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Can Oversteering and Understeering change by Weight?

Can Oversteering and Understeering change by Weight?

Can Oversteering and Understeering change by Weight?

(OP)

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.

RE: Can Oversteering and Understeering change by Weight?

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

RE: Can Oversteering and Understeering change by Weight?

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.

RE: Can Oversteering and Understeering change by Weight?

(OP)
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.

RE: Can Oversteering and Understeering change by Weight?

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.

RE: Can Oversteering and Understeering change by Weight?

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).  

RE: Can Oversteering and Understeering change by Weight?

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

RE: Can Oversteering and Understeering change by Weight?

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.

RE: Can Oversteering and Understeering change by Weight?

Tire size and relative wheel width?


Norm

RE: Can Oversteering and Understeering change by Weight?

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

RE: Can Oversteering and Understeering change by Weight?

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.

RE: Can Oversteering and Understeering change by Weight?

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

RE: Can Oversteering and Understeering change by Weight?

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.

RE: Can Oversteering and Understeering change by Weight?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Can Oversteering and Understeering change by Weight?

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.

RE: Can Oversteering and Understeering change by Weight?

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 ...

RE: Can Oversteering and Understeering change by Weight?

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?

RE: Can Oversteering and Understeering change by Weight?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Can Oversteering and Understeering change by Weight?

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??

RE: Can Oversteering and Understeering change by Weight?

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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Can Oversteering and Understeering change by Weight?

Ok, So yes I agree linear range Understeer, but until a very low speed where the rear will let go, as the rear tyres have such a small amount of load on them, they hardly deflect over the tarmac and will suddenly let go and the car will oversteer at a very low speed

I guess this is because the friction coefficient in the sliding region must drop off more for lightly loaded tyres. So linear range, more weight on the front, more understeer. But as for the lightly loaded tyres, the slip angle for maximum efficiency is much less, and as such a variation in slip angle could easily send the tyre into the sliding region.
Also a small variation in normal force ("small relative to the car with more "normal" corner weights) means a large variation in % of normal load on the really lightly loaded tyre, so again easy to send the tyres into sliding with low efficiency, So yes agree that it would be easier for the rear to "bounce" and let go.

so linear understeer to a small lateral acceleration at which point unpredictable O/S

Again this isnt a "normal" cars weight distribution, but you might get th same situation in running narrow but high corner stiffness front tyres and wide lower corner siffness rear tyres..ie even with a 50/50 weght distribution the car will oversteer to a point the front starts sliding at which point it drops off efficiency dramatically.


This is also "normal for say a motorbike where a 100kg rider on a 100kg bike can move the weight distribution around very dramatically......

RE: Can Oversteering and Understeering change by Weight?

Ok, I'm lost.

"the car will oversteer to a point the front starts sliding at which point it drops off efficiency dramatically".  This ain't physics.  A vehicle that is oversteering has a rear slip gradient higher in magnitude than the front. BFD.  This doesn't register in Vehicle Dynamics Nation.

How about this:
1 gokart, 50% wgt dist. Front axle steer, linear FY tires.
2 gokart, 50% wgt dist. front axle steer, nonlinear FY tires.
3 gokart. 50% wgt dist, front axle steer, nonlinear FY, MZ tires, no steering compliance.
4 gokart. 50% wgt dist, front axle steer, nonlinear FY, MZ tires, linear steering compliance.
5 gokart. 50% wgt dist, front axle steer, nonlinear FY, MZ tires, nonlinear steering compliance.
6 gokart. 50% wgt dist, front axle steer, nonlinear FY, MZ tires, nonlinear steering compliance, roll DOF with front and rear unequal roll steers. Pick your own realistic values.

Each of these is a cause for Depends (not the adult diaper kind).

Extra credit for another load condition (as in GVW).

Don't mention camber, racing, compliant body, Milliken (sorry my friends Bill & Doug), or motorcycles.

Pick a test procedure: Constant radius, constant steer, step, or frequency chirp.

Then pose a question, not a simulation loop train of thought.

This is usually followed by:

"Students will show all their work", or, "The driver will indicate front or rear Great Grip conditions".

RE: Can Oversteering and Understeering change by Weight?

Ok, I meant the car will oversteer (rear slip angle more than the front as corner stiffness is less, until the FRONT tyre (as it is very "unforgiving", ie sharp drop off of later force after the peak grip level with increased slip angle) loses efficiency (by moving into the sliding region) and now with the front sliding and the rear still in the linear region it NOW can supply a higher lateral force than the cheese cutter front tyres that are now sliding towards the outside of the corner giving UNDERSTEER, not linear range understeer, comeone im talking about a racing car I dont care about driving around a shopping lot.

Im talking constant radius, slowly increasing the speed to approximate steady state, and my question is "will the front of the car hit the wall or will the rear?!!"

RE: Can Oversteering and Understeering change by Weight?

The answer is... it depends.

To be honest I think you can answer your own question if you look in Millken and set up a bicycle model. Then you'll know what assumptions you are making. As it is, as cibachrome has indicated, you are just waving your hands around in the air.

Cheers

Greg Locock

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

RE: Can Oversteering and Understeering change by Weight?

Yes, Sorry for all the hand waving and muliple examples, you can make different assumptions and come out with opposite results.

To be honest In an underhanded way im trying to find the reason why in a single track vehicle it is almost 100% correct that putting extra weight on the front will make the vehicle turn, whilst in almost 100% of the cases (according to the bicycle model) the exact opposite case is true.

Surely motorbikes also obey the laws of physics, and have tyres that touch the road. there has to be a link between a simple "bicycle" model of a car, and a real bicycle.

Surely the enlightened ones should in there great wisdom be able to bestow upon me an answer more befitting of an engineering forum than.."depends"

RE: Can Oversteering and Understeering change by Weight?

I did made a full scale experiment a year ago. The car has 60% rear weight, tire size somewhat matching this. I mounted a wire at the location of CG and pulled the car sideways to see what end is sliding first. Without rollbars but a wheelrate matching the weight distribution. The outcome was pretty neutral, either end could start sliding.
This was just a mockup test, really nothing to depend on, ill just mentioned it as it is sort of topic related.
Regards
Goran Malmberg

RE: Can Oversteering and Understeering change by Weight?

If this car has a high power to weight ratio, then you are assisting turning it about the front axle using tire aligning moments.  It takes a Really "New School" driver to do this and a tire that will cooperate. Verify by adding some air to the front tires (dropping the 2 MZ moments) and lowering the front pressures (raising the rigid body MZ moments). This should be almost the same as the nose weight increase.  You can measure this too:  The instant center of the turn will be way behind the geometric center.  You may have heard of this at the track as "tighty-loose".  The car is tight (understeering) and to free it up under power you tighten it up even more in the front (which is counter to intuition).  Do the same experiment in the garage by putting grease plates under the rear wheels and turning the steering wheel. The car will rotate about the front axle pivot point.

This is the same mechanism in play when contesting what a supercar's weight distribution ought to be.  With rearward bias, there is more ultimate front control available for use by a Really Good driver. For Neutral and Understeering nose weights, the front is all used up, therefore no forward control is available. Under these circumstances, an experienced driver will stand on the throttle to get the car to turn.  This takes some nads and is a major reason you never see family pictures on the front of dash.

That's why loose is fast !

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