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Old 10-13-2009, 05:22 PM   #1
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Default RC Tire Characteristics, Weight and Cornerspeed

Thought i'd make a seperate thread to discuss something that came up in the Xray thread about rubber tire characteristics as they apply to RC cars.

All the full size car literature and teachings talk about lateral forces generated at or beyond the limits of the tire, where the more you load a tire the lower its effciency gets and the end result is less overall grip. The primary goal being to REDUCE roll and REDUCE loading on outside tires to increase overall lateral grip in corners. Think NASCAR wedge & preload techniques for ovals making a car faster by reducing the outside tire load.

Now for RC cars it seems this is not (or may not be for most cases) true as the tire is operating well below this region and is operating where more vertical load equals more overall grip.

So the goals for RC racing is to have INCREASED roll (assuming camber is kept in control), and INCREASED tire loading to get more grip.

Anyway, i'm interested in the technical aspects and whether in fact this is true for RC tires. Most of us can dial a car for front/rear balance correcting for oversteer/understeer off and on power using shocks, springs, rollbars etc. but how many of us have a real handle on increasing overall grip (and hence cornerspeed) of the car?

Some part of me refuses to believe that a heavier RC car is faster thru corners than a lighter one... if i'm wrong i'd like to know why!!

it would be cool to build a tire testing rig, but i'm not sure if it would take a month or the rest of my life to complete!
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Old 10-13-2009, 06:31 PM   #2
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I understand what your saying. But unfortunately this might be out of the scope of most rc racers.

I work for a top Indy car team and before a Le Mans prototype team doing carbon work. I always pay attention to the engineers on how and why they decide to make certain setup changes. But some of it just doesn't seem to transcend to its scale counterpart.

It would be nice to have a scale motec system or full telemetry (active ride hight, tire temps, roll, compression rebound, and virtual track layouts to see where each were doing what, etc...) but yeah.. some luxuries are ment to stay in the full size world.
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Old 10-13-2009, 06:42 PM   #3
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I will add though that since I deal more with aerodynamics in full scale that a lot of aero tricks can be applied to scale cars. I've played with splitters and full under wings to keep the air speed up under the car and gaining downforce. I have actually felt the difference in high speed corners with flat floors, splitters, and rear diffusers. But since we tend to hit the walls a little more than full scale cars do, these aero bits will get damaged quite easily. Not to mention, carbon is the only suitable material which is out of reach for most to fabricate with. Fixing the rear wings to the chassis somehow in a supportive way, instead of the body, adds some downforce, too. Since you would not losing some from the rear of the body flexing.

going off on a tangent here.. sorry
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Old 10-13-2009, 06:54 PM   #4
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Quote:
Originally Posted by Blackstring View Post
I understand what your saying. But unfortunately this might be out of the scope of most rc racers.

I work for a top Indy car team and before a Le Mans prototype team doing carbon work. I always pay attention to the engineers on how and why they decide to make certain setup changes. But some of it just doesn't seem to transcend to its scale counterpart.

It would be nice to have a scale motec system or full telemetry (active ride hight, tire temps, roll, compression rebound, and virtual track layouts to see where each were doing what, etc...) but yeah.. some luxuries are ment to stay in the full size world.
yeah i know what you mean i've opened this can of worms in drag racing and gokarts before and yeah its complicated, but it would be nice to know to the simplest approximation the load/grip situation. For example the Xray setup book says things like this:

"The amount that a chassis rolls in a corner depends on the position of the roll axis relative to the car’s center-of-gravity (CG). The closer the roll axis is to the center of gravity, the less the chassis will roll in a corner. A lower roll center will generally produce more grip due to the chassis rolling, and the outer wheel “digging in” more"

"More chassis roll equals more grip"

but then things like this:

"Anti-roll bars are used to adjust the car’s side (lateral) grip. They can also be used in conjunction with a softer spring rate to handle bumpy tracks more efficiently without excessive chassis roll at mid-corner. Anti-roll bars resist chassis roll and by doing so transfer wheel load from the inside wheel to the outside wheel. The stiffer the anti-roll bar, the more load is transferred. However, as the outside wheel is not able to convert the extra wheel load into extra grip, the sum of the grip of both wheels is actually reduced. This changes the balance of the car to the axle at the other end of the car; increasing the stiffness of an anti-roll bar on one particular axle (front or rear) decreases the side grip of that axle and increases the side grip of the axle at the other end of the car."

so which is it?
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Old 10-13-2009, 07:38 PM   #5
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Both are true, roll without transfering weight is traction. Soft setups have more traction. anti roll bars control roll, but reduce traction. Once you have enough traction controlling roll becomes more important than how much traction you loose.

Look at it this way, if you have 100 lbs on both inside and outside wheels and the coeff on friction is say 1, then you have 200 lbs of traction. As you load the outside wheel traction increases in a linear manner for a while until you load the tire so hard that the rubber can no longer support the traction requirements. Say if at 150 lbs this happens. So when the outside tire has 175 lbs on it you only get 150 lbs of traction and the inside tire has 25 lbs, so you only have 175lbs total. Less than you started with. Roll bars control roll by reducing the load on the inside tire while the outside is still highly loaded. For real tire this is called a carpet plot.

Look at full sized trophy trucks, they are so soft to handle 800 hp in the dirt that they almost roll over on pavement. But on a low traction surface they still get lots of traction.

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