911/930 Camber/Caster for track

[jimmcc]

What's the latest thinking on 911/930 caster settings for track use? Less or more? There are advantages to each. I've been looking at caster settings for the Group B turbos or the early 80s which ran to lower caster numbers but they were on bias ply tires. I'm not sure if that makes a difference.

What is the thinking about camber settings differences front to rear for a 911 or 930? I know it's all relative to but was looking for some basic rules of thumb.

I'm cross posting this on the 930 list.

Thanks

Jim
1980 930
1995 993

[ChkbookMechanic]

Without my alignment numbers near me I can't tell you exactly what I run but... I like to get as much caster as possible. I think on my last alignment I got about 7*. In terms of camber I run about -1.6 to -1.7 on the front and about the same on the rear. Some people like to dial in an additional -.5* on the rear from what the front is.

[175K911]

It really depends on a whole host of factors like suspension setup, tire choice, driver ability etc.

FWIW, my car is almost exclusively track, 2508 lbs, 23/33 torsions bars, custom valved Bilsteins, Elephant Racing everywhere there's a pivot point, 17" Nitto R-comp tires, 350+ track days behind me (PCA instructor for 15 years). Here's my alignment:
-2.5 degrees camber front
+6 degrees caster
1/16" toe out front

-3.0 degrees camber rear
1/16" toe in rear

As you can see, my outside tires are pretty well flat to the ground even though the inside front is daylighted and the rear has just 25% of it's tread on the ground:

[911st]

Originally Posted by ChkbookMechanic

Some people like to dial in an additional -.5* on the rear from what the front is.

As a general rule if you the adjustment range to obtain what ever setting you might want this is not neccessary with a 930. Front and rear might be better set close to the same as a 930 has more camber gain in the rear than a 911.

However, as 930 and 911's are limmited as to how much neg camber thay can get, it may actually come to that.

On my 85 I ended up with about -2.25 neg (max camber in the rear but could only get about -1.1 to .1.2 in the front. I could use more. My springs are basically stock 86 so it is a pretty soft set up and still rolls about 4 deg in a corner.

[911st]

Ed,

Excellent data point. What is your sway bar set up please?

Thx.

[hobieboy]

Going from memory, I am running ~1.75 deg camber in front which is less than what most runs.

However, when I started measuring tire temp, its fairly even across from outside to inside (~5-7 deg from outside->middle then middle->inside).

Based on that, I didn't increase camber. YMMV.

[175K911]

Originally Posted by 911st

Ed,

Excellent data point. What is your sway bar set up please?

Thx.

Tarett 25mm bars front and rear. Settings depend on the track but in general I run the front one about 1/4 of the way from full soft and rear bar at it's midpoint.

For example- these settings work for MidOhio. At a slower tighter track like Blackhawk Farms or Grattan I'll run the rear bar stiffer, but at a high speed track like VIR or the Glen, I'll run the front bar at it's midpoint with the rear bar at midpoint as well.

But my settings are also influenced by the fact that I run a Guard LSD set at 50/80 lockup which causes push in some low speed 2nd gear turns, but also allows me to comfortably trail brake into most turns as well.

[911st]

Ed,

Cool info. Can I ask what tire/rim sizes (stager).

Sounds like it is not only a georgous car but very well developed.

Thx again.

[38D]

Steup will vary based on driver speed/level and preference. I run well over -3 degree of camber front & rear and the max caster I can get and still be even left to right.

[911st]

While camber needs are somewhat determined by the tire choice (think R888 v RA1) it is more determined by how much the car rolls in a corner (spring, sway, center of gravity).

It is also effected by the geometry of the car. With 911/930's for example the 930 has a more aggressive camber curve because of the changed inner mount. Another example is raising the spindle on the front struts on a lowered car. This restores the front end to a more beneficial area of the camber curve. On a stock car those two things can be good for about another .5 deg of effective camber.

A stock car rolls about 4 deg. With this it looses about 3.2 deg of camber. A car with upgraded sways and aggressive torsion bars might sway about 2.5 deg and lose about 2 deg of camber.

I do not know what the best tire angle is but it is probably something between square to -1 deg to work its best.

Thus a stock car would need about -3.2 to -4.2 to get the best performance out of a tire. For most of this it more comes down to living with what we have and mostly using the outside of the tires on the track, especially in the front where we can only get about -1 deg of camber.

A torsion bar race car might be able to use something between -2 to -3.5 deg of camber. (This fits the data points we are getting.)

A race car with 400/600 springs might reduce neg camber needs by another deg or so.

At least that is what I belive from what I know so far.

[38D]

Originally Posted by 911st

A race car with 400/600 springs might reduce neg camber needs by another deg or so.

400/600 would be very soft for a full race car. The 964 cups have 600/800 progressive springs, and it is way too soft (rules prevent spring changes). GT cars are running in the 900-1000f/1000-1400r rates, and those are only 2200-2400lb cars.

As for camber, top 996 cup drivers will run over -5 degrees in the front. In my class (964 cups), we run over -3.5 but less than -4.5 (specific number is a trade secret ). So I think your estimates are way off, and as I said before, largely depend on how good the driver is.

[911st]

Colin,

I was referring to torsion bar 911's. The 964 + after have different geometry, tend to run at higher weights, and are full race cars on different tires.

Also referring to DOT R type tyres that most DE/TT drivers are using.

For a 2200 lb early 911 I have heard 400/600 is a starting point for cars converted to coil overs. As a point of reference, 23/33 torsion bars are about 250/400 lbs per inch. Not drought many run higher rates than this.

As to my estimates being way off, Ed's car is a good data point for the type of car I was referring to and fits my thinking.

However, I am surely not an expert on this, just trying to help and learn.

Thank you for the info.

[fredmeister]

Originally Posted by 911st

Colin,

I was referring to torsion bar 911's. The 964 + after have different geometry, tend to run at higher weights, and are full race cars on different tires.

Also referring to DOT R type tyres that most DE/TT drivers are using.

For a 2200 lb early 911 I have heard 400/600 is a starting point for cars converted to coil overs. As a point of reference, 23/33 torsion bars are about 250/400 lbs per inch. Not drought many run higher rates than this.

As to my estimates being way off, Ed's car is a good data point for the type of car I was referring to and fits my thinking.

However, I am surely not an expert on this, just trying to help and learn.

Thank you for the info.

you have to remember too that torsion bars are acting at different points on the control arms so what you really should be quoting are wheel rates to compare apples to apples.
Thus a 31mm torsion bar may be equivalent to a 600in-lb coil spring for example once the actual wheel rate is calculated. unfortunately getting the dimensions from the suspension to run the numbers is the hard and tedious part, and most of us are relying on the previous experience of others to recommend specific spring/bar sizes that have hopefully already done the math.
FWIW, I had met Ed at the track this summer and seen his car. wow, it is nice and he is fast. i thank him for the info.

[911st]

I have been thinking about "Wheel Rates" v "Spring Rates".

I can see this calulating wheel rates form spring rates could be a very comples calulation.

However, I think we might be able to get close enought to use for comparisions if we apporach this in a more simplistic way.

Here is what I am thinking:

The front seems easy. If we look at the front the wheel goes up and down very evenly with the strut. Thus, the spring rate and wheel weight for the front should be prety close to the same. The length of the A arm is mostly irrelivant as all it dose is locate the strut.

The rear is a little more complicated in that the shock is mounted a little over 4" past the centerline of the wheel. The wheel center is about 18.5" from the center of the spring plate arm link. Thus, the rear shock point moves foughly 15% furtner than the wheel at any time. Thus, the rear wheel rate should be about 15% higher than the rated spring rate.

Elephant Racing now offeres coil over kits for 911's and has a spring rate calulator depending on use and weight.

See: http://www.elephantracing.com/suspension/coiloverkits/911coiloverkits.htm

For a 911 of my weight on a full race car they are recomending a starting point of 400front/550rear.

If my assumptions as to how we might convert spring rate to wheel rates, that would put me at about 400# front and about 630# for the rear "wheel rates".

For compairison I belive the wheel rates for a 23/31mm torsion bars is 250/332#.

Thus, 400/550 rated springs should be about 60% stiffer.

Here are what I belive to be the wheel rates for different torsion bar sizes as being:

Front: (lbs/inch) (% of stock)
19 110# ---------(18.8mm)
21 173 157%
22 210 191%
23 250 227%
24 290 264%
Rear
24 122# ----------(24.1mm)
25 140 115%
26 165 135%
27 191 157%
28 221 181%
29 254 208%
30 294 241%
31 332 272%
33 427 350%

Note that rear TB was 24.1mm till 86 then 25mm. 930 rear's were 26mm.

I would like to know if this is close to being valid and appricate input.

Thx.

[Flieger]

For torsion bars, the A-arm length is crucial for wheel rate. I tkae it you were referring to coil-overs in your above approximation.

[911st]

Yes, yes, the A arm length is important with torsion bar front 911 in computing rate.

It should not be relevant to coil over as it is more of a locating link, not a lever.

Thx.

[fredmeister]

yes but how are you calculating the torsion bar spring rates then? you need to compare the lever arm length and the point of load application between coils and t-bars to make a valid comparison.
that is why breaking both down to wheel rates gives a common denominator.

[Flieger]

This is how I see it:

Torsion bar rate is "constant" for degrees of movement. The torsion bar's force vector changes both magnitude and direction throughout suspension movement. The A-arm length does not change so the torsional force increases linearly. The wheel rate takes into account the effective control arm length for the torsion bar- the perpendicular distance from the ball joint to the torsion bar. This value decreases either side of a horizontal A-arm. This means that the torsion bar effectively gets stiffer (more than linearly) as the A-arm moves by degree. The effective arm length is just to cosine of the angular displacement multiplied by the actual arm length.

Coil-overs act in a near-vertical plane and therefore transmit a relatively constant spring rate force vector direction to the wheel for any A-arm displacement.

[911st]

Flieger

Yes, in theory the angle of the arm on a torsion bar system relative to force will change the spring rate. However, why make it overly complicated. At the angles and range of movement we are dealing with this rate cange is probably very small.

There are also some small changes in the angle of the rear shock to the rear arm that will have some effect on the wheel rate but again, this is going to be small.

Anything with in about 25 lbs per inch should be usable as we end up balancing the system further with sway bars.

Another thought. I have seen 21mm front bars run w 26, 27, 28, 29, at 30mm rear bars. Most tuners used to recommend 22/28's as a solid set up. Now many are recommending 22/29's. That is about a 30# difference. Start looking at higher spring rates and the gap is larger.

fredmeister

I picked up what I believe to be torsion bar wheel rates from two sources noted here some where and some time ago. I guess there could be some error in them but I have seen them quoted by others before. One I think might have been from a Wil French or some such and the other was from someone that I believe tested the rates at the wheel but not 100% sure. If my numbers are incorrect I would love to know this.

Here is one reference as to percentage differences between torsion bars and something on calculations.

http://www.pelicanparts.com/techarticles/911_torsion_bars/911_torsion_bars.htm


Again, I realise this is a bit simplistic but getting overly complcated may not yeald any more usable information.

[911st]

Found one some documentation:

http://tech.rennlist.com/911/pdf/torsionbars.PDF

http://instant-g.com/Data/911CoilConv.html