Why does it work this way?

[Tim Lewis]

I've been thinking about the differences in tuning the chassis on a car and a kart. It bothers me because the 'rules' are opposite of each other. You have to figure that the tires are acting about the same, they don't know whether they are on a kart or a car, so that must mean there are other factors on a kart that are over-riding, but I have yet to figure it all out.

Many of the common tuning adjustments are used to time the lifting of the inside rear wheel. Chassis stiffness, axle stiffness, castor, rear track, front track, ackerman etc. all effect when and how that tire gets off the ground (and it has to at least get light to reduce understeer).

The rear track effects a couple things. 1 degree of twist in the chassis will raise the inside tire off the ground more with a wide track than a narrow one, so that's probably a factor. With a wide rear, once you are on only 1 rear tire on the asphalt and you hit the gas, that tire will provide more of an oversteering moment on the chassis. It will literally drive the kart around on that rear. So that is also going to be a player.

Camber on the front is used to get the most effective footprint out of the tire, so at least that's the same as a car.

Ballast and seat position allow you a direct influence for tuning the handling balance, but opposite of a car. A rear weight bais normally makes a car loose (i.e. Porsche 911) while a nose heavy car will push (anything from Detroit). Why is there a difference?

I don't have a problem realizing the rules are different for a kart. The truth is it just bugs me that I don't know physics behind why the difference exists. There are a few of you out there (Braun and Dis. Jr. come to mind) that have dealt with both cars and karts. You guys have to have some pet theories, care to share them?

Tim Lewis

[Mike Unger]

Tim
In regards to your general questions about why a kart behaves differently than a car...
You seem to have a good handle on the solid rear axle issues but what you need to realize with changing the weight forward and rearward is that karts are generally have more tire that they can use and cars are the opposite. This is because looking at a graph of tire lateral grip vs normal force shows that lateral force genally increases with more normal load until a point were the tire becomes overloaded and begins to loose grip. This is why wider is always better with a car and why a front heavy car will understeer. When a car looses grip it is generally becuase it is overloading the tire. But a kart operates more on the lower end of the normal force vs lateral grip curve. Meaning more normal force means more grip. The problem you run into with karts is you can end up with too much grip which robs horsepower, which slows you down. This almost never happens with a car.

[Mark Dismore Jr]

Lack of suspension, drivers overall weight percent, overall weight and a locked rear axle are all reasons why karts are so different.

We are constantly tuning the chassis to use the tires in the right way. That's why guys are always throwing around 32/30, 30/30 and every other tubing combination out there. We are accomplishing the same thing as a car but in a different way. Race car shocks are an amazingly complex piece (Jeff knows more than I) between shim stacks, bleed and other factors you can unweight the car for different corners. Race car shocks have "nose" usually built in the shock. If you look at a shock dyno sheet you will see on some shock a very sharp "stiff compression" at first. This is the nose, that way the car is initially held up well under turn in, so the weight transfer doesn't overload the tire. In this way we use stiffer axles for more grip. It helps use both tires and doesn't overload the axle/tire on turnin.

Castor is used in race cars as well. F2000 cars usually have 2-4 degrees of castor, unless your a wuss!

As far as weight ballast and effecting handling, you have to remember that a kart is overtired compared to a sportscar, heck, a 7.10 is nearly as wide as a F2000 tire. So you don't see the effect so much so weight tends to enhance the kart to a point. But go scale out your kart and put 55% nose weight, I bet that bad boy will push, you have to think in relative terms a little bit on this stuff. You also have to include reasons why a car gets loose with rear weight and a lot of it is body roll, something that you never deal with in karts. With a suspension, body roll really influences the chassis, especially on turn in.

Now don't get me started on oval racing, 60% cross to loosen the kart up? Not on an Indycar, you'd crash that badboy on the first lap!

Don't compare karts and cars it gives me a headache! Seriously, weight and suspension is such a huge factor on racecars that it makes the comparisions hard to relate to each other. I also find it weird that some things that seem opposite to racecars works on a kart. Hopefully the above makes some form of sense!

Mark

[Steve F]

I really struggled the first two years of karting because of my sportscar and engineering background. Weight transfer is a big enemy in car racing, yet in karts it is a very necessary part of getting the kart to work correctly.

If you do the math of the lateral dynamic load transfer in a turn, you will find that increasing track width reduces the amount of weight transfered to the outside wheel. Thats a true physics thing regardless of whether its a kart or a car. Following that theory, decreasing the track width would load the outside tire more and produce more grip (up to the point when the tire is overloaded). Honestly, I do not know if we karters are in the range where we are overloading the tires. I think what Mark Jr. said about the relative size of tires has a lot of merit.

But, you and I both know that lots of karters
INCREASE the front track to get more grip. After pondering this for a long time, I finally conceded that the kart's front end geometry plays a far more significant role in determining how the kart handles. Moving the front wheels in or out can have a remarkable amount of influence on how the rear of the kart works.

There are so many factors going on simultaneously when the kart is turning, that its no longer a matter of straight ahead physics. If it were, chassis manufacturers wouldn't be spending the insane amount of hours they do building prototypes and testing.

If you are still interested in the physics and trying to figure out why, there are a number of good books to help you with the physics. "How to make your car handle", "Race Car Engineering and Design", "Tune to Win" and "Race Car Vehicle Dynamics". Some are much more readable than others. Engineering degree not included.

Steve Frank

[Tim Lewis]

Well, as far as the physics goes, I have a little bit of a background in that sort of thing.

In a car, you tune the handling balance mostly by changing the front to rear roll couple. Essentially, do you want the front to carry more or less of the cornering load.

In a kart, you really don't even have the ability to change that roll couple. It's built into the chassis. I suppose different bars bolted into and out of the chassis change the roll couple a little, but honestly, I rarely use them.

I know that the tires on a kart have a big footprint, but for arguments sake, let's say we are looking at the outside tires taking all of the load in the middle of the corner. If it's a 300 lb kart, the front is probably carrying 140 and the rear is carrying 160. My guess is that at that kind of load it should be pretty much on line any other race tire. I've looked at several tire slip angle plots, and I have never seen one that has a higher coefficient of friction with increasing load. This is assuming some constant slip angle.

Steve's take on the effect of track to handling is entirely my case in point. I guess what this shows is that, for a kart, roll couple distribution isn't a big player for handling balance. This is a pretty unusual thought for me.

Along a different train of thought, Diz Jr. mentioned the shocks influence on handling. Seeing as our 'suspension' is completely undamped, is this the reason that karts 'hop' particularly when really gripped up. That would be my guess.

I don't really know where this all is going...I'm just thinking out loud and trying to gain a little insight on the whole deal.

Thanks to everyone who put in comments.

Tim Lewis

[Paul Kish]

What I have found with karting/racing over the years is that you first need to learn what you want each of your tires to do for you. You need to learn the specific job each tire needs to do when the kart/car goes around a turn.

Then and only then can you look at how what you learn about different adjustments will do. You can learn about roll centers, you can learn about the center of gravity and the verticle center of gravity, you can learn about stagger, ackerman, kpi, castor, camber, etc., the list goes on and on. But unless you first learn what you need to accomplish with all the adjustments available to you your setup job will be a tough one.

First and formost, it all boils down to what you need each tire to do for you at every place in the turn.

The second thing you need to know or have a very good feeling for is where your weight is located on the kart/car when you need to transfer it and what direction it is going to go. Weight moves around, you may need to think in terms of moving it more than once to get something done. By that I mean you move it first to where it can be directed at a particulat tire to do a particular job for you and then move it back where it can do another job for you at a different part of the track or a different part of the corner.

You need to first know how you are going to use each tire at each part of the track. Then you need to know how to move or transfer the weight of the kart/car to make each tire do its job at each part of the turn or track.

Sorry if this doesn't fit in... I just got to reading the above posts and started talking out loud on here too. Just cause it's fun and I enjoy it and learn from writing stuff down. No other reason.

[John Learmonth]

quote:

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Originally posted by Tim Lewis:

In a car, you tune the handling balance mostly by changing the front to rear roll couple. Essentially, do you want the front to carry more or less of the cornering load.

In a kart, you really don't even have the ability to change that roll couple. It's built into the chassis. I suppose different bars bolted into and out of the chassis change the roll couple a little, but honestly, I rarely use them.

Steve's take on the effect of track to handling is entirely my case in point. I guess what this shows is that, for a kart, roll couple distribution isn't a big player for handling balance. This is a pretty unusual thought for me.

Along a different train of thought, Diz Jr. mentioned the shocks influence on handling. Seeing as our 'suspension' is completely undamped, is this the reason that karts 'hop' particularly when really gripped up. That would be my guess.

Tim Lewis

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Tim,
I suspect that many or even most of the chassis adjustments we make with our karts do actually have a strong affect on the roll couple, changing the ratio of roll / roll stiffness of the individual load paths between the CG and any particular contact patch. I think it likely that these effects on roll couple have at least as big an effect on changing the handling and handling balance as any other effect of any particular chassis adjustment.

On cars, adjustments to roll couple are achieved by altering suspension geometry and/or mechanical suspension rigidities. This will affect how any particular contact patch is loaded at any moment in time under any specific dynamic circumstance. On karts, adjustments to roll couple are also achieved by altering ‘suspension’ geometry and/or mechanical rigidities. However the exact means by which this is done is not the same as with cars, and the roll couple can also be fiddled by moving the CG.

Rather than use softer or stiffer anti roll / sway bars as on a car, with karts you can change rigidities in various parts of the chassis. Rather than rearranging suspension geometry, with karts we can widen or narrow track widths (changing the orientation of the roll axis, which can increase / decrease roll / roll stiffness at either end), or move weight around, both of which will alter the relationship of the main masses to the actual and theoretical ‘suspension’ geometries (chassis rails and the geometries extrapolated from the chassis rails)). As such, moving weight up / down / forward / backward moves the centre of mass / weight / gravity closer or farther from the centres and axis of ‘roll’, which will alter the roll couple.

Raising or lowering the chassis at either end will also affect roll couple. Raising / lowering the chassis will raise / lower the CG, but won’t have exactly the same affect as just raising / lowering the CG the same degree by raising / lowering the seat. Eg assuming the chassis rails to act as ‘suspension’, then raising the front of the chassis will slightly raise the CG, but more strongly it will raise the front roll centre, which will increase roll stiffness at the front of the kart.

Of course in order to agree with any of my ideas on this, you need to also agree that karts do in fact have roll centres, and must have two of them.

Tim, you say: “I guess what this shows is that, for a kart, roll couple distribution isn't a big player for handling balance”, then you say: “this is a pretty unusual thought for me”. I would guess that your instincts are telling you that roll couple SHOULD be a big player in handling balance, for karts as much as any four wheel vehicle. I would go with your instincts!!

PS the driver’s body acts as a ‘suspension damper’ on a kart chassis. The ‘floppy’ nature of your bod helps your kart to handle!

[Paul Kish]

As usual the more I learn the more often I realize how dumb and ignorant I have been previously. It's too bad that learning teaches you how dumb your are.

But anyway I have a question about roll and weight transfer.

Since just prior to last season we have been helping and working mainly with sprint cars and will probably be doing it next year too.

The roll centers I did not care about all of a sudden from learning about suspended cars and reading on here have become more important to me. When I think about the body rocking in the suspension on a car now I see the need and want to know where the roll centers are.

At this point in time I am interested in how the location of the roll center when moved closer to a wheel effects weight transfer. I'm mainly talking about the pannard bar in the front of a sprint car. I am now sure that if we could find the roll center on a kart the actions would be the same.

Back to my question. As always in my questions i'm going to be making some statements. They are part of my question and if the statements I make as part of my question are incorrect, Please straighten me out. thank you very much

If I were to move the front roll center of the sprint car to the left by shortening the pannard bar, I think, that would shorten the lever between the front roll center and the left front tire? Shortening the lever between the front roll center and the left front tire will quicken the reaction and weight transfer off the right front and to the left front.? One of the objectives is to along with getting weight off the right front is to get weight on turn in to the left rear. Since shortening the lever between the front roll center and the left front tire increases how quickly weight is placed on the left front "less weight will be able to or have time to travel to the left rear where we can use it to adjust entry. On the other hand if we lengthen the lever between the front roll center and slow down the rate at which weight is transfered to the left front we are also at the same time increasing the rate at which we remove weight from the right front.???? Because we have now slowed down the rate of transfer to the left front and increased the rate of transfer from the right "more weight will be moved to the left rear".? That is because x amount of weight is going to be transferes no matter what and where it goes is going to be determined by the length of the levers to the two front tires and resulting front roll center.? Move the front roll center to the left and more of the weight that "is" going to leave the left front is initially going to end up at the left front and less will be available for the left rear.???? many ???? Move the front roll center to the right and it will increase the rate at which the weight is going to leave the right front and slow down the rate at which the weight is going to be applied to the left front.???? Since the weight "is" going to transfer more of it will then be applied to the left rear.?????

I sure hope I have some of the above correct, all of it would be "GREAT" ! but i'm not that confident about what I know.

The end result in my above example will be that adjusting the front pannard bar will effect the amount of weight presented to the left rear at turn in. ?????? After that other forces come into play.

Thank you very much
Paul

[John Learmonth]

quote:

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Originally posted by Paul Kish:

But anyway I have a question about roll and weight transfer.

Since just prior to last season we have been helping and working mainly with sprint cars and will probably be doing it next year too.

Paul

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Paul,
I’m afraid you lost me with your question, I just found it too difficult to decipher. I probably couldn’t have answered it anyway!

FWIW, I gather you’re considering shortening the panhard rod(s) on a sprint car. The best length for any PR is; as LONG as is practically possible! The reason being that as either end of the PR rises or falls relative to the other end it moves in an arc, causing the axle / chassis to move laterally (relative to each other) with suspension deflection. For the same degree of deflection, the shorter the PR the worse this problem will be.

This lateral displacement will also be ‘seen’ at the contact patches as a ‘spike’ in lateral force applied against the contact patches, and could be enough to destabilise grip when cornering at the limit. Even if it doesn’t compromise grip it can cause instability in the handling and steering inputs required (ie bumpsteer). The greater the suspension deflection and the shorter PR length the bigger this problem will be.

All this is the main reason PRs have fallen out of favour with suspension designers, most preferring to use a Watts Linkage instead (which has no lateral displacement with vertical movement of the chassis or axle) when laterally locating a beam axle. I suppose it might be possible that sprint cars are an exception to this, and that in this application PRs may have some advantage that I’m not aware of over other locating devices. After all, sprint cars are the only kind of modern car I know that use beam axles as the preferred suspension.

I don’t know whether or not you’ll be able to move the roll centres laterally by changing the length / lateral attachment point positions of the PR, but you can easily (relatively) change the roll centre height simply by raising or lowering the PR. It would generally be best to keep the PR horizontal in the static state.