Just wondering. After the big fuss about rearwards weight distribution for MS and how the MGP had the longer wheelbase for a more rearward distribution etc, the general consensus was that rearwards = oversteery and forwards = understeery, as the momentum/inertia would be greater.
But surely in slow speed etc, the additional normal force pressing the car down is greater than the momentum? Such as hairpins etc? I mean, that is what drivers do under heavy braking, no? Transfer the weight forward?
we need to seperate two things weight/mass & load/force.
under braking you will get load transfere towards the front tires, but there is no additional mass at the front tires.
So your tire sees an increase in vertical load, but at the same time the lateral accelaration acts in your CoG (mass centre) which is still at the same point (save for a few milimeters).
if you move your CoG (mass centre) more forward, your front tires will carry additional vertical force (F=m*a), but at the same time, they will need to support this additional mass under lateral acceleration (again F=m*a)- sideways
If you like, you have to pay a price for this extra vertical force (cost function).
The increase in load due to load transfere does not move your CoG (mass centre) forward at the same amount (it does move forward slightly, but not dramatic).
It´s the same with downforce, you get an extra vertical force/load on your tire, but you don´t have to support it under lateral acceleration - it´s a free lunch if you like.
The statement that a forward CoG causes understeer and a rearward CoG causes oversteer is a bit too general.
You have to see it in relation to your tires, there size, width and stiffness (lateral and vertical).
If you would have a car which uses the same tires front and rear, your statement would be more or less correct.
But if you use different tires (such as in F1) you have an "ideal" weight distribution in regards to your tires.
This would be slightly rearward (geometrical) for a current car to be "neutral".
"Make the suspension adjustable and they will adjust it wrong ......
look what they can do to a carburetor in just a few moments of stupidity with a screwdriver." - Colin Chapman
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Is there an equation for weight distribution for the size of a tire? like what would the teams do if we go back to the skinny tires of the classic F1 cars?
The first order relationships are pretty simple. Race Car Vehicle Dynamics covers them in the chapter on Force-Moment Analysis.
Basically you need to now the cornering stiffness of the front and rear tyres (Cf and Cr), the distance between the front axle and CG (a) and rear axle and CG (b), and the sliding (i.e. limit) friction coefficient of the front and rear tyres (muF and muR)
For linear range the following holds:
Cf x a = Cr x b = neutral
Cf x a > Cr x b = oversteer
Cf x a < Cr x b = understeer
Weight transfer onto the front axle and off the rear axle are second order effects that you can tune with once you got the first order effects nailed.
Basically you want a touch of U/S both linear range and limit by this analysis. The higher power you have the more U/S you need in the car to allow you to get on the power.
Shrek wrote:Is there an equation for weight distribution for the size of a tire? like what would the teams do if we go back to the skinny tires of the classic F1 cars?
To answer your question you'd need to know the cornering stiffness and friction levels of the tyres. In the case of skinny 1960s crossply F1 tyres both parameters will be smaller than today's tyres.
Ben has posted the bare facts (if temperature effects & down force are neglected). I think there are a few more issues that affect the performance of a vehicle on-track.
A vehicle that sideslips such that the nose points into a corner will cue "oversteer" to a driver, & "understeer" if the nose points away from a corner. I used quotes here to indicate that this has nothing to do with what Ben was talking about. Sideslip depends upon the lateral stiffness of the front and rear tyres and the longitudinal position of the c.g. It is a very powerful cue & should be avoided if the vehicle is to be driven close the the actual lateral limit with confidence.
The aero centre of pressure should be held close to the centre of gravity if the lateral balance of the vehicle is to be consistent through corners of differing speeds. Moving the wheels relative to the chassis is a neat way of moving the centre of gravity without having to re-work the aero balance.
The performance of slick tyres is affected by the way they are "worked", & the observable measure of that is tyre temperature. One way of changing the way tyres are worked is to move the c.g. (& c.p.) - towards the under-worked tyres.
The maximum torque that can be transmitted through the driven wheels depends upon the vertical force acting on the tyres. Increasing the vertical force will increase the maximum torque. Hence, moving the c.g. (& c.p.) away from the driven wheels will reduce "traction".
The missing piece of the jigsaw (for me) is the relationship between lateral & vertical tyre stiffness. I believe there is normally a strong correlation between the two, so tyres that minimize sideslip will also tend to be "worked" evenly although, to be fair, suspension set-up can also help in that respect & tyre designers can blow any assumption into the weeds.
Now, starting from a vehicle with good balance, suppose that it is decided to increase the vertical stiffness of the front tyres (for aero reasons, perhaps). This will (I contend) increase front tyre lateral stiffness, which will cue "oversteer" to a driver. It will also tend to work the front tyres less, and so lead to actual (temperature-related) understeer & a confused driver. The simple "fix" to both problems is to move the wheels back, hence moving the c.g. & c.p. forward. But this will unload the rear axle, thus reducing traction. It will also keep an understeering balance if/when the tyres are working correctly - as per Ben's post.
F1 have, since 2007, used front tyres that are too stiff vertically & rear tyres that are too soft by a margin, to judge by statistics. This fact has resulted in substantial changes in set-up philosophy since that year. The goal posts were moved again for 2010 with the introduction of revised tyres & no mid-race refuelling.
Any set-up solution for tyres that are intrinsically mis-matched to the vehicles is likely to be an uneasy compromise, so it is not surprising (in my view) that some teams & drivers have struggled to find a competitive compromise. The "fix" for 2012 is to regulate c.g. position implying, presumably, that a unanimous chorus of protests will push the unfortunate tyre supplier towards a product that is better matched to the vehicles - if the aerodynamicists will allow it....
WilO wrote:Is there any hope that Pirelli will take heed of the vertical stiffness issue and supply tires that are better suited??
Who knows, Wil? The good news is that the tyres used by the open-wheeled series currently supplied by Pirelli appear to be well-matched. The bad news is that a) GP2 tyres appear to be a fairly reasonable match to GP2 vehicles & b) F1 teams have prepared a specification for the 2011 F1 tyres, apparently.....
DaveW wrote:A vehicle that sideslips such that the nose points into a corner will cue "oversteer" to a driver, & "understeer" if the nose points away from a corner. I used quotes here to indicate that this has nothing to do with what Ben was talking about. Sideslip depends upon the lateral stiffness of the front and rear tyres and the longitudinal position of the c.g.
Hmm? That's exactly what he wrote out.
And while a subtle point, for accuracy's sake probably worth pointing out sideslip and sub-limit under/over-steer are entirely dependent on the cornering stiffness of the tires, rather than lateral stiffness. Related but not identical.
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Jersey Tom wrote:Hmm? That's exactly what he wrote out.
And while a subtle point, for accuracy's sake probably worth pointing out sideslip and sub-limit under/over-steer are entirely dependent on the cornering stiffness of the tires, rather than lateral stiffness. Related but not identical.
I don't wish to argue the point too strongly, because you may be correct. However, would you, for example, include compliance steer in cornering stiffness? That has a similar effect on driver perception. My understanding is that cornering stiffness describes what happens at the contact patches. If that is correct, then my point is not directly related to cornering stiffness, I think.
DaveW wrote:A vehicle that sideslips such that the nose points into a corner will cue "oversteer" to a driver, & "understeer" if the nose points away from a corner. I used quotes here to indicate that this has nothing to do with what Ben was talking about. Sideslip depends upon the lateral stiffness of the front and rear tyres and the longitudinal position of the c.g.
Hmm? That's exactly what he wrote out.
And while a subtle point, for accuracy's sake probably worth pointing out sideslip and sub-limit under/over-steer are entirely dependent on the cornering stiffness of the tires, rather than lateral stiffness. Related but not identical.
If you mean sideslip as "beta" i.e. body sideslip angle - the first order effect is the cornering stiffness. The actual attitude of the sprung mass relative to the contact patches will be affected by the lateral stiffness though. I tend to agree with Dave here and I've found that lateral stiffness of the rear tyres has a big impact on driver perception of oversteer.
You get different sorts of comments about oversteer; "flat slide", "inertial loose", "roll oversteer", etc. For me there are clear differences between the slip angles at steady state and the attitude of the sprung mass and how that is perceived by the driver.
Dave's post is a good addition to mine - I wanted to ignore aero and tyre temp initially, particularly the latter following a previous thread
Thanks, Ben. Sebastien Loeb spoke to an Autosport reporter (see here) after driving an F1 vehicle a couple of years ago. The interesting comment was "...I began to get a good feel for the car and enjoyed the sensation of it moving around on the track.", suggesting he had been warned about the characteristic before his test. Not a surprise that he enjoyed it, given his day job.
Incidentally, I didn't address the Merc/MSC problem specifically. It would appear that Mercedes changed suspension geometry so as to move the c.g. rearwards (contrary to my initial guess), because the vehicle was judged to have excessive understeer. My logic suggests that the change would probably have increased the understeer.
If memory serves, I think page 29 of RCVD contains a graph displaying load (x axis), lateral force (y axis), with curves of different slip angles. Apologies if this is vague, but I'm unfortunately working at this hour and don't have my copy of the bible with me.
Forgive the stupid question, but I'm wondering what this could tell us about tire performance on one axle. Obviously load will be transferred to the outside tire while cornering. Does it therefore follow that this load increase also increases the slip angle and therefore the lateral force? I should think that it does....
I guess I'm a bit confused as to what this graph really tells us, though I think I may have just answered my own question in thinking through this. Be interested in any thoughts though.
Wil: The plot you referenced might have been presented as a carpet, I suppose. It simply describes how the lateral load generated by a tyre would be expected to vary as a function of vertical load and slip angle.
What will actually happen during a manoeuvre requires the solution of a set of vehicle (non-linear) dynamic equations. An accurate solution would require tyre characteristics, including information similar to the plot referenced, & much else besides (damper models, spring non-linearities, etc).
The distribution of vertical loads between the tyres during a manoeuvre can be manipulated by the vehicle suspension (e.g. arb settings) whilst four tyres remain in contact. The non-linear nature of the referenced plot is (one of) the reason(s) that vertical load distribution during a manoeuvre affects whole vehicle lateral balance.
Thanks kindly for that Dave, you've reaffirmed what I thought. I was reading a bit more into it, as the accompanying text made reference to the graph as being useful for determining what happens across an axle. I was thinking front tires, and the difficulty of knowing vertical load, slip angle (taking into account Ackermann steer, if any)...Thanks for fleshing out that plot for me.
