I’ve been worried about setting up my front and rear suspension. I have designed all the A-Arms so that I have massive adjustment; which is good. But not knowing where and what to adjust is bad. Therefore, the books were dusted off and I began to read up. Primarily, I wanted to know How much negative camber should I start with?
The books however, all seemed to start with tyres and vehicle weight.
Tyre Traction vs Load
I’ve read several articles on Tyre Traction vs Load, but I struggled to find an detailed information / figures from manufacturers about tyre performance. Should I be worried about graphs and numbers?
Obviously, a cars’ tyres are a major component in a vehicles handling. So getting things like camber right must be really important?
If the tyres aren’t in perfect contact with the road surface; with maximal contact area; then handling will be compromised. But what else can be done to help?
- If the weight on a tyre is minimal, what happens to traction?
- If tyre weight is overloaded, what happens to traction?
- When the camber angle changes, what happens to traction?
- How much negative / positive camber should be set at normal ride height?
I could spend ages surfing for precise tyre characteristics, but I decided to use ‘generalised’ characteristics. Cornering efficiency will be different for every tyre, yet the relationship between Vertical load and Traction will always follow the same shape curve. It’s the shape of that curve that is important to me.
As Vertical load increases the increase in traction per pound drops, until the relationship becomes more linear. We’ll call this relative traction a loss in the cornering efficiency.
In the above graph, we can see the answer a couple of my Questions
- There would be minimal (Lateral Load) Traction available – 100lbs.
- If you overload the tyre, it’s efficiency will drop. When Lateral Load exceeds the amount of Traction, the car will skid / slid.
The above graph is in pound(lbs). I have taken my corner weights to be 175Kg / 386lbs which is much lighter than what the typical tyre is designed to handle. Therefore, a Locost or Haynes Roadster will be working in the left curved section of the above graph.
This is simply the weight on the tyre. Whilst stationary, this can be measured by placing each wheel on scales.
For my Locost / Haynes Roadster, I used a weight of 700Kg and a perfect 50/50% weight distribution.
Vertical Load ⇒ 175Kg / 386lbs
During motion the vertical load on each wheel will change. By tuning chassis weights and characteristics it is possible to change tyre loading during cornering, braking and acceleration.
This is as obvious as it sounds. The ‘stickier’ the tyre, the more traction or grip it has. How sticky a tyre is, will depend on the road condition, temperature and weather. A hot slick tyre will be sticky on a race track in summer, yet rubbish when the temperature drops and the skies open. That’s when your winter tyre has more ‘traction’.
By noting values from the above graph, we can generate a Vertical Load vs Cornering Efficiency chart.
Vertical Load vs Cornering Efficiency
According to the above chart, cornering efficiency would be at 100% at 1000lbs load on each wheel. For a light car that’s a lot.
Tyre Deflection vs Camber
At any given vertical load, a tyre will give maximum grip / traction when it is perpendicular to the ground. Now there is the system developed by D.J. Sportscars (DAX) that keeps the wheels vertical no matter what the lateral cornering forces are, but for us mere mortals, the tyres contact patch will be bigger when the wheel is perpendicular to the ground.
Therefore, it makes sense to have zero camber when the maximum amount of grip is needed. A degree or two of negative camber is sometimes added to compensate for deflection is suspension bushes and A Arms and as a Haynes Roadster or Locost with it’s strong suspension and urethane bushes / rose joints is unlikely to experience much deflection zero camber seems like a good starting point. For a road driven car, any more than 1° negative camber will start to wear out the inside edge of your tyres. Race cars, particularly ones that don’t have huge braking and acceleration forces to cope with may have 2 or even 3° negative camber depending on factors such as track conditions, tyre temperatures, weather etc. For a road car you don’t want to be changing your tyres and suspension settings everytime the sun comes out. (ok bad example, as it hasn’t stopped raining in weeks)
I will keep reading, as all I have found out is a light car, with good tyres has got a huge advantage over a lorry, and anything more than 1° is too much camber for a road car.