Caster angle/ride height formula

[Claus]

@ howard
Sent you a privat message

[curtis]

Hi
I was the author of the caster angle correction article published in flat out in 2003
Here is a copy. The charts and figures are available if anyone would like to see them.

Caster Angles & Ride Height

1 The Geometry of the 2CV chassis front suspension is such that Caster angle varies
according to ride height

2 Ride height is dependent on
(a) Wheel rolling radius.( Wheel bearing centre distance above ground)
(b) Wheel load, (Causing suspension deflection, including tyre deflection)

3 Citroen designed the 2CV to have a specified ride height with a caster angle of 15degrees. This geometry gave reasonable steering wheel loads and centering forces with the original steering wheel. The standard ride height for the front axle is 192.5 to 197.5 mm (7.5 to 7.8in), with 125 tyres.
The condition of the vehicle specified as unladen and with one gallon of fuel in the tank.

Citroen Specials builders generally want to lower the suspension to improve the look of the car and to reduce roll on cornering. They also want to reduce the diameter of the steering wheel to improve looks and to facilitate easier entry and exit of the cockpit.

How low do we want to ride?
The specified Citroen mean set up with 125 tyres is as follows (for comparison deduct 10mm if your car has 135 tyres)
Ride height of 195 mm (7.7in) producing a caster angle of 15 degrees
From the article submitted by John Nash for the 2CV, uncorrected geometry,
Ride height of 125mm (4.9in) produces a caster angle of 25 degrees.
Ride height of 100mm (3.9in) produces a caster angle of 28 degrees.

To maintain the 15 degree caster angle, it is necessary to cut and rotate the kingpin socket in the suspension arm by a correction angle. This can, for convenience of measurement, be related to a shift at the circumference. As the diameter of the tube at the knuckle is 56mm. So the dimension is 0.4887x the angle change. See Chart2

With reference to John Nash’s geometry drawing and taking dimensions from my pre-modified chassis, the following dimensions have been used in the derivation of the relationship between ride height and caster angle.
e (wheel centre above ground) = 315.5mm
d (Bottom of Chassis below axle bearing centre) = 160mm
x (Radius of wheel bearing from axle bearing centre) = 390mm
a (Radius of kingpin knuckle centre from axle centre) = 432.5mm
g (Distance from wheel bearing to Kingpin Knuckle centre) = 98mm
c (Ride height of chassis) Variable
Theta (Caster angle) Variable
Alpha( angle at the Knuckle) 58.2 degrees

See figure 1 where the fixed angle Alpha at the knuckle can be derived by use of the cosine rule and then the ride height “c” can then be expressed in terms of Theta thus.

C = 315.5-98 cos.Th + 432.5 sin. (90-58.2-Th) –160 or C = 155.5 - 98CosTh + 432.5 sin ( 31.8 -Th)

See Chart 1, which shows how the 2CV caster angle varies with ride height.
John Nash used a ride height of only 100mm (3.94in.) and a caster angle adjustment of 12.5 degrees. (6.2mm circumferential correction)
I have used a ride height of 114mm (4.5in) with a caster correction angle adjustment of 10.6 degrees. (5.2mm circumferential correction)

My experience with a 13in steering wheel, hand loads have been reduced to a comfortable level and ground clearance has not proved to be a problem.

It is interesting to note that the caster angle approaches zero at a ride height of 280mm (11in)
So don’t over adjust the caster and under adjust the ride height if you intend to tow your adjusted special. Tyre wear will start to occur long before zero caster angle is reached. This is because the self-centring forces have gone and the wheels don’t know which way to point. The reason for this is probably that as tow hooks of cars are much higher than tow bar attachment points on 2CV chassis and the centre of gravity of the cars is much higher than the tow bar, acceleration of the combination results in a large nose up pitching moment causing the caster angle to approach zero.
Towing with a negative angle is impossible because the wheels will immediately go to full lock.

Graham Curtis 1448 26th July 2003

[JieKie]

hi curtis

i was wondering if you could send me those charts and figures.

thanks in advance

[Paul]

Hi,

Does anyone know the logic behind the Bijou ride heights quoted in the document Maintenance Book 2CV Twins by John P Richards C.Eng MI Mech E, see the extract from page 66 below.

Ride Heights - Maintenance Book 2CV Twins - John P Richards.jpg (42.02 KiB) Viewed 11024 times

I’m interested in the ride height for the Bijou because they would appear to suit my trials car as I need maximum ground clearance. As I understand it, the Bijou used the same rolling chassis as the 425 cc 2CV (see http://en.wikipedia.org/wiki/Citro%C3%ABn_Bijou) in which case you would expect the Bijou to have an identical ride height to the 2CV. However, there is an increase in excess of 100 mm.

There is more information on the Bijou (see http://www.citroenet.org.uk/foreign/slo ... bijou.html) that shows what appears to be the standard 2CV rolling chassis.

Assuming that the Bijou used the identical front suspension arms as the 2CV (there is nothing to indicate otherwise), then the KPI must have been approaching vertical and the steering would be very light.

Regards Paul

[Ken Hanna]

Paul,
safest bet would be to assume that there's a typo in that information.
Citroen's workshop manual quotes a front ride height for the early cars of 195 +/- 2.5mm. For the early vans, that was increased to 205 +/- 2.5mm.

Having once driven an Acadiane pickup which had been wound up well beyond the recommended ride heights, I found the absence of self-centring, particularly on tight corners, to be downright dangerous...

Ken

[Paul]

Hi Ken,
I thought it might be too good to be true. I'll experiment with the Arcadiane settings of 212 mm front and 317 rear. I'm currently set to 300 mm at the rear; I don't seem to have made a note of the front ride height, but I'm sure it can increase, particularly as I've now got 30 lb of ballast on the front.
Regards Paul