Rotation
 

Hi all,

when a set up guide or people talk about rotation. what does this actually mean?

does it mean that the car will rotate more as in a tighter turning circle?

Thanks

Rotation, to me, is how the car piviots when it's turning, and more specifically the behaviour in the middle of the turn.

A car that doesn't rotate, feels slow to get through the middle of the corner normally, leaving a delay for getting back on power. The opposite is a car that rotates too quick, and you have to correct the steering in the middle.

It's different from over/understeer, and different cars rotate about different places (some from around the rear axle, some from around the front). Ideally, good rotation means it get through the centre of the turn quick, and back on power as soon as possible.

Of course, will mean different things to others, but thats how I tend to see it.

HiH
Ed

+1 This is also what I think about it.

Regards Robert

A car will rotate around the centerpoint of the rear driveline. Getting the balance of rotation that is drivable is what you are looking for. With perfect steering geometry and steering pivot points and correct ackerman that will turn correct diameters at the given wheelbase and track width of the car, the car will rotate around the center of the drive line with little to no resistance or scrub. Then you can control how aggressive the car is by placing wieght further away or closer to the rear axle or driveline. The further away you place weight from the rear axle, the more smooth the car will feel but ill not rotate as freely because in order for the car to rotate, it has to pull that weight around the rear of the driveline. The more weight placed rearward or close to the rear axles will make the car more aggressive to a point as the front is lighter and will not take as much to plant the front tires or rotate around the rear driveline. Too light in the front however will make it very hard for the tires to maintain traction. So therefor, you need a good balance based on the given track conditions.

+1 on this!

Can you explain what the trade off is between weight forward/backward and weight inline/out wide?

Say you are on high bite carpet, a car that rotates freely and effortlessly can be very hard to drive. Moving weight forward will slow the rotation down. Think of it like this, if you go to the supermarket and throw a 50 lb bag of dog food in the shopping cart, it is harder to steer than when it is empty. Move the bag toward the back of the cart, it is harder than when the cart is empty but much easier than when you have the bag pushed forward. That is how the front to rear weight placement helps based on how aggressive the car is on the given track. You want the weight as far back as you can without being hard to drive.

Centering the weight helps transition speeds. The more centralized the weight is, the faster the car can change directions. The only trade off is the car will roll more as there is less weight off center of the car to counter balance roll. Realistically though, an ideal setup will have some weight off center to give you the best of both worlds.

Very good explanation

Thanks. These things can get confusing. :)

thanks guys, good reading although my head is now slightly mashed :)

protc3,

I'm having trouble relating what you've said to my own experience, observations, and layman's understanding of physics.

I get the shopping cart analogy, but shopping carts have casters on the front wheel, and you steer them from the handlebar at the back. Because they're casters, the front wheels don't really generate any lateral grip, they just align to roll freely. If shopping carts had steering wheels, they'd handle best with the weight in the middle of the cart, no?

I've always thought of it being that, when the car is at speed on the track (that's what really matters) the point around which it rotates is defined by the CG location, and the lateral grip being generated by the tires. Of course, the lateral grip being generated by the tires is dependent on a whole boatload of things.

A car rotates because steering input causes the front wheels generate lateral force, which creates a yaw moment around the CG, which causes the rear tires to develop a slip angle and generate a lateral force to keep the car from spinning out.

The further back the CG is, the stronger the yaw moment the front wheels can generate is, and the more lateral force the rear need to generate to keep the car from spinning out. I guess when I think of it that way, your first explanation makes a bit more sense.

But I still don't get the statement "A car will rotate around the centerpoint of the rear driveline." At my track, the fastest cars (with the fastest drivers) are always visibly rotating around a point just a bit forward of the center of the car. It's especially noticeable on a 1/12 scale with a rollover antenna. It looks like the antenna is following a line around the track, and the car is just pivoting around it.

-Mike

Think of it this way...if the car were rotating around the CG then that would be the fixed point and the front of the car would pivot in and the rear would pivot out. If that were happening you would see 2 things...first the rear tires would lose grip and secondly the front and rear tires would travel in closer to the same arc the closer the CG got to the center point of the car. But this does not happen as the rear tires always travel in a tighter arc than the front.

The natural tendency is for the car to rotate around it's CG...however the grip of the rear tires changes this pivot point to the center of the rear axle. Once the rear tires lose grip and a spin is initiated then the car does rotate around it's CG.

I have experienced this, but with a case of beer ;)

Great explanation!

rotation? always pass to the left. :lol::p:sneaky:

The car's CG effects the center of rotation as the tires begin to slip but does not function as a center of rotation.

Jason is correct, cars (shopping carts too) rotate around a point where the lines from the center of the rear axle intersect with lines perpendicular to each of the front wheels IF you have perfect traction/no tire slip (dotted lines in the image).

The point of intersection is called the Instantaneous Center of Rotation or ICR .

The topic of this thread is what to we R/C Racers mean by Rotation.

In R/C Cars "Rotation" is the degree to which the rear tires slip in a controlled manner in order to most efficiently proceed through a corner and point the car to effectively exit a turn.

We set up our cars to intentionally increase the rear tire slip angles to induce rotation. This shifts the ICR forward (solid lines on the image).

Full scale race cars do this but R/C cars use this effect to a much larger degree.

Cars that do not rotate are too locked in and will lack corner speed. You have to drive them around corners. This is slow.

Cars that rotate perfectly are 100 predictable, consistent and easy to drive as a locked in car. You drive them through corners and point them out of corners. This is fast.

Cars that rotate uncontrollably are inconsistent and terrible to drive. You have to pitch them into corners and catch them. This overheats tires and while you may get a magic fast lap with a car like this you wont be able to do that consistently for a full race.

Racers alter their setups to get the balanced sweet spot of rotation.

Note that Ackermann has to change in order to be in sync with the slip angles and their relative ICR. This is a big deal in sedans and I think its one of the main reasons double bell crank steering systems are better than singles in these cars. They allow for more efficient Ackermann curves that better relate to the ICR of sedans.

http://www.smithees-racetech.com.au/images/Image2.jpg