Can Oversteering and Understeering change by Weight?

[mjmghdm]

There is a question about oversteering issue.

If I add some (or very much) masses to the rear end (rear axle) of my car, Can this alteration chage my understeer vehicle to an oversteer one?

Actually I have read alot about oversteering, but I can't take into account all of them together. Now this question is bothering me!
I think there should be no difference, excluding a graph I have seen in "Reimpell"'s book of "Chassis Engineering Principles" that shows a reduction of lateral friction coefficient due to increment of the weight (shown for an individual tire).

I ought say that I don't want to add other axle effects such as compliance-oversteering or roll-understeering or ....
I'm already asking about the effect of distribution of weight (between front and rear axle) on oversteering behavior of a car.
In the other words, Can I move some weight from the back to the front to make an understeer vehicle from my oversteer car?

Thanks.

 

[hemipanter]

Som race car use ballast, and if located in front of the front wheel the car will understeer. For two resons, less µ number and the location of the mass. The oveer-understeer effect may turn out different using front weights than from another swaybar. What method I should use mmyself depend on if I want any special behaviour of the steering tendecy. However, I should use the skidpad test to measure what in my eye:s is over-understeer.
Regards
Goran Malmberg

 

[cibachrome]

For every question, there is an answer:

Yes same total weight with different rear weight gives more oversteer. Also, adding payload to back of a vehicle gives more oversteer. Different effect than anti-rollbar because axle weight affects the straight ahead response. Anti-roll bars don't generally start their effects until some significant axle sideforce builds up. Yes there are some straight ahead effects of a bar but that's not what the poster was talking about. This subject is only about the load sensitivity of tire cornering stiffness. Tires are softening springs with load, so their sideslip rate increases as you add load, therefore increased rear sideslip rate produces increased oversteer.

 

[mjmghdm]

Thank you tow;

Dear hamipanter:
Thanks for your illustration. Your example validates my conception. But my question is exactly the secoend reason of yours: "location of the mass". How dose this happen? Is this observable (1.)in a simple bicycle model or (2.)in a four wheel model or (3.)is this exactly an axle based effect and only can be seen in a full car model?
(In addition; you spoke about steering tendency. Is it the same steering effort? Or not)

Dear cibachrome:
Thank you for your explanation. But I don't have question about what I know "load sensitivity of tire cornering stiffness". I'm asking; If I compensate this effect of tire (for example by increase the rear tires pressure), Will the oversteering effect still remain? (because of the weight)?
(In addition; What is "straight ahead response"? I'v never heard of that. Does it mean Understeering? If not, Can you (or everyone) explaine what it means?

Thanks who read, who think and who answer.

 

[bikesrfast]

The car will 'slide' more at the end you add the weight too IF the attitude of the car remains unchanged from the weight increase.

Example if you introduce increased pitch because of the weight then other variables may enter the picture such as changed roll center/s.

 

[cibachrome]

You won't be compensating weight change effect on understeer with air pressure. As you add air pressure, tire stiffness gradient goes down.

If you plot a vehicle's understeer function vs. lateral acceleration, it has a value at zero g's and extends to other values, generally either increasing or decreasing. The "straight ahead response" is the value at the zero g intercept. Sometimes other g levels (like 0.1g) are used instead because of the method used to get understeer (its a derivative and susceptable to endpoint conditions in the math) and because the understeer at low g levels can be very nonlinear due to steering system traits. Given this function, axle masses and distribution raise and lower the function. Roll bars don't change the zero g ("straight ahead") value very much. No effect if the roll gradient is maintained or the roll steer and camber fators are small. Roll bars change the width characteristics of the function and the ultimate value (increasing or decreasing).

 

[hemipanter]

Lets say we have a vehicle with a weight distribution of 50-50% front to rear weight showing neutral steer. In such case we might have two different situations, low or high polar moment of inertia. In the case of high polar moment of inertia the car will be slower to change direction. We still keep the weight of the vehicle the same but this time we change the weight to 40-60 rear to front distribution. In this case we will still have the polar moment of inertia to take in to concideration plus another weight distribution.

If we run on the skidpad the polar moment wont play any big difference since the car will not be forced to change direction very fast. If we to be able to stay on the circle line have to increase steering input more and more by the speed increase the car is understeering. So, what does the 40-60 weight do then? If we compensate the weight cange by tire friction area to keep the µ number the same there should be no difference in the over-undeersteer effect.

I did not take other parts of influence in to conciderations here, camber gain or whatever one might think of. The car could as well be four parts of rubberarea
on the pavement loaded by a weight that alter its location.

Hope I understand your question right?
Regards
Goran Malmberg

 

[cibachrome]

Polar Moment is non sequitur to an understeer discussion. The weight distribution can be compensated by normalized cornering stiffness {"friction rate") changes, but how that is done remains the task. Not with roll bars and not likely with tire pressure.

 

[NormPeterson]

Tire size and relative wheel width?


Norm

 

[hemipanter]

I mentioned tire size as "rubber area". But there are different opinion about the size and shape of the footprint using different size tires and inflation. However, what I meant was that higher axle weight takes a larger rubberarea for the same load per area.
Regards
Goran Malmberg

 

[cibachrome]

Hemipanter:

Your website was very interesting. I looked at the tire grooving methods you show.

For this discussion, the tread contact area and pressure distribution are only small players in the understeer/oversteer equation. They are limit/maximum g influences. For 'normal' driving range ( under .75g ) the tire carcass materials and tire and wheel dimesntions are dominant in tire cornering stiffness determinaton. Reciprocal normailzed cornering stiffness is the tire contribution to understeer. Understeer is simply the difference in front and rear axle sideslip stiffness.

 

[hemipanter]

Cibachrome.
Right, I am probabley talking to much racing oriented effects. In fact I dont have performed very much testing using regular cars with street tires, or better put, no such testings at all.
I am glad you liked my site.
Regards
Goran Malmberg

 

[Yawing]

I have pondered this issue for a while now. It depends if you are talking about tyres that are operating purely in the linear range, or one axle is in the transitional region,etc, and of course what tyres are fitted and how they react to increased normal load.

if purely in the linear range, theortically by adding extra rearward weight% for example would yield more oversteer. increasing the rear tyre pressure in general should also increase the oversteer, not reduce it. It increases actual vertical stiffness and hence rear axle roll resisting stiffness so more weight transfer on rear versus front,etc. Also higher pressure means smaller contact patch area,etc.

 

[GregLocock]

So, why do you put a couple of bags of cement into the tray of a tailhappy pickup?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Yawing]

Are the bags above, forward of, or behind the rear axle centreline?

I can imagine the bags behind the rear axle CL giving understeer as the front is lited off the ground and the rear gains load. now this brings me to my conundrum of motorbikes where you lean forward to gain front grip, which is opposite to the theory of milliken in the simple bike model.

Could it be for most tyres that within a range (as a function of the rated load capacity) the tyre will lose efficiency as vertical load is removed, ie when the tyres vertical load is much less than its rated capacity?

Ie the vertical load sensitivity is mostly due to the tyre compound deflecting over the stones in the asphalt such that at first the tyre gains lots of extra surface area and deflection upon initial loads, but the more and more it is loaded the less gain it gets to a point of total saturation of the tyre on the tarmac...

Now conversely start at the same point and REDUCE the load, for each increment of vertical load removed you will get a sharp drop off of surface area of the tyre touching as the tyre comes out of the "asperities" of the road..

Ie if you have a pickup that has little or no weight in the back, and it has tyres (and springs but that another story) that are rated to take 1000kg of payload over and above the static weight of the car, then there will be little deflection of the the tyre into the asperities. Sooo when you add weight on the rear, the tyres are pushed into the tarmac actually gaining efficiency!!

just a theory but one that tries to join the theory of motorbikes and cars.

 

[cibachrome]

Checkmake Greg !

The effects from adding the weight to the pickup isn't a tire thing at all. Truck tires generally have high load sensitivity (they compensate for load proportionally). That's why they are on there.

Trucks usually are designed with lots of roll steer load compensation: They can have 0% empty and 15 to 20% laden. On this side of the pond, Its not uncommon for a light truck to have more linear range understeer with some load in it than when its empty.

The load at the back also increases the polar moment. This changes the yaw natural frequency. When the yaw peak frequency goes farther away from the roll and sideslip natural frequency (i.e. they decouple). The vehicle gets less oscillatory with less feed-forward feedback. You can tell which mode is more happy by driving at different speeds. Roll dynamics are not speed dependent, Yaw velocity and sideslip are. This same phenomenon is the reason some claim that roll oversteer (in the total vehicle sense) is good for handling when you drive REALLY fast. Because the roll and yawrate modes can cross over in the case of high roll natural frequency and high sideslip frequency (stiff tires with short relaxation), their roll oversteer is actually feeding back as roll understeer in the systems engineering view.

This makes a great Simulink demo if any of you are so inclined ...

 

[Yawing]

Ok,

what if we simplified the question now by saying we have a 1000kg car with its suspension welded up, same tyres all the way around, each corner weight the same. We do a test in putting 100kg either on top of the bonnet (hood) directly above the front axle centreline, then directly in the centre of the car, then in the boot (trunk directly over the rear axles, or even then try the weight as far back as possible to reduce the front weight and increase the rear.

now drive that car around a skidpan. What would happen?

 

[GregLocock]

What level of complexity do you want as an answer?

If you just take the bicycle model and have a tire that just has a constant cornering stiffness, then the weight forward will increase the required steer angle for a given latacc, aka linear range understeer.

Now make the tire more realistic, and make its lateral force proportional to the vertical force, and the steer angle, and then the weight forward has no effect.

Now make the tire more realistic and make the lateral force fall off as a function of vertical force, and the weight forward will cause understeer again.

Now make the bicycle a 4 wheeler, so that load transfer becomes important, and the understeer caused by adding the weight forward will increase yet again, partly due to the CGZ change.

Additional complexity, for a 'normal' car, will carry on with the same trend. Generally weight forward, and high up, will increase understeer.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Yawing]

Yea thats fine, but how far can you go?

can you keep moving the weight forward such that now you have a car with 90% of its weight on the front?

will it still understeer??

 

[GregLocock]

Does that sound like a 'normal' car?

Yes, I expect it would have more linear range understeer , but ultimately in reality it will reach a point where the rear tires bounce off the ground and the back will go.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.