This isn’t the standard Haynes Roadster Rear Upright.
I’ve modified several of the dimensions to get a small degree of anti-squat.
Because the chassis is 4 inches wider than stock, some of the other dimensions have changed too.
I wanted the A-Arms as long as possible, so the mount holes for the A-Arm are further outboard than normal too.
The biggest change is that the dimensions for the rear A-Arm mounts are different to the front.
Instead of one A-Arm mount at the top there are 2. It would not be possible to use urethane mounts as the upright no longer moves up and down in a perfect vertical line. The Upright now travels in an arc with a small degree of rotation.
I’m not that good at explaining these things and too be honest it took a while for me to get my head around quite a few issues.
It’s all very well getting the dimensions for the modelling software, it’s another putting them into practise.
The clevis and rose joint arrangement will allow a small degree of camber, caster and toe in/out adjustment
Notice that the inner joints have clevis arrangements and urethane mounts. The outer joints are all 5/8 UNF rose joints. In ride terms, I ideally wanted all urethane joints, but I made few sacrifices. I just hope it’s not too hash, after all I don’t want a car that is so hash I dread driving it.
Anti – Squat Rear Suspension
(Note. Wheelbase dimensions differ from the standard Haynes Roadster)
When a car accelerates forward, weight is transfered to the rear. The weight transfer is dependent on many aspects including the weight of the vehicle, the weight distribution front to rear and the height of the centre on gravity. The vehicle can be seen to ‘squat’ at the rear under hard acceleration.
With Anti Squat the driving forces of the rear axle can be used to counteract this squatting force. This counteraction is known as ‘Anti-squat’.
The amount of anti-squat depends on application, many off-road race vehicles run as little as 15-20% anti-squat (due to high Centres of Gravity), whilst Top Fuel Drag racers have up-wards of 150% anti-squat.
It is possible to stop this squat and if even you watch a full blown Drag Racer, the rear actually rises. This forces the tyres into the ground even harder for greater traction off the line. During a corner a certain amount of anti-squat is desirable. The car remains level, yet the wheels are being forced down into the ground, greater traction and hence faster cornering speeds are available. Too much can cause the rear to lift, along with the centre of gravity and over-steer will result.
Greater than 100% Anti – Squat Rear Suspension
If the instantaneous centre (red dot above) falls above the 100% anti-squat line then there will be greater than 100% anti-squat. Upon hard acceleration the rear of the car will lift. This will give greater traction but the energy used lifting the car will result in a loss of horsepower at the wheels.
Less than 100% Anti – Squat Rear Suspension
When the instantaneous centre falls below the 100% anti-squat line the rear of the car will squat, causing the wheels to spin. With 100% or neutral anti-squat more power can be put down out of corners and the car will not lurch so much during gear changes. However; particularly with cars of high centres of gravity; achieving 100% anti-squat means bringing the Instantaneous Centre (I.C.) close to the rear wheels. This can cause wheel hop during breaking. In the above diagram the I.C. is approximately 1.75 meters from the rear wheel. Anything less than 1.1 is said to produce ‘break hop’. The squatting action can delay the weight transfer to the rear wheels; don’t think of the rear squatting as much as the wheels being pulled away from the tarmac.
Squatting whilst traversing rough surfaces can also be bad. Not only is acceleration causing the car to squat but the bumps are also compressing the suspension, too a point where other handling characteristics are compromised and shocks are inefficient.
Rear end squat can cause the front wheels and steering to go light, loosing ‘feel’ and possibly traction. When a vehicles Centre of Gravity (CG) is shifting all over the show, and the car lurches fore and aft, control can-be somewhat tricky, compared to a vehicle that remains perfectly linear.
Calculating Anti – Squat Rear Suspension
I’ve taken the weight distribution to be even front to rear and the CG height to be about 18″ (complete guess). Draw a vertical line through the front wheel axis until it hits a horizontal line drawn through the CG. A diagonal line is now drawn from this intersection to the contact patch of the rear wheel (Anti-Squat Line). If the point where the two axis lines of the rear A-Arms intersect (Instantaneous Centre) is above the diagonal ‘Anti-Squat line’ then there is greater than 100% anti-squat, below less than 100%. You can see, that I have exactly 100% (as suggested by my suspension modelling software) but I may well reduce when I drive it. I figured a lot of low slung race cars have close to 100%. The Haynes Roadster is pretty low and has a similar 50/50 weight distribution.
100% or Neutral Anti – Squat Rear Suspension
Theoretically, with 100% anti-squat the car will neither lift nor squat. If you think your car squats too much increase your anti-squat and if it lifts too much reduce it.
For more on anti-squat suspension see my other blogs.