Usable range for roll center height in racing conditions 1

[sierra4000]

Hello ,
How is defined the usable range for roll center height in racing conditions?
How is possible to fight against bad RCH ? (how effective is?)

Thanks for opinions
Radek

 

[GregLocock]

I'd have thought we've got 7 different threads on RCH already, and most of them terminate with some sort of conclusion that it is just horses for courses.

I worked on a car that had either an IRS or a watts link live axle. We did not change the front suspension, and I'm damned if I could pick the difference between the two on public roads unless they were rough. The difference in RCH would be 200mm or so.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

Zero (ground level) is a practical minimum. Going below in some conditions isn't the end of the world.

Independent suspension + high roll center = jacking. Jacking + soft spring rates = cars that are prone to rolling over by tripping over their own swing axles.

Beam axle will tolerate high roll center but it still has bad side effects in one-wheel bump.

You can make any bad suspension design work by not letting it move. (high spring and damping rates)

Pick something reasonable and run with it.

 

[sierra4000]

OK ,
i understand upper level when car start lifting,
but where is real bottom level? can be any lower value always compensate with extra roll bar stiffness?
i think then suffers opposite wheels independency
or coming transitient problems???

 

[BrianPetersen]

Yes, a lower roll center will increase the roll couple (based on the vertical distance between roll center and center of gravity) so if controlling roll is a priority, it would want stiffer springing and/or anti-roll bar rates.

Consider what happens to the contact patch in one-wheel bump if the instant center is below ground level and what that might do to the steering.

Roll centers / instant centers near ground level aren't the end of the world. Look at Formula 1 cars with their upper and lower wishbones out in plain view. (The arms are almost horizontal). There's not much camber change with roll, and the roll center at nominal ride height is near ground level. There's also not much functional suspension travel at all - the spring and antiroll rates barely let it move. And they're pretty fast on a smooth track. Might not work out so well if you tried to run one of those through a WRC gravel section, or go off-road desert racing with it.

And that begs the question ... what type of racing and on what type of track. I can guess, based on your posting history, but I don't want to guess. The design criteria for Formula 1 racing aren't the design criteria for WRC racing and those aren't the design criteria for the Baja 1000 racing.

Just saying "roadracing" doesn't fully address the question, because I'll guarantee that a F1 car will have difficulty with the rough, frost-heaved surfaces of accessible-to-amateur-level racetracks in my area (Canada). My thing is motorcycle roadracing, and I set mine up to have plenty of compliance and suspension travel. Too stiff beats up the rider (me) and sends the bike chattering off into the weeds. When you have an application that needs compliance and suspension travel, that's when it all starts getting tough.

 

[sierra4000]

Yes Brian,
let's say the interest is on the saloon cars on the asphalt track around 2-2,5 Hz ride frequency up to 100 mm total wheeltravel

 

[BUGGAR]

"..........lower value always compensate with extra roll bar stiffness?"
In other words, designing in a lower "roll center" (that is, lower suspension roll resistance due to suspension geometry) to compensate for a too-stiff roll bar?

 

[sierra4000]

Buggar,
in other words:
when car is lowered, RCH go down.............is possible corrected with more stiffer anti roll bar? again and again?

(always is easyer increase ARB stiffness before restore RCH)

excuse my english

 

[BrianPetersen]

Given the ease of changing antiroll bars, I would think it's the other way around: having to use a very heavy antiroll bar in response to having a roll center too low (i.e. below ground level).

MacPherson front suspensions are rather well known for having lots of instant-center movement with suspension travel. The "tuners" who install suspension kits that slam the car to the ground end up placing the roll centers below ground level (and, often, with the instant-center on the "wrong" side of the car so that camber change on initial bump travel is the "wrong" way). 'Course, this is the same crowd that also installs lowering springs so low that the car is riding on bump stops thus having spring rates approach infinity, and antiroll bars that can only be described as "excessive", and ultra-low-profile tires with no sidewall, thus ending up with the aforementioned "you can make any bad suspension geometry work if you don't let it move" ... and a car that hops and skips and chatters over the slightest imperfections.

Don't do that.

 

[NormPeterson]

Is a too-low geo RC the unavoidable consequence of lowering when there are competition rules prohibiting things like the relocation of suspension pivot locations and tall ball joints? Is there some other overriding reason?

One thing you clearly don't want is a RC so low that the camber gain starts going in the wrong direction on the outboard wheel (this being consistent with a badly placed front view instant center as noted by Brian). Camber gain that only slows to a small value in the right direction probably isn't going to be optimal either.


Norm

 

[BrianPetersen]

Lowering something with MacPherson geometry without relocating pivot points and/or dropped spindles will lower the roll center by more than the amount that the car is lowered, and can cause camber gain with bump travel to go the wrong way. What you can do about that, depends on careful reading of the rulebook. The easiest fix may be to not lower it by enough to cause trouble.

If you can install adjustable upper strut mounts, you can get some static camber and you can play with strut inclination angle (hopefully both fore/aft and in/out). If you can get some clearance in the bolt holes where the strut attaches to the spindle, you can get some static camber. Sometimes there may be a different car (which may be platform-related) that has parts that will interchange but have different geometry that can be swapped - Audi TT versus VW Golf, for example. Sometimes you can play with the way the subframe attaches to the unibody (which has the effect of moving the control arm attachment points and the steering rack together). You can have "collision damage".

World Rally cars have MacPherson geometry that can be ... interesting.

 

[sierra4000]

Norm,
Yes rules
and
Relocation of suspension pivot locations often is not easy,
Usually used the upright extensions brings new problems with upright extra load and compliance or is not possible due to upright configuration (steering rack relocation not possible also)

There exist only one right way which is only new uprights which is really expensive,
that is this other reason

From that point of view seems stiffer ARB only one way to maintain car out of bumpstop

I'm trying to find this acceptable RCH bottom level

The camber gain seems like a reasonable boundary,
But really we want to have CoG height above when chassis to ground gap or wheel arch area still are sufficient?

 

[BrianPetersen]

With MacPherson, when the front-view angle between the chassis-end pivot, the ball joint, and the strut mount is at 90 degrees, that is the spot where the camber gain with further bump travel starts going the wrong way. With normal geometry (a few degrees of front-view inclination of the strut), this will already have the roll center below ground level. If the lower arm is horizontal (ball joint center same height above ground as the chassis-end control arm pivot) that will place toe roll center a little above ground level and still have a little bit of camber gain left in it with further suspension compression. In the VW world, the latter scenario is commonly regarded as being as low as you can go without starting to cause trouble.

I betcha there is not a whole lot of difference in ride height between these scenarios ... I'll guess 20-30mm ...

 

[sierra4000]

Yes Brian,
I know McPherson problems and kinematic,
and yes I know TT vs Golf differences this can little help but is insufficient for lowering over 50mm

WRC is different (money) level

 

[cibachrome]

TT Front roll center HF = 0, Rear HR = 150 mm
1999 Golf HF = +2 mm HR = 12.4 mm
2000 Golf HF = -2 mm HR = 11.5 mm (Probably the same as 1999 but higher roll stiffness f & r same distribution.

Hyundai (N=16) all have HF ~ 100 mm, HR = ~100 mm

 

[BrianPetersen]

Perhaps it would help the rest of us if we knew exactly which vehicle we are talking about here, in case someone knows something more about it.

At 50mm lowering ... How much reserve travel in compression do you need (to allow for braking, cornering, and bumps)? With most normal cars, 50mm lowering won't leave much left. You would need high spring rates ... except that high spring rates with shorter springs might lead to zero preload in jounce, or even the spring coming loose entirely.

The fix may be "don't lower it that much" ...

 

[sierra4000]

Brian,
is general question

no problem go 50mm down with shorten dampers
around 30mm bupstop gap is sufficient with high rate springs of course helper springs is need

 

[sierra4000]

excuse me Cibachrome,
i do not understand
although Your data are always interesting
what Your opinion? is camber gain realy limit?

 

[cibachrome]

You mentioned TT and Golf. I stated their measured roll centers at 2 pass load. My approach is always different. Start with: "What roll per g can you tolerate ?", then, "What suspension travel do you have ?". Then: "What ride rates do you need to accomplish track bumps ?", "What caster do you need to align tire MZ peak with tire FY peak for a pair of tires?", "What extra roll stiffness do you need to fulfill your roll per g target, given the roll per g from springs ?", then its front and rear bar sizes that are not heavy, easy to change, efficient and fit. What's left is roll centers to provide missing roll stiffness AND +- 5% tunable TLLTD with the bars in your hauler. Very simple actually. Consider moving some weights around to help you: battery, gas tank, driver seat, etc.

 

[sierra4000]

Perfect!
then TT vs Golf have different ride height (have different uprights)

for clarification:
"What suspension travel do you have ?"
i think direct related to "What ride rates do you need to accomplish track bumps ?"
"What roll per g can you tolerate ?"
less roll=less camber gaing requirement?
"What caster do you need to align tire MZ peak with tire FY peak for a pair of tires?"
is resultant (max acceptable) static camber due to camber gain (or camber loss?)

"What extra roll stiffness do you need to fulfill your roll per g target, given the roll per g from springs ?"
i understand

"then its front and rear bar sizes that are not heavy, easy to change, efficient and fit."
this probably i not understand, this mean for ARB stiffness no exist limit? (or maximum % stiffness contribution?)

"What's left is roll centers to provide missing roll stiffness AND +- 5% tunable TLLTD with the bars in your"
OK

seems that real limit is maximum acceptable static camber related to roll
yes?