Spring rates for circuit Alfa Romeo 156 Twin Spark

[warsch]

I know I'm opening a can of worms but I'm really trying to figure out what would spring rates would be the best for track dedicated Alfa Romeo 156 2.0 Twin Spark.

I've checked Twin Spark cup controlled suspension if offered with 10/7 kg/mm springs which equates roughly to 1.3 Hz and 1.82 Hz front and rear suspension frequencies respectively.

The general consensus is that 2 - 2.5 Hz is suitable for a circuit car so 1.3 seems a bit on the low side.

I've checked what some coilover companies offer

stock 3.5/2.65 kg, 0.77/1.12 Hz
BC Racing 10/3.5 kgг, 1.3/1.29 Hz
Yellowspeed Dynamic Pro Sport 12/5 kg, 1.42/1.53 Hz
XYZ Racing Tarmac Rally 16/8 kg, 1.64/1.95 Hz
Yellowspeed Premium Competition 20/14 kg, 1.84/2.57 Hz

I currently have 11.5/5 kg spring and is somewhat happy. But what if I'm missing something?

The car is no/low aero and will be running semislicks next year.

What rates seem reasonable for QLD tracks?

QR and Lakeside aren't particularly smooth in some parts.

[Colin Edwards]

The 1.3Hz front springs are probably chosen to suit the grip characteristics of the Federal tyres.  TS Cup is intended to be an entry level category.

I'd be going progressively softer in the front and harder in the rear until the rear lifts an inside wheel then go back one or two steps.  Use bar stiffness and dampers to fine tune balance.

[warsch]

That's an interesting approach but it's way too unpractical, at least for me. As I would need to have heaps of different springs to try and then swap them over a few times. Especially so that I need to order a set of new coilovers and they would revalve them to match spring rates. Thus it's really great to know ahead, at least the ballpark. Hence the question.

[Colin Edwards]

Given 1.82Hz rear springs are specified for TS Cup, why not give that a try first up.  Fair chance you will (or should) end up greater roll stiffness in the rear. 
Coilovers featuring externally adjustable bump and rebound should be a priority.

[warsch]

Thanks Colin.

TS cup setup is in fact stiffer in the rear if accounted for front suspension motion ratio (~0.75). That is also true for all the other setups listed.

So if I use their setup as a baseline, how do I adjust it for grippier tires?

Coilovers with separate compression/rebound adjustments are out of my price range unfortunately. I perfectly understand the benefits but the price almost doubles with this feature for all off the shelf kits for 156/147/GT platform.

[Colin Edwards]

Hi warsch,

Unfortunately the old Catch 22 kicks in here.  In order to "adjust for grippier tires" you need to have "adjustable" suspension!  If you don't have fully adjustable dampers (at least), your unlikely to release the full potential of "grippier tires"!

Given you intend to develop a "track dedicated Alfa Romeo 156 2.0 Twin Spark", a degree of suspension adjustment will be needed if you want to effectively realise the full potential of the car, its suspension, its tires and the driver!

I'd certainly install the stiffer rear springs, install a budget datalogger / laptime recorder, beg borrow or steal a quality tyre pressure and temperature gauge (the Alfano T36 is my choice) then play around with tyre temps and pressures. 

[warsch]

That's all true. But I can only afford one-way adjustable suspension at the moment however I can order any spring rates to come with them. So I will have some limited degree of adjustability. I will get two-way dampers eventually but just not yet.

I have basic pressure gauge and infrared pirometer and a phone data logger app. And I plan to invest in this too. I will be working on driver potential separately with some coaches too.

But for now I'm trying to figure out spring rate ballpark as I can change them later on within +- 2 kg.

[105gta]

Hi Warsch, as Colin has pointed out it's a bit of a catch 22, I'd suggest if you're happy with the current setup, leave it as it is for the time being and save for better/more adjustable suspension. There are so many variables (including the driver) that make it near impossible to guess what is required.
Like most of us, money is a real consideration. Maybe it's best to choose the tyre you want to run and think you can afford to replace as needed (proper semis will wear out way faster then you expect) try them out with your current setup that you're happy with (and know how it behaves). Drive it as often as possible, play with wheel alignment, pressures etc and improve your self until you honestly can't go any faster. Then let a pro drive your car and let them show you where the extra time is.  Hopefully by then you'll have the money aside for decent suspension and a good idea of what you want in spring rate. Spending money on low end single adjustable 'coil overs' is probably not the best financial move. It'll be a big learning curve and throwing in too many variables at once will only complicate things more.
You're happy with your car. Get the better tyres. Learn them and consider your next move.
Ben

[bazzbazz]

Also remember THE most important thing about suspension tuning . . . . . . change one thing at a time! 

[warsch]

I have single adjustable coilovers already and they were really a step up from "sporty" suspension I had before. However I bent both front shock shafts so I need a new set. Hence the question. While I'm ok with current setup I still keep wondering what if I just don't know any better?

I do drive a lot, I had 19 track days this year so now I'm at point of needing a coach as my progress is slowing down. And that's the plan for the next year.

[105gta]

It sounds like you're on the right path.
Seeing as though you're in Brisbane, have a chat with MCA suspension. Murray may be able to repair your existing shocks and revalve to suit your needs and car setup.
Keep in mind that vehicle weight is important, so if you plan on changing things(any lightening, roll cage etc...) try to have that done before trying to fine tune the suspension.

[warsch]

My current coilovers are of unknown brand and a bit of unknown mileage. I also damaged both front inserts. I guess I can buy just the new front inserts but I rather want a brand new set of BC Racing or Yellowspeed. Both brands can valve the dampers to match the spring rate of my choice. Hence the question.

The car is a constant work in progress as building it is also part of the fun for me. That's why I want to learn and try to understand as much as possible myself.

I need to boast a bit too - I was the first one in regularity with AROCA club this and the previous year.