suspension, roll centre height, camber and scrub

[Ntrprise]

I am building a Formula 750 car in the UK and am looking into the design of the suspension and i am not sure what is critical and what is not.

For most conditions i could tell you the roll centre height, camber and scrub. I know to get the camber as close to zero for the entire range of chassis movements.
What i do not know is what else is important? We obviously do not want lots of scrub as this would effect the tyres but is that more or less important than having a roll centre a certain distance above the ground?

Any views?

[SpeedLite]

From what I can remember about roll centres, ideally you should try to get it below ground level with the centre of gravity as low as poss.
Running with camber reduces friction in a straight line as less of ther tyre is in contact with the ground but at the cost of extra heat in the tyers. But also it means that when cornering the outside loaded tyres will have the greatest contact patch as camber will always change with suspesion movement when running wishbones.
Another thing to think about is bump steer. You do not want the steering angles to change with suspension movement. Try to get the steering arms and track rods to move in the same arcs as the wishbones by changing the height of there pick up points.
There are lots of other things to think of aswell such as anti-dive, anti-squat, castor etc.
Have you tried reading any books for help? There are plenty out there.

[HiRich]

Yeah, get a book! Many years ago I did suspension design, and:
a) it is very complicated
b) there are no perfect solutions

My bible used to be a book written in the early 70's by Colin Campbell.

Some areas:

Camber: Actually, you don't want this as close to zero. You really are looking to keep the pressure across the contact patch the same at all loads and articulations (a subtle difference, you need to consider the wheels alignment compared to the road surface, accounting for body roll, AND deflection of the tyre). The ultimate measure is therefore surface temperature across the contact patch. As you are clearly taking this all very seriously, you would be wise to incorporate camber adjustment.

Bump steer: As mentioned

Roll Steer: Not really an issue with wishbones, but certainly with trailing arms and other systems, especially live axles/de Dion. As the outer wheel rises into the body, it moves back (or forward, depending on geometry). the 'loose' inner wheel moves forward. In plan view, the axle points out of the corner. If it's the rear doing this, you are producing roll oversteer.

Front/Rear balance:
Matching front & rear roll centres, roll stiffness, CoG's. You aren't looking to get them the same, just to work in tandem.

The list is endless, so a good book is really essential. For a race car, I would suggest two tips:
- Include simple (single-point) adjustment wherever you can.
- Remember to focus on the articulation you will use most of the time, not the extremes.

[SpeedLite]

HiRich sounds more of an expert than myself.
I can't remember the name of the books I used at Uni but I will dig them out and let you know.

[avsfan733]

you will want to run in the range of 1-3 degrees of negative cambver maybe more and it varies front to back and by track

[HiRich]

Expert? Err, no

[Mattracer]

When it is said that driver X has requested a roll centre adjustment, exactly how is this done, particularly during the race, when time is crucial. I'm specifically talking about touring cars (V8 Supercars) but would like info on open wheelers too. I'm OK with camber, castor etc but I don't know the connection between that and a "roll centre". I would presume that means the Cof G of the vehicle and it seems like the central nervous system of the chassis dynamics. Does altering this imply changes to the camber/castor too? Thanks.

[GTV27]

Ususally requires alternate mounting points for the suspension (eg the centre point on a watts linkage is a common adjustment point to raise or lower the roll centre on a solid axle)

Its the sort of thing that can be done in practice, but not (to my knowledge anyway) DURING a race. Adjustable anti- roll bars however, are a diffent kettle of fish- was that what you meant?

[Onlooker]

Originally posted by Mattracer
When it is said that driver X has requested a roll centre adjustment, exactly how is this done, particularly during the race, when time is crucial. I'm specifically talking about touring cars (V8 Supercars) but would like info on open wheelers too. I'm OK with camber, castor etc but I don't know the connection between that and a "roll centre". I would presume that means the Cof G of the vehicle and it seems like the central nervous system of the chassis dynamics. Does altering this imply changes to the camber/castor too? Thanks.

It is the rear roll centre and as they all use a "watts link" rear axle location (not panhard bar)it is just a matter of raising and lowering the pivot point.It is like a little rocker wit 2 arms going to the diff kind of like steering arms stopping the axle from moving left to right.(PS this pivot bolt is normally what is broken when you see a car limping back to the pits wheels sticking out of one gaurd and not the other.
Most cars have a little threaded arangement that can just be wound in/out to raise and lower the pivot point.With the aid of a couple of uni joints it would be posible for the air jack guy to operate it.The thread is generally located in front of the fuel tank,in the boot,behind the rear firewall.

[Onlooker]

Originally posted by GTV27
Ususally requires alternate mounting points for the suspension (eg the centre point on a watts linkage is a common adjustment point to raise or lower the roll centre on a solid axle)

Its the sort of thing that can be done in practice, but not (to my knowledge anyway) DURING a race. Adjustable anti- roll bars however, are a diffent kettle of fish- was that what you meant?

You can actually adjust them whilst driving the car,a cable (similiar to anti R/B) and a well lubed and precision slider and away you go.Once on the srtaight there is very little load on the link.
NASCAr have an adjustable panhard bar the teams adjust during the pit stops through a hole in the back window and a threaded slider.

[Mattracer]

Thanks for all that guys!

[GTV27]

I stand corrected on the ajdustable watts link...no wonder they keep breaking the things as an immense amount of lateral force would be fed through that point, which if adjustable would probably be somewhat weaker than otherwise.

[Mattracer]

Perhaps an IRS would be be more in-keeping with the purist's psyche, hopwever I fear we may bore the OS contingent in the audience...

[Onlooker]

Originally posted by GTV27
I stand corrected on the ajdustable watts link...no wonder they keep breaking the things as an immense amount of lateral force would be fed through that point, which if adjustable would probably be somewhat weaker than otherwise.

As the link is a pivot there is actually very little force on the bolt in normal use.Whacking walls breaks them all as it is only around a 16mm bolt.