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Thread: Wavetrac Differential

  1. #21
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    Quote Originally Posted by Tyler Jones View Post

    They have already tried most configurations for arbs, including stiff arb front and no arb rear.
    This is what sticks out to me. If they have tried big changes to front & rear roll stiffness (I don't know what values you consider "stiff" and "soft") but the chassis doesn't respond to it, it's an indicator that your chassis' torsional stiffness is too low. Could be a global stiffness issue or it could be a local stiffness problem right at the bellcranks/bellcrank posts. You said 1800 ft*lb/deg was your torsional stiffness from FEA. If that includes the whole chassis & suspension assembly (hub-to-hub stiffness in other words) then it is probably acceptable, but if that's "frame only" then I'd consider it a bit more soft than desirable. If your analysis was over-constrained in any way to inflate your results, or any errors in welding/assembly could mean you're even further away from a good target.

    I'd recommend doing a physical test of your hub-to-hub torsional stiffness. Or an alternative that may be easier to set up, you can measure your wheel rates on scales - if they're significantly softer than what your wheel rate calculations would predict, you've most likely got problems with installation stiffness of your belcranks.
    Last edited by JT A.; 07-16-2015 at 04:49 PM.

  2. #22
    Quote Originally Posted by Tyler Jones View Post
    I think some of the focus shifted on to figuring out the suspension issue. Which while I appreciate it, is not exactly why we need a different differential. I do not think it is a "band-aid fix". While I see the point that the suspension is causing an issue, I don't see how a differential that will handicap the car in odd situations is acceptable. A different differential isn't the solution to this problem, its a solution to a few different problems. One of which, is the diff is currently untuneable.
    Well I don't see how you can justify spending x-hundred of your sponsor's hard earned dollars when you: a) haven't even attempted to determine and address the root cause of the issue, and b) can't quantify (or even make estimates of) the gains fixing this 'issue' will bring you.

    Your chassis setup causes the 'unloading of your rear inside wheel'. You described the 'unloading of your rear inside wheel' as an issue and the main reason for wanting to buy a new diff in your very first post. We are trying to help you cure this issue, but you seem to have no interest in this, and have it in your mind that you just have to have a new diff. If you are not interested in curing the issue, and are actually only interested in having a tuneable diff you can mess about with, or you just want to piss away money, don't try and gain support for this approach under the pretense of trying to objectively solve a problem.

    Quote Originally Posted by Tyler Jones View Post
    I have a few hunches to our wheel lifting issues. One of which is an overall roll gradient vs wheel travel issue. The car was allowed to roll quite a bit in the last cars, and I think it rolls more than calculated. Which is easy to prove in this car, but I haven't had a chance to yet." - Suspension guy 2
    Curious reasoning. I don't see how excess body roll in isolation can be the root cause of lifting a wheel. My VW Polo rolls a shed-load but doesn't jack a wheel. Therefore roll gradient alone is not the metric that will explain this behaviour.

    Quote Originally Posted by Tyler Jones View Post
    We have scales and the car is on them frequently for testing. I don't have a data sheet with me to record actual corner weights.
    ????? Why post this?? Don't you have budget for pen and paper??

    Quote Originally Posted by Tyler Jones View Post
    I guess I will have to track down the rest of those numbers myself when I find the time.
    ????

    Quote Originally Posted by Tyler Jones View Post
    Steering Geometry: Look at the design tech sheet. Those are all "designed" numbers
    ?????

    Quote Originally Posted by Tyler Jones View Post
    Ignore the ARBs and do not consider them into wheel rates.
    Just how exactly should we be quantifiably considering the varying stiffnesses of ARB's, then???

    Quote Originally Posted by Tyler Jones View Post
    Wheel Rate: 20n/mm seems low, maybe a conversion error? Check the tech sheet we turned in for lincoln, that will give you a closer estimate.
    "Wheel rates are about 112 lbs/in on all corners" is what you posted on Page 1. Why don't you check the 'conversion error'??

    Quote Originally Posted by Tyler Jones View Post
    CG height: 10.2 measured without wings, 10.5" with wings
    said the cg raises about 1.5 in with wings
    There is inconsistency between what your 2 suspension guys are saying. Why aren't they aligned?? Are they not on the same team?

    Quote Originally Posted by Tyler Jones View Post
    "I think our actual weight dist. was 49%, -Suspension guy 1
    Static weight distribution: 45% front actual - Suspension guy 2
    I appreciate the diligence in relaying the info as you receive it Tyler, but come on! Weight distribution is what it is, it is not open to interpretation, this value shouldn't vary depending on source! Measure it!
    _____________________

    Some constructive notes:

    Quote Originally Posted by Tyler Jones View Post
    Also we haven't explored 100% of the bar options, there are some left which could likely fix the problem by adding a lot of roll stiffness, but the car would probably understeer a ton (since we'd be adding most of that to the front).
    The theory implies that more front bar/springs than rear gives more understeer. The theory more specifically says that a lateral load transfer distribution is what adjusts your balance. Lots of practical issues can interfere here, so that adjusting springs and bar do not actually change your LLTD as you might expect. These practical issues can be extremely significant, to the point you may not believe. Understanding what the theory should tell you is one thing, but don't be blind to what you are actually seeing, even if it disagrees with your current level of understanding of the theory.

    As many have already stated, compliance if a practical consideration that can make basic spring/bar LLTD predictions redundant. If your chassis is too soft, stiffening front springs / bars won't actually adjust your lateral load transfer distribution. Have you verified that adding lots of front spring and front bar actually gives your car excessive terminal understeer? Try it. If it doesn't, your chassis is too soft, and all those LLTD predictions you are taking from Milliken equations under the assumption that your chassis is rigid are meaningless and far from the truth.

    Quote Originally Posted by Tyler Jones View Post
    The suspension does not have bushings and does not carry much friction at all. Essentially is negligible for most calculations we do
    More friction on one axle than the other will also take your car far away from the LLTD predictions you have based solely on spring/bar stiffness. I am curious to know how you are so confident that you have negligible levels of friction. I don't just mean control arm friction when unloaded, I mean net system friction when loaded, including your damper seals. Total system levels can add up to be quite significant. Even if you can see your suspension actuating, your friction levels may be considerably higher on one axle than another, contributing to an unexpected balance difference.

    Quote Originally Posted by Tyler Jones View Post
    From what I've been learning, I'm pretty suspicious of our roll centers and how they work with the rest of the car
    Yes. This is as Tim said, and is a possible cause. Why not explore it.

    Quote Originally Posted by Tyler Jones View Post
    how the travel available is mostly for compression.
    As Z suggested to you, be careful you are not hitting rebound stops (over-extending your damper). Once you reach the level of lat acc that causes on one of your inside dampers to full extend, you will see full, immediate load transfer across this axle. If this is happening on the rear axle of your car, not only will it completely reverse the base chassis 'understeer' balance you believe you have, but it could also very well contribute to lifting a wheel..

    Quote Originally Posted by Tyler Jones View Post
    I don't remember our exact lateral g's but our best skidpad time was 5.381s. From our data acquisition we have seen up to 2.1g's during Endurance. I drove the skidpad event at Formula North and from what I could tell (I'm by no means a professional driver) the car wanted to over-rotate at the limit. The car doesn't appear to be jacking at all during steady-state. The time we noticed the problem most is the hairpin at the end of the slalom at the FSAE Lincoln Endurance course.
    If you reach 2.1 g then your issue is not big. What kind of % reduction in lap time do you really believe curing this issue will bring to the team? Will that % be worth the amount of money you seem set on spending?

    If you depart with oversteer on the skidpad, adding more torque to your outside wheel will not increase cornering capacity, so again a new diff will not benefit you here. Also, check that the terminal oversteer you appear to be seeing actually aligns with your predictions for steady-state behaviour. Why don't you have limit understeer? Is what you see versus what you expect acceptable?

    Hairpins are usually the most steady-state of all the FSAE endurance event corners. What exactly do you think it is about the hairpin that is different to the skidpad that might help to explain why you see this inside wheel spin at the hairpin but not at the skidpad? Does the jacking only occur during turn-in at the hairpin? If so, why are you worried about getting torque down here, again, consider how you will actually be faster by removing this symptom.
    Last edited by CWA; 07-16-2015 at 06:29 PM.

  3. #23
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    Tyler,

    Based on your last few posts I reckon I now know your root problem(s).

    A car a few years ago at FSAE-Oz had something similar. It was a DISASTER! From memory, it finished top five. Don't take this the wrong way, but poorly built and poorly set-up cars can still do well in FSAE competitions. You are only competing against other students. You only have to drive 20 miles at an average speed of ~35 mph to finish top of the ladder...

    Anyway, some of the above clues stick out to me like a sore thumb. I would be more certain if I saw the car in the flesh. Even some photos would make it easier to know for sure.

    But (!), the above mechanical details are not your MAIN problem. Your biggest problems include these:

    * Your Suspension-Guys are incapable of instantly quoting the most important numbers in their area of responsibility. This is BAD. (What happens when they have to remember birthdays, and wedding-anniversaries, etc.!)

    * Too many theoretical numbers, too few measured numbers. Given the typically overconstrained FEA analyses of frames (as mentioned by CWA [Edit: Oops ...by JT A.]), I take your quoted frame torsional stiffness as MEANINGLESS. (BTW, once you have made the few simple bits that are needed, a hub-to-hub torsional test should take about ten minutes. Every initiate Team-member should be required to do such a test, along with all the other important measurements, before they are accepted as official Team-members.)

    * You are chasing a complicated solution of a "tuneable" diff, even though your Team is incapable of tuning what they already have in front of them! A tuneable diff will simply add one more riddle to your already long list of unsolved puzzles. (Yep, I know of one mid-field Oz-Team that bought a Drexler a few years back (at ~Aus$4K!) because of "spinning inside-rear". They are still mid-field, the inside-rear still spins, and the Drexler has never been opened ... because no one knows how to adjust it!)

    Decide whether to go with an open or locked (=spool) diff, then adjust the rest of the car to suit.

    Oh, and it is perfectly acceptable to "look over the shoulders" of the winning Teams. This is called "researching the prior-art". For example, my records show that 1st*, 2nd, 3rd, 4th, and 5th at last year's FSAE-Oz-14 all ran spools. Now, what's the name of that Team that recently won the big US Michigan-15 comp...?

    (*Edit: Oops! NOT 1st (Monash had a Drexler), but the others all had spools. And so does that Team that keeps winning all the Northern hemisphere comps...)

    Z
    Last edited by Z; 08-16-2015 at 10:24 PM. Reason: Oops...

  4. #24
    Quote Originally Posted by CWA View Post
    Well I don't see how you can justify spending x-hundred of your sponsor's hard earned dollars when you: a) haven't even attempted to determine and address the root cause of the issue, and b) can't quantify (or even make estimates of) the gains fixing this 'issue' will bring you.

    Your chassis setup causes the 'unloading of your rear inside wheel'. You described the 'unloading of your rear inside wheel' as an issue and the main reason for wanting to buy a new diff in your very first post. We are trying to help you cure this issue, but you seem to have no interest in this, and have it in your mind that you just have to have a new diff. If you are not interested in curing the issue, and are actually only interested in having a tuneable diff you can mess about with, or you just want to piss away money, don't try and gain support for this approach under the pretense of trying to objectively solve a problem.
    The initial purpose of the thread was just to obtain some technical information about the Wavetrac, not to ask for suspension advice. This is why I'm appearing biased, I was already decided on switching to the Wavetrac if I could obtain the necessary drawings and info. I'm also not very knowledgeable about vehicle dynamics, so I can't comment much myself. I will try to get my suspension guys to respond more directly to your questions, as they are the ones against keeping our current differential.

    Quote Originally Posted by CWA View Post

    ????? Why post this?? Don't you have budget for pen and paper??
    We are not all in direct proximity to the car at all times, I will take the time to try to get corner weights for the suggested scenarios this weekend.

    Quote Originally Posted by CWA View Post

    ????
    My suspension guys apparently don't have the time to get some of the numbers you want so I will have to try and find some of them myself, most should be on spreadsheets on our network harddrive.

    Quote Originally Posted by CWA View Post

    Just how exactly should we be quantifiably considering the varying stiffnesses of ARB's, then???
    I'm not sure they fully understand how ARB's contribute to overall wheel rates.

    Quote Originally Posted by CWA View Post

    "Wheel rates are about 112 lbs/in on all corners" is what you posted on Page 1. Why don't you check the 'conversion error'??
    According to that number, 20n/mm is accurate. Not sure where he is coming from.

    Quote Originally Posted by CWA View Post

    There is inconsistency between what your 2 suspension guys are saying. Why aren't they aligned?? Are they not on the same team?
    They don't work directly together as much as I think they should. This is more a team organization issue which I will try to resolve this coming competition year.

    Quote Originally Posted by CWA View Post

    I appreciate the diligence in relaying the info as you receive it Tyler, but come on! Weight distribution is what it is, it is not open to interpretation, this value shouldn't vary depending on source! Measure it!
    I stated before that before Formula North the car weighed in at 45% front and at Lincoln the car weighed in at 49% front. Ride heights and aero were adjusted between those two competitions so that may be contributing to the difference. Again, they don't work directly together as often as they should. The whole team has some communication issues I'm trying to resolve.

    Quote Originally Posted by CWA View Post
    _____________________

    Some constructive notes:



    The theory implies that more front bar/springs than rear gives more understeer. The theory more specifically says that a lateral load transfer distribution is what adjusts your balance. Lots of practical issues can interfere here, so that adjusting springs and bar do not actually change your LLTD as you might expect. These practical issues can be extremely significant, to the point you may not believe. Understanding what the theory should tell you is one thing, but don't be blind to what you are actually seeing, even if it disagrees with your current level of understanding of the theory.

    As many have already stated, compliance if a practical consideration that can make basic spring/bar LLTD predictions redundant. If your chassis is too soft, stiffening front springs / bars won't actually adjust your lateral load transfer distribution. Have you verified that adding lots of front spring and front bar actually gives your car excessive terminal understeer? Try it. If it doesn't, your chassis is too soft, and all those LLTD predictions you are taking from Milliken equations under the assumption that your chassis is rigid are meaningless and far from the truth.
    We have experimented with somewhere around 150-225lb/in but have springs up to 400lb/in. I will try to verify when we get a chance to test, which may take at least a week before we have the opportunity. I would hope our chassis is reasonably stiff as it weighs ~83lbs with powdercoat and the whole car weighs 405lbs without driver, but I understand that may not be the case.

    Quote Originally Posted by CWA View Post

    More friction on one axle than the other will also take your car far away from the LLTD predictions you have based solely on spring/bar stiffness. I am curious to know how you are so confident that you have negligible levels of friction. I don't just mean control arm friction when unloaded, I mean net system friction when loaded, including your damper seals. Total system levels can add up to be quite significant. Even if you can see your suspension actuating, your friction levels may be considerably higher on one axle than another, contributing to an unexpected balance difference.
    Do you have a suggestion on how we can verify levels of friction from one axle to the other? The control arms pivot about sphericals and the rockers/bellcranks pivot about needle bearings so they shouldn't hold much friction. I don't see this being a major contributor to the overall issue, but feel free to correct me.

    Quote Originally Posted by CWA View Post
    Yes. This is as Tim said, and is a possible cause. Why not explore it.
    What should I tell my suspension guys to look into specifically? Are the roll centers too low or too high? Again, I'm the wrong person to be answering these questions. Is there a good resource to familiarize myself with vehicle dynamics on a basic level? (I've tried to read Milliken's RCVD but it is way too long and dry for me to get very far, I start falling asleep at slip angles)

    Quote Originally Posted by CWA View Post
    As Z suggested to you, be careful you are not hitting rebound stops (over-extending your damper). Once you reach the level of lat acc that causes on one of your inside dampers to full extend, you will see full, immediate load transfer across this axle. If this is happening on the rear axle of your car, not only will it completely reverse the base chassis 'understeer' balance you believe you have, but it could also very well contribute to lifting a wheel..
    How do we resolve this, if it is happening?

    Quote Originally Posted by CWA View Post

    If you reach 2.1 g then your issue is not big. What kind of % reduction in lap time do you really believe curing this issue will bring to the team? Will that % be worth the amount of money you seem set on spending?

    If you depart with oversteer on the skidpad, adding more torque to your outside wheel will not increase cornering capacity, so again a new diff will not benefit you here. Also, check that the terminal oversteer you appear to be seeing actually aligns with your predictions for steady-state behaviour. Why don't you have limit understeer? Is what you see versus what you expect acceptable?

    Hairpins are usually the most steady-state of all the FSAE endurance event corners. What exactly do you think it is about the hairpin that is different to the skidpad that might help to explain why you see this inside wheel spin at the hairpin but not at the skidpad? Does the jacking only occur during turn-in at the hairpin? If so, why are you worried about getting torque down here, again, consider how you will actually be faster by removing this symptom.
    Personally I think we perform very well as is, it is my suspension guys that wanted to get a new differential in the first place. The Wavetrac is the most economical differential that solves the issue of an unloaded wheel. I'm more or less trying to facilitate the switch should the rest of the team decide that it is worth the extra $200 and time spent. As is, myself and the Technical Director think we should stick with the Torsen unless I can get those Wavetrac internals drawings before the school year begins. I'm still not certain why everyone in this thread is so against the switch for financial reasons when $200 is next to nothing compared to the ~$20,000 we raise each year to spend on the car. I actually reduced the cost of the 2014-2015 car by $600 compared to the 2013-2014 car by switching away from a Tilton bias bar to one constructed with McMaster hardware, so you could almost say I covered the cost of a different differential already.

    I guess I didn't consider that a hairpin is steady-state. I feel as though the extra speed carried followed by hard braking before turn in might contribute to more load transfer/roll but as I said before, I'm not a professional driver nor am I an expert on Vehicle Dynamics. The jacking is only really noticeable in the hairpin, but when it is noticeable it is really noticeable. The inside rear wheel actually completely lifts by 1-2" off the ground. This makes me think that the issue might be related to over-extending the damper as you said. Let me know what you think.
    Last edited by Tyler Jones; 07-17-2015 at 09:57 AM.
    2014-2015 Iowa State University FSAE Project Director
    2014-2015 Iowa State University SAE Vice President

  5. #25
    Well it is good that you can accept that your team can be better aligned. I won't harp on about this any more, there is only so much you can do and I appreciate what a challenge things like this can be. I know I'm no good at these kinds of things. There is a reason that there are so many team / project management roles in industry.

    Quote Originally Posted by Tyler Jones View Post
    The initial purpose of the thread was just to obtain some technical information about the Wavetrac, not to ask for suspension advice
    Well, perhaps. But by including your reasoning behind your decision to change the differential in your initial post, you have faced some scrutiny. This scrutiny, whilst unexpected, and probably not what you wanted, is exactly what you will be given by the design judges at competition, is it not? You'll be asked to justify many decisions you have made regarding hardware purchase / calibration in exactly the same way. If you don't like it / can't handle it now, you will surely struggle in the design event. You should know as well as I do that "we had some spare cash left over so decided to blow it on a new diff without knowing if it will actually make us faster or not" will not cut it in front of a design judge. Even if you do have the cash lying around, this is not what FSAE engineering, nor engineering in the real world is about.

    Your team should embrace this problem as a chance to develop their knowledge and understanding. Instead, your suspension group at least, seems rather defensive and unwilling to act on an issue which should certainly be of great concern to them. Also, I can tell Tyler that you are curious as to whether you can make a simple tweak that will negate the need for a new diff. If this is something you want to know, you'll have to ensure your suspension team collaborate in providing this basic vehicle setup info. If this is something you don't want to know, and you just want to buy the diff anyway, then ok, but on a personal level there is nothing more I can contribute. And I'm sure a lot of other members will frown if this is the process you decide. But this is completely up to you.

    Quote Originally Posted by Tyler Jones View Post
    I'm not sure they fully understand how ARB's contribute to overall wheel rates
    If not this way, how else are your suspension guys numerically predicting an ARB's contribution to LLTD? They must have some idea, or how else would they be able to state that the largest ARB they have will give 'too much understeer'? How do they know that doubling the largest bar they have won't still deliver understeer that is within an acceptable range? If you have not been quantifying how each ARB contributes to 'roll stiffness' or 'wheel rate' at each axle (these metrics effectively describe the same thing), then you can't possibly predict your balance as you have claimed.

    Quote Originally Posted by Tyler Jones View Post
    I stated before that before Formula North the car weighed in at 45% front and at Lincoln the car weighed in at 49% front. Ride heights and aero were adjusted between those two competitions so that may be contributing to the difference
    OK. As a matter of interest, does your car lift it's wheel on a hairpin with and without the wings? Also, for the record, it's not too important in this case, but know that ride height changes (or 'rake' / static pitch angle changes) do not alter your static front / rear weight distribution. At least not by any percentage the useful side of a decimal point, given the pitch angles feasible to a race car with 2" of suspension travel. This is a filthy myth that somehow seems to be quite rife on the racing scene.

    Quote Originally Posted by Tyler Jones View Post
    Do you have a suggestion on how we can verify levels of friction from one axle to the other? The control arms pivot about sphericals and the rockers/bellcranks pivot about needle bearings so they shouldn't hold much friction. I don't see this being a major contributor to the overall issue, but feel free to correct me
    I know from experience that friction levels can be great in spherical bearings and dampers. Our damper rod end seals were horrendously stiff (bad product purchase, cheap brand), and our brand new control arm spherical bearings were staked into their housings too tightly, resulting in lots of friction.

    To quantify the friction levels on your car; a) lift car up so that all four corners droop when unloaded, b) lay the car back down on the ground until springs support it, c) roll it a couple of metres to relieve the tyres of any jacking force, d) measure ride height / damper travel (this is 'position A'), e) have a person stand on each end of the car at the same time so that the suspension is in bump, f) have these people stand back off the car, so that the springs try to raise the car back up to ride height, g) again roll the car a couple of metres to relieve the tyres, h) re-measure the ride height / damper travel, call these new readings 'position B' because they will sure be different values to position A.

    If you know your actual wheel rates, you should now be able to mathematically derive some friction levels for each axle. Then when you start to compare these friction force values to your known normal load variations caused by dynamic load transfer, you can start to consider whether this friction is 'too much', or not. Also consider the the difference in friction levels between axles, this delta is what will contribute to unexpected balance changes away from target.

    Quote Originally Posted by Tyler Jones View Post
    What should I tell my suspension guys to look into specifically? Are the roll centers too low or too high? Again, I'm the wrong person to be answering these questions. Is there a good resource to familiarize myself with vehicle dynamics on a basic level? (I've tried to read Milliken's RCVD but it is way too long and dry for me to get very far, I start falling asleep at slip angles)
    Milliken is a great starting point, hopefully your suspension guys are living out of RCVD right now. If you can't stomach Milliken, definitely don't try Pacejka or Matchinsky.

    RC's too high, as I think Tim might have said. There are some great threads in this forum on control arm jacking, the most useful posts I've found on the subject are by Z. Try and find these, they helped me understand jacking with more clarity than any book I've read. Z also explains the limitations of the RC metric too, which should also be heeded when considering control arm geometry.

    Quote Originally Posted by Tyler Jones View Post
    How do we resolve this, if it is happening?
    Have a look good hard look at your system, you should be able to creatively figure this out on your own (or rather, tell your suspension guys to). If you are lucky, you may have adjustable spring seats, allowing you to retain your spring and wheel rates but change the resting place of the damper rod relative to the body for a given ride height. If this is not an option, things may be more awkward for you.

    Quote Originally Posted by Tyler Jones View Post
    I guess I didn't consider that a hairpin is steady-state. I feel as though the extra speed carried followed by hard braking before turn in might contribute to more load transfer/roll but as I said before, I'm not a professional driver nor am I an expert on Vehicle Dynamics. The jacking is only really noticeable in the hairpin, but when it is noticeable it is really noticeable. The inside rear wheel actually completely lifts by 1-2" off the ground. This makes me think that the issue might be related to over-extending the damper as you said. Let me know what you think
    No you are correct, the entry and exit of a hairpin are transient. To be honest, I think I was a bit liberal in deeming an FSAE hairpin as steady-state, as it will be barely be, I think I gave the wrong impression here. Here are three bunches of some further, loose thoughts from me. But for them to develop any further, and lead into a potential solution, investment from your team is required:

    1. If the wheel is only jacked at the hairpin during braking / turn in, you shouldn't be trying to apply drive torque here, so why are you concerned by it? As soon as you release the brakes / longitudinal load transfer reduces to zero / reverses direction, the jacking should cease / wheel should grip the road again. Is this the case? If so, again, think about how much you will benefit by addressing the issue here.
    - If the jacking only occurs during turn in, but you still want to address it, it could be that your inner rear damper is becoming over-extended due to combined cornering and braking load transfer. Or it could be that your rear damper rebound rates are significantly higher than your bump rates, and the jacking is a consequence of the transient body roll velocity during turn in. There are some tests you can perform to rule each of these mechanisms out.

    2. Perhaps the wheel remains jacked for as long as you hold the steering wheel at roughly the same angle (the tightest it ever needs to be = to negotiate the hairpin) for the duration of the turn, regardless of whether you are braking or applying drive torque. Should this be the case, this does indicate that steering system jacking could be your strongest contributor. Post up those corner scale measurements / your KPI and trail values asap! If the hairpin turn radius is less than the skidpad turn radii (I can't remember the event formats off the top of my head), this may explain why your wheel remains jacked during the hairpin but not during skidpad.

    3. If the jacking behaviour is only ever seen on one corner of one track, consider, is the track skewed or uneven at this corner where you see this behaviour? If you were to set up some cones and repeat the braking / hairpin manoeuvre on a known flat test surface, does your wheel still jack? Even if you deduce that the jacking is caused by an uneven road surface, there is still an aspect of your car's setup that is facilitating this behaviour - I'm sure no other cars jack their wheel on the same corner.
    Last edited by CWA; 07-17-2015 at 12:12 PM.

  6. #26
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    ...(I've tried to read Milliken's RCVD but it is way too long and dry for me to get very far, I start falling asleep at slip angles)
    We never expected anyone to read it cover-to-cover, I'm always amazed (and pleased) when someone says they have.

    Off topic:
    This reminds me, there was a funny picture of a guy on his back with RCVD opened over him -- as if he fell asleep and it fell on him (or maybe it was real and not a pose?) I didn't snag a copy when I first saw it, and haven't seen it since. Anyone know where this was posted? Link please!

  7. #27
    Reason from the fundamentals:

    F=ma, a =v^2/R, T =Iα. Newton's laws with undergrad mechanics of materials can get you a long way in FSAE if they are understood well across the vehicle. I.e. it is worth asking, what supplies the F, a, and T and how much of these do you want in what directions and magnitudes? How do you maximize the F, a and T? Do you even want to maximize them? What combinations of these do you want over the course of a lap? You've got this 550lb mass navigating mostly in 2-D, what needs to happen to this force-moment model to reduce the time it takes for this mass to get from the start to finish line? How does suspension influence this? What does the differential do to influence the forces, accelerations, and yaw moment? Do you even care? Will the differential solve the problem you're trying to solve?

    My best advice would be to find a way to make learning engineering not seem like a chore. Be inspired to read RCVD, go to/pay for OptimumG seminar, Carroll smith books, etc.. (An FSAE car with good aesthetics help this and can be a good motivator, also a great way to recruit new members!)
    Last edited by Steve Krug; 07-24-2015 at 03:42 AM.
    Steve Krug
    Wisconsin Racing

  8. #28
    We tell our guys to start with Tune to Win before going into RCVD. Tune to Win when read in its entirety gives a great foundation for the kinematics of a vehicle and how the systems work together. I use RCVD as a textbook of sorts. The important thing I think your suspension guys need to see is that the entire car works as a system, and there's probably not one quick, easy fix to the problem you're seeing. Something I try to drill into my members heads is that the big picture matters more than the details. It could be that the entire system of your chassis is causing the problem, it could be a setup adjustment that was overlooked. You'll only find out through extensive testing.

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