Suspension Design

[Bob Putnam]

Originally Posted by nota2266

Russ,
How about

" Parallel to the median gradient of the mass centroid "

but there exists differing schools of thought in this regard as to whether or not this is the ideal

IMHO, there's no good reason to match the roll center to the mass centroid when roll resistance can be adjusted with springs and anti-sway bars. Don't forget that a high roll center sends the bottom of the tire in an outward arc. This has a destabilizing effect when the car encounters bumps, plus it jacks the car up under side loads.

[Trevor Booth]

Russ,
momentary brain fade is a regular occurrence here in Kurrajong , keeps me going !!

[Trevor Booth]

As I say Bob

"there exists differing schools of thought in this regard as to whether or not this is the ideal"

[Russ Noble]

Bob,

I don't think it's usually matched to it. It's usually below it but roughly parallel to it. It doesn't necessarily even have to be high, in fact some roll centres can be below ground level.

Do you think independent suspension is a good thing? The use of antiroll bars inevitably means you no longer have truly independent suspension. More so with really heavy bars.

There is no 'correct' design, it's all a compromise. You have to decide what features are important to you and accept the downsides that this will cause in other areas.

I hope I've got this right, I'm starting to run out of excuses....

Cheers

[Bob Putnam]

Unless the wheel rate is effectively zero, there is no such thing as totally independent suspension. Even without anti-sway bars, a bump at one corner will (non-instantaneously) reduce the load at two other wheels. Anti-sway bars actually reduce this tendency. Where anti-sway bars fail is their lack of resistance to pitch, which is why race cars typically use them only to finely tune the front/rear balance, and let the springs do most of the roll resistance.

[Eric]

Originally Posted by andys

Eric,

Would you be kind enough to explain why the C5 geometry is all wrong? Also, what are the guidelines for roll centers for mid-engined cars vs. the C5.

Thank you,

Andy

Sure,
The chassis packaging and therefore weight distribution of a C5 is different than a mid engined gt40. The height of the CG is also different, both at the individual axle planes and the one true CG point.

The C5 is also designed with a great deal of anti squat and anti dive in the suspension geometry. More than is needed for a GT. For the front, there can be a slight bind to the suspension that would be unnecessary for a mid engined car.

A GT simply transfers weight and loads far differently than a C5 to the contact patches by design. Any suspension geometry is designed to facilitate and regulate this load transfer. The wrong one will very simply create a car that does not roll, yaw, or pitch in the controlled manner that it could with the correct geometry.

Alll this happens around the CG point which is in a very different place longitudinally and in height between these 2 cars. We are talking about controling moment arms, so it is critical that we prepare for them.

As for roll centers, the C5 Front RC point is much higher than the GT, and in fact the RC point could be high enough to create a jacking effect in the front in roll movements--very bad! There is an imaginary longitudinal axis through the CG of any car about which the car will roll. This axis is not level, and is affected by the weight distribution. A general guideline is to keep the RC points equal distant from this roll axis in height. This requires that the suspension arms be angled and the lengths correct in order to place the RC in the correct location. This will keep the car feeling "natural" as it rolls about an axis which is correct. If it is wrong...the car gets "spooky".

RC manipulation is a black art of racecar design, and it is a very open and broad subject. There are many many books written about suspension design and theory. So I will leave more in-depth discussion for them rather than fill up pages here online.

I hope this has been of some help, let me know what more I can add.

Cheers
Eric

[Jac Mac]

Been looking into this for my own car and have come to the conclusion that the vette uprights might not be the ' ideal' donor part for the GT40 for the following reasons;
1. The distance from axle c/l to top/bottom ball joints( particularly the bottom ball joint as this dictates the position of the brake rotor/caliper/wheel center) which of course influence the scrub radius acheivable.
2. The sheer size-bulk of the aluminum upright particularly in the steering arm/ball joint areas.
3. On the original GT40 tub the pickup points for the lower wishbone are approx 3" to 4" above the floor level with the rear pickup point slightly higher- when you use the vette uprights you have to mount them approx 1.5"/2" above floor level to achieve similar geometry.
4. While this does not create a huge problem with narrow 8" rims, you end up with a large scrub radius once you try to make it work with 10" or 11" wide rims, while I realise some folk are not too concerned about the larger scrub radius, it does limit the amount of lock available as the tyres start to foul on the front of the fuel tanks & inner guard.

Now to overcome these issues it would appear that either a fabricated upright with reduced axle c/l to lower ball joint distance along the lines of Jim C--CAV etc would help or a production upright with similar dimensions.There are plenty of these about.

Jac Mac

[Trevor Booth]

Eric,
Do you not mean--
The roll axis is the imaginary line through the front and rear roll centres.

It is this imaginary line which some people believe should be parallel or near parallel to the gradient of the mass centroid. (another imaginary line)

It is the relationship between these two imaginary lines which govern mass transfer in roll. (the roll couple) Another controlling factor is the transverse shift of the Roll centres as the car rolls.

Roll stiffness as mentioned by Bob P dictates the amount of mass transfer and how it is distributed.

This is the " black art" part assisted these days by 3D computer simulation.

[Trevor Booth]

Jac Mac,
You could always fabricate your own in stainless steel, very light , very strong

[Russ Noble]

Originally Posted by nota2266

Eric,
Another controlling factor is the transverse shift of the Roll centres as the car rolls.

Trevor, do you mean the transverse shift of the Instant Centres?

That being the case I have never been able to find out, or understand, how the transverse movement of the IC affects the car. I know with a long effective Swing Axle Length you can get a massive shift in IC, not so much with short SAL. What practical effect does this shift have?

Thanks,

[Eric]

Originally Posted by nota2266

Eric,
Do you not mean--
The roll axis is the imaginary line through the front and rear roll centres.

It is this imaginary line which some people believe should be parallel or near parallel to the gradient of the mass centroid. (another imaginary line)

It is the relationship between these two imaginary lines which govern mass transfer in roll. (the roll couple) Another controlling factor is the transverse shift of the Roll centres as the car rolls.

Roll stiffness as mentioned by Bob P dictates the amount of mass transfer and how it is distributed.

This is the " black art" part assisted these days by 3D computer simulation.

Yes, I do mean that as well. Some engineers would argue that the gradient of the mass centroid as a roll axis is false, but it is still the basis of many suspension designs. I happen to agree with the concept.

As a rule of thumb the place to start with a clean sheet is to keep the roll couple equal lengths in front and rear. In fine tuning it generally changes depending on the vehicles functional envelope, and multiple other factors of the realities of individual vehicle dynamics. There is no perfect formula for where to put the roll centers initially, its usually an educated guess based on the vehicle parameters.

the amount of weight transfer in the vehicle is not exactly controlled by spring rate, the mass will shift based on CG position, the springs are reactionary devices, not preventitive.

I work with many very sophisticated design level suspension CAD/CAE programs. They are very useful in initial phases, but vehicles always go through many changes when they hit the road as mules, and many unpredictable dynamics evolve.

Finding the final relationship between design type, geometry, alignment, spring-damper-swaybar, and ackerman settings is still very much an art that every manufacturer prides itself in having.


I am including an image of a simple mid-engine mass centroid and roll axis diagram so that it may help illustrate the concept.

Also, I have design level experience with the C5, and measured a C5 suspension with a 3 axis measuring system on a surface plate in the past, and I include images of it (notice the amount of front anti dive and the huge difference in the front vs rear arm orientation)
Also included is the simple CAD arm and steering arm position at ride height.

Cheers
Eric

[Trevor Booth]

Eric,
we are on similar wavelengths

"the amount of weight transfer in the vehicle is not exactly controlled by spring rate "

I am not saying this, yes springs are reactionary devices,- reacting to the mass transfer.

I wouldn't so much say that roll centres are based on the vehicle parameters but rather on how you want the vehicle to behave, nor would I say it is an educated guess. The kinetics may be calculated, tyre data is more readily available and 3D mechanism dynamics programmes make it even easier to narrow down Rc locations. Naturally some fine tuning is always necessary on rollout.

I have been playing with suspensions since about 1968 from motorbikes to mining trucks and every thing in between. Over that period the advances in the knowledge of suspension behaviour have been enormous driven mainly by a better understanding of what the tyre is actually doing and how best to extract max grip from the tyre.

I note that you are running Catia

Cheers

[Trevor Booth]

Russ,
transverse shift of RC has a corresponding shift in Roll Axis.
There is a challenge for you, as the vehicle rolls you have a constantly changing roll axis skewing about the longitudinal centreline in plan view.

What do you think that is doing

[Russ Noble]

In end view I can see the RC raising or lowering slightly but would have thought that was relatively insignificant.

In plan view? Hmmmm......that is a challenge for me...... I might have to think about this for a while.....

Thanks Trevor.

Cheers,

[Jac Mac]

Originally Posted by nota2266

Russ,
transverse shift of RC has a corresponding shift in Roll Axis.
There is a challenge for you, as the vehicle rolls you have a constantly changing roll axis skewing about the longitudinal centreline in plan view.

What do you think that is doing

Hey Trevor,
Stop playing with Russ on this stuff, next time I visit him the car will be in a thousand pieces for a total rethink at this rate!!

Jac Mac

[Trevor Booth]

Jac Mac,
I have the greatest admiration for the ingenuity of Kiwi's, he will work it out and besides I thought you were the reinstated pit crew.

[Russ Noble]

Yeah, Jac Mac won't accept my lucrative redundancy package! He still insists on being on the team, says he needs to fix my scrub radius, whatever that is....????

Cheers

PS Trevor, I've always struggled trying to figure this sideshift effect. Somewhere along the line my understanding of the basics of the dynamics involved comes up short. May have to go up to the library and do some more reading. But in the meantime I've got a car to build, so that's not going to happen any time soon. Maybe one morning I will wake up and the penny will drop..........