I wonder if anyone would bother reading my posts on this forum after I question this .. but...
If you shift CofG longitudinally (more weight to front or rear), how this affects oversteer/understeer tendency?
My gut instinct tells that by moving weight to front you would have more oversteer, but then I think that this also would increase centrifugal force on the front tyres, which would result in traction deficit - understeer.
a lighter rear end will cause oversteer as you have less mechanical grip (less mass creating less pressure on the contact patch); lighter front end will cause understeer for the same reasons.
for excess mass at the front to induce understeer, you're talking extremes in terms of mass moved there, as well as an extreme circumstance of poor driving to cause this to happen (ie: understeer would be induced moreso by the driving than the weight balance adjustment).
downforce in F1 quantities (as well as suspension setup) overrule all this somewhat though, making the shifts in mass more of a finetuning of car balance than the prevalent factor.
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More mass at the front will cause understeer - if you draw the FBD you will see that more force is required from the tyres to have the same radius of curvature as the rear therefore the front will lose grip before the rear. Oversteeer is the opposite - the rear end has more mass and it will break away before the rear. You can help this problem with employing a different driving technique - if you are is understeering you should drive in such a manner that allows you to get on the throttle early therefore shifting the CoG back. The opposite is true of a car that will oversteer. Of course it is better to have a balanced car from the outset.
Downforce doesn't work the same way as mass; more downforce on an end, the more grip that end has.
What you want is to have more downforce at the end which is going to slide - i.e. 2009 F1 cars have a CoG that is more rearward than previous years so they require more rear-downforce compared to the front. Unfortunately (particularly evident by Mclaren) the rear wing/diffuser on 2009 cars isn't quite suitable to counteract the extra mass.
In a long bend the heavier end will always want to overtake the lighter end, that's basic vehicle dynamics. So in a long constant-speed bend a rear-heavy car will definately have more oversteer than a front-heavy car. The extreme case is NASCAR racing on a fast oval, where more or less everything about the setup (wedge, track-bar, ARB, springs) is aimed at fine-tuning the weight-balance so that the car "rotates" through the bend at the perfect rate.
What Fil is saying is only true under heavy acceleration or heavy trailbraking. In these situations, a light rear can cause snap oversteer because the rear tires will not have enough traction. This is a different kind of oversteer than the above, caused by spinning rear wheels (under acceleration) or locking rear wheels (under braking).
A rear wheel drive car with a lot of front weight can be quite tricky to drive, because it will never be balanced: Under braking it will tend to snap-oversteer when the rear gets very light, in the middle of the bends the front end will plow because of the weight is "pushing" the front end outwards, and on the exit the car will again be prone to snap-oversteer as the rear tires lose traction.
Similarily, a rear wheel drive car with lots of rear weight (think Porsche 911) can also be tricky to drive. At first the front end will feel light, the inside front wheel will lock up easily under braking and the car will hesitate to turn in. Then in the middle of the bend the heavy rear swings out and wants to overtake the front, and the driver has to apply some power to settle the car. As more power is applied towards the end of the bend the rear plants itself and the car doesn't want to turn at all, possibly the inside front wheel will even lift from the ground if the rear suspension is too soft.
Talking in 2009 F1 terms, it seems like McLaren are having a problem with too much rear weight. If you compare the cars you will see that Ferrari for example have moved the cockpit forward a lot compared to McLaren, giving a more balanced weight-distribution. I wonder if McLaren should consider removing that KERS unit alltogether, and adding ballast further to the front of the car. Unless they can modify that diffuser to give more rear downforce, that is.
Last edited by Roland Ehnström on 16 Mar 2009, 14:51, edited 1 time in total.
Roland you say that a light rear will give you snap oversteer under braking and in mid corner. Which is 100% correct but how would you go about making the rear more heavy so that it looses that rear end snap? The car i run has fixed front and rear springs and is a single seater with a monogue tub and double wishbone suspension with push rod to bell crank.
Thanks hope you can tell me because it will solve my problem straight away.
Alonso
Well, if there is no ballast to move rearwards I guess there is not much you can do to actually change the weight-bias... Unless you can move the whole front suspension forwards to give the car a longer wheel-base and more rear weight-bias, but I suspect that is not possible with your car?
There are surely some setup changes that could improve things, but that is a bit out of my league.
Alonso:
I assume your car is a rear wheel drive. In that case, I guess a simple solution would be to increase the rear downforce. This should reduce the snap oversteer drastically in your car just like it will in the 2009 Mclaren .
Otherwise if the snap oversteer is the problem and not that much the mid corner understeer then maybe moving the weight bias forward may solve the problem. The car will definately be less "jerky".
First off it isn't a "Stupid Question" but there are many and varied answers.
There are examples of rear weight bias, rear drive cars that were built to understeer (Chev Corvair) and many front bias front drive cars that can and will oversteer(try an old mini in a long corner and lift-off....wheeeeee).
Your specific question should be addressed by starting out....without changing anything else.....and then, Yes, I would agree with your statement. Weight to the rear, oversteer and to the front, understeer.
Problem is there are an enormous number of other variables that can come into play to completely change the behavior of the chasis under various conditions. Turn-in, power on, power off, under braking and with F1 cars, all this changes with downforce which is a function of speed^2.
It's all about compromise. You want fast starts, move the weight back, tyre issues, move it forward, braking ...see starts....
Hats off to the chasis guys who need to balance all these issues and convince the drivers it is the best approach. Sometimes it isn't about how fast you go but how you go fast.
Personal motto... "Were it not for the bad.... I would have no luck at all."
If you move more weight on the rear axel the car will understeer more and if you add more on the front axel it will oversteer more, simple as that although the behavioral diffrences that Roland talked about also plays it´s part.
In a car with downforce it´s quite intresting.
If you move more weight to the rear axel the car will get more understeer in mechanical grip, BUT, you also increse the diffrence between the center of pressure and the center of gravity if you then also don´t add more wing to the rear or less in front.
Think of it like this. A car with 50f/50r weightdistrubution and downforce with 48 in front and 52 in rear.
Now, if you change the weight to 45f/55r and still have that same 48/52 downforce level it will start to oversteer since there´s now less force needed to turn the car but you´re still having the same downforcelevel needed to give the frontend the amount of grip needed to turn it when it had more weight in front.
So, with more weight on the rear axel the mechanical grip-balance will be more Understeer but the downforce balance, like in highspeed corners, will get more Oversteer.
This is a VW Gol, "Turismo Nacional Argentino". Its a 1.6L, 220HP and 900Kg car.
The engine is at the front as the traction. The rear of the car is literally empty. A typical setup?
Tyre pressure (hot):
Front 30psi
Rear 40psi
Rear spring rate is damn stiff if you ask.
Downforce: what is that? Lift you may say...
Its all about making the front end to get some grip. The rear goes as loose as you can. Take your conclusions.
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna
sdimm wrote:If you move more weight on the rear axel the car will understeer more and if you add more on the front axel it will oversteer more, simple as that although the behavioral diffrences that Roland talked about also plays it´s part.
In a car with downforce it´s quite intresting.
If you move more weight to the rear axel the car will get more understeer in mechanical grip, BUT, you also increse the diffrence between the center of pressure and the center of gravity if you then also don´t add more wing to the rear or less in front.
Think of it like this. A car with 50f/50r weightdistrubution and downforce with 48 in front and 52 in rear.
Now, if you change the weight to 45f/55r and still have that same 48/52 downforce level it will start to oversteer since there´s now less force needed to turn the car but you´re still having the same downforcelevel needed to give the frontend the amount of grip needed to turn it when it had more weight in front.
So, with more weight on the rear axel the mechanical grip-balance will be more Understeer but the downforce balance, like in highspeed corners, will get more Oversteer.
If you are referring to mass, rather than weight then you are incorrect.
Belatti good example. FWD cars are designed to have as much front grip as possible as all the mass is over the front wheels.
sdimm wrote:If you move more weight on the rear axel the car will understeer more and if you add more on the front axel it will oversteer more, simple as that although the behavioral diffrences that Roland talked about also plays it´s part.
In a car with downforce it´s quite intresting.
If you move more weight to the rear axel the car will get more understeer in mechanical grip, BUT, you also increse the diffrence between the center of pressure and the center of gravity if you then also don´t add more wing to the rear or less in front.
Think of it like this. A car with 50f/50r weightdistrubution and downforce with 48 in front and 52 in rear.
Now, if you change the weight to 45f/55r and still have that same 48/52 downforce level it will start to oversteer since there´s now less force needed to turn the car but you´re still having the same downforcelevel needed to give the frontend the amount of grip needed to turn it when it had more weight in front.
So, with more weight on the rear axel the mechanical grip-balance will be more Understeer but the downforce balance, like in highspeed corners, will get more Oversteer.
If you are referring to mass, rather than weight then you are incorrect.
Belatti good example. FWD cars are designed to have as much front grip as possible as all the mass is over the front wheels.
I´m talking about the weightdistrutubion of a car that´s standing still. It just simply is true... ask any professional racedriver about it, drive a gokart and see what happens when you move the seat forward and backwards.
Cars like the 911 Porsche has the same philosofy as a FWD car with much weight on the weight only the other way around.
Since they´re RWD the idea is to have as much grip as possible over the rear to get good traction. A 911 for instace has awesome acceleration compared to other car with a more 50/50 distrubution from 0-30kph because they get so much more traction.
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