I'm not exactly sure where to start with this one..
Let me tell ya where I'm coming from...
On the rear suspension... I've been told to increase spring rate when I step up to a non-poly spherical type bearing.
I was told that without the drag of the poly bushings, I would need to go up 100ish lbs to compensate for that drag.... and keep close to the same effect suspension rate back there.

So... Here is the actual question. If the above is true, would it not then be true that with a front bearing suspension (like Mike Muellers).. that the front spring rates (be it torsion or coil over) would also need to go up....
In order to keep the front rear balance...

Brad..... On the track only cars you set up with 300... 400... rear springs... what kind of front spring rate do you end up with...

somewhere I once saw a conversion chart to compare torsion bar milimeters to coil spring foot pounds.

brant

Brant,
First I have to say that I don't understand why the effective spring rate would go up with Poly, I believe it would be the shock damping rate that would go up. Second, here's a post I saved from Rennlist that covers 911 torsion bars. Ignore the rear , and double check the arm distance.
Mark S.
'70 914-6

Oops, here's the old post:
----- Original Message -----
From:
To: 911/993/996 <911@rennlist.org>
Cc:
Sent: Saturday, January 22, 2000 8:42 PM
Subject: [911] Torsion bars


> Listers:
> Every once in a while, questions come up on torsion bar sizes , and of
> the suitability of certain size bars. I've compiled the following
information
> that can be used for all pre-G50 911's.
> Torsion bar spring rates can be determined by the formula:
>
> K= 1,178,000 ( d)*4 / (L) (A)*2
>
> -d is to the fourth power, and A is to the second power.
> -d is diameter of torsion bar
> -L is torsion bar length
> -A is the lever arm length
>
> A fellow Rennlister, Joe Winn, measured torsion bars he had for sale,
> and the following is for the FRONT bar:
> -length of 611 mm ( or ~24" )
> -spline length at each end of about 1".....this makes for an
> "effective" length of 22" , assuming full length spline engagement as
> installed. Also, I'm using a lever arm measurement of 12"...the distance
from
> the torsion bar centerline, to the brake caliper centerline. With this , I
> come up with:
> SIZE ( mm) SPRING RATE ( Lb/in)
> 18.8 110
> 21 173
> 22 210
> 23 250
> 24 296
> 25 350
>
> Using the same principle for the REAR bars, we have:
> - bar length of 626 mm ( 24.65 ")
> - spline length at one end of 19 mm, or 0.75"
> - spline length at other end of 26 mm, or 1"
> - "effective" bar length of 22.9", assuming full spline
engagement
> - lever arm of 18.5"...centerline of torsion bar to centerline
of
> rear wheel.
> With these, I get:
>
> SIZE (mm) SPRING RATE ( lb/in)
> 23 100
> 24 120
> 25 140
> 26 165
> 27 191
> 28 221
> 29 254
> 30 294
> 31 332
> 32 377
> 33 427
>
> There a few interesting observations about this. First , notice
how
> powerful that "4th power" influence has on spring rates. Also, look how
cars
> get delivered by Porsche. Let's use my car, a stock 85 Carrera, as an
> example. It uses an 18.8 / 24.1 combo . This results in about equal
> front/rear spring rates ( 110 / 120). The popular upgrades to 21/27

> yielding 173 /191) , and 22/28 ( yielding 210 / 221) , although stiffer,
> maintain about the same front and rear balance. Keeping in mind that
Porsche
> probably came up with spring rates to fight inherent oversteering
> characteristics ( in other words, purposely dialing in understeer), I
would
> think that a more reasonable upgrade for most cars would be 21/28 ( 173/
> 221).....or 22/29 ( 210/254), to reduce some of that dialed-in
understeer.
> Caution: going too far in that direction may make for a more neutral
handling
> car at low speeds, but all setings will tend to more oversteer at high
speeds
> ! ...so let's not over-do this. High speed oversteer is something you
> definately don't want.....it's hard to catch and very user unfriendly..
Once
> the basic balance is set, you can still fine tune the front / rear
cornering
> balance with anti -roll bars. Also, too stiff of a rear bar won't allow
the
> car to squat and "hook-up", you may even get a skittering effect by not
being
> able to put the power down. You need a little bit of suspension
deflection.
> Hope some of this helps.
> Regards,
> Wil Ferch - 85 Carrera
> WFerch6687@aol.com
>
> +++
> The 911 List is proudly sponsored by Vertex - http://www.vertexauto.com/
> Vertex is a worldwide supplier of quality auto parts for your Porsche.

Brant,

4 or 6 cyl car ??

I run anywhere from 250's to 300's in the front.

I need to leave (deliver a race engine for a FP car) but I will re visit this topic tonight.

B

Mark,

that is exactly the old email I was thinking of...

regarding the spring rate and poly bushings...
the way it was explained to me...
the drag and binding of the bushing itself, took more force to raise or lower the suspension.. This added force thus acted like a bigger/stronger spring...

So if a trailing arm was frozen or 100% solid... (from the bushing)... that would equal a solid chassis with infinite spring. If the trailing arm only drug somewhat that might be the equivalent of 100ft lbs of additional spring....

I think your right though... dampening and rebound.

hope this makes since to somebody out there....
brant

Your on the right track.

Problem is: the drag is only about 25-30 lbs. The springs normally sell in 25lb increments.

Step up 25-50 on your regular springs.

We run 250's in the rear of a 9146 narrow body car on Hoosiers.. it also has a 31mm bar.. meaning I could step up to 275's and back off on the bar a little.

B

Brad,

In your expert experience...
lets say its a 4 wheel coil over car...
are you running similar front and rear rates to balance the vehicle...

something like 250 front/ 250 or 275 rear?
------------------------------------------------------------
here is what is driving all of my questions fellas...
I want to do mikes front bearings...
I intend to keep my rear about the same spring rate.
I like the balance I have now..
I was thinking that I might need to bump my front rate to compensate for the reduce in drag on my front suspension with mikes bearings...

I run 22 mm front bars and was thinking about stepping up to 23 mm front bars...

the conversion email (looks like walt Fricke's work).. shows a 22 bar at: 210ft lbs
and a 23mm bar at: 250 ft lbs...

I am realistic, and know that I'm just looking for a starting point and then will have to dial it in on the track...

hey have I thanked you all yet today for all of the free advice?????

thanks....
brant

Yes on a 6cyl car. No on a 4cyl car.

The sixes tend to be very close front to rear spring rate wise when they are "dialed".

My fast 914 4 cyl drivers tend to have a stiffer front than rear. This allows us to dial in the rear with the front bar and adjustable shocks.

Start low. 90% of the questions you are asking have to do with driver comfort. Some older guys can drive the wheels off a highly sprung race car.. others like them a little softer.

B

Funny, I just had this exact conversation this afternoon. We were talking about spring rates, shock valving and swaybars, front vs. rear. Guy who is setting up my car is in the camp of big front swaybar, no rear bar, and relatively soft rear springs. Bilsteins are usually custom re-valved. Let the front end keep the car planted and let the rear move a bit.

The two "older guys" I was chatting with are on other ends of the spectrum. One prefers the softer setup with a bit of roll and the other is into the super stiff "I wanna feel every pebble" setup, like 300-600lb spring rates on some cars.

I am too much of a newbie to know what I like best and drive quickest...good thing springs are cheap. I need to switch to the Smart-Racing front bar setup shortly. The Welt 22mm bar may end up being too small, and the stories of them breaking give me the heebiejeebies.

Make sure you go through Tony or I for your Smart bar. DO not let anybody talk you out of the 31 bar. This allows you to run a softer rear spring.

This is exactly why I told Brant to start soft. It sucks setting up a car rock solid and going backwards with your setup as it bounces over everything.

B

Yep, the guy I was talking to today is running (I think) a 26 or 28 bar and said it probably isn't enough. I guess one good thing about the Smart setup, is you can change just the bar and keep your mounts and droplinks (right?).

I don't think I would like the car totally rock solid, but not soft in the stock 60's-70's style either. Somewhere in between...

One good thing, I heard that two of the fastest guys in my POC class are bumping (one of them is the guy who kicked my a$$ last weekend). I might actually be competitive in my little 914 after all.

Yes. Correct. You can just buy the bar if you need to step up later. All of their bars are designed to fit all the pieces sold with each bar. Somebody was thinking.

B