Can we talk about Rear Camber Settings - UPDATE

[Tazio]

Can someone in the know explain the impact on changing rear camber settings?

The reason I ask is when mine was flat floored recently, the rear camber was reduced from 2.5 degrees to 1.5 degrees, the explanation being that it was to counter some of the slow speed understeer I was experiencing ( the change worked).  However I have also noticed that high speed stability ( say 90 leptons) has been compromised/lost, it feels like the rear seems to want to self steer...feels weird.

[Jonathan Kay]

"Why have negative rear camber?"

De Dion? What tyres?

Jonathan

PS: Are you sure you mean camber rather castor? Did they change or machine or shim or the de Dion ears?

[Richard Price]

Tazio,

Toe is more likely to impact on stability than camber.

I think the standard specification is supposed to be around 30' toe in. I've heard folk about running with toe out, but that really does make the car feel vague in a straight line, and gives the impression of better turn in.

Bent dedion tubes are not at all uncommon, normally giving greater toe in. 

Have the ears been changed to adjust the camber or shims added?

Standard 1.5° ears normally measure more like 1.75°, and 2° R300 race ears are available.

 

[Tazio]

Thanks Richard,

As you know the car was originally campaigned by Ian Keen, and was set up quite aggressively, loads of front toe, 2.5 degrees front camber, and near as dammit 2.5 degrees rear camber ( standard 1.5 degree ears plus shims), and 21 mins of toe.

The car now has 2.2 degrees front camber, 3 mins toe out, the rear has 1.5 degrees camber 14 mins of toe.

At speed, and in a straight line,  it feels as though I am chasing the car, it moves, I correct...unsettling.

And Jonathon, the car was flat floored on CR 500s on 6 and 8 inch wheels, but shows the same symptoms on A021s on 6 inch all round.

 

On a general point, I would like to understand the theory behind rear camber, and the implications of making changes, increasing or decreasing.

 

[Mark Durrant]

To save confusion for others 'the caster angle or castor angle is the angular displacement of the steering axis from the vertical axis of a steered wheel in a car, motorcycle, bicycle or other vehicle, measured in the longitudinal direction.' Therefore this does not apply to the rear on a Caterham.

[Richard Price]

Tazio,

is that toe in or out at the back?

The idea of camber is that the tread should be as flat as possible specifically when cornering. With radials tyres, the tread section stays as quite a rigid band, then, in cornering, the side forces make the tyre roll on the wheel. Assuming you don't have too much toe, if you have too much negative camber, you'll wear the inside edges of the tyres, and won't work the outside edges.

If you have too little negative camber, you'll work the outside edges too much.

Similarly, the more negative camber you have, the less grip you'll have on the brakes in a straight line.

Where that sweet spot is depends on your use and your driving style.  

Camber does not tend to affect stability.

[Mark w]

I had my car flat floored today .

The rear camber was -1.6 degrees ( both sides ) and was left as it was .Toe + 0.6mm ( not sure what that is in mins ) 

Front set at 2 degrees negative , -1mm toe  .

Had to adjust the castor on one side to get matched figures at 6 degrees  .

Apart from my inability to convert mm of toe to mins the above seem similar to your settings . Car tracks true and certainly stable at speed .

May sound daft but tyre pressures ?

[F355GTS]

If I've read your post correctly I'm surprised you're running with toe out rather than in on the front, that will certainly make the car feel nervous in a straight line and potentially cause it to tramline

[Superwhite R283]

Had mine flat floored a few years ago:

Front camber: -1 1/2

Front toe: 1mm toe out

Front ride height: 5 5/8 inch

Rear camber: -1 3/4

Rear toe: 2.5mm toe in

Rear ride height: 6 inch

Weight distribution set up diagonals even

75mm to sump

[Tazio]

Thanks everyone,

In answer to the points raised, the car has 14 minutes toe in at the rear, tyre pressures are OK, I do run with a touch of toe out on the front, and am used to the effects this has.

The 'instability' let's call it that became noticeable after the changes were made on the rear, as I said it feels as though the rear is trying to steer the car, changes were a reduction of camber from -2.5 degrees to -1.5 degrees and a reduction in toe in from 21 minutes to 14 mins toe in...

Weird, unless there is something loose, and if there I can't find it lol.

As it is I'm going to increase the rear toe in and see how it goes.

Appreciate the input.

[Tazio]

Update.

Took the car back to Northampton Motorsport, they rechecked all the settings, nothing had changed, checked all bushes no wear found ( all this for no charge)...only left the shock absorbers as potential culprit for the instability.

Today have swapped the rears, with another set I have...the original osr is definitely soft ....so I'm quite hopeful that's the culprit for the feeling of 'instability' .

[Wrightpayne]

Bilstein only at meridian J21 M1 perhaps time for a refresh. On my list to do when I have some spare cash :-)

Ian

[Tazio]

Ian,

They are going there, the osr has no rebound..., was that or a set of new ones...but the house extension comes first.....timing eh ? lol