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Last Post 02 Mar 2013 12:07 PM by  TeamRX8
Help me fix my under steer!
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ratt_finkel
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29 Jan 2013 03:24 PM

    Hopefully one of your guru's would be willing to help me get to the bottom of this. I had a chance to drive a similiarrly prepped Evo mid-last year. It was one of the most well balanced, neutral and easy to drive cars I have ever had the pleasure of driving in. It was at that point I knew there was something terribly amiss in my setup. Here are the stages I've gone through in an effort to get my car there.

    Original setup - 2011

    Stock sway bars

    600f/800r spring rates.

    18x9.5” wheels.

    Excellent rotation. Good toss ability and point ability. Average to good in transitions.

    Worked great right up until Lincoln Spring events where the soft rates and ride height had us bouncing off the bump stops and rolling over like a stock car.

    Stage two. - 2011

    Up rates to 750f/900r

    Revise ride height to mimic other SP evo’s. With some slight rear rake.

    18x10.5” wheels.

    Rotation reduced. Toss ability Reduced. Point ability increased. Good to excellent in transitions.

    Overall the car felt way more planted. But at nats had a terminal under steer that resulted in decreased performance.

    Stage three – 2012

    Up rates to 900f/1000r

    Upgrade front and rear bars to stiffer units.

    Change front camber from -3.0 to -3.5 degrees camber.

    Change rear camber from -1.5 to -2 degrees.

    No push. No Toss ability. Excellent point ability, but washes out past turn in. Superb in transitions.

    Still very planted. But doesn’t want to do anything more than go straight.

    Stage 3.5

    Replaced front helical diff with Quaiffe unit.

    Rebuilt rear diff.

    Still no help.

    Final Stage

    Did test event at local venue with the 9.5” wheels and 275/35/18 Continental Cup tires and it transformed the car. It was all of the good and none of the bad.

    Car was still not as compliant as I’m shooting for. But had the overall balance I was looking for.

    I have dyno'd my shocks VS the ones on the other car. And by the numbers I have vastly better valving, range, everything. But still feel like my car lacks the compliance. And still doesn't have that smooth mid-corner balance we all dream of.


    Thanks!


    oinojo
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    29 Jan 2013 03:59 PM

    Sure, The Evo 9 has a front motio ratio that is close to 1:1, I think it is 0.98. So as you kept increasing the spring rates, the more push you introduced into the equation since the rear wheel rate is not increasing the same rate. The rear suspension is Multi-link and so its actual wheel rate is lesser than you think. I'd say leave the 1000 in the rear and try your 750's up front if you are not bottoming out.Mid-corner balance is tuned with swaybars and corner exit performance can be controlled using rear ride height. Adjust the swaybar until you have decent mid-corner performance and raise the rear until you start to get power oversteer from the rear.

    - Jonathan Lugod

    IntegraR0064
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    29 Jan 2013 04:24 PM

    ^what Jonathan said. I can't quickly find the MR for the rear, but I did see the Evo X has a 0.84 motion ratio in rear, so just to illustrate assuming you had an Evo X and assuming MR of 1 in front due to Macpherson struts the wheel rates would be:

    600/800: 600/565
    750/900: 750/635
    900/1000: 900/706


    Notice how in each iteration you increased the front rate relative to the rear....and thus more understeer.

    Also you didn't mention exactly what you did with the bars, and toe is big factor as well. But just the spring rates alone would do it.
    EricH
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    29 Jan 2013 05:01 PM
    I'm not an EVO guy, but looking at your info you posted it looks to me like your front to rear roll stiffness ratio is out of whack. Just based on spring rates alone, you started at 600/800 and now you are at 900/1000.

    And you said the car is understeering. So, try running softer front springs.

    As for negative camber, that should be dictated by temps using a probe type pyrometer, but -3.5 in the front sounds about right for a heavy car on R-comps.
    ACM
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    29 Jan 2013 05:05 PM
    Who rebuilt the rear diff - when did you last rebuild the rear diff prior to the most recent rebuild ?

    How did the grip from the Cup tyres compare to whatever you usually run ?
    ratt_finkel
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    29 Jan 2013 05:52 PM
    oinojo wrote:

    Sure, The Evo 9 has a front motio ratio that is close to 1:1, I think it is 0.98. So as you kept increasing the spring rates, the more push you introduced into the equation since the rear wheel rate is not increasing the same rate. The rear suspension is Multi-link and so its actual wheel rate is lesser than you think. I'd say leave the 1000 in the rear and try your 750's up front if you are not bottoming out.Mid-corner balance is tuned with swaybars and corner exit performance can be controlled using rear ride height. Adjust the swaybar until you have decent mid-corner performance and raise the rear until you start to get power oversteer from the rear.

    - Jonathan Lugod

    I was originally increasing front rates to counteract body roll. And the rear then as you said to counteract the difference in MR. I don't think the car would bottom out on anything but mabye 600's. But I could always raise the front. It's not exactly low now. That's an interesting comment about the bars. The front is a non adjustable Tanabe. The rear is in the middle on my Hotchkiss Bar. I can get power oversteer now. But it doesn't come easy.

    IntegraR0064 wrote:

    ^what Jonathan said. I can't quickly find the MR for the rear, but I did see the Evo X has a 0.84 motion ratio in rear, so just to illustrate assuming you had an Evo X and assuming MR of 1 in front due to Macpherson struts the wheel rates would be:

    600/800: 600/565
    750/900: 750/635
    900/1000: 900/706


    Notice how in each iteration you increased the front rate relative to the rear....and thus more understeer.

    Also you didn't mention exactly what you did with the bars, and toe is big factor as well. But just the spring rates alone would do it.

    Currently Running zero toe everywhere.

    js154 wrote:
    I'm not an EVO guy, but looking at your info you posted it looks to me like your front to rear roll stiffness ratio is out of whack. Just based on spring rates alone, you started at 600/800 and now you are at 900/1000.

    And you said the car is understeering. So, try running softer front springs.

    As for negative camber, that should be dictated by temps using a probe type pyrometer, but -3.5 in the front sounds about right for a heavy car on R-comps.

    If my real roll stiffness was out of whack, wouldn't the car be tail happy? We ran softer springs and still had the issues. I think ultimately the problem begins in the rear.

    -3 to -3.5 seems to be about right for this car. I went up to -3.5 based on pics of the car loaded up.

    ACM wrote:
    Who rebuilt the rear diff - when did you last rebuild the rear diff prior to the most recent rebuild ?

    How did the grip from the Cup tyres compare to whatever you usually run ?

    Shep rebuilt the diff. The last time it was touched was shortly before I bought the car. I had it rebuilt because the car was behaving like a stock rear diff. Leading me to believe it may be that. But now that we know it's good I'm moving on.

    The grand am cup tires are supposedly rebadged R6's. These examples were not new. But I would say they felt like R6's. Just took more heat to get going. The interesting thing on that experiment was different sized wheels. Or more notably different offset. These wheels give the car significantly narrower track width. especially in the rear.Submit

    ACM
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    29 Jan 2013 06:17 PM
    Rats - I was hoping it would be someone other than Shep or TRE :-)

    Did the rebuild elicit any change in the car ? If it wasn't a Shep/TRE build beforehand the change should have been significant. I'm guessing the switch from the factory helical to Quaife had no effect.

    Have you tried different ACD settings to see how they affect the car - that may help point you in a direction ?

    For each of the listed stages did you make all the changes listed at the same time or did you step through each change a couple of events at a time ?

    Davidss
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    Posts:181


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    29 Jan 2013 06:20 PM
    Going from 600/800 to 900/100 would make the car pushier/less loose.

    Before you spend money on anthing else I would try turning up the rear bar to the max and disconnecting one of the front endlinks. What is the stiffest rear bar you can run? You want to increase the amount of weight transfer at the rear compared to the front up until the point that the car starts to pick up a wheel in the rear. Can you run a speedway type splined swaybar?
    IntegraR0064
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    29 Jan 2013 06:35 PM
    Given how it was described, I don't think the diff has anything to do with it. He changed that after it already was pushing.

    To maintain the same spring balance as you had in the beginning and keep the 900 lb front springs you'll need 1200 lb/in rear springs. That's why you're understeering. I'd try that. Or drop your fronts to the 750 springs in front (if you still have them) and keep the 1000's in the rear.

    Again this is all assuming that 0.84 motion ratio, not sure if that's exactly right. All of this is generic advice, I don't know Evos.


    That'll at least get your spring balance back to what it was in the beginning.
    mitchman
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    30 Jan 2013 09:28 AM
    Another vote for changing your spring rate (softer in the front and/or stiffer in the rear). I agree with Davidss and others.
    solo-x
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    30 Jan 2013 09:37 AM
    If you have a test day, take what you have now and just dial out some rear camber and dial in some rear toe out. It'll get loose, I promise. ;)
    SVT199
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    30 Jan 2013 11:36 AM
    I think for Solo that the higher rate springs are hurting you. If you want the smooth mid corner feel you will need some compliance rather then relying on the tire side wall to do all the work. Also if the car has no roll I feel that the higher camber in the front negatively affects entry and transitioning into mid corner grip and predictability because the contact patch takes longer to achieve maximum foot print. With smoother road race conditions the stiffer works because everything is slower and longer. The constant transitioning of Solo needs to maximize grip fast to work and getting the tire flat faster is better for grip and smoothness front to rear. I also don't like masking handling issues with rear bar stiffening but that is just my opinion, letting the suspension work "independently" with less bar and proper rate springs will help, especially over bumps.
    Davidss
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    30 Jan 2013 12:16 PM
    SVT199 wrote:
    I also don't like masking handling issues with rear bar stiffening but that is just my opinion, letting the suspension work "independently" with less bar and proper rate springs will help, especially over bumps.

    In many situations I would agree but remember we are talking about something that is mostly a FWD car.

    oinojo
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    30 Jan 2013 01:50 PM
    Ideally, the spring rates overall need to lowered to regain some compliance that he is looking for. Jeremy, under throttle tip-in the weight transfer will mainly go towards the rear of the car because of your wheel rates. While this is a good approach for putting power down on a RWD car, I'm not sure how it would apply on the AWD. Most AWD car's I've driven drive like a FWD. Remember we are talking about wheel rates. IMO anything more than 500 in/lbs of wheel rate is too much for autocross as it would need significant aero to make it work well.
    RedEvo8
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    30 Jan 2013 04:56 PM

    The primary downside of going with a softer front spring -- is the 60% front weight distribution of a 3000 pound car on r-comps. I went to a 900lb spring in the front to fix this problem. The picture below was last April. Now it's much flatter and I think it might even be faster. Would you be shocked if I told you it was a 850lb spring in the rear?




    ratt_finkel
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    31 Jan 2013 11:55 AM
    I have decided to do a test and tune with the car. Hopefully that will get this enigma figured out.
    rotis
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    31 Jan 2013 12:30 PM
    For the test and tune make sure you have a plan that has some math behind it. The number one thing is to think about ride stiffness in terms of NF, not spring rate. Start with steady-state, a skidpad would be ideal and also gives the opportunity to optimize camber and tire pressures if you have access to a pyrometer. Tuning suspension is fun, wheee!
    ratt_finkel
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    31 Jan 2013 01:12 PM
    rotis wrote:
    For the test and tune make sure you have a plan that has some math behind it. The number one thing is to think about ride stiffness in terms of NF, not spring rate. Start with steady-state, a skidpad would be ideal and also gives the opportunity to optimize camber and tire pressures if you have access to a pyrometer. Tuning suspension is fun, wheee!

    There will be lots of experts there to help things go properly.

    My plan is start with some toe out in the rear. Then reduce the neg camber to -1.5. I have no doubt this will aid in rotation. But I am skeptical it will give me the balance I am looking for. And does nothing for the harshness. Next I'll work on sway bar settings. And last we will start with spring rates. If things go as planned, I may be able to do some shock revalving also. Tire pressure will go last. And eventually I'll have to see how this all effects front grip. Which at this point we don't eve know if it's optimized LOL!

    TeamRX8
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    01 Feb 2013 02:21 AM
    sticking my neck out here, but you might want to consider not driving in so deep .... :-/

    Otherwise agree with rotis, start steady state and work from there is the smart plan of attack i.e. good turn in and washes out is screaming the obvious at you - spring/bar balance is likely off. You're not in a stock class so tricks like running less than optimum camber and such are just kluges that will have negative consequences in certain other scenarios

    .

    RichJones07
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    11 Feb 2013 02:01 AM
    FYI, I believe the motion ratios for the Evo 8 and 9 are 0.91 front and 0.73 rear.
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