924srr27l

ok, But why is the rear calculation squared then, only the front isn't ?


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SloMo228

The rear spring (the torsion bar) has a lever (the trailing arm) acting on it, whereas the front spring has road forces more or less directly applied. Because of the lever effect of the trailing arm, you have to calculate the "effective rate" at the wheel in order to match front and rear spring rates.

924srr27l

Yeah I get all that, each component does not give a 100% Spring rate due to the positions, angles and leverage points etc...


So the front struts on the early and later 944's are different unyet the calculation is .9 so 90% of the Spring rate is the wheel rate, and the rear arms leverage is 65%, so again where and why does a a "Squared" calculation come into the rear only ?


Hence my question:


how does a .65 ratio work out to be 63.4Lbs from a 150 Lb spring?

.65 of 150 is 97.5 ?



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mel_t_vin

Correct...thanks, Mike.

924srr27l

It's like trying to get blood out of a stone !


why does a a "Squared" calculation come into the rear only ?



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Noahs944

Rotational Leverage, no?

924srr27l

Just to clarify, I'm referring to Coil Over Springs here, not the Torsion Bars.


They are taken at the designated rate with no reductions, according to the previous page (melvtwin) so my 27mm Hollow are 220 Lbs.


But I want to clarify what deductions to take off a Coil spring rate to get the actual wheel rate if I also use a Coil over Damper as well as the T bars.


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Noahs944

Thanks to my Homeys in E-Town for hooking me up withthe used springs. Gonna blast em & paint em.
Also got my 28mm T-Bones...

https://www.youtube.com/watch?v=0hiU...Rus19o&index=4

mikey_audiogeek

It applies to the front as well. Front motion ratio is about 0.94. Square that and you get the rate ratio of 0.9.

924srr27l

Ok, so what's the actual Squared figure when fitting Rear Coil over Springs?
and retaining the T bars

And the Front Strut Figure for the 924 must be different to the 944 & 968's because the wishbones and wheel offset? differences

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mikey_audiogeek

People will stop answering your questions if you don't pay attention to the answers already given...

Rear spring rate ratio is 0.4225 (multiply spring rate by 0.4225 to get wheel rate). Front spring rate ratio is 0.9.

Offset doesn't make much difference, the main effect is the angle of the strut which is approximately the same for all 924/944/968. Wishbone length has no effect.

Add the rear coilover calculated wheel rate to the torsion bar calculated wheel rate to get total wheel rate.

These calculations are to only get you in the ballpark, there are other factors that have more effect on handling balance.

Google "wheel rate spring motion ratio" and start reading!

Cheers,
Mike

924srr27l

I'm looking for more accuracy Mike, not ball park.


Pay attention and start reading.


What other factors are there ? that might change these 2 fundamental figures when calculating & changing springs on the front & rear


There are some cracks and uncertainty and I'd like some clarification if possible as much as one can get from a forum.


So some calculations were declared:
But there was no mention of a front figure which had been or was to be squared?
hence why I asked.


"Have always seen" may not be that reliable ?


So only then was a .94 figure declared but "About" sounds vague, so does anyone know an accurate figures and for both wishbone lengths?




The question of the early v late Strut angles is a good question, because I do know the strut angle and wishbone length are used to calculate the wheel rate an they are significantly different (25mm?), so there's no question to my untrained knowledge that the ratio numbers cannot be the same.






Approximately is not good enough, an inch longer wishbone will produce a different figure due to the higher angle hence they are not the same.


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kevingg

Thanks for such a great thread here a lot! I want to lower my car soon too, and I have already got some nice springs from a good spring manufacturer. I want to do it on my own. Is it difficult to replace everything?

porscharu

924ssr27l, if you were not happy with the answers supplied then take your own measurements and calculate to your satisfaction. This might be a good time for you to help the community.

MAGK944

Please stop highjacking this thread, the op has his answer, bought the parts and has probably fitted them and is enjoying his car. Start you own thread about the "exacting figures" you seem to need to perform what is, at best, a guesstimate anyway.

You have been given good figures by others here to use for your calculations, so use them or do your own measurements. I gave you advise earlier in this thread and I said you would probably ignore it and do youir own thing anyway, which you did.

In truth each car will have slightly different figures anyway depending on wheel offset, ride height, camber, weight, etc., therefore the only way to figure your "number" is to measure your car. However, in parctise it will not make any differnce when you do measure your car and use your figures. For example I've seen figures from 0.92 to 0.96 quoted for the front MR and guess what, they are all correct, so pick one or do your own specific meaurements. If you calculate your own number it still is relatively meaningless as there are tollerences in effect beyond your control. I shall demonstrate, lets see how it works out with 250lb spings using two extreme MR's:

250*(0.92^2) = 212 WR
250*(0.96^2) = 230 WR

Now lets add the fact that coil spings are made to a nominal number and an acceptable tollerance standard of +/-10%. So the first calculation for wheel rate could be anywhere between:

For 250 nominal springs: 225*(0.92^2) = 190 and 275*(0.92^2) = 232

I think you can see where I am going here. However, iof you really want to calculate your own number for MR, it is relatively easy. The number is simply the cosine of the angle of your strut , its otherwise know as the Angle Correction Factor (ACF). For the 944 its around 15 degrees at the front and what do you know, cos15 = 0.96

Finally consider that all your static balancing and calculations mean very little in real life, they are a basic starting point at best, nothing more so dont get too caught up in it. Dynamically, the way you drive and how you like the car to handle will affect things much, much more. Its a personal thing.

Appologies to everyone reading for the long post, I will continue to bore you...

The best suspension advise I have seen on this forum is from Chris White a few years ago. I keep all of these posts and take notice of them. I am posting it here for everyone to take note of as it a good a place as any for some expert advise:
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Chris White , 11-03-2012 09:59 AM
Time to debunk another old ‘myth’….
Choosing the ‘best’ spring rate is not a math calculation to get the same rate for the front and rear. People get caught up in the ‘perfect 50/50’ balance concept. In reality it just doesn’t matter that much. In fact for the best grip you want a rear bias for braking and acceleration – people that like to poke fun at 911’s for their heavy rear bias don’t understand that under barking with a lot of weight transfer the 911’s end up with a nice balance and those of use with static 50/50 balance end up very nose heavy. Look at classic racing 911s –close to the same rotor and caliper size front and rear. Then when its time to accelerate the 911 transfers lots of weight back to the driving wheels….and the rest of us have to deal with tire spin!
In a track situation a well driven 944 should only have an actual 50/50 weight distribution when it is parked. If you are driving well you should be either accelerating or braking – not coasting.

SO, in reality you need to select the best spring rate for suspension control for each end of the car and then use sway bars and other technics to balance the car. There is some grey area here, you can ‘t go way overboard with different rates front to rear. Most well set up club racers end up with full coil overs running 6-800 front and 600 rear. 600 in the rear seems to give the best balance of stiffness to reduce roll and suppleness to absorb road surface variations. The front spring rate is more determined by driver style and preference. I have seen fronts set up at 600 – 1200 that give good performance.

If you do the math you can see that this is far from the ‘balanced’ approach that the typical equal effective rate would dictate.

As for street set ups – the nice balance I have found is a closer to equal than the track set up. 350/550 spring rate feels good to me – but that really depends on the condition of the roads in your area. 350/550 over expansion joints is obnoxious. On smooth paved roads its nice.

A good piece on news is that springs are relatively cheap – try a couple out and see what works for you.

Chris White , 11-07-2012 09:50 AM
The first piece of advice is to ditch the T bars.
The torsion bars are a nice theoretical solution since they do not add any unsprung weight to the suspension; unfortunately the way Porsche chose to use them in the 944 makes them a problem. There is no easy adjustment for height and its very time consuming to change the bars for different spring rates.

Corner balancing a 944 with T bars is a very time consuming effort…and not much fun! The problem with the ‘helper’ spring concept is that it can only raise the rear of the car (unless you reindex the torsion bars). You also will end up with an interesting situation where you have a dynamic difference in spring rates from one side to the other. When cornering you are compressing the Torsion bar and helper spring on the outside wheel but on the inside you are relaxing only the torsion bar (the helper spring will be rattling around on the shock with no effect on the suspension). Its not a huge problem, but it does make the sway bar deal with multiple effective spring rates.

If you ditch the torsion bar you can change the rear springs in under 30 minutes, and that is pretty handy compared to half a day to swap or reindex torsion bars!

Another thing to keep in mind when looking at spring rates that have been used by other people, especially track people; the main reason you see production cars use high spring rates for track use is to limit the movement of the suspension. The more you allow your suspension to deflect the great the camber and toe may change. On cars with not well developed stock suspensions the change in camber can be quite significant and the best way to control it is by using very stiff springs so that the suspension will not move from the static position. If you look at a picture of a 944 cornering hard on a track you can see that the inside front wheel has horrible negative camber and is usually scrubbing the inside shoulder of the tire. Look at the pictures to see this in action (the Garnet Red car is a picture of my street car from 10 years ago running the 350/550 combo with 2.5 degrees of negative camber and weltmeister sways – it still shows inside front bad habits!
My advice would be to keep the 400 fronts, ditch the Tbar and go with 600 rear coil overs.
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Chris White , 11-07-2012 10:35 AM
You may be able to calculate the best ‘theoretical’ spring rate…but that does not equate to the best spring rate in the ‘real’ world. I know that some of the readers get all bent out of shape when I say that, but the ‘real’ world has to take into account track conditions, weather conditions, driver style and other intangibles that are not in the theoretical world. 

If there existed a track that was as smooth as a pool table with absolutely no imperfections I would set a 944 up with very high spring rates. The 944 is a street production car and as such has a suspension that displays unwanted (for the track) characteristics through its range of movement.

If the track is very rough (like Bridgehampton before it closed) you need a much softer spring rate to avoid skipping down the track! Braking into the heal of the boot at Watkins Glen does not work well with a very stiff suspension – and it can’t be fixed by just tweaking the shocks!

Dry vs rain….big difference in spring rates.

Driver style – smooth vs overly aggressive, trail braker vs hard turn in….

Tires – brand new Hoosiers or 30 heat cycle Victoracers.

Suspension components – solid vs rubber bushings.

Way, way too many factors in the ‘real’ world to be able to calculate the one best spring rate. You can calculate a good starting point but that’s about it – or you can see what has been working for other successful drivers/teams and start with that set up and fine tune it to your needs.