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'Driving The Circle'


nikpro

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This thread has been ammended - a Big thanks to Kevin Jones for pointing out my errors in the Original thread  thumbs-up.gif
(....and humble apologies to those it confused  blush.gif)

This is the last in the three part series I have written for the FAQ section of the website, the next article will be on how and why we set-up a vehicle the way we do, however to understand these future articles it is vital that you have read and understood these three preceding articles. I again apologise to the people I am ‘teaching to suck eggs’

The Articles should be read in conjuction and in this order:

(1) Anti-Roll Bars (Including Load Transfer)
(2) Tyres - Gripping Stuff
(3) This Article!

In the last two articles we have talked about Load transfer and Tyres. We have seen how the grip available from the tyres is inter-linked with the load placed upon them and how tyres generate lateral grip through slip angles.

A tyre’s grip can be split into longitudinal (acceleration & braking) and lateral traction (cornering), because of the way the tyre is designed it will produce slightly more lateral grip than longitudinal.
We have, however, seen that with load transfer that the tyres on an axle must be considered as pairs and that the overall cornering force from an ‘axle pair’ will be slightly lower than the tyre could provide individually, so for all intents & purposes the tyres will be able to provide an equal amount of traction in any direction.

Let’s consider we fit a tyre to our vehicles that can provide 1.4g of traction (This is roughly what a tyre like an R888 can provide on a Westfield). This means that the tyre can either give us 1.4g of acceleration, braking or cornering but not all at the same time.

The diagram below represents what we refer to as the G-Circle Plot of the Tyre:

17711.jpeg

Anywhere inside the Circle the Tyre will have traction:-

(1)If we Look at Vector (arrow) F2 we can see that all of the tyres available grip is being used for braking; if you were to turn the steering wheel at this point the car would just ‘plough’ straight on – we are using 1.4g of grip..

(2)Now look at Vector F1; here we are not braking quite as hard as in the previous example (1g) so we can use some of the tyres grip for cornering (1g). We are now using 2g of grip from the tyre – we have ‘cheated it’ in to providing an extra 0.6g of Force!



So What Does This Mean?

It doesn’t take rocket science to deduce that the more grip we can exploit from the tyres the quicker we will be able to drive round a circuit.
It is quite natural for most drivers that, as they pass an apex of a corner, they will accelerate whilst ‘unwinding the lock’ from the steering wheel; the more ‘lock’ you take out the harder you can accelerate. This is perfectly correct and is exploiting the ‘G-Circle’ of the Tyre to its best.
However, when we analyse a driver’s braking performance we find a large proportion of them only brake in a straight line, releasing the brakes before ‘turning-in’ to the corner.

…….Why?

(a) Probably because it is drummed into us when we first learn to drive a car and by some track day instructors.

(b) The other reason is probably because the car doesn’t feel as stable under braking; It takes a very special car to accelerate any where near 1.4g in anything other than 1st gear so there is a lot of grip left from the tyres to turn, however braking at 1.4g on a track is relatively easy, the closer you get to this limit the less grip is available for turning – hence the car ‘skids’ quite easily.



Improving the drivers braking performance is fundamental to improving his lap times; the better and more consistent they become at ‘trail braking’ the quicker their lap times will be; it allows a person to brake later and deeper into the corner and utilises the full grip available from the tyres.

Braking should 'mirror image' the acceleration phase out of the corner. We have very few 'steady state' corners on UK tracks and there really should be very little time on a 'balanced throttle'.

The Corner entry sequence should be:

(1) At braking Point apply brakes building to maximum pressure smoothly and quickly (not suddenly).
(2) As turn in point approaches ease off the brake pedal slightly, at the same time start applying steering lock to induce the turn in.
(3) Applying less and less brake pedal pressure, keep applying lock to hit apex 'clipping point'.
(4) As maximum steering angle for corner is obtained no more brake  pressure should be being used and the car will be on a 'balanced throttle'; Apex is clipped and power is applied for corner exit.

'Trail Braking' is used at Stages (2) & (3).

The above sequence will use the full grip available from the tyre; I do appreciate it is a theoretical corner but the rules apply to most corners on UK circuits.

(Gerrards at Mallory Park is Completely different and will be discussed in Corner Weighting later).


I have attached a ‘G-Circle’ Plot taken from our MX5 Race Car:

17710.jpeg

This was taken from two laps of the Anglesey International Circuit, the Red Trace shows significantly better ‘trail braking’ and hence utilises more of the Tyres full Grip, this  resulted in a lap time almost a full second quicker than the Lap shown by the Black Trace. (The difference in lap time is not solely down to ‘trail braking’ but is part of the ‘jigsaw’).

-----------------------------------------------------------------

If you use Race Technology Hardware/Software (DL1 etc.)
'G-Circle Plots' can be drawn the following way:

For RT Users:
Select Graph
(Data Options)
Y Axis - Long accel
X Axis - Latt. accel
(Uncheck tickbox; Display Track Markers & Join Graph Points)

(View Options)
Display Grid
Display Circular Grid


---------------------------------------------------------------

An interesting discussion took Place on the original thread in which I probably Caused a lot of confusion  (Thankfully Kevin picked up on it straight away  thumbs-up.gif) about whether it's better to have a left or right hand drive vehicle on a  UK Circuit so I thought we would look at the answer here:


Going through it step by step:

(1) Using a theoretical car that weighs 400kg on the Right hand wheel pair (due to driver sitting on right hand side) and 300kg on Left hand Wheel Pair, with a 50/50 Front/Rear weight split.

(2) The Static weight distribution would be as follows:

          FL  150Kg's         FR  200Kg's

          RL  150Kg's         RR  200Kg's

(3) Asuming Load Transfer across the axle's during a 1g corner is 200 kg, split 50/50 Front to Rear; the weight distribution through a 1g  Right Hand Corner would now be:
     
            FL  250kg's        FR  100kg's
   
            RL  250kg's        RR  100kg's

(4) In a 1g Left Hand Corner:

            FL  50kg's          FR  300Kg's

            RL  50Kg's         RR  300Kg's

In a Right hand corner there is, therefore a more even weight distribution across the axles (difference of 150Kg's instead of 250Kg's in the Left Hand Corner.

From the ARB article:

ARB's - The Theory

'An axle will have more grip as a whole if the weight is carried more evenly by the axle wheel pair.'
(As Kevin has highlighted).


Hence a Right Hand drive car will have more cornering power (generally) turning Right than turning Left.

N.B - Further thanks to THOS who has highlighted that my Data Logger and his Data Logger (both Race technology) show different for Right/Left Corners - mine shows a '-g' for Right hand corners where as his shows '+g' - ensure you work out which is which for your application.

 

Information for the above articles taken from the following sources: Milliken & Milliken Race Car vehicle dynamics, Optimum G, Dan Kozakewycz blackartdynamics.com, Smithees Race Car technologies, Herb Adams Chassis Engineering, Alan Staniforth Competition Car Suspension.
 

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Glad to be of service, very useful article.

One question I had was on when/if you should try cheat the g-circle, so for example I might 'blip' the throttle when hard cornering to generate some oversteer to make a bend/corner at a higher speed than I think I could otherwise, that often feels quicker but not sure if it really is or is it just more fun  :devil:

If that works then should you aim for some understeer on entering a corner too assuming you have space to make it round and trail break as well. If it works that must be harder to get right to avoid locking?

Guess I am asking is neat and tidy the way or is cheating the done thing for best laptime, in theory?

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Glad to be of service, very useful article.

One question I had was on when/if you should try cheat the g-circle, so for example I might 'blip' the throttle when hard cornering to generate some oversteer to make a bend/corner at a higher speed than I think I could otherwise, that often feels quicker but not sure if it really is or is it just more fun  :devil:

If that works then should you aim for some understeer on entering a corner too assuming you have space to make it round and trail break as well. If it works that must be harder to get right to avoid locking?

Guess I am asking is neat and tidy the way or is cheating the done thing for best laptime, in theory?

Very, very Good question Kevin and I think it depends on several factors such as 'type of bend', car, car set-up and driving style.

The simple rule is:

the less the tyres are turned the faster the speed because tyres 'scrub' speed during Cornering.

I can only really comment on the MX5 race car as this is where my experience lies.

Generally the car doesn't have much power and the faster laps are obtained, as with most cars, by who has the highest corner exit speed. to obtain this in the MX5 the good drivers would 'back the car' into the corner by inducing oversteer on turn in. This would then point the car at the apex and exit so the wheels could be straightened and power applied long before the corner apex. Hence carrying higher 'corner speed'.

On a Car with significantly more power this may well be different and a smoother more 'text book' approach would pay divedends.

The only real answer is the stop watch and different approaches at different corners may well work.

So simply Put..........

You have to test to perfect set-up/driving style.

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The Articles should be read in conjuction and in this order:

....

This probably is the best post I've ever read about vehicles dynamics.  

It's clear, concise and precise.

It reminds me about the Vehicles Dynamics chapter of Carroll Smith's "Tune to Win" book, which is, in my opinion, the best book about track oriented chassis tuning.

But this articles are much easier (and shorter) than the above mentioned Chapter, and tell just everything any non-professional car enthusiast might need to correctly balance (and get the best out of) his car.

Very well done and thankyou!

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  • 3 weeks later...

Nikpro,

Thanks for the PDFs.  Whilst I'm not into competition driving, I do like driving quickly and found them fascinating and easy to digest.

Mike.

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  • 2 weeks later...
  • 2 months later...

I've just re-read this, and it really is very informative. Must admit, I haven't practiced trail braking, I tend to lift off the brakes before turning in. Self-preservation instincts I guess. However I did have my car set up overall to eliminate understeer, and I think I may have overdone it a bit. I think it will be better balanced this season, so might grit my teeth and try it. So it'll be Fraser's fault if I spin! :D  :D  :D

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So it'll be Fraser's fault if I spin! :D  :D  :D

And I'll blame Troy for giving you ridiculous amounts of Power!   :D  :t-up:

Seriously - it makes these posts enjoyable to write when people appreciate them and gain some benefit from my ramblings!

Thanks again.

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  • 5 years later...

The links are broken as they're carried over from the old Boardroom software

Article 1 is here:

http://forum.wscc.co.uk/forum/index.php/topic/83455-anti-roll-bars/

Article 2 is here:

http://forum.wscc.co.uk/forum/index.php/topic/83563-tyres-gripping-stuff/

For those that weren't aware, Nikpro is the old Boardroom name for Fraser, in the pre BLiNK Motorsport days.

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LOL

Read this thinking who the 'ecK is 'Nicpro', reads a bit like Fraser@BLiNK!

& it was! :d

So thanks for that Dave!

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