The Nature of Tyre Grip
Breakthrough Technology – How is Grip Created at the Tyre?
In the first of our “Key Performance Concepts”, we discuss creating grip at the tyres through optimising the suspension spring stiffness (ie stiffer springs create more grip).
How did we come to this conclusion? Why is it a central plank in our thinking about suspension set-up?
When you are doing set-ups for a living, there is one driving force – you get a big tick if the car is faster and pretty much a zero if it’s not. After a while, the evidence was staring us in the face – the cars with the optimized spring stiffness were seconds faster per lap.
Furthermore, to help us with our thinking about set up, we need some explanation over and above the empirical evidence. The following relates to the properties of tyres:
What happens with tyre grip is that the tyre rubber keys into the tiny peaks and valleys of the asphalt surface. Stiff springs result in higher frequency vibrations at the tyre contact patch. This has two very important effects:
- Firstly, more heat is created in the rubber, and so increases friction (or grip).
- Secondly, the rubber dissipates an increasingly large proportion of the energy of the higher frequency vibration, thus helping the rubber stay keyed into the road surface.
- Optimizing spring rates is the single biggest contributor to improved cornering performance, where previously they have been running springs way too soft. Our clients reduce lap times by seconds per lap.
- Cars on the road need softer spring rates, for reasons of ride comfort. (The lower frequency, higher amplitude vibrations do not heat the rubber, nor key the rubber into the asphalt surface to the same degree.)
- These two effects are largely explained by a property of rubber known as “hysteresis”. For the best description of this, read Chapters 2 and 3 of Paul Haney’s Book, “The Racing & High Performance Tire”. Paul's book has detailed descriptions and diagrams that fully explain the effects of the higher frequency vibrations.
So, we can keep increasing spring stiffness from road car stiffness, getting grip improvement with each step, until it becomes too stiff. Based on our experience, we are able to pretty much home in on what will work for any race car. This recommendation is a starting point; the first part of your a baseline set up. It could be that further testing at the race track is required.
Most of our clients don't do any further testing, especially if the car is winning races. For example, David Stone’s winning Ford Falcon XY remained unchanged in set up from 2006, when we first did the car all the way through to 2015, when it was still winning races and claimed the club championship in the hands of it's new owner.
On the other hand, David’s equally race winning Mazda MX5 has been in development on and off since our initial setup in 2012.
It’s a question of competition really. If needed, there would easily have been more development left in the set-up of the Ford.
Notes on the Picture of a Race Tyre Above.
This picture is a Hoosier slick tyre on David Stone's Mazda MX5. There are two reasons for excessive graining. The first, that the conditions and loadings are too much for the tyre - stinking hot day, for instance. The second reason is running too hard on cold tyres. This is what's happening here.
David can often get off the start line and lead the field into the first corner, as long as he starts from the first or second row. Then he needs to gap the field on the opening lap before the the turbo cars and those with larger capacity engines (more horse power) get into stride.
This "need to run hard on cold tyres" results in the graining you see in the picture - small bits of hot rubber tearing off and re-adhering to the cold tyre surface. (Not to be confused with the "tyre pickup", the larger bits of rubber (light grey in the picture).
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