Roll axis inclination

[2SlowJoe]

What side view roll axis inclination do you guys usually target and why?

Here are some observations:

The inclination of the roll axis effects the roll-yaw coupling. Having the axis inclined so that it goes up as it goes rearward causes the body to yaw out of the turn as the body rolls.

The Maurice Olley notes suggest that due to the roll yaw coupling, a roll axis that goes up as it goes rearward adds roll damping. The roll causes a yaw out of the turn. A roll velocity causes a yaw velocity. The yaw causes a lateral accel that tries to unroll the vehicle. This is all proportional to vehicle speed so it could be thought of as a form of roll damping. The increased roll/yaw damping can reduce occupant head toss.

An inclined roll axis will also effect turn in behavior. The more it is inclined up as it goes rearward, the slower the turn in response should be since the body is yawing out of the turn.

If the axis is inclined so that it goes down as it goes rearward, the yaw roll coupling will cause some self feeding. The roll will cause a yaw that turns into the turn which creates a lateral accel that also turns into the turn which adds to the roll angle…

Inclining the roll axis also has effects on TLLTD. If it is inclined so that it goes up as it moves rearward, there will be a larger TLLTD to the rear suspension from the RCH contributions. This could probably be easily mitigated by tuning the Anti Roll Bars, but it is something to think about. With a high enough rear roll center a rear Anti Roll bar may not be necessary.

Does any of this matter if your front suspension is a strut? Struts typically have RCH vertical migrations of ~2:1 in ride motion. This also causes a RCH lateral motion in roll. If the front RCH is moving all over the place (OK up and to the outside wheel) as you roll does the initial Roll axis inclination mean much?

What do you guys do?

 

[hpfreak]

Interesting post, and quite a mouthfull!

"Inclining the roll axis also has effects on TLLTD. If it is inclined so that it goes up as it moves rearward, there will be a larger TLLTD to the rear suspension from the RCH contributions. This could probably be easily mitigated by tuning the Anti Roll Bars, but it is something to think about. With a high enough rear roll center a rear Anti Roll bar may not be necessary."

This is a noble cause, reducing pieces and $. This has been tried here, and the vehicle rigidity must be very high. Also, the roll feel gets weird. The front "dives" and the rear "stands" during a turn.

"Does any of this matter if your front suspension is a strut? Struts typically have RCH vertical migrations of ~2:1 in ride motion. This also causes a RCH lateral motion in roll. If the front RCH is moving all over the place (OK up and to the outside wheel) as you roll does the initial Roll axis inclination mean much?"

I believe the migration is more important than initial RCH, but then again, they are directly related. The front needs to be matched to the rear to get the right roll feel. Making the rch and rc migration compromise is one of the many things simulators can help with.

 

[Joest]

Nice discussion.

My experience with heavy sedan (>2800lb) with live rear axles, is that the rear anti roll bar can be deleted if the roll couple distribution is managed properly. We typically run a front RCH of about 2" and a rear RCH of about 8-9" (WB of ~102"). The front is a strut setup and the rear uses a watts link mounted on the axle. Our roll couple distribution is about 80-85% front (15-20% rear) biased. This configuration does not require a rear anti roll bar, even with relatively soft (250-300lb/in) rear springs. Additionally, it should be noted that this setup has been good enough to dominate the American Iron series that last few years. I don't know the specifics of your application, but it sounds like you are on the right track.

 

[NormPeterson]

Inclination of the roll axis affects the “feel” of the vehicle during transients, since the TLLTD does not remain constant over time. Lateral load transfer through the RC’s occurs almost immediately, while lateral load transfer that is based on suspension roll stiffnesses has to wait for the chassis to attain some finite roll angle (and for the suspension to move finite amounts) and hence lags in time as some function of chassis inertia in roll and suspension frequencies. So if you’ve got a vehicle with a roll axis that slopes upward toward the rear you start out the transient with most of the TLLTD at the rear (oversteer, assuming all else equal). But assuming that the vehicle is tuned to some reasonable steady state balance (meaning that we can probably ignore the “drifting” crowd!), as the roll does develop the TLLTD shifts forward. So that initial oversteer shifts toward final understeer. You mostly adapt to this without conscious thought and simply dial in a little bit more steer angle. That’s far easier for most folks than the reverse situation, with some initial understeer shifting toward oversteer. Step changes in steer input exaggerate this, but then again, the times when you are trying to make those kind of inputs you kind of want the car to be predictable and feel "stuck down".

I think that at least with moderately or more powerful stick-axle RWD cars you probably are best off if you can satisfy your roll requirement without needing a rear sta-bar if you can do so without the ride becoming intolerable.

I’m having a little difficulty seeing front strut RC migration being upward and toward the outside, at least in pure roll. It seems to me more like it would be downward and toward the inboard side. But I’m assuming left vs right side suspension symmetry . . .

Norm

 

[GregLocock]

I think it was John Miles (ex Lotus F1 driver and R&H guru) who told me that a properly designed car would not need a rear anti roll bar. We always ended up with one, often of infinitesimal effect, because the customer expected to see one.

Cheers

Greg Locock

 

[CRidnour]

Message for JOEST
Hate to be stupid but let's see
sedan > 2800
struts forward and
running AI?
I crew chief in Spd GT,GT1,AS, T2 and the world's fastest F body SSB. Every run VIR, Summit Point, Watkins Glen or Lime Rock? We'll be at the Speed GT race at the end of the month (LR)
I'd like to talk -
Craig

 

[Joest]

CRidnour,

Just to clear things up, AI refers to the American Iron series (AI and AIX). Specifically, we run in the AI West (there is also AI east) in both the AI class. The 2 different classes are AI, which has a power to weight ratio limit and the AIX, which does not. Our AIX car runs a detuned GT1 small block Ford with approx. 600 crankshaft HP. I don't plan to go East much this season. Maybe you will be here in our backyard at Infineon (Sears) for the Speedvision WC in July?

-Joest

 

[chassismatt]

wow Joest! 80-85% looks really high compared to the front driver stuff I am used to (60-65%F TLLTD)- just enough to maintain limit understeer in a steady state turn. Do you typically run that much front TLLTD for rear drivers to offset the oversteer tendancy of the rear tires being driven with a WHOLE LOTTA power?

By the way- great topic, I am in the middle of this subject on an experiment right now, and have noticed a quicker turn in with the roll center height lower in the rear.

In my suspension, in order to lower the RCH, I have to drastically reduce the rear roll steer, so by the numbers, rear cornering compliance increases and understeer decreases which tells me that lateral acceleration response time should decrease (due to large rear compliance). The other thing I guess you guys probably already know is that as you raise the rear roll center, you need to pay attention to the amount of jacking that is being introduced.

Matt

 

[Joest]

Matt,

Q:
"Do you typically run that much front TLLTD for rear drivers to offset the oversteer tendency of the rear tires being driven with a WHOLE LOTTA power?"

A:
Yes, the non-driven axle must provide the majority of the roll resistance so that the driven wheel maintain as near-to-equal weight distribution as possible. Since our cars are designed to not require a rear anti sway bar, tuning is accomplished by increasing the front roll stiffness until understeer begins to occur (along with anti squat). We then back off a bit so that the car oversteers just a bit during limit cornering.

The procedure is reversed for front wheel drive apps. Cars that lift one of the rear tires off (common with a high RC) are set up for 100% weight transfer at the rear. In this case, a larger rear bar will provide no additional benefit. If body roll is still unacceptable, stiffer spring or a larger front anti-sway bar must be used. Make sure to only lower your RC as much as needed to avoid needing too much roll resistance. Going too low will inevitably compromise acceleration out of corners.

Note: The inside front wheel has also been know to lift on rear engine Porsche such as the ones that run ALSM GT3 and older 911. The chassis is stiff enough to support full weight transfer at the front and the outside tire is not loaded enough to cause the car to understeer. This is the advantage of a Porsche with the engine in back. With a 60/40 F/R weight split static, under full braking this changes to about 50/50 just as is required. Now, as the driver immediately gets back on the throttle exiting the corner, the front tires see a lot less load than most front engine cars. This lets the front outside tire handle the loads induced by the heavily biased roll couple distribution freeing up the rear tires’ traction budget to be used for acceleration.

-Joest

 

[gielhansen]

I'm confused.

Norm Peterson says: "So if you’ve got a vehicle with a roll axis that slopes upward toward the rear you start out the transient with most of the TLLTD at the rear (oversteer, assuming all else equal)."

This seams logical.

But Matt Purdy's statement seems to say the opposite: "I am in the middle of this subject on an experiment right now, and have noticed a quicker turn in with the roll center height lower in the rear."

Can anyone explain this to me?

Giel

 

[GregLocock]

Matt is not doing a single variable experiment. As in most prcatical experiments he can't just change one thing.

Also, he may have changed the front/rear roll balance, maybe his front tyres like the new setting.

Also, if he has changed how the power gets onto the ground then all bets are off (if he is turning in under power, which seems unlikely)



Cheers

Greg Locock

 

[NormPeterson]

I think what Matt is dealing with is most likely either a torque arm or a 3-link suspension with a PHB or a Watts link. Or perhaps a fancier version involving birdcages and/or decoupled torque arms. Anyway, the amount of lateral weight transferred across the rear axle is not the only effect that you get when you drop a RCH that's primarily defined by a PHB or WL even if that's the only physical change that you make. Note that he also mentioned drastically reducing the roll steer, which I read as being a change from some amount of rear axle oversteer/vehicle understeer toward rear axle understeer/vehicle oversteer. And that's exactly what you'd expect if you look at what is happening to the rear axle's own roll axis rather than what a single point along it is doing.

It's quite possible to push the vehicle balance past neutral steer clear into oversteer if you lower the PHB or WL without regard to where the other point that defines the axle steer is located. The term "loose" does seem to fit here . . .

Norm

 

[chassismatt]

Guys- I am dealing with a torsion beam rear axle. The experiment was meant to focus on the effects of reducing rear roll steer, so perhaps I misled you a bit with my statement- I apologize for moving the thread off topic (roll axis inclination to roll steer effects)!!.

Because it is a torsion beam axle, the roll center heigt and roll steer are coupled, so my RCH dropped 10mm with a reduction from ~9% to ~1% rear roll steer. I upped the stabilizer bar size in the rear in order to maintain the same roll gradient and TLLTD. The quicker turn in I mentioned also comes with a pretty large sideslip overshoot with a constant speed step steer.

 

[Tmoose]

Hi Greg,
A while ago you said " snip......... who told me that a properly designed car would not need a rear anti roll bar. "

I've been thinking about that, and I wondered if the manufacturer got the "proper design" for one passenger car model, the station wagon (weight distribution), or the GT (lower and stiffer springs) might require some brand new suspension parts designed? I've added bars to a few cars, and believe that sometimes the change was to how the car "felt" more than an a real cornering power increase.

 

[GregLocock]

For sure, one suspension architecture that has to support more than one centre of gravity location/principal axes inclination will probably need a roll bar. That discussion occurred long ago when working on nice cars that were (or should have been) well optimised, not compromised at every turn!


Cheers

Greg Locock