Using more power

[Martin A]

Hi All

I've been looking through some old threads and about 9 months ago

StressedDave said:

There are some really useful interactions between power and cornering grip that skew the importance even further towards using less of the cornering grip and using more power to fill up the budget. But that's straying way, way beyond this discussion.

Maybe Stressed Dave could stray way, way beyond that discussion now and let those of us who are interested in "using more power to fill up the budget" know a bit more about those interactions.

Over to you Dave

Best regards

Martin

[7db]

I read that (*holy thread resurrection - batman*) thread as well and noted a quote from Mark K

"For the given road conditions, you have an amount of grip available which you can 'spend' on either acceleration (+ve or –ve (braking)) or steering, or a combination of both. However, you have only a certain amount, say £10. So if you spend £5 on steering, you can only spend £5 on accelerating"

Since budget has been brought up in the OP, it's worth noting that if you spend £5 on steering, you still have £8.66 left for braking, vectors doing what they do.

[StressedDave]

Martin A said:

Hi All

I've been looking through some old threads and about 9 months ago

StressedDave said:

There are some really useful interactions between power and cornering grip that skew the importance even further towards using less of the cornering grip and using more power to fill up the budget. But that's straying way, way beyond this discussion.

OK then, are we all sitting comfortably?

Let start with a big disclaimer: I'm not talking about limit handling, but about normal on-road driving.

This is quite an important factor, as you're using a lot less of the total available grip, which means that you've got quite a bit of leeway for combining cornering with acceleration (or indeed braking) before you get close to the limit of adhesion. Ad db said above, even if you're cornering at 50% grip, you've still got 86% of the total grip left. Well, sort of:

1) While a tyre on its own can generate about 1g of grip on a dry road surface in good condition, its ability to generate cornering grip is dependent on three main factors:

a) The slip angle - the angle between the direction the tyre is pointing (its heading angle) and the direction it is actually travelling (velocity vector).
Under normal road driving conditions, the tyre is more or less linear - i.e. cornering grip = constant x slip angle. At the top end of road driving, this relationship falls away : cornering grip < constant x slip angle

b) The camber angle of the tyre - with modern radial tyres this has less effect than slip angle. Ratios of 10-20:1 are often quoted(i.e. 1 degree of slip angle generates as much cornering force as 20 degrees of camber), but trying to extract real tyre data from a manufacturer require the sacrifice of virgins and the donation of limbs, so what the tyres fitted to your car do is a matter of conjecture or the expenditure of lots of cash to do the rig testing yourself.

c) The vertical load on the tyre. The more load carried by the tyre (so think weight transfer), the more slip angle is required to generate a cornering force as a percentage of weight. That sounds confusing (it is to me and I'm typing it out) - what I mean is that to generate a cornering force of 20% of the weight carried by the tyre, you need a bigger slip angle as that weight goes up.

If you're clever you can tune how much weight is transferred at each end of the car when it corners by changing suspension components - that's why you have adjustable anti-roll bars on racing cars. On the road this relationship is set up to be inefficient for stability and numpty-proof reasons. It's rare to see a car with 1g tyres capable of consistently generating that because you can't get all four wheels both correctly loaded and with the right slip angles at the same time.

But coming back on to the Zen art of doing what sounds wrong to aid cornering:

When you corner you develop cornering forces in relation to the slip angles of all four tyres. This is good. However, this force is developed perpendicular to the slip angle. This is not so good because effectively the direction of the force is not towards the centre of the turn but to a point rearwards of that. So you get a combination of cornering force and drag, so even though you might be really good at setting the car up for the corner, you still don't get 100% efficiency in cornering grip - some has to be wasted.

Now comes the clever bit. If you add power, thus developing a traction force as well, this is produced parallel to the heading angle of the tyre and you get a component which counteracts the drag you've developed and a component which acts towards the centre of the corner - effectively topping up the cornering grip you've lost.

But consider what happens if you add more power than is necessary to 'top up' the grip - you get extra cornering grip but you also start increasing the speed of the car. Depending on the rate of application and the amount of extra power you apply, you can end up using less slip angle for a corner. The skill comes in balancing the increased cornering grip with the fact that the increased speed is demanding either more cornering grip or a wider line with the same grip.

I'll leave you to practice... the real key is being very good at bend assessment and disciplined enough to take a couple of extra mph off to give you a) the confidence to use bigger throttle openings and b) the ability to increase the speed gradually through the corner without running out of limit point.

[Martin A]

Hi All

Thanks for that Dave. It's largely what I understood anyway but was concerned there might be some vital snippet I was not aware of. The camber / slip angle ratio was new to me though. I'm sure the bulk of it will be very useful for those who have more recently joined the forum.

Thanks again

Best regards,

Martin

[StressedDave]

There are other snippets, including the effect of traction on the bushes but these are secondary effects but also variable from car to car. I rarely go into the theory with clients - letting them experience it for themselves normally does the trick...

[7db]

The issue of tyre efficiency is one that troubled me a lot - "The more load carried by the tyre (so think weight transfer), the more slip angle is required to generate a cornering force as a percentage of weight"

The key here is the difference between vertical tyre load and weight / inertial mass.

When considering what lateral acceleration will be produced, it is convenient to consider that the load is proportional to the inertial mass, and so the acceleration is proportional to the slip angle. With variable loading (complicated by spring/damper arrangements), it is most certainly not. Superior accelerations can be produced way in excess of 1g by loading a tyre with more weight than its share of inertial mass.

Unfortunately across the whole vehicle the total load is the total weight, except where there is a compression in the road, or additional aerodynamic effects adding load without weight.

As tyres lose efficiency with extra load, the next effect is that the overall vehicle has less available grip unless the weight is even balanced on all four corners.

This neatly squares the dilemma that emerges when cornering near the limit of grip, putting the weight on the nose to aid turn-in does increase the grip at the front but at disproportionate expense for the grip at the back. By mid-corner the weight needs to be balanced again to prevent terminal oversteer taking the back away.