True Coilover Rear Damper Or Not
 

There has been a lot of discusion here about moving the rear spring to the OEM location to a true coilover. Some argue for it and some against it.

I found this online yesterday. This is the JUN Time Attck AWD 350Z. As you can see this is a pure race car and has little to do with a street driven 350Z. They could have gone with whatever they wanted to use as as a suspension solution. From the pics it looks like they went with a true coilover.

This car is very competetive on the track.

http://img.photobucket.com/albums/v2...3_8T2Jved2.jpg

http://img.photobucket.com/albums/v2...7_1OaRlrYf.jpg

Also the 380RS has a true coilover in the rear!

http://www.nismo.co.jp/en/products/c...sc_img09_b.jpg

JET

I don't see a problem with a true coilover design in the rear...just keep an eye on the bushings...

so does the Nismo 370RS race car or whatever, but thier rear suspensions are anything but stock. any pictures that aren't Xs? :D

Fixed!

JET

why are they using helper springs on a track car?

as long as the rear is not inverted tube design u are fine. else you wont be able to 'slam' the car

The argument has been concerning weather the upper shock mount can take the load which it was not intended to do with the spring in the OEM location. And Resolute had expressed concern about the effect on spring rates.

JET

Since these are all full race cars, they could have reinforced the upper mount area, and use different shock mount bushings on the spindle, reinforced the spindle, and I can almost guarantee these cars run very high spring rates (given how low they are), so the suspension does not travel enough and generate enough camber change to significantly affect effective spring rate. What works on these race cars, do not necessarily apply to mildly modified street cars. Remember, Nissan developed the suspension this way for a reason, and the midlink is a relatively expensive piece for Nissan to produce, so it is that way for a pretty good reason.

Not so much the spring rates, my concern is with the kinematics.

I thought that the spring mounted directly on the upright would cause an issue in lateral weight transfer, where the kinematics would create a higher wheel rate from reaction torque placed through the upright, whereas the moment would not exist if the load were on the lower control arm. I could not confirm this through ADAMS modeling, though I still suspect there is more occurring than ADAMS modeling can show. (ADAMS does have some flaws)

That being said, what I can confirm is this:

The spring mounted on the damper assembly WILL cause a linear spring to be progressive, since the wheel rate is a function of spring rate multiplied by the cosine of declination. Since the spring will move as an arc, the cosine function will increase through travel, affecting spring rate.

The damper's upper mount is not idealized for carrying the lateral load transfer if the spring is also utilizing this mount. The direction of the mounting bolts is perpendicular to this moment to allow flex, if the weight is transfered through this position, a wider mount would be better, such as the front upper mounts. I would not be surprised, without reinforcement, if the two upper mount bolts would break under high lateral loads.

The anti-squat on the 350Z is very high. The reaction torque in the longitudinal plane from acceleration forces is very pronounced, and acting through the top of the upright increases this affect, since there is a static moment created by the damper's mount from the upright's center. This is the primary reason spring rates on the Z must be fairly low- to accommodate the high wheel rate found under acceleration. If the traction arm could be adjusted to a new position, which I have seen done by re-welding the chassis mount, then the deficiencies of the spring rate in the damper location (in terms of wheel rate and anti-squat) could be resolved.

The JUN car you posted does not use the stock kinematics, and the traction arm is missing altogether. The Nismo race car does not appear to use the stock mount and also has adjusted the traction arm.

I would imagine that the upper mount of the Nismo car has been re-worked to survive the high lateral loading from competition.

Inverting the rear damper would be the best way for the damper to endure the lateral loading, if the spring is mounted around it. It would also better handle the friction increase in the damper's motion from lateral loading.

Mounting the spring in the OEM position, provided stock kinematics are considered, reduces this friction considerably (60% according to Nissan) and allows the spring rate to better match the wheel rate by reducing the affects of anti-squat (compared to a damper mounted spring).

I would not run the spring on the damper without the changes I mentioned, and have seen no benefit in ADAMS modeling, or using Optimum K, with using a spring mounted on the damper. If the changes I outlined were made, then mounting the spring on the damper has the benefit of reducing unsprung mass, and reducing the motion ratio, but these are only benefits if the kinematics are properly dealt with first.

That's really all I have to say about it. Do what you want, but be careful comparing photographs of cars who's suspension design you know nothing about. There is more than meets the eye to proper design. Nissan did it the way they did on purpose.

Will

You responded while I was typing, but pointed out the same things: I'd be willing to bet they did, for reasons I posted above.

I didn't mention the bushing on the upright, since it seems to be well known as a deficient design for carrying the sprung mass, but machining Al bushings wouldn't be difficult. A nuisance for a street car, though, and doesn't solve the upper mount problem (far more intensive problem)


Exactly. I was answering JET from the perspective of a street-driven car using a spring on damper, but a race car can negate the cosine function of the wheel rate with limited travel.

Well said.

Will

The JUN car uses every control arm in it's stock location. The stock ones in the JUN car were swapped for the adjustable ones as seen in the pics. I run the exact same suspension setup and I have had no problems with my upper shock mounts and I do track my car. Nothing needs to be modified in respect to the upper shock mounts. I also run 16/14 spring rates on my car.

http://img.photobucket.com/albums/v2...6_xNef072i.jpg

http://img.photobucket.com/albums/v2...8_sXImXBD4.jpg

JET

no point commenting, resolute and kuah did a great job already - great posts guys

resolute - either you're the worlds greatest student, or you have way too much time on your hands ;)

So the concensus is moving the spring to the damper is ok as long as your spring rates are rasied and suspension travel is reduced assuming the rear upper shock mount is adequate? That's what I'm reading here.

JET

In the first pic, I thought the subframe had been modified, and the linkage was different from stock. Thanks for the other pics. I wish I could acid dip my chassis and make it look so clean.

I don't know if you'll find a consensus, JET. Not sure what the point is to seeking one. If you like what you have, and it works for you, then I'm sure you'll be happy with it.

I mentioned the benefits to running the spring on the damper:
1.) less unsprung mass, and 2.) no motion ratio.

Problems with that design:
1.) requires higher spring rate to minimize effect of the damper's arc travel
2.) higher spring rate is not conducive to good traction under acceleration with stock linkage (anti-squat)
3.) stock bushing on upright isn't designed for load (albeit easily remedied), and upper mount not designed for the moment created by placing sprung mass there.

As far as your upper mounts being "fine", I remain dubious. The FEA I ran was a simple COSMOS analysis that was originally set-up for a graduate student's work on a Mitsubishi Evo for a local team. The moment on the two mounting bolts is huge when you consider the lateral forces possible with R-compound tires. I still see fatigue being an issue. While the stock design places an equal load on both bolts, it was not designed for the sprung mass and a third equilateral mounting bolt would be ideal (and an easy upgrade for a racing team so inclined).

Be that as it may, I see no real benefits to mounting the spring on the damper unless the concerns I noted are addressed. That's my view unless some data convinces me otherwise. I have done quite a bit of personal analysis of the stock geometry (as a substitute for not being able to actually upgrade the car), and don't see any gains that outweigh the cons.

Why does JUN do this, or the 380RS? Who knows. I didn't design them so I don't pretend to know. I also do not assume that what works for them translates into what works best for my needs. I merely get as much data as I can, and go from there. Right now, the data I've seen supports the OEM spring location as superior for my needs.

I'm not sure what your needs are, what dampers you are running, or why you have such high spring rates. In the damper location, that rear spring gives a really high wheel rate and high suspension frequency. I'll come out and say I'm on the lunatic fringe of Z owners that don't like a lot of Japanese suspensions, and having run a few of them on a shock dyno, find their general tuning to go against what I was taught. Whatever it is you bought, though, if they work for you, then that's all that matters. You don't need a consensus to enjoy them.

Will

Some 350z race cars have been runing coilovers in the rear for some time now and even endless has made there own for the Z

Function X - - Front - - Rear
Spring rate - - 12kg - - 10kg
Spring Length - 178 - - 178
Upper mount - - PB - - PB

I'm sure Adam can hook you guys up, but if you guys are looking for "GOOD" track suspension then you should look into ProFlex, Reiger, Moton or even Ohlins :)

I would love a set of Penske shocks... want to sponsor me? :rolleyes:

JET

I've always been a lurker, but this thread made me come out of hiding.

The term kinematics is mostly used to refer to the effects of moving a suspension through the arc of its travel on alignment settings such as camber, caster, toe, etc. Moving the spring onto the damper at the upright does not change any of these settings.

If you’re referring to changes in wheel rate caused by moving the spring, then again, your assumption has errors. While moving the spring to the upright does increase the wheel rate, moving the spring to the upright does not create an additional moment that does not exist if the spring is mounted on the mid-link as stock. Moving the spring’s mount point on the unsprung mass farther outboard (to the upright, in this case) simply causes the moment on the suspension generated by a set spring force to be greater than if the spring was mounted farther inboard (on the midlink, as is stock). Remember your fundamental moment equations? Moment = Force * distance from pivot point. Increasing the distance increases the moment for a set force. That is the effect that increases the wheel rate when the spring is moved to the upright, not creation of an additional moment.

Increasing the wheel rate is not uncorrectable. You can compensate by running a softer spring.

I hate to break it to you, but the stock setup is progressive as well. The stock setup is rising-rate progressive until the point where the spring is perpendicular to the mid-link, at which point the suspension becomes falling-rate progressive with additional bump travel. There is no avoiding this progressiveness short of moving to an inboard spring/damper setup with a pushrod or pullrod configuration, or an outboard spring & damper with a nik-link configuration.

I don’t know what you’re talking about – if I remember correctly, the damper shaft rides in a spherical bearing mounted in the rear upper mount. This makes the rear damper a two-force member, meaning that it physically cannot transfer bending moments to the chassis at the rear upper mount.

The upper mount bolts are not particularly load bearing. They are only there as positioning studs, and to keep the damper from falling out of the rear shock tower if the suspension goes to full droop. The load is actually supported by the plate of the rear upper mount itself, as it loads the shock tower in compression in reaction to wheel loads. An overloaded rear damper would not break the bolts, but would rather tear the sheet metal unibody at the shock tower.

A wheel rate is a wheel rate is a wheel rate, and does not depend if the wheel is being loaded by longitudinal acceleration or lateral acceleration. I will agree that there is a good deal of anti-squat on a 350z, but changing the spring location will not change anti-squat effects short of the way you load bushings when the wheel is in ride. A spring & damper mounted on the rear upright will increase the load on that rubber bushing, causing increased bushing deflection, at worst reducing bushing life, and at worse possibly leading to a decrease in the spring & damper’s ability to control wheel travel.

As far as the “static moment” you talk about, the mounting of the spring on the mid-link (behind the upright) also creates the same moment, in a GREATER quantity, as the horizontal distance between the rear hub axis and the mid-link mount on the upright is greater than the horizontal distance between the rear hub axis and the damper mount on the upright.


The 350z does not have a macphereson strut setup – the shocks are never loaded in bending and are two-force members short of bearing drag in the upper and lower mounts. I’d be willing to be the bending loads in the dampers are negligible compared to axial loading.

Changing the spring location DOES NOT CHANGE ANTI- GEOMETRY! Anti dive and anti squat are determined by the declination of the lower and upper a-arms towards the vehicle center of mass. Changing the spring packaging has nothing to do with this! Also, what friction are you talking about specificially?

Spring rates don’t “match” wheel rates. Spring rates DETERMINE wheel rates. Wheel rate = spring rate * (installation ratio)^2. You don’t match your spring rate to your wheel rate, your spring rate DETERMINES your wheel rate!


Mounting the spring on the damper does not reduce unsprung mass except for the fact that you can run a smaller diameter spring with a lower spring rate (thinner spring coils) which, all else equal, probably weighs less.

Moving the spring to the damper does not mean there is no motion ratio between the suspension and the spring. There is still a motion ratio which changes based on the instantaneous inclination of the suspension. Unless you are referring to no motion ratio between the spring and damper, in which case you are correct.

Number 1 is wrong. Moving the spring farther outboard to the upright means you can run a softer spring and have the same ride and roll rates as a stiffer spring mounted inboard on the mid-link.

Spring rate does not mean anything – wheel rate does. You can run the same wheel rate regardless of spring packaging and have more or less the same characteristics under longitudinal acceleration.

I agree with the bushing comment, as well as your postulation that the chassis must be reinforced at the upper mount, either by adding metal thickness or tying the upper mount into a substantial roll cage (not the cusco ******** everyone runs on this site).

Hahah, this seriously made me LOL…you ran sheet metal unibody analysis on COSMOS? REALLY? I can’t imagine Cosmos would give you any meaningful results considering the complex geometry of the unibody. You realize that auto manufacturers spend millions developing proprietary chassis FEA models and solvers, right?

There is no moment on the upper mounts! Both ends are supported in such a manner that there is no bending load on the damper! The lower mount pivots on a bolt, and the upper mount rides in a captured spherical bearing.

Fatigue is an issue, I agree – the shock tower on the chassis must be reinforced. I disagree with the need for a third bolt, but agree with the necessity of chassis bracing in the area of the rear upper mount.

I see some advantages…
1) You can run a softer spring
2) Spring-on-damper suspension is less annoying to analyze than a suspension when the spring and damper are decoupled
3) Mounting the spring on the damper removes bending loads from suspension arms when using stiffer springs. Loading arms in bending causes unwanted changes in alignment when the wheel is loaded
4) Spring-on-damper mounting allows for easy adjustment of ride height and preload without needing additional parts to adjust spring assembly length


Please see above…

JDM stuff is ridiculous. Most of the dampers are low quality made in China garbage, have no low-speed damping, and run excessive compression damping. Remember, it’s the SPRING’s job to control the suspension in compression, not the damper’s job. The damper’s job is to control the suspension in rebound. Compression damping should always be lower than rebound damping. Most JDM suspension are overdamped in compression and the ride is excessively rough.

Seriously? Is that your only concern with modifying your car's suspension? If its a street car, sure, being slammed looks awesome. However, there is no point for a race car to be slammed unless aero grip is more of a concern than mechanical grip (i.e. high speed formula-style & Prototype racers)

The change in rate discussed by Resolute is due to the change in inclination angle of the coilover assembly (affecting the installation ratio) under bump, due to change in camber. The camber curve affects the inclination angle because the coilover mounts to the top part of the spindle assembly, instead of the middle where it should be for cars that are designed for coilovers.

I question your statement there. The midlink connects to the middle of the spindle. Therefore, wheel travel is related to spring travel geometrically by a simple ratio. Other than the effect due to the spring bending, which is a relatively small correction, the rate should be linear.

However mounting spring on the damper creates bending loads on the shock, increasing friction. This is one of the main performance advantages of moving the spring from damper.

In comparison, the midlink is designed to be loaded. Properly designed for the load, the actual amount of actual deflection will be insignificant.