Psuedo-physics question re: ride quality of coilovers

K, I just swapped the Moostang from stock location springs to a set of coil-overs. The internet lore was that in spite of the increase in wheel rate, the COs with their lower rate springs, would provide better ride quality. I've found this to be absolutely true, however I can't wrap my brain around why.

I mean, if I have a 12" lever, why would the force transmitted though it be different whether it was resisted by a 50 lb/in spring at the end or a 100 lb/in spring at the point of 1:2 motion ratio? The net force against the end of the lever should be identical.

What am I not taking into account here?

Hrm.

It doesn't answer the question, but I would point out that you need the square of the leverage ratio to get the difference in rates. When you displace the wheel 1", not only are you only "feeling" half the spring force, you've only displaced the spring 0.5", so your 100 lb/in spring on a 1:2 leverage ratio creates a wheel rate of 25 lb/in.

So your 1:2 motion ratio results in a 1:(2-squared) or 1:4 wheel rate.

But since you've started with the given that the wheel rates are the same, and assuming that part is correct, that's all a red herring with regard to your question.

Could it be anything to do with not having a big, ringing mass of a heavy spring increasing NVH over bumps, where the coilover spring is considerably lighter?

Something flexing?

The coilover has a better quality damper than you had previously?

Giant Purple Snorklewacker wrote: The coilover has a better quality damper than you had previously?

Yeah, I'd bet that this has way more to do with the damping than the springs.

Giant Purple Snorklewacker wrote: The coilover has a better quality damper than you had previously?

Exact same dampers (literally the same specific ones that were on the car before the CO conversion).

So all you've done is to move the top spring perch. You have not moved the top damper location? If that is the case then the mass of the spring is the only significant difference.
If you have changed the physical spring rate or moved the top of the damper pick-up location then you have changed wheel-rate and/or motion ratio. Both of those could affect the ride with the same damper settings.

stafford1500 wrote: So all you've done is to move the top spring perch. You have not moved the top damper location? If that is the case then the mass of the spring is the only significant difference. If you have changed the physical spring rate or moved the top of the damper pick-up location then you have changed wheel-rate and/or motion ratio. Both of those could affect the ride with the same damper settings.

Keep in mind this is a foxbody Mustang. I removed the stock location springs, which sit inboard of the strut, and replaced them with an adjustable coilover setup that mounts on the strut. Spring rate went down from (est) 600 lb/in to 300 lb/in, however the CO springs have an equivalent wheel rate as something like 700 lb/in stock-location springs. Struts are non-adjustable Bilstein HDs. CC plates were not adjusted, nor was the position of the camber bolts I use (yes, I have both, crash bolts were used to make up for the reduction in neg camber from using SN95 spindles). Ride height was brought back up ~1", which reduced static neg camber by ~1 degree.

My guess is that with the worse leverage, the stock spring needs to exert twice as much force to give you the same force at the wheel. So you get twice as much force on the body because of the spring location.

From a corner carvers thread:

Originally Posted by Jack Hidley
David sort of touched on something that is important, but seems to be ignored a lot in chassis design, in his last post.

If you can mount the spring with a motion ratio closer to 1:1, you will end up using a softer spring for a given wheel rate. This means that for a given ride or roll rate (stiffness at the wheel), the suspension forces directed through the spring and into the chassis will be lower. If mounting the spring in a location that gives a 1:1 motion ratio also connects it to the chassis in a spot that is stiffer than the original location, this gives an even greater benefit. The ratio of chassis stiffness to roll stiffness becomes even higher, which is almost always good. This is one of the reasons that installing coilovers on the front of a 1979-2004 Mustang improves ride quality over the FCA mounted springs.

Gimp wrote: ...you will end up using a softer spring for a given wheel rate. This means that for a given ride or roll rate (stiffness at the wheel), the suspension forces directed through the spring and into the chassis will be lower

Yes, but why? That's exactly what I'm not understanding.

Is this just a case of me WAY overthinking it, and the answer is as simple as "because the spring rate is lower"?

I don't know why, but if I think of the spring as a bushing, it makes a little more sense. A soft rubber bushing would transmit less vibration than a poly bushing, even if it were in a more leveraged position.

If you have ground controls or similar......the only reason it is softer is due to the added leverage.

Real quality coilovers will ride even better as long as you don't go nutters with the stiffness of the dampening

Any spring will have a softer rate than a hard-rubber bumpstop! If you were bottoming out before, your car would ride like a buckboard.

From Gimp's quote, I'm having a little trouble getting my head around the idea that the seat pressure of the spring against the body makes a difference, but the mention about where the forces are being introduced to the body makes me curious.

Are you running any sort of strut tower brace? I can't picture the stock upper spring perches on a foxbody, but if the upper strut mounts are more solidly tied into the chassis, could a reduction in body flex and cowl shake be the improvement you're feeling?

Had a STB, but I removed it about 6 months ago. Coilovers were installed last weeked, STB is still sitting on the shelf.

Picture might help...

In this shot you can see where the coilover is. The circular thing you can see behind the swaybar is the upper spring perch for the stock spring location. The lower spring perch is in the middle of the A-arm.

ReverendDexter wrote:

Gimp wrote: ...you will end up using a softer spring for a given wheel rate. This means that for a given ride or roll rate (stiffness at the wheel), the suspension forces directed through the spring and into the chassis will be lower

Yes, but *why*? That's exactly what I'm not understanding. Is this just a case of me WAY overthinking it, and the answer is as simple as "because the spring rate is lower"? I don't know why, but if I think of the spring as a bushing, it makes a little more sense. A soft rubber bushing would transmit less vibration than a poly bushing, even if it were in a more leveraged position.

It is pretty much "because the spring rate is lower." A lower spring rate doesn't have to "work as hard" to absorb a bump, so it transmits less vibration to the chassis.

All I can see is the spring from the CO is acting in the arc of the suspension travel while the OEM spring is not, it is offset. Draw a line through the center of each... and I believe you will see that the springs are not in the same operational plane

Maybe this will help?

Mustang Suspension

On a side note, I have been very seriously considering MM front coilovers on my 95 GT. I've heard that coilovers improve the ride and my Mustang needs all the help it can get...

is the shock tower of a Fox mustang designed to take spring loads like that? i know that strut tower braces are popular on those cars with the stock springs.. the softer ride could be from the body structure moving around.

If your previous suspension was binding or locking up in any location, or you put one wheel on the bumpstop, you'll have effectively an infinite wheel rate there.

Looking at the page stanger_missle linked.

http://www.miracerros.com/mustang/t_animated_coilovers.gif

With the coilovers (on the left), the spring and shock work together in a 1:1 action. For every motion of the shock, the spring sees the exact same motion, and all forces are joined together to the wheel, and at the top mount.

With them separated (on the right), the forces are not the same, nor is the motion. The spring is acting through the lower arm at the bottom, and through a different frame perch at the top. This brings in flex and frequncy, and the resulting delays and amplifications. Hit a bump and the wheel and shock move instantly. The spring sees a smaller motion because of the leverage, and it sees it just a little later because the arm flexes a little. The chassis or frame sees the force at two different locations as well, the shock top and the spring top. This certainly works, but it doesn't work together as smoothly because of the linkage and leverages and time delays and such.

As another way of looking at it, this is why an overhead camshaft riding directly on the valves works better than a pushrod & rocker arm valve train.

novaderrik wrote: is the shock tower of a Fox mustang designed to take spring loads like that? i know that strut tower braces are popular on those cars with the stock springs.. the softer ride could be from the body structure moving around.

There's a pretty big thread on Corner-Carvers about just that. The really, really, short version is that only triangulated braces are worth a damn, and even then there's so little flex between the strut towers that what you're actually bracing is the slight flex that occurs between the strut towers and the firewall under hard braking.

chaparral wrote: If your previous suspension was binding or locking up in any location, or you put one wheel on the bumpstop, you'll have effectively an infinite wheel rate there.

Nope. I definitely wasn't hitting the bumpstops. This is a comparison of street driving to street driving, as there's nowhere 'round here I can safely drive this car that hard, at least not on my Star Specs, lol. Haven't had a chance to AX this setup yet, that comes next weekend.

I have a basic dumb coilover question and this is as good as anywhere to see if I can get some feedback.

I got a deal on a pair of used Koni coilover shocks for the Jimmy. I made a picture:

A is normal ride height. B is compressing over a bump in the road. C is fully uncompressed as if the wheel dipped into a pothole.

C picture is where I don't fully understand??? I hope I say this right.

Is it common for the spring to release from the seat and not work in reverse to help stop the shock from traveling beyond the spring distance? Am I missing a retainer to keep the spring seated? Add limiters to the A-arms? Don't worry, it is normal! and the coilover spring only works one way (compression). Thanks in advance. T

In reply to Gasoline:

My understanding is that it is normal for the shock to be the limiter, not the spring. I think the "correct" setup with a shock that can droop further than full extension of the spring is to use a "tender" spring to take up the extra room. The tender spring is very soft, and goes to coil bind pretty much instantly on compression; it's just there to keep the main spring aligned correctly when it's off the perches on extension.

Of course, if they're random shocks not meant for a Jimmy, you'll need to make your own assessment of how much droop is too much. If the shocks allow too much droop, you may need droop limiting measures. If that point is short enough that the springs don't come off the perches, that saves you mucking about with tender springs...

I suppose if the length of the springs coincides with the droop you want, you could find a way to attach the perches to the spring so it did act as the droop limiter, but I don't think that's the norm...

Hopefully if I've gotten any of that wrong, someone with more experience will correct me...

In reply to ransom:

Gotcha, Big thanks! I'm getting a construction plan formulated in this area.