Oh, keep in mind if you switch from 350 lb/in springs to 300 lb/in front springs it's not like it's going to ride like an old Cadillac. The front springs on the US-spec NSX's (non Zanardi of course) were between 180 and 200 lb/in.
You need to keep in mind that the stock 180-200lb springs are at a much higher ride height that has far more compression travel than lowered cars. The lower you run the car, the less travel you have before hitting the bumpstop and causing a harsh and jarring ride. This is why I recommended against JJM4life lowering the spring rates on his V3s, especially at the ride height he wants to run - based off of the contact between the lower spring perch and the UCA.
*As you lower a car, you need higher spring rates to avoid hitting the bump stops. This is why I recommended running stiffer spring rates at his ride height. I also cut down my bump stops to increase compression travel. Removing 0.5" of bumpstop will make the KW damper bottom out into the bumpstop as the tire starts to touch the inner fenderwell. This gives a more compliant ride over most bumps and still prevents damage when you hit harsh bumps.
Theory is great, but the recommendations don't understand his specific setup. As I said before: I have his exact suspension, am familiar with his issue, and have first hand experience solving it. (Unless you have V3s?)
And on to this:
Haven't missed any points - I've just chosen to ignore the incorrect ones.
You have a basic grasp of motion ratios but you do miss a big part of the understanding of how motion ratios work.
You can solve the problem with shorter springs but that is not the ideal solution for everyone. But it is for HIS KWV3s at HIS ride height. Simply switching to shorter springs is a bad idea because you are loosing compression travel before you hit the bump stops, or worse, the springs coil bind.
Shorter springs raise the location of the lower spring perch for the SAME RIDE HEIGHT -which is his goal to fix his problem. From there he has the room with the spring perch to lower the car further, in which case lowering it from his current ride height via shorter springs or softer spring rates (as you recommend) both will reduce total compression travel of his suspension.
Let's circle back Motion ratios for a second:
The average NSX has a front corner weight of ~600 lbs. correct. A 350 lb/in spring will compress ~1.7 inches when loaded with that weight. A 300 lb/in spring will compress 2 inches when loaded with that same weight INCORRECT (read below) and will therefore compress approximately 0.3 inches more than the stiffer spring. Now, the motion ratio of the front NSX suspension is ~0.75 to 0.7 (actually 0.67) for a slammed front end. Therefore, a 0.3 inch drop for the front coilover will translate to an ~0.4 inch drop at the front wheel. That's the math behind my previous example for anyone interested. So, leaving the lower spring perch alone in the front and just switching from a 350 to a 300 lb/in spring will drop it ~0.4 inches. Is that enough for JJM4life to give him the drop and spring perch clearance he needs?
Your travels are off due to a misunderstanding of how motion ratios work. If you have a 600lb corner weight resting on a strut with a perfect 1:1 motion ratio; yes a 350lb spring will compress 1.71 inches. However, we do not have struts on the NSX. We have an SLA suspension with a 0.67 motion ratio.
Wheel Rate = spring rate*(Motion Ratio)^2.
350*(0.67)^2 = 157.1lb wheel rate.
600lb corner weight acting on 157.1lb wheel rate = 3.82" of travel at the wheel (assuming zero preload on the spring at full droop and no excess droop travel. This results in 2.56" of travel at the damper (3.82"*0.67),
compressing the spring 2.56" at static ride height.
A 300lb spring = 134.67lb wheel rate and will cause the wheel to compress 4.46" (0.64" lower ride height with the same spring perch height) and the spring will compress
2.98" - 0.43" at the spring/damper.
For example, an Eibach 7" long, 2.5" ID spring with a rate of 350 lb/in has an effective travel of about 4.25" before the coils touch together and bind. That same Eibach spring but nominally 6" long will bind within 3.75" of travel. You're loosing half an inch of compression travel in the shock. Incorrect; the 6" spring will coil bind 0.5" sooner; it has no effect on the damper/'shock's travel. This is huge on a street car.
Take the example previously for a typical front NSX... The front 350 lb/in spring will compress ~1.75" when loaded with the static weight just sitting on the street. The 7" long spring before now only has 2.5 inches of compression travel (4.25" - 1.75" = 2.5"). Don't forget your typical 1.25" thick bump stops to protect the damper, and you only have about 1.25" of compression travel on street bumps before you impact your bump stops.
Now, switch to a 6" spring that is 1" shorter. Your compression travel for the damper is only 0.75" before you hit the bump stops. This will severely impact the ride quality observed on a typical North American street.
As we learned above; on an NSX, a
350lb spring will compress
2.56" from 600lbs of static weight. That leaves a
7" spring with
1.69" travel before coil binding, and a
6" spring with
1.19" of travel before coil binding. Most coilover bumpstops are harder in durometer and never compress the full thickness of the bumpstop. So it's not accurate to take the thickness of a bumpstop off of the travel of the damper.
A
300lb, 7" spring will sit 0.64" lower with the same spring perch height as above, compress the spring 2.98", leaving
1.27" before coilbinding with a softer spring rate.
By comparison: My car has 80nm springs (
457lbs) = 205lb wheel rate. That = 2.93" of wheel travel, 1.96" of spring compression. Given my 140mm (
5.5") spring, which should have ~3.5" before coilbinding, I should have
1.5" of travel at the spring.
Remember that 1.5" at the spring = 2.23" of tire travel before coilbinding.
NOW, going with my idea of just switching to a softer spring yields the following:
An Eibach 7" long, 2.5" ID spring with a rate of 300 lb/in has an effective travel of 4.5" before the coils bind. WHAT? That's even more travel than the springs I had before! How'd they do that?! Different coil diameter and possibly geometry that they've fine-tuned for spring linearity and maximum travel.
If you're not coilbinding; changing to a spring that has more travel before coilbinding won't magically give your damper more travel. The only way to get more travel is to cut down or remove the bumpstop, or get a shorter damper body.
Keeping your 350 lb/in spring rate and simply going to a 1" shorter spring will decrease your allowed coilover compression travel by ~0.5" and negatively impact the ride quality on bumpy roads. It will allow you to raise your lower spring perch by ~1" though from the perch position you have now to maintain the current ride height. Not true. As said above, for the KWV3, you'll hit the bump rubber before you coilbind the shorter spring.
Going from a 350 lb/in spring to a 300 lb/in spring will increase your allowed coilover compression travel by ~0.25" and improve the ride quality on a mostly street-driven NSX. It will allow you to raise your spring perch by 0.3" from the perch position you have now to maintain the current ride height.
A 300lb spring will hit the bumpstops sooner at a low ride height, resulting in a harsh ride. The only way a 300lb spring will ride softer is at a higher ride height that does not hit the bumpstop. The issue is compression travel to the bumpstop, not a coilbinding limitation.
You can certainly go to shorter springs, but I've offered an alternative that hasn't been discussed before.
because there are more negatives to a softer spring at the ride height JJM4life wants
It all depends on what JJM4life desires. If they're happy with the current ride or want to go slightly softer, then the softer springs will do that in addition to lowering the car to where they possibly want it. It would be a win/win as opposed to simply going to a 1" shorter spring.
Not true. You need to look at the whole picture (above)
Now they have enough info to make an educated decision :wink:
NOW he has.
In reality, you will hit the KW bumpstop well before you coilbind the spring at this ride height, especially with a 300lb spring. I've tested my 456lb 5.5" spring with 0.5" cut from the bumpstop and it does not coilbind, and the tire just starts to touch the fenderwell at full compression (against the bumpstop) and the ride quality is great without slamming into the bump stops.
It may sound counterintuitive, but when you want to run very low, stiffer springs that keep you out of the bumpstops will make the car ride 10X better than a soft spring that's sitting on the bumpstop.
CLIFFNOTES for those that get lost in the above:
-Lower ride heights will bottom the damper into the bumpstop sooner.
-Lower ride heights need stiffer spring rates to stay out of the bumpstop.
-Cutting down the bumpstop or shortening the damper body is the only way to get more suspension travel (all the examples above bottom the damper out before the spring coilbinds)
-Softer springs need higher ride heights to prevent the damper from bottoming out in the bumpstop.
-If you want your KW V3 lower than the stock setup where the spring perch hits the upper control arm:
-----Use a shorter spring and increase spring rate at least to 457lbs. 514lbs or 571lbs (like NSX-R) would be preferable over the stock V3's 343lbs.
Here's the motoIQ article from many years ago, testing the V3:
https://motoiq.com/project-nsx-part...and-nitto-tires-in-pursuit-of-a-track-record/
And here's the thread back when FXMD was one of the first to sell and support KW's in the NSX market:
http://www.nsxprime.com/forum/showt...Spring-Packages-by-FX-Motorsports-Development
