Understeer

[SootySport]

2 hours ago, AdamR said:

Yep, as has been said, a gentle, progressive lift off the power will encourage some weight transfer gradually to the front (a sharp lift will cause it to 'dig in' and you may end up with lift off oversteer - which is fun but maybe not what you need in panic mode), and reducing lock will usually allow the front tyres to recover.

This may be tricky to explain in text, but basically... Slip angle is the difference in angle between the direction the car is travelling and the front wheels are pointing. When you are in an understeer condition, the front tyres are in the 'Exceeding the limit' part of the following graph:

Therefore, by reducing the amount of lock - and thus slip angle - you can get them back into the green range. We're taking in the order of 2-3 degrees of front wheel angle here though, so it's a fine line and takes a while to learn how to 'feel' what the tyres are doing through the steering wheel. You can also listen to them though, as tyres will have a chirp in the green position of the graph and a distinct squeal once you start to push them into the red bit.

As a related aside, this is why road tyres are more fun / easier to learn to drive (properly!) on than sticky ones. They have a wider working window for slip angle, so you have more of a 'back of a spoon' than a 'knife edge' to play around with:

 

As you said though Russ, a big part of driving quickly is in keeping the car out of wild understeer - usually by using the brakes and throttle to steer the car instead of the steering wheel.

By the time I've done all that I would have hit the wall. 

I do know the theory but it does depend on how long you got before you hit something.

[Beaker_Bob]

I was looking for some tyre pressure info a while back and came across this on the Locost builders website.  The background and pressures suggested are specific to the Toyo R888's, but the theory and what to change based on what you want to achieve from handling may be of some benefit in avoiding under steer in the first place.

Bob

SET UP ADVICE FOR THE TOYO PROXES R888 

What I would like to do is to offer some advice on tyre temperatures and pressures and how to achieve the best from our tyres when taking part in either trackdays or racing. 

The R888 has a semi race construction (very stiff) and a race tread compound. The optimum tread temperature range is between 85C and 95C measured using a probe type pyrometer, and ideally a maximum difference across the tread of 9C. The maximum hot pressure we recommend is 40psi. Camber angles up to 5 degrees are permissible but the final setting will depend on tread temperatures. It is advisable to have as much positive castor as practical as castor induces a beneficial camber change during cornering. I recommend that the tyres be put through a heat cycle before hard use, this will scrub them in and remove any mold release agent. 

The pressures you use will initially depend on the weight of the car, too little pressure on a heavy car can lead to over deflection of the tyre and subsequent failure. 

Below are some basic settings: 

VEHICLE WEIGHT COLD PRESSURE HOT PRESSURE 
Very Light < 800kg 17 - 22 psi 22 - 29 psi 
Light 800kg - 1000kg 20 - 26 psi 24 - 32 psi 
Heavy 1000kg - 1400kg 23 - 27 psi 28 - 40 psi 
Very Heavy > 1400kg 27 - 35 psi 37 - 40 psi 


As a tyre gets hotter the pressure increases, this is due to the moisture in the air. The cold pressure you set to achieve a desired hot pressure will depend on the conditions on the day i.e. ambient and track temperature, wet or dry. If the day/track is cold you will need to start with a higher cold pressure as the tyre will not get as hot therefore the pressure increase will not be so great. 
Hot pressures must be balanced side to side. Once the tyres have cooled you will find that you will have a difference in pressure side to side, if you have been racing on a right hand track you will find the offside pressures will usually be higher than the nearside. 

Changing hot inflation pressures by small amounts can be used to fine tune handling. 

Reduce Oversteer Reduce rear pressures or increase front pressures 
Increase Oversteer Increase rear pressures or reduce front pressures 
Reduce Understeer Reduce front pressures or increase rear pressures 
Increase Understeer Increase front pressures or reduce rear pressures 


Achieving the required tread temperatures will depend again on the conditions on the day i.e. ambient and track temperature, wet or dry. 

You often here competitors saying “My tyres started to go of towards the end of the race”, this is usually due to the tread getting to hot. 

The tread temperatures are constantly changing through out a race, hotter when cornering and cooler when on the straights and cooling even more when you are slowing to come into the pits. Therefore the temps you record in the pits will be lower than those during the race. So if you record temperatures within the range given above the probability is the temps will be too high during the race. 

Increasing your tyre pressures will cause your tread temperatures to increase, more pressure stiffens the tyre’s casing which results in the tread having to do more work resulting in the tread getting hotter. Lowering your pressures will cause them to decrease. 


Inevitably changing one thing will affect other things, the whole set up of your car is a compromise between anything that is adjustable. 



Alan Meaker 
Technical and Motorsport Manager Toyo Tyres (UK) Ltd