After reading an article about tire friction something confused me. Since the Fmax = G*W (Fmax being maximal horizontal force before the tire slides, G being static friction coefficient and W behing the vertical force on the tire) So when there is no acceleration or braking going on and a car goes thruw a corner at constant speed this would mean the center of gravety wouldn't really matter. A high center of gravety would put more load on the outside tires but also a higher Fmax. With G being constant the potential speed of the car should be the same with a car with a lower center of gravety. (Because W and F are linear and the total W of the car being constant.) It seems hard to believe but is this thruw?
The problem is exactly that for a tyre G isn’t constant with the load, it actually decreases as the load increases, that’s called tyre load sensitivity.
If you are putting more load on the outer tyre and less load on the inner tyre, in the outer tyre G is smaller than in the inner tyre and the result is a loss in the maximum lateral force.
Thanks for the clear answer Reca;) Though it raises a wierd question right away:
A common rule is that a softer suspention gives more mechanical grip. Though it also means the chassis will roll more in a steady state corner, wich means the center of gravety will move further to the outside wheels. That would mean a higher vertical force on the outside wheels than with a stiff suspention. Than why does a soft suspention give more mechanical grip? According to this it should actually be less. (since an even share of the gravety forces over all 4 wheels would be optimal according to the previous post)
(BTW let's think of a smooth road surfcase to keep things simple..)
The "problem" with soft suspension is that there is a higher chance the tire will "bounce" off the track, reducing grip. Even if it weren't that soft, the stiffer suspension will ensure a much larger and constant contact patch compared to a softer suspension. A bad thing about stiff suspension, besides ride quality, is that if it is too stiff, the car acts as if it doesn't have any dampers at all (or is it springs?). In that case, the tire will lose contact with the road as well.
What my friend tells me in autocross is that stiffer springs and suspension are used to keep the car from bottoming out. Since the car is so low, he wants a stiffer suspension to avoid this dilemma. I always assumed that stiff suspension will make the car less likely to lose its contact w/ the ground. Given, I'm no suspension expert...
Suspension at Monaco is a little softer because the bumps will often become too harsh for the drivers to take.
Well that does make sense why race cars are so stiff. Still it's hard to get why drivers will go for soft suspention on tracks with loads of slow corners. Okay I understand about the bumbs and road surfcase. But still a general rule in car setup is softening the rear when there is over steer in slow corners just to provide the rear of more grip, and that's not always because of bumbs.
I must admit you made it a bit clearer but there is still something missing because I am pretty sure a softer suspention in F1 means more slowcorner grip. (Okay also a highe/ less responsive car I admit) Wich brings me to the question in my previous post again
At these times I realize it's useless to study law...
I don't know anything about soft and stiff suspensions. I know stiffening the car will lessen the chance of bottoming out, such that you can lower the car a little more. And that too stiff means the car acts as if w/o suspension, because of lack of compression.
My friend in autocross said stiffening the rear end of his car (Acura Integra) w/ a rear sway bar stiffens the rear end such that he gets a little oversteer in an otherwise power understeer-biased car. I guess this bar is to keep the suspension flexing and again ensuring that the wheels stay at an optimum contact patch (no not the Michelin-thingy).
Last year's BMW.Williams website said that a stiffer suspension ensures the wheel stays in contact w/ the ground, especially when bumps are present. So I'll have to take whatever I can get.
I'll take a big guess then on soft suspension and slow corners. Under braking the car will pitch forward, and I guess w/ a softer suspension up front, the front will be able to handle or "accept" this loading more than a stiffer suspension? In other words, if it was stiff the car would readily "dampen" this loading and therefore the brake loading would be short lived... and then the wheels would not be able to take advantage of the front weight biasing towards braking. I guess with a softer suspension, a "window of opportunity" to take advantage of the weight upfront increases.
This is a wild guess; I'm NOWHERE near sure.
Bring back wider rear wings, V10s, and tobacco advertisements
Plato wrote:
Than why does a soft suspention give more mechanical grip?
Plato wrote:
But still a general rule in car setup is softening the rear when there is over steer in slow corners just to provide the rear of more grip, and that's not always because of bumbs.
I think it's connected with the relative stiffness of front and rear end. The total rolling moment is distributed between the two ends depending by the relative stiffness : the stiffer end receives the higher % of the moment.
Softening the front (or stiffening the rear) you are reducing the % of the total rolling moment going to the front end. Since part of the load transfer is proportional to the rolling moment, as a result you are reducing the load transfer between the two front tyres.
The opposite happens increasing the stiffness of the front end or reducing the stiffness of the rear.
I asked my friend about suspension and says he runs stiffer in the back, relative to the front. He says that the stiffer side reflects the weight transfer, or in other words more of the weight is retained in the softer side. He gets a little bit of oversteer w/ a stiffer rear suspension; I must say that it has to be softer in the front and therefore he is getting a little more grip up there.
Yes but the rules for using soft springs/hard springs on the front or back are totally different depending on whether the car is FWD, RWD or 4WD.
If we take an example:
Using softer springs on the front of the car relative to the back will induce more weight transfer to the front of the car. This load will enhance grip on the front of the car, under normal, straight braking conditions. However, we know that braking is not always normal which is why other elements of setup are used. But, this is not to say that you can just put softer and softer springs on the front, because there needs to be a balance between the front and rear damping that isnt too disimilar.
Secondly, the speed that a damper rebounds at is dictated primarily by the shock fluid viscoscity. If the fluid is thick and goopy, then the rebound will be slow. If it is watery and 'light', then the rebound will be faster. This will govern how the car deals with bumps.
The coefficient of friction for a racing tyre varies with slip, camber, temperature etc and is not a linear relationship through any of the variables. The trend is that (in CF v slip) the CF will increase (approximately) linearly to a point, then dip down slightly and stay constant for all other values of slip . F1 tyres can reach values of 1.5 I read somewhere.
The common rule that softer suspension gives more mechanical grip is a bit misleading. Soft suspension gives good traction out of slow corners (which answers another question of yours). Unless you have a whacky tyre or suspension setup, you want to reduce roll as much as possible. A car can only accelerate as much as the tyre with the least traction allows it (unless it spins of course) so reducing weight transfer in cornering is preferable. This occurs even more so in F1, as the cars are so pitch sensitive. Too much roll (lateral or longitudinal) causes a loss of aero load. Suspension in F1 is basically only to absorb bumps.
In regard to the Acura, stiffening the rear is the way to reduce understeer as you said and works wonders for FWD cars. The rule of thumb is to set the rear roll stiffness high enough so the inside wheel just lifts up at the apex. Adding a front roll bar usually has less of an effect; it increases understeer by overloading a front tyre, but decreases it as the heavily loaded tyre is also driving rather than being dragged.
Wow, must say most of your comments make sense, but there are other factors to consider. First generally the softest setup possible is best as it follows bumps in the road better. The difference in F1 cars and other high aerodynamic force cars is that ride height is crucial to produce the correct amount of downforce, therefore they use the springs to control the ride height of the car in order to better use that dfownforce. For a non-aero car you want to try to use as much jounce travel as possible in order to get the best grip over bumps(with correct dampening forces of course). Now onto the Integra and other road cars turned autocrossers. First of all on a race car the suspension is usually setup so that the optimum camber occurs around the same time a max lateral grip. So say you have a total rollstiffness of 2deg/g and your limit of adhesion is 1 g, then if you going around a circle at one g the the outside tires are at about whatever the optimal camber for that tires is. Road cars arent typaically designed this way. Theres a famous quote from Colin Chapman that 'any suspension will work, if you dont let it), so onto the stiffer roll stiffness used by the autocrossers. Since the suspension geometry in these type of cars arent developed for racing they dont use that extra movement in the chassis as it will pset other things such as camber and a whole slew of effects that road cars are designed to do. So by using stiff chasis they only look to control weight transfer and throw out controlling camber change and take the easy way out and only change the static camber. Another problem associated with autocrossers is when they lower their car, good idea, but many bad effects. Firstly, the car was not designed to sit this low, screwing with the suspension geometry even more, so stiffen it up so that you can control weight transfer and whatever else bad that happens with your wacky geometry and you not letting the suspension work, but you are allowing it to transfer weight in a way that is desireable. Another thing, usually overlooked is that a car that rolls does not feel good to the driver, think about it, wouldnt it feel alot more comfortable if you were in a car that felt as steady as a rock in terms of roll, sor of like playing a video game id imagine. But anywho, these autocrossers if they went out there with professionally designed suspensions you would definatly see the top cars running much softer setups, we often times run springs in the range of 100lb/in, as we have 4in of jounce travel and little aero, but with a purpose built chassis and suspension. Use the jounce, it is your friend.
4 inches?! Woah thats some wheel travel. Most of my dealings have been with open wheel cars (FSAE, F Ford). Given their light weight and low COG wheel travel is usually no more than 1-1.5 inches in either direction.
Like you said with non-aero cars, you can use the springs and dampers to improve mechanical grip. Im not too flash with dampers, trying to learn about them now.
My comment in regard to pitch and roll sensitivity sort of did take ride height into account. The drop of in downforce due to pitching or rolling (both of which change height and wing angle) is huge, Sauber and Toyota had huge problems with it. But normal open wheelers without huge wings, its not such a huge problem.
How do you mean stiff chassis? Do you mean with an overall higher roll stiffness (or roll gradient) or stiff springs and dampers? Anti Roll Bars are a good way to control roll characteristics without effecting your individual corners. You could soften the springs to give better traction out of slow corners and handle bumps better, while not allowing too much roll on medium to fast corners.
So when there is no acceleration or braking going on and a car goes thruw a corner at constant speed this would mean the center of gravety wouldn't really matter. A high center of gravety would put more load on the outside tires but also a higher Fmax. With G being constant the potential speed of the car should be the same with a car with a lower center of gravety. (Because W and F are linear and the total W of the car being constant.) It seems hard to believe but is this thruw?
