posted on October 05, 2009 23:44
Project EVO X: Part 1 - Suspension
By Mike Kojima
What does the guy that has the Ultimate Streetcar do for a daily driver? Cheston Chiu is no stranger to building some really sick iron. His super clean, built to the hilt twin turbo 350Z is no show car. The Z placed second, missing first place by less than a point in Sport Compact Car Magazine’s Ultimate Street Car Contest and repeated the performance when the USCC torch was handed to Modified Magazine after SCC’s demise.
Since the Z is a bit extreme to be a daily driver, Cheston wanted something nice but not as boring as his Maxima beater. The car still had to have some potential for some nice modifications but nothing that would render the car impractical as a daily driver. After considering several different options, Cheston settled on a 2010 Mitsubishi EVO X. The biggest reason was the twin clutch TC-SST transmission.
The TC-SST transmission is an advanced transmission. It is actually a computer controlled manual transmission with electronically controlled twin clutches. What is cool about this system is that the next gear can be pre-selected for lightning fast seamless shifts--faster than humanly possible. The computer can control the shifting and clutch like an automatic transmission or you can manually shift via steering wheel mounted paddles. The clutches lock up solidly and there is no power-sucking, slipping, slush-box torque converter to waste power, nor are there inefficient power-dissipating planetary gears like a conventional automatic.
The TC-SST transmission has the comfort and the convenience of an automatic transmission with the efficiency and directness of a manual transmission. Although manual control is nice, the computer actually does a pretty good job of shifting. In sport mode, it hits the downshifts perfectly and holds gears when driving in the twisties just like you would yourself. The electronics rip off perfectly rev matched downshifts better than you can. After driving the car hard we feel that it might do pretty good if you let it do the shifting! Cheston liked the idea of a super automatic!
Cheston also liked many of the EVO X’s other advanced driver's aids such as Super All Wheel control that vectors torque around the chassis to help steer and stabilize the car as it does its thing, ABS brakes with electronic force distribution and active stability control that makes for a fast and stress free drive without compromising performance. Unlike many other performance cars, the electronic aids on the EVO X are relatively transparent to the driver, just the thing for a serious performance enthusiast.
|We had a pretty tough job in our quest to improve the EVO's suspension. The stock MR hardware features genuine Eibach springs with Bilstien struts calibrated to them--nice!
Cheston’s goal with project EVO X is rather complex. While ultimate performance is the goal, since the car will remain Cheston's daily driver, the car cannot simply be built for all out track day performance. It must be both fast and livable in daily use--a difficult challenge to be sure.
Thursday, October 01, 2009 8:56 AM
That teflon tape trick works brilliantly, btw. I use it everywhere I can where there's polyurethane - metal contact. Funny thing is that some of my friends still don't believe that it works. Their poly bushings all squeak and bind. Mine don't.
Thursday, October 01, 2009 9:06 AM
First post after being a long time reader of "beyond the dyno" which followed on to this blog. Instigated by cheap whiskey and crap at work today(which sortof involves valves), I felt compelled to post this.
You might want to re-clarify cavitation vs flashing. Cavitation involves the collapsing of bubbles.
With bubble formation, not sure if piston/flow velocity is high enough to choke the flow in a typical automobile shock, but if it does, suppose it'd cause major screwups with the chassis control.
Looking forward to more articles from motoiq.
Thursday, October 01, 2009 11:54 AM
It is cavitation and it makes a hell of a lot of foam in a shock. I have a test shock with a clear lexan body from my days as a chassis engineer. You can actually see the foam being created inside the valve packs, even at low speeds that you can by moving the rod by hand! Its possible to foam the entire fluid volume within seconds at ambient pressures with just hand pressure. This drops drastically with pressurization though, but on a shock dyno its easy to see at the pressure limits (about 30 psi) of my plastic shock.
30-50 Psi is about what the gas pressure in a typical twin tube is
The phenomena of caviation being an issue in automotive dampers is not opinion, its fact and the reason why there are things like gas shocks, floating separator pistons and remote reservoirs.
Monotubes and the V3's can run higher pressures and fight off foaming better.
Friday, October 02, 2009 3:45 AM
Thanks for the reply. Think you misunderstood me. Hope this clears up.
Agree than bubbles/foam will form when local pressure drops below vapor pressure(Pv) of the liquid. That is vaporizing/flashing. Boiling is not exactly the right word since the liquid is heated reducing the Pv.
IF the bubbles do collapse/implode when pressure climbs back beyond Pv, then it's cavitation. Cavitation damage is very severe.
Foam formation is a different issue.
Since hydraulic fluid is a mixture, and in some cases may contain air contaminant, foaming/bubbling from the pressure drop and other reasons are not to be unexpected. As you said, these bubbles/foam linger around, so it's not imploding instantaneously although pressure has risen over Pv, i.e. not cavitation.
Sorry if I appear anal about the choice of terms/words, since we are engineers, it only seems logical.
Friday, October 02, 2009 4:58 PM
Can you refer me to the source of the correct definition? I want to use it as a reference if I have to write something about this subject again.
Here is my understanding:
Cavitation is the formation of vapor bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure.
Hydrodynamic cavitation describes the process of vaporisation, bubble generation and bubble implosion which occurs in a flowing liquid as a result of a decrease and subsequent increase in pressure. Cavitation will only occur if the pressure declines to some point below the saturated vapor pressure of the liquid. In pipe systems, cavitation typically occurs either as the result of an increase in the kinetic energy (through an area constriction) or an increase in the pipe elevation.
Its not specific about the time to collapse or the ratio of immediate collapse to collapse a few seconds or factions of a second away from the bubbles creation.
Friday, October 02, 2009 5:07 PM
Searching some more, I have found many technical white papers about shock design referring to the phenomena I am describing in the article as cavitation as well.
Can you show me some supporting documentation regarding your points? Not trying to be a smart ass, just curious.
In my clear test shock you can clearly see the bubbles and foam being generated by the valves in the moving piston, the piston trails streams of bubbles and they are being created by the washer stack and orifices. It simply not foaming air inside the shock. The shock starts with no air in the damping fluid, its a decarbon type monotube with a floating separator piston.
Friday, October 02, 2009 11:17 PM
Got an email address to send stuff and continue discussion? This is straying slightly off topic from the article (suspension vs flow/fluids).
Mine is email@example.com (dont laugh).
Friday, October 02, 2009 11:47 PM
Big swaybars with stock endlinks?
Saturday, October 03, 2009 8:17 AM
The stock endlinks on the EVO X are spherical, no rubber!
Saturday, October 03, 2009 8:19 AM
I think our readers might find this discussion interesting, its one of the reasons why we have this comments area. I think this is interesting, I am hoping to lean something!
Saturday, October 03, 2009 12:20 PM
Ok, this is going to make me feel like my ex yapping non-stop, but here goes.
I see that you've turned to wiki for your definition of cavitation. It is commonly accepted that "bubble formation" is cavitation since it comes from the word cavity which sort of imply voids in the liquid. I'm not exactly sure if that is technically correct, but from my limited work experience with flow instruments, bubble/foam formation can be cause by many things, and cannot be simply called "cavitation".
In the control and process industry, it is important to differentiate these different types of phenomenon.
Allow me to quote a Shell guideline on selection of valves. I have to link pictures because I dont know how to host a pdf, nor I'm comfortable posting the whole thing to a public domain because they will sue my pants off.
For simplicity sake, lets imagine a pure liquid which is not a mixture. Water is a good example.
First, we have flashing, or plain vaporization.
Then, after the liquid flashed, IF surrounding pressure is higher than Pv, the water vapor (which is in bubble form now) collapses.
Here's another source talking about the difference between flashing and cavitation.
In a shock absorber, you have the pressure drop and subsequent recovery, and in your case, you see bubbles. It seems natural to assume cavitation is happening since all pre-requisites are met. The question now is, why are the bubbles/foam lingering? Cavitation says that the bubbles need to collapse.
If you fill your shock absorber completely with pure water (without dissolved air or any impurities), I dare to bet that even if you produce bubbles, they will collapse almost instantaneously and you will not see foaming. That is cavitation.
So far so good for pure liquids which have a very precise cutoff where/when things happen. But in a mixture, things are different as different components have different properties.
When the pressure of a mixture drops, a few things can happen.
-First, dissolved air, gases, or low density stuff might free themselves from the liquid (I dont know the proper term) something like when you shake/stir a glass of cola. These bubbles are very different from the vapor bubbles, and might float or stay intact in the fluid.
-Then some parts of the mixture with high Pv might flash into vapor bubbles.
When the pressure recovers, the vapor bubbles collapse. The other bubbles might remain and linger depending on liquid viscosity/surface tension and other bubble related properties.
Now, we know hydraulic fluids are a mixture of stuff. Sometimes water is even in the mix, and air contamination of hydraulic fluids is not uncommon, irregardless of whether it's in a closed circuit or not. Since Pv for hydraulic fluids chosen for such applications are very low, it stands to reason that only the first portion is happening.
Sidenote - "On some actuators for valves that I work on, when filling in the hydraulic fluid, after bleeding, we run it through a machine that cycles and removes the entrained gases (air). Even in a closed circuit with high pressure, there's still much bubbles to be seen and removed. Do you guys have the same thing for shocks?"
Pdf on some hydraulic fluids with properties, most of it have really low Pv in the region of <0.1mmHg.
If you know exactly which fluid you are working with, there's bound to be a MSDS (material safety data sheet) for it with Pv stated inside. Quick google will show that the a low Pv for hydraulic fluids is a highly desired quality (special applications not withstanding).
Other important qualities include air-release and anti-foaming tendencies which ultimately determine how much bubbles/foam you see. They've got guidelines to measure that I suppose, but I'm not a hydraulics guy.
Based on the above points, and the few below, I do not agree that it is cavitation in a shock, but just simply bubbling/foaming.
-The bubbles do not implode but rather linger as described in your reply
-most shock manufacturers should know how to choose the proper oil (with regards to Pv) based on the type of pressures expected
-there is no perfect hydraulic fluid
-if it was really cavitation, I doubt any shock absorber would survive. Google cavitation damage.
It is however arguable that a small small small portion of it is really cavitating after foam formation, but i doubt it. Some may argue that the bubbles will take up space in the valve orifice so liquid velocity have to further increase to meet a certain flow rate (i.e. piston speed in a shock), but that only happens up to a limit before the flow is choked, and in the case of a automotive shock, very likely damage something.
All this aside, it could be that "cavitation" is the accepted word for "forming a void", so all the above would be irrelevant. Really depends on how one defines the word I guess.
Once again, my apologies for the long winded post. Please feel free to comment on any section where you are in disagreement.
Saturday, October 03, 2009 4:51 PM
I find this to be very interesting and I think many of this articles readers do as well so there is no need to apologize.
When considering the definition of instant recovery, is that totally independent of the viscosity of the fluid?
So what I might be seeing is the release of dissolved gasses in the fluid? When fooling around with my test shock I have noted that you can dissolve the gas in the fluid with pressure. I have purposely left some air in the shock and pressurized it to see what would happen as sure enough the air dissolves into the fluid at around 40 psi or so.
I have called up a few of my damper engineer friends (since I don't design dampers, just involved with set up and calibration) that they have told me that the phenomena is caviation.
I asked them about your points and they said that its cavitation, but I have to research some more and look at your references in detail, I am thinking that you are correct about entrapped gasses being released and thats what I have observed in the past.
Sunday, October 04, 2009 12:57 PM
I love the teflon tape on the sway bar, that's just brilliant.
The Motorsports Engineering department at Colorado State had a lexan monotube damper which was run on the school's Roehrig for the chassis dynamics and racecar suspension classes. Cavitation was plainly visible during high-speed runs, and the effect of adjusting gas pressure was easily demonstrated. I wish I had a video of it. I know when Claude Rouelle visited, he always had a similar video. I'll see if a buddy who interned with him has a copy.
Sunday, October 04, 2009 3:15 PM
Was it made using Bilstien internals? That's the part I have. I used it to demonstrate the phenomena in Sport Compact Car Magazine.
Sunday, October 04, 2009 4:10 PM
I'm not sure who made the unit, but we used Bilstein internals. It was a monotube, and low-speed was adjusted with a needle valve while high speed was controlled with shim stacks. Of course, that's how most monotubes are. We adjusted the gas pressure, as well as altered the shim stack components and recorded changes on the dyno. The clear case allowed cavitation to be seen behind the piston's stroke under various conditions. It was pretty cool, and easy to work on.
Monday, October 05, 2009 10:15 AM
I'd imagine pressure transmission in any liquid to be "relatively" instantaneous. Speed of sound in liquids is super fast, and sound is nothing more than pressure waves. Since viscosity, density, compressibility all interrelate, it would seem that viscosity can play a part in the speed of sound/pressurewaves, and hence bubble implosion, however, I doubt it'd cause any significant delays. You wouldnt put compressible bouncy and slow reacting fluid into your shocks anyways, would you?
I did some goggling and it seems that for shocks (and many other things as well), the word "cavitation" is just simply used when bubbles/foams are produced with no mention if bubbles are collapsing or lingering.
Another word that pops up often alongside foaming of hydraulic oil is "aeration" which sounds more accurate to me.
This again brings us back to the nomenclature issue. I'm really starting to think this is just as simple as a definition problem.
Dear Will, do you have a picture of the "cavitation" bubble stream/pattern? How do they look like compared to the propeller cavitation videos found on youtube?
Monday, October 05, 2009 10:57 AM
I will try to get some pics using my test shock later in the week. Keep up the interesting posts!
What you see is a cascade of bubbles emanating from the piston. At ambient pressure they hang around for a few seconds but if the shock is stroked rapidly, the entire fluid volume becomes foamed and will take about 20 seconds to clear.
As the shock is pressurized, the bubbles almost instantaneously collapse to where they are only visible for a fraction of a second. At 40 psi or so, you don't see them anymore.
Wednesday, October 07, 2009 6:44 AM
Asked about this issue with the hydraulic guy in the company, and it seems I'm half wrong. Cavitation CAN still happen in a hydraulic system, but it does not contribute to the foaming. The bubbles from cavitation will still collapse just outside the orifice and cause damage. This however is rare as there are easy ways to solve (i.e. pressurise the system and take care of velocities), and there must be a major screwup somewhere for an engineered system to cavitate.
The reason you notice less bubbling and thus foam with higher tube pressure is contributed by two factors.
One, the higher pressure forces the air(foam) to dissolve back into the liquid more readily.
Two, higher pressures mean that dissolved and entrained air have a hard time "escaping"
Other than these two, naturally there are other properties of the oil that affect the release/absorption rate of air.
Simplest solution seems to be to remove the air, but it can be limited by the design of the circuit or the availability of such a device. (See sidenote in my 4th post on the 4th of Oct)
Two links for your quick reference
An example given by the guy is to shake a coke bottle and then when the bottle is full of bubbles, squeeze it so that pressure increases further and note the difference in time for bubbles to dissolve back into the liquid compared to one that is just left there after shaking...i think...
His talk made sense to me hence why I'm relaying it here.
Thursday, October 15, 2009 9:36 AM
trust me mike the modification is really an art of science.so much to learn nuff respect mike always wish i was around to learn more
Tuesday, October 20, 2009 5:02 PM
No one noticed the mistake I made on the roll center height? Its fixed!
Friday, October 30, 2009 8:55 AM
Hi Mike. Great article.
As mentioned you do need stronger endlinks. Those who have tracked their Evo Xs with a lowered suspension have snapped the stock endlinks in half. some have made their own custom endlinks. The stock endlinks are weak and cannot handle the stiffer bars.
You will also need to upgrade the tranny internals on the SST before you track the car. At high speed tracks, the tranny will overheat and go into limp mode. This is a well known problem on the SST. Some have ysed tranny coolers to remedy the problem, but some are saying that this is not enough. Rebuilding the internals is needed. I would never buy the SST for this reason. Mitsu needs to do a better job on this tranny before it becomes track worthy.
Friday, October 30, 2009 9:08 AM
We know about this and are working on it. The endlinks have survived a couple of track events but we are watching them closely.
Friday, October 30, 2009 12:44 PM
How did the tranny hold up on the track?
Friday, October 30, 2009 4:29 PM
Limp mode after half a session, just stock with an air scoop. Bigger cooler coming.
Monday, November 02, 2009 12:43 AM
Hey Mike, what did you guys do to reach the rear rebound adjustors? Did you just leave one of the knobs on or something? I had KW V3s on my car and could never get to the rebound adjustors. (Mine were the clubsports and I could not get them to ride well on the street.) I switched to the A'pexi S1 dampers and they work pretty decently, except that the rear springs are way too soft. (8kg/mm front/6kg/mm rear)
Monday, November 02, 2009 1:08 AM
I wasn't paying attention honestly, its Cheston's car! Good to see you here Kit.
Saturday, November 28, 2009 8:17 PM
Are Those KW V3's regular or clubsport?
Saturday, November 28, 2009 10:25 PM
Tuesday, December 01, 2009 8:01 PM
how are they compared to stock?
Tuesday, December 01, 2009 8:09 PM
They ride smoother in some respects than stock even with higher spring rate due to digressive valving. The calibration was developed on KW's six post shaker rig.
Wednesday, December 09, 2009 11:36 PM
what is the real difference of the standard and clubsport? Is it the same shock but different springs? would you happen to know the front and rear compression for both?
Thursday, December 10, 2009 6:44 AM
Different spring rates and damping.
Sunday, December 27, 2009 7:01 PM
I'm set on the KW coilovers. Would you guys recommend the regular factory KW V3's for around $2k or is it really worth spending the extra $400 for the Robispec Street KW v3's? My X is a daily driven car with weekend canyon carving.
Sunday, December 27, 2009 7:56 PM
I like the Robispec parts, he changes the foot valve for a more digressive compression curve.
Sunday, December 27, 2009 10:05 PM
Mike, I don't understand what that really means.
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