[Editor's Note: This article originally appeared in the November 2012 issue of Grassroots Motorsports.]
When I started my pro career in the Touring Car class of the Pirelli World Challenge (then known as Speed World Challenge), I, like many drivers before me, had visions of big, powerful GT cars dancing through my head—big, powerful, rear-wheel drive GT cars to be …
People always talk about caster and how it adds camber as you turn... This is true however it also jacks the chassis in a bad way by moving the outside tire upwards and the inside tire downwards which results in more body roll.
Dick Shine used to say that caster was evil and to never increase it. I forget where I ran upon the discussion... but there is some kind of analysis that was done at some point to see why he might have thought this... And it had to do with the tire's "trail" and where it fell in relation to the contact patch causing a disconnect in steering feel as grip is exceeded. In a nutshell, you could more easily sense the limits of the car with less caster, so while more caster might in theory give you more grip, the point at which traction is exceeded is different than where you feel feedback in the wheel or some such. I wish I could find the discussion again.
In reply to flatlander937 :
Caster will never add negative camber on turn in, because steering axis inclination takes camber away.
Well, you could if you had caster higher than the 10-15 degrees of SAI your car may have.
Either way, the effect is only noticable at high steering lock. When you are driving on course you are rarely over 10 degrees of steering angle and usually under 5. Changing camber with caster adjustments is like shuffling the deck chairs on the Titanic, technically you are making a change but we're talking a tenth of a degree or so.
In reply to Pete. (l33t FS) :
I want to say I've heard that before, but for some reason the way you said it just clicked with me.
I'm thinking that if you decrease caster / minimize it, it would have the effect of keeping the chassis flatter when turning. Perhaps jacking the rear inside up making it looser is why this is so common for people to add caster on a front wheel drive car? Generally speaking it will feel better.
I really enjoy my R53 as a daily runabout and occasional track car, but RWD will always be my go to for toys. It's just the way God intended it.
I've auto-crossed FWD for decades and the most SOTP effect with more (positive) caster is steering feel as it relates to the self-centering effect, if both sides are equal. In autocross, additional caster does lead to valid camber gain as the steering inputs are much greater on smaller, tighter course. The camber/toe comments are dead-on though. The one significant drawback to the loose rear is during rapid transitions (slalom) as local club course designers seem to really like them. If the rear is loose enough to corner well, it is generally too loose for best time in transitions. Road racing, where slalom doesn't really apply, additional caster isn't nearly as effective. Suspension set is always a compromise with every setting, so it really depends on the driver "feel" for the given setup.
Another point to make is that professional race cars have to run with very, very firm/hard suspension bushings, often metal or Delrin® to be competitive. They know something. Now, to make an enthusiast’s competition vehicle more able to maintain your desired competition alignment settings you need to solve that with performance suspension bushings. Performance urethane/polyurethane to the rescue! Pictured is a complete PROTHANE performance urethane suspension bushing kit for a Honda Civic (on the left) and their stiffer than stock rubber rear sway bar bushings (on the right). Remember in the article, about the rear sway bar being more important than the front? Also, using them is very close to installing a more robust suspension. Of course, every type of vehicle is different, but to remove the suspension slop/looseness overall is desirable!
This article has been my guide for modifying, my Fiesta, nice to see it out again. When you do ALL of this, and then lower the car and stiffen the suspension and upgrade the brakes, no bar up front is no-bueno. My bigger front bar was from a GRM article or post somewhere, I'd love to see a Phase II follow-up article on what happens after you do what's in this article.
This is a pretty sophisticated FWD set up story and conversation but I did have one question. I too have been racing FWD's for many years and early on, the folks that did my initial alignments, a race shop in LA, set the front toe out, and the rear in, not a Robb suggests, rear out.
My understanding was the rear toe-in steered better into the corner much like the Honda Passive rear steering, as the suspension loads up, the inside rear tire toes in. The sooner the car rotates, the sooner the throttle can be applied.
In reply to Brock2 :
Nope. Toe in is more stable under braking but that is because it stops the rear from moving around. Toe out in the rear is what helps the car to rotate. If your shop is saying different then take your car to another shop
As someone who spent 20 years driving as "right wheel drive" purist, having recently started driving/racing a fwd car, I will admit that I was wrong and completely agree with the opening of the article. Given I was never exposed to anything other than sloppy econo-boxes, (nothing wrong with econo-boxes), I had a strong bias based on things I believed to be true, but never experienced firsthand.
flatlander937 said:People always talk about caster and how it adds camber as you turn... This is true however it also jacks the chassis in a bad way by moving the outside tire upwards and the inside tire downwards which results in more body roll.
Dick Shine used to say that caster was evil and to never increase it. I forget where I ran upon the discussion... but there is some kind of analysis that was done at some point to see why he might have thought this... And it had to do with the tire's "trail" and where it fell in relation to the contact patch causing a disconnect in steering feel as grip is exceeded. In a nutshell, you could more easily sense the limits of the car with less caster, so while more caster might in theory give you more grip, the point at which traction is exceeded is different than where you feel feedback in the wheel or some such. I wish I could find the discussion again.
Isn't the body roll in a favorable direction? The weight moves off the front outside and onto the front inside. You have to run some amount of caster and like most setups it's about finding the optimal amount.
https://youtu.be/XxZzUS7NmWs
I've been very pleased with camber plates run sideways for castor. I believe I have it set with 5deg caster, 1.5seg camber, and 0 toe. With the rear set for some toe, I don't remember but think it was around 1/16". Perfect for autox and spirited street driving. I wouldn't want anything that tail happy on a road course as it does change directions rapidly.
Robb just won the GT4 class in GT America in Nashville last night.
djhedges said:flatlander937 said:People always talk about caster and how it adds camber as you turn... This is true however it also jacks the chassis in a bad way by moving the outside tire upwards and the inside tire downwards which results in more body roll.
Dick Shine used to say that caster was evil and to never increase it. I forget where I ran upon the discussion... but there is some kind of analysis that was done at some point to see why he might have thought this... And it had to do with the tire's "trail" and where it fell in relation to the contact patch causing a disconnect in steering feel as grip is exceeded. In a nutshell, you could more easily sense the limits of the car with less caster, so while more caster might in theory give you more grip, the point at which traction is exceeded is different than where you feel feedback in the wheel or some such. I wish I could find the discussion again.
Isn't the body roll in a favorable direction? The weight moves off the front outside and onto the front inside. You have to run some amount of caster and like most setups it's about finding the optimal amount.
https://youtu.be/XxZzUS7NmWs
Weight yes.
But look at the entire body when you turn the wheel. You're essentially shortening a leg of a table(outside front) and lengthening another (inside front). Of course it has "more weight" on the inside... But now the whole thing flops over even easier towards the outside.
Nobody ever looks at the ENTIRE CAR. FWD cars almost always lift the inside rear tire. So based on that, your front struts are DIRECTLY tied to how far over the body rolls. What else happens when the body rolls more? You lose more camber in the rear too!
What happens if you gain camber up front while turning due to added caster... the entire body rolls more, and your rear outside tire is now leaning over even more (assuming you didn't make any changes out back). So the car feels looser.
Is it faster? Probably (because most FWD stuff sucks to begin with). Is it fastest? I don't think enough people think for themselves to actually test and find out (and it certainly is a pain to do, so I understand it).
jb229 said:Robb just won the GT4 class in GT America in Nashville last night.
And we chatted while I was there.
Robb was like, I still get comments on this article. And physics haven't changed since.
For my fwd street stud tire class ice racers I've morphed into adding castor and leaving the camber stock.
We have standing starts,our track is biased with only one left/right hand corner depending on track direction that day so I used to run staggered camber for track direction.
The studs bite better when pushed into the ice with camber vs scratching across it.
Lots of other tricks off course but for low grip adding castor works for me.
I was just rereading this article yesterday, with the plan to take my econobox for an alignment, before Sunday's autocross.
I just started road racing a FWD 1993 Mazda MX6/Ford Probe with a 2.5 L V6 and I'm taking all this advise to heart. What I'm curious about is what front spring rates are recommended. What the car came with is 1400k/800lbs in front which seems too stiff. Also I adjusted the spring preload by jacking up the front and just taking the slack out of coilovers, is this a good starting point?
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