.

Best,
Grady

[rfix'n]

Since Superman has an SC, alright my motives are a little more selfish as I drive an SC, how about some SC owners who have modified their suspensions for both street / track tell what they used for torsion bars and how happy they are with the setup.

Thanks in advance!

[Jdub]

Supe:

I went 21/26 with Sports. Neatrix rear, poly/street front.

But to be truthful, it is simply *replacing* the suspension bits that will amaze you. Since I dumped all of the old, dryed-up rubber, tbars, tierods, ball joints, and clapped-out shocks, my stiffer-than-stock setup is far easier on my molars than the original stock setup.

That's what'll blow you away: just having a fresh suspension. My tbar selection (note 9"er rears allied with 26 tbars) really smooths out the worst of it. Come on up and take my car for a spin.

John

[Schrup]

Originally posted by Superman
But what if you take a zero-travel suspension onto I-5 just North of Fife? I suspect that, in addition to being a nearly unbearable ride, the tires may not agree to staying on the pavement all the time. That is, the horrendous problems with the road surface on certain terrible patches of road, may in fact allow a "street" suspension better performance than a "track" suspension.

When I was driving home from the vintage races Sunday, I got boxed into the far right lane south bound from hyw 18 to the Pierce County line on said road. Think I chipped 3 teeth, it was horrible, reminded me of my old Bobcat days on frozen construction sites getting pounded.

[Schrup]

I hope he takes you up on your offer John. He has driven mine & I would be curious what his conclusion would be. I feel my suspension might be a little harsh for the amount of commuting he does. 22/30 Torsion, 22/21 sways, revalved sports, polygraphite bushings, 24/25" ride height, with 15x205/55 & 225/50s.

[jluetjen]

John I agree with most all your comments but take issue with your opening comment on transfer and stiffness.

Good catch Chuck. Note that I've edited my earlier posting to say that stiffer suspensions transfer weight faster. Cars with no suspension will transfer the weight almost instantaneously (less tire flex).

(yes this will shift with body roll, but that is a very minor consideration)

OK, Now I've got you on this one -- if it was minor, why would people bother to widen the track of race cars? The downside with a wider track is that it increasese frontal area and aero drag. Given that most cars are roughly 2x as long as wide, the weight transfer front to rear is roughly 2x the transfer side-to-side (less chassis flex).

The amount of suspension travel and the spring rate (among other things) will determine how long it takes for the car to settle and transfer the its weight. Shocks will also affect this. Generally cars that transfer their weight instantaneously have limited "feel" and can be difficult to drive. Any bump in the road (and all tracks have them, even the wonderfully smooth F1 tracks) will upset the traction of the affected tire (and possibly the others) if the car is too stiffly suspended. Having a car that transfers it's weight too slowly results in the car lagging the driver's inputs and potentially the track. I go back to "Just Right" being the ideal.

What constitutes "Just Right" is going to be different based on the track, the driver's skill level and the combination of suspension settings. To define it proactively requires a discussion of suspension frequency, weight transfer equations, careful measurements of the car and it's roll centers, CG and an understanding of the speeds being travelled.

[Randy Webb]

Superman - Grady is right, it is a compromise - it might be the "ideal" compromise for a certain range of road conditions and driver desires -- make sense?

Phase I:
Be sure your shocks are good and the tires too. Stick with 15" wheels. Then replace every piece of rubber in the suspension - including the top shock mounts. Consider using the Elephant poly bronze items F & R. Chuck says they are fine for street use and give better feel w/o harshness.

Phase II is to modify for stiffer swaybars, T-bars, and shocks. Evaluate how you like this - you can always change back -- it's only money (ha ha says my indebted wallet...).

* * *

Phase n+1 is to put an erp setup on the front and a 993 suspension into the rear -- probably costs more than just buying a 993... there are some posts on Rennlist re these. Even Cary Eisenlohr chimed in.

Whatever you do, read BA's book first and good luck.