Haynes Roadster Cornering G Force

Pin It

How much advantage is there is having a light car when it comes to cornering? Sure a light car will accelerate better, but does the lack of weight mean a lack of traction?

Everybody knows you can set up a car to handle differently for different situations. For a drag racer, virtually all of the traction is directed to-wards acceleration, whereas an oval circuit racer has most of its’ traction directed towards cornering.

A drag racer has a lot of traction targeted to-wards acceleration, but when the power is too high, the rear end brakes loose due to the lack of lateral forces from the tyres. With the wheels spinning, the back end fish tails and the power has to be radically dropped to re-gain control. This same car, does not require huge front brakes, as large braking forces force cause the front wheels to lock and skid.

A Haynes Roadster / Locost type vehicle is rarely fitted with ABS brakes. Therefore, whilst braking hard, there still needs to be some traction available to maneuver around obstacles. A Formula 1 race car would try to spend as much time as close to the ‘limit of traction’ circle as possible, whereas a road car would spend most of its’ time well inside the traction zone, so as to have plenty spare to avoid those unexpected obstacles, puddles etc.

Nb. The ‘Circle of Friction’ diagram above is an over-simplified explanation. The Relative availability of sideways and longitudinal forces depends upon the parts of the contact patch where actual sliding first develops, with the development of different forces.

For a Haynes Roadster, Locost or Caterham style car, there needs to be a happy balance between, left and right turns, acceleration and braking. Therefore, a set-up close to the centre of the circle would be good.

G-forces

It always seemed strange to me that when I picked up an American car review magazine they always had a G-force performance figure for cornering, yet they are some of the biggest softest suspension cars I have ever driven. Cornering, never seemed high on the designer priority list. Take a European or Japanese hot hatch and G-force figures were rarely mentioned. Maybe this is because good cornering was taken for granted. A bit biased? Well Maybe. I don’t tend to read car review mags these days and I haven’t been to the US for a fe years, so I guess times may have changed.

How are these figures taken?

A car is driven around a fixed circle as quickly as possible and a time recorded for a 360° lap (T). If the circle is 200 feet wide, the radius is 100 feet (R).

The simplified formula for cornering power is:

g = 1.225 x R
T2

using a lap time of 11 seconds we get :

g = 1.225 x 100 = 122.5 ≈ 1 g
11 x 11          122

Take a Haynes Roadster weighing 700Kg / 1543lbs. If it is subjected to 1g, then it is said to be pulling 700Kg of lateral force.

How do I predict cornering g-Forces?

For this to be accurate, then tyre performance curves are needed.

Tyre Performance Curve

If you know the weight at each wheel, how much traction is available, you can not only predict how much cornering power is available you can predict other characteristics such as understeer and oversteer.

Remember………

Corner weights of a car are taken at rest. But as a car goes around a turn, accelerates or brakes, weight is transferred from one wheel to another. For instance in a right hand turn, weight is transferred to the left or outside outside tyres. This is called lateral load transfer. The more weight on a tyre, ultimately, the less traction it will have.

Example – The ideal situation (This situation only exists whilst car is static)

Car Weight 1543Lbs
Front – Rear weight distribution 50/50%
Left – Right weight distribution 50/50%
Load transfer from cornering 0

This car, would have corner weights of 175Kg / 386lbs. As can be seen in the above graph, tyres are generally designed for much heavier vehicles.

Vertical Load vs Cornering Efficiency
Vertical
Load (lbs)
Traction
Available
Traction
Vertical load
Factor Cornering
Effeciency
300 500 500
300
1.67 167%
400 600 600
400
1.5 150%
500 700 700
500
1.4 140%
600 800 800
600
1.33 133%
1000 1000 1000
1000
1 100%
1500 1250 1250
1500
0.83 83%
2000 1500 1500
2000
0.75 75%
Theoretical cornering forces
Tyre Location Static Weight on Tyre Traction Available
Front Left 386Lbs 575Lbs
Front Right 386Lbs 575Lbs
Rear Left 386Lbs 575Lbs
Rear Right 386Lbs 575Lbs
Totals 1543Lbs 2500Lbs

 

Total Cornering Force = Traction
                                       Weight

Total Cornering Force = 2300 = 1.49g’s
1543

Initial Conclusion

Using the above tyre performance curve(the only one I’ve found) as a target I have approx 1.5g’s to look for whilst cornering. This is never going to be achievable however, as corner weights change as the vehicle moves around

Right, we have our perfect car, but it is still stationary. What happens when this ‘perfect’ car goes around a bend and weight shifts from one side to another?

To do this we need a couple more variables. I’ve taken these from my kangaloosh calculations and from pure guess work based on similar cars.

  • CofG (Centre of Gravity) = 600mm/ 23.6in
  • Track width = 1524mm / 60in
  • Cornering Force = 1g

Lateral Weight Transfer = Weight(W) x Cornering Force (g) x CofG
Gravity x Track Width(T)

Lateral Weight Transfer = 1543 x 1 x 23.6 = 36414.8 = 607Lbs
1 x 60               60

This means that 606.9Lbs will be transfered from one side of the car to the other. Split this across the front and read axles and you get : 303.5Lbs

Theorectical cornering forces
Tyre Location Static Weight on Tyre Lateral weight transfer Weight on Tyre During Cornering Traction Available
(from graph)
Front Left 386Lbs -303.5Lbs 82.5Lbs 150
Front Right 386Lbs +303.5Lbs 689.5Lbs 720
Rear Left 386Lbs -303.5Lbs 82.5Lbs 150
Rear Right 386Lbs +303.5Lbs 689.5Lbs 720
Totals 1543Lbs 1543Lbs 1740Lbs

Total Cornering Force = Traction = 1740 = 1.12g’s
Weight      1543

Therefore, we can see that traction has been reduced from 1.49g’s to 1.12g’s. It also shows that in a 1g corner, there is 1.12g of cornering force – basically this car is inside its’ limit of traction. To be honest, that’s not too shabby. Particularly when you consider a lot of top sports cars struggle to reach this figure with their fancy traction and stability control systems. The hot hatches are left dreaming.

What happens if my corner weights aren’t even?

A Caterham / Haynes Roadster / Locost has a pretty good weight distribution. Some owners have managed to balance corner weights within a Kilo, even side to side with the driver on board. But what happens when corner weights are off. Lets try a front heavy vehicle, with 60% of it’s weight over the front axle.

With the same 1543 (700Kg) we would have 925.8Lbs on the front and 617.2 on the rear wheels. The load transfer would still be 607Lbs (see above), but that now splits to 364.2 on front wheel and 242.8 on rear wheels.

Theorectical cornering forces
Tyre Location Static Weight on Tyre Lateral weight transfer Weight on Tyre During Cornering Traction Available
(from graph)
Front Left 462.9Lbs -364.2Lbs 98.7Lbs 180
Front Right 462.9Lbs +364.2Lbs 827.1Lbs 780
Front Total 925.8Lbs 960
Rear Left 308.6Lbs -242.8Lbs 65.8Lbs 100
Rear Right 308.6Lbs +242.8Lbs 551.4Lbs 650
Totals 1543Lbs 617.2Lbs 750Lbs

Total Front Cornering Force = Traction = 960 = 1.03g’s
Weight     925.8

Total Rear Cornering Force = Traction   =   750 = 1.21g’s
Weight         617.2

What this shows is the front end will not stick as well as the rear. The car will understeer, placing higher wear on the front tyres. A car car only corner as fast as the lowest traction figure available, which in this case is 1.03g’s.

Conclusion

Getting your corner weights is important if you are into racing. There is a 0.09g difference in cornering. This might seem a small number but lets place it back into this equation:

g = 1.225 x R
T2

As before the radius is still 100 feet.

Re-arranging:

T = √1.225 x R  = √ 1.225 x 100 =  10.90 seconds
g                        1.03

T = √1.225 x R = √ 1.225 x 100 = 10.45 seconds
g                     1.12

That’s potentially 0.45 seconds a corner. Over a whole lap of a track that’s several seconds instant advantage over other cars, or in real life, enough to avoid that dog as it jumps into the road.

Back to my ‘how much negative camber should I have?’ question……

If my weight distribution is off then I need to adjust the camber so that on the limit of traction (in this case the rear wheels), the front wheels are closer to perpendicular to the ground. e.g. dial in a little negative camber. If I dial in some negative camber, hopefully I’ll be able to get nearer to the ideal cornering efficiencies at a given loading.

What I need to find out, is the relationship between tyre deflection and camber angle, take these percentages and add them to the cornering percentages below and voila! Or I could have some fun by hacking it around a track, then comparing lap times.

However, if I spend some time getting my corner weights more even, then I won’t need the negative camber. Basically, I need to get the scales out.

My other option would be to have Anti-Roll bars front and rear.

Vertical Load vs Cornering Efficiency
Vertical
Load (lbs)
Traction
Available
   Traction    
Vertical load
Factor Cornering
Effeciency
300 500 500
300
1.67 167%
400 600 600
400
1.5 150%
500 700 700
500
1.4 140%
600 800 800
600
1.33 133%
1000 1000 1000
1000
1 100%
1500 1250 1250
1500
0.83 83%
2000 1500 1500
2000
0.75 75%

Click here for more info on  – Camber angle and Cornering Efficiency

Pin It

Web Design by Go Web Solutions