go kart chassis setup

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Geoffgeoff
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go kart chassis setup

Post by Geoffgeoff »

Anyone know of a good guide (book? online?) to dirt kart chassis setup? Starting points for left side, cross, and front end weights? Effect on weight at each wheel due to raise/lower each spindle or rear axle bearing? I've done my share of racing, but most of it's been ATV harescrambles and kart chassis setup is new to me.

Thanks for your help,
Geoff
David Redszus
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Post by David Redszus »

Don't feel bad. When it comes to tuning kart chassis, everything is a matter of trial and error. While technical analysis can and has been done, karters would rather believe the next racer than the laws of physics.

The important concept to tatoo onto your forehead, is that everything that is done is to keep the tires planted on the ground. Never mind lift due to steer angle and all that junk; never let any tire get off the ground.

Then teach the driver to minimize the turning of the steering wheel.
Bubstr
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Post by Bubstr »

You will find that center of gravity, front to rear, side to side and hight is the one biggest tool you have. You can soften or stiffen a corner or front to rear with seat mounts and floor pan mounts and thickness and tire off set, but how you mount the driver forward back hight is the most important. Getting the COG right for the roll centers built into the kart is where the real money is made. For the beginner as everyone was once, the best advice I ever got was the heavy end slides out change it. This means two things weight and spring rate. Static weight is fairly simple, the spring rate is the chassis and driver. If you move tires out it softens rate, in stiffens rate. rubber seat mount grommets or how many grommets and where the seat struts run to can also soften or stiffen from front to rear. This is the simple part. Most have this down pat. the edge usually comes in tire choice. Contact area, slip angles. If your running dirt speedway, grooving and sipeing can give an edge. Picking a compound for track surface and length of race is important also.
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David Redszus
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Post by David Redszus »

Getting the COG right for the roll centers built into the kart is where the real money is made.
Where are the front and rear roll centers and roll axis on a kart?
How are they measured?
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Post by Bubstr »

David Redszus wrote:
Getting the COG right for the roll centers built into the kart is where the real money is made.
Where are the front and rear roll centers and roll axis on a kart?
How are they measured?
It really don't matter, you can only change them slightly by moving wheels or increasing the flex. They would be found much the same way as a straight axle car. It seems the important issue is the results of having the COG too high or too low.
Loose in and out and threw the corner= too low COG.
Loose in tight off= rear bias heavy.
Tight in loose off= front bias heavy.
Wheel hop, chatter or bicycling= too high.
Skating or the feeling of being on ice= too low COG

Each kart or class of engine and weight has it's own best balance. A box stock 4 cycle restricted class for juniors is not the same as a dual 2 cycle outlaw class, because of different forward and side loads on tire contact patch.

On dirt especially the flexibility of the kart is important, depending on track surface. While a very flexible kart may maintain tire compliance very well on a wet rough track, one that isn't as flexible can be more effective at it on hard slick. So starting the qualifying with a flexible chassis and taking some of the flex out as the night goes by can gain some.

Here is something to think about. I was involved with a factory team, All karts raced where for sale. they got karts less seat and mounts period, the reason was they fit driver (lol) and less engine or engines unless they paid extra. Does that tell you where the speed secrets was? They once sold the dual outlaw kart and couldn't get a replacement for three weeks. We took different brand trade in kart, put our stuff on it and won the second week out against the one we sold. The selling of karts was a two fold promotion to have confidence to sell winning karts and to keep the race team in fresh karts that where not work hardened. The engines was good but nothing special. They discouraged the sale because of blue printing and dyno time.
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Post by David Redszus »

Bubstr wrote:
David Redszus wrote:
Getting the COG right for the roll centers built into the kart is where the real money is made.
Where are the front and rear roll centers and roll axis on a kart?
How are they measured?
There are no roll centers on a kart. Roll centers should not be confused with gravity centers. Roll centers and the resulting roll axis are found on rigid body vehicles with descrete suspension components.

Gravity centers must be determined laterally, longitudinally and vertically. They are used to determine weight transfer due to gravitational forces in any direction.

Karts utilize a tubular ladder frame chassis which behaves as a torsional plane. It is subject to bending and twisting depending on the forces being applied. Vertical deflection is determined by bending forces due mainly to frame stiffness, weight of the driver, wheelbase and vertical Gs.

Torsional, or twisting forces are determined by lateral G forces, driver weight, track width and center of gravity height. Which specific part of the chassis that is bending or twisting is the real secret to building and tuning a kart correctly. It is seldom, if ever, done.

Ideally, a kart frame should be designed and built to fit the size and weight of the driver since the driver represents the largest sprung mass. This means changing tube diameters, wall thickness, seat locations and seat attachment points, and frame linear dimensions.

But rules and ignorance usually do not permit the ideal design objectives to be met. Consequently, tuners must use the frames as built and resort to minor adjustments in order to obtain better handling than their competitor. It doesn't matter if it is right or wrong; it just has to be better than the other guys.
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Post by Bubstr »

I realize that a kart custom made for each competitor would eliminate a lot of tuning problems, but it would lighten your wallet so much. You would have to have a second one built for the weight change.

Your right in that each kart is different. The basic seen difference is the width of the main frame rails compared to the tread width. This is one of the things that is adjustable by sliding hubs and different off sets in wheel halves. I suppose if you was looking to tying this down to real numbers, you could relate it to two opposing sway bars or torsion bars. But then you would have to add in the ends, front and rear and seat mounts that all act as sway bars also. I suspect the manufactures only loosely use hard numbers and depend on test and tune and feed back for changes. I think they get a call from Indiana or Iowa or Texas saying we did this to your kart and it really helped. Lets try that and see what it does.

I have tried locking down a kart in a jig and using a torque wrench to measure chassis loading and deflection, but it seems the numbers are just loosely connected to real handling. I guess it still comes down to where the rubber meets the road.

Geoff, I believe you have found the best part of karting. Racing on dirt is a big equalizer. This is a place where smart goes fast and that takes some of the money equation out of it. Learn to read the track, you won't finish on the same track you start on. You will probably change a couple teeth on gear at least one grooving pattern and some stagger to be king of the hill. Some don't like it because they can't buy the top spot, you get mud on your stuff and wear gears out. But some like it for them reasons.

One of the things as a driver or the guy that sets up the kart, is to know when to run the cushion, when to go to the bottom and when to skate the middle. This is something only experience can tell you. I will tell you the key to running the cushion is having a little good stagger and getting the right rear wheel with in 6 inches of the berm going in and it's a free ride around the corner. If you want off the cushion bip the throttle it will set the right rear and you almost automatically go about a kart width down on the track. Don't do tis till after apex of corner. The key to bottom feeding is having the front hooked uo and a light right foot. The key to skating the middle is harder compound tire and neutral balance. There are several different kinds of tracks, You have momentum tracks, these are the ones with wide sweeping turns, usually a bank They like cushion running, some so much the cushion goes to the wall. there are ones with tight corners flat and strait a ways that no one can run a cushion. The end up being a drag race between two corners, but don't discount the corners, because any extra corner speed is what wins the drag race. There are countless tracks in between.
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Post by BillyShope »

If I were asked to point out the roll centers on a kart, I'd throw up my hands and walk away. But, the kart has them! Or, at least, it acts like it has them. What I'm getting at is that, if you use the technique described on Page 30 of my site, the spreadsheet will spit out the locations of the roll centers and the roll stiffness distribution. In other words, the kart acts like it has a roll axis in the XZ plane with roll centers located as indicated. Knowing this, you can then evaluate the effect of that extra shim under the seat or a change in spindle height or whatever. All you need are your scales and 3 locations for attachment of a tether.

If you have back issues of Racecar Engineering, there was an article in the summer of '07. Unfortunately, there was a typo in one of the equations, but my spreadsheet cranks out the correct answers.
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Re: go kart chassis setup

Post by j-c-c »

I have not raced a kart in over 50 years, but I see no mention above of the sometime dated(?) advantage of lifting ever so slightly the inside rear tire on a solid axle kart in the tighter turns on asphalt tracks. Has this thinking been debunked?
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Re: go kart chassis setup

Post by David Redszus »

j-c-c wrote: Mon Apr 12, 2021 9:49 am I have not raced a kart in over 50 years, but I see no mention above of the sometime dated(?) advantage of lifting ever so slightly the inside rear tire on a solid axle kart in the tighter turns on asphalt tracks. Has this thinking been debunked?
Many years ago, we used FEA to evaluate the movement and chassis forces in a shifter kart.

We found the corner lifting forces were a function of where the seat mounts were attached to the
frame longitudinally. The lateral weight transfer force of the driver was the biggest tuning factor.

In addition, we found that a belly pan attached as a stressed skin to the frame could
tune torsional flex and keep all four wheels on the ground.

Bounce was another important factor but could not be determined from our FEA study.
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Re: go kart chassis setup

Post by Rick! »

j-c-c wrote: Mon Apr 12, 2021 9:49 am I have not raced a kart in over 50 years, but I see no mention above of the sometime dated(?) advantage of lifting ever so slightly the inside rear tire on a solid axle kart in the tighter turns on asphalt tracks. Has this thinking been debunked?
No. When I was at the Mannheim indoor track somewhere near 2001, I was instructed to lean to the outside of the turn to loosen up the inside rear to get around tight corners faster. This was on a Biland chassis with a Swissauto 250cc 4 smoke.

Bubstr, your info is money! Thanks for sharing.
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