Crosskart Xtreme best center of gravity height?

[BCjohnny]

I think that the track cars that lift the front wheel when cornering on track likely have the front roll center higher than rear roll center. All those cool photos with the inside front wheel in the air…

No ..... it's not directly a result of front RC height

When a vehicle lifts the inside front wheel when cornering, 100% of the lateral weight transfer is being take across that 'axle', and none effectively at the rear

The handling on such a set up is balanced usually by raising the rear roll centre ,,,,,, thus creating 'sheer' across the rear tyre footprint so the vehicle corners more neutrally ...... obviously taking grip away from one axle to compensate the other isn't the hot set up

Raising roll centres to crutch other problems is generally bad, and can make the vehicle feel like it's skating around corners

There's more to it than that, but that's the bones of it in my understanding of the job

[ptuomov]

What’s the specific input?

[ptuomov]

I think that the track cars that lift the front wheel when cornering on track likely have the front roll center higher than rear roll center. All those cool photos with the inside front wheel in the air…

When a vehicle lifts the inside front wheel when cornering, 100% of the lateral weight transfer is being take across that 'axle', and none effectively at the rear.

I disagree with this. If a car lifts the front inside wheel, it might have 85/15 lateral weight transfer in the rear and 100/0 in the front. Change the setup slightly and it might have 90/10 and 95/5 lateral weight transfer and the inside front wheel no longer lifts.

[ptuomov]

Roll center.

EDV-CR110.jpg

Cars don't body roll around a center.

These ones almost did, until they start to roll and the slides on an angle.

Ok so how I read this is that you think the roll center is not a useful concept.

[BCjohnny]

I think that the track cars that lift the front wheel when cornering on track likely have the front roll center higher than rear roll center. All those cool photos with the inside front wheel in the air…

When a vehicle lifts the inside front wheel when cornering, 100% of the lateral weight transfer is being take across that 'axle', and none effectively at the rear.

I disagree with this. If a car lifts the front inside wheel, it might have 85/15 lateral weight transfer in the rear and 100/0 in the front. Change the setup slightly and it might have 90/10 and 95/5 lateral weight transfer and the inside front wheel no longer lifts.

Yes, you are correct, apologies ...... what I should have said is 'excess roll stiffness is being taken across that axle, and too little effectively at the rear'

Regardless, it's not fundamentally a 'roll centre' problem, in itself ...... and raising roll centres to balance handling is usually the wrong thing to do

[Rick!]

We can talk and postulate about roll center until somebody’s blue in the face. Without the coordinates of the left side corners, both front and rear, as measured from the ground plane and longitudinal center plane, it’s all conjecture. Get me those hard points and I’ll create a simple mbd car model and create all the necessary points to analyze all the suspension kinematics. Include the inner and outer tie rod coordinates and the hub centers also.
If I understand the “problem description”, when the kart handles better due to weight placement, the holeshot performance is compromised? Yes? Sounds like basic weight jack principle should be considered. A clever person can meet the ballast requirements in spirit of the rule and still get a holeshot and handling. (A set of Intercomp scales is invaluable for this exercise. 4 accurate bathroom scales could suffice in a pinch.)
Just so everyone is on the right understanding, is the reaction point where the cg “force” and outside tire/ground force moments act about considered the roll center? Also, as the car leans, the roll center moves. A proper roll center plot shows the roll center lowering as the suspension compresses - think concave down.
Also, roll stiffness and roll center are two distinct properties and are not so much coupled. The roll center position doesn’t rely on corner springs and roll bars and roll stiffness is not perfectly beholden to a geometric instant center.

[ptuomov]

"Get me those hard points and I’ll create a simple mbd car model and create all the necessary points to analyze all the suspension kinematics."

I will likely take you up on the offer once I come back from the travels, thank you!

"If I understand the “problem description”, when the kart handles better due to weight placement, the holeshot performance is compromised?"

I don't know what holeshot means in this context. There are other cars that take off faster from the standing start, that's the only spot where we're losing. Out of the corners, the car accelerates very well as it is. "Holeshot" devices that actively alter shocks or springs are banned and easily discovered.

"A set of Intercomp scales is invaluable for this exercise. 4 accurate bathroom scales could suffice in a pinch."

We have an AccuSet II scale that will give us the weights at each wheel. We just have to get those exactly level with a long-construction level, the shop floor has a fairly steep slope for the drains. We also have blocks that (with some trigonometry) will allow us to measure the CG height.

We can talk and postulate about roll center until somebody’s blue in the face. Without the coordinates of the left side corners, both front and rear, as measured from the ground plane and longitudinal center plane, it’s all conjecture. Get me those hard points and I’ll create a simple mbd car model and create all the necessary points to analyze all the suspension kinematics. Include the inner and outer tie rod coordinates and the hub centers also.
If I understand the “problem description”, when the kart handles better due to weight placement, the holeshot performance is compromised? Yes? Sounds like basic weight jack principle should be considered. A clever person can meet the ballast requirements in spirit of the rule and still get a holeshot and handling. (A set of Intercomp scales is invaluable for this exercise. 4 accurate bathroom scales could suffice in a pinch.)
Just so everyone is on the right understanding, is the reaction point where the cg “force” and outside tire/ground force moments act about considered the roll center? Also, as the car leans, the roll center moves. A proper roll center plot shows the roll center lowering as the suspension compresses - think concave down.
Also, roll stiffness and roll center are two distinct properties and are not so much coupled. The roll center position doesn’t rely on corner springs and roll bars and roll stiffness is not perfectly beholden to a geometric instant center.

[BCjohnny]

Also, roll stiffness and roll center are two distinct properties and are not so much coupled.

I don't agree, unless I'm reading that wrong, or it's an emphasis thing ...... in any case I'm not trying to provoke an argument, just clarity

All things being equal roll stiffness related to roll centre (height), as a function of CoG (height), are inextricably 'coupled', otherwise changing the springs on a car would not change the weight transfer across each axle, and you wouldn't be able to balance the handling

The roll center position doesn’t rely on corner springs and roll bars and roll stiffness is not perfectly beholden to a geometric instant center.

Yes, roll centre position doesn't rely on the other factors, and will usually move around relative to roll, yaw & braking ....... unless it's a physical bolted position on a live / dead axle, and even then it will move fractionally

The sharpest guy I ever had the privilege of talking to ....... he fooled around with that F1 stuff ....... said that if you weren't designing the chassis around the roll centres, you were starting from the wrong place

So 'postulating' about roll centres IMO is key to optimising set up, unless the chassis was designed properly around their consideration in the first place ...... and that's not always the case

OP ...... if the 'car' is handling properly and you're only trying to improve the 'holeshot', you might be going down a rabbit hole with that, unless it's like UK 'grasstrack' racing where whoever wins the standing start 'drag race' into the first corner can become almost impossible to pass, even by superior handling cars

Anyway ...... enjoy the holidays

[ptuomov]

OP ...... if the 'car' is handling properly and you're only trying to improve the 'holeshot', you might be going down a rabbit hole with that, unless it's like UK 'grasstrack' racing where whoever wins the standing start 'drag race' into the first corner can become almost impossible to pass, even by superior handling cars.

Passing is difficult but not impossible in this series. I'd say that if you drop to P2 in the start but have a better car for a flying lap, you'll win. If you drop to P4 in the start, you won't win. Ideally, I'd like the driver to qualify to front row and be P2 or better after the first turn.

[BCjohnny]

Sorry, my semantics ........ 'almost impossible' equals ' can be very difficult'

I flicked through the rules you posted and am still at a bit of a loss as to what suspension you're actually running

If it's no great secret what is the type of rear suspension you have ....... IRS double A arm / multi link, live chain / prop driven ?

Not claiming to be an expert by any means, but each responds to different set up as regards traction from a standing start

[Rick!]

Sorry, my semantics ........ 'almost impossible' equals ' can be very difficult'

I flicked through the rules you posted and am still at a bit of a loss as to what suspension you're actually running

If it's no great secret what is the type of rear suspension you have ....... IRS double A arm / multi link, live chain / prop driven ?

Not claiming to be an expert by any means, but each responds to different set up as regards traction from a standing start

From the engine thread:

Without a side view with the wheel removed, it is hard to envision the king pin axis. The toe adjuster is just that as it has its own set of pivots. It seems to look like a hybrid type of swingarm with the front frame mounting points positioned to move the IC rear and upwards, depending on whose chassis you can find on the interwebs.
The shocks are mounted upside down with the coil springs inside the bags/sleeves.
Looking at more Youtube videos, there is very little suspension movement for weight transfer on the starts. There is little suspension travel in general so it does mostly resemble a go-kart with a high CG. There is considerable front end dive on hard braking that I can see. I stopped going down the cross kart chassis/suspension rabbit hole at that point.

[Rick!]

Here are a couple links to roll center definitions and how to use them for handling tuning.
(I can see why on an F1 car that defining the roll center is paramount and defines the chassis hard points early in the design. Since the F1 car is pretty mature, roll center windows are very well defined and may not be used for handling tuning. But on other vehicles, it may be suitable to change knuckle/a-arm attach points for tailoring vehicle response.)
One link does go into setup for a drifter which the cross karts seem to emulate when cornering.
Roll center and roll moment
Roll center tuning parts
Drift car geometry setup
Do we have the CG positions for with and without ballast yet? Inquiring minds need to know.

[BCjohnny]

Out of interest ..... and I have come across this site before ...... I flicked through the first article linked, and it states this :

For a front wheel drive car, the front roll centre will generally be lower down than the rear roll centre. Also, for a rear wheel drive car the rear roll centre will often be lower than the front roll centre. This gives the driven wheels more roll and therefore more grip through the corner. A four wheel drive car will tend to have roll centres at a similar height front to rear. They will be tuned from this position to distribute more or less roll to the front or rear axles.

The bit in bold goes against what most people who have a grasp of the subject would be familiar with ...... and I'm talking 'production' stuff here, not just F1

As written it's not authoritative, if the writer maybe meant 'rear engined, rear wheel drive' it might make more sense, but not always

I do agree that a lower roll centre will induce more 'grip' ...... it usually does, within reason, all things being equal ...... due to the way the lateral (cornering) weight transfer is reacted at the (outside) tyre contact patches

Anytime you raise a roll centre you run the risk of losing grip, which is why it's sometimes used to balance handling, and is the wrong way to go about it, unless you have little other choice

[Tom68]

Roll center.

EDV-CR110.jpg

Cars don't body roll around a center.

These ones almost did, until they start to roll and the slides on an angle.

Ok so how I read this is that you think the roll center is not a useful concept.

Instant centers are more important.

Select a handling deficit, then address it.

[BCjohnny]

ptuomov

I flicked through the rules you posted and am still at a bit of a loss as to what suspension you're actually running

Not claiming to be an expert by any means, but each responds to different set up as regards traction from a standing start

My understanding is that you're generally happy with the handling, but would like to improve standing start traction ..... if you can post up some details then maybe some improvements can be made

Ok so how I read this is that you think the roll center is not a useful concept

Although not directed to me ...... roll centres are not a 'useful concept', they are fundamental to how any vehicle goes around a corner