Tyre grip-understeer and slip angle

[Spiros-Foivos]

Hello , This is my first post in the forum so far , i am 17 years old and currently studying to get to the mechanical engineering university where i live at Greece . I have some questions regarding tyre grip and slip angle . As far as i understand grip the same tyre is determined by a number of factors , the main ones are the weight on the tyre and slip angle while cornering. I have read that weight shifting helps a the tyre (that the weight is shifted to ) to produce more grip and the slip angle gets bigger resulting in more grip .
But i dont understand why they say that when a car is nose heavy it tends to understeer on cornering . As far as i understand a nose heavy car while have more weight on the front tires thus producing more grip ? I thought about polar moment of inertia but still i can't quite understand why this happens ...
Also when cornering why it is sayed that the center of gravity should be closer on the wheels of the drive axle ? Wouldn't this cause in a rwd car the front tires to raise and reduce available grip for turning ?
And to clear things up what are the all the factors that determine tyre grip in the same track-car
and does more slip angle mean generally more grip ?
Thanks in advance ! I have these thoughts eating me and can't study maths and physics for school

[Jersey Tom]

Think you need to be careful using the word "grip" as it is not well defined. "Force" may be a better word for what you're talking about.

But yes, typically as you go to a more forward biased static mass distribution, you will have more understeer. Yes, as you put more mass above the front tires they will be able to generate more cornering force. However, don't forget that the front tires then have more mass they have to pull around!

Presumably you are familiar with Force = Mass * Acceleration. Basically the general behavior of tires is that by moving mass forward, force will increase less than mass. A 5% increase in mass may only give you a 4% increase in force, so your acceleration [performance] decreases.

[godlameroso]

There is an inertia that needs to be overcome when you place weight at an extremity of a car, it's why Subarus are so stable, they have a wide, and low center of gravity over the front wheels.

[Greg Locock]

I think the OP has got slightly confused, by this confusing topic. As you increase the vertical load on a tire, it does develop more lateral force/vertical force, but to do so it needs a greater slip angle. This is one aspect of https://en.wikipedia.org/wiki/Tire_load_sensitivity

So, if you move the cg towards the front of the car, the front tends to slip more to attain a given lateral acceleration, and so it understeers more, all other things being equal.

[smellybeard]

...and the slip angle gets bigger resulting in more grip.

No. The slip angle gets bigger as a result of more 'grip'. The slip angle is a function of the distortion of the tyre onto a fresh contact patch as it rotates. More cornering force (or less tyre rigidity): more tyre deflection.

[Spiros-Foivos]

So if i understand it right if you just add more load on the tire on a given slip angle it won't produce any more lateral force but , it will if you increase the slip angle . So if a tire can produce a given max force on a given load at it's optiumum slip angle for this load , if you increase the load ,this optimum slip angle will increase allowing you to increase the slip angle and thus allowing the tire to produce more force than before

[Jersey Tom]

So if i understand it right if you just add more load on the tire on a given slip angle it won't produce any more lateral force

Not quite. See the following graph... a representative plot of lateral force versus slip angle at three different vertical loads:



Disregard the arrow there pointing to a peak, not sure why was being shown initially. In any event, you can see quite clearly that as you go from one vertical load to another at a constant slip angle, the lateral force will (usually) change. There are however certain conditions where increasing vertical load will have no change in lateral force, or even a decrease.

[Spiros-Foivos]

If that's the case then it seems thatthis is in contrary to what "Greg Locock" sayed . And moreover imagine the situation where you are midway a corner and you brake a little just to shift the weight on the front of the car , then considering the graph the slip angle should not increase and the tire would create more lateral force . .. would that be correct ?
Thanks
Edit: in other words , is it possible to make more lateral force by loading a tire more while still maintaining the same slip angle and thus the same distortion ?

[Jersey Tom]

If that's the case then it seems thatthis is in contrary to what "Greg Locock" sayed .

Not at all. Just need to keep in mind the distinction between force, and mass.

Like I said earlier on, as you move mass distribution forward, the front tires will be able to produce more maximum force but not in large enough proportion to totally offset the additional inertia the front tires are trying to accelerate. Front tires work worse, rear tires work better = more understeer.

And moreover imagine the situation where you are midway a corner and you brake a little just to shift the weight on the front of the car , then considering the graph the slip angle should not increase and the tire would create more lateral force . .. would that be correct ?
Thanks

With the tire force curves shown in that graph, yes, the fronts will create more lateral force and the rears will create less, so the weight (not mass) shift to the front will make the car want to rotate more.

[Greg Locock]

Spiro, that's why it is easier to talk about the ratio of lateral force to vertical force, at a given operating point. We often call the cornering stiffness mu/degree, which is rather horrible scientifically, but a very handy way of plotting tire stuff out. So in Tom's graph, if you divided each datapoint by the Fz, then you'd get 3 curves that are almost identical, and the differences between them are telling.

[Greg Locock]

The legend bug strikes again, low is red, medium is black, blue is high. As you increase Fz then you need more slip angle to generate the same Fy/Fz.

[Spiros-Foivos]

Thank you all, i understand now !!
So if you lower the ride height in the front of a rwd car you you will create more lateral force but not high enough to balance the extra mass placed forwoard and make it corner faster and moreover you will cause the tires to work harder and wear out faster to get that extra force because of the extra slip angle needed .

[g-force_addict]

Hello , This is my first post in the forum so far , i am 17 years old and currently studying to get to the mechanical engineering university where i live at Greece . I have some questions regarding tyre grip and slip angle . As far as i understand grip the same tyre is determined by a number of factors , the main ones are the weight on the tyre and slip angle while cornering. I have read that weight shifting helps a the tyre (that the weight is shifted to ) to produce more grip and the slip angle gets bigger resulting in more grip .
But i dont understand why they say that when a car is nose heavy it tends to understeer on cornering . As far as i understand a nose heavy car while have more weight on the front tires thus producing more grip ? I thought about polar moment of inertia but still i can't quite understand why this happens ...
Also when cornering why it is sayed that the center of gravity should be closer on the wheels of the drive axle ? Wouldn't this cause in a rwd car the front tires to raise and reduce available grip for turning ?
And to clear things up what are the all the factors that determine tyre grip in the same track-car
and does more slip angle mean generally more grip ?
Thanks in advance ! I have these thoughts eating me and can't study maths and physics for school

Hi, welcome.
IMO slip angle is more of an effect.
Tire grip depends on rubber compound, temperature, camber, inflation pressure, rim width, tire construction, road surface, etc.

Google weight transfer.
The more vertical load a tire has the more grip it has, true but
the less efficient it gets, so if you double the vertical load you get a bit less than 2x the grip. Thus a net loss.

In an ideal world the center of gravity CoG should be in the wheelbase center.
In performance or race cars the front tires are usually narrower and smaller than the rear ones to ease turn in and for aerodynamics reasons. So being the rear tires fatter and bigger this allows for more grip thus allowing the CoG to be moved a BIT to the rear and just a bit.

A higher slip angle doesn't necessarily mean more grip.
As you can see here

The red line is a race tire and the blue one is a street tire.
In fact most (if not all) high performance tires produce their maximum cornering force at lower slip angles than street tires.
Of course the maximum cornering forces are higher in high-performance/race tires.

As for math, you'll only need formulas for weight transfer.
To determine tire grip you need to look at tables/charts for an specific vertical load.
AFAIK no math formula can accurately predict the real world performance of a tire, at least not a highly developed race tire.

[Jersey Tom]

Thank you all, i understand now !!
So if you lower the ride height in the front of a rwd car you you will create more lateral force but not high enough to balance the extra mass placed forwoard and make it corner faster and moreover you will cause the tires to work harder and wear out faster to get that extra force because of the extra slip angle needed .

Dangerous phrasing. If you lower front ride height you are changing camber, toe, jacking forces, CG height, front weight distribution, etc.

Keep it simple. If you move front mass distribution forward, by itself, you could potentially expect what you describe.

[smellybeard]

IMO slip angle is more of an effect.

Exactly. It is not part of the grip mechanism and correlating it to grip is pretty pointless unless it is the slip that you are actually studying.

Managing the slip angle gives the driver an apparent alteration in grip but this is just his subjective perception viz understeer or oversteer.