My problems…

So how do I go about curing body-roll?

I’m using push rod in-board rising spring rate suspension, but currently the spring rates are even front and rear. So do I need anti-roll bars as well?

Plus, I’m still trying to work out if I need some static negative camber on the front to cure under-steer.

So out came the books again.

Roll Angle

When a car goes around a bend the wheels change their camber angle to the road. The outside wheel gains positive camber and the inside wheel negative. When a wheel isn’t vertical to the road, grip is lost. If you can limit body roll, then the camber change will also be limited.

There are many factors that effect camber gain such as:

• Roll centre height
• Swing Arm Length
• Height of suspension mounts
• Height, length and angle of control arms

I’ve used Kangaloosh to design my suspension, but there were many limiting factors I had to incorporate, such as using existing Sierra 4×4 uprights, Sierra track width, chassis width, ground clearance etc. So the camber gain per degree of body is almost a fixed constant I can’t do much about.

Nb. It is hard to get front suspension geometry to work when there is more than 0.75° camber change per degree of body roll. (My design has 0.36°)

Negative Camber

I have already worked out that I can compensate for the camber gain by adding in some static negative camber.

But I have also learned that negative camber is not without its’ problems:

• Excessive tyre wear on inside edges
• High tyre temperatures (ok for racing, but no good for long term wear)

So for a road going car, the limit is 1°. In the case of my Locost / Haynes Roadster type car, I don’t expect to need even this much, but I have several books to finish reading before I’ll know.

How do I control Roll Camber Gain?

As mentioned above there are several factors that affect roll camber gain vs body roll. Some of which, for a kit car, there is limited control over

Centre of Gravity (CofG)

This one is fairly obvious. A tall car will have a high centre of gravity and a low slung racer, a low one, however, there is not a massive amount that can be done to change this, as Kit Cars are generally already designed to have a low a centre of gravity. In the case of my Haynes Roadster I have taken my CofG to be 600mm (a complete guess, based on those published for similar cars)

Roll centre Height

Raising the roll centre height will reduce the roll angle.

Track width

I am using 4×4 Sierra front suspension components, therefore the track width of this build is more or less fixed by those components. I can change the offset of the wheels, but apart from that I limited to using Ford dimensions. I do have a wider track than a standard Locost / Haynes Roadster and even a Caterham and that will give my car a smaller roll angle.

Tyres

A car equipped with road tyres will go around a corner slower than if it was re-fitted with sticky race tyres. The faster a car goes around a corner the higher the cornering forces and body roll. Therefore, simply fitting sticky tyres might not be the answer as body roll may be excessive. There will need to be something done to control roll stiffness.

Roll Stiffness

Ok, so I might have to add some extra means of controlling Roll stiffness. I have three obvious choices.

1. Harder Springs
2. Increase the rising spring rate ratio on my push rod suspension cantilevers
3. Anti Roll Bars

1. Stiffer Springs

Increasing spring rate to control body roll often involves huge hike in spring rates. Controlling roll in this manner will serious affect ride comfort, but there are other problems. If the spring rate is too high a car will crash into bumps, without absorbing them. Adhesion with the ground is compromised or even lost. Plus there may be other problems such as bump steer and under steer in corners.

2. Increasing Rising Spring Rate Ratio

Having a rising spring rate is a huge advantage over a constant spring rate or indeed a falling one (like I’ve seen on several cantilever set-ups). One advantage of this kind of set-up is a softer ride under normal conditions and a harder one when large bumps are hit. Huge bumps are not normally found on the average race track so a sharply rising rate is less of a problem, however for a road going car you will experience the same problems a 1. Stiffer Springs. Another problem is; depending on design; a small tweak in the cantilever design will produce huge changes, so the sharper the rising spring rate the harder it will be to match left to right and front to back.

3. Anti-Roll Bars

This has to be the best way to increase roll stiffness and limit body roll. Anti-Roll bars are also known as ‘Stabiliser Bars’.

If both wheels hit a bump or are compressed under braking or acceleration, they have no action. However, when a car is roll with one wheel up and the other down, the anti-roll bar will twist, providing torsional stiffness. The stiffer the bar the more resistance there is to body roll. However, it doesn’t take much of a change in the bar diameter to give large changes in the resistance.

Stiffness = D4

So the only real option is to change the length of the swing arms on each end. In the case a push rod system like below, either the cantilever ratio or the length of the bar will need changing.

Many production cars have their anti-roll bars mounted on soggy rubber mounts that limit their effectiveness.

The effectiveness of an anti-roll bar will be limited by:

• Frame mounts
• Stiffness of of swing arms
• Stiffness of drop links (where used)
• Stiffness of A Arms

What size Anti-Roll Bar do I need?

• Car weight : 700Kg (1544Lbs)
• Front / Rear Weight Distribution : 50 / 50%
• Left / Right Weight Distribution : 50 / 50%
• Cornering Load Transfer : 276Kg / 607Lbs
• Roll Stiffness (Front): 36Kg/ 80lbs
• Roll Stiffness (Rear): 0Kg / 0lbs

From previous ‘theorectical’ calculations:

Theoretical cornering forces
Tyre Location Static Weight on Tyre Lateral weight transfer Weight with Front Cornering Load Lateral weight transfer with front anti-roll bar Weight with front bar Traction Available(from graph)
Front Left 386Lbs -303.5Lbs 82.5Lbs -80 2.5Lbs 5
Front Right 386Lbs +303.5Lbs 689.5Lbs +80 769.5Lbs 750
Front Total 772Lbs 755
Rear Left 386Lbs -303.5Lbs 82.5Lbs 0 82.5Lbs 150
Rear Right 386Lbs +303.5Lbs 689.5Lbs 0 689.5Lbs 720
Rear Total 772Lbs 870
Totals 1544Lbs 1544Lbs  1544Lbs 1625

Total Cornering Force = Traction = 1625 = 1.05g’s average
Weight     1544

Front Cornering force = 755 =  0.98g’s
772

Rear Cornering force = 870 =  1.12g’s
772

There is more grip at the rear than the front, so this car will under steer. The stiffer the anti-roll bar, the more this under steer will be apparent.

What if I fit front and rear Anti-Roll Bars?

For this theoretical example, I’ve used a similar configuration to before but fitted with front and rear anti-roll bars. Notice the change in weight distribution.

• Car weight : 700Kg (1544Lbs)
• Front / Rear Weight Distribution : 60 / 40%
• Left / Right Weight Distribution : 50 / 50%
• Cornering Load Transfer : 276Kg / 607Lbs
• Roll Stiffness (Front): 22Kg/ 50lbs
• Roll Stiffness (Rear): 18Kg / 40lbs
Theoretical cornering forces
Tyre Location Static Weight on Tyre Lateral weight transfer Weight with Front Cornering Load Lateral weight transfer with front anti-roll bar Weight with front bar Traction Available
(from graph)
Front Left 386Lbs -303.5Lbs 82.5Lbs -50 32.5Lbs 50
Front Right 386Lbs +303.5Lbs 689.5Lbs +50 739.5Lbs 740
Front Total 772Lbs 790
Rear Left 386Lbs -303.5Lbs 82.5Lbs -40 42.5Lbs 50
Rear Right 386Lbs +303.5Lbs 689.5Lbs +40 749.5Lbs 740
Rear Total 772Lbs 790
Totals 1544Lbs 1544Lbs  1544Lbs 1580

Total Cornering Force = Traction = 1580 = 1.02g’s average
Weight     1544

Front / Rear Cornering force = 790 =  1.02g’s
772