Publication: Total Magazine Article 2009
Our last article found us exploring the concept of single input steering – minimising driver control inputs to optimise tyre slip angles. We discussed best practice techniques, minimising weight transfer through the platform of the vehicle, maximising the available grip from the tyre through a corner at speed. So how do we squeeze the last nugget of grip from our tyres consistently lap after lap? Let’s go to work on circuit and find out.
We have put in a few steady laps and warmed up the engine oil and transmission fluids, tyres and brakes are now up to temperature. We are mapping the circuit and beginning to feel comfortable with the complexity of the corners. At this point the temptation is to push on and find the vehicles limit of adhesion (Grip Limit) ……….. Hold the speed for a moment!
We need to confirm a key point: are you driving to a system, or relying on instinct and what you think is right? Without a systematic approach to cornering you increase the potential for an “off” and make it difficult to find the vehicles true limit of adhesion throughout the corner. Let’s think systematically and see if we can get these words to leap out of the page and be of use on your next track day.
The corner starts a long way before we make an input with the steering wheel. Braking in a straight line is the safest technique and should be carried out over the shortest possible stretch of tarmac. If I brake in a straight line without steering input applied, I am using the tyres available grip to brake, I am not sharing the braking with steering, so I will stop in the shortest possible distance. What about trail braking I hear you all shout!!! No worries, we will cover that next time when the whole feature is dedicated to braking.
Having retarded the vehicle to the correct entry speed for the corner, it is time to settle the platform with the gas pedal. Now let’s consider a few of the small detail points that have a huge effect on the balance of the vehicle as you turn in. The relevance of a light pressure applied to the gas pedal is this: we need to compensate for the frictional loss between the tyre and the road as we apply the steering. If I turn into a corner off gas, the vehicle will slow. If the vehicle slows, the platform will experience increased body roll. There is then the potential to overload the tyres on the weighted side of the vehicle and you will experience a potential reduction in grip. Think of the gas pedal as a levelling device, keeping the platform as flat as possible as you roll into the turn.
Secondly think of your foot speed – don’t be lazy with the pedals. Dial into how long it takes you and how sensitively you slide from the brake to the gas pedal. Take too long and you allow a torque build up through the platform and into the tyres. The additional weight transfer experienced by the tyres may induce a reduction of grip, or a spin as you turn in. Mid and rear engined vehicles are particularly sensitive to foot speed – the inertia created by the engine position creates a pendulum effect, and the potential for a negative experience for the untrained.
Finally be sensitive to how aggressively you get back onto the gas – remember we are looking for perfection every lap, do not squeeze the gas down too far. If I am on and off the gas, I am shifting weight and just like unnecessary steering input, you will reduce tyre grip. Aim to apply enough gas to settle the car and then not have to adjust again until you are leaving the corner.
So to recap BRAKE IN A STRAIGHT LINE, BACK ON THE GAS, TURN IN. Yes that does sound systematic doesn’t it? A mantra to recite over and over again as you work the circuit.
To perfect the braking and turn in you must allow enough tarmac to get the job done. Experience tells me if you are new to circuit driving you will probably brake a little late – the result being a frantic scrabble on the controls with a feeling the corner is approaching too fast to get everything done that you need to do. Ok we have turned in and settled the car with the gas, described a nice steady state single radius turn and we are approaching the corners apex. So when do I accelerate? We can look for a fixed point on the circuit, but let’s try to be a little more flexible.
The most effective way to decide on an acceleration point is to simply ask yourself: “When am I leaving the corner?” It sounds simple doesn’t it? You may already use this technique – but really focus on it and you will find time in the lap if you do. Why? Too early on the gas and you will have to back off or run out of road on the exit, too late and you will allow the platform to roll unnecessary weight to the heavy side of the vehicle promoting understeer.
Get it right and our levelling device – the gas pedal – will allow balanced lateral acceleration and forward motion to the tyres optimum potential: true grip limit. It sounds counter intuitive but think the process through. We can reduce understeer by squeezing the gas down and shifting a little platform weight to the rear wheels, thus reducing the weight on the loaded front wheel. That is much easier to demonstrate in car than write about. I trust I have got my point across successfully. Matching the unwinding of the steering wheel with the squeezing down of the gas will complete the turn. So let’s complete my cornering mantra: BRAKE IN A STRAIGHT LINE, BACK ON THE GAS, TURN IN, SQUEEZE AS I LEAVE WHILE UNWINDING THE WHEEL.
Now we are cornering systematically with an appreciation of steady state inputs, let’s have a more polarised look at “Grip Limit Signals”. The subtle messages the car communicates with us to determine the available grip remaining from the tyre as we negotiate a turn. A road tyre on a modern family saloon will offer in the region of 0.8 to 0.9g of grip between the tyre and the road. A performance car breaks the 1g barrier, add sticky track day tyres to the mix and we could be pulling 1.5g lateral acceleration through the turns and under braking.
Armed with this information, we should be prepared to experience high cornering and braking forces. Full harness belts and contoured seats promote a good secure seating position and aid car control. Don’t forget to use the foot rest between gear changes and support your body through the turns – a detail point, but the benefits are obvious when you experiment.
Be aware of the audible and physical messages communicated by the car and the tyres as we negotiate a corner at speed. Road and track tyres chirp to us screeching as they approach grip limit. Listen for a constant pitch – it will confirm your inputs are steady state. A change in pitch of the chirp or screech can signify unnecessary adjustment of the controls. Approaching grip limit, the weight or torque of the steering wheel will reduce as the tyres grip on the road reduces.
This can be accompanied with a feeling the vehicle is not following the path you have described with the steering. Ultimately if you have gone in too hot, the tyre will roll onto the side wall. With the tread block running against its intended direction of travel, you will experience a vibration through the steering wheel. This can be a fine tingle through your finger tips to a full blown vibration and depends on the level of enthusiasm applied to the steering when turning in. Remember our mantra and focus on the detail, the less you do the faster and safer you will be.
We will discuss braking next issue, in the mean time safe driving and keep safe in these challenging winter months.
CAT Driver Training Ltd.
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