Suspension Question: Spring Length vs. Spring Rate in Tein Flex Coilovers
I found a set of new Tein Flex coilovers for a ridiculously low price, so picked them up and just finished the install. I’m a bit of a MonStar when it comes to suspension… but I plan to stick with these for a good long while.
I knew what I was getting into with the spring rates, and they really aren't too bad, but in the interest of drivability and handling balance, I'm planning on dropping the front spring rate from 10kg/mm to 8kg/mm, and leaving the rears at their factory 6kg/mm.
The car has ST sways front and rear - so there's already some extra roll stiffness in the front, which is another part of the reason I'm thinking about reducing the front rates.
Replacing springs is simple enough... you can order just about any length/rate you want from a Tein dealer for about $50 a spring.
The trouble is, should I order replacement springs that are slightly longer in free length than the original to compensate for the extra sag in the suspension that will result from the softer springs? Or should I raise the spring perch to compensate? Or should I just lengthen the shock body to compensate?
Since the Flex kit is adjustable both in shock length and spring perch height… it isn’t an issue of keeping the spring preloaded when the suspension is at full droop. Still, I’m leaning towards a slightly longer replacement spring so the extra sag won’t eat into bump travel.
The original front 10kg/mm Tein springs are 175mm long, and replacements are available in assorted lengths in 25mm increments. (e.g. 200mm, 225mm, etc.)
Curiously, the Tein SS kit has 8kg/mm rates in the front, but the springs are much longer, about 275mm. The SS is a very different design from the Flex, however, and since damper length isn’t adjustable I suppose they need the extra length to stay seated.
Also, any general discussion on adjusting preload vs. shock length is appreciated. I understand the advantage of shortening the shock instead of just lowering the spring perch to lower the car, but since shock length can take care of ride height, I’m not sure when one ever really needs to adjust the perches beyond keeping enough tension on the spring to make sure it can’t unseat.
IN SUMMARY: Installing softer springs in fully adjustable coilovers – should the replacement springs be longer than the originals to compensate for the slight increase in sag?
Thanks!
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Don't bother with a longer spring unless the spring perches are near the top of the travel. If you can fit a longer spring, it's fine if you buy one. It won't change the handling characteristics of the car or anything. Actually a longer spring is preferred as it will have a longer distance from the free length to coil bind, and will have less "twisting" force on the shock itself. However, longer springs are also heavier.
What exactly do you mean by "sag"? You should adjust the spring perches to achieve the ride height you desire. With the "adjustable" shock body length, just adjust it so that the spring is in contact with both perches at full droop.
Adjusting the preload on the spring as NO effect (other than changing the ride height) unless you preload the spring to a loading greater than the load on the spring when the car is on the ground at rest (i.e. you run the suspension with zero droop). Now if you preload the spring and shorten the shock the corresponding amount, that again will not have any effect other than having less droop (ride height should be the same).
I should mention that most GC/Koni users use 8" free length springs. With my Koni 3011s, I used 8" free length springs. With my new setup, I'm planning to use 7" free length springs.
I'll have to re-read this later to make sure I haven't screwed something up. I just got home from work.
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Billy - 98 Prelude
#27 H2 NASA TX HondaChallenge
#27 PTB NASA TX Performance Touring Racing makes heroin addiction seem like a vague wish for something salty. -- Peter Egan
What he means is that when you go from 10kg/mm springs to 8kg/mm springs, with a fixed vehicle weight you will get more drop (springs compress more with the same weight and same perch height).
Theoretically, you will probably be looking at a 20% increase in static compression which should be in the ballpark of 10mm or so. You should be able to raise the perch 10mm to account for this. A different free length should not be needed unless, as Billy points out, you already have an issue of the spring not being loaded when the suspension is fully unloaded.
The free length of the spring has nothing to do with whether or not the spring is loaded at full droop, given the SAME ride height.
Actually by my calcs, you get a 25% increase in compression of the spring. In other words, if the spring compressed 40mm with the 10 kg/mm springs, the new 8 kg/mm springs will compress 50mm.
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Billy - 98 Prelude
#27 H2 NASA TX HondaChallenge
#27 PTB NASA TX Performance Touring Racing makes heroin addiction seem like a vague wish for something salty. -- Peter Egan
He wants to know if he needs to get longer springs to account for this. I can see this being an issue if your free length was just right to take up the gap between the perch & the cap with the suspension fully unloaded AND you wanted to change the ride height. For instance, if you soften the suspension and want to raise the car, you would have to adjust the perch a lot more to counteract the softer springs and get above your current ride height. You could get away with less adjustment with longer springs.
Right now, the generic coilovers I have on the Prelude are dialed up to the top of the sleeve to get the ride height I want with the springs that I have- a longer spring would help this.
Noobs, note that the # of coils or material have to change to keep the same rate for a different length.
A different free length should not be needed unless, as Billy points out, you already have an issue of the spring not being loaded when the suspension is fully unloaded.
Is that an issue? I got GC 8" springs and they unload at full droop. They have about an inch of space at the top, and my car isn't dropped that much. [/hijack]
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"The only thing that needs adjusting is the nut behind the wheel."
How many times do I have to say it? Spring length will not change now much of a gap you have between the spring and the perch at full droop (keeping ride height constant). The only way to fix that issue is to use droop limiters, a shorter damper, or helper springs.
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Billy - 98 Prelude
#27 H2 NASA TX HondaChallenge
#27 PTB NASA TX Performance Touring Racing makes heroin addiction seem like a vague wish for something salty. -- Peter Egan
He wants to know if he needs to get longer springs to account for this. I can see this being an issue if your free length was just right to take up the gap between the perch & the cap with the suspension fully unloaded AND you wanted to change the ride height. For instance, if you soften the suspension and want to raise the car, you would have to adjust the perch a lot more to counteract the softer springs and get above your current ride height. You could get away with less adjustment with longer springs.
Yes, if you're adjustment is close to the next longer/shorter spring length. If he needs to adjust the perch 10mm, getting a 25mm longer spring might not be the best solution.
Quote:
Originally Posted by marcucci
Right now, the generic coilovers I have on the Prelude are dialed up to the top of the sleeve to get the ride height I want with the springs that I have- a longer spring would help this.
Yes, this is one instance where longer springs would be useful. The general rule of thumb is to use the longest spring you can to lessen the twisting loads on the damper itself. Longer springs also have more compression travel until full coil bind, which is a plus. However longer springs weigh more. On a 3k lb Prelude, that's not a big deal, but on a pure race car, it could be.
That being said, for high spring rates, it can be difficult to find springs that are a long and have a high spring rate. This is one reason why I'm going to go with 7" long springs with my new shocks (I can't get the rate I want in the rear with an 8" long spring, but they are available in 7" long springs). The 7" long springs should have plenty of travel so that I won't have to worry about coil bind with the shorter spring.
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Billy - 98 Prelude
#27 H2 NASA TX HondaChallenge
#27 PTB NASA TX Performance Touring Racing makes heroin addiction seem like a vague wish for something salty. -- Peter Egan
Agreed, but keep in mind very few people on this board are running/developing pure race cars or have the funds to drop >$1k into springs/shocks. Sometimes the best solution requires it also be a cheap one.
I don't know where you're getting that from, but i'm trying to give some general guidelines for picking spring length. I'm not trying to discuss the development of a pure race suspension.
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Billy - 98 Prelude
#27 H2 NASA TX HondaChallenge
#27 PTB NASA TX Performance Touring Racing makes heroin addiction seem like a vague wish for something salty. -- Peter Egan
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In other words, if the spring compressed 40mm with the 10 kg/mm springs, the new 8 kg/mm springs will compress 50mm.
Spring length will not change now much of a gap you have between the spring and the perch at full droop (keeping ride height constant). The only way to fix that issue is to use droop limiters, a shorter damper, or helper springs.
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