small 1/4mile oval tips, transitioning from road racing

[Dirtmod13]

First paragraph is an effect, not a cause. Second paragraph is an effect, not a cause.Third paragraph, holy crap, you just offered up a theory on why. Now if we have the bars and bite set to balance the weight on each of the rear tires at corner exit, why does the LR get so much less pressure? Wouldn't it get the same pressure as the RR since it has the same weight on it at exit?

Less pressure to make larger contact patch which gives more traction. It's a simple way to get more use out of that tire

[Confused?]

The rear tires are not equally loaded on exit. That would be true IF the car was headed in a straight line, but it's not. It's still turning as you are picking up the throttle. Generally, the LR tire will be loaded more than the RR, under acceleration. Think of it this way, if 500lbs are transferred to the rear, do you want 250lbs on each tire? Or would you rather have 350 on the LR and 150 on the RR? The extra load on the LR helps it "break" through the dust and gain traction.

The right side tires have more air because there is more lateral loading than the left side. The left side, usually has less because because of increased footprint and deflection. A loaded LR, with a low air pressure, will wrinkle the side walls like a drag slick. This aids in maintaining traction under throttle. Think about how a drag slick works.

The air pressure affects wheel weights, but more importantly, affects sidewall and tread deflection. What works for me may not work for you. I like to run 16 on the rights and 12 on the lefts in the tack. For dry, I drop the rights to 14 RF and 13 RR. The left I will drop to 10 for both tires. That change will affect the LR bite in your set-up. You need to know what other changes to make after dropping the air pressures.

Hope this makes it a little more clear.

[rob burgoon]

Less pressure to make larger contact patch which gives more traction. It's a simple way to get more use out of that tire

Sure, but why not less pressure on the right side tires then? Traction is good!

The rear tires are not equally loaded on exit. That would be true IF the car was headed in a straight line, but it's not. It's still turning as you are picking up the throttle. Generally, the LR tire will be loaded more than the RR, under acceleration. Think of it this way, if 500lbs are transferred to the rear, do you want 250lbs on each tire? Or would you rather have 350 on the LR and 150 on the RR? The extra load on the LR helps it "break" through the dust and gain traction.

The right side tires have more air because there is more lateral loading than the left side. The left side, usually has less because because of increased footprint and deflection. A loaded LR, with a low air pressure, will wrinkle the side walls like a drag slick. This aids in maintaining traction under throttle. Think about how a drag slick works.

The air pressure affects wheel weights, but more importantly, affects sidewall and tread deflection. What works for me may not work for you. I like to run 16 on the rights and 12 on the lefts in the tack. For dry, I drop the rights to 14 RF and 13 RR. The left I will drop to 10 for both tires. That change will affect the LR bite in your set-up. You need to know what other changes to make after dropping the air pressures.

Hope this makes it a little more clear.

At corner exit, you're still turning as you feed in the power, so you still have weight transfering to the right side and the RR. To keep the RR from getting overloaded, you use bite and bars to add weight to the LR. I'd expect them to be equalish once all is said and done. However, if what you're saying about more total load on the LR than the RR helping it push down through the dust is right, why wouldn't he same be true for the RR pushing down through the dust if we let the RR stay really heavy?

For lateral loads, that makes some sense. If the right side tires are experiencing more load, I could see an issue where lack of sidewall stiffness becomes a problem. That might explain the higher pressures for tacky tracks.

Great post! Thank you!

[powerslide]

Side walls do not have grooves that is why you can only go so low on right side pressure, also the driver stops feeling the changes when you get too little air in the tires.

[Confused?]

The RR is heavy, as you put it, during the corner. That is also why the trend towards stiffer RR springs than LR. The weight is transferred to the right during cornering. To offset that, more angle in the bars, more indexing, more LR bite, and less air are used. Typically, you would not want to drive off the RR in the slick. It causes the car to loosen up on exit. In the tack, sure, but not dry slick. Equalizing the rear wheel loading happens down the straight. Or, it should. That's where you should feel the RR starting to raise up, if the chain allows it.

At corner exit, the weight should already be on the right side. As you pick up the throttle, the weight starts to move to the rear. The LR get loaded more due to bar angles and roll steer.As the car travels down the straight, the weight starts to move back towards the left side of the car. Think of the weight, during racing, as being a liquid. It get slung all around the car. The driver is the one who tries to control it and "put" it where he/she wants it.

Everything that is designed into one of these cars is done for a specific reason, at a specific point on the track. They are all "matched" together, as a compromise, to make the car work everywhere. That's why you don't run the same bare angles on the right side as you do the left. Also why we run a j-bar rather than a watts linkage. Air pressure are just one more variable in the mix.

If you can find some books on Late Models, it may be worth your time to read them. The rules for those cars apply to Mods as well.

Good luck

[rob burgoon]

I've got this one and have read through it a few times. Good stuff but kind of light on the "why" sometimes.

http://www.amazon.com/DIRT-MODEL-CHA.../dp/B00QVBTSAA

[rob burgoon]

The RR is heavy, as you put it, during the corner. That is also why the trend towards stiffer RR springs than LR. The weight is transferred to the right during cornering. To offset that, more angle in the bars, more indexing, more LR bite, and less air are used.

So we want the pressure as low as we can get it until the footprint goes to crap. Too much lateral load does that. In the middle of the corner, the right side tires have most of the lateral load and as such, those pressures need to be higher to help stay off the sidewall. Then, at exit, we have decreasing lateral load and want to go forwards. At this time, the LR being low pressure is a good thing since tire crinkle is good when you're pretty much going straight.

The only problem with that theory is the fact that the LR is up half a foot in the air almost the whole corner and carrying a fair amount of weight. Maybe the RR still carries more midcorner despite that?

[Confused?]

The LR is up half a foot? Are you referring to the chassis? If your tire is up, then you have other problems. The chassis should be up on the left rear. It's a bi-product of the weight transfer and suspension designs. About the only times the RR is loaded more than the LR is entering and through the center of the corner. That's where you get the sidebite from, loading the RR enough to plant it to the track's surface. If the car is set-up correctly, and the driver is doing his/her job, the RR will be the most loaded tire in the center of the corner. The RF will be at entry and weight should be shifting to the LR at exit.

Too low air pressures can also create too much heat in a tire. The same is true for too much air. A friend of mine runs 12 on the right and 10 on the left for all conditions. He has a few IMCA national championships. I respect his opinions very much. Generally, I, as well as most, lower the pressures for dry slick because you don't carry as much speed into or through the corners. Traction is hard to find and the lower pressures help with that. Also as said above, there are no treads on the sidewalls. The air needs to be high enough so that you aren't running on the sidewalls.

[oldtrackchamp4x]

A good way to see if your right side pressures are to low is to take a marker, white shoe polish works well, and mark from edge of tread toward wheel in a few spots. That way you can visually see how far tire is rolling under. Do the same on the left side tires but mark on the inside.JMO.