n smikle's LMP car

[mep]

Lets start with your CoG height.
Do you have a rough idea where it is going to be?

My way of thinking goes along this:
Basically the main figure to think about is RC height in relation to CoG height.
Higher RC results in less roll but more weight transfer. As a rough value you can put it at 20% CoG height.
Maybe you can also set up some mathematical models to find optimum location. I would love to see that.

The next step is to make sure it does not migrate very much under roll and ride. As a rough value you can say it should not migrate much more than the size of your fist.

Yes I'm guilty because it's all BS, and how can I know if that's better? I don't, it's just a starting point. After all KRC is just.... ok, no, I don't want to start that discussion again.

What there was a discussion on RC's?
I missed that. Come on tell what you think.

[Caito]

Lets start with your CoG height.
Do you have a rough idea where it is going to be?

My way of thinking goes along this:
Basically the main figure to think about is RC height in relation to CoG height.
Higher RC results in less roll but more weight transfer. As a rough value you can put it at 20% CoG height.
Maybe you can also set up some mathematical models to find optimum location. I would love to see that.

The next step is to make sure it does not migrate very much under roll and ride. As a rough value you can say it should not migrate much more than the size of your fist.

Yes I'm guilty because it's all BS, and how can I know if that's better? I don't, it's just a starting point. After all KRC is just.... ok, no, I don't want to start that discussion again.

What there was a discussion on RC's?
I missed that. Come on tell what you think.

Higher RC(but below CoG) reduces roll but not weight transfer. Weight transfer reamins the same. What you would change is if the weight is reacted via the control arms("instant" weight transfer) or via the spring(not instant, depends on shock+damper)

My quote, was it ironic?

[PlatinumZealot]

If the roll center is in the transverse plane which contains the center of the wheels for a given end of the car, what happens when the wheels are turned? - Since the plane has now rotated and no longer transverse what do I do?


And what about where the front and rear roll axes intersect? What is the implication of that?

What is a good range of roll angle to analyse? And finally what Is a good range of change in ride height to analyse? (for say a GT car?)

[mep]

Yep Caito
Actually when I wrote these few lines I was not sure about the weight transfer issue but I kept it there for the reactions. I should spent some time to read trough this again. I remember that there was a major disadvantage, the reason why it is low on so many cars, of a high RC.

My quote, was it ironic?

You confuse me now. Was it ironic?
I didn't notice irony. You started with "After all KRC is just..." which leads me to think you don't attach much importance to it and I just want to know why.
Let's quickly forget F1 cars and other high downforce cars now. I think a proper suspension design is very important. Many things have to be fine tuned to each other and RC is a thing where you should spent some time with.

@n_smikle:
I don't really get your question. Can you specify it more?

[PlatinumZealot]

mep, from wiki:

"The SAE's definition of the force based roll center is, "The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll"."

After I steer the front wheels (or if i bump on wheels and it steers), the center lines of the front wheels can no longer lie in a transverse plane. So what do I do then? Project them on another transverse plane half way between?

[Tim.Wright]

keep it at the same plane as where you started. Its a completely arbitrary location anyway.

Tim

[PlatinumZealot]

OK. I just put in the transverse planes.

I am assuming:
- the ground is flat from now on.
- the tyres keep the same shape, and wheel centre height is the same
- the car body flexing is negligible

So I will put in the same roll angle front and rear.
I will measure the R.C heights at a given roll angle (is 5 degrees good? too much?)

[Tim.Wright]

How much will your car roll? That would give you the answer. Have you figured out your target vertical and roll stiffness yet?

Tim

[Richied76]

Just as a note.. If your backsides sat on the transmission tunnel, make sure you've a nice thick plate over it! I knew someone that had a dragster where your bum was an inch over the diff!

[PlatinumZealot]

How much will your car roll? That would give you the answer. Have you figured out your target vertical and roll stiffness yet?

Tim

I am a very lazy person, Tim.

I will use 3 degrees. Will show a pic of how the roll centers are now in the "as is" state.

Just as a note.. If your backsides sat on the transmission tunnel, make sure you've a nice thick plate over it! I knew someone that had a dragster where your bum was an inch over the diff!

hehehe.. good heads up.

I'm a little flattered by thoughts of this car actually getting built though.

[PlatinumZealot]

This is the car as is. 3 degrees roll. (right click "view") correction to the rear R.C it is 1.2 inches above ground. I guess you can just plug in the CoG wherever.




Note that:

- the pick up points are very high above the ground. So it looks a little weird.

- There is some anti dive in the front arms so the roll axis is skewed

- This is "As is" I have not dialed in anything yet.

- The rear has a different track, and control arm width..


What do you guys think should be done to tighten it up?

[Ciro Pabón]

The transverse vertical plane passes through the center of the wheels. It's not necessarily perpendicular to wheels, according to your definition (although when wheels point straight ahead it happens to be this way).

So, when you rotate the wheels, the plane remains in the same place, if wheels rotate around their centers when steering.

I hope you get it (or you explain to me why I am wrong).No caster, please, I'm lazy too, as a good engineer should (after all, premature optimization is the root of all evil). That's why I studied civil instead of mechanical. I would use a horse and a chariot... or better a stallion for lower roll center.

[PlatinumZealot]

Ah.. you share my philosophy.. laziness breeds innovation.

I just remembered that the inspiration for this car is the Lotus 7. So I did some googling and used some Lotus values. I did some tuning to mimic some of these values. I feel a little better now.

Experimental Caterham (Independent rear suspension)

Front suspension
roll centre height 30mm
50% anti dive

Rear suspension
roll centre height 65mm
toe in on bump
anti squat geometry used

Lotus Elise

Front suspension
roll centre height 30mm
travel 50mm bump / 60 mm rebound
camber gain in bump 0.31 degrees per inch
frequency 90cpm
KPI 12.0 degrees
Castor 4.25 degrees
Trail 4mm
Scrub radius 10.5mm

Rear suspension
roll centre height 75mm
travel 50mm bump / 70 mm rebound
camber gain in bump 0.45 degrees per inch
frequency 98cpm

[Caito]

Remember that antidive % depends on CoG height. So you'll not be able to calculate that until you have a good approximation of cog height.

[PlatinumZealot]

The Percentage anti-dive. Is it a result? or is it an input into another equation?

What use does the number itself have?