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Discussion Starter #1 (Edited)
So what spring rates are actually RIGHT for our cars, for performance? Thats what I have been trying to figure out lately. A friend of mine did a bunch of calculations of ride frequency while we were designing our SAE car last semester, and I decided to try to do similar for our cars. Detailed discoussion of this (and a whole lot of other stuff too) is available in Race Car Vehicle Dynamics by Miliken. However, for our purposes the tech tips section on www.optimumg.com, and specifically the first of the springs and dampers articles will be sufficient. The rest of the articles are useful, but to do any of the calculations would require data that we don't have (CG location, damping rates, ect).

They start by giving recomended ride frequencies for passenger cars, low downforce race cars, and high downforce race cars. For low downforce race cars (where a performance oriented MR2 is going to fall) they recommend ride frequencies of between 1.5 and 2 hz. They also sugest that a slightly higher frequency in the front vs the rear can be good on a race car.

For weight I used 2800lb with a 42/58 weight distribution.

For a ride frequency of 1.75hz I calculated:
Front: 184lb/in
Rear:254lb/in

For 2.0hz
Front: 240lb/in
Rear: 332lb/in

Notice two things. One, these rates are MUCH lower than what most off the shelf coilovers for the MR2 run. The idea behind these is to give maximum grip over bumps. Roll stiffness should then be made up by sway bars (Article #2 covers this). Two, the front/rear ballance is much closer than any of the off-the-shelf springs provide, with the exception of the H&R, which are still not very close to these calculated rates.

I think if you wanted to match these with a strut / spring setup the best you could do would be to use the front H&R springs (230-260lb/in) and the rear RS*R Heavy Duty Race (346lb/in, no range given). That would give you a very nearly 2hz ride frequency, and roughly equal lowering front and rear (From what I have read, RS*R lowering rates are based off the 93).

It would also be possible to pair the H&R rears (235lb/in) with the RS*R Race fronts (201 lb/in) to create a setup that would be a bit closer to the 1.75hz frequency, although with a bit of a higher frequency in front. The actual frequencies would be roughly 1.85f and 1.7r. The article I linked to states that 10-20% higher front ride frquency is common on race cars, and this setup would create a front that is in fact about 8% higher.
 

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Alex,

As always, thanks for sharing. These discussions are always fun and I always learn a lot.

I think the theory and your starting spring rates make sense, and might get someone to a good starting point for a road race car. But I think there are a lot of shortcomings on our cars that we have to compensate for.

First, the writeup assumes a motion ratio of 1:1, which we're close to with our struts, but I think it's closer to 0.9 or so. That will cause the spring rate to be about 10% higher I think.

Second, as we have been discussing in other threads, we don't have an ideal camber curve (especially in the rear), so body roll tends to hurt us more than it would on a car with SLA suspension. This usually means we spring up a little more in order to keep a decent contact patch when cornering. Something like the Elise, on the other hand, really shines with fairly soft suspension and some body roll, but it also has very well designed suspension and nice camber curves.

Lower frequencies produce a softer suspension with more mechanical grip, however the response will be slower in transient (what drivers report as ?lack of support?). Higher frequencies create less suspension travel for a given track, allowing lower ride heights, and in turn, lowering the center of gravity.

I think the above quote sums it up nicely. It's always a compromise, and a lot of us go with lower ride heights and stiffer springs. I come from the autox world, and I think we tend to put a premium on response since we are changing direction so quickly (averaging a significant steering input every 2-3 seconds--slaloms sometimes require 6 inputs in about 3 seconds). Road racers may be able to run lower frequencies because they have more time to set the car in a turn, and probably average more like one input per 5-6 seconds or more (10 turns a minute or so?).

As an example, I've successfully run spring rates higher than most coilovers come with. I, and others, have also found that running about equal rates f/r works well on our cars, which is probably right inline with the 10-20% higher in front they refer to. I've always been baffled why coilovers come with front springs so much softer than rears. My only conclusion is that they're trying to make the ride more comfortable on the street.

I think if you take all these things into account, you could probably get into the 350-400# range pretty quickly, which is probably the lower end of what most folks are running on race cars (road race and autox). If you've got a severely lowered car, it may take even more spring rate to manage body roll if your roll centers aren't brought back up.
 

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Discussion Starter #3 (Edited)
I think thats exactly why coilovers (and in fact the stock springs) are so much stiffer in the rear. As that article mentions at the beginning, stiffer rear springs keep the chassis from pitching over bumps, improving ride comfort.

I had kinda forgotten about our lack of camber gain when I was doing this, you are right, a bit more spring is probably helpful due to that. On the other hand, from what I was able to gather from those articles, you would want softer springs to make the car maintain traction over bumps, and stiffer roll bars to keep your camber in the sweet spot durring body roll.

As for motion ratio, optimum K will spit that out too, and it tells me it is about 1.035 front 1.05 rear. Now I'm not sure which way that is going, weather its wheel:spring or spring:wheel, but either way its so close to 1 as to be almost insignificant. Its also very close front to rear, so it shouldnt have a huge impact on ballance.

On a side note, the H&R springs provide a front ride frequency that is roughly 18% higher than the rear... right in line with what that article is recommending.
 

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Alex W said:
I had kinda forgotten about our lack of camber gain when I was doing this, you are right, a bit more spring is probably helpful due to that. On the other hand, from what I was able to gather from those articles, you would want softer springs to make the car maintain traction over bumps, and stiffer roll bars to keep your camber in the sweet spot durring body roll.
There are plenty of debates over springs vs. sway bars. They do act as you're suggesting, but only if the bump is encountered by both the left and right tires at the same time (so both sway bar arms move up at the same time keeping the sway bar out of the equation). On a street car, I think this makes a lot of sense. On a race car, the line gets blurred.

Depending on how much you rely on them (how big they are), sway bars can also do funky things like lift tires off the ground. Not always a bad thing, often those tires aren't doing much anyways (inside front coming out of a corner) but something to keep in mind. They essentially act against the springs on the inside wheels.
 

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Discussion Starter #5
That, and what I just got to thinking, you can only do so much with bars... If your running ST sways and still need more roll stiffness then stiffer springs are the only real solution.
 

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for someone with choices between bars only and springs only, would you say springs only is the better option? from what I've read I seem to think so, but I'm not able to 100% grasp all that you guys are throwing out
 

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Discussion Starter #7
I think it depends on the course you are running on. If its a 100% glass smooth road course with no bumps of any kind then by all means, go with crazy stiff springs. On the other hand, if you need to be able to handle bumps without losing grip while still maintaining a high roll stiffness then that is where sway bars come into play. Atleast thats how I understand it.
 

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see thats the thing, most of the tracks I'm on are on the smoother side, but you really need to start hopping the curbs to get that last few tenths... which kinda makes it bumpy again...
 

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Discussion Starter #9
kbrew8991 said:
see thats the thing, most of the tracks I'm on are on the smoother side, but you really need to start hopping the curbs to get that last few tenths... which kinda makes it bumpy again...
I think the tried and true spring rates in the 400-500 lb/in range are probably where you need to end up on a race car, as randy has said.

The point of this thread was really to say that the rear springs are way to stiff (or the fronts are way to soft) on 95% of the off the shelf springs and coilovers out there.
 

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Alex W said:
I think it depends on the course you are running on. If its a 100% glass smooth road course with no bumps of any kind then by all means, go with crazy stiff springs. On the other hand, if you need to be able to handle bumps without losing grip while still maintaining a high roll stiffness then that is where sway bars come into play. Atleast thats how I understand it.
If you are on a bumpy course, with super stiff sway bars and lack spring rates, you're probably crushing the bumpstops and have no more suspension travel. Thus having you skip across the pavement.

It seems that stiff sway bars work better on glass surfaces and high spring rate would be more beneficial to a bumpy road course. The higher spring rates would allow you some additional suspension travel to take in the bumps, no?
 

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Alex W said:
I think thats exactly why coilovers (and in fact the stock springs) are so much stiffer in the rear. As that article mentions at the beginning, stiffer rear springs keep the chassis from pitching over bumps, improving ride comfort.

I had kinda forgotten about our lack of camber gain when I was doing this, you are right, a bit more spring is probably helpful due to that. On the other hand, from what I was able to gather from those articles, you would want softer springs to make the car maintain traction over bumps, and stiffer roll bars to keep your camber in the sweet spot durring body roll.

As for motion ratio, optimum K will spit that out too, and it tells me it is about 1.035 front 1.05 rear. Now I'm not sure which way that is going, weather its wheel:spring or spring:wheel, but either way its so close to 1 as to be almost insignificant. Its also very close front to rear, so it shouldnt have a huge impact on ballance.

On a side note, the H&R springs provide a front ride frequency that is roughly 18% higher than the rear... right in line with what that article is recommending.
Very interesting! What is "K" for?

I have H&R coilovers that I'll install. I was going to just run TRD fsb and 19mm OEM rsb for a while and then add the TRD rsb later. In theory, would this not be necessary with the H&R coilovers? (running stiffer fsb)
 

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Alex W said:
As for motion ratio, optimum K will spit that out too, and it tells me it is about 1.035 front 1.05 rear. Now I'm not sure which way that is going, weather its wheel:spring or spring:wheel, but either way its so close to 1 as to be almost insignificant. Its also very close front to rear, so it shouldnt have a huge impact on ballance.
Assuming that is wheel:spring, then the more familiar spring:wheel would be 0.97 front and 0.95 rear. Those are close. I use 0.95 at both ends. Try plugging some shorter struts into your program and lean the tops in some more. Then, check out what happens to the front motion ratio.
 

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Discussion Starter #13
traffic said:
If you are on a bumpy course, with super stiff sway bars and lack spring rates, you're probably crushing the bumpstops and have no more suspension travel. Thus having you skip across the pavement.

It seems that stiff sway bars work better on glass surfaces and high spring rate would be more beneficial to a bumpy road course. The higher spring rates would allow you some additional suspension travel to take in the bumps, no?
I'm not sugesting springs so soft that you are bottoming out all the time, just not super stiff. Like in the 300-400 lb/in range, whereas on a super smooth track you can get away with 700lb/in range, for example.

As an example, although not really the same, I have noticed on some of the rougher autocross lots I run on that I am quite a bit faster with my struts (tokicos) set softer rather than stiffer.
 

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So again with my limited knowledge, it is better to get the spring rate correct. Then fine tune the balance with sway bars.
 

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traffic said:
So again with my limited knowledge, it is better to get the spring rate correct. Then fine tune the balance with sway bars.
Yes. Springs control side to side and for/aft movement. Bars only control side to side. I would spring the car appropriately and use adjustable bars to fine tune.

Also, one thing often overlooked, as you increase spring rate you are decreasing body roll, which is also decreasing your reliance on the bar, which only comes into play when the body rolls. The same bar change on a car with 500# springs will have less of an impact on handling than on a car with stock springs. The bar is a smaller % of the effective spring rate.
 

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this is quite a sensible discussion, id agree with quite a lot of what has been said regarding wheel rates (aka ride frequencies).

nobody has mentioned the important of damping though. damping can be used to compliment spring rates, and also to improve the "feel" and response time of the car. if you run a lot of low-speed compression damping then intial turn in can be made to feel much sharper and more pronounced, but at the same time allow softer springs to be run, with less rebound damping, and gain large increases in mechanical grip and traction

try not to use sway bars to tune the roll of the car until the springs and damping are spot on in every other regard. you dont want to go overly stiff on the sway bars to compensate for problems elsewhere

with regards ride height, youd like to be as low as possible, but that comes at the expense of geometry. lowering the car changes the static inclination angles of the control arms, which in turn alters the roll centre (for the worse) and you can screw up the handling whilst trying to improve the CofG and end up with negative results. you can correct suspension arm static angles with roll centre adjuster blocks, but its best not to get into a situation where they are required in the first place unless there is a significant gain to be made there

suspension is always a giant compromise. ideal spring rates are rarely the fastest in reality, especially on macpherson struts. as has been discussed, McP suspension exhibits poor camber change characteristics, and you end up running stiffer to control the camber as the tyre contact patch is more beneficial than the ideal mechanical grip from the softer springs. you then end up running more rebound damping to control the spring and sacrificing traction as well. you just find a happy medium where the car is fastest, which comes as the result of testing and development

dont overlook damping though, it is by far the most important aspect of the suspension
 

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Jim2109 said:
this is quite a sensible discussion, id agree with quite a lot of what has been said regarding wheel rates (aka ride frequencies).

nobody has mentioned the important of damping though. damping can be used to compliment spring rates, and also to improve the "feel" and response time of the car. if you run a lot of low-speed compression damping then intial turn in can be made to feel much sharper and more pronounced, but at the same time allow softer springs to be run, with less rebound damping, and gain large increases in mechanical grip and traction

try not to use sway bars to tune the roll of the car until the springs and damping are spot on in every other regard. you dont want to go overly stiff on the sway bars to compensate for problems elsewhere

with regards ride height, youd like to be as low as possible, but that comes at the expense of geometry. lowering the car changes the static inclination angles of the control arms, which in turn alters the roll centre (for the worse) and you can screw up the handling whilst trying to improve the CofG and end up with negative results. you can correct suspension arm static angles with roll centre adjuster blocks, but its best not to get into a situation where they are required in the first place unless there is a significant gain to be made there

suspension is always a giant compromise. ideal spring rates are rarely the fastest in reality, especially on macpherson struts. as has been discussed, McP suspension exhibits poor camber change characteristics, and you end up running stiffer to control the camber as the tyre contact patch is more beneficial than the ideal mechanical grip from the softer springs. you then end up running more rebound damping to control the spring and sacrificing traction as well. you just find a happy medium where the car is fastest, which comes as the result of testing and development

dont overlook damping though, it is by far the most important aspect of the suspension
Bilstein HD dampers are a great example of this. It's initial dampring rate is higher than that of the KONI yellows. It makes up for softer springs at turn in. Then allows you to settle into the corner. As long as you are smooth, it works very well. If you're a little rough, the higher ultimate damping rates of the KONI help lessen some of the extra motion.

The HDs can also make up for some of the progressive rate springs that are available. Again, it makes up for the softer initial spring rates at turn in.

Of course, the downside is that your rippled sections (small consecutive bumps) are a bit rougher.
 

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traffic said:
Bilstein HD dampers are a great example of this. It's initial dampring rate is higher than that of the KONI yellows. It makes up for softer springs at turn in. Then allows you to settle into the corner. As long as you are smooth, it works very well. If you're a little rough, the higher ultimate damping rates of the KONI help lessen some of the extra motion.

The HDs can also make up for some of the progressive rate springs that are available. Again, it makes up for the softer initial spring rates at turn in.

Of course, the downside is that your rippled sections (small consecutive bumps) are a bit rougher.
what youre referring to is "low speed" compression damping, and this is the way that most good saloon race cars are setup (unfortunately that doesnt account for all that many, too many people still favour masses of rebound and no bump damping, on super stiff springs. and its just not faster). additional low speed damping to give increased driver feel and compensate for softer spring rate (which aids mechanical grip in the slower corners and overall traction), and also allows much less rebound damping to be run due to the softer spring rate
 

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Discussion Starter #19
So I just revisited this while trying to decide on a new set of springs for my car, and noticed that the Eibach prokits fit this theory almost exactly... The high end of the rate range (210f and 304r) puts them almost exactly between the 1.75hz and 2.0hz ride frequencies, with a slight frequency bias toward the front...
 

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Alex W said:
So I just revisited this while trying to decide on a new set of springs for my car, and noticed that the Eibach prokits fit this theory almost exactly... The high end of the rate range (210f and 304r) puts them almost exactly between the 1.75hz and 2.0hz ride frequencies, with a slight frequency bias toward the front...
You know, one thing I noticed on the Eibach springs, their advertised rates are very similar to the TRD. However, the pro-kit has a slightly higher ride height in the front than the TRD.

I'm wondering how much progression there is? Obviously there is a bit more suspension travel available on the pro-kit for the front. But I'm curious at how the different ride height affects handling as well.

I'm still trying to dial in the proper ride height of my H&R coilovers. Following my friend's advice, I've got the front dumped on the threads. So now the front and rear have about the same fender gap. I should probably take proper measurements. But again, I'm not sure how to calculate the relative ride height since I have different tire heights than OEM, different spring rates as well. ARghh.....
 
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