Alignment Caster problem - getting -ve caster
10-02-2010, 01:59 AM
#1
Three Wheelin'
Thread Starter
Alignment Caster problem - getting -ve caster
Checking my front camber with recently installed camber plates and found that although I get the desired -3.0 degrees camber at wheels straight ahead, at a 20-30 degree wheel angle the outside wheel is "losing" rather than gaining camber - ie the camber measurement is around -2.4 degrees at the increased steering angle. The inside wheel goes into positive camber as expected, but measuring outside camber for left and right turns is still losing -ve camber.
Tabs are pointing outwards and camber plates are set with shock shaft to rear. Car is lowered around 1.5". Is this a result of the A arms alignment (ie levelling out or pointing slightly up)?
Tabs are pointing outwards and camber plates are set with shock shaft to rear. Car is lowered around 1.5". Is this a result of the A arms alignment (ie levelling out or pointing slightly up)?
10-02-2010, 07:23 AM
#2
Three Wheelin'
Hi Eric, it's as a result of the "KPI" or kingpin inclination, as it used to be called. The strut inclines when viewed from the front at about 16 degrees (guess), which means that the angle between the strut and the wheel is about 13 degrees (16 - 3 camber).
Now imagine the strut turned at 90 degrees steering angle. The side view strut inclination is about 4 degrees (Caster) which would result in +9 degrees of camber (16 -3 -4) at 90 degrees steering angle. Therefore the camber would vary from -3 to +9 as the steering moves from 0 to 90 degrees (if that were possible).
Hope my description is clear enough!
Cheers,
Mike
Now imagine the strut turned at 90 degrees steering angle. The side view strut inclination is about 4 degrees (Caster) which would result in +9 degrees of camber (16 -3 -4) at 90 degrees steering angle. Therefore the camber would vary from -3 to +9 as the steering moves from 0 to 90 degrees (if that were possible).
Hope my description is clear enough!
Cheers,
Mike
10-02-2010, 08:07 AM
#3
Three Wheelin'
Thread Starter
Hi Eric, it's as a result of the "KPI" or kingpin inclination, as it used to be called. The strut inclines when viewed from the front at about 16 degrees (guess), which means that the angle between the strut and the wheel is about 13 degrees (16 - 3 camber).
Now imagine the strut turned at 90 degrees steering angle. The side view strut inclination is about 4 degrees (Caster) which would result in +9 degrees of camber (16 -3 -4) at 90 degrees steering angle. Therefore the camber would vary from -3 to +9 as the steering moves from 0 to 90 degrees (if that were possible).
Hope my description is clear enough!
Cheers,
Mike
Now imagine the strut turned at 90 degrees steering angle. The side view strut inclination is about 4 degrees (Caster) which would result in +9 degrees of camber (16 -3 -4) at 90 degrees steering angle. Therefore the camber would vary from -3 to +9 as the steering moves from 0 to 90 degrees (if that were possible).
Hope my description is clear enough!
Cheers,
Mike
10-02-2010, 08:18 AM
#4
Rennlist Member
Mike are you describing that look that Mercs used to (or still do) have when parked and the wheel is turned full lock and they look kinda wrong?
10-02-2010, 06:59 PM
#5
Three Wheelin'
Yes - but wouldn't the camber go from -ve 3 to say -ve 9 in that case, as -ve is with the bottom of wheel further out than the top. Turning the wheel should result in the camber increasing by the negative of the caster angle. So if you had +3 degrees caster, camber should go from -3 to -6 at 90 degrees.
So, outer wheel would go to +9 and inner wheel would go to -17.
With strut suspensions KPI is (almost always) greater than caster so you ALWAYS end up with a camber "loss" (meaning you lose negative camber).
Amuses me that so many people worry about the rear suspension (which actually has a half-decent RC height and camber curve) when the front end is so compromised. Going to a decent double wishbone setup or even SLALS (Audi A6 anyone?) would be a BIG help in the handling stakes. In 911's, the legendary snap oversteer is caused by the front suspension, not the rear.
If you want to visualise this stuff, get a styrofoam plate and stick a sharp pencil through it. Helped me understand why semitrailing arm suspensions toe-in on bump!
Last edited by mikey_audiogeek; 10-03-2010 at 07:40 PM.
10-02-2010, 07:00 PM
#6
Three Wheelin'
When caster = KPI, the outer wheel will still look fine (caster and KPI cancel out) but the inner wheel will look broken!
10-02-2010, 07:58 PM
#7
Rennlist Member
Yes that's the look I was thinking of...broken. Is this the similar effect Eric is seeing?
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10-02-2010, 08:23 PM
#8
Three Wheelin'
Thread Starter
Depends which wheel you're looking at. At 90 degrees steering angle, the outer wheel camber will increase by (KPI-caster) and the inner wheel camber will decrease (get more negative) by (KPI-caster).
So, outer wheel would go to +9 and inner wheel would go to -17.
With strut suspensions KPI is (almost always) greater than caster so you ALWAYS end up with a camber "loss" (meaning you lose negative camber).
So, outer wheel would go to +9 and inner wheel would go to -17.
With strut suspensions KPI is (almost always) greater than caster so you ALWAYS end up with a camber "loss" (meaning you lose negative camber).
The 944 (like most cars) normally has positive caster. Positive caster will make the outside wheel gain additional negative camber to aid cornering. Mine will lose negative camber. The outer wheel should not generally go to positive camber in cornering, however the inside wheel may.
If the axis of the strut is inclined forward (positive caster), as the outside wheel is turned in, it should start to gain -ve camber due to the strut inclination, no? Picture this as if the strut was horizontal (90 degrees +ve caster), when you turned the outside wheel in you would have -90 camber wouldn't you?
I would be interested to see what the camber difference is on your cars for the outside wheel straight vs turned in.
Eric
10-03-2010, 05:13 PM
#10
Three Wheelin'
Mike,
The 944 (like most cars) normally has positive caster. Positive caster will make the outside wheel gain additional negative camber to aid cornering. Mine will lose negative camber. The outer wheel should not generally go to positive camber in cornering, however the inside wheel may.
If the axis of the strut is inclined forward (positive caster), as the outside wheel is turned in, it should start to gain -ve camber due to the strut inclination, no? Picture this as if the strut was horizontal (90 degrees +ve caster), when you turned the outside wheel in you would have -90 camber wouldn't you?
I would be interested to see what the camber difference is on your cars for the outside wheel straight vs turned in.
Eric
The 944 (like most cars) normally has positive caster. Positive caster will make the outside wheel gain additional negative camber to aid cornering. Mine will lose negative camber. The outer wheel should not generally go to positive camber in cornering, however the inside wheel may.
If the axis of the strut is inclined forward (positive caster), as the outside wheel is turned in, it should start to gain -ve camber due to the strut inclination, no? Picture this as if the strut was horizontal (90 degrees +ve caster), when you turned the outside wheel in you would have -90 camber wouldn't you?
I would be interested to see what the camber difference is on your cars for the outside wheel straight vs turned in.
Eric
Like I said, KPI has the opposite effect to positive caster. If KPI is greater than caster, you will lose negative camber as lock is applied.
10-03-2010, 05:38 PM
#12
Three Wheelin'
Thread Starter
10-03-2010, 07:33 PM
#13
Three Wheelin'
Here's some more thoughts: http://www.eng-tips.com/viewthread.c...106192&page=18
Greg Locock is always good value.
Pictures here: http://www.not2fast.com/chassis/revoKnuckle.pdf
Greg Locock is always good value.
Pictures here: http://www.not2fast.com/chassis/revoKnuckle.pdf
10-03-2010, 07:38 PM
#14
Three Wheelin'
Slightly off topic, but: on our racecar with power steering, we used to run huge steering offset and huge caster. As steering lock is applied, it weight-jacks the suspension. Really helps cure understeer/oversteer conditions. Only possible with power steer!
10-04-2010, 01:40 AM
#15
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It might be interesting to try adjusting the caster via the plates and measuring the effect on your "camber loss". How much caster adjustment do the plates allow?