22/28 vs. 22/30

[Rusty Heap]

I going to try 21/28 t-bars in my 911, thanks for the info.

[schnellmann]

I have the 21/28 combo from Sander/Elephant Racing. It's a really good setup. Not sure I'd want any more spring rate for a car I drive on the street very much.

[RWebb]

Originally Posted by dd74

Souk and Walt - thanks for the information. BTW: if the shocks are mismatched to the torsion bars, what's the inherent result? Bounciness, really bad handling, etc?

This thread got bumped back up & I happened to see your query, David. I'll add to the other answers that if you troll the Internet for the terms over-damped, underdamped, and optimal damping, you should find a nice diagram somewhere that will explain things well.

Socks are really "Dampers" - the Brits got that one right.

[dd74]

Hi Randy - thanks for the help. I'm really more concerned about my rear-end squatting. I think John (JuletJen) suggested my car might be too low!

Any thoughts?

Here's a photo of what I'm talking about:

[RWebb]

IT kinda looks low... what are the measurements -- using the factory manual method?

That will tell the tale...

[drsimonwong]

I ran 23/30 for a while and found them too stiff for my mixed road/track combo (85% road). Softened up to 21/28 and I feel it's perfect now. My wife certainly noticed the difference and appreciated the change.

A touch more body roll, but the control over rough road surfaces is significantly better. Tyres stay glued rather than hopping about. The front end of our cars is so light that any stiffer on rougher surfaces is hard work indeed.

Good luck.

[Noel]

One thing that wasn't mentioned was the notion of loosing weight, especially at the back of the car and the affect this will have on the relative spring rates. Does anyone know any correlation for this. For example, loosing X number of pounds equals Xmm of torsion bar. Or is such a correlation not accurate?

[84_Carrera]

Originally Posted by Jack Olsen

..600#-coils on my 72, which is actually probably too stiff in the back. I keep meaning to drop down to 500# springs.

Jack, I ran 600# fronts on my FWD track SHO & 400# rears. I had the extra weight of the battery, steering, supercharger, etc. also, and it was WAY too stiff. Turn-in was tough, too, as I'd skip rather than digging in. Anyway, I was told to lose the swaybar also, and was considering going to 500/525# hypercoils at the time I decided to sell the car.

FWIW only.

As for the 911, I'm running 22/29's on mine.

[burgermeister]

Originally Posted by Noel

One thing that wasn't mentioned was the notion of loosing weight, especially at the back of the car and the affect this will have on the relative spring rates. Does anyone know any correlation for this. For example, loosing X number of pounds equals Xmm of torsion bar. Or is such a correlation not accurate?

An approximation of your rear ride frequency is sqrt(2*k/m) / 6.28 Hz (did I remember that right?), k being the wheel rate for one side, and m being the rear axle weight divided by "g" (you want mass, not weight). Your unpsrung frequency is probably dominated by the tire rate, so torsion bar size doesn't matter much for it.

So, keeping 2*k/m constant will keep your ride frequency the same. So, for a 10% mass reduction and 28mm bars, 27.3mm bars would be similar ...

Or so I remember from classes take long long ago in a galaxy far far away ...

[Noel]

Interesting. Many thanks. 10% seems like a lot in this case, just from a high level gut check. I'll run some numbers once I figure out how to apply that formula.

Thanks again.

[88 Club Sport]

My scarrera is set up for DE's and spirited mountain blasts. It probably weighs 2600 or so. When I put it together, I went with:

22/29's
Poly bushings up front and Neatrix in the rear
Carrera sways 22/21mm
Bilstein sports

It handled good but I was not convinced it was the best for the intended use. I then went with:

23/31's
Poly bushings all around with zerks
Tarrett adjustable sways
Bilstein sports revalved for the bars etc by Bilstein

It handles great on the track and for fun rides but I would not want to drive it too far on the street.

[pdqcarrera]

Originally Posted by dd74

Hi Randy - thanks for the help. I'm really more concerned about my rear-end squatting. I think John (JuletJen) suggested my car might be too low!
Any thoughts?
Here's a photo of what I'm talking about:

Accelerating hard up the hill there at Willow will give you squat and side load so you can expect to bury that right rear tire a bit unless you have a whole lotta spring rate and sway bar and it doesn't look like you do. You should see photos coming down the cork screw at Laguna if you want to see a wheel buried into the well! LOL ;-) I figure you're only too low if you're running out of suspension and hitting the bump tops, although there's a lot of factors involved. In SoCal I highly recommend a visit to Johnson Alignment in Torrance. Steve Alarcon will set you up with the alignment and corner balance best suited for your car and your driving style. Cheers!

[dd74]

Originally Posted by pdqcarrera

Accelerating hard up the hill there at Willow will give you squat and side load so you can expect to bury that right rear tire a bit unless you have a whole lotta spring rate and sway bar and it doesn't look like you do. You should see photos coming down the cork screw at Laguna if you want to see a wheel buried into the well! LOL ;-) I figure you're only too low if you're running out of suspension and hitting the bump tops, although there's a lot of factors involved. In SoCal I highly recommend a visit to Johnson Alignment in Torrance. Steve Alarcon will set you up with the alignment and corner balance best suited for your car and your driving style. Cheers!

TRE is handling my handling. We decided to stay with the 28mm rear torsions, and instead put revalved Bilstein sport shocks in the rear; corner balance, align, and that's it. The car is more street than race.

[Trackrash]

Very interesting read.

I have noticed from my own experience that the width of the wheels and tires as well as the offset (including spacers) will have an affect on the spring rate. It makes sense that a wider wheel, or one with more offset, would create more leverage on the torsion bar. The result is a softer sprung car.
I recently changed from 7" to 8" wheels in the rear of my longhood. The result was a wallowy, noticeably softer rear suspension.

It seems to me that if one says "I have X size torsion and it feels like ..." should be in relation to what size tires and wheels are being used.

Wouldn't a narrow body car not need as large of torsions as say an RSR styled car?

Gordon

[pdqcarrera]

Dave B/TRE is a good choice IMHO. He's just so darned far away for us behind the Orange curtain! LOL ;-) Has he talked you into adjustable sway bars as well? He should, and you should let him!

[Wil Ferch]

Gordon says..."It makes sense that a wider wheel, or one with more offset, would create more leverage on the torsion bar. The result is a softer sprung car...."

Yes..for the front. The TB runs longitudinally with the car. Wider wheels or track will impart more leverage and create a softer "effective" wheel rate.

However...doesn't compute for the rear , where the TB runs transverse across the car.

[Trackrash]

Wil, I see your point. (lots of geometric images going through my head). Could it be that the affect I had was because the tire/wheel width increase changed the roll stiffness on the rear?

Gordon

[burgermeister]

Originally Posted by Wil Ferch

Gordon says..."It makes sense that a wider wheel, or one with more offset, would create more leverage on the torsion bar. The result is a softer sprung car...."

Yes..for the front. The TB runs longitudinally with the car. Wider wheels or track will impart more leverage and create a softer "effective" wheel rate.

Not claiming to know much about setting up a 911 suspension...
If I draw a freebody diagram of a front A-arm & knuckle, and pretend the front A-arm is horizontal, how does a longer spindle (ie. tire with greater offset) change the spring rate? The vertical force at the tire patch must be equal to the vertical force at the balljoint (assuming the strut does not carry vertical load). The longer spindle will cause a greater overturning moment on the knuckle, which can only be reacted by a lateral load on the lower A-arm and on the top of the strut... so how would a longer spindle change the spring rate?

Is it the compliance effects of the knuckle and chassis structure that make it softer? (Can't be the bushings, everyone has PB in their car )

[Walt Fricke]

Burgermeister

My modest understand of levers (or arms) says that no matter what the shape of the lever arm is, its effective length (assuming rigid materials) is the distance between where the load is applied (tire contact patch center) and where the lever articulates. In the case of a 911 front end, this is the axis of the A arm through the chassis mounts. Which for all practical purposes is a perpendicular from the patch to the centerline of the torsion bar if bushings are reasonably stiff.

So if you move the patch out (widen the track of the car), you are using a longer lever. Easier to twist the T bar. Softer.

Now what this ignores is the fact that the spindle is not a rigid part of this total "lever" length. We know from scrub effects that as the tire moves up and down it can move in and out, changing track. And this can mess up handling if the geometry is bad and there is much of it, and so on.

Still, is it a reasonable first approximation to treat the ball joint and strut side loading as if they were frictionless? So, other than accounting for track changes with suspension movement, you could take the perpendicular length from the A arm mount axis to the center of the contact patch as your lever length when computing wheel rate based on a given torsion bar diameter and length?

Your long ago (so you say) learning has to be a couple of steps ahead of my lack of formal engineering education. Free body diagram? Gee, I've seen engineers use the term.

Walt

[burgermeister]

Walt,
I do admit to being an "enginer". I'll post my free body diagram at the risk of getting ridiculed off the board. It's been a long time (late 80's) and I am beyond rusty, though it is fun doing these things. If I am being a PITA, please let me know - it is not my intent. Mostly I'm trying to learn from everyone here on the board....



There is a term that increases force based on spindle length and kingpin inclination, but it appears that it would be small.

That assumes I did everything right, which I would not bet lunch on.

?????