Relation between roll stiffness and slip ?

[Ashish_FSAE]

assuming no longitudinal weight transfer, if the front roll stiffness of my car is more than that or rear, will the front of the vehicle slip more or rear will slip more ?

the FSAE cars are rear heavy, so if i want my car to over-steer on turn entry with braking, the weight transfer will cause the CG to shift towards the front and hence both front and rear of my car will slip almost equally giving almost a neutral steer situation. Now if i want it to be over-steer, i have to increase the rear slip of the car, two ways to do it:

1. reducing the cornering stiffness of rear tires - this was possible if i had braking only on rear wheels, increasing the longitudinal skid in rear tires, its cornering stiffness will reduce giving over steer, but FSAE car has 4 wheel brake, so this option ruled out.

2. by appropriate body roll at the rear end, i want the rear wheels to slip more, so should I keep the rear roll stiffness less, so that more weight is transfered to the outer rear wheel and it rolls more ?? Or should i adjust the rear camber gain in such a way that the rear axle slips more ???

[Mikey Antonakakis]

Originally posted by Ashish_FSAE:
the FSAE cars are rear heavy
I wouldn't be so sure of that...

Also, the CG only moves as much as your suspension allows. Weight transfer is different. And it's not safe to assume that you'll have a neutral steer situation. I think our 08 car had a 47/53 weight distribution and 09 had 49/51, and 08 had some pretty strong trailing throttle oversteer (no braking required, just lift off the throttle and steer and the back end would come out). 09 was set up to have a rear ARB but that was never built, so it had less of a tendency to oversteer under deceleration (it also understeers a lot under steady state cornering, does this answer your question about front vs rear roll stiffness?).

[The_Man]

Originally posted by Ashish_FSAE:
assuming no longitudinal weight transfer, if the front roll stiffness of my car is more than that or rear, will the front of the vehicle slip more or rear will slip more ?


Imagine your car to have an infinitely rigid chassis and the 2 axils to have a torsional spring to represent the roll stiffness of the front and rear separately(Kf and Kr). Not with lateral acceleration your car will tend to roll = T/(Kf+Kr) where T is the moment torque produced about the roll centre but the lateral pseudo force. Now since you have a infinitely rigid chassis your roll angle is same front and rear but stiffness's are different so the load transfer is different. So for example a higher roll stiffness in front will give you a lesser weight transfer in the rear pushing your car towards <STRIKE>oversteer</STRIKE> understeer and visa versa.

[Ashish_FSAE]

The man,

If my front roll stiffness is more, then more amount of load will be laterally transferred while cornering, resulting in an more un-even load distribution between the front inner and outer wheel. this will result in less lateral force developed by both front tires, if i assume the front and rear tires are slipping at the same angle, say 5 degrees. But as the front tires are producing less cornering force due to lateral weight transfer, they will slip more (say 10 deg) to match the cornering force developed by rear wheels. Hence a higher roll stiffness at the front end will give me a under-steer vehicle and visa versa.

What do u say ?

[The_Man]

Sorry I meant to write understeer. My bad.

[Ashish_FSAE]

Oh come on, good we discussed, we both got some clear idea.

[exFSAE]

Load transfer to the front (under braking) will, all things being equal, tend to yield a balance transition towards oversteer.. not the other way around..

[Pete Marsh]

[QUOTE]Originally posted by Ashish_FSAE:
The man,

But as the front tires are producing less cornering force due to lateral weight transfer, they will slip more (say 10 deg) to match the cornering force developed by rear wheels.



I think you might have missed the point here. There is no reason the front tyres are going to do anything to "match" the cornering force of the rears. Rather if the forces are not matched there will be a yaw torque which may, or may not, be a good thing.
It is your job to make a system that CAN be balanced by YOUR driver, in such a way that it can be guided around the track at, or near, its limit of grip. This means the driver must be able to control the yaw torque on the chassis, and hence its direction, to their satisfaction. It is also good if you can achieve this and also have more grip than everyone else on the day.

Pete

[Ashish_FSAE]

Pete,

I said that assuming there is no longitudinal load transfer, just to keep things simple at this stage of understanding. So with no longitudinal load transfer, and the rear and front tires having same cornering stiffness and load distribution being 50-50, i concluded that a stiffer front axle will produce understeer and visa versa.

No with the weight transfer, say, braking during turn entry, the weight distribution will be say 65-35. Now in this case, load on front axle being more, hence the front will slip less as compared to the rear (assuming no longitudinal load transfer). Hence the car will over steer on braking and understeer on accelerating.

How about that ?

[Brian Smith]

I dont think you have a clear deffinition of understeer.

My simple deffinition of understeer- The front of the car has less maximum grip avaiable than the rear.

Note this has nothing to do with slip angles, just ultimate grip. We care about oversteer or understeer at the cars fastest possible cornering speeds. If the front runs out of grip first, the car is understeering. It dosent matter what the slip angles of the front or rear were before the front end reached its limit.

With more weight transfer comes less grip and higher roll resistance at either end will give higher weight transfer. I think you understand this.

[The_Man]

Aren't we all saying the same thing?

[t51r]

ashish, I think you misunderstand the meaning of understeer and oversteer.

Understeer happens when front tires' grip is less than the rear tires', the radius of path you car going is becoming bigger, usually driver tends to give more steer angle to try to get back to desired radius. (front slip angle > rear slip angle)

[Yellow Ranger]

My simple deffinition of understeer- The front of the car has less maximum grip avaiable than the rear.

Remember, its not just less/more grip - its that grips relation to the CG location, longitudinally. Its more about sum of moments from lateral and long forces than it is about sum of forces. I always think of understeer/oversteer in terms of vehicle yaw, not lateral grip.

[Mikey Antonakakis]

I have a different definition. Oversteer is when the passenger is scared. Understeer is when the driver is scared.

[Yellow Ranger]

haha

[flavorPacket]

I'm surprised nobody has quoted Claude here yet, so I will.

"There is no such thing as oversteer or understeer. There is only over yaw moment or under yaw moment."

Pete is right.

[Yellow Ranger]

that's what he says, i was trying to remember it

[STRETCH]

"There is no such thing as oversteer or understeer"

I see... so that's why it's the most talked about thing in vehicle dynamics!

[exFSAE]

Originally posted by Brian Smith:
I dont think you have a clear deffinition of understeer.

My simple deffinition of understeer- The front of the car has less maximum grip avaiable than the rear.

Note this has nothing to do with slip angles, just ultimate grip. We care about oversteer or understeer at the cars fastest possible cornering speeds. If the front runs out of grip first, the car is understeering. It dosent matter what the slip angles of the front or rear were before the front end reached its limit.

With more weight transfer comes less grip and higher roll resistance at either end will give higher weight transfer. I think you understand this.

Gotta disagree here. I'd say understeer or oversteer occur at all non-zero lateral acceleration. Just a difference in front and rear axle slip angles and how they build up.

When you saturate an axle and run out of grip I'd use "plow" or "spin."

Understeer and oversteer are not entirely about grip.

Regarding Claude's comment... don't take everything he says as gospel. Same goes for Carroll Smith, Milliken, etc. The reason I don't like his way of putting it is as follows. On a skidpad you can have the car "set" tracking a constant radius, constant speed turn. Yaw rate acceleration is zero. Yaw moment is zero. You're sub-limit. But you can still have two different setups that result in different amount of steering and chassis angle.. varying degrees of balance and understeer.

If he meant AVAILABLE yaw moment (headroom) at either axle, that's a different story.. but still not the whole story.

[poe21]

Just to clear up a few things. Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit the wall, and torque is how far you take the wall with you when you hit it. Got it... good. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Brian Smith]

Remember, its not just less/more grip - its that grips relation to the CG location, longitudinally. Its more about sum of moments from lateral and long forces than it is about sum of forces. I always think of understeer/oversteer in terms of vehicle yaw, not lateral grip.

Yeah I agree about it being the sum of the moments and not merely lateral force. I was just keeping it simple with a longitudinally centered (about the WB) CG.


Originally posted by exFSAE
I'd say understeer or oversteer occur at all non-zero lateral acceleration. Just a difference in front and rear axle slip angles and how they build up.


What if you have different tires on the front and rear. The fronts have more ultimate grip but less slip stiffness. The car will "understeer" untill the rear reaches its peak and then the car will "oversteer". What do we say the car does, oversteer or understeer or a combination?

I guess what Im asking is, does oversteer/understeer refer to what happens before the limit or after the limit is reached? Or both?

[exFSAE]

Originally posted by Brian Smith:
What if you have different tires on the front and rear. The fronts have more ultimate grip but less slip stiffness. The car will "understeer" untill the rear reaches its peak and then the car will "oversteer". What do we say the car does, oversteer or understeer or a combination?

I guess what Im asking is, does oversteer/understeer refer to what happens before the limit or after the limit is reached? Or both?

Both, absolutely. Vehicle balance ("oversteer / understeer / whatever") changes through the whole range of vehicle speed, cornering, driving, and braking acceleration.

There's on-center balance and response, at low lateral G.. there's limit cornering balance.. there's on- and off-throttle balance. Hell you can have on-throttle oversteer at low speed, and on-throttle understeer at high speed.

A good example just about any FSAE driver should be familiar with is as follows:

You're either on a skid-pad, or going through a quick wide sweeper. Let's say a left turn. As you're trying to get into the throttle you can feel the car wants to push a bit (from rear weight transfer).

If you lift off the throttle or tap the brakes, even if you don't spin out (thus not at limit of traction) you feel the balance change to oversteer. Rear end takes some more yaw, you use a little less steer angle, and the car gets "pointed" better.

Having a driver come in from a run and say "She's! tight!" or "She's loose!" is worthless. Gotta know what the balance is and how it changes with every input you make.

[Brian Smith]

Having a driver come in from a run and say "She's! tight!" or "She's loose!" is worthless. Gotta know what the balance is and how it changes with every input you make.

Just heard an F1 driver radio into the pits that the car was understeering and I thought, is it corner entry or exit, or mid corner, tell your engineer something.

[Stocky Fast 1]

Originally posted by poe21:
Just to clear up a few things. Oversteer is when you hit the wall with the rear of the car, understeer is when you hit the wall with the front of the car. Hp is how fast you hit the wall, and torque is how far you take the wall with you when you hit it. Got it... good. http://fsae.com/groupee_common/emoticons/icon_wink.gif

I think I may have to use this line with the new people on the team just to mess with them, it's too good.