Koni sports in conjunction with Boss 302 springs?

So let's suppose I get a set of Boss 302 springs for my GT. It's pretty darned cheap when you get right down to it, about $160 for all 4 corners, it gets me the ride height I want in the front and the rear (I want to keep the geometry in the rear about the same as stock so as to avoid pinion angle and anti-squat geometry issues, but want the rear to be reasonably well matched with the front. And since Ford has basically done the engineering work with respect to matching the front and rear springs, it seems logical to take advantage of that).

I want adjustable shocks, and the Koni sports are widely regarded as excellent.

My question is this: since Koni Sports don't seem to have model-specific versions, is it likely that the compression setting in the shocks are going to be mismatched to the springs? How much tolerance is there in that? Koni Sports can supposedly be revalved (though doing so is, from what I've seen, rather pricey), but that raises the question of how one would be able to figure out what to revalve them to. Is that something that can be determined through calculation, based on the suspension travel, the spring rate, and the corner weight? If so, how does one go about calculating the proper valving?

a not so quick response from koni na guru lee grimes that i got years ago when i questioned him about the max spring rates to pair with sports/OTS valving:

"As nice as it might be to have a hard answer to this, there is no real answer to “what is the max spring rate for valving X? and even moreso if it is adjustable. The difficulty is that valving and spring rate along are the only “knowns” in the equation with a huge number of variables such as the usage of the car (daily driver, autoX, track, mixed, etc.), other mods to the car, and definitely driver preferences. Anyone who says “spring rate X is fine for valving Y but X+50 or X+100 is not” is either fooling you or themselves. It is akin to asking what amount of ground pepper is best for scrambled eggs. Too many variables about the consumer other ingredients that might be used (cheese, salsa, etc.). I like a lot of ground pepper on eggs but that is my personal taste preference.

Although the shocks are usually more expensive than the springs, I am a firm believer that you should not under spring your car for a certain activity just because the shocks might be perceived as not as capable. The springs are upstream in the suspension equation so the answer is to use the best spring rate for the activity and rest of the car then put the best shocks that you can on it. We know that a Stock class autocross car on stock springs will almost always do better with a custom valving that is adapted more to autocross than to the mix of street performance use that it was designed for from the factory. The large range of rebound adjustment lets the driver match it to an extremely wide range of uses, preference and upgrades including spring rates.

So the answer it that when it comes time to step up your spring rates, go for it. I believe that with almost any spring rate, the non-customized KONI Sport is going to be the best commonly available option and can be made even better suited if custom decisions are made from there depending on the many variables. Certainly there is a point that if you put huge spring rates on the car it will do better with a customer valving but the off-the-shelf KONI Sport is still going to be the next best choice in its price range.

I hope that clears it up. As common as the question is, it really puts the cart before the horse and leaves out many important variables that should be considered in the big picture. There is no point where the performance is going to drop off the edge or do quick damage to it mechanically."

tl;dr yes. you'll be fine. speaking from experience with much higher rates on yellows than boss 302 springs, too

The only thing I could say about the Boss springs is that they still use the stiffer rate in the back and softer in the front combination like the regular GT/Brembo cars use and the Boss also uses a stiffer rear bar and staggered tire combination to dial in its handling.

Its seems like many Boss owners who wish to better their handling are doing away with the stiffer rear/softer front spring combination and replacing that with the more common stiffer front/softer rear combination offered by most aftermarket brands (Steeda/Eibach..etc).

The Koni Yellow's have a pretty wide range of spring rates they can support so I don't think using a Boss spring would be of much concern as they are pretty soft in the grand scheme of things.

My concern isn't so much having too little shock for the spring rate being chosen, but perhaps having too much. Having too much compression will result in a very harsh ride even with relatively soft springs, since the spring rate and the shock compression rate are additive. But, of course, too little compression results in poor transient response to cornering, so it winds up being a balance.

I guess if the Koni Sports are designed to be useful with relatively stiff springs, it seems logical that the compression setting must be relatively mild.

2013DIBGT said:

The only thing I could say about the Boss springs is that they still use the stiffer rate in the back and softer in the front combination like the regular GT/Brembo cars use and the Boss also uses a stiffer rear bar and staggered tire combination to dial in its handling.

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Well, there seems to be quite a lot of confusion about whether the fronts are really softer than the rears, seeing how these cars have more weight in the front than in the rear, and especially, seeing how there is much more sprung weight in the front than in the rear. I would expect changing from a softer front/firmer rear to a firmer front/softer rear would result in all sorts of handling balance issues, seeing how the cars seem to be very nicely balanced from the factory.

Do we have any confirmation from any source that the fronts are truly softer than the rears as these cars come from the factory?


As for the balance of the Boss 302, it appears to me that the rear sway bar was changed to counteract the staggered setup, to preserve the neutrality of the car. As such, since I'm going to be running with a square setup, my plan is to leave the sway bars alone. I will go to adjustable sway bars if I find the balance to be off.

Its seems like many Boss owners who wish to better their handling are doing away with the stiffer rear/softer front spring combination and replacing that with the more common stiffer front/softer rear combination offered by most aftermarket brands (Steeda/Eibach..etc).

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Are there any aftermarket brands that offer the reverse? I've not heard of any.

The Koni Yellow's have a pretty wide range of spring rates they can support so I don't think using a Boss spring would be of much concern as they are pretty soft in the grand scheme of things.

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But the problem is that Koni Yellows are adjustable in rebound only. That's the most important kind of adjustment that can be made, of course, but compression will control cornering transient response on the outside corners and will control the harshness upon hitting a bump. It probably also has a lot to do with how likely you are to hit the bump stops when encountering larger bumps in the road. Of course, I'm sure it's quite a bit more complicated than that.

My concern is with whether or not the Koni Sports have too aggressive a compression setting, but suspect that, given they are designed to work with relatively stuff springs, they probably don't. But there's also the possibility that they don't have sufficient compression and that, when used with relatively mild springs, they will tend to bottom. I guess the only way to find out is to try them out.

There is no confusion about whether they are softer in the front vs rear.

There are many sources with the list, here is one:
http://brembo50.com/Default.aspx?pageId=1130035

Every model has a softer front and stiffer rear. The most drastic being in the Laguna Seca which has 137/191.

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NoTicket said:

There is no confusion about whether they are softer in the front vs rear.

There are many sources with the list, here is one:
http://brembo50.com/Default.aspx?pageId=1130035

Every model has a softer front and stiffer rear. The most drastic being in the Laguna Seca which has 137/191.

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But if I'm not mistaken, all those "sources" are actually using the same source, Ford's published specifications.

My question is whether or not those specifications have been independently verified.

The guy that developed the Brembo 50 springs for the rear has independently confirmed them at least for the brembo car.

The confusion comes from people not understanding that sometimes cars have softer springs in the front and spreading misinformation.

If you have anything but a stock GT, (which has very close rates front and back) you can go confirm the softer fronts by driving over a speed bump.

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I haven't seen any independent verifications anywhere but I don't see why Ford would publish the rates incorrectly on purpose across all models of mustang.

Even if they did publish them with bogus numbers to throw off the competition in order keep the secret recipe from getting into the wrong hands its not like any flavor of mustang in factory trim is a handling benchmark IMO. If it was so good in its current state then all of us wouldn't be here changing the suspension out for something else.

Even those who own the LS/Boss models find the FRPP Handling kit an upgrade to what it comes with from the factory and the FRPP kit is nothing special really.

2013DIBGT said:

I haven't seen any independent verifications anywhere but I don't see why Ford would publish the rates incorrectly on purpose across all models of mustang.

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I've seen speculation that they accidentally reversed the column headings on the specification sheet, which would result in exactly the issue in question.

But if the spring rates have been independently measured and have confirmed the numbers, then Ford's numbers are correct and that's that.

Even if they did publish them with bogus numbers to throw off the competition in order keep the secret recipe from getting into the wrong hands its not like any flavor of mustang in factory trim is a handling benchmark IMO. If it was so good in its current state then all of us wouldn't be here changing the suspension out for something else.

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I always figured that you guys were changing the suspension because your use of the car is more track-oriented than the use to which most purchasers put it, and that you're willing to compromise other areas of use (e.g., daily driving) in order to achieve it.

Even those who own the LS/Boss models find the FRPP Handling kit an upgrade to what it comes with from the factory and the FRPP kit is nothing special really.

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I'd not seen much discussion of the FRPP kit with respect to use on the Boss Mustangs, particularly as regards improved performance on the track. Can you point me at those discussions?

kcbrown said:

I'd not seen much discussion of the FRPP kit with respect to use on the Boss Mustangs, particularly as regards improved performance on the track. Can you point me at those discussions?

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Here's two threads from Boss owners who swapped in an FRPP Handling kit and seemed very pleased over the factory Boss handling:

http://bossmustangsonline.com/index.php?topic=2041.0

http://bossmustangsonline.com/index.php?topic=1769.0

I've seen other reports from Boss owners using different packages with similar results.

We have tested them, and that brembo5.0 site's info matches our results. Our #'s came very close, within 1-2%..

That said - the Boss 302 Springs will feature a pretty decent rake, from front to rear. The Koni front struts will lower the front an additional 1/4"....so I don't think it would look, or perform the best with that combo.

BMR Tech said:

We have tested them, and that brembo5.0 site's info matches our results. Our #'s came very close, within 1-2%..

That said - the Boss 302 Springs will feature a pretty decent rake, from front to rear. The Koni front struts will lower the front an additional 1/4"....so I don't think it would look, or perform the best with that combo.

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The use of Koni struts will result in lowering the car?? That suggests that the Konis are at full extension with the full weight of the car on the springs.

How much lowering would I get in the front if I used them with the stock GT track package springs?

1/4", regardless of spring choice.

BMR Tech said:

1/4", regardless of spring choice.

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But how?? Does the strut have a different (shorter by 1/4") top spring perch to strut mount distance?

You got it. Actually, lower spring perch mount, is 1/4" lower than factory.

BMR Tech said:

You got it. Actually, lower spring perch mount, is 1/4" lower than factory.

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The location of the lower spring perch shouldn't have any effect on the ride height if the location of the upper spring perch relative to the strut mount is the same between the two.

The reason is that the car's weight will compress the spring to the same length in both cases, as that is based on the amount of weight, the starting length of the spring, and the spring rate.

Moving the lower spring perch on the strut without changing anything else only means that the initial spring length after pre-compression by the strut is a little longer than it was before (because by moving the lower perch without moving the upper perch, you change the amount of pre-compression), but that doesn't change the spring length you get after compression from the weight of the car. The sole exception to that is if the length of the spring after pre-compression is less than it would be if it were compressed by the weight of the car, which means that the strut assembly would be in full extension with the full weight of the car on it at that point.

The bottom line is that unless the distance from the top of the spring to the strut mount has increased, you won't get a lowering effect from the strut unless the strut is fully extended with the full weight of the car on it, something that shouldn't be the case.

There is so much wrong or half wrong spring and damping information in this thread I don’t even know where to start.

Readers BEWARE!

kcbrown said:

The location of the lower spring perch shouldn't have any effect on the ride height if the location of the upper spring perch relative to the strut mount is the same between the two.

The reason is that the car's weight will compress the spring to the same length in both cases, as that is based on the amount of weight, the starting length of the spring, and the spring rate.

Moving the lower spring perch on the strut without changing anything else only means that the initial spring length after pre-compression by the strut is a little longer than it was before (because by moving the lower perch without moving the upper perch, you change the amount of pre-compression), but that doesn't change the spring length you get after compression from the weight of the car. The sole exception to that is if the length of the spring after pre-compression is less than it would be if it were compressed by the weight of the car, which means that the strut assembly would be in full extension with the full weight of the car on it at that point.

The bottom line is that unless the distance from the top of the spring to the strut mount has increased, you won't get a lowering effect from the strut unless the strut is fully extended with the full weight of the car on it, something that shouldn't be the case.

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Sorry...but...you're...uh...wrong.

Treat the loaded strut assembly as two line segments, A and B.

Ride height=A+B

A=distance from strut mounting surface to lower spring perch, at ride height
B=distance from strut-knuckle bolt hole to lower spring perch (this is what Kelly has measured)

We agree that A will not change from strut to strut, assuming the spring is the same in all cases, and that the upper spring perch is the same in all cases.

However, as Kelly has shown, if you reduce the length of segment B (in this case, about .25") then ride height will lower by the amount it's reduced. This is how coilovers work.