Linear vs. Progressive springs
02-04-2013, 12:47 PM
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Linear vs. Progressive springs
I have been trying to educate myself on suspension workings before pulling the trigger on a new setup for my '95 cab. I am admittedly a rookie on this subject and am fully aware that not everything you read on internet forums is necessarily "true", however, this explanation that I found on another site seems to make sense to me. It also confirms a lot of the recomendations that I have received to install progressive springs for a mostly road setup if I want a fairly comfortable ride in regular use and yet a firm, planted feel for more spirited driving.
Here is the psot for your feedback:
This is perhaps one of the most common items of upgrades enthusiasts pursue to improve their cars - Replacement Springs to lower their rides. IMO, this is one critical component of the car that needs to be done correctly to ensure YOUR handling is truly improved and not a preception of improvement over what you think is "best".
Biggest question if you don't know till now, what is the difference between straight and progressive springs? Let me explain to you what they truly mean. Fact is, not many people know the differences in behavior between straight and progressive springs. This thread will provide you a basic informed understanding.
Basic Differences - Straight (Linear) vs Progressive Rated Springs
Straight Springs have a spring rating that is linear across the entire length of the spring that it can be compressed. These types of springs can be easily recognized because the space between each winding or spring coil is equal to one another. While Progressive Springs have a spring rating that increases or changes as the spring compression changes. How much it changes depends on the spring type. A progressive spring can be recognized by the fact that the space between every winding or spring coil of the spring is different. It will go to small on one end to large on the other.
Key Characteristics - Straight (Linear) Springs
The most significant characteristic of a straight spring is that it is easy to understand and set up for just about anyone with a basic knowledge on how spring rates work. (The more spring rate there is, the harder the spring will be, the harder the spring, the stiffer the ride.) There is really only one variable to take into account and that is the spring rating, and since the spring rating is consistent. You can calculate how the spring will behave under various "conditions".
This is an advantage that should not be underestimated, specifically under racing conditions. On the tarmac, you want to know exactly what the suspension is doing, how it's doing, how much it's doing it and where it's doing it. The only way of doing that is when the spring behaves the same under different conditions around the track to deliver a consistent handling pattern specific to what you want the car to handle. If comfort is not a part of the equation, it is possible to calculate exactly what spring rate you want at a certain tarmac condition to give you the best response, feel and ultimately - Traction.
While time for setup for road and racing use is somewhat not comparable. How you set up your ride does not change either way. The "informed" consumer will often if not always base their homework and buying research based on the manufacturer/tuners research and development, racing experiences and most importantly race proven results. This will also explain why most of them will if not always recommend straight springs, because that is what they know. However, a sales-pitch like ‘proven on the racetrack’ does not always mean the best solution for the open road. After all, a road car and a track car is somewhat different in demand and performance specifications.
Key Characteristics - Progressive Springs
The most important characteristic of a progressive spring is of course it's variable spring rating value. This makes it harder to calculate or predict it's behavior on any tarmac, but it's much more significantly designed for road use. To understand this, we look at what we want to achieve, just like any race car. The ideal situation for the majority would be the straight-line comfort, combined with the razor-sharp turn-in, feel and control of a race car. Suspension wise, this is somewhat impossible because as soon as you focus the bias on comfort, you automatically lose out on the handling aspect.
This is a balance scale that goes either way. However, with a progressive spring we can get pretty close to a 50/50 setup. A progressive spring will have it's lowest spring rating when it's the least compressed and the highest spring rating when it's the most compressed. When we are driving in a straight line, the majority would not want to feel or obtain feedback on too much of the road.
The majority want it to be a comfortable and relaxed ride but as soon as a twisty road is spotted again, the attention changes to the need for a firm, planted feel with lots of driving feedback from the tires. In a corner, the increased load causes the car to ‘squat’, meaning the suspension gets compressed both front and rear. Because of this compression, the initial comfortable springs stiffen up and start giving you the planted feel and feedback. And the faster you drive, the more feedback and feel will the springs give you, exactly as our race car example but for a road car application.
Choice Factoring - Driving Experiences/Condition Differences
Whatever the spring type that is used, it has to be correctly setup for your car. Factors affecting how you want the car to handle point on what the car is to be used for, whether a road car, weekend sprited/track car or purpose tuned race car. If not, your suspension will not perform at it's optimal peak. A badly setup straight spring is just as bad as a badly setup progressive spring (vice-versa).
It is good to keep in mind that progressive springs don't have infinite adjustability. There is a range in which they will operate optimally, and outside that range performance will drop. You can’t just keep adding load and expect the springs to magically keep up forever. The optimal range however, is much larger than a straight spring will ever have, as the examples have shown.
Does this all mean progressive springs are better than straight springs and the various suspension tuners are talking crap? Not one bit. It ultimately comes down to vehicle application for whatever conditions but for a primarily road use car, a progressive spring will no doubt be a clear vantage over a straight spring setup. A good example is - Racing specified tires. They don’t work very well on the road, because you often cannot get or keep them within their optimum "working" temperature range, much like a race suspension setup will not optimally work on the road if it's been primarily setup for the tracks. If your application is the open road and you want to enjoy it to the best of your ability, you have to chose the right type. And for that application the right type would no doubt be - Progressive Rated Springs.
Here is the psot for your feedback:
This is perhaps one of the most common items of upgrades enthusiasts pursue to improve their cars - Replacement Springs to lower their rides. IMO, this is one critical component of the car that needs to be done correctly to ensure YOUR handling is truly improved and not a preception of improvement over what you think is "best".
Biggest question if you don't know till now, what is the difference between straight and progressive springs? Let me explain to you what they truly mean. Fact is, not many people know the differences in behavior between straight and progressive springs. This thread will provide you a basic informed understanding.
Basic Differences - Straight (Linear) vs Progressive Rated Springs
Straight Springs have a spring rating that is linear across the entire length of the spring that it can be compressed. These types of springs can be easily recognized because the space between each winding or spring coil is equal to one another. While Progressive Springs have a spring rating that increases or changes as the spring compression changes. How much it changes depends on the spring type. A progressive spring can be recognized by the fact that the space between every winding or spring coil of the spring is different. It will go to small on one end to large on the other.
Key Characteristics - Straight (Linear) Springs
The most significant characteristic of a straight spring is that it is easy to understand and set up for just about anyone with a basic knowledge on how spring rates work. (The more spring rate there is, the harder the spring will be, the harder the spring, the stiffer the ride.) There is really only one variable to take into account and that is the spring rating, and since the spring rating is consistent. You can calculate how the spring will behave under various "conditions".
This is an advantage that should not be underestimated, specifically under racing conditions. On the tarmac, you want to know exactly what the suspension is doing, how it's doing, how much it's doing it and where it's doing it. The only way of doing that is when the spring behaves the same under different conditions around the track to deliver a consistent handling pattern specific to what you want the car to handle. If comfort is not a part of the equation, it is possible to calculate exactly what spring rate you want at a certain tarmac condition to give you the best response, feel and ultimately - Traction.
While time for setup for road and racing use is somewhat not comparable. How you set up your ride does not change either way. The "informed" consumer will often if not always base their homework and buying research based on the manufacturer/tuners research and development, racing experiences and most importantly race proven results. This will also explain why most of them will if not always recommend straight springs, because that is what they know. However, a sales-pitch like ‘proven on the racetrack’ does not always mean the best solution for the open road. After all, a road car and a track car is somewhat different in demand and performance specifications.
Key Characteristics - Progressive Springs
The most important characteristic of a progressive spring is of course it's variable spring rating value. This makes it harder to calculate or predict it's behavior on any tarmac, but it's much more significantly designed for road use. To understand this, we look at what we want to achieve, just like any race car. The ideal situation for the majority would be the straight-line comfort, combined with the razor-sharp turn-in, feel and control of a race car. Suspension wise, this is somewhat impossible because as soon as you focus the bias on comfort, you automatically lose out on the handling aspect.
This is a balance scale that goes either way. However, with a progressive spring we can get pretty close to a 50/50 setup. A progressive spring will have it's lowest spring rating when it's the least compressed and the highest spring rating when it's the most compressed. When we are driving in a straight line, the majority would not want to feel or obtain feedback on too much of the road.
The majority want it to be a comfortable and relaxed ride but as soon as a twisty road is spotted again, the attention changes to the need for a firm, planted feel with lots of driving feedback from the tires. In a corner, the increased load causes the car to ‘squat’, meaning the suspension gets compressed both front and rear. Because of this compression, the initial comfortable springs stiffen up and start giving you the planted feel and feedback. And the faster you drive, the more feedback and feel will the springs give you, exactly as our race car example but for a road car application.
Choice Factoring - Driving Experiences/Condition Differences
Whatever the spring type that is used, it has to be correctly setup for your car. Factors affecting how you want the car to handle point on what the car is to be used for, whether a road car, weekend sprited/track car or purpose tuned race car. If not, your suspension will not perform at it's optimal peak. A badly setup straight spring is just as bad as a badly setup progressive spring (vice-versa).
It is good to keep in mind that progressive springs don't have infinite adjustability. There is a range in which they will operate optimally, and outside that range performance will drop. You can’t just keep adding load and expect the springs to magically keep up forever. The optimal range however, is much larger than a straight spring will ever have, as the examples have shown.
Does this all mean progressive springs are better than straight springs and the various suspension tuners are talking crap? Not one bit. It ultimately comes down to vehicle application for whatever conditions but for a primarily road use car, a progressive spring will no doubt be a clear vantage over a straight spring setup. A good example is - Racing specified tires. They don’t work very well on the road, because you often cannot get or keep them within their optimum "working" temperature range, much like a race suspension setup will not optimally work on the road if it's been primarily setup for the tracks. If your application is the open road and you want to enjoy it to the best of your ability, you have to chose the right type. And for that application the right type would no doubt be - Progressive Rated Springs.
02-04-2013, 01:06 PM
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Good summary - you may want to add a bit about combo units - like the PSS type spring set, where there are two springs, one a tender for the other, to give a softer ride until the tender is in compression, where the secondary and stiffer spring comes into play... in this case both are linear (I think), but together are a sort of step-progressive spring.
02-04-2013, 04:12 PM
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Mike- As I have considered the PSS10, KWv setups overkill for my needs I haven't spent much time investigating how they work, the engineering behind them and why they are an upgrade to the standard setup. Would anyone care to take a shot at a rundown for the group?
02-04-2013, 07:35 PM
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To add to what Mike said, I believe the idea with combo setups is to select the tender spring so that it has say 0.5" of travel at a standstill. I believe that's how the PSS9 was designed (not sure about PSS10). Then under braking or cornering, the tender would lock up, and your spring rate at that corner increases to whatever the main spring's rate was. So let's say you select 500 lb/in as your rates for both the tender and main, but the tender is much shorter than the main. At normal driving, the spring rate is Keq = (500*500)/(500+500) = 250 lb/in. After your 0.5" or so of travel, the tender locks up and the spring rate becomes 500 lb/in. I believe that selecting the tender spring simply boils down to selecting a spring where the force required to lock it up (see, e.g. Eibach's 2004 pdf catalog for detailed data) is slightly higher than the corner weight less unsprung mass. For example, my rear corner weights are 960 lbs
less 110 lbs unsprung = 850 lbs. Then you might choose a tender that locks up at 1,100 lbs or whatever.
I'm pondering such a setup for my motons, but there's not enough shaft length and the shocks would have to be revalved.
I found the combo setup works very well on PSS9s. The problem with that shock was lousy damping. Like Ken said, damping is crucial. Motons with 600F/800R are like twice the spring rate as PSS9 but give a similar street ride due to the dramatically better damping.
less 110 lbs unsprung = 850 lbs. Then you might choose a tender that locks up at 1,100 lbs or whatever.
I'm pondering such a setup for my motons, but there's not enough shaft length and the shocks would have to be revalved.
I found the combo setup works very well on PSS9s. The problem with that shock was lousy damping. Like Ken said, damping is crucial. Motons with 600F/800R are like twice the spring rate as PSS9 but give a similar street ride due to the dramatically better damping.
02-04-2013, 07:40 PM
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By the way, I didn't even try to cover progressive tenders, LOL! I'm not sure Eibach still sells them, and even if they do, they're only available in 60mm, which I'm not sure would fit anything we use.
02-04-2013, 11:32 PM
#7
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Rising rate (progressive) suspensions are fine for street cars, however they are damned unpredictable on track and race cars when you are really pedaling,...... 
Mains & tenders, OTOH, are dual-rate suspensions and much easier to drive at the limit; all things being equal.
Mains & tenders, OTOH, are dual-rate suspensions and much easier to drive at the limit; all things being equal.
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02-06-2013, 06:27 PM
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If the PSS-9's are not very good at damping, any comments about the KW Variant 3 Coil Over's? I know that you can adjust the rebound- and compression damping - are they that much superior to the PSS-10 setup? I think the 993's variants come with a single variable spring on the KW's, not the two stage setup like the PSS-10's.
Cheers,
Mike
Cheers,
Mike
