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Thread: Wavetrac Differential

  1. #11
    Quote Originally Posted by Tyler Jones View Post
    "Man that is a lot of questions, I don't have access to all those numbers at the moment and I might hesitate a little bit to put that much info on the forum (probably not a big deal, but he's asking for almost our whole design haha). I can answer all of those though, and it sounds like he is wondering whether the front wheel motion in steering is jacking the inside rear too much, and also if our bar setup is lifting it. I wouldn't suspect too much jacking from the front (although I wouldn't totally rule it out) since we have 3 deg of caster and about 0.8 inches of scrub radius, which doesn't displace the wheel downward very much (a lot of caster would be more like 5-7 deg). Wheel rates are about 112 lbs/in on all corners, and since we've tried almost all the bar combinations I don't think he can help us too much otherwise. It's got low friction and it's good on the scales, so we can pretty safely rule those things out."
    First, there is no magic formula that your suspension guy might have happened upon, so don't be so worried in sharing your setup details. Even if your setup did give the best handling car in existence (which by the sounds of it, no offence; it does not), true value is found in the understanding of the relevant mechanisms and knowing exactly why the setup gives good performance. It is entirely possible that the setup that might be golden on one car would be considerably less effective on another car. But knowing how to adjust the setup slightly depending on your situation is where the real skill lies. Point being, your setup details are pretty arbitrary - you should have no qualms with sharing this info.

    Second, one way or another your car is lifting a rear inside wheel. This is truth, this is reality, so you say. Don't forget that THIS is the root cause of your issue, and replacing the diff that doesn't fully lock is effectively a band-aid fix. In my opinion, until you've exhausted all of your options in chassis tuning, you should not be spending more money on hardware. Even if you have tuning limitations on your own chassis, I'd say you have a duty to figure out the mechanism causing this behaviour so that you can: a) justify to your budget providers why exactly you need a new diff, and b) pass on this knowledge to next year's team as a warning. If you guys are designing the next car, this is an even bigger reason to want to figure this out. Don't forget, plenty of teams do not lift their I/S/R wheels round corners, and so manage to make do with lesser diffs. You should strive to find out why that is, even if it turns out you can do nothing about it.

    Now, I am 90% sure that real answers to all of the questions I asked will expose the cause of this behaviour (by 'real answers' I mean actual, measured values, rather than nominal design-intent values. The car certainly won't have turned out exactly as the ideal spreadsheet solution showed you it should - friction for one can have significant effects). There may be some obscure mechanism causing your I/S/R wheel to lift that isn't related to control arm jacking ("RCHs"), steering system jacking, or roll moment / lateral load transfer / static weight distribution, but I will be surprised if this is so. As said, there are plenty of cars out there that do not jack their inside wheel; it should not be difficult to determine why your car is different, it abides by the same rules of physics after all.

    See if you can post back with the vales you haven't answered: Static Weight Distribution, estimates of F&R Roll Centre Heights and CG height. Also do you know what your chassis torsional stiffness might be? For starters, if this is too low, it will explain why bar tuning has not been as effective as you were perhaps expecting. Do you really have just 20N/mm wheel rates front and rear? To confirm this, please don't consider me patronising in checking; what are your spring rates and what motion ratio do you have? How much 'stiffer' does your range of bar combinations supposedly make your wheel rate in roll (a number would be useful, for you and us both)? Also, more out of curiosity than much else, how much does your car actually weigh?

    Yes as your suspension guy recognised, I was trying to determine whether steer jacking might be the main contributor to your issue. You've suggested your caster angle and scrub radius are within a reasonable range, but don't forget that these aren't the only contributors to steer jacking. Your KPI and trail contribute to this too - what are these values? Also, the stiffness of your tyres / suspension / bars / chassis contribute to this mechanism - an infinitely stiff car will theoretically lift its inside wheel with far less caster and scrub than you have mentioned. Your quoted wheel rates do not seem too stiff if they are accurate, but you need to be sure that you don't have too much friction that acts to ruin these rates - the friction will act as a false suspension stiffness, at least in that it will serve to increase the effects of steer jacking. Also, your 'scrub radius' in CAD is a hypothetically ideal value, which technically isn't even a kinematic dimension (it should be based on the contact patch centre of pressure, not just the centre line of your wheel). As can be imagined, in real life, exactly where this centre of pressure is in relation to your steering axis ground level intersect depends for starters on the width of your wheel, it's camber angle with the road (with steer and roll), and tyre sidewall stiffness's.

    So can you also post up: static front camber, camber gain of your front suspension, wheel widths, tyre type and sidewall sizes? Do you have your own scales? If so, you might as well get the car on them, record and then post here: a) your static corner loads with driver and steering straight ahead, and b) your static corner loads with driver and max steering angle in any direction (might as well do both directions to assess asymmetry). I believe this would be a great starting point.
    Last edited by CWA; 07-15-2015 at 01:01 PM.

  2. #12
    Quote Originally Posted by Tyler Jones View Post
    I'm aware of most of the options available for FSAE differentials. From what I have found we can choose between:
    • A spool (causes understeer)
    • Detroit Locker (causes understeer)
    • A Cam and Pawl a.k.a. front diff from atv (also causes understeer but doesn't work with a lifted tire)
    • Torsen (<$1000, bias adjustable, doesn't work with a lifted tire)
    • Quaife (<$1000, bias adjustable, doesn't work with a lifted tire)
    • Wavetrac (<$1000, bias adjustable, works with a lifted tire)
    • Taylor (>$2000, bias adjustable, based on Quaife so doesn't work with a lifted tire?)
    • Salisbury-style differential (expensive, bias adjustable, can be made to work with a lifted tire)
    • Drexler (>$2000, bias adjustable, can be made to work with a lifted tire)


    Goals in order of importance:
    1. price <$1000
    2. bias adjustibility
    3. works when one wheel is unloaded
    4. weight < ~10 lbs (not a huge driving factor here)

    The Wavetrac is cheap (~$600), the bias can be adjusted by changing the material of the friction plates, it works when one wheel is unloaded, and with a custom housing the weight increase over the Torsen would be negligible. The only drawbacks that I can see are the lack of the drawings necessary, the time invested in designing a new housing, and having to partially disassemble the housing every time you want to adjust the torque bias.

    Feel free to bring up any negatives to the Wavetrac I haven't thought of, but from what I can see it it meets all my goals.
    Personally from my stand point the fact that you have to make a custom housing and do not have access to a drawing are very big negatives, but that is from the perspective from where I stand on this issue. Yours may well be very different and that is the beauty of FSAE.

    I would love to see how you weight those thing of importance and maybe develop a Pugh matrix of the different options.

    Also Why would you want to completely un-load any of your tire during the events, i dont think i would want to do that on my car.(maybe i am not thinking clearly on this) If this is something that is happening in your car and you are developing jacking effects in the rear. Then a spool would be a great option but your suspension would have to further develop this plan of action, but it doesnt do anything with you second goal. It could super exceedon the other 3 of the goals though. Or your suspension guy may want to get rid of the jacking and the need for the new style diff will go away.
    Craig Kellermann
    -Construction Science and Management (This is my Major)
    -University Of Texas-San Antonio
    -2011-present

    " Charge It To The Game "

  3. #13
    Quote Originally Posted by CWA View Post
    First, there is no magic formula that your suspension guy might have happened upon, so don't be so worried in sharing your setup details. Even if your setup did give the best handling car in existence (which by the sounds of it, no offence; it does not), true value is found in the understanding of the relevant mechanisms and knowing exactly why the setup gives good performance. It is entirely possible that the setup that might be golden on one car would be considerably less effective on another car. But knowing how to adjust the setup slightly depending on your situation is where the real skill lies. Point being, your setup details are pretty arbitrary - you should have no qualms with sharing this info.

    Second, one way or another your car is lifting a rear inside wheel. This is truth, this is reality, so you say. Don't forget that THIS is the root cause of your issue, and replacing the diff that doesn't fully lock is effectively a band-aid fix. In my opinion, until you've exhausted all of your options in chassis tuning, you should not be spending more money on hardware. Even if you have tuning limitations on your own chassis, I'd say you have a duty to figure out the mechanism causing this behaviour so that you can: a) justify to your budget providers why exactly you need a new diff, and b) pass on this knowledge to next year's team as a warning. If you guys are designing the next car, this is an even bigger reason to want to figure this out. Don't forget, plenty of teams do not lift their I/S/R wheels round corners, and so manage to make do with lesser diffs. You should strive to find out why that is, even if it turns out you can do nothing about it.

    Now, I am 90% sure that real answers to all of the questions I asked will expose the cause of this behaviour (by 'real answers' I mean actual, measured values, rather than nominal design-intent values. The car certainly won't have turned out exactly as the ideal spreadsheet solution showed you it should - friction for one can have significant effects). There may be some obscure mechanism causing your I/S/R wheel to lift that isn't related to control arm jacking ("RCHs"), steering system jacking, or roll moment / lateral load transfer / static weight distribution, but I will be surprised if this is so. As said, there are plenty of cars out there that do not jack their inside wheel; it should not be difficult to determine why your car is different, it abides by the same rules of physics after all.

    See if you can post back with the vales you haven't answered: Static Weight Distribution, estimates of F&R Roll Centre Heights and CG height. Also do you know what your chassis torsional stiffness might be? For starters, if this is too low, it will explain why bar tuning has not been as effective as you were perhaps expecting. Do you really have just 20N/mm wheel rates front and rear? To confirm this, please don't consider me patronising in checking; what are your spring rates and what motion ratio do you have? How much 'stiffer' does your range of bar combinations supposedly make your wheel rate in roll (a number would be useful, for you and us both)? Also, more out of curiosity than much else, how much does your car actually weigh?

    Yes as your suspension guy recognised, I was trying to determine whether steer jacking might be the main contributor to your issue. You've suggested your caster angle and scrub radius are within a reasonable range, but don't forget that these aren't the only contributors to steer jacking. Your KPI and trail contribute to this too - what are these values? Also, the stiffness of your tyres / suspension / bars / chassis contribute to this mechanism - an infinitely stiff car will theoretically lift its inside wheel with far less caster and scrub than you have mentioned. Your quoted wheel rates do not seem too stiff if they are accurate, but you need to be sure that you don't have too much friction that acts to ruin these rates - the friction will act as a false suspension stiffness, at least in that it will serve to increase the effects of steer jacking. Also, your 'scrub radius' in CAD is a hypothetically ideal value, which technically isn't even a kinematic dimension (it should be based on the contact patch centre of pressure, not just the centre line of your wheel). As can be imagined, in real life, exactly where this centre of pressure is in relation to your steering axis ground level intersect depends for starters on the width of your wheel, it's camber angle with the road (with steer and roll), and tyre sidewall stiffness's.

    So can you also post up: static front camber, camber gain of your front suspension, wheel widths, tyre type and sidewall sizes? Do you have your own scales? If so, you might as well get the car on them, record and then post here: a) your static corner loads with driver and steering straight ahead, and b) your static corner loads with driver and max steering angle in any direction (might as well do both directions to assess asymmetry). I believe this would be a great starting point.
    We have scales, I can try to get the static corner loads some time this week. The suspension guys are working on providing the other info.

    Quote Originally Posted by craigorydean View Post
    Personally from my stand point the fact that you have to make a custom housing and do not have access to a drawing are very big negatives, but that is from the perspective from where I stand on this issue. Yours may well be very different and that is the beauty of FSAE.

    I would love to see how you weight those thing of importance and maybe develop a Pugh matrix of the different options.

    Also Why would you want to completely un-load any of your tire during the events, i dont think i would want to do that on my car.(maybe i am not thinking clearly on this) If this is something that is happening in your car and you are developing jacking effects in the rear. Then a spool would be a great option but your suspension would have to further develop this plan of action, but it doesnt do anything with you second goal. It could super exceedon the other 3 of the goals though. Or your suspension guy may want to get rid of the jacking and the need for the new style diff will go away.
    A big reason I'm a proponent of the Wavetrac is that the work required to get it on the car is the exact same as what we have been doing for the past several years with the Torsen. Yes, not having the drawings means it is basically impossible to switch to this differential, so if I can't obtain them in a timely manner we will probably stick with the Torsen.

    The jacking issue is definitely something to attempt to resolve. I would not want to intentionally design jacking into the system to use a spool as I think that would compromise the overall performance of the car compared to one with a working differential and some small jacking issues in very tight corners. The 2014-2015 car with its current issues did end up placing 3rd in Endurance and 3rd overall at Formula North and was going to place very well at FSAE Lincoln until we were black flagged during endurance (lost the endcap of our muffler, dang new sound rules). This is partially why I think simply switching to a differential that can lock with an unloaded wheel is a good idea (if it ain't broke, don't fix it? I guess that depends on what you view as broken, which for most is probably the chassis setup).

    I'm grateful for everyone's advice, even if it looks like I'm dead-set on the Wavetrac. If I can't get the necessary technical info for the Wavetrac we will be sticking with the Torsen anyways and will be forced into trying to solve this unloaded wheel situation via chassis tuning, like it or not.
    2014-2015 Iowa State University FSAE Project Director
    2014-2015 Iowa State University SAE Vice President

  4. #14
    If you're getting third place at comps, the issue can't be too bad. That fact does help put things in perspective. Do you have any vids of this behaviour on track?

    What is your maximum achieveable steady-state lat acc? (Do you have instrumentation to check this?)? Is the' inside rear wheel lifting / limited amount of drive torque' issue what dictates this limit? So during a steady-state skidpad test, rather than ploughing or spinning at the limit (ie rather than saturating an axle of its capacity to give out any more lateral force), do you just reach a given speed and maintain it, with your inside wheel spinning up if you give it more gas? Meaning you can never get your outer wheel to spin up / the car to spin out?

    If this is not the case, make sure you are clear about what makes you think that this wheel lifting / wheel spinning is even an issue. Your car is obviously quite quick at the moment. You could find a way to send more drive torque to the outer wheel than you are already doing - whether you do this with your existing diff by solving the jacking, or whether you find a better locker. But perhaps your outer wheel has been close to saturation anyway (and is also balanced well with your front) hence your competitive results. In this case, adding more locking torque will actually allow the driver to saturate the outer rear tyre with drive torque (rather than spinning up the inside as you are currently doing), which will end up making a worse contribution to consistently lower lap times than before (car is more of a handful, harder to handle).

    This is of course pure speculation / mental masturbation on my account, but food for thought at least, something to be aware of when you begin making changes.
    Last edited by CWA; 07-15-2015 at 06:20 PM.

  5. #15
    Senior Member
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    Australia
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    Tyler,

    It seems you are using the standard "because racecar" approach to solving your problem. Namely, chase the most complicated and expensive solution you can find. (And if you think spending yet another ~$600, AND making a new diff-case, is NOT expensive, then please send the money to me! )

    As noted by others above, there are many simpler and cheaper fixes to your problem. Please re-read CWA's posts, and also get your suspension guys to read them several times. There are many clues there that can help your Team build a faster car.

    Regarding your list of options/goals for selecting a diff, I think you are somewhat missing the point. It is important to realise that you need the "whole package" (= diff + suspension + everything-else) to work well, and that means you cannot just choose a diff based on a simplified list of "diff characteristics" taken in isolation.

    Because cost is so important (to me!) I would immediately narrow my diff choice down to these two cheapest options.

    1. A "spool" diff.
    This is about the simplest machined part you can make for the car (= lathe + drill-press?). (Or you can turn your current Torsen into a spool by squeezing some extra washers in there, or filling with epoxy, or weld...) Your current car seems to be already "set-up" for a spool, so no extra work there. And, most importantly, there are countless cars, both past and present, that have won FSAE comps with a spool. The "spool causes understeer" is one of those lame arguments that might be true of a spool in general, but has turned out to be irrelevant with the car set-up used on many winning FSAE cars.

    2. An "open" diff.
    Your Torsen is close to this, though cheaper ones are available from any car wrecking yard. The set-up changes required on your car, to suit a fully open-diff, are quite easy. From reading your above posts I suspect your suspension guys are, ... ummm, a little lost. [Well done Z! The politeness lessons are working.] Short list: Fit stiffer front springs, and/or softer rears. Or a VERY stiff front-ARB. Make sure frame is torsionally stiff enough for this to work!!! Make sure front dampers reach droop limit before rears. Reduce excessive "steer-wedging" from steering geometry. Beware of VERY short end-view Rear-Virtual-Swing-Arm-Lengths. And some others ... but the above should cover it. Personally, this is my preference because I like the idea of having all four wheels always planted.

    The choice is yours, of course, but throwing money around does not equal good engineering. Besides, I am sure you will eventually meet a girl who will teach you how to really squander your hard-earned dollars!

    Z

    (PS. "A good engineer is someone who can do with one dollar, what any fool can do with ten!")
    Last edited by Z; 07-15-2015 at 10:07 PM.

  6. #16
    Senior Member
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    Location
    Brighton, MI
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    In Difference.

    You can turn an open diff into a limited slip diff with some really bad bearings and some worn out side gears. It only differentates when you are at lots of load trasfer and most often its when you are at full throttle.

    Push some brake fluid into the spinning wheel using a rotary valve in the steering system.

    Keep the rubber on the road.

  7. #17
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    Join Date
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    Would also recommend a spool. It's cheaper and lighter (lower polar moment) and there's less stuff to break/look after. If your car is already lifting the inside and you can't tune it out, adding a spool will not cause understeer (vs. a car that has the inside planted).
    Jay

    UoW FSAE '07-'09

  8. #18
    Quote Originally Posted by Z View Post
    Tyler,

    It seems you are using the standard "because racecar" approach to solving your problem. Namely, chase the most complicated and expensive solution you can find. (And if you think spending yet another ~$600, AND making a new diff-case, is NOT expensive, then please send the money to me! )

    As noted by others above, there are many simpler and cheaper fixes to your problem. Please re-read CWA's posts, and also get your suspension guys to read them several times. There are many clues there that can help your Team build a faster car.

    Regarding your list of options/goals for selecting a diff, I think you are somewhat missing the point. It is important to realise that you need the "whole package" (= diff + suspension + everything-else) to work well, and that means you cannot just choose a diff based on a simplified list of "diff characteristics" taken in isolation.

    Because cost is so important (to me!) I would immediately narrow my diff choice down to these two cheapest options.

    1. A "spool" diff.
    This is about the simplest machined part you can make for the car (= lathe + drill-press?). (Or you can turn your current Torsen into a spool by squeezing some extra washers in there, or filling with epoxy, or weld...) Your current car seems to be already "set-up" for a spool, so no extra work there. And, most importantly, there are countless cars, both past and present, that have won FSAE comps with a spool. The "spool causes understeer" is one of those lame arguments that might be true of a spool in general, but has turned out to be irrelevant with the car set-up used on many winning FSAE cars.

    2. An "open" diff.
    Your Torsen is close to this, though cheaper ones are available from any car wrecking yard. The set-up changes required on your car, to suit a fully open-diff, are quite easy. From reading your above posts I suspect your suspension guys are, ... ummm, a little lost. [Well done Z! The politeness lessons are working.] Short list: Fit stiffer front springs, and/or softer rears. Or a VERY stiff front-ARB. Make sure frame is torsionally stiff enough for this to work!!! Make sure front dampers reach droop limit before rears. Reduce excessive "steer-wedging" from steering geometry. Beware of VERY short end-view Rear-Virtual-Swing-Arm-Lengths. And some others ... but the above should cover it. Personally, this is my preference because I like the idea of having all four wheels always planted.

    The choice is yours, of course, but throwing money around does not equal good engineering. Besides, I am sure you will eventually meet a girl who will teach you how to really squander your hard-earned dollars!

    Z

    (PS. "A good engineer is someone who can do with one dollar, what any fool can do with ten!")
    Z,

    The Wavetrac is $600, compared to $400 for a Torsen (which we gut and use a custom housing with anyways), or ~$300 for an ATV front differential. In my mind the price difference is almost negligible over our current Torsen since we would be spending $400 out of that $600 either way. I personally programmed and ran the CNC turning to make the custom Torsen housing using donated 7075, so other than time invested the housing was free. Yes, time is a very valuable resource for FSAE but in most cases I would rather invest my time over straight cash in the car.

    I do agree that we should try locking the Torsen and testing with it as a spool, but I don't know of any methods to achieve that which aren't destructive for the internals (its not cheap to destroy the gears in pursuit of something which may turn out not to be beneficial). To manufacture our own spool would require some method of hobbing or wire EDMing the correct spline for our stub shafts, which we don't have the resources for, meaning it is definitely not cheap. We could buy some stubshafts that have a bolt pattern instead of a spline, but the only ones I can find are $185.00 per stubshaft. Perhaps we could manufacture our own but that's more time invested in something the car isn't tuned for at the moment.
    I would also like to see a list of recent results for cars using spools as I haven't seen a single car in the top ten at any North American competition using spools to my knowledge.

    My suspension guys are the ones that initially suggested switching to a locking differential because they are afraid that standard options for tuning the chassis to keep that wheel down would cause the car to understeer significantly. They have already tried most configurations for arbs, including stiff arb front and no arb rear. The car currently has stiffer springs front than rear, though there are options to try different springrates there. I can't answer for droop limits, steer-wedging, etc but I will have my suspension guys look into it.

    Don't get us wrong Z, we are a very fiscally conservative team. Most of our design is centered around keeping the car cheap (usage of a lot of laser cut steel, aircraft fabric body, lots of 3D printed components). We took 2nd in cost at Formula North with a very accurate cost report (I know this doesn't exactly equate to real world cost but it at least correlates to a cheap car).

    More quotes from my suspension guys:

    "I think our actual weight dist. was 49%, and one of the aero guys said the cg raises about 1.5 in with wings (i'd like to measure that to see for sure). From what I've been learning, I'm pretty suspicious of our roll centers and how they work with the rest of the car, and also how the travel available is mostly for compression. 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)." -Suspension guy 1

    "Static weight distribution: 45% front actual
    CG height: 10.2 measured without wings, 10.5" with wings (estimated, This is what I think it was, can probably get a better estimate later)
    Roll Center: 3.0" front 3.8" rear
    Chassis Torsional stiffness: Not sure off my head. (Somewhere around 1800lbs/degree simulated)
    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. Springs are 225s.
    Ignore the ARBs and do not consider them into wheel rates.
    Car weight is 405lbs?

    Steering Geometry: Look at the design tech sheet. Those are all "designed" numbers,
    Static Front Camber: -2*
    Wheels: 6in width

    The suspension does not have bushings and does not carry much friction at all. Essentially is negligible for most calculations we do. (Not always negligible, but its low enough that I'm ignoring it for now and focusing elsewhere)

    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.

    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.

    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

    I guess I will have to track down the rest of those numbers myself when I find the time.

    Thanks for the advice, keep it coming!
    2014-2015 Iowa State University FSAE Project Director
    2014-2015 Iowa State University SAE Vice President

  9. #19
    Quote Originally Posted by CWA View Post
    If you're getting third place at comps, the issue can't be too bad. That fact does help put things in perspective. Do you have any vids of this behaviour on track?

    What is your maximum achieveable steady-state lat acc? (Do you have instrumentation to check this?)? Is the' inside rear wheel lifting / limited amount of drive torque' issue what dictates this limit? So during a steady-state skidpad test, rather than ploughing or spinning at the limit (ie rather than saturating an axle of its capacity to give out any more lateral force), do you just reach a given speed and maintain it, with your inside wheel spinning up if you give it more gas? Meaning you can never get your outer wheel to spin up / the car to spin out?

    If this is not the case, make sure you are clear about what makes you think that this wheel lifting / wheel spinning is even an issue. Your car is obviously quite quick at the moment. You could find a way to send more drive torque to the outer wheel than you are already doing - whether you do this with your existing diff by solving the jacking, or whether you find a better locker. But perhaps your outer wheel has been close to saturation anyway (and is also balanced well with your front) hence your competitive results. In this case, adding more locking torque will actually allow the driver to saturate the outer rear tyre with drive torque (rather than spinning up the inside as you are currently doing), which will end up making a worse contribution to consistently lower lap times than before (car is more of a handful, harder to handle).

    This is of course pure speculation / mental masturbation on my account, but food for thought at least, something to be aware of when you begin making changes.
    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.
    2014-2015 Iowa State University FSAE Project Director
    2014-2015 Iowa State University SAE Vice President

  10. #20
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    As CWA alluded to, you are trying to substitute your diff to solve a suspension problem.

    First things first, what is your calculated/simulated LLTD? What is your calculated/simulated US (using what ever metric suits you, i.e. Static margin, stability factor, UG)?

    This is the place to start. Only once we have this information can we start a discussion. If your suspension guys can't tell you these numbers, then I wouldn' be paying any attention to their predictions of too much understeer. Especially not given your subjective feedback points in the direction of instability/oversteer.

    By the way, your roll centres appear to be nearing the height where the outer wheels will "lock" its travel near limit lat acc. Not necessarily a bad thing, but it causes very asymmetric roll movements which raises your CG when you corner. Can make the LLTD/balance tricky to predict using the normal simple calcs too. Though I'm sure your suspension guys are aware of this right? Are you looking at your damper positions when you are testing?
    Last edited by Tim.Wright; 07-16-2015 at 04:43 PM.

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