CG location on balance

[Yawing]

Sorry if its already been discussed, but search function isnt working for me..


Say you can adjust the ballast of a race car to move 100kg (say out of 1000) from the rear tyres static load to the front. does this increase understeer?

obvious answer is yes, but say if the tyres are such that with a higher vertical load the optimum slip angle for max lateral load diminishes, therefor optimum steering angle diminishes, and the driver "feels" less U/S.

Now the car is cornering slower, there is less steering angle, and driver comments less U/S so possibly O/S feeling. Now wouldnt say a stiffer rear spring make the car faster. As the front is the limiting factor in grip, reducing the difference in front CP load so increasing maximum lateral acceleration??

 

[GregLocock]

"Say you can adjust the ballast of a race car to move 100kg (say out of 1000) from the rear tyres static load to the front. does this increase understeer?
"

The exact answer is : sometimes.

(a) what do you mean by understeer

(b) what is the balance of the car in its baseline configuration.

" with a higher vertical load the optimum slip angle for max lateral load diminishes" is this not always the case? (genuine question - the data I've seen says yes)








Cheers

Greg Locock

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[Yawing]

Hi,

Ok, I shouldnt have asked decreasing the understeer, but more so decrease the ratio of front to rear slip angle at maximum cornering force, and hence the driver requires less steering angle so he may comment of "less Understeer" .

assuming the car is biased towards mid corner understeer to start with, so increasing the front weight bias might feel like it improves the balance to a more neutral setup, yet the min corner speed is reduced. BUT as the front is still the limiting factor, winding on steering lock will show it is still an U/S car.

With regard to the optimum steering angle versus normal load, I think about ackermann V pro-ackermann. As I believe many F1 cars run pro-ackermann (or have in the past), could this point to the fact that with the lighter loaded tyre on the inside they try to reduce the slip angle for optimum grip (or conversely the outer higher loaded tyre have a higher slip angle)

 

[Yawing]

* edit: many F1 cars run reverse-ackermann.

 

[GregLocock]

I think you know far more about this than I do. The answer is still sometimes, in my opinion!


Cheers

Greg Locock

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

 

[Yawing]

Just bouncing the idea around. and I do agree with sometimes, as it appears to be highly tyre sensitive, and where each tyre is operating (ie linear range or optimum, or past optimim slip angle,etc).

just trying to correlate weight distributions effect on maximum cornering levels (not neccessarily "balance") of a car V a motoGP bike for example. As it seems clear that on a motorbike, more weight over the front means more "grip" on the front.

 

[cavedweller]

Not sure about GP bikes, but im a very keen DH mountain bike rider and this is the general consensus.

The fastest cornering technique is to weight the front wheel so that the rear wheel breaks traction easier. This causes the bike to enter into a drifting condition which can be controlled by the rider shifting his weight forwards or backwards and also left to right, increasing or decreasing the lean angle into the corner. The slowest cornering technique is to slow down and steer the bike through the corner. I would say GP bikes do a similar thing, drifting through the corner and leaning the bike instead of steering.

This is something not applicable to cars, they can't lean to corner (well, they do but in the wrong direction), and the car weighs so much that having the driver lean does nothing I would say increasing the weight to the front on a car would make it oversteer fairly bad as the back end would want to break traction and the car body would lean to the outside of the corner making the problem worse, controlling this with a steering input only would be difficult.

But then again, i know nothing about the topic.

 

[WolfHR]

Shenanigans, sorry to digress- I take it that Your searches found no results. It just happened to me, and it seems default type of search is 'Find a forum', as indicated in drop-down box right below the search field. Choose e.g. 'Search posts' and it will trawl through entire posts to find Your keywords (I take it, that's what You were after)

 

[GregLocock]

If you look in Milliken there's some very helpful graphs of tire performance (by some Japanese sounding guy) that explore the traction/sideforce/vertical load/slip angle tradeoff.

From this you could fairly easily build a model of the car in the two conditions, and work out what is going on.

I have a suitable model, but I am not giving it away, and it is not yet ready for sale.



Cheers

Greg Locock

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

 

[GregLocock]

http://www.geocities.com/greglocock/friction_circle.png

Is my rather hard to understand interpretation of that data.

Cheers

Greg Locock

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

 

[Yawing]

Yeah, Milliken is where my confusion started.

basically the bicycle model and moving the CG forward and aft. BUT he assumes the corner stiffnesses are the same (which they certainly arent with changing normal loads) and assuming te tyres are operating in the linear range (which again for a racing vehicle is a poor assumption).


Greg, Im trying to even simplify matters more and pretending the car is on a large radius corner (such that no longitudinal force other than just enough to oppose the longitudinal component of side slip), so purely looking at the tyre in steady state balance.

 

[ubrben]

Before you thoroughly confuse the issue by talking about bikes, two points.

1. the "bicycle model" is crap at simulating bikes

2. The rear tyre on a large motorbike steers the machine. They undoubtedly suffer from the front "pushing" in some circumstances, but in my experiences if normal bicycle model yaw balance applied they'd be massively understeery.

Ben

 

[GregLocock]

On the other hand the bicycle model is an excellent model for cars...

Cheers

Greg Locock

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