Adjusting ARB's to improve handling.

[MikeW]

Just trying to understand the theory on this a bit better. Typically if I'm suffering understeer I will tend to soften the front by either changing to a softer anti roll bar or removing all together- in practise this works fine. If it's oversteer then I would soften the back...but IIRC I could also look at stiffening the back and stiffening the front in response to the understeer/oversteer issues. Can anybody explain the theory and also the pros and cons of softening the front versus stiffening the back for the understeer?

 

MikeW

Mega Grad Race No 22

[Peter Carmichael]

When the car is loaded up in a corner, the body rolls until the cornering loads are countered by the righting forces in the suspension. The total righting moment is the sum of the contribution of the front and rear suspension (i.e. the difference in the left and right wheel loads at both the front and rear of the car).

 

When you stiffen up, say, the front ARB only. You get less roll angle at the stage where the car reaches equilibrium. The difference in the wheel loads at the REAR are less than they were before (less roll angle, same stiffness). The difference in the wheel loads at the FRONT are more than they were before (total righting moment MINUS the reduced rear wheel righting moment).

 

There is a quirk in the frictional behaviour of real tyre materials. The simplistic view of friction is that grip is proportional to load. It turns out that frictional grip is non-linear with load. The effect of this is that you get less grip out of an axle the larger the weight transfer. So in our example above, the stiffer front ARB has concentrated the weight shift at the front axle: the front axle will grip less; the rear axle will grip more.

 

IF ONLY IT WAS THIS SIMPLE.

 

There is an additional effect widely ignored by tinkerers. When you stiffen up the chassis' roll behaviour, the car takes up less extreme roll angles. This gives the front suspension an easier time keeping the front tyres flat to the ground, which in turn generates more front grip. IT COULD BE THE CASE THAT THE TWO EFFECTS WILL CANCEL EACH OTHER OUT. The rear axle (deDion or Live) is immune to roll affecting the orientation of the rear tyre contact patch.

 

The net result of this is that stiffening up the front ARB may or may not result in a more pronounced understeer characteristic.

 

By contrast, stiffening up the rear ARB will always result in a more pronounced oversteer characteristic (as far as this theory goes).

 

Just to make it more complex still, none of this has bothered with the complexities of suspension dynamics (geometry changes of the steered front wheels, damping etc.)

[Jason Plato]

also , dont tbe tempted to reduce the effective grip at one end of the car to match the other .

 

So If you are suffering understeer , dont reduce the grip at the rear to match the front .

 

Try and increase the grip at the front to match the rear

[thinfourth]

so a stiffer rear gives more over steer 😬

 

autotest sunday will try max rear

 

Sod the heater wheres my shades

[metal mickey]

Peter C , thats explained a conversation I had with a very fast and experienced sprinter/hillclimber a few years ago. He said he'd stiffened the front of his car and found more grip thus reducing understeer. I'm suffering with understeer after a change to wider and stickier rubber on the rear(cr500).I've softened the front to try and gain grip but may try stiffening it.

 

[Peter Carmichael]

also , dont tbe tempted to reduce the effective grip at one end of the car to match the other

 

Although many other effects come into play, this sort of comment is irrelevant when talking about the simplistic ARB behaviour I have described. ARBs transfer grip from one end of the car to the other. They do not reduce grip per se.

 

Now there are issues to do with roll compliance that affect dynamic grip and a stiffened up set of ARBs may lessen roll compliance. This is an entirely different effect to the one I described before and is to do with the car regaining grip when it loses it. This has a lot more to do with roll centres and hence ride heights.

[Jason Plato]

my "irrelevant" comment wasnt anything to do wth your script Peter . It was a simple observation .

 

So good to see you back .......

[Peter Carmichael]

I'm glad to find out the old abrupt, offensive writing style is still working. 😳

[MikeW]

Thanks guys. A bit wiser now....I think 😬

Peter, as a matter of interest if you had understeer say on the first practise of a sprint what would you alter first 🤔

 

MikeW

Mega Grad Race No 22

[Peter Carmichael]

Well, I'm probably the wrong person to ask as the only change in setup I have made over the last two years has been tyre pressure.

 

There are lots of different types of understeer, depending on where it happens in the corner. You also have to consider where your setup is compared to a last known good handling setup. "What has changed?"

 

Are we talking classic Seven handling slow speed understeer? What tyres? What tyre pressures? What current springing? What ARBs? When did you last have your toe setting checked? What did you fiddle with last?

 

You have to take a balanced approach to all the possible parameters.

 

I've probably got the least experience of anybody around here with list 1A tyres... etc.... so the answer is probably: "stick it on A24 compound ACB10s and it will stick like the proverbial"

[MikeW]

Thanks Peter and points taken. To put my question into more perspective. I've got a good handling 7, flat floored, raced regulaly and competitive. However there are times at circuits or sprint venues where there can be a certain amount of understeer where I sense i could be losing time. Thinking about it probably characterised as mid speed corners where the power needs to be back on well before the apex. Springs are fixed by the regs, tyres are 48R's. With this in mind I have ( I think) a limited number of variables to play with ARB's, rake, tyre pressures and importantly a limited amount of time. So I guess I'm just appreciative of any inputs that might help to dial in a more optimised set up a bit more quickly.

 

MikeW

Mega Grad Race No 22

[RESOLVIWOLF]

"Well, I'm probably the wrong person to ask as the only change in setup I have made over the last two years has been tyre pressure."

 

LMAO

[Mr Woo]

Maybe I'm just stupid but shouldn't all this theory have been done to death now and all the different ways suspension can be set up and the effect be well documented? Perhaps it is just a black art but I have found doing other hobbies there are usually right and repeatable ways to do things but unravelling myth and guesswork from genuine knowledge and experience can be pretty tricky

[Richard Price]

Mike,

How much negative camber do you run?

You saw me adjusting mine at Llandow - after adding another half degree negative, understeer was reduced and I took over a second off my previous best time.

Unlike some folks who get quicker and quicker through the day, my times at sprints are normally quite consistent (if I don't go off 😳), with my third timed run 0.27sec slower than the second. I'd also add that both my practice runs were slow becuase they were affected by fuel surge - sorted by adding a couple off gallons of petrol 😳.