Roll Steer Plot
Roll Steer Plot
(OP)
Can someone post, or point me in the direction of a "proper" roll steer plot. What I am looking for is an example of ideal conditions for a REAR multilink (or other similar) suspension design. The project I am currently working on has some issues with roll oversteer and I what some fuel to back up what I am feeling in the car. Basically I am looking for a toe patern plot in relationship with body/suspension roll.
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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
RE: Roll Steer Plot
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
Taking the rear ARB off is a good move, we only put them on for marketing half the time. (well, exaggeration)
Cheers
Greg Locock
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
Regards
pat
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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
RE: Roll Steer Plot
Thanks.
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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
RE: Roll Steer Plot
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.
RE: Roll Steer Plot
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.