Difference in Camber Gain between road car and GT3 car

[Nero]

I am interested in finding out some base data (front/rear camber gain) about the camber gain of a production sports car and what if any changes were made for the same car to run in GT3?
Can anyone assist with this data?

[AU N EGL]

Originally Posted by Nero

I am interested in finding out some base data (front/rear camber gain) about the camber gain of a production sports car and what if any changes were made for the same car to run in GT3?
Can anyone assist with this data?

WHich cars?

I race a Corvette and have a corvette convertable as a street car.

The alingment are:

Street

Front:
Camber: -0.75*

Rear: -0.5*

The race car

Front
camber -2.5*

Rear:
Camber -1.75*

Depending on the track configuration

Some of the race cars run as much as -3.5* in the front and -3.0* in the rear with the same car in street configuration -0.2 to -0.9 Street cars keep negative camber to a minium so tires last longer.

[Nero]

Thank you, but by camber gain I mean the increase in camber above the amount of roll, so the car rolls deg and the wheel is at 1.25 deg, so a camber gain of .25.

[phoenix]

Hi Nero

Just a guess here, but to get camber gain not only does the geometry have to be correct but the suspension has to move - obviously. A competition GT3 car will be on far stiffer springs than a road car so in roll there will be very little compression of the spring and movement in the suspension will be minimal, so camber compensation is likely to be less than a road car.

For this reason the teams probably put on more static negative camber so that the camber lost through roll leaves the tyres with the correct amount of negative camber when cornering.

I have no data for a GT3, but on our chassis (a GT of sorts) very roughly if we double the wheel rate we get half the roll compensation. However we will also have half the amount of roll. On Avons we run 3 degrees negative front and 1.5 degrees negative rear to compensate. On Pirellis we ran previously we ran 4.5 degree negative front and 2.5 degrees negative rear to get the same tyre temps.

[phoenix]

Just to add, you can also add camber in a corner to the front wheels by increasing the castor angle, though there can be negative effects from doing this.

[Nero]

Good points Phoenix, good points. I am building a spaceframe mid engined car that will have a reasonable amount of travel with track work being the hardest it will ever run so geometry will be built to suit. I am naturally interested in what the manufacturers actually do and why and then what changes are done for circuit work.
www.racemagazine.com.au

[phoenix]

Originally Posted by Nero

I am naturally interested in what the manufacturers actually do and why and then what changes are done for circuit work.
www.racemagazine.com.au

Manufacturers normally make cars that have very safe handling - i.e. they tend to understeer before cornering forces get too high.

The manufacturers may design this into the suspension geometry by having less favourable roll compensation on the front suspension than on the rear, or they may choose a different approach, such as much higher roll stiffness at the front which will mean the front tyres are overloaded and slide long before the rear tyres.

Most road cars when pushed round a circuit wear the outside shoulder of the front tyres very badly - this is a sign of little or no roll compensation at the front.

Limiting chassis roll is a good starting point, but if you want to use the car on the road you won't be able to go too stiff before the car becomes unbearable to drive on the road for any distance.

On the track you want a car where the grip available from both the front and rear tyres are exploited fully so I suggest that you design the front and rear roll recovery to suit your designed suspension travel, but if your design gives the same camber recovery front and rear, also design the car to have more front roll stiffness than at the rear or it could be very nasty to drive.

Design the camber recovery to be adjustable if you can - at the front anyway - then test it.

You could also use different springs and bars at the circuit (if they are quick and easy to change) so you can have two cars for little more than the price of one!

[Nero]

I have noticed that many manufacturers are now shaping not just the kinematics' but also the other aspects of grip...the most obvious being the VX220's very slim front tyres. It is difficult for a customer to change settings and get into trouble when there is more limited mechanical grip regardless...or maybe I'm a cynic!
My car is designed to be a compromise for different events and road use and I understand and accept that it will not be optimal for the track...lets face it I'm the biggest speed hump. However what I want to build is a vehicle with relatively benign behaviour at the limit, even if this means the limit is 5% lower than it would otherwise be. My rationale being that if a driver such as I with limited talent is not afraid of approaching the limits then I will do so and be consistently faster. Of course it is possible that the vehicle will be more forgiving in changing conditions too.
What do you feel about the theory of rubber contact patch equalling weight distribution?
Changing settings will not be that easy at one level as camber gain is interdependent on other settings.

[phoenix]

Originally Posted by Nero

What do you feel about the theory of rubber contact patch equalling weight distribution?

Most mid and rear engined two wheel drive cars with lots of power have larger (wider) rear tyres than front tyres. From what I have read this is less to do with weight distribution and more to do with traction required under acceleration, particularly when the tyre is required to give lateral grip at the same time, as in accelerating out of a corner.

[VT-R]

No I wouldn't say you were being cynical regarding the VX's front tyres...

If you look at most current MR production cars (and also some single seat race cars) they tend to towards having 15% wider tyres on the rear than on the front.

I think the VX is closer to 25% which I would say is to do with the fact that Vauxhall would prefer to build a sports car for Joe Public that's a little less focused, with more understeer, than gain a reputation for building a ditch finder.

[Splatz the Cow]

Yeah, for mid engine sports cars, having fronts tyres with a width closer to the rears makes the front over-tyred.

If the engine has plenty of grunt then you can make throttle adjustments to balance the handling as necessary.

If the turn is ok, then having front tyres a bit slimmer helps the rear grip balance considerably, meaning less entry oversteer.

[Y.N.M]

Originally Posted by Nero

I am interested in finding out some base data (front/rear camber gain) about the camber gain of a production sports car and what if any changes were made for the same car to run in GT3?
Can anyone assist with this data?

I think that it would vary from car to car? Definately depends on what type of suspension the car would have ie. McPherson strut or Double Wishbone. A Double Wishbone system would give you more control over your camber during roll, bump and droop but that will also depend on the geometery of your design.

I"m not 100% sure but I think if your fabricating your car the best option would be an Unequal length Double Wishbone setup with shorter upper arms. That should be able to give you good camber gain depending on your design.

If I'm incorrect someone please correct me!

[phoenix]

Originally Posted by Y.N.M

I think that it would vary from car to car? Definately depends on what type of suspension the car would have ie. McPherson strut or Double Wishbone. A Double Wishbone system would give you more control over your camber during roll, bump and droop but that will also depend on the geometery of your design.

I"m not 100% sure but I think if your fabricating your car the best option would be an Unequal length Double Wishbone setup with shorter upper arms. That should be able to give you good camber gain depending on your design.

If I'm incorrect someone please correct me!

You are absolutely correct in saying that unequal length wishbones (set at the correct angles) give the most camber compensation.

However, as mentioned above, the suspension has to allow the wheel to move up in order for the wishbones to increase the camber angle. If the suspension is virtually solid, by the fitting of very hard springs, very little negative camber will be added. Hence the need to set the static camber more negative.

[Y.N.M]

Your right Pheonix, but in saying that, in reply to Nero's original question about camber gain, wouldn't the gain be the same per unit of wheel travel regardless on the spring rates?

[phoenix]

Originally Posted by Y.N.M

Your right Pheonix, but in saying that, in reply to Nero's original question about camber gain, wouldn't the gain be the same per unit of wheel travel regardless on the spring rates?

Yes indeed, the rate of change would remain the same, but the rate of change is not linear unless using equal length parallel wishbones.

So, for example, with unequal length wishbones the rate of change in the first 10mm of suspension movement may not be the same as in the second 10mm of movement; because the upper and lower links are describing dis-similar arcs, more camber is added exponentially as the suspension deflects, so the first increment adds less camber than the next. This is not what is required, as roll increases linearly with the cornering force.

With a softer spring, the non-linear rate of change could be anticipated and the wishbone lengths and angles set so that as at 3.5 degrees of chassis roll, and whatever suspension displacement that results in, 3.5 degrees of negative camber are added.

However, doubling the spring rate resulting in 1.75 degrees of roll and half the suspension displacement, would result in less than 1.75 degrees of additional negative camber, with the suspension links set the same. So a different rate of change would have to be tuned in to the suspension to compensate. There are a number of ways to achieve this; a shorter upper link, a longer lower link, lowering the inner pickup of the top wishbone, raising the inner pickup of the lower wishbone. All of these have an effect on roll centre location and swing arm length, so it's either back to the drawing board and calculator to re-design the supension geometry, or time to add some more static negative camber.

[Al Weyman]

Lowering the upper inner mounting point on a double wishbone suspension will increase camber gain, look how many old historic Mustangs are modified like this by redrilling the mounting point an inch or so lower on the fletch panel, known as the Shelby mod I believe. A similar thing is done on 1st gen Camaros known as the Guilstrade mod but that is far more complex needing cutting and shutting of the main frame and would be illegal in historic racing but bet its still done (not on my 2nd gen as the factory already did it)! ;-)

[phoenix]

Originally Posted by Al Weyman

Lowering the upper inner mounting point on a double wishbone suspension will increase camber gain

I think I covered that in my previous post:

Originally Posted by phoenix

There are a number of ways to achieve this; a shorter upper link, a longer lower link, lowering the inner pickup of the top wishbone, raising the inner pickup of the lower wishbone.

[Al Weyman]

OK and I just gave a couple of real life examples, :=)

[Y.N.M]

Agree with that pheonix.

Nero if you are building a car using why are you interested in what camber gains from a particular production to GT3 car it would produce? I would imagine it what you need would be specific to your car?

Would it be correct to say Nero would need to figure out what kind of weight transfer and roll angles he will encounter before looking into the camber gain needed and then calculate that too suit?

[phoenix]

Originally Posted by Y.N.M

Would it be correct to say Nero would need to figure out what kind of weight transfer and roll angles he will encounter before looking into the camber gain needed and then calculate that too suit?

Yes, indeed. Without knowing the roll angle at, say, 1G, and the weight transferred onto each outside wheel at 1G (so that the spring deflection and therefore suspension movement can be calculated) then Nero would not have sufficient information to define the camber compensation required.

[dtype38]

Is camber compensation not purely a funtion of suspension geometry. IE, say the suspension arms are designed to give 1 deg camber gain/loss (outer/inner tyre) per 1 deg of chassis roll, does it matter if the car rolls by 1/2 deg or 3 deg?

[phoenix]

Originally Posted by dtype38

Is camber compensation not purely a funtion of suspension geometry. IE, say the suspension arms are designed to give 1 deg camber gain/loss (outer/inner tyre) per 1 deg of chassis roll, does it matter if the car rolls by 1/2 deg or 3 deg?

If you plot the camber compensation on your own car, I am sure you will find that the camber gain is not in a 1:1 correlation with suspension travel, whereas roll angle does have a 1:1 correlation with suspension travel. Therefore you may have set the geometry to give 1 degree of camber compensation at 1 degree of roll, but at 1/2 degree and 3 degrees of roll you will have a different amount of camber compensation (in degrees) than you have roll. As ever, with chassis tuning, this is all about compromise and optimisation.

[dtype38]

Ok, I see your point. Just in my case I spent many hours on my cad system finding positions and lengths for my wishbones and their pivots so that they pretty much gave constant camber gain per degree of roll from 0 to 3 deg.

Handled like the dogs danglies mid corner, but sadly it put my roll centre in a strange place and braking and turn in issues meant that it wasn't a very good compromise.

And so continues the search for a perfect set-up

Anyway, back to the thread....

I think Nero is asking if anyone has any suggestions for setup data for his GT3. AU N EGL bunged a few up. Anyone else got any?

[phoenix]

Originally Posted by dtype38

I think Nero is asking if anyone has any suggestions for setup data for his GT3. AU N EGL bunged a few up. Anyone else got any?

Frankly, I would give Porsche a call - or one of the teams running a GT3 - for some suggestions. So much depends on the tyres used too - so I would also speak to the racing department of the tyre brand being used - they collect a lot of useful information. For example, on the same car Pirelli wanted 4.5 degrees negative (to get heat into the tyres) and Avon wanted no more than 3 degrees - 2 1/2 ideally. The tyre requiremets are really the starting point for this.

[johnlear]

Neil,
I assume this is for the front suspension on the 'Godiva' project car? (i.e. nickname for the car being designed).

Isn't this car going to be using a front SLA suspension? If so then any numbers from a Mac Strut car will be irrelevant since you should easily be able to do get a far superior camber curve with SLA.

SLA should also mean you ought not need to use as much static neg camber because (obviously) you'll gain more camber in roll than with a Mac Strut (Mac Strutted racing cars seem to require very substabtial static neg camber in order to keep the outside wheel near vertical when cornering hard, at the expense of poor contact patch size / loading per sq cm when braking, and very disadvantageous camber at the inside wheel when cornering).

Lesser static neg camber is a 'good thing' because it will be better for braking performance, can't hurt tyre wear, as well as improving inside tyre grip whenever the inside tyre happens to be significantly loaded (this is particularly important for FWD racers that are typically set up so that most of the lateral weight transfer occurs at the rear in order to maximise front grip, less important for a RWD car where the inside front does relatively little work).

A bit off topic, but I would also suggest that you might consider using a whack of caster angle for the steered camber gain it gives (something like 10° or so, an arbitrarily biggish number...), but also for the steering response created by the greater trail that is typically concomitant with larger caster angles.

It would be best to design the suspension from scratch with higher caster in mind rather than build it with lesser caster then later decide to increase caster. This is because increasing caster also creates bump steer (the tie rod end height changes with changes in caster), and addressing bump steer can be a PITA.