Need REAR caster settings, road race set-up experiment
01-19-2010, 03:54 PM
#1
Heel & Toe
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Need REAR caster settings, road race set-up experiment
HELP!,
I'm a "newbie" to these forums, but I have significant road race experience (just not in a Corvette). Does anyone have ANY data relating to best set up for REAR caster angle on 2007 Z06, 18" wheel, Michelin slicks 27.9" x 13"??? BTW, rear camber is 2.5 degrees, & car is only 2600 lbs. It would seem that some amount of negative caster (opposite of front wheels) would be in order, just based on the physics, but I would greatly appreciate any & all input, especially from someone who has experimented with different settings!
Thanks, Dale
PS just a short thanks to a couple of members for their responses over in "C6 Tech"
I'm a "newbie" to these forums, but I have significant road race experience (just not in a Corvette). Does anyone have ANY data relating to best set up for REAR caster angle on 2007 Z06, 18" wheel, Michelin slicks 27.9" x 13"??? BTW, rear camber is 2.5 degrees, & car is only 2600 lbs. It would seem that some amount of negative caster (opposite of front wheels) would be in order, just based on the physics, but I would greatly appreciate any & all input, especially from someone who has experimented with different settings!
Thanks, Dale
PS just a short thanks to a couple of members for their responses over in "C6 Tech"
01-19-2010, 06:14 PM
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On certain kind of race car chassis' (for example, my open-cockpit, tube-framed, single-seater - "Le Plastique Pig"), I will diddle the rear caster to effectively gain some extra wheel-base. It's not much, but in a really sensitive chassis, it can make a bit of a difference. Really.
In a Corvette ? Nah...I don't think you could feel a difference in the wheel-base.
In a Corvette ? Nah...I don't think you could feel a difference in the wheel-base.
01-19-2010, 08:18 PM
#5
Instructor
I played around with rear caster when I was racing a Pontiac Solstice that has adjustable rear caster from the factory. The factory recommended setting was -4 deg and I believe I was running -5 to -5.5 deg to zero out the bump steer in the rear. I have not played with this parameter on the vette since the modification would require significant effort within the class ruleset.
Eric
Eric
01-19-2010, 09:21 PM
#6
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Thanks to all responses!
As I had already posted on "tech", but forgot to mention here, I suspect that dialing-in some NEGATIVE caster in the rear would help the torque thrust transmitted thru the control arms "plant" the rear suspension (sorta like anti-squat, independent of pick-up geometry). I've never tinkered with rear caster, but I'm sure that ALMS & DP cars have rear uprights that can be adjusted for variable caster settings----& I have to guess THEY have a reason for it.
Re the Solstice; it is just a little curious how caster would affect bump steer. That is a condition that has more to do with the anchor point of the toe link being in the plane of a line intersecting the upper & lower control arms axis----but I'm willing to learn something here.
Anybody know any engineers on race teams like Pratt & Miller, or TRG?---would like to here input from a designer's perspective.
BTW, I'm fabbing custom control arms, & have custom billet uprights, so the "stock" limitations don't apply in my situation.
Dale
As I had already posted on "tech", but forgot to mention here, I suspect that dialing-in some NEGATIVE caster in the rear would help the torque thrust transmitted thru the control arms "plant" the rear suspension (sorta like anti-squat, independent of pick-up geometry). I've never tinkered with rear caster, but I'm sure that ALMS & DP cars have rear uprights that can be adjusted for variable caster settings----& I have to guess THEY have a reason for it.
Re the Solstice; it is just a little curious how caster would affect bump steer. That is a condition that has more to do with the anchor point of the toe link being in the plane of a line intersecting the upper & lower control arms axis----but I'm willing to learn something here.
Anybody know any engineers on race teams like Pratt & Miller, or TRG?---would like to here input from a designer's perspective.
BTW, I'm fabbing custom control arms, & have custom billet uprights, so the "stock" limitations don't apply in my situation.
Dale
01-19-2010, 10:40 PM
#7
Race Director
I guess it's just me, but I don't understand the concept of "caster" applied to the rear suspension of a Corvette. As it says in the definition of caster below, it has to do with a "steered wheel".
And from another source:
So.....I'm not quite sure what LS7NSX is talking about, because my Corvette rear wheels don't "steer". Since all the cars I've ever driven only have front wheel steering, I've always understood positive caster of the front wheels to be as mentioned above - improve steering stability, damping and return, and to take advantage of camber changes as the wheel is steered into a turn.
At least that's my understanding of caster - YMMV!!!
Bob
From Wikipedia, the free encyclopedia
Caster angle....is the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations.
This topic when specifically applied to bicycles and motorcycles is more commonly referred to as rake and trail. This is discussed in the article on Bicycle and motorcycle geometry.
The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel casters around so as to trail behind the axis of steering. This makes a car easier to drive and improves its directional stability (reducing its tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although, in racing, large caster angles are used to improve camber gain in cornering. Caster angles over 10 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.
The steering axis....does not have to pass through the center of the wheel, so the caster can be set independently of the mechanical trail, which is the distance between where the steering axis hits the ground, in side view, and the point directly below the axle. The interaction between caster angle and trail is complex, but roughly speaking they both aid steering, caster tends to add damping, while trail adds 'feel', and returnability. In the extreme case of the shopping trolley wheel, the system is undamped but stable, as the wheel oscillates around the 'correct' path. The shopping trolley/cart setup has a great deal of trail, but no caster. Complicating this still further is that the lateral forces at the tire do not act at the center of the contact patch, but at a distance behind the nominal contact patch. This distance is called the pneumatic trail and varies with speed, load, steer angle, surface, tire type, tire pressure and time. A good starting point for this is 30 mm behind the nominal contact patch.
Caster angle....is the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations.
This topic when specifically applied to bicycles and motorcycles is more commonly referred to as rake and trail. This is discussed in the article on Bicycle and motorcycle geometry.
The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel casters around so as to trail behind the axis of steering. This makes a car easier to drive and improves its directional stability (reducing its tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although, in racing, large caster angles are used to improve camber gain in cornering. Caster angles over 10 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.
The steering axis....does not have to pass through the center of the wheel, so the caster can be set independently of the mechanical trail, which is the distance between where the steering axis hits the ground, in side view, and the point directly below the axle. The interaction between caster angle and trail is complex, but roughly speaking they both aid steering, caster tends to add damping, while trail adds 'feel', and returnability. In the extreme case of the shopping trolley wheel, the system is undamped but stable, as the wheel oscillates around the 'correct' path. The shopping trolley/cart setup has a great deal of trail, but no caster. Complicating this still further is that the lateral forces at the tire do not act at the center of the contact patch, but at a distance behind the nominal contact patch. This distance is called the pneumatic trail and varies with speed, load, steer angle, surface, tire type, tire pressure and time. A good starting point for this is 30 mm behind the nominal contact patch.
And from another source:
CASTER ALIGNMENT
The third most important wheel alignment angle is caster [this article discussed camber and toe in previous sections], which is the forward (negative) or rearward (positive) tilt of the steering axis as viewed from the side. Caster is usually measured in degrees, and only applies to the front wheels because they are the only ones that steer (except for the few oddball Japanese cars that had four-wheel steering).
Caster is a weird angle because it doesn't affect tire wear directly. It's greatest effect is on steering stability, steering effort and steering return.
The third most important wheel alignment angle is caster [this article discussed camber and toe in previous sections], which is the forward (negative) or rearward (positive) tilt of the steering axis as viewed from the side. Caster is usually measured in degrees, and only applies to the front wheels because they are the only ones that steer (except for the few oddball Japanese cars that had four-wheel steering).
Caster is a weird angle because it doesn't affect tire wear directly. It's greatest effect is on steering stability, steering effort and steering return.
So.....I'm not quite sure what LS7NSX is talking about, because my Corvette rear wheels don't "steer". Since all the cars I've ever driven only have front wheel steering, I've always understood positive caster of the front wheels to be as mentioned above - improve steering stability, damping and return, and to take advantage of camber changes as the wheel is steered into a turn.
At least that's my understanding of caster - YMMV!!!
Bob
Last edited by BEZ06; 01-19-2010 at 10:43 PM.
01-19-2010, 10:50 PM
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Changing caster will affect bump steer....93ASP makes a good point. As you move the angle of the vertical axis i.e. caster, you cannot avoid moving the pickup point of the toe link vertically (and to some extent, laterally), which will change the bump steer. Depending on the geometry in the normal range of travel, you can possibly develop a bump steer problem...and it always seems to end up as "toe out in bump", which is scary as ****, unless it's intentional (I used to do that on the fwd VW's I raced two decades ago...but I had to be "committed to the throttle" when I turned the wheel...if you're not, you're going to get an ambulance ride). On the current race fleet, only 1 allows me to change rear caster, and I always have to check...and re-shim...the rear toe link if I jack around with rear caster.
In a word....no.
Anti-squat isn't independant of pick-up geometry. It can't be.
Think about it...torque angle on a screw doesn't change when you switch hands on a screwdriver. Anti-squat is defined by the geometry where the suspension arms attach to the chassis...and varying the caster angle won't change that.
I suggest that you read a bit on the subject...either the Milliken stuff(William Milliken..."Race Car Vehicle Dynamics", or any of his writing...he is America's unquestionable authority on the subject, and for a 90-something-year-old, he's quite a hoot in person !), or Carroll Smith's books (Tune to Win, I think). They both discuss anti-dive and anti-squat extensively.
I suspect that dialing-in some NEGATIVE caster in the rear would help the torque thrust transmitted thru the control arms "plant" the rear suspension (sorta like anti-squat, independent of pick-up geometry).
Anti-squat isn't independant of pick-up geometry. It can't be.
Think about it...torque angle on a screw doesn't change when you switch hands on a screwdriver. Anti-squat is defined by the geometry where the suspension arms attach to the chassis...and varying the caster angle won't change that.
I suggest that you read a bit on the subject...either the Milliken stuff(William Milliken..."Race Car Vehicle Dynamics", or any of his writing...he is America's unquestionable authority on the subject, and for a 90-something-year-old, he's quite a hoot in person !), or Carroll Smith's books (Tune to Win, I think). They both discuss anti-dive and anti-squat extensively.
Last edited by Don Keefhardt; 01-19-2010 at 10:59 PM.
01-19-2010, 11:03 PM
#9
Race Director
it is easily modified on the C6Z06/ZR1's, since they have adjustable upper control arms.
I don't think it would help anti-dive, or anti-squat, since the control arm angles would remain the same. It could effect bump steer, but since all motion would change fore/aft, not up/down, I think it would be minimal.
You could increase wheel base ever so slightly, but enough accomplish anything?????
I may have to play around with this at some point. Maybe test the extremes and see what happens, just for fun. VIR will be ready for us in 3 weeks!
I don't think it would help anti-dive, or anti-squat, since the control arm angles would remain the same. It could effect bump steer, but since all motion would change fore/aft, not up/down, I think it would be minimal.
You could increase wheel base ever so slightly, but enough accomplish anything?????
I may have to play around with this at some point. Maybe test the extremes and see what happens, just for fun. VIR will be ready for us in 3 weeks!
01-19-2010, 11:42 PM
#10
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Thanks for the discussion---will bone up on Milliken, then probably set rear @ Zero caster. Don't have any data that supports my hypothesis, & the rationale becomes diluted with zero-compliance, minimal-travel, rear suspension anyway. Just wanted to hear what other racers are doing in the real world!
Dale
Dale
01-19-2010, 11:59 PM
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Dale -
Just keep doing what you're doing. Ask questions. Listen to answers. Learn the "science of race cars". It's really rewarding.
Experimentation at the track is REALLY rewarding (Really....REALLY...IT'S HUGE!!!!). Screw around with all the adjustments. Find out what all the adjustable stuff does. Take lots of notes. Setting up race cars is REALLY subjective. What you like, and what makes YOU go fast, doesn't necessarily transfer to another driver/car combination. I do lots of endurance racing, and my team mates don't always appreciates how I like the set-up. Every driver is diffferent. Of course, I'm the fastest (dodges rotten tomatoes from my team-mates !)
There is no substitute for testing. Invest the time, and you'll reap HUGE rewards.
Just keep doing what you're doing. Ask questions. Listen to answers. Learn the "science of race cars". It's really rewarding.
Experimentation at the track is REALLY rewarding (Really....REALLY...IT'S HUGE!!!!). Screw around with all the adjustments. Find out what all the adjustable stuff does. Take lots of notes. Setting up race cars is REALLY subjective. What you like, and what makes YOU go fast, doesn't necessarily transfer to another driver/car combination. I do lots of endurance racing, and my team mates don't always appreciates how I like the set-up. Every driver is diffferent. Of course, I'm the fastest (dodges rotten tomatoes from my team-mates !)
There is no substitute for testing. Invest the time, and you'll reap HUGE rewards.
