I'm trying to learn about spring rates. I've been looking at this page, which shows the effective spring rates for various diameters of torsion bars.

For balanced handling, should the front vs rear spring rate be proportional to the front-rear weight balance of the vehicle? In other words...if a vehicle had a f/r weight balance of 60/40, should that vehicle have front springs that are 1.5x stiffer than rear? Or am I completely not understanding it...

If that is correct, then that should mean 944s should have nearly equal front and rear spring rates, since their f/r weight balance is nearly even. And looking at the effective rate of torsion bars as given on that link, then front 250lb springs should be matched with 28mm rear torsion bars...right?

Again, I'm just trying to learn and understand...if I've completely missed the concept, please teach me.

 

It's important to understand that there's a difference between "spring rate" and "wheel rate". Due to the placement and angle of the spring/strut relative to the control arm, only a percentage of the spring's rate is actually holding up a car.

Also do a little research on suspension frequencies - this has more to do with what you're asking about - the ratio of sprung weight to wheel rate.

Through trial and error, many racers have found that 350# front springs and 30mm torsion bars are well balanced for a stripped out 944 - that's why it's the SP1/Spec 944 setup.

So, I suppose it's safe to say that if you're looking for a baseline starting point, you can get a front spring that's roughly equal to the torsion bar's effective rate. But there are other factors like front splitter, ballast, weight distribution, alignment and driving style that also come into play.

 

Regarding wheel rate vs. spring rate, I've come across a couple different numbers for that. One is 60%, the other I've seen is 65% (that the wheel rate is 60% or 65% of the spring rate). Regardless of the actual conversion, wouldn't it be constant, therefore something that can be cancelled out when comparing front and rear rates, as long as the type of rate being compared is the same? In other words, compare front spring rate to rear spring rate, OR compare front wheel rate to rear wheel rate.

My car is not going to be raced. Oh, maybe some day I'll take it to a drag strip just to see what it can do, or maybe to some kind of timed event or something. But in general, it's not going to be raced, but it will be (and is) my daily driver. But I do like quick and fun driving, and while I'll need a certain level of comfort for daily use, I'll also want to know that the car will respond relatively quickly if I ask it to.

The research I've done suggests that for this type of driving, "spirited street", front spring rates of 220 to 260 are good. I've seen a used set of 250 lb springs for sale, and I was curious what size rear torsion bar I should get to complement. The link in my first post says that 28mm is equivalent to 254 pounds, if I remember correctly, so I was thinking that might be the size to use, if the front and rear spring rates should be nearly equal, given the nearly equal weight distribution of these cars.

I've also read the article by Pasha on suspension prep, which does suggest using 210 lb front springs with 26mm rear torsion bars, or 260 lb front springs with 27mm or 28mm rear torsion bars. It's the "or" part of that equation I don't like, I want to know exactly what to get, which is why I was doing more research. The 250 lb springs I just saw for sale are a bit less than 260 lbs, so maybe I should err on the lower end of that range and get the 27mm bars. I just don't know. I don't want to trial-and-error this, I just want to get it right and get it done.

 

Hiya!
Unfortunately, the oft-quoted ratio for wheel rate to spring rate of 65% is incorrect. It's actually the motion ratio that is 65%. The spring rate to wheel rate ratio is (motion ratio) squared, or 42.25%. This info is corroborated by Porsche documentation.

Hope this helps,
Mike

 

It is interesting, although to be honest I don't really understand it. The main question is what should the rear rate be compared to the front rate. Am I correct that the front vs rear proportioning should be roughly the same as the weight proportion of the vehicle, or does that have nothing to do with it?

 

That's only for the rear, if you're using a coil over. I believe the front is much closer to 90%.

 

Yes I think van is correct. The fronts are around 92% rears are around 56% with coilovers, however with torsion bars iirc the effective rate of the torsion bar is the wheel rate.

 

Porsche's 42% rate was with the stock shock bolt right?
Would the extended racers edge bolt have an effect on the wheel rate as well?

 

Now I'm completely lost after the last three posts.

 

More goes into it than just weight distribution
28mm bars with 250lb springs will probably make it too tail happy

 

It sure does. Search my posts for this, where I documented the effect on wheel rates of the re shock mounts.

 

Would 27mm bars be a better balanced match for 250lb front springs then?

 

Even early and late offset cars have different motion ratios... and so do steel vs aluminum rear control arms (measuring from the shock mount).

And, while measurements of all suspension components and calculations will get you close, nothing quite beats doing a physical test... take the springs off, put the control arm at ride height, then move it exactly 1" and see exactly how much the shock compresses.

 

To understand just forget about weight distribution at this time, set the car up level or with a slight forward rake and you are pretty close to 50/50.

Now think about the front spring, it is inboard and at an angle to the wheel so it doesn't transmit the same rate to the wheel. If the front spring was vertical and was somehow bolted directly to the center of the wheel, then the wheel rate would equal the spring rate. As it actually sits at an angle and inboard of the center of the wheel the spring rate is about 92% of the wheel rate. So if you have 240lb front springs fitted the wheel actually sees them as 240x90%=220lb springs.

Now turn to the rear. Let's say you want the rear wheels to also see 220lb. With torsion bars it's easy as they effectively act directly on the wheel, so you want an effective rate of 220lb which equals a 27mm torsion bar.

Say you have coil overs at the rear. Well depending on how they are mounted again like the fronts they will sit at an angle and inborad of the wheels. Some way of mounting cause a bigger angle than others. For example Escort Cups mount directly to the trailing arm at an acute angle whereas others use an adapter on the trailing arm that makes the coilover more vertical. Because of that the rear wheels will see anything between 42-56% of the spring rate.

Say it's 56%, which is about right for the Ground Control coilover mounting and you want the rear wheel to see the same as the front wheel. 220/56%=393lb so you are looking at a 400lb spring at the back to match the 220lb at the front.

Hope that explains things for you.

 

+1 Exactly what I do to determine spring rates except I do one extra step. I get the corner weights first then remove the springs to measure the actual deflection of the wheel hub with 1in movement between the spring plates. That determines the spring rates, however then I jack/move the control arm parallel to the ground and measure the spring plate gap again to determine the length of the spring I need. I can do this because I know how much the spring will compress from the spring rate I calculated previously and the corner weight I measured beforehand. I select my spring length to allow full adjustment on my coilover, so when it sits on the ground the adjusting collar is about mid way on the coilover adjustment.

Van, you should do one of your great videos on this