Crosskart Xtreme best center of gravity height?

[ptuomov]

Any thoughts on the chassis setup for Crosskart Xtreme Speedcar?

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This is not a regular race car. On paved track and long races with regular (rear differential) race cars, my rule of thumb is that you want the center of gravity as low as possible and about 60% weight in the rear and 40% in the front. With the appropriate suspension, center of gravity below the wheel hub center is money. But this is different.

This car has a live rear axle without a differential (so the inside wheel must lose traction in corners) and is driven on a circuit that is 1/2 gravel and dirt and 1/2 paved. Furthermore, the races are very short (3-5 laps with 1km lap) with a standing start so it's about 1/2 a drag race at the start and 1/2 a circuit race afterwards. Clockwise track but a lot of turns in both directions. Where would one want the center of gravity for a car like this? Especially in terms of height?

[modok]

I don't know,
But I am very interested in the subject.

In these SMALL kart cross thingies tho pretty sure it's going to be just too high anyway.

[BCjohnny]

.....my rule of thumb is that you want the center of gravity as low as possible and about 60% weight in the rear and 40% in the front ......

Where would one want the center of gravity for a car like this? Especially in terms of height?

It'll be suck it and see ........ but that wouldn't be the worst place to start

Racing tarmac and loose ('rallycross') there'd be a temptation to maybe want the CoG a little higher for the latter, but as said with a 'car' of that intrinsic design I'd think you might struggle to get it too low, within reason

If you're not introducing camber gain or roll steer problems etc (all too easy with independent suspension), my preferred instinct is usually to get the CoG and roll centres as low as practical, while obviously retaining the front / rear balance, and use the springs / bars to set the handling, as long as you don't run into too soft / hard control issues

Indeed the usual CoG over roll centre height ...... roll couple ...... consideration, not that it's ever that simple

I think of more overall importance might be the shocks, but they're likely restricted, depending on the formula rules

JMO

[Tom68]

It's a weird question, what are you going to change ?

You need ground clearance, you need suspension travel, the buggy has been built with arm angles to suit a certain ride height.

[BCjohnny]

You need ground clearance, you need suspension travel, the buggy has been built with arm angles to suit a certain ride height.

Yes ...... the whole thing has been designed at a certain ride height and arm angles, which dictate roll centres etc

It's difficult to see what major improvements can be easily made ...... and it would be easy to get it wrong, and fast, if you're not paying attention

I'd be looking at what everyone else was doing and buy the best shocks the rules would allow ....... wheel control is probably were the improvements mostly lie, so also get as much weight out of the unsprung masses as practical

[ptuomov]

In these SMALL kart cross thingies tho pretty sure it's going to be just too high anyway.

If it weren't for the standing start, I agree that you'd likely benefit from the lowest practicable CG height during a flying lap. However, the start is very important and if you don't have enough weight on the rear wheel then the only way (that I can think of, after the suspension is set up) get traction is to move the CG up.

It's a compromise between a drag race and a circuit race, with about equal importance.

[ptuomov]

I think of more overall importance might be the shocks, but they're likely restricted, depending on the formula rules.

They are three way adjustable shocks with free coil-over springs. In my opinion, giving me anything fancier to stare at would be casting pearls in front of a swine.

[ptuomov]

It's a weird question, what are you going to change ? You need ground clearance, you need suspension travel, the buggy has been built with arm angles to suit a certain ride height.

The cart is an older, lighter model that we bought from a heavier driver and our driver is young and light (and very fast). Consequently, we can install heavier components anywhere as the previous owner got the cart without a driver as light as possible, to the edge of reliability. This gives us some ability to shit the center of gravity height, from installing a heavier rear wing to adding lead plate ballast to the floor plate.

The ground clearance is measured at the end of the suspension travel if I recall correctly. Since the springs are free, it's not usually bottoming; except in the big jumps on some of the Swedish tracks.

[ptuomov]

I'd be looking at what everyone else was doing and buy the best shocks the rules would allow ....... wheel control is probably were the improvements mostly lie, so also get as much weight out of the unsprung masses as practical

If we just make everything else as light as possible, load the ballast under the driver's seat, and install springs to match that weight distribution, the driver loves it and the flying lap times are very competitive. The problem is that this setup is not competitive at the standing start due to less traction than the competition. I believe that the slow standing start simply comes from either too little static weight on the rear axle and/or too little weight transfer to the rear axle due to the low CG height.

(But I'm more of an engine guy and a total noob with the chassis setup and I'm just trying to understand what's happening here. If what I'm saying makes no sense to you that's likely just because it doesn't make sense.)

[Rick!]

Some things need to be identified/measured before swagging suggestions.
What's the front roll center height? A high roll center height is not necessarily bad.
Rear roll center height? A high roll center is not a bad thing as long as its below the vehicle CG.
Front and rear bump steers dialed in to near zero or 1/4*-1/2* max? The one guy mentioned roll steer - if the front suspension has minimal bump steer, then concentrate on the rear suspension. Take off the springs and measure toe change per unit of suspension vertical travel. If the rear toes the opposite of the front, like the front toes in and the rear toes out or vice versa, then you need to fix it.
Whats the side view swing arm height, distance from front axle? Is it providing any anti-dive?
With a rear engine and a close driver, anti-squat is more than likely built in and minimally adjustable. Can you incorporate an adjustable anti-squat mechanism that allows max transfer for the start and then is moved to "normal" once up to speed? (This is a racing snowmobile device as normally oval sleds have no weight transfer in the "handling" configuration.
As for the kart CG, you have 3 major mass chunks - the driver, the engine/trans and the frame. Evaluate how hard it is to move the driver down, or the engine/trans down, or re-configuring the frame to lower its CG. My guess is that lowering the driver would be the easiest. Will the kart become OMG better if the new CG is 1/4" lower? Or does it make the handling response more sluggish. You don't have the advantage of using tire traction and slip angle to motivate better cornering so you use weight transfer and suspension geometry.
As the other poster mentioned, you are left with springs and sway bars. Get plenty of each and use the driver feedback to get the best compromise.
Don't try to stick the tires on ice, remove studs to create the same slippage and feel as dirt and then dial in the bars/springs.
For bars, try to use just straight tubing so they are easy to calculate the torsional rate. You can math out the bending in non-straight tubular bars and come up with the composite rate. Rate can be altered by the "leg" length and where you attach it to the controls arm. The idea is to make them easy to swap so the driver remembers the feel of comparing A to B.
Lastly, are there any key frame bars that add torsional rigidity to the chassis? Can they be made removable? A former engineer I worked with used different "cross bars" to tune his dirt modified for different tracks. Might be worth a thought exercise.
Why is there a wing on that car? You're trying to get the nth degree of engine performance and that contraption is probably sucking 1-3hp on the high speed straights. Look up the wing cross section in NACA or other resources and find its performance curves and figure out the lift and drag and make a judgment from there.

[Rick!]

There are hydraulic spring seat adjusters that one could get clever with to loosen the front springs at launch - easily activated with another cluster master cylinder and controlled with a steering wheel button and hyd solenoid (think Line Lock). Loosening the front springs allows easier weight transfer and a better holeshot. Let go of the button and it goes back to handling mode. Or, magneto rheostatic shocks (MR) that would soften rebound at launch for a better holeshot.
The thing is, if you create enough of a competitive advantage with your driver that already wins in an improperly set up kart, you will have tech enforcement up your fanny and new rules until your guy/car, is slowed down to "parity."

[ptuomov]

There are hydraulic spring seat adjusters that one could get clever with to loosen the front springs at launch - easily activated with another cluster master cylinder and controlled with a steering wheel button and hyd solenoid (think Line Lock). Loosening the front springs allows easier weight transfer and a better holeshot. Let go of the button and it goes back to handling mode. Or, magneto rheostatic shocks (MR) that would soften rebound at launch for a better holeshot.

Explicitly banned in the rules and easy to tech.

[ptuomov]

Why is there a wing on that car? You're trying to get the nth degree of engine performance and that contraption is probably sucking 1-3hp on the high speed straights. Look up the wing cross section in NACA or other resources and find its performance curves and figure out the lift and drag and make a judgment from there.

It’s a decoration from the factory. On commonly raced 1km tracks and with the optimal gearing, the speeds aren’t high enough to create any appreciable downforce. You can see that the wing isn’t doing anything from the flimsy attachment points to the plastic cover. The most function I’ve gotten from it is as a surface on which to place your coffee cup at the pits.

Some people reduce ballast at the floor and make the wing heavy to increase the CG height. But I don’t know if that helps or hurts in net.

[ptuomov]

The thing is, if you create enough of a competitive advantage with your driver that already wins in an improperly set up kart, you will have tech enforcement up your fanny and new rules until your guy/car, is slowed down to "parity."

After two races this fall, our driver already received several (poorly thought out) rules changes targeted at him for this coming season.

[ptuomov]

Can you incorporate an adjustable anti-squat mechanism that allows max transfer for the start and then is moved to "normal" once up to speed?

Also explicitly prohibited by the rules. No adjustments whatsoever to the suspension during the race.