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.

 

[Yawing]

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......

 

[cibachrome]

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".

 

[Yawing]

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?!!"

 

[GregLocock]

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 for tips on how to make the best use of Eng-Tips.

 

[Yawing]

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"

 

[hemipanter]

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

 

[GregLocock]

So... it depends.

Cheers

Greg Locock

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

 

[cibachrome]

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 !