How much castor?

[graham bahr]

[QUOTE=Denis Bassom]It's tight corners I have a problem. I'm RWD and the FWD boys are all over me in them.

QUOTE]

more front anti roll bar worked wonders for the beemer, i simply drilled an extra couple of holes in it which gives me two stiffer settings than std.

[Al Weyman]

I have found on my camaro unequal length wishbone set up that camber increases at full droop (not good I would have thought). Would playing with castor help this situation?

[graham bahr]

i wouldn't of thought so, how much does it increase? it has to be remembered that the wheel thats drooping would be the inside one which if you cornering with any speed will only be lightly loaded, is its probably not as bad a situation as it sounds, after all if you had a cortina or one of my beemers you would find the inside front wheel is in the air! if you were to enlarge my advatar you'd see the cars three wheeling

[Al Weyman]

Just by eyeballing I would say it increases by a couple of degrees maybe more . I have it up in the air at the moment I will try to measure it. I hear what you say about the inside wheel but was more concerned that he outside wheel would be decreasing camber as it rises approaching full bump.

[Tim Falce]

[Tim Falce]

Originally Posted by Al Weyman

Just by eyeballing I would say it increases by a couple of degrees maybe more . I have it up in the air at the moment I will try to measure it. I hear what you say about the inside wheel but was more concerned that he outside wheel would be decreasing camber as it rises approaching full bump.

I have this problem and have had to set the camber to an average setup for the amount of roll I would expect in corners. The problem is that it doesn't work well in hairpins or tightish corners and with the spring rates I have it is frightening in the wet.
I tried altering the caster to compensate but it didn't have much effect.
D-type38 spent quite a few weeks on his computer redesigning his unequal length wishbone set-up to good effect and despite his lack of power is now one of the fastest cars in the two race series we enter.

[Al Weyman]

Taking that on board Falcemob and trying to work a compromise does anyone have any ball park figures of expected camber increase in relation to castor settings. I wonder if this is why the handling has got hurt bad on my car as having to raise the suspension some 1.5 inches to comply with CTCRC regs to achieve 4" clearance with smaller diameter wheels. Maybe I have too much camber now (?) as I have not measured it since I raised the suspension. I will say raising the rear suspension 3/4 inch and shedding some weight off the front and reducing front toe in (thanx by the way to whoever it was that suggested this) the car felt more stable at Silverstone yesterday although I never realy pushed it because of clutch problems. I have to find two seconds a lap but I reckon we can get there.

[Goran Malmberg]

We might be awheare that changing caster affect bumpsteer in moost cases. So if a 3 dgr change is selected, bumpsteer should be checked.
Goran Malmberg

[dtype38]

Oh, my word! Al, if I raised my ride height by 1.5 inches I'd get something like 3 deg of camber change!

Anyway, there's a simple way of checking what camber adjustment your caster setting gives. Sit your car straight and chalk a line parallel to your front wheel. Then turn the steering wheel half a turn and draw a second line. Measure the angle between the two lines in degrees and divide it by 90. Now multiply this number by the amount of castor you have and that will give you the number of degrees (of fraction of a degree) of camber change made by turning your wheel half a turn. Subtract this value from your outer wheel and add it to your inner wheel to find out how much additional camber change the castor is giving on top of your usual wishbone change. This is, of course, at half a turn of the wheel, say a hairpin. If you are in a quicker bend and only have 1/4 of a turn on the wheel, then the correction will only be half of that.

And if you like to four wheel drift around corners with the steering wheel straight and using your right foot to steer, then the castor isn't doing anything

[johnh875]

To answer the earlier question SAI = steering axis inclination (seeing as king pins have not been used for many years)

Al, might you get more neg camber also as your suspension compresses? I mean from the original height not the 4x4 setting... (worth going to taller tyres instead?) The unequal length wishbones you describe sound like they might benefit from the "Shelby mod" used on old Falcons & Mustangs, where the pivot for the upper wishbone is lowered to increase camber both static and under bump travel (also moved back for castor). I don't think it would be terribly beneficial under droop travel however this is not normally as important, affecting the inside tyres.

[graham bahr]

Originally Posted by Al Weyman

Just by eyeballing I would say it increases by a couple of degrees maybe more . I have it up in the air at the moment I will try to measure it. I hear what you say about the inside wheel but was more concerned that he outside wheel would be decreasing camber as it rises approaching full bump.

arhh full bump, you said full droop that might well make a difference then

[Al Weyman]

When I take a hairpin as often as not I am opposite locking :-).

John I have to run the Dunlop Formula R 225 x 55 x 15 control tyre that is the biggest they make and smaller than what the car originally was fitted with for the street! Which is why I feel that a control tyre is not always the way to go if you are catering for a variety of different machinery. I am not allowed to move the mounting points either so I am a bit stuffed really apart from that the set up on a Camaro would not allow the tweak that is used on the Mustang.

Graham what I mean to say (I may not be making myself clear) I have for example got the car up in the air chamging a clutch and the suspension at full drop now has about 7 degrees negitive camber an increase of say 4 degrees from the 3 degrees at the standard ride height. So I am assuming when the wheel continues through its travel when hard cornering and compresses near to the bump stop then presumably the camber will continue to decrease, not a good situation.

[Peter Mallett]

Originally Posted by Al Weyman

I am not allowed to move the mounting points either so I am a bit stuffed really apart from that the set up on a Camaro would not allow the tweak that is used on the Mustang.

You're not trying Al. I run to the same rules but I've got a shed load of castor available but sadly I don't have Hercules' arms!

[Al Weyman]

No Pete the Mustang conversion John was talking about involves redrilling the top suspension mounting hole one inch higher on the flitch panel which would not be legal or possible on the Camaro as it has a full front subframe and all the suspension is built off of this (no structual flitch panels). However camber and caster adjustment is a breeze, you just add or remove spacers but John was talking about actually moving the top wishbone up one inch.

[greenamex2]

Al, at static height, does your top arm point up, down or level from the chassis to the wheel?

How much suspension travel do you have?

PS I am in the process of testing a formula to indicate what is happening to the camber when the wheel is turning. Very basic A level applied maths stuff but I'll publish it when it's checked. This will have the benefit of being able to play with caster, camber and KPI from the comfort of your own spreadsheet.

[Al Weyman]

Hi Dennis, I know the lower arm is pararell to the ground not sure about the upper one I believe it would point down which is the one that is moved up or down (can't remember which) on the Mustang conversion. As for suspension travel, quite a lot I would have thought especially now I have had to jack it up a bit but I have not actually meaasured it

[greenamex2]

Sounds like the upper are is dropping outside of it's usable range and SERIOUSLY pulling in the top of the upright.

If your car is anything like a Corvette then it's probably got pretty short upper arms.

Lowering the mounting point on the body or raising it on the upright would reduce/solve it but is probably outside the scope of your regs. This would also help the camber curve under compression.

As it's occuring on the unloaded side it would probably be useful to check some photo's to see if you are getting that much droop on track. If you aren't particularly close to it then it's probably not worth worrying about.

[Al Weyman]

Yes very similiar set up to the Corvettes as you say I will probably have to grin and bear it but I would say you are spot on in your analasis. I'll just have to get me one of them god darned 740 bhp motors and make it up that way .

[johnny yuma]

TAKE your front springs out reassemble everything to rotors,move your suspension and steering to every available setting and graph it.

[greenamex2]

I'll build you one, £2.50 to you sir and I'll even supply a dyno sheet saying it's got 850BHP.

[Al Weyman]

Wow really just £2.50? 840bhp then I can really kick ass!

[greenamex2]

Back to the point of the thread, plotting the camber curve as somebody (can't see the entry, sorry) would give a good idea of where the camber change happens.

If it is just the last inch or so then you could do something about it (lower the car??!!). If it is over a much broader range then you may have some form of inherent problem/mismatch of components.

I would also be interested in seeing the figures just to verify how bad my car is (I'll be doing the same).

[graham bahr]

Originally Posted by Al Weyman

Graham what I mean to say (I may not be making myself clear) I have for example got the car up in the air chamging a clutch and the suspension at full drop now has about 7 degrees negitive camber an increase of say 4 degrees from the 3 degrees at the standard ride height. So I am assuming when the wheel continues through its travel when hard cornering and compresses near to the bump stop then presumably the camber will continue to decrease, not a good situation.

i see, although i doubt your suspension will be moving through its full travel on circuit, many cars will do what you describe, but the real question is how much does it change during use, a couple of cable ties around your shocks will soon give you this info, most likely is that the camber change will be minimal at normal ride height/ usage assuming your wishbones are fairly paralell ( there goes my morning bad speeling again) to the ground

[graham bahr]

Originally Posted by Al Weyman

Wow really just £2.50? 840bhp then I can really kick ass!

just think what you could do if you saved up a tenner!

[greenamex2]

Are we sitting class? Good then I shall begin.

The formula for calculating camber as your wheel turns is -

Camber = Normal camber + KPI * (1 - COS(Wheel angle)) - Castor * SIN (Wheel angle)

Note that it is Wheel angle and NOT Steering wheel angle.
Also note that KPI (King Pin Inclination) is the same as SAI, whatever than stands for.
Also also note that some spreadsheets/calculators work in Radians and NOT degrees. I some really daft numbers until I worked out Pocket Excel was one of them!

So for example, for my car at a 20 degree angle we get -

-4+8*(1-COS(20)) - 4*SIN(20) = -4.9 degrees

Or in other words, a 0.9 degree increase in negative camber.

If I added 1 degree of castor I then get -

-4+8*(1-COS(20))-5*SIN(20)=-5.22 degrees

Or, in other words, an extra 1.22 degrees of negative camber or 0.32 degrees more that I started out with.

What I have done on my PDA is plot out the existing curve at 5 degrees increments for the existing castor, then plus 0.5 and plus 1 degree.

The main thing you notice, as well as more castor give a 'steeper' rise in gained camber, is at a certain point your start LOSING camber. The point this happens at is again dictated by the castor, the more castor the greater wheel angle it occurs at.

So what, the boy with acne at the back asks?

If you are in a hairpin you turn the wheel a lot. In my case I am turning it PAST the point of best camber and start losing grip the more I turn. This explains why my car turns into hairpins well then starts to wash out, as well as why any hairpin circuit rips large lumps out of my tyres. But on fast corners (less wheel turning) the car is fine.

OK, my car is more sensitive to camber than many (I can detect a change of less than a quarter of a degree) but this probably also applies to many others.

So, in answer the question "how much castor", the answer is enough to stop you losing camber when turning the steering wheel going round the tightest corner that you are likely to encounter.

As a rule of thumb, something like 75% of your KPI seems quite good.

For me it is looking like AT LEAST a 1 degree increase.

Have a fiddle with a spreadsheet and some graphs, it was quite enlightening.