The most effective way to set caster on a single seater

[Alonso]

Hey guys i was woundering if you guys would give me some helpful ways to setup and check castor on a single seater car.? And what the effect of to much and to little castor will have on the handeling of the car. As well as camber on the front and rear of a wing slick car running on dunlop radial 13/08 inch tyres. And how to much and to little will effect the handeling front and rear.
Thanks guys
Alonso

[ReubenG]

You need to supply a little more information on the suspension - it's reasonable to assume that a single seater racer means the front suspension is double wishbone (double A-arms to some).

How do the suspension arms connect to the chassis and how do they connect to the upright (kingpin / bearing carrier)? If you can supply a photo of your suspension then it would help a great deal.

From my distant memory of suspension dynamics ( I was the transmission / braking guy on our team) I recall that camber is on of the more difficult suspension parameters to set. Camber can be changed by altering the length of the wishbones or the position of the suspension / chassis attachments. Castor angle can be measured from the positions of the top and bottom hinges of the upright. Frequently theses positions are fixed and adjusting castor can mean machining a whole new upright.....

I will leave the questions as to how changing castor and camber angle will affect handling to those more knowledgeable / experienced than me. If you do a search on this forum (or google) you will turn up a fair amount on this topic.

[Alonso]

This car is a double wishbone system with a pushrod and shimmed camber spacers front and rear. The wishbones are rosejointed to the chassis on mounting points. The top wishbone in front and rear is a spherical joint with aluminium block bolted to the upright. Between the block and upright there are shimms for the camber adjustment. Castor would then be increased or decreased by the rosejoint points on the wishbones.
If that gives you a better idea of the suspension layout
Thanks guys
Alonso

[RacingManiac]

I assume you can set caster by extending the rod-end(rosejoint) on one of the a-arm leg, or shimming the pickup in and out on the chassis side. Measurement though I am not too sure. Aside from trying to locate the suspension upper/lower ball joint of the upright inspace, I think you can only doing it incrimentally if you don't know what you have to start with. If you do then you can try to calculate it by how much you are adjusting the adjustment.

As to the effect of caster, the main function of caster is to provide some steering camber compensation, and to somewhat enhance the effect of static camber built into the vehicle by cambering the inside wheel "into" the corner so that the camber on the inside wheel does not fight the outside wheel and reduce the grip(and also increases the outside camber). It also increases the self-aligning force on the steering, because it physically "jack" the car up as you steer one oneside, which changes your static cornerweight. Because its effect is steering angle dependent, it lends itself more useful in lowspeed corner as you have much bigger steering input. The problem with too much caster I think is it will make the car more difficult to drive, as with the jacking effect you will be changing your corner weight and might make the car more twitchy. And it might also dilute the steering infomation through the wheel as the "feel" from the contact patch feedback from the pneumatic trail and scrub radius might be masked by the physical jacking of the caster. Which further makes it more difficult to drive....

[Carlos]

M Costin had a few comments on castor angle in his book Racing & Sports Car Chassis Design.

"Castor Angle. The angle between a line drawn through the kingpin top and bottom bearings, or equivalents, and the vertical line through through the wheel centre in side elevation."

"The position in height of the inboard pickups is governed by suspension geometry in front elevation, which can be fixed after practical experiment to get the desired amount of steering feedback. Some castor is certainly necessary, and as the limits on most designs are between 3 degrees and 8 degrees, a mean of 5 degrees would seem to be a reasonable compromise."

It's a very old source, written before he joined with Kieth Duckworth forming Cosworth to design the Ford Cosworth DFV 3000cc V8, in about '67, an F1 engine of some note which some considered very good. Although most famous as an engine designer, his opinions on suspension and chassis design are often quoted. Perhaps these references will lay credence to his stature as an engineer.
http://en.wikipedia.org/wiki/Cosworth
http://www.cosworth.com/content.php?pag ... ntentid=75

[Jersey Tom]

Changing your a-arms is typically the only way to do it.

[sdimm]

Caster is which force the steeringwheel is trying to keep the steeringwheel straight or countersteer if the car is oversteering.
A good way to determine how much caster a car should have is to put it in a big drift with just enough throttle to keep the speed. Then feel how much the steering will try to countersteer the drift by it self. If the car snapps very fast and tries to contersteer the car to much, there´s to much caster.

Try putting a normal roadcar in a drift on snow for instance and let the car turn the wheel it self and it will give you an idea of the effect.
Don´t know how to compensate if at all compensate on a car with downforce.

I´ve talked with one of the topdrivers in WTCC´s raceenginner about exactly this and that was his answer.

That more caster whould give more camber during turning isn´t quite true either since the wheels turn with such a small number of degrees that it doesn´t matter.


// Mattias

[Alonso]

Sdimm so what you saying is that to much castor would create a very nervous car because the front is to good? Would this create a snap oversteer threw fast corners such as high speed sweeps and exiting slow to medium corners power on. And also what you saying is that castor does not effect camber when turning in to corners because the degrees that you need to turn is not enough to effect it.
Thanks Alonso

[sdimm]

Sdimm so what you saying is that to much castor would create a very nervous car because the front is to good? Would this create a snap oversteer threw fast corners such as high speed sweeps and exiting slow to medium corners power on. And also what you saying is that castor does not effect camber when turning in to corners because the degrees that you need to turn is not enough to effect it.
Thanks Alonso

Well the car would try to center the steering with to much force meaning that you loose the feel in the steering and it can´t help you to countersteer a drift. Sure, I suppose that if you have to steer into the corner with great force that could make the car more nervous if it starts to oversteer since you can react to if fast enough.

It´s not about good or bad but feel.
A great handling car doesn´t understeer but oversteers just enough without making the car drift (or maybe skid is what you call it.)
So you want to be on that edge were the car is about to oversteer so much that it almost go into a drift but without actually doing so. By having the caster set as I said it can help you to countersteer if the car is about to or has started drifting just a little so much. If you for instace have to much caster in that situation it could make the wheel feel like it´s jiggeling back and forth a bit.

If you have snapoversteer it might be the differential that´s is set to weak and need more preload or whatever you can adjust on it. If not that, it might be that the car is rolling to much and you need harder front- AND rear antirollbars, maybe just rear might to it.

And the last part you wrote about caster basicly, yes. I meant If you turn the frontwheels say 10 degrees in a corner, how is caster going to affect that?

// Mattias

[geordie racer]

Kinematic camber induced from castor is more important than you think - especially when running near full lock on hairpins. Because it works for both wheels so positive camber is created on the inside and negative on the outside. So the camber thrust effect is effectively double that of just running static negative camber.

Also depending on scrub radius, positive castor can help weight transfer to the front outside wheel during breaking when the car rolls about the diagonal from the inside front to the outside rear. We found this was really effective at reducing understeer on our formula student car simply by adding a 5mm spacer on the front axle giving us a scrub radius of about 30mm. (running about 7 degrees of castor)Obviously the more positive castor/scrub radius, the more effort you need to put in at the steering wheel though.

We simply just changed the lengths of the upper WB to set castor. typically between 5-8 degrees on our FS car. Then we added shims to the lower WB point/upright which was in 2 sections to variate static camber.

[sdimm]

Kinematic camber induced from castor is more important than you think - especially when running near full lock on hairpins. Because it works for both wheels so positive camber is created on the inside and negative on the outside. So the camber thrust effect is effectively double that of just running static negative camber.

Also depending on scrub radius, positive castor can help weight transfer to the front outside wheel during breaking when the car rolls about the diagonal from the inside front to the outside rear. We found this was really effective at reducing understeer on our formula student car simply by adding a 5mm spacer on the front axle giving us a scrub radius of about 30mm. (running about 7 degrees of castor)Obviously the more positive castor/scrub radius, the more effort you need to put in at the steering wheel though.

We simply just changed the lengths of the upper WB to set castor. typically between 5-8 degrees on our FS car. Then we added shims to the lower WB point/upright which was in 2 sections to variate static camber.

Intresting, but have you tried to crank the caster way down with the spacers still on to see the difference, still getting the same grip according to datalogger then? or does the steering only get lighter?

[geordie racer]

Yeah we reduced the caster when we were running on faster circuits which made the steering quite noticably lighter, and easier! We havnt had datalogging equipment working on our car yet but to drive it definatly felt better to run some positive caster with a healthy amount of scrub radius. Although this tended to only help on sharp corners where we were braking into them - a more prominant feature on an FS style circuits but for real circuit racing - probably not that effective it i reckon.

[Jersey Tom]

Lotta stuff getting mixed up here. I'll come back to this later, but for starters... caster and mechanical trail are related but not the same.

[sdimm]

Yeah we reduced the caster when we were running on faster circuits which made the steering quite noticably lighter, and easier! We havnt had datalogging equipment working on our car yet but to drive it definatly felt better to run some positive caster with a healthy amount of scrub radius. Although this tended to only help on sharp corners where we were braking into them - a more prominant feature on an FS style circuits but for real circuit racing - probably not that effective it i reckon.

But if you don´t measure the diffrence how do you know it´s giving more grip versus just feels like more grip?

Lotta stuff getting mixed up here. I'll come back to this later, but for starters... caster and mechanical trail are related but not the same.

Nice! Someone to help us clear things up

[Scuderia_Russ]

In most instances we are able to set ours with a digital bubble on the upright before turning from lock to lock to see what readings we are getting. Obviously different vehicles have different optimum values.