In theory you need to work backwards from your vehicle understeer plot, to give you an understeer budget.

However, in practice IRS' have compliance oversteer ie steer in under lat acc. They also have roll oversteer. Bear in mind that rear axle oversteer is vehicle understeer.

Roll steer is typically linear, and I have seen figures of 0.1 deg steer/deg roll.

Cheers

Greg Locock

The reason I am asking this question is because of the feeling you get from driving the car.  During a turn-in or slalom type manuever, the rear feels like it is steering to the outside of the turn radius.  I call this lack of rear firmness in evaluations.  It actually feels like the outside wheel is in a toe out condition and steering the rear of the car.  I have seen some ADAMS plots of the roll steer and it is showing toe-out during bump for the first 2 degrees of body/chassis roll then going to toe-in, and likewise toe-in for the entire body roll during rebound.  This basically says the inside wheel is toe-in and outside wheel is toe-out, giving the car a "rear steering" feeling, instead of being firm and the rear staying behind you during a turn.

Am I wrong to assume the proper curve should be somewhat linear and bump travel should be toe-in, and rebound travel should be toe-out?  I know most of the time you only look at bump steer plots, this is why I need other opinions and knowledge.  It's hard to find actual roll steer plots and design criteria to back up what I am thinking.

Mainly the problem is that the spring force and ARB reaction force are NOT apllied to the centerline of the rear wheel.  I believe this is causeing the knuckle to rotate and cause a very strange toe pattern during roll.  I think if I move the knuckle side toelink pivot point up 2mm it should help the initial toe-out on bump become toe-in initially, and likewise help the rebound become more toe-out.  Once again this is all being forcast from a roll steer situation rather than just a bump steer condition.

Roll oversteer on turn-in was very common on prototype /beam/ axles and takes a while to develop out. It can be useful to look at a trace of steering wheel angle vs time to see if you are having to apply a negative correction. Also look at your yaw velocity curves to see if you are getting a nice smooth buildup of yaw velocity.

The ADAMS plot you describe could (and probably would) produce such a feeling - it seems reasonable to dislike ANY kinks in these curves.

"the proper curve should be somewhat linear and bump travel should be toe-in, and rebound travel should be toe-out"

yes, exactly.

"Mainly the problem is that the spring force and ARB reaction force are NOT apllied to the centerline of the rear wheel. "

that is bad


" I believe this is causeing the knuckle to rotate and cause a very strange toe pattern during roll.  I think if I move the knuckle side toelink pivot point up 2mm it should help the initial toe-out on bump become toe-in initially, and likewise help the rebound become more toe-out."

It is hard  to use geometry changes to compensate for compliance errors, as they are not robust against tuning changes, ie if you change your ARB you'll have to change the tie rod outer height.

"  Once again this is all being forcast from a roll steer situation rather than just a bump steer condition."

Sounds like you are on the right track.

Cheers

Greg Locock

Thanks Greg.... now it's time to get people to spend some money and fix what I consider fundamental errors by the chassis design deptment.  It's very frustrating being a young engineer and seeing things that are so obviously wrong in design, and at the same time not have the total confidence to speak up and tell another older/higher level engineer that he is wrong, and his work is unacceptable from basic engineering principles standpoint.  It really makes me wonder how we can build cars sometimes.  Do truely basic and fundamental engineering errors happen at all companies???????????

"It is hard  to use geometry changes to compensate for compliance errors, as they are not robust against tuning changes, ie if you change your ARB you'll have to change the tie rod outer height."

I know exactly what you are saying!  Because of the reaction force of the ARB being offset from the centerline of the wheel, diameter of the ARB affects the toe pattern.  This is what got me thinking in the first place of what is wrong.  The car actually feels better without an ARB in the rear, but body roll becomes way to large for the concept of the vehicle.  And furthermore, I cannot apply more spring force or damping force, becasue it to is offset from the centerline.  This car is very important for us, and I don't want to be the one that goes down in flames for it's poor handling, but I am the lead ride and handling tuner for the project and I will be the one that has to answer to the project managers and upper managment.

You are probably working for the opposition, but if by   chance that you aren't, contact me via our internal email.

Taking the rear ARB off is a good move, we only put them on for marketing half the time. (well, exaggeration)

Cheers

Greg Locock

"You are probably working for the opposition, but if by   chance that you aren't, contact me via our internal email."

My parent company is so big and has multiple companies attached to it, that I wouldn't even know where to begin to figure out if we are on the same team.  The parent company is one of "the big 3" if that helps, but then again....not many car companies are not part of the big 3 anymore!

"Taking the rear ARB off is a good move, we only put them on for marketing half the time. (well, exaggeration)"

ha ha... that is the other problem....marketing.  The previous generation has a rear ARB and so does the family sedan off the same platform.........the sportier car with out a rear ARB may be a hard sell to the sales guys.

It sounds like you are slowly but surely tracking down this problem. I would agree that any kinks or changes in a toe curve are a bad thing and you should minimize that first. I think that will help you out. Do you know the toe pattern w/o roll bar? Does it smooth the toe curve?

Don't limit yourself to geometry changes only. If you can help the situation by removing the roll bar, maybe you should consider tuning other pieces. Tires are a big tuning knob and sometimes you get a "lag" or "gain" from tires. Removing the rear bar would saturate front tires more and produce more understeer, and improve the "lack of rear firmness" you are feeling.

yes, I know the toe pattern w/o Rear ARB, and it does help smooth the toe pattern.  But the big problem I am having is roll stiffness.  This car is rather sporty, and needs a solid feel and low roll amount.  The problem is the only way to get the roll stiffness I need is to have a rear ARB and high spring rate, but both of theese cause the knuckle to rotate and create a poor toe pattern because they are both applied to the knuckle at a location that is offset from the centerline of the wheel.  If the roll stiffness is reduced, it feels like a family sedan, and is not fun to drive, BUT the toe patern is better and the rear feels more firm, but the body roll is unacceptable.  Shifting the balance to the front tires does not work, because then the front reacts to quickly and creates a yaw delay.... this is also not acceptable... so then if you reduce the front spring to balance that, you are back to the original case, but now with more body roll.

This is really turning out to be a no win situation.

Update:  Moving the outer toe link rotation point looks like it works in ADAMS, but the evaluation results on an actual vehicle is not a noticeably good result.


Still trying many things..... but getting more frustrated every day by this suspension design.

Work with your tire supplier to help you tune this condition. Does this sportier version get a new tire?

Can you change body mounting points? If you raise the rear roll center, it will speed the response of the rear and decrease the roll bar rate required. This could decrease the toe change.

Can you regain the lost roll resistance at the back with proportional increase in the front bar, and retain overall roll resistance. The only problem I envissage with that is that it imparts more torsional stress to the body structure.

Regards
pat

Pat he tried that : "Shifting the balance to the front tires does not work, because then the front reacts to quickly and creates a yaw delay.... this is also not acceptable... "

Obviously it would be nice to raise the rear roll centre to reduce this delay, but changing hardpoint locations is not always an easy option in production cars.

How about increasing rear tyre pressure to reduce the yaw delay? (Long shot)

OK, life is bad. Realistically you've played with springs and bars and got to a good understanding. Since this is a transient condition look at your shocks. I'd be thinking about bumping the rear shocks up a bit in their low velocity jounce section.

The other thing to look at to reduce yaw delay is a constant contact jounce bumper (a foam cylinder over the shock rod) and reducing your road spring rate. This increases the effective roll rate while maintaining the same bounce rate.

Cheers

Greg Locock

"Since this is a transient condition look at your shocks. I'd be thinking about bumping the rear shocks up a bit in their low velocity jounce section. "

Got a set coming in the morning with increased low speed D/F!  One step ahead of you!

"The other thing to look at to reduce yaw delay is a constant contact jounce bumper (a foam cylinder over the shock rod) and reducing your road spring rate. This increases the effective roll rate while maintaining the same bounce rate."

Long Jounce bumpers have been tried!  It definatly make the car more "solid", and I can lower the spring rates.... BUT Life is never simple!  2 Problems.  Problem 1 is endurance... the jounce bumpers have shown to decrease in spring rate by almost half.  So they "wear out" and then the vehicle starts handling badly and we have warrenty work that I am responsible for in the long run.  Number 2 is body accuracy!  Our vehicle height has a tolerance of about +/- 7mm.  In actual production it never sees that much, but some preproduction runs on other cars off this platform have been as much as 5mm different.  That doesn't seem like a lot, but this platform is very sensitive.  So if the contact point of the Jounce bumper to the S/A changes from car to car....they all handle different, and I have to fix that.....more work for me!  So I have almost given up on using long jounce bumpers, but it is still in the back of my head.  But actually, YES it does decrease the yaw delay, but it doesn't fix the big problem of toe-out rear oversteer feeling.

I did make a little progress today by using stiffer bushing on the toe link, but the response is way to much, so I will try to decrease the stiffness of the trailing arm bushing to help the toe link and lower arm move together.  Currently the lower arm seems to remain fixed, but the just the toe link moves vertically(knuckle rotating about the B-point of the lower arm).  Hopefully the weaker/more compliant trailing arm bushing will help the lower arm and toe link move together in the longitudinal direction instead of forceing the toe link to move vertically.  Hopefully it brings the response back down to a resonable level, and not to an unaacceptably low level.


BTW... you guys have been great!  I don't have alot of help at my job, and I am the only one in the states doing my work for ALL our cars, so it is very nice to bounce ideas and questions off others with similar backgrounds.  Sounds like a lot of work, but at least I have got to work on 3 totally different vehicle classifications in the past 3 years.  I just hope my hard work pays off in the long run.

"Work with your tire supplier to help you tune this condition. Does this sportier version get a new tire?"

Of course it gets a new tire!  Much stiffer and lower sidewall to offset the higher roll stifness.  The tires are actually pretty good at the moment.  We have had 2 submisions, and they are amost what we want, so tires are not that big of an issue at the moment.

"Can you change body mounting points? If you raise the rear roll center, it will speed the response of the rear and decrease the roll bar rate required. This could decrease the toe change."

As Greg mentioned above.... it would be nice, but changes like that require alot of money in production cars when you want to use as many carry-over parts on a platform...including the rear subframe/crossmember.  As I said earlier, the design is crap, but I have to fix it because the chassis design department has more "pull" in the company, and nobody wants to listen to a young engineer on only his third vehicle!  And if I can mask thier mistakes, I come out looking like a hero, and in the long run I can say that they need to start checking thier designs a little more closely in the future, so this NEVER happens again.

"How about increasing rear tyre pressure to reduce the yaw delay? (Long shot)"

We have a new standard for tire pressure that was developed with the tire manufacturers.  This is basically in response to the Ford Explorer issue here in the US.  The US government is really watching what car companies and tire manufacturers do after that incident.  I don't care who's fault it was, but at least the rest of us got a better guidline to follow, and we have to stick with it.  So basically our pressures are set to ideal manufacturer's recomendations.  So the days of ride and handling tuning with air pressure is gone for good in the US.

Can you do what Jaguar did with their IRS way-back-when on the XK-E's?  Two spring/shock units per side, one set ahead of the axle and one set behind it, to provide better symmetry of resistance.  Barring single spring or shock failure, that would leave only the antiroll bar with significant asymmetry.  There might even be some marketing benefits to it.

A fairly ancient paper by a Mr. Bergman suggests in part that 0.1 deg/deg of roll underteer is a little too high.  My own thoughts on rear steer in an IRS are towards minimizing it, in the interest of minimizing lift-throttle oversteer where toe-in in jounce (bump) at least partly goes away without steering input.

basically our pressures are set to ideal manufacturer's recomendations.  So the days of ride and handling tuning with air pressure is gone for good in the US.

What constitutes "ideal manufacturer's recommendations"?  I'm basically curious here, and I'm not sure how completely limiting this might be, as inflation pressures would have to be at least high enough to be associated with capacities somewhat above the static tire load anyway.  Is there also some arbitrarily defined upper limit as well?.  What I'm getting at is that while something as extreme as the early Corvair tire pressure differentials are probably out the window (something like 15/26 or 15/28 IIRC) it seems that there should remain a range of a couple of psi either way for a little fine tuning room.

Norm

Hang on, we're talking about a production vehicle here, and, from the sounds of it, one where ride and handling has no real priority. Adding twenty-fifty bucks and 4 kg a car in extra springs and shocks and brackets is NOT going to be on the cards, even if there is room for them.

The toe/roll figure varies quite a lot even for vehicles that handle well, but 0.1 deg/deg is a typical figure near design height. It has to be considered as part of the total understeer budget - pulling numbers out of a hat without considering the compliance understeer and the front axle is not especially helpful. Modern cars will have more u/s than older ones for stability reasons (and you wait and see what happens when the new NHTSA rollover rules come in). There seem to be two schools of thought with this curve - there's the 'minimise it' school, who end up with an s shaped curve, and there's the 'linearise it' school, who end up with a straight line, at the cost of greater total roll understeer. A very broad bush statement based on K&C results for 8 recent large sedans cars is that luxury sports tend to be minimised, whereas handling sports tend to be linearised. I wouldn't treat that as gospel - that is not a very scientific summary.

The tire pressure limitation is an interesting, and fairly sensible, proposal. I can't see why we'd be allowed to lower the pressure - if the manufacturer says that is what is needed to handle a particular load then it is hard to argue.

Cheers

Greg Locock

The tire pressure limitation is an interesting, and fairly sensible, proposal. I can't see why we'd be allowed to lower the pressure - if the manufacturer says that is what is needed to handle a particular load then it is hard to argue.

Let me try again.  "ideal manufacturer's recomendations" suggests that other settings might at least be permissible (most likely over a fairly small range) even though slightly different settings within such a range might not meet somebody else's definition of "ideal", interpreted to mean "optimum".  And that's where I was asking what the basis for the manufacturer's recommendations might be (I'm more or less hoping that it's based on a logicical evaluation rather than being a somewhat arbitrary cutoff point).  

While it makes sense to require the pressure to be adequate to carry the load (ummm, would this be the static load or some max value envelope of transient loading?), the first mention of this tire pressure issue came in response to whether or not a higher rear tire pressure could be specified.  Short of said pressure being in excess of the rated maximum tire pressure, I don't see where an increased pressure would not be a legitimate tuning tool.

Norm

I agree, higher should be fine.

I don't know how they'll cope with real conditions. Obviously, particularly for the rear tyre of a front wheel drive car, the load could be very near kerb weight for 99% of the time, yet that other 1% it could be 5 up, full cargo, towing a trailer. This could almost double the static load.

Anyone who believes that the average owner will modify their pressures to suit hasn't met many average owners.

Cheers

Greg Locock

Hi Greg,

Anyone who believes that the average owner will modify their pressures to suit hasn't met many average owners.

Agreed.  It's doubtful that many even realize that some cars already have more than one set of mfr-recommended pressures posted, depending on the loading (at least two of my cars' owner's manuals and/or pressure stickers contain such info).  And I'd expect that most of the folks that do are involved in some form of motorsport competition or high performance on-track lapping.

I can also picture that various loadings of a transient or quasi steady-state nature such as that due to lateral and longitudinal accelerations could affect the loading used as a design basis, if only as a simplified concession to fatigue.

Norm

Is it possible for you to tune the lateral force steer to counter the roll oversteer?

Thanks.

Yes, but it is going to lead to the possibility in real events where the two will be fighting each other. This is less important on the race track, where your roll and lat acc should be in phase.


I just had a long conversation with someone who thinks about this stuff long and hard and he said:

1) Roll oversteer of 0.1 /is/ on the high side (sorry Norm, you were right), and "proved" it by pulling the plot of the best handling car (tm) which was more like 0.03. Fair enough, but I still have plots showing 0.1 for good handling cars.

2) People do strange things to their roll and bump steer plots, depending on which they think is most important. This may explain why luxury sports have funny roll steer curves - they were trying to optimise the bump steer response.

3) We managed to persuade ourselves that compliance oversteer (at the back) should preferably  be as linear as possible, and MUST account for variations in pneumatic trail. This doesn't apply to cheaper suspensions that may need a bit of help.

Cheers

Greg Locock

This is a very interesting thread. I realise that in this instance it is probably too difficult to justify a change in the rear roll centre height, however I wanted to propose REDUCING the rear roll centre height rather than increasing it.

Several things happen when you adjust the roll centre height, including effecting the load transfer across the axle. Reducing the roll centre height should reduce the load transfer across the axle which will increase the effective cornering stiffness of the axle and inturn reduce the slip angle and add a little bit more understeer. It will also mean that a higher percentage of the load transfer goes through the springs, shocks and ARB hence the vehicle should hopefully become a little more responsive to tuning of these components. The total roll moment will have been increased so you will need to re-check your roll stiffness to ensure the roll angle is still within your target range.

Out of interest what is the roll axis of the vehicle? I have done some tests in a driving simulator where I changed the roll axis by changing the rear roll centre height. The base condition had a 5degree inclination. I dropped the rear roll centre to get a parallel roll axis and the vehicle felt very 'crisp', however when I increased the roll axis to 10degree and again drove through the lane change, the vehicle went into a series of large 'fish tails' as the rear tried to break away.

I hope this helps, yes I realise that there are many coupled effects as a result of changing the roll centres etc...

Andrew

This is a fairly old topic, but I thought I would wrap it up a bit and give some insite to others what I found.

This is still an issue with the vehicle, but I think I masked it pretty well.  What has been done since this is started:

Redistributed the roll stiffness:  I reduced the front spring rate by more than I reduced the rear, and I increased the rear ARB.  The vehicle is now about 57.1% Front roll stiffness dist, with a load transfer rate ratio of about .90.  This is a sporty FF vehicle.  I wish the load transfer rate ratio was less, but basically the gyometry does not allow it.

Increased the front compliant understeer:  after reviewing the front compliance steer, I noticed it was very stiff.  I reduced the front crossmember mounting bushing stiffness.  I wanted to just change the rear mounting stiffness(like the VW golf), but two different bushing rates was not accepted by the chassis dept.  And this way we can commonize with another vehicle.

So basically I have reduced the front suspension response and let the rear do more of the work.  This also has reduced the yaw rate acceleration to become more linear, which in turn masks the sudden change in roll steer.  Having more rear roll stiffness has also reduced the actual amount of roll, so it limits the amount of roll steer.

Biggest problem NOW:  A 4-5Hz bounce in the rear.  It makes a very uncomfortable jiggle in your stomach.  I am assuming the damping force around 0.1-0.2 m/s is too much and not allowing the rear suspension to stroke.  The problem is we can't use a long jounce bumper, so I need damping force to creat a reaction for the steering feel.  I need to reduce that area in the rear and in the front to balance it back out to regain the steering response.

I also increased the bushing stiffness of the inner/rearward upper arm bushing of the rear suspension.  This helped a bit to reduce the knucle rotation of the multilink system.  

Anyone who reads this:  NEVER offset the spring/damper knuckle fitting point from the centerline of the rear knuckle if the vehicle is not AWD.  And if possible, NEVER offset the rear ARB reaction fitting point from the centerline of the rear knuckle.  These two issues have created so MANY headaches this past six months.  On a rear multi-link suspension, fix the knuckle to the centerline of the knuckle and mount the ARB to the shock or similar centered position.

I think I follow most of what you're saying.

I wonder if maybe you also want the lines of actions of the springs/bars/shocks to be consistent with the intended path of the loaded ball joint, at least as seen in side view.  That's one of the immediately noticeable things about the C5's rear suspension when you get to see one up on a lift, BTW.

Regarding the "tinkering" with compliances - what about local chassis structural compliances?  It's easy to assume that the chassis is rigid, but it's really just another spring in series, and will deflect under load.  Maybe you could get some help from the structural guys?

Norm

"Regarding the "tinkering" with compliances - what about local chassis structural compliances?  It's easy to assume that the chassis is rigid, but it's really just another spring in series, and will deflect under load.  Maybe you could get some help from the structural guys?"

That is a very good point. My models account for body (and arm) complance in a variety of ways, but as of the next program we'll be switching to a fully compliant model of the body, which is derived from the finite element analysis of the bodyshell. It is not unusual to see mounting points that are only slightly stiffer than the bush that bolts into them. This means that very large changes in the bush's stiffness are needed to get a measurable difference in the car, and that the isolation at that bush is very poor.

Cheers

Greg Locock

Many people assume that for an independent suspension, the plot of toe vs wheel center travel that results from a ride event is the same as the one that results from a roll event, or a single wheel event.  This assumption assumes that the left side of the vehicle is independent of the right.  This is not true.  

Consider the poster’s rear multilink suspension.  In a ride event the wheel may toe in with jounce travel, yet in a roll he observed the event the effect as the opposite.  As the vehicle rolls, the ARB applies forces to the knuckle that will tend to rotate it and cause a steer effect.  

The original poster posed a solution of linking the spring and ARB as close to wheel center as possible to minimize the moments that it produces on the knuckle and therefore minimize the steer effect.

I have a different view on things.  With careful thought to the geometry, you can have the stab bar moments on the knuckle help you.  If you link the ARB away from wheel center, but in a fashion that causes toe in with jounce (for the rear) you can achieve a toe curve that has little to no toe change in a parallel wheel travel event (good for tire wear) and has the level of toe in you require during a roll event.  

In practice you can not really achieve zero steer in ride and your target roll steer in roll for all vehicles because you will have different spring rates and different ARB rates on different vehicle trim levels.  For a multilink rear, what you can do is link the stab bar in a manner that toes the wheel in the opposite direction that the offset spring force toes the wheel.  This should bring your compliant roll steer number closer to the one predicted kinematically.

Another thing...

If this was a front suspension things get even more complicated since the left side is linked to the right through the steering system compliance.  With rack compliance, a net rack force will cause a steer effect.

Anything that could put a moment about the kingpin can cause a net rack force.  In a roll event if there is a net rack force, it will cause a steer effect.